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This is the stable version of the full-notch creep test ontology (OntoFNCT) that ontologically represents the full-notch creep test. OntoFNCT has been developed in accordance with the corresponding test standard ISO 16770:2019-09 Plastics - Determination of environmental stress cracking (ESC) of polyethylene - Full-notch creep test (FNCT).
The OntoFNCT provides conceptualizations that are supposed to be valid for the description of full-notch creep tests and associated data in accordance with the corresponding test standard. By using OntoFNCT for storing full-notch creep test data, all data will be well structured and based on a common vocabulary agreed on by an expert group (generation of FAIR data) which is meant to lead to enhanced data interoperability. This comprises several data categories such as primary data, secondary data and metadata. Data will be human and machine readable. The usage of OntoFNCT facilitates data retrieval and downstream usage. Due to a close connection to the mid-level PMD core ontology (PMDco), the interoperability of full-notch creep test data is enhanced and querying in combination with other aspects and data within the broad field of materials science and engineering (MSE) is facilitated.
The class structure of OntoFNCT forms a comprehensible and semantic layer for unified storage of data generated in a full-notch creep test including the possibility to record data from analysis and re-evaluation. Furthermore, extensive metadata allows to assess data quality and reliability. Following the open world assumption, object properties are deliberately low restrictive and sparse.
With this presentation, the push-out technique is explained. The focus of the experimental work is on the characterization of the fiber-matrix interface of short fiber reinforced composites. The reinforcing component was glass fibers and the matrix polymer was PA6.6 and PPA.
It is demonstrated for the first time that the push-out technique ca be applied on injection molded short fiber PMC and is sensitive to the mechanical interface properties. Further studies are planned on the influence of multiple processing, the temperature and humidity.
The dataset provided in this repository comprises data obtained from a series of full-notch creep tests (FNCT) performed on selected high-density polyethylene (PE-HD) materials (for further details, see section 1 Materials in this document) in accordance with the corresponding standard ISO 16770.
The FNCT is one of the mechanical testing procedures used to characterize polymer materials with respect to their environmental stress cracking (ESC) behavior. It is widely applied for PE-HD materials, that are predominantly used for pipe and container applications. It is based on the determination of the time to failure for a test specimen under constant mechanical load in a well-defined and temperature controlled liquid environment. The test device used here also allows for continuous monitoring of applied force, specimen elongation and temperature.
Lightweighting as a cross-cutting technology contributes significantly to achieve the European Green Deal goals. Based on, but not limited to, advanced materials and production technologies, the demand for natural resources and CO2 emmissions are reduced by lightweighting during production, as well as use phase. Therefore, lightweighting is a crucial transformation technology assisting in decoupling economic growth from resource consumption. In this manner, lightweighting contributes significantly as a key technology of relevance for many industrial sectors such as energy, mobility, and infrastructure, towards resource efficiency, climate action and economic strength, as well as a resilient Europe. To strengthen international partnerships, addressing global issues of today at the edge of science with high performance lightweight material based on carbon fibers, an overview about the BAM expertise in carbon fiber reinforced materials is given.
Im statistischen Mittel steht eine Windkraftanlage (WKA) in Deutschland alle 6 Jahre wegen eines Schadens an den Rotorblättern und verursacht Kosten von 20k€-40k€ infolge Ertragsausfall und Reparaturkosten. Bei über 30.000 WKA in Deutschland verursacht dies jährlichen Kosten von 100-200 Mio. €. Zu 70% sind die Ursachen Fertigungsfehler. Diesem Themenkomplex widmet sich FB-5.3 in Kooperation mit FBs der Zerstörungsfreien Prüfung (Abt. 8) schon seit Jahren.
Im aktuellen Projekt geht es um einen vorzeitigen Verschleiß der Rotorblätter infolge der Verwendung gealterter Glasfaser-Rovings. (GF). Die Entwicklung moderner Windkraftanlagen hatte in Europa seinen Ursprung in den 90er Jahren. OEMs und GF-Hersteller waren dicht beisammen. Die Globalisierung des Marktes führt heute zu einer weltweiten Verschiffung der GF in feucht-warmen Containern. Je nach Beschaffenheit (Chemie) der Schlichten (Oberflächenbehandlung) von GF können diese stark altern (Hydrolyse-Effekte) und die Festigkeit kann im Bauteil um bis zu 50% abgemindert sein.
In einer Kooperation mit der Rotorblattallianz, einem Zusammenschluss der OEMs, Halbzeugherstellern und Forschungsinstituten, wurden schon vor 5 Jahren in einem Forschungsprojekt (FB-5.3; VH 5538) die chemischen Mechanismen der Alterung exemplarisch für eine Glasfaser-Roving-Type aufgeklärt. Im aktuellen Projekt (FB-5.3; VH 5304) wird ein beschleunigtes Alterungsverfahren (erhöhte Temperatur und Feuchte) am Roving im un-impregnierten Zustand entwickelt und die Festigkeit nachfolgend nach Einbettung in eine Epoxid-Harz-Matrix bestimmt. Je nach Alterungszustand zeigt sich eine Abnahme der Zwischenfaserbruchfestigkeit von bis zu 50%. Nicht alle Glasfaser-Roving-Typen zeigen diesen Effekt. Es ist gelungen für OEMs und Halbzeughersteller ein handhabbares Verfahren zur innerbetrieblichen Qualitätssicherung zu entwickeln, da die wenigsten Firmen über komplexe Analyseverfahren, wie an der BAM, verfügen.
Epoxy R-Glass Fiber-Reinforced Polymer (GFRP) composite plates were hydrothermally aged at 60 ◦C for 23, 75, and 133 days. The water content reached 0.97 wt%, 1.45 wt% and 1.63 wt%, respectively. The studied GFRP matrix was inert to hydrolysis or chain scission, allowing for investigation of irreversible changes in the fiber-matrix interphase due to hydrothermal aging upon re-drying. During each period, a subset of the specimens was removed from the water bath and dried in a chamber. The weight loss upon drying was explained with epoxy leaching (impurities), sizing-rich interphase hydrolysis, glass fiber surface hydrolysis, accumulated degradation products escaping, and water changing state from bound to free. The influence of hydrothermal aging on the fiber-matrix interfacial properties was investigated. Lower interfacial strength of hydrothermally aged (wet)
samples was attributed to plasticization of the epoxy, plasticization and degradation of the sizing-rich interphase (including formation of hydrolytic flaws), and hydrolytic degradation of the glass fiber surface. The kinetics of epoxy-compatible epoxysilane W2020 sizing-rich interphase hydrolysis provided an estimate of ca. 1.49%, 4.80%, and 8.49% of the total composite interphase degraded after 23, 75, and 133 days, respectively. At these conditions, the interface lost 39%, 48%, and 51% of its strength. Upon re-drying the specimens, a significant part of the interfacial strength was regained. Furthermore, an upward trend was observed, being 13%, 10% and 3% strength, respectively; thus, indicating a possibility of partial recovery of properties.
Wind turbine rotor blades are produced as aerodynamic, three-dimensionally formed sandwich shell structures using mainly glass fiber reinforced plastics (GFRP). The GFRP used today have the potential to withstand the harsh operating conditions of rotor blades over the projected service life of 20 years. Premature damages that occur can be traced primarily back to design and/or manufacturing imperfections, as well as unusually high load scenarios (for instance lightning strikes, or strong winds combined with system control errors). Therefore, rotor blades have to be repaired several times (on average 2-3 times) during their projected service life. Approximately 70% of these are repairs of small- to medium-sized sandwich shells, carried out by technicians directly accessing the blade by suspended roping at the wind turbine location. The goal is to execute repairs that will last over the remaining service life, i.e. be sustainable.
Within the framework of a research project carried out at BAM, Division 5.3 Polymer Matrix Composites, Department 5 for Materials Engineering, not only was the geometric shape (round vs. square in relation to the top view) varied, but also the layup of the scarf repair structure, variables that have not been studied systematically to-date. Sub-component scale sandwich shell specimens with representative repairs were used in this work in addition to coupon-scale specimens that are more commonly seen in literature. These sub-component shell specimens were tested using a unique shell test bench under loading conditions representative of wind turbine blade shell operating conditions with respect to fatigue strength. Using non-destructive testing methods (field strain measurement and thermography), damage development and distribution was monitored and analyzed in-situ. As a result, a concept was developed in which the repaired areas showed at least equivalent if not higher fatigue strength than the reference shells that were not repaired.
Slow crack growth (SCG) under the influence of external fluid media, usually termed environmental stress cracking (ESC), is still one of the most frequent origins of severe damage and failure of polymeric materials. For polyethylene (PE) this is relevant for mass products, like bottles for consumer products, but also for high-performance materials for pipes or industrial packaging, including containers for chemicals and other dangerous goods. Especially for high-density polyethylene (PE-HD) the susceptibility to SGC and ESC is depending on the complex interplay between molecular weight and architecture and the resulting semicrystalline morphology, especially the formation of tie-molecules.
A reliable assessment of the resistance against this damage phenomenon is essential for demanding as well as safety-critical applications and has to take into account suitable testing methodologies and conditions in combination with environmental media reflecting the properties representative for typical fillings and relevant components causing ESC. In this context a better understanding of different influencing factors, such as sorption, swelling or surface activity is necessary together with a detailed characterization of different stages of crack propagation and underlying mechanisms. Therefor well-established testing methods, such as the Full Notch Creep Test (FNCT), were complemented by detailed fracture surface analysis using e.g. LSM and SEM in combination with a time-dependent optical monitoring of the progressing crack growth.
For the crack growth also the ratio of fibrillated craze zone to crack opening is important for the deeper understanding of crack propagation and related material parameters which can be additionally addressed by X-ray computed tomography.
In Leichtbauanwendungen sind Materialien mit geringen Dichten und hohen Festigkeiten gefragt. Insbesondere Faser-Kunststoff-Verbunde (FKV) erfüllen diese Anforderungen und werden zunehmend für Strukturbauteile eingesetzt. Die Oberflächen moderner Rotorblätter von Windkraftanlagen werden aus FKV gefertigt. Das Versagen dieser Materialien hat massiven Einfluss auf die Einsatzfähigkeit der gesamten Windkraftanlage. Daher ist die Kenntnis des Versagensverhaltens dieser Komponenten unter statischer und zyklischer Belastung von großem Interesse, um die Betriebsbeanspruchung abzuschätzen. Im Idealfall wird die Schadensentwicklung unter Betriebslast mit zerstörungsfreien in-situ-Prüfverfahren ermittelt. Hier berichten wir über die Beobachtung der Rissentwicklung in Carbon-Faserverstärkten Kunststoffen (CFK) unter kontinuierlicher Zugbelastung durch in-situ Synchrotron-Röntgenrefraktionsradiographie. Eine selbst entwickelte elektromechanische Zugprüfmaschine mit einem Kraftbereich bis zu 15 kN wurde in den Strahlengang an der BAMline (BESSY II) integriert. Da in herkömmlichen (Absorptions-) Radiographien Defekte wie Zwischenfaserbrüche oder Faser-Matrix-Enthaftung keinen ausreichenden Kontrast verursachen, wird zur Kontrastanhebung die Röntgenrefraktion benutzt. Hier wird das sogenannte Diffraction Enhanced Imaging (DEI) angewandt, um Primär- und gebrochene Strahlung mit Hilfe eines Analysatorkristalls zu trennen. Diese Technik ermöglicht schnelle Messungen mit einem Gesichtsfeld von einigen Quadratmillimetern (hier: 14 mm × 7.2 mm) und ist ideal für in-situ Untersuchungen. Die Bildgebung und der Zugversuch erfolgen mit einer Bildfrequenz von 0.7 / s und einer Dehnrate von 0.00055 / s . Bei 0°/90° Fasergelegen treten die ersten Zwischenfaserrisse bei 380 MPa (Dehnung 0.8 %) auf. Vor dem Versagen bei ca. 760 MPa (Dehnung 2.0 %) beobachten wir die Ausbildung eines nahezu äquidistanten Rissmusters (Risse in ca. 1 mm Abstand), das sich im geschädigten Zustand über die gesamte Probe erstreckt.
In Leichtbauanwendungen sind Materialien mit geringen Dichten und hohen Festigkeiten gefragt. Insbesondere Faser-Kunststoff-Verbunde (FKV) erfüllen diese Anforderungen und werden zunehmend für Strukturbauteile eingesetzt. Die Oberflächen moderner Rotorblätter von Windkraftanlagen werden aus FKV gefertigt. Das Versagen dieser Materialien hat massiven Einfluss auf die Einsatzfähigkeit der gesamten Windkraftanlage. Daher ist die Kenntnis des Versagensverhaltens dieser Komponenten unter statischer und zyklischer Belastung von großem Interesse, um die Betriebsbeanspruchung abzuschätzen. Im Idealfall wird die Schadensentwicklung unter Betriebslast mit zerstörungsfreien in-situ-Prüfverfahren ermittelt. Hier berichten wir über die Beobachtung der Rissentwicklung in Carbon-Faserverstärkten Kunststoffen (CFK) unter kontinuierlicher Zugbelastung durch in-situ Synchrotron-Röntgenrefraktionsradiographie. Eine selbst entwickelte elektromechanische Zugprüfmaschine mit einem Kraftbereich bis zu 15 kN wurde in den Strahlengang an der BAMline (BESSY II) integriert. Da in herkömmlichen (Absorptions-) Radiographien Defekte wie Zwischenfaserbrüche oder Faser-Matrix-Enthaftung keinen ausreichenden Kontrast verursachen, wird zur Kontrastanhebung die Röntgenrefraktion benutzt. Hier wird das sogenannte Diffraction Enhanced Imaging (DEI) angewandt, um Primär- und gebrochene Strahlung mit Hilfe eines Analysatorkristalls zu trennen. Diese Technik ermöglicht schnelle Messungen mit einem Gesichtsfeld von einigen Quadratmillimetern (hier: 14 mm × 7.2 mm) und ist ideal für in-situ Untersuchungen. Die Bildgebung und der Zugversuch erfolgen mit einer Bildfrequenz von 0.7 / s und einer Dehnrate von 0.00055 / s . Bei 0°/90° Fasergelegen treten die ersten Zwischenfaserrisse bei 380 MPa (Dehnung 0.8 %) auf. Vor dem Versagen bei ca. 760 MPa (Dehnung 2.0 %) beobachten wir die Ausbildung eines nahezu äquidistanten Rissmusters (Risse in ca. 1 mm Abstand), das sich im geschädigten Zustand über die gesamte Probe erstreckt.
Packaging containers for dangerous goods that include aggressive liquids require that any packaging material that is based on high-density polyethylene has a high degree of stability and durability. This work is focused on testing the environmental stress cracking of the high-density polyethylenes used for such containers in contact with crop protection formulations, in particular, two model liquids established in Germany as standardized test media representatives for crop protection formulations containing the various admixtures typical for such products. One of the liquids is water-based and contains mostly surface-active ingredients, while the other is solvent-based and includes some emulsifiers. Originally established for pin impression tests, these model liquids and their individual components were here used for the first time as environmental media in the Full Notch Creep Test, which addresses the resistance against environmental stress cracking. The Full Notch Creep Test was carried out on five high-density polyethylene types with both model liquids, and also on one selected material with its components. The evaluation was focused on the fracture surface structures, which were visualized by a scanning electron microscope and by optical in situ imaging of the notch opening. While the water-based model liquid and its surface-active individual components induced environmental stress cracking with the characteristic pattern for a craze-crack mechanism and so-called brittle fracture on the surface, the solvent-based model liquid and its soluble ingredients exhibited rather ductile failure behavior, caused by the plasticizing effect on the polymer that reduced the yield stress of the high-density polyethylene.
For both cases, fracture surface analysis, together with side views of the crack opening, showed a clear relation between surface pattern, notch deformation (e.g., by blunting), or crack opening due to crack growth with time to failure and the solubility of the liquids in high-density polyethylene.
Self-healing agents have the potential to restore mechanical properties and extend service life of composite materials. Thermoplastic healing agents have been extensively investigated for this purpose in epoxy matrix composites due to their strong adhesion to epoxy and their ability to fill in microcracks. One of the most
investigated thermoplastic additives for this purpose is poly(ethylene-co-methacrylic acid) (EMAA). Despite the ability of thermoplastic healing agents to restore mechanical properties, it is important to assess how the addition of thermoplastic healing agents affect properties of the original epoxy material. In this work, EMAA was added to epoxy resin and the effect of the additive on fracture toughness of epoxy was evaluated. Results indicate that although added in low concentrations, EMAA can affect fracture toughness.
Manufacturing and deformation behavior of alumina and zirconia helical springs at room temperature
(2023)
Ceramic helical springs with identical dimensions were produced by hard machining from alumina, alumina toughened zirconia (ATZ), and tetragonal zirconia polycrystals (TZP) stabilized with different oxides. According to the results of the spring constant determination under deformation rates of 3 mm/min, the deformation behavior of all ceramic springs obeys to Hook’s law. However, variation of the deformation rate, tests under constant load, and spring recovery behavior revealed differences in the deformation behavior of alumina, TZP, and ATZ springs. Alumina springs exhibited time-independent deformation in all tests. In contrast, anelastic deformation at room temperature was demonstrated in all springs containing TZP. This deformation is completely reversible over a period of several days. Anelastic behavior is particularly pronounced in Y-TZP springs, whereas Ce-TZP springs exhibit comparatively very low but still reliably detectable anelasticity. Oxygen vacancies in the TZP ceramic are considered the most likely explanation for the anelastic behavior of TZP springs at room temperature.
A new patented dynamic mechanical analysis (DMA) is presented, where the tensile, bending- or torsional stiffness of a media can be characterized in-situ during the phase transition from liquid to solid. An epoxy system, e.g. Hexion L285/H287, is filled into an elastomer container, such as a silicone tube. This can be mounted into a conventional OMA and, based on a linear viscoelastic approach, the storage modulus (E';G'), the loss modulus (E'';G'') and the loss angle tan(δ) can be measured at constant temperature as a function of time in order to investigate the liquid to sol-gel to solid transition. With this new method, the stiffness increase as a result of the cure process can be directly measured more precisely than with a rheometer in a shear plate set-up, because using an elastomer container gives a defined cross section for calculating the Young's modulus.
The interest in repair technologies for polymer composites has increased significantly over the last decades, due to the growing use of these materials in structural applications. In this study, poly (ethylene-co-methacrylic acid) (EMAA) was used as self-healing agent to glass fiber/epoxy composite. Materials with EMAA contents of 2 wt% and 5 wt% were manufactured using Resin Transfer Molding (RTM) and the effects of the healing agent on the properties were investigated using tensile tests and Dynamic Mechanical Analysis (DMA). Results show slight variation of properties, which was more pronounced as the content of EMAA increased. In addition, the healing efficiency was investigated through fatigue tests and the addition of higher content of EMAA increased the number of cycles to failure after the healing activation cycle.
Fully-reversed fatigue behavior of scarf joint repairs for wind turbine blade shell applications
(2022)
To enable a quick and cost-effective return to service for wind turbine blades, localized repairs can be executed by technicians in the field. Scarf repairs, shown to be highly efficient with a smooth load transition across angled joint walls and a restored aerodynamic profile, are the focus of this work. The failure mechanisms of these structures were examined under quasi-static tensile and fully-reversed cyclic loading. While the scarf ratio was held constant at 1:50, the repair layup was varied between large-to-small and small-to-large. The effect of the presence of resin pockets and the fiber orientation mismatch between parent and repair material on the restored strength of BIAX ±45° glass fiber reinforced polymer scarf joint structures was studied.
Prolonging structural integrity—Fatigue of scarf repairs for wind turbine blade shell applications
(2023)
The influence of scarf repair variables on the mechanical properties of glass fiber reinforced polymer sandwich structures is examined for the application of wind turbine blade shell repairs. Reference shell specimens are tested under fully-reversed cyclic loading to understand the fatigue behavior of the sub-component structure within a range of in-service loads, to which shell specimens with a scarf joint repair are compared. A fatiguesuperior repair material is shown to improve the fatigue behavior of the repaired structures compared to the reference specimens. In separating layup and geometry, damage development is examined on the subcomponent scale for a structural understanding of a scarf repair using in-situ digital image correlation-based strain field measurements as well as passive thermography. Within scarf repair variables, the geometry is shown to play a larger role than layup in the fatigue performance of the repaired structure.
Among the strategies to produce healable thermosetting systems is their modification by the addition of thermoplastic particles. This work investigates the influence of poly(ethylene-co-methacrylic acid) (EMAA) on fibermatrix interfacial properties of a glass fiber reinforced epoxy matrix composite. Epoxy-EMAA interactions were evaluated using differential scanning calorimetry (DSC) and infrared spectroscopy. The effects of EMAA on the epoxy network formation were evidenced by changes in glass transition temperature, cure kinetics and alteration of chemical groups during cure. Interfacial shear strength (IFSS) measurements obtained by single fiber pull-out tests indicate similar interfacial properties for pure and EMAA modified epoxy. Additionally, the potential for self-healing ability of an EMAA modified epoxy was demonstrated. However, IFSS after a healing cycle for the EMAA modified epoxy was lower as compared to the pure epoxy, because of the lower fiber-EMAA interfacial shear strength. So, thermoplastic healing agents has not only to fill cracks in the matrix material, but also have to be optimized regarding its interface properties to the reinforcing fibers.
The fracture surface of slow and continuous crack propagation during environmental stress cracking of a semicrystalline polyethylene exhibits isotropic roughness exponents at the local scale but resolved across the macroscopic fracture surface a clear position dependence is found. The spatially resolved roughness exponent admits values in the range between 0.1 and 0.4, demonstrating nontrivial exponents in the small length-scale regime. Instead, they vary across the fracture surface according to the stress-state distribution, which suggests that the exponents are intimately linked to the locally dominating dissipation processes during craze cracking.
Fully-reversed fatigue behavior of scarf joint repairs for wind turbine blade shell applications
(2022)
Due to manufacturing imperfections which can propagate to damage under in-service loads, wind turbine rotor blades, made primarily of glass fiber reinforced polymers (GFRP), often fail significantly before their design life. To enable a quick and cost-effective return to service, localized repairs can be executed by technicians in the field, directly accessing the blades by suspended roping. Scarf joint repairs, shown to be highly efficient with a smooth load transition across angled joint walls and a restored aerodynamic profile, are the focus of this study. The damage mechanisms of these structures were examined under fully-reversed mechanical cyclic loading with a load ratio R = -1, which was made possible on the coupon scale by a custom designed anti-buckling support. The number of cycles to failure were compared across load levels. While the scarf joint ratio was held constant at 1:50, known from industry and literature to be a good compromise between restored mechanical properties and repair size, the layup sequence was varied between small-to-large and large-to-small. Hereby the effect of the presence of resin pockets and fiber orientation mismatch between the parent and repair structure on the failure mechanism as well as fatigue strength of ±45° GFRP scarf joint repair structures was studied. Strain development across the joint length was measured to assess stiffness degradation in addition to the fatigue strength recovery of scarf joint structures with respect to pristine reference specimens. Grayscale analysis was used to monitor the damage state leading to fracture across the specimens. Post-mortem fractography analysis with light microscopy described the global failure mechanisms as well as local damage distribution within the structure, all of which provided insight to critical variables in scarf joint GFRP structures under fully-reversed mechanical cyclic loading, allowing for the tailoring of such repairs on wind turbine blade shells for maximum restoration of service life.
The phenomenon of environmental stress cracking is still a major issue in materials engineering as well as from a scientific perspective. Especially in the case of packaging materials made of high-density polyethylene, assessing the potential of premature damage due to environmental stress cracking is complex because of the large number of components in possible liquid filling goods. As a first guideline, the well-known effect of detergents and dispersants is usually considered, but the interplay with other components, such as organic solvents is often unknown. Particularly challenging in this respect are crop protection products, consisting of several different admixtures in addition to the biologically active ingredients. This study is based on two model liquids representing these admixtures that were established for testing in this context. The model liquids as well as their constituting components were used as liquid media in the Full Notch Creep Test to characterize the environmental stress cracking behavior of a typical high-density polyethylene. Complementary to the time-to failure of a notched specimen obtained from these tests, the crack opening was monitored optically, and the fracture surface was analyzed post-failure by laser scanning microscopy as well as scanning electron microscopy. Based on the results, the effect of various surfactants as well as the influence of organic solvents on environmental stress cracking are discussed.
A new patented dynamic mechanical analysis (DMA) is presented, where the tensile, bending- or torsional stiffness of a media can be characterized in-situ during the phase transition from liquid to solid. An epoxy system, e.g. Hexion L285/H287, is filled into an elastomer container, such as a silicone tube. This can be mounted into a conventional DMA and, based on a linear viscoelastic approach, the storage modulus (E’;G’), the loss modulus (E’’;G’’) and the loss angle tan(delta) can be measured at constant temperature as a function of time in order to investigate the liquid to sol-gel to solid transition. With this new method, the stiffness increase as a result of the cure process can be directly measured more precisely than with a rheometer in a shear plate set-up, because using an elastomer container gives a defined cross section for calculating the Young’s modulus.
The crack propagation rate of environmental stress cracking was studied on high-density polyethylene compact tension specimens under static loading. Selected environmental liquids are distilled water, 2 wt% aqueous Arkopal N100 solution, and two model liquid mixtures, one based on solvents and one on detergents, representing stress cracking test liquids for commercial crop protection products. The different surface tensions and solubilities, which affect the energetic facilitation of void nucleation and craze development, are studied. Crack growth in surface-active media is strongly accelerated as the solvents induce plasticization, followed by strong blunting significantly retarding both crack initiation and crack propagation. The crack propagation rate for static load as a function of the stress intensity factor within all environments is found to follow the Paris–Erdogan law. Scanning electron micrographs of the fracture surface highlight more pronounced structures with both extensive degrees of plasticization and reduced crack propagation rate, addressing the distinct creep behavior of fibrils. Additionally, the limitations of linear elastic fracture mechanisms for visco-elastic polymers exposed to environmental liquids are discussed.
Poly (ether ether ketone) (PEEK) has a high continuous service temperature, excellent mechanical properties, and good solvent and abrasion resistance, which can be further improved through the addition of carbon nanotubes (CNTs). CNT-PEEK nanocomposites are promising matrices for continuous carbon fibre composites; powder processing can mitigate the high melt viscosities in these systems. In this study, model single fibre (hierarchical) composites were produced by embedding sized and desized carbon fibres in nanocomposite CNTPEEK powders followed by single fibre pull-out tests to assess interfacial characteristics. Carbon fibre-PEEK interfacial shear strength is typically 40–45 MPa. Increasing CNT loadings increased fibre-matrix interfacial shear strength linearly up to ~70 MPa at 5.0 wt%, which was attributed to the CNT-based mechanical modification of the PEEK matrix. Apparent interfacial shear strength was inversely correlated with the embedded fibre length irrespective of carbon fibre sizing or CNT loading, indicating brittle fracture of the fibre-matrix interface.
Pulled out carbon fibres were still coated with the matrix, which indicated strong adhesion at the interface in all samples, likely related to a transcrystalline region. Adhesion was, however, negatively affected by the presence of epoxy sizings. Frictional shear strength was independent of embedded fibre length and CNT content for all samples.
Rotorblätter von Windenergieanlagen (WEA) weisen häufig nach wenigen Jahren, lange vor dem Erreichen der prognostizierten Lebensdauer von 20 bis 30 Jahren Risse in der Blattschale auf. Die Folge sind aufwendige Reparaturen am installierten und schwer zugänglichen Rotorblatt und der kostenintensive Nutzungsausfall durch den Stillstand der WEA.
Als mögliche Initiatoren für die Schäden in der Blattschale der Rotorblätter gelten fertigungsbedingte Imperfektionen. Für die Untersuchung des Einflusses dieser Imperfektionen auf das Ermüdungsverhalten der Rotorblätter wurde an der BAM (Bundesanstalt für Mate-rialforschung und -prüfung) ein Prüfstand für statische und zyklische Versuche von Schalensegmenten im intermediate scale entwickelt und betrieben. Die untersuchten Schalensegmente in Sandwichbauweise sind der Rotorblattschale von WEA im Hinblick auf die Strukturmechanik, die eingesetzten Halbzeuge, den Laminataufbau und dem eingesetzten Fertigungsverfahren ähnlich. Als Imperfektionen wurden verschiedenen Variationen von Lagenstößen in die Hautlagen und Schaumstöße mit Breitenvariation in den Stützkern reproduzierbar eingebracht. Die Überwachung des Schädigungszustandes während der Schwingversuche unter realistischen Lastszenarien erfolgt über eine kombinierte in situ Schädigungsüberwachung mittels passiver Thermografie und Felddehnungsmessung.
Mit den durchgeführten Schwingversuchen und der begleitenden Überwachung des Schädigungszustandes ließen sich die Schadensinitiation und die signifikante Herabsetzung der Lebensdauer durch die eingebrachten Imperfektionen zweifelsfrei nachweisen und entsprechende Konstruktionshinweise für die betriebssichere Auslegung von Sandwichstrukturen ableiten.
Das langsame Risswachstum (slow crack growth, SCG) sowie der umgebungsbedingte Spannungsriss (environmental stress cracking, ESC) sind relevante Schädigungsmechanismen für teilkristalline Werkstoffe auf Basis von Polyethylen hoher Dichte (PE-HD).
Der Vortrag gibt einerseits einen Überblick über die grundlegenden Struktur-Eigenschafts-Beziehungen in diesem Kontext, andererseits werden auch verschiedene praxisorientierte Prüfverfahren vorgestellt. Letztere wurden in gemeinsamen Forschungsprojekten von BAM und LBF mit dem Schwerpunkt Gefahrgutbehälter bzw. Pflanzenschutzmittel vergleichend untersucht und durch weitergehende Analytik ergänzt.
This presentation is a summary of the work from the past 20 years’ development of PMC-testing at the BAM-FB 5.3 with respect to safety-relevant design of advanced light weight structures in aircraft, wind turbine and automotive applications. The talk begins with wood as an example from nature, and emphasizes that load case, fiber architectural design and the production process and quality have to go hand in hand to generate an advanced light weight structure. Since PMC-relevant basic findings of mankind span across hundreds of years, high-performance composite applications today are based more on long term experiences than on breakthrough inventions of modern days.
In the second part of the talk, future plans and projects of FB-5.3 are presented, specifically addressing H2-safety, circular economy, recycling by design and digitalization of PMC-technologies.
Im Rahmen des Vortrags werden die neusten Ergebnisse aus dem Fachbereich 5.3 zur Beschreibung des Ermüdungsverhaltens von FKV mittels Mikromechanischer-Modelle präsentiert. Explizit wird der theoretische Ansatz am Beispiel von GFK unter thermomechanischer Beanspruchung hergeleitet und an Hand von Versuchsergebnissen verifiziert.
Polyethylen hoher Dichte (PE-HD) ist ein weit verbreitetes Material für Transportbehälter, die oft für eine längere Nutzungsdauer vorgesehen sind. Insbesondere in diesem Fall können mikroskopische Schäden im Material auch weit unterhalb der Streckgrenze auftreten, die durch eine Spannungskonzentration verursacht werden, deren Ursprung in intrinsischen Materialfehlern oder äußeren Kratzern liegt. Mit fortschreitender Schädigung bilden sich Rissstrukturen, die von verstreckten Fibrillen aufgespannt werden, bis es zum Versagen dieser Fibrillen kommt und sich der Riss ausbreitet. Dieser Schadensmechanismus des langsamen Risswachstums kann durch eine Vielzahl von Umgebungsmedien beschleunigt werden und wird dann als "environmental stress cracking" (ESC) bezeichnet. Eine international normierte Methode zur Validierung der Spannungsrissbeständigkeit von PE-HD Materialien ist der „Full Notch Creep Test“ (FNCT).
Anhand von Bruchflächenauswertungen mittels Rasterelektronenmikroskopie und Laser-Scanning-Mikroskopie, werden die typischen Schädigungsstrukturen des ESC sichtbar. Die fibrillierten Strukturen des ESC auf der Bruchfläche von PE-HD ergeben sich besonders bei oberflächenaktiven wässrigen Umgebungsmedien. In Lösungsmittel-Umgebungsmedien, im Zusammenspiel mit PE-HD, kommt es aufgrund der plastifizierenden Wirkung zur Herabsetzung der Streckspannung und Bruchflächen aus dem FNCT veranschaulichen keine signifikanten Anzeichen des Schädigungsmechanismus ESC.
The research presented here attempts to assess the potential for re-using carbon fibre (CF) fabrics recovered from recycling infusible acrylic thermoplastic carbon fibre reinforced polymer composites (CFRPs) in a universal manner, i.e. by combining with a wide variety of matrices to manufacture 2nd generation composite laminates by resin infusion. The 2nd generation composites have been compared in terms of bulk and interfacial properties against counteparts processed with virgin carbon fibre fabric infused with the same matrices. Generally, an increase in damping (tanδ) was observed in all 2nd generation composites, which can be attributed to a residual thin thermoplastic layer present on the recovered fibres. The interfacial adhesion of the 2nd generation Composites was investigated by shear tests and scanning electron micsoscopy, and also appears to be less influenced by the type of matrix.
Currently, the Full Notch Creep Test (FNCT) [1] method is used by material suppliers and end users in industry for the approval of container and pipe materials based on high-density polyethylene (PE-HD). The resistance to environmental stress cracking (ESC) of the material is evaluated using the time to failure of the specimen in an aqueous solution of a detergent [2, 3]. Usually specimens made of sheets with isotropic material properties, manufactured by hot pressing, are employed in order to obtain intrinsic properties of the material in terms of ESC failure. In contrast, the processes used in manufacturing to form containers and pipes, such as extrusion blow molding or extrusion, impose anisotropic properties to the material. These are mostly due to a microstructural orientation (polymer chains or crystallites) [4]. Furthermore, the different cooling conditions significantly affect the size distribution of crystallites as well as the overall morphology. It is therefore essential to understand the influence of process-induced material characteristics on failure due to ESC.
A large number of studies on material properties as a function of microstructural preferential orientation have already been conducted [5-7]. However, effects on ESC as the major failure mechanism of containers and pipes are still rather unexplored [8, 9]. The most important factor is whether primarily intramolecular high-strength covalent bonds or the substantially weaker intermolecular van der Waals forces are predominantly loaded.
In addition to the widely established classification by time to failure, the strain or crack opening displacement (COD) provides valuable information about the evolution and progression of damage as a function of time [10, 11]. Optical strain measurement using digital image correlation allows the differences in COD for isotropic and different angles of orientation of anisotropic specimens to be discussed. Also, a post-fracture surface analysis provides clarification on the craze-crack mechanism of the ESC. These different ESC-related properties of extruded and hot-pressed specimens have been investigated at different environmental medium temperatures and different initial stresses to provide a broad characterization of the fracture behavior of PE-HD.
Molecular orientation has a significant effect on the material properties of polymers. Preferential orientation of the microstructure (polymer chains or crystallites) in a specific direction or plane often enhances the material properties, especially if the high-strength covalent bonds are primarily exposed to loads instead of the weaker van der Waals bonds. However, the orientation-dependent microstructure and its mechanical behavior is in general already well understood by many scientific studies [1-3]. Isotropic materials are frequently required for an intrinsic material characterization without prevailing processing-induced properties, as is the case for Full Notch Creep Test (FNCT) [4] addressing environmental stress cracking (ESC) in high-density polyethylene (PE-HD) [5, 6]. Since ESC is one of the major limiting issues for long-term performance of PE-HD pipes and containers [7], which in contrast have a production-related preferential orientated microstructure due to extrusion or extrusion blow molding, it is important to additionally investigate the ESC resistance of such anisotropic microstructure.
Investigations of the slow crack growth (SCG) with respect to the molecular orientation generally obtain a factor of 1.2 up to 4.7 between crack growth perpendicular to the extrusion direction and crack growth parallel to the extrusion direction 8. Based on FNCT investigations with an aqueous detergent solution as environmental medium, hot pressed sheets with isotropic morphology are compared with extruded sheets from which specimens with different orientation angles are taken. However, the time to failure obtained by FNCT is also significantly influenced by the different cooling conditions under which the final morphology is formed. The tendency of the specimen to fail due to ESC is investigated as a function of environmental medium temperature. For a more detailed analysis of the affecting parameters in the manufacturing process, the ESC resistance is discussed considering the differences in crystallinity as revealed by thermal analysis.
Die Werkstoffgruppe der Faser-Kunststoff-Verbunde (FKV) hat sich aufgrund ihrer hervorragenden Leichtbaueigenschaften unter anderem im Sportgerätebau, in der Luft- und Raumfahrt und in der Windenergieindustrie etabliert. Die so hergestellten Strukturen sind in der Regel nicht nur mechanischen Belastungen, sondern auch thermischen Lasten in einem breiten Temperaturspektrum ausgesetzt. Dennoch ist die Auswirkung des Temperatureinflusses bei einer Kombination von thermischer und mechanischer Last auf die Lebensdauer von Strukturen aus FKV bisher nur wenig untersucht.
Im Rahmen dieser Arbeit wird der Einfluss von Temperaturen zwischen 213 K und 343 K auf einen Glasfaser-Epoxidharz-Verbund experimentell untersucht. Das Material wird in diesem Temperaturbereich eingehend charakterisiert: Es werden sowohl die thermomechanischen Eigenschaften von Faser- und Matrixwerkstoff als auch die des Verbundes ermittelt. In einem weiteren Schritt wird dann der Einfluss der Temperatur auf die Schädigungsentwicklung im quasi-statischen Lastfall sowie unter schwingender Ermüdungsbeanspruchung bei verschiedenen FKV-Mehrschichtverbunden analysiert.
Basierend auf den experimentellen Daten wird ein Zusammenhang zwischen der Schädigung und der Anstrengung der Matrix innerhalb der Einzelschicht demonstriert. Die Matrixanstrengung wird mithilfe eines mikromechanischen Modells unter Berücksichtigung der thermomechanischen Eigenspannungen analytisch berechnet. Bei Querzugbeanspruchung kann gezeigt werden, dass eine Vorhersage der Schädigung in Abhängigkeit der Volumenänderungsenergie innerhalb der Matrix getroffen werden kann.
Mithilfe des Konzepts der Matrixanstrengung ist eine Vorhersage der Lebensdauer des Werkstoffs unter schwingender Ermüdungsbeanspruchung in Abhängigkeit der Einsatztemperatur möglich.
Polyethylen hoher Dichte (PE-HD) wird als Werkstoff für Rohre und Behälter für den Transport und zur Lagerung von Gefahrgütern verwendet. Für die Beurteilung und technische Freigabe dieser Materialien ist insbesondere das Verständnis des Schädigungsmechanismus des langsamen, umgebungsbedingten Spannungsrisses (engl.: „environmental stress cracking“, ESC) essentiell. ESC tritt bei relativ geringen auf einen Werkstoff einwirkenden mechanischen Spannungen auf. An lokalen Fehlstellen (z.B. Defekte, Inhomogenitäten, Kerben) beginnend wächst ein Riss langsam durch das Material und führt nach gewisser Zeit zu einem charakteristischen, pseudo-spröden Bruch. Dabei wird das Risswachstum durch äußere Medieneinwirkung zusätzlich entscheidend beeinflusst. Dieses langsame Risswachstum wird als Hauptursache für das plötzliche und unerwartete Versagen von Polymerwerkstoffen angesehen.
Eine etablierte Prüfmethode zur Bewertung des Materialverhaltens gegenüber dieses Schädigungsmechanismus ist der Full-Notch Creep Test (FNCT), der für PE-HD Behältermaterialien üblicherweise unter Verwendung von wässrigen Netzmittellösungen (Arkopal N 100) durchgeführt wird. Die aus dem FNCT erhaltene Standzeit dient dabei als Bewertungskriterium für verschiedene PE-HD-Werkstoffe. In einer Studie wurden neben einer typischen Arkopal-N-100-Netzmittellösung praktisch relevante, organische Flüssigkeiten wie Biodiesel und Diesel als Testmedien verwendet, um deren Einfluss auf das ESC-Verhalten von PE-HD-Behältermaterialien zu charakterisieren. Neben der klassischen Standzeit-Auswertung erfolgte eine erweiterte Bruchflächenanalyse mittels Licht- (LM), Laserscanning- (LSM) und Rasterelektronenmikroskopie (REM). Insbesondere die LSM erlaubt eine schnelle und einfache Unterscheidung pseudo-spröder und duktiler Bruchbilder, die zur Beurteilung der Repräsentativität des FNCT für das dem Spannungsriss zugrundeliegenden langsamen Risswachstum von Bedeutung ist.
Glass fiber reinforced polymer (GFRP) materials in practical applications have to endure cyclic mechanical loading in a wide temperature range (e.g. aircraft applications, automotive, wind turbine blades). In this study the static strength and fatigue behavior of GFRP was investigated in a temperature range from 213 K to 343 K. Therefor the coefficients of thermal expansion of the composite as well as the matrix are measured in this temperature interval. The inverse laminate theory was extended and used to calculate the inter fiber-failure effort for a virtual UD-layer according to the layer wise strength approach. The experimentally determined results are compared with the micro-mechanical model according to Krimmer, which has been enhanced to include the effect of temperature and fiber-perpendicular failure modes. A correlation between matrix effort, the dilatational strain energy of the matrix and the damage state of the specimen is demonstrated. It is shown that a fatigue life assessment can be performed with the aid of a temperature-independent master fatigue curve, as it was similar done for the fatigue behavior of CFRP and GFRP to very high load cycles at room temperature.
Wind turbine rotor blades commonly fail before their projected 20-year lifespan largely due to defects that originate during manufacturing and are propagated by operational fatigue and environmental conditions. The cost-intensive replacement outcomes lead to a high loss of earnings, and are one of the inhibitors of wind turbine production. A potential repair alternative to restoring the mechanical properties of such lightweight fiber reinforced polymer (FRP) structures is to locally patch these areas with scarf joints. This type of repair allows for a smoother load distribution across the joint, and is favored especially on structures where minor aerodynamic contour changes are key. The effects of such repairs on the structural integrity, however, is still largely unknown. Building upon an understanding of the static load failure mechanism of GFRP scarf joints, presented at the ICCS23 Joint Event in 2020, the influence of the fiber orientation mismatch between parent and repair materials of 1:50 scarf joints on the failure mechanism of monolithic glass FRP specimens under cyclic fatigue load were examined in this study. Specimens with various layups were produced with the vacuum-assisted resin infusion (VARI) process using biaxial E-glass non-crimp fabric (NCF). The patch layers were then joined directly to the parent structure with the VARI using biaxial E-glass NCF with half the areal weight of the parent side to allow for better drapability. This mimics the soft-to-hard patch style utilized in wind turbine blade shell field repairs. The specimens were tested under uniaxial fatigue load, during which they were periodically monitored for damage onset. A comparison of the +45/-45° and 0/90° layups allowed for an understanding of the role of a highly mismatching fiber orientation in the transition zone between parent and patch material on the failure mechanism of the scarf joint. In addition to the tensile strength and stiffness property recovery assessment, a grayscale analysis using in-situ camera images determined the damage state leading to failure in each region across the scarf joint, which varied in the parent material versus scarf joint region, providing insight to the critical regions in this composite structure under cyclic loading.
In this study, the thermomechanical damage behavior of a glass fiber reinforced polymer material is investigated. The coefficients of thermal expansion of the composite as well as the matrix are measured in a wide temperature range. Quasi-static experiments with neat resin, unidirectional and multidirectional laminates are performed as well as fatigue experiments in a temperature range from 213 K to 343 K. This study focusses on the matrix damage due to fiber-parallel loading. A correlation between matrix effort, the dilatational strain energy of the matrix and the damage state of the specimen is demonstrated. It is shown that a fatigue life assessment can be performed with the aid of a temperature-independent master fatigue curve.
In this work, a closed loop recycling process is investigated, which allows polymerised bulk thermoplastic matrix (Elium 150) from production waste (also referred to as recyclate) to be reused as additive in composite manufacturing by vacuum assisted resin infusion (VARI) of virgin Elium 150 monomer. It is shown that this process can save up to 7.5 wt% of virgin material usage in each processing cycle. At the same time, the thermal stability and stiffness of the composite increases with the proportion of recyclate introduced. Contemporarily, the shear and bending properties have also been observed to improve. Gel permeation chromatography (GPC) showed that the changes observed are due to an increase in molecular weight with the recyclate content. In particular, a correlation between the molecular weight and the shear properties of the composite was discovered using single fibre push-out tests.
Four typical high-density polyethylene container materials were used to investigate damage or stress cracking behavior in contact with model liquids for crop protection products. These model liquids are established in German regulations for the approval of dangerous goods containers and consist of typical admixtures used for crop protection products but without biological active ingredients. This study is performed with the standardized method of Full Notch Creep Test, adapting the media temperature to 40 °C according to the usual conditions where these test liquids are applied. The two model liquids differ into a water-based solution and a composition based on different organic solvents which are absorbed by the material up to significant levels. Therefore, extensive sorption measurements are performed. The fracture surfaces obtained are analyzed in detail not only by light microscopy, but also by laser scanning microscopy as well as scanning electron microscopy. Influence of pre-saturation and applied stress are addressed by respective systematic series of experiments.
As a type of high-performance composite material, glass-fiber reinforced plastics (GFRP) are favored for the construction of wind turbine rotor blades due to their high specific strength and stiffness properties (Grasse et al, 2010). During the blade manufacturing process, however, imperfections are often introduced, then further propagated due to harsh environmental conditions and a variety of loads (Caminero et al, 2013; Trappe et al, 2018). This leads to failure significantly before their designed lifespan. Since replacement of entire blades can be a costly potential outcome, localized repairs of the damaged region to restore structural integrity and thus lengthen its lifespan can executed in the field by technicians accessing the blades directly by suspended roping. These methods involve replacing the lost load path with a new material that is joined to the parent structure. In recent years, considerable studies have been conducted to investigate the influence of different repair parameters on the stress distribution, ultimate strength, impact behavior, and residual stresses of bonded repaired structures [Caminero et al, 2013; Trappe et al, 2018; Shufeng et al, 2014; Harman and Rider, 2011; Ahn and Springer, 2000; Lekou and Vionis, 2002). However, there currently do not exist any standardized repair procedures for wind turbine rotor blades. Namely, there is a lack of understanding about the effects of the layup of various repair methods, especially on the damage mechanism and fatigue life of the shells of rotor blades (Caminero et al, 2013; Trappe et al, 2018).
This work therefore aims to begin to enrich this knowledge gap by testing the influence of different variables among repair patches on the mechanical properties of sandwich composite structures. Manufactured with the vacuum-assisted resin infusion (VARI) process, the test specimens are produced as a GFRP structure to represent the outer shell portion of a wind turbine blade, then repaired with a scarf joint. Scarf repairs are favored as the most efficient of the common structural joints, as the removal of the damaged area with angled walls leads to a nearly uniform shear stress distribution along the bond surface and no eccentricity in the load distribution (Caminero et al, 2013; Lekou and Vionis, 2002; Siener, 1992).
The performance of specific layup methods of repair patches, namely a large-to-small versus small-to-large scheme of repair layers, is studied with static and load-controlled fatigue testing, then compared to pristine test specimens as well as to each other in terms of mechanical property restoration. The transition layer between repair and parent material is especially of interest in the performance of the structure. Damage onset, crack development and eventual failure are monitored in-situ with non-destructive testing methods, including thermography with an infrared camera system and a 3D deformation analysis system, to develop a more robust understanding of the effects of these repair concept variables on wind turbine blade shell structures.
A potential repair alternative to restoring the mechanical properties of lightweight fiberreinforced polymer (FRP) structures is to locally patch these areas with scarf joints. The effects of such repair methods on the structural integrity, however, are still largely unknown. In this paper, the mechanical property restoration, failure mechanism, and influence of fiber orientation mismatch between parent and repair materials of 1:50 scarf joints are studied on monolithic glass fiber-reinforced polymer (GFRP) specimens under tensile load. Two different parent orientations of [-45/+45]2S and [0/90]2S are exemplarily examined, and control specimens are taken as a baseline for the tensile strength and stiffness property recovery assessment. Using a layer-wise stress analysis with finite element simulations conducted with ANSYS Composite PrepPost to support the experimental
investigation, the fiber orientation with respect to load direction is shown to affect the critical regions and thereby failure mechanism of the scarf joint specimens.
High-performance fibre-reinforced polymer composites are important construction materials based not only on the specific properties of the reinforcing fibres and the flexible polymer Matrix but also on the compatible properties of the composite interphase. First, oxygen-free (a-CSi:H) and oxygen-binding (a-CSiO:H) plasma nanocoatings of different mechanical and tribological properties were deposited on planar silicon dioxide substrates that closely mimic E-glass. The nanoscratch test was used to characterize the nanocoating adhesion expressed in terms of critical normal load and work of adhesion. Next, the same nanocoatings were deposited on E-glass fibres, which were used as reinforcements in the polyester composite to affect its interphase properties. The shear properties of the polymer composite were characterized by macro- and micromechanical tests, namely a short beam shear test to determine the short-beam strength and a single fibre push-out test to determine the interfacial shear strength. The results of the polymer composites showed a strong correlation between the short-beam strength and the interfacial shear strength, proving that both tests are sensitive to changes in fibre-matrix adhesion due to different surface modifications of glass fibres (GF).
Finally, a strong correlation between the shear properties of the GF/polyester composite and the adhesion of the plasma nanocoating expressed through the work of adhesion was demonstrated.
Thus, increasing the work of adhesion of plasma nanocoatings from 0.8 to 1.5 mJ·m−2 increased the short-beam strength from 23.1 to 45.2 MPa. The results confirmed that the work of adhesion is a more suitable parameter in characterising the level of nanocoating adhesion in comparison with the critical normal load.
A phenomenological criterion for an optical assessment of PE-HD fracture surfaces obtained from FNCT
(2021)
The full-notch creep test (FNCT) is a common test method to evaluate the environmental stress cracking (ESC) behavior of high-density polyethylene (PE-HD), e.g. for container materials. The test procedure as specified in ISO 16770 provides a comparative measure of the resistance against ESC using the time to failure of PE-HD specimens under constant mechanical load in a well-defined liquid test environment. Since the craze-crack damage mechanism underlying the ESC phenomenon is associated with brittle failure, the occurrence of a predominantly brittle fracture surface is a prerequisite to consider an FNCT measurement as representative for ESC, i.e. a time to failure dominated by craze-crack propagation.
The craze-crack propagation continuously reduces the effective residual cross-sectional area of the specimen during the test, which results in a corresponding increase of the effective mechanical stress. Thus, a transition to ductile shear deformation is inevitable at later stages of the test, leading usually to a pronounced central ligament.
Therefore, an optical evaluation of FNCT fracture surfaces concerning their brittleness is essential. An enhanced imaging analysis of FNCT fracture surfaces enables a detailed assessment of craze-crack Propagation during ESC. In this study, laser scanning microscopy (LSM) was employed to evaluate whether FNCT fracture surfaces are representative with respect to craze-crack propagation and ESC. Based on LSM height data, a phenomenological criterion is proposed to assess the validity of distinct FNCT measurements. This criterion is
supposed to facilitate a quick evaluation of FNCT results in practical routine testing. Its applicability is verified on a sample basis for seven different commercial PE-HD container materials.
Die Bereitstellung von zuverlässigen Werkstoffdaten stand für die Werkstoffprüfung seit dem 19. Jahrhundert in zentraler Stelle. Mit der zunehmender Digitalisierung unserer Gesellschaft gewinnt das Datenmanagement insbesondere im Hinblick auf die Modellierungs- und Simulationsaktivitäten an Bedeutung.
In unserem Beitrag beschreiben wir die gegenwärtigen Aktivitäten in Deutschland – insbesondere die NFDI-MatWerk und die BMBF-Initiative MaterialDigital – sowie in Europa und Übersee. Abschließend heben wir die Auswirkungen auf die technische und akademische Aus- und Weiterbildung.
The mechanical behavior of fiber reinforced composites with polymer matrix is governed by several mechanisms operating at different length scales. In this contribution we describe first non-destructive techniques which are adequate for the characterization of the fiber-matrix interphase at a microscopic level. In a second step we describe on a mesoscopic level the influence of manufacturing related elements on the mechanical properties of rotor blades for wind turbines. We concentrate on thermography, laminography and ultrasound in connection with mechanical testing systems. Finally we present methods for monitoring rotor blades by using embedded optical fibers.
Moderne Segelflugzeuge sind voll-verklebte tragende FKV-Strukturen. Über die Schwingfestigkeit der Faser-Kunststoff-Verbunde (FKV) und die Lebensdauerabschätzung dieser Werkstoffklasse auf Basis verschiedener Lastspektren wurden insbesondere von Christoph Kensche verschiedenen Arbeiten durchgeführt und auch auf dem Symposium für Segelflugzeugentwicklung vorgestellt. Hingegen ist die Betriebsfestigkeit von Struktur-Klebungen im Anwendungsbereich der CS22 vergleichsweise wenig untersucht worden.
Über die Jahre wurden in verschiedenen Projekten immer wieder Probleme in Struktur-Klebungen (u.a. Betriebsbelastungsversuche an Versuchs-Holmen) identifiziert, analysiert und spezielle Prüfverfahren weiter entwickelt (Projekt „Tragflügel neuer Technologie für die allgemeine Luftfahrt“, FK SIF765). Im Rahmen des LuFo-Vorhabens GeAviBoo (General Aviation Booster) wurde in zwei Arbeitspaketen das Thema erneut aufgegriffen und Betriebsbelastungsversuche an Klebproben durchgeführt. Im Rahmen des Vortrages werden die Erkenntnisse der letzten 10 Jahre aufbereitet und die neusten Ergebnisse vorgestellt.
Modern air-liners and wind turbine rotor blades are made up primarily of fiber reinforced plastics. Failure of these materials heavily impairs the serviceability and the operational safety. Consequently, knowledge of the failure behavior under static and cyclic loads is of great interest to estimate the operational strength and to compare the performance of different materials. Ideally, the damage evolution under operational load is determined with in-situ non-destructive testing techniques. Here, we report in-situ synchrotron X-ray imaging of tensile stress induced cracks in carbon fiber reinforced plastics due to inter-fiber failure. An inhouse designed compact tensile testing machine with a load range up to 15 kN was integrated into the beamline. Since conventional radiographs do not reveal sufficient contrast to distinguish cracks due to inter-fiber failure and micro cracking from fiber bundles, the Diffraction Enhanced Imaging (DEI) technique is applied in order to separate primary and scattered (refracted) radiation by means of an analyzer crystal. This technique allows fast measurements over large fields-of-view and is ideal for in-situ investigations.
Modern air-liners and wind turbine rotor blades are made up primarily of fiber reinforced plastics. Failure of these materials heavily impairs the serviceability and the operational safety. Consequently, knowledge of the failure behavior under static and cyclic loads is of great interest to estimate the operational strength and to compare the performance of different materials. Ideally, the damage evolution under operational load is determined with in situ non-destructive testing techniques. Here, we report in-situ synchrotron X-ray imaging of tensile stress induced cracks in carbon fiber reinforced plastics due to inter-fiber failure. An inhouse designed compact tensile testing machine with a load range up to 15 kN was integrated into the beamline. Since conventional radiographs do not reveal sufficient contrast to distinguish cracks due to inter-fiber failure and micro cracking from fiber bundles, the Diffraction Enhanced Imaging (DEI) technique is applied in order to separate primary and scattered (refracted) radiation by means of an analyzer crystal. This technique allows fast measurements over large fields-of-view and is ideal for in-situ investigations. Imaging and the tensile test are run at the highest possible frame rate (0.7 s-1 ) and the lowest possible strain rate (5.5∙10-4 s -1 ). For 0°/90° non-crimp fabrics, the first inter-fiber cracks occur at 380 MPa (strain 0.8 %). Prior to failure at about 760 MPa (strain 2.0 %), we observe the evolution of nearly equidistant (1 mm distance) cracks running across the entire sample in the fully damaged state.
Modern air-liners and rotor blades of wind turbines are basically made of fiber reinforced plastics (FRP). Their failure heavily impairs the serviceability and the operational safety. Consequently, knowledge of the failure behavior under static and cyclic loads is of great interest to estimate the operational strength and to compare the performance of different materials. Ideally, the damage evolution under operational load is determined with in-situ non-destructive testing techniques. Here, we report on in-situ synchrotron X-ray imaging of tensile stress induced cracks in carbon fiber reinforced plastics (CFRP) due to inter fiber failure. An in-house designed compact-tensile testing machine with a load range up to 15 kN was integrated into the beam path. Since conventional radiographs do not reveal sufficient contrast to distinct cracks due to inter fiber failure and micro cracking from fiber bundles, the Diffraction Enhanced Imaging technique (DEI) is applied in order to separate primary and scattered (refracted) radiation by means of an analyzer crystal. In the laboratory, scanning X-ray refraction topography of CFRP has been applied long before but it comes along with several disadvantages: the long total measuring time hampers real time (in-situ) measurements and the required small beam size hinders end-to-end imaging. The introduced technique overcomes both drawbacks. Imaging and tensile test rig are run unsynchronized at the greatest possible frame rate (0.7 s-1 at 28.8 µm pixel size) and smallest possible strain rate (5.5∙10-4 s-1). For 0°/90° non-crimped fabrics (ncf) the first inter fiber cracks occurred at 380 MPa (strain 0.7 %). Prior to failure at about 760 MPa (strain 2.0 %) we observe the evolution of a nearly equidistant 1 mm grid of cracks running across the entire sample in the fully damaged state before total failure.
Wind turbine rotor blades, made of fiber reinforced polymers (FRP), often fail before their projected 20-year lifespan, largely due to defects that originate during manufacturing and are propagated by operational fatigue and environmental conditions. The cost-intensive replacement outcomes lead to a high loss of earnings, and are one of the inhibitors of wind turbine production [1]. A potential repair alternative is to locally patch these areas of the blades with adhesively bonded structural repairs. However, the effects of such repair methods of the outer shell region on the structural integrity of the rotor blades are still largely unknown, and are thus investigated in this project.
The shell components of rotor blades are made of FRP composite material sandwiching a lightweight core, often a rigid foam or Balsa wood. The repair methods involve replacing the lost load path with a new material that is joined to the parent structure [2]. Repairs in this project focus on the scarf method, which allow for a smoother load distribution across the joint, aiming to study the damage mechanism of glass FRP scarf repairs for wind turbine blade shell applications. Namely, the source and path of the damage initiation and propagation, role of the interface between parent and patch material, and the role of the fiber orientation mismatch at this interface are examined. Biaxial ±45° and 0/90° FRP specimens are produced with the vacuum-assisted resin infusion (VARI) process using E-glass non-crimp fabric. The patch layers are then joined using VARI with a scarf ratio of 1:50, using glass FRP fabric with half the areal weight of the parent side to allow for better drapability. The methods and practices in specimen production are based on common industry practice in rotor blade shell manufacturing and repairs.
The specimens are tested under uniaxial tensile load, during which they are periodically monitored for damage onset. A comparison of the ±45° and 0/90° specimens allows for an understanding of the role of a highly mismatching fiber orientation in the transition zone between parent and patch material on the failure mechanism of the scarf joint. Although failure in both orientations begins as delamination at the joint edge, the difference in the mechanisms at play in the two different specimen types leads ultimately to different fracture paths. Namely, in the inter fiber failure mechanism of the ±45° specimens, the higher interlaminar strength compared to the intralaminar strength of the laminate leads to intralaminar failure of the ±45° scarf joint specimens. Alternatively, the competition in the 0/90° specimens lies between the interlaminar strength and fiber failure strength, and here we experience failure primarily across the scarf joint length. The scarf joint in the 0/90° specimens disrupts the continuity of the load-carrying 0° layers, directing the failure path to remain primarily along the scarf joint. Experimental results are compared to finite element analyses of scarf patch repairs on glass FRP sandwich specimens with the same respective layup orientations, where the damage initiation regions are identified and correlated to fiber orientation, serving as a bridge to future work which will experimentally examine the scarf repair patches on sandwich shell specimens.
In the present study, a simple approach was used to investigate the effect of UV-exposure on two high density Polyethylene materials (PE-HD), commonly used for storage tanks, on fuel sorption behavior and colonization by microorganisms. The aim was to investigate whether the sorption behavior of the fuels (diesel/biodiesel) and the colonization by microorganisms, frequently occurring in the fuel, is affected and may lead to undesirable or safety-relevant material changes. We showed that the UV-irradiation leads to significant changes of the sorption behavior due to chemi-crystallization and crosslinking. The fuel Sorption is affected by the UV-induced formation of polar carbonyl and hydroxyl groups predominantly occurring at the surface. With respect to microbial colonization behavior for Bacillus subtilis and Pseudomonas aeruginosa, isolated from a contaminated diesel sample, differences of the initial adhesion could be shown depending on the initial type of polyethylene as well as on the degree of UV-induced degradation.
Environmental Stress Cracking (ESC) and Slow Crack Growth (SCG) of PE-HD induced by external fluids
(2020)
High-density polyethylene (PE-HD) is widely used as a packaging material. Typical applications are pipes and containers for storage and transport of dangerous goods. For these applications, the understanding of the craze-crack damage mechanisms slow crack growth (SCG) and environmental stress cracking (ESC) is of importance. Since these mechanisms are considered to be the major causes of failure, their understanding is essential for inspection and release of those materials.
A well-established test method for the assessment of these damage mechanisms is the full-notch creep test (FNCT). It is used in this study for a detailed investigation of crack propagation phenomena in PE-HD container materials under the influence of different fluids such as air, water and aqueous detergent solutions (Arkopal N 100) as well as biodiesel and diesel. Based on the results of the FNCT, a classification scheme of different fluids is proposed, which allows for an assignment of the respective damage mechanisms. Hereby, it is differentiated between (i) inert, (ii) purely surface-active and (iii) additionally sorptive, bulk-active fluids with respect to SCG. If the test fluid changes the intrinsic properties (at the surface or in the bulk), the damage mechanism is addressed to ESC behavior.
In FNCT investigations, stress, temperature and specimen geometry were varied systematically. In addition to the time to failure as common measure for the resistance of a PE-HD type against crack propagation, specimen elongation was considered in detail.
Several imaging techniques were applied for fracture surface analysis of specimens tested in FNCT to gain novel information on SCG and ESC behavior. From height profiles obtained by laser scanning microscopy (LSM) and information on surface structures from scanning electron microscopy (SEM), indicators for the differentiation of the crack propagation mechanisms could be derived. Based on the LSM data, an algorithm for the distinction between ductile shear deformation and brittle crack growth as dominating failure mechanism was developed. Imaging techniques were also used for determination of crack propagation rates, which were related to time-resolved FNCT elongation data. From the time-resolved determination of crack lengths of partly damaged FNCT specimens, an increasing length of craze zone with a progressively propagating crack was revealed for the first time. This relation of crack and craze zones was specified by fracture mechanical considerations.
The full-notch creep test (FNCT) is a common method to evaluate the environmental stress cracking (ESC) behavior of high-density polyethylene (PE-HD) container materials . The test procedure as specified in ISO 16770 provides a comparative measure of the resistance against ESC using the time to failure of specimens mechanically loaded in a well-defined liquid environment. Since the craze-crack damage mechanism underlying the ESC process is associated with brittle failure, the occurrence of globally brittle fracture surfaces is a prerequisite to consider an FNCT measurement as representative for ESC . Therefore, an optical evaluation of FNCT fracture surfaces concerning their brittleness is essential. Due to the experimental setup, an inevitable increase of the true mechanical stress and the associated appearance of small ductile parts on fracture surfaces is induced in any case. Hence, an FNCT experiment is considered as 'valid', if the corresponding fracture surface is predominantly brittle . Based on laser scanning microscopy (LSM) height data of FNCT fracture surfaces , a universal and easy-to-use phenomenological criterion was developed to assess the validity of distinct FNCT experiments. This criterion is supposed to facilitate a quick evaluation of FNCT results in practical routine testing.
During their lifetime, polymer components subjected to mechanical loads and environmental influences show a loss of their mechanical properties required for their specific applications. In this respect, the craze-crack damage mechanism slow crack growth (SCG) is relevant for PE-HD components used in high-performance applications such as pipes and containers for the storage and transport of dangerous goods. SCG is considered to be the major failure mechanism in polyolefins and it typically occurs suddenly and unexpectedly. Due to the fields of application, SCG is a safety relevant issue. To test for the resistance of PE-HD pipe and container materials against SCG, the full-notch creep test (FNCT) is widely applied in Europe. In this study, SCG phenomena in PE-HD are investigated in detail based on an improved FNCT, especially including the consideration of the influence of environmental liquids effecting the damage mechanism. Using an enhanced fracture surface and a crack propagation analysis with imaging techniques such as light microscopy (LM), laser scanning microscopy (LSM), X-ray computed tomography (CT-scan) and scanning electron microscopy (SEM), detailed data concerning SCG are obtained.
The combined application of FNCT and such imaging techniques is explicitly advantageous and recommended to gain important information on damage occurring to PE-HD induced by mechanical stress and the influence of environmental liquids, which is essential within the Fourth Industry Revolution.
The impact fracture behavior of two common high-density polyethylene grades for container applications were intensively studied by the instrumented Charpy impact test after well-defined exposure to UV-irradiation. Individual stages of the impact event, such as crack initiation and crack propagation energy as well as maximum impact load, were investigated from the recorded load–deflection curves. UV-induced material property changes were further investigated by infrared spectroscopy, differential scanning calorimetry, and dynamic-mechanical analysis as well as density measurements. Based on the results of the Charpy impact test, three indicators were identified to describe the extend of photooxidation on high-density polyethylene: (a) a reduced Charpy impact strength—at least to half of its initial value for a distinctly brittle impact fracture, (b) a marked decrease in the crack propagation contribution to the impact strength, and (c) an increase of the brittle features of the fracture surface.
Due to the different thermal expansion of the constituent materials, cyclic thermal loading of FRP induces alternating stresses in the material at two scales: at the micro scale (level of fibre–matrix-interaction) and at the macro scale (level of the multidirectional laminate). Especially the micro scale effect is not comprehensively investigated yet. Additionally, computational investigations mostly neglect this effect due to the homogenous modelling of the composite material. As this effect is assumed to significantly contribute to the fatigue of FRP at thermal loads, the present paper suggests an experimental and numerical multiscale approach including Experiments at the different involved material scales to separately observe the effects acting at these scales. The approach also includes numerical modelling for each scale to complement the knowledge gained from the Experiments and to create a basis for the consideration of the micro effect even in macroscopic fatigue models treating homogeneous modelled composites. The main focus of the contribution is to bring the overall Approach up for discussion, rather than to present the multiscale modelling details.
Thermoset materials characterization is often limited to solid state analytical techniques such as IR, NMR, DSC, TGA and mechanical testing. Alternatively, their off-gassing behavior can also be evaluated using GC based techniques such as TD-GC-MS, allowing this method to be applied to thermoset materials analyses such as identification, aging characterization, and formulation optimization. As an overview, common thermoset materials were evaluated by analyzing their gaseous degradation products via TGA-based pyrolysis and subsequent TD-GC-MS for the identification of representative volatile signatures. It is thereby possible to distinguish different classes of phenolic materials or cured epoxy resins, as well as their amine or anhydride curatives. Additionally, this method enabled quantification of a volatile fragment (bisphenol A, BPA) which is associated with oxidation of epoxy/amine thermoset materials. The amount of evolved BPA increased linearly with aging time and this trend exhibits linear Arrhenius behavior over the temperature range (80–125 °C) studied, in agreement with oxidation sensitivies based on oxygen consumption data. Further, TD-GC-MS was used to explore how off-gassing of residual anhydride curative from an epoxy/anhydride material depends on formulation stoichiometry. Even in formulations that theoretically contained enough epoxy to consume all anhydride (1:1 stoichiometry), an imperfect final cure state resulted in residual anhydride which could evolve from the material. For such materials, a slightly epoxy-rich formulation is required to ensure that the material contains no residual unreacted anhydride. Analysis of volatiles generated by thermal exposure is an attractive characterization approach enabling compositional analysis as well as complementary diagnostics for materials degradation.
A shell test bench was developed at BAM 5.3 which allows for static and fatigue testing of curved fiber-reinforced plastic (FRP) structures, during which in-situ the damage state can be non-destructively inspected by thermography and strain-field measurement techniques. Sandwich shell specimens with typical wind turbine blade manufacturing defects were designed and tested. The tested imperfections show a fairly significant reduction (up to 90%) of the shell test specimens‘ lifetime, depending on the type of imperfection. Using the in-situ NDT methods incorporated in the shell test bench, the location and cycle time of the initial defects and the damage evolution was investigated.
The common fatigue life certification of aircrafts according to the certification Standards 23 and 25 follows a building block approach. Static tests at room temperature as well in humid and high temperature conditions are done on the coupon level. Additionally, a full-scale static and fatigue test must be performed on the complete airframe (minimum on the fuselage together with the wing). For each type-certificate the complete building block approach test program must be performed.
Traditionally in Germany, the certification of sailplanes (Certification Standard 22) follows rather a family concept. A shared data base was created over the last 50 years based upon a large number of material testing. In addition to static tests at room temperature and hot-humid conditions, fatigue tests are also done on the coupon level. Additional static and fatigue tests were done on complex structures such as spar-beams, fuselages and full-scale wing structures. However, for each type-certificate, only static tests should be performed in full-scale. This concept is determined by the certification memorandum CM-S-006 “Composite Lightweight Aircraft” 2017.
The presentation was given as an introduction to the discussion about the future expectations and developments of the EASA concerning the type-certification of lightweight aircrafts according to CS22 at the OSTIVE Sailplane Development Panel Meeting at the EASA in Cologne on the 11th of October 2019.
Diese DIN SPEC legt ein Verfahren für die Prüfung von unverstärkten und partikel- und kurzfaserverstärkten Kunststoffen mittels Zug-Druck-Schubprüfung fest. Dieses Verfahren findet Anwendung zur Bestimmung der Schwingfestigkeit im Bereich von 103 bis 107 Schwingspielen und der Nachgiebigkeiten unter statischer Belastung sowie der Zugfestigkeit. Alle Beanspruchungen und berechneten Größen sind nennspannungs-basiert.
Grundsätzlich besteht bei Rohrprobekörpern auch die Möglichkeit eine Beanspruchung in Längs- und in Umfangsrichtung durch Innendruck zu erzeugen. Dies wird in dieser Spezifikation nicht berücksichtigt, um die Anwendung überschaubar zu halten. Bei Rohrproben mit ausgeprägter Querschnittsverkleinerung (z. B. der Prüfkörper Typ B), entsteht eine Umfangsspannung auch aufgrund einer Zugkraft (siehe Anhang C). Die Umfangsspannung muss dann mittels FEA lokal berechnet werden.
Das Verfahren gilt für rohrförmige Prüfkörper deren spezifische Form den Anforderungen der Werkstoff-prüfung und bei Kurzfaserverstärkung einer hohen Vorzugsausrichtung in Probenlängsrichtung genügen.
Diese DIN SPEC gilt nicht für endlosfaserverstärkte Kunststoffe. Wegen der vollkommen unterschiedlichen Fertigungstechnologien und der zu erwartenden hohen Prüflasten, sind hier die Anforderungen an die Prüfkörpergeometrie gesondert zu betrachten. Es wird nicht zielführend sein, beide Werkstoffklassen in einer Norm zu behandeln.
Der Fachbereich Mechanik der Polymerwerkstoffe befasst sich mit der Charakterisierung der mechanischen Eigenschaften und der Degradation von Polymeren und Faserverbundwerkstoffen, die in hochbeanspruchten, sicherheitsrelevanten Konstruktionen wie jenen des Flugzeugbaus, des Automobilbaus und der Windenergie-Industrie eingesetzt werden. Auch die Analyse der chemischen und physikalischen Eigenschaften von Polymeren zählt zu unserem Aufgabenbereich.
Das Spektrum reicht von der statischen Festigkeit über die Betriebsfestigkeit, die Messung bruchmechanischer und viskoelastischer Parameter bis hin zur Formulierung molekularer Schädigungsmechanismen.
Statistically the mean time between damage events on rotor blades is 6 years (Deutscher Windenergie Report 2006). Due to imperfection in the production the shell structures get cracks after a few years fare before the designed life time. A shell test rig was built at BAM for efficient research on the effects of defects in production.
In-situ and ex-situ NDT give a better understanding from degradation processes in composite materials.
With advanced methods in the research on the fatigue behaviour of FRP it was found a load level of infinite life for GFRP and CFRP. This is in the range of typical strain values of airliners and rotor blades in normal operation.
Due to the fibre-composite nature NDT techniques have to be suitable to a wide length scale to image micro cracking as well as bigger defects. Therefore different techniques have to be applied and developed.
Even for the basic measurements of material data for design and engineering of composite structures there is a need to upgrade standards. With a new shear frame test rig more precise values can be obtained.
With advanced methods in the research on the fatigue behaviour of FRP it was found a load level of infinite life for GFRP and CFRP. This is in the range of typical strain values of airliners and rotor blades in normal operation.
Statistically the mean time between damage events on rotor blades is 6 years (Deutscher Windenergie Report 2006). Due to imperfection in the production the shell structures get cracks after a few years fare before the designed life time. A shell test rig was built at BAM for efficient research on the effects of defects in production.
Test blades of ~10m are an efficient way for SHM research and evaluation of NDT-methods and blade geometry.
A shell test bench was developed at BAM 5.3 which allows for static and fatigue testing of curved fiber-reinforced plastic (FRP) structures, during which in-situ the damage state can be non-destructively inspected by thermography and strain-field measurement techniques. Sandwich shell specimens with typical wind turbine blade manufacturing defects were designed and tested. The tested imperfections show a fairly significant reduction (up to 90%) of the shell test specimens‘ lifetime, depending on the type of imperfection. Using the in-situ NDT methods incorporated in the shell test bench, the location and cycle time of the initial defects and the damage evolution was investigated.
The described investigation of carbon-fibre-reinforced plastics (CFRP) documents that damage evolution can be observed by means of X-ray refractography. Comparative investigations with synchrotron technique on CFRP and grey-scale analysis on glass fibre-reinforced-plastics (GFRP) confirm these results. Moreover it was found that the fracture mechanical properties of the matrix system influence damage nucleation and propagation in the laminate during static and fatigue loads. Single-step fatigue tests were carried out on Laminates with RIM135 and LY556 matrix systems made from non-crimped fabric (NCF) or twill weave in different fibre orientations. The damage to the LY556 laminates was characterized by laminate cracks growing rapidly over the whole specimen width, whereas the damage on the RIM135 laminates was characterized by an earlier onset of micro-cracking followed by laminate cracks. The specimens were fatigued up to 108 (very high cycle fatigue (VHCF) regime) load cycles. S-N-curves of damage initiation were drawn and boundaries were identified for endurance within the VHCF regime. A phenomenology based model focusing on matrix stress was applied to reproduce the first inter-fibre failure (IFF) under static and fatigue loads.
An effective method for evaluating the fatigue strength of thick unidirectional composite laminates of wing spar caps has been presented here. A typical width-tapered bending specimen has been developed for a four-point loading set up. Static and fatigue loading was performed at a load ratio R = -1. The complex stress state has been investigated numerically by finite element analysis. Finally, the concept is proven experimentally on specimens made of pultruded fibre rods.
The fatigue behavior of the continuously tapered width by water Jet cutting has been compared to discrete tapering via rod-drop. The new method enables a screening of material fatigue behavior caused by normal and shear loading.
Due to their high strength-to-weight ratio and excellent fatigue resistance, glass fiber reinforced polymers (GFRP) are used as a construction material in a variety of applications including composite high-pressure gas storage vessels. Thus, an early damage detection of the composite material is of great importance. Material degradation can be determined via measuring the distributed strain profile of the GFRP structures. In this article, swept wavelength interferometry based distributed strain sensing (DSS) was applied for structural health monitoring of internal pressure loaded GFRP tube specimens. Measured strain profiles were compared to theoretical calculation considering Classical Lamination Theory. Reliable strain measurements with millimeter resolution were executed even at elongations of up to 3% in the radial direction caused by high internal pressure load. Material fatigue was localized by damaged-induced strain changes during operation, and detected already at 40% of burst pressure.
Wind turbine rotor blade shells are manufactured as sandwich structures with fiber-reinforced polymer (FRP) due to the material’s high specific stiffness and strength. With a growing renewable energy industry and thereby a spread of wind energy farms, especially in offshore applications, the need to fully utilize turbines through their designed lifespan is becoming increasingly essential. However, due to imperfections during manufacturing, which are then propagated by harsh environmental conditions and a variety of loads, blades often fail before their projected lifespan. Thus, the need for localized repair patch methods for the outer shell portions of the blades has become of greater interest in recent years, as it is crucial to the optimal compromise between continuation of wind energy production, cost efficiency, and restoration of structural performance. To increase the understanding of the effect on the fatigue life of the rotor blades, this study tests localized repair patch methods and compares them to each other as well as to reference, non-repaired specimens. Manufactured with the vacuum-assisted resin infusion process, the shell test specimens are produced as a curved structure with glass FRP sandwiching a polyvinyl chloride foam core to best represent a portion of a rotor blade shell. Patch repairs are then introduced with varying layup techniques, and material properties are examined with cyclic fatigue tests. The intermediate scale test specimens allow for the observation of material as well as structural variables, namely of interest being the stiffness and strength restoration due to the repair patches. Damage onset, crack development, and eventual failure are monitored with in-situ non-destructive testing methods to develop a robust understanding of the effects of repair concepts.
Biopolymers are very efficient for significant applications ranging from tissue engineering, biological devices to water purification. There is a tremendous potential value of cellulose because of ist being the most abundant biopolymer on earth, swellability, and functional groups to be modified. A novel, highly efficient route for the fabrication of mechanically stable and natural hydrogels is described in which cellulose and glycine are dissolved in an alkaline solution of NaOH and neutralized in an acidic solution. The dissolving temperature and the glycine amount are essential parameters for the self-assembly of cellulose chains and for Tuning the morphology and the aggregate structures of the resulting hydrogels. Glycine plays the role of a physical crosslinker based on the Information obtained from FTIR and Raman spectra. Among the prepared set of hydrogels, CL5Gly30 hydrogels have the highest capacity to absorb water. The prepared CL5Gly30 gels can absorb up to seven times their dry weight due to its porous 3-D network structure. CL5Gly10 hydrogel exhibits 80% deformation under 21 N force executed. The method developed in this article can contribute to the application of heavy metal adsorption in aqueous solutions for water purification and waste management.
Fiber-reinforced-polymers (FRPs) are in current research focus in the lightweight construction industry, because of their extraordinary characteristics (stiffness and strength-to-density relation). The structure of polymer matrix and the interaction with reinforcement are crucial for optimization of the mechanical and thermal properties of FRPs. Due to the macromolecular chain structure, the mechanical properties of a polymer strongly vary with temperature: Below the glass transition, the chain segments of a polymer are “frozen”. Regarding fracture, the total changed energy during fracture if only dissipates for the generation of the new surfaces. However, in the region of the glass transition, the polymer chain segments start to get “unfrozen”, and the energy is not only required for generating new surfaces, but also for irreversibly deformation. This irreversible deformation is affected by the global temperature and the local temperature near the crack tip, which is affected by the local strain rate and crack propagation velocity.
Hence, in this research project, the irreversible deformation of neat and reinforced polymers will be controlled by changing the global temperature as well as the local temperature. With using different fracture experiments, the amount of energy required for creating new surfaces and for the irreversible deformation will be separated. In this presentation, I summarized of the first 15 months the whole project. In this period, the basic crack propagation theory for neat polymers is established and the special fracture experiment sample is prepared and tested at room temperature. In addition, the model of the specimen is first established.
Crack propagation in polymers: Separation of surface energy and irreversible deformation energy
(2019)
Fiber-reinforced-polymers (FRPs) are in current research focus in the lightweight construction industry, because of their extraordinary characteristics (stiffness and strength-to-density relation). The structure of polymer matrix and the interaction with reinforcement are crucial for optimization of the mechanical and thermal properties of FRPs. Due to the macromolecular chain structure, the mechanical properties of a polymer strongly vary with temperature: Below the glass transition, the chain segments of a polymer are “frozen”. Regarding fracture, the total changed energy during fracture if only dissipates for the generation of the new surfaces. However, in the region of the glass transition, the polymer chain segments start to get “unfrozen”, and the energy is not only required for generating new surfaces, but also for irreversibly deformation. This irreversible deformation is affected by the global temperature and the local temperature near the crack tip, which is affected by the local strain rate and crack propagation velocity.
Hence, in this research project, the irreversible deformation of neat and reinforced polymers will be controlled by changing the global temperature as well as the local temperature. With using different fracture experiments, the amount of energy required for creating new surfaces and for the irreversible deformation will be separated. This poster is the summary of the first part of the whole project. In the first part, the basic crack propagation theory for neat polymers is established and the special fracture experiment sample is prepared and tested at room temperature. In addition, the fracture experiment at room temperature is validated numerically.
Fiber-reinforced-polymers (FRPs) are in current research focus in the lightweight construction industry, because of their extraordinary characteristics (stiffness and strength-to-density relation). The structure of polymer matrix and the interaction with reinforcement are crucial for optimization of the mechanical and thermal properties of FRPs. Due to the macromolecular chain structure, the mechanical properties of a polymer strongly vary with temperature: Below the glass transition, the chain segments of a polymer are “frozen”. Regarding fracture, the total changed energy during fracture if only dissipates for the generation of the new surfaces. However, in the region of the glass transition, the polymer chain segments start to get “unfrozen”, and the energy is not only required for generating new surfaces, but also for irreversibly deformation. This irreversible deformation is affected by the global temperature and the local temperature near the crack tip, which is affected by the local strain rate and crack propagation velocity.
Hence, in this research project, the irreversible deformation of neat and reinforced polymers will be controlled by changing the global temperature as well as the local temperature. With using different fracture experiments, the amount of energy required for creating new surfaces and for the irreversible deformation will be separated. The fracture tests include the conventional tensile test, the macroscopic peel test and the single fiber peel – off test.
Modern wind turbine rotor blades consist of sandwich shell segments made from glass fiber reinforced polymers. During manufacturing, defects can arise which could lead to failure of the whole component under dynamic mechanical and thermal loads. Hence during operation defects can arise which, if detected, can be repaired locally and in-situ by applying repair patches instead of taking the whole rotor blade down and repair it remotely. This method is much more time and cost effective, since the shut-down time of the energy converter is limited to a minimum. These repair patches can, however, also lead to new defects if not applied optimally. Therefore, it is necessary to control the quality of the repair patches to ensure the best possible restoration of structural integrity of the component. As a rotor blade is an object with a large aspect ratio, X-ray laminography is predestined to provide 3D information of the objective volume. To enhance the amount of information gained from laminographic reconstruction, we use in this study a photon counting and energy discriminating X-ray detector and apply a material decomposition algorithm to the data. By inherently separating the incident spectra within the detection process into two distinct energy bins, the basis material decomposition can provide material resolved images. Choosing glass and epoxy resin as basis materials and numerically solving the inverse dual-energy equation system, the reconstructed laminographic datasets contain highly valuable information about the distribution of the basis materials within the structure. Furthermore, cross- artifacts arising from the limited angle of the projection data can be reduced by this method which allows to investigate structures that were hidden underneath the artefacts.
Modern wind turbine rotor blades consist of sandwich shell segments made from glass fiber reinforced polymers. During manufacturing, defects can arise which could lead to failure of the whole component under dynamic mechanical and thermal loads. Hence during operation defects can arise which, if detected, can be repaired locally and in-situ by applying repair patches instead of taking the whole rotor blade down and repair it remotely. This method is much more time and cost effective, since the shut-down time of the energy converter is limited to a minimum. These repair patches can, however, also lead to new defects if not applied optimally. Therefore, it is necessary to control the quality of the repair patches to ensure the best possible restoration of structural integrity of the component. As a rotor blade is an object with a large aspect ratio, X-ray laminography is predestined to provide 3D information of the objective volume. To enhance the amount of information gained from laminographic reconstruction, we use in this study a photon counting and energy discriminating X-ray detector and apply a material decomposition algorithm to the data. By inherently separating the incident spectra within the detection process into two distinct energy bins, the basis material decomposition can provide material resolved images. Choosing glass and epoxy resin as basis materials and numerically solving the inverse dual-energy equation system, the reconstructed laminographic datasets contain highly valuable information about the distribution of the basis materials within the structure. Furthermore, cross- artifacts arising from the limited angle of the projection data can be reduced by this method which allows to investigate structures that were hidden underneath the artefacts.
Behälter aus hochdichtem Polyethylen (PE-HD) werden zur Lagerung und für den Transport von Kraftstoffen eingesetzt. Beim Kontakt beider Medien diffundiert der Kraftstoff in das Polymer, erhöht dessen Volumen und Duktilität und ändert damit das Fließverhalten des Polymers unter Zugbelastung. Zudem weist PE-HD in Luft im verstreckten Bereich das sogenannte strain-whitening (Lichtstreuung) auf, während in Diesel gelagertes PE-HD optisch transparent wird. Zur Untersuchung dieses Phänomens wurden ortsaufgelöste Röntgenrefraktion und –diffraktion (Topographie) eingesetzt. Aus Sicht der molekularen Struktur (Diffraktion) weisen die beiden Probentypen keine Unterschiede auf: im verstreckten Bereich liegt eine „Fasertextur“ der Polymerketten in Belastungsrichtung vor, im unverstreckten Bereich nahezu ideale Isotropie. Die für mikroskopische Strukturen empfindliche Röntgenrefraktion zeigt außerdem für die Probe in Luft im verstreckten Bereich eine ausgeprägte Orientierung von Grenzflächen in Zugrichtung, während die in Diesel gelagerte Probe keine Grenzflächen zeigt. In Analogie zum optischen Verhalten bewirken die ähnlichen Brechungsindizes beider Medien im Röntgenbereich, dass eindiffundierter Diesel als Immersionsflüssigkeit die Brechungseigenschaften herabsetzt.
Behälter aus hochdichtem Polyethylen (PE-HD) werden zur Lagerung und für den Transport von Kraftstoffen eingesetzt. Beim Kontakt beider Medien diffundiert der Kraftstoff in das Polymer, erhöht dessen Volumen und Duktilität und ändert damit das Fließverhalten des Polymers unter Zugbelastung. Zudem weist PE-HD in Luft im verstreckten Bereich das sogenannte strain-whitening (Lichtstreuung) auf, während in Diesel gelagertes PE-HD optisch transparent wird. Zur Untersuchung dieses Phänomens wurden ortsaufgelöste Röntgenrefraktion und –diffraktion (Topographie) eingesetzt. Aus Sicht der molekularen Struktur (Diffraktion) weisen die beiden Probentypen keine Unterschiede auf: im verstreckten Bereich liegt eine „Fasertextur“ der Polymerketten in Belastungsrichtung vor, im unverstreckten Bereich nahezu ideale Isotropie. Die für mikroskopische Strukturen empfindliche Röntgenrefraktion zeigt außerdem für die Probe in Luft im verstreckten Bereich eine ausgeprägte Orientierung von Grenzflächen in Zugrichtung, während die in Diesel gelagerte Probe keine Grenzflächen zeigt. In Analogie zum optischen Verhalten bewirken die ähnlichen Brechungsindizes beider Medien im Röntgenbereich, dass eindiffundierter Diesel als Immersionsflüssigkeit die Brechungseigenschaften herabsetzt.
High-density polyethylene becomes optically transparent during tensile drawing when previously saturated with diesel fuel. This unusual phenomenon is investigated as it might allow conclusions with respect to the material behavior. Microscopy, differential scanning calorimetry, density measurements are applied together with two scanning X-ray scattering techniques: wide angle X-ray scattering (WAXS) and X-ray refraction, able to extract the spatially resolved crystal orientation and internal surface, respectively. The sorbed diesel softens the material and significantly alters the yielding characteristics. Although the crystallinity among stretched regions is similar, a virgin reference sample exhibits strain whitening during stretching, while the diesel-saturated sample becomes transparent. The WAXS results reveal a pronounced fiber texture in the tensile direction in the stretched region and an isotropic orientation in the unstretched region. This texture implies the formation of fibrils in the stretched region, while spherulites remain intact in the unstretched parts of the specimens. X-ray refraction reveals a preferred orientation of internal surfaces along the tensile direction in the stretched region of virgin samples, while the sample stretched in the diesel-saturated state shows no internal surfaces at all. Besides from stretching saturated samples, optical transparency is also obtained from sorbing samples in diesel after stretching.
Localized patches are a cost- and time-effective method for repairing fiber-reinforced polymer (FRP) sandwich wind turbine rotor blade shells. To increase the understanding of their effect on the fatigue of the blades, this study examines the effect of various layup methods of localized repair patches on the structural integrity of composite sandwich structures. Manufactured with the vacuum-assisted resin infusion (VARI) process, the shell test specimens are produced as a curved structure with glass fiber reinforced polymer (GFRP) sandwiching a polyvinyl chloride (PVC) foam core. Patch repairs are then introduced with varying layup techniques, and material properties are examined with cyclic fatigue tests. The transition region between patch and parent material is studied in greater detail with finite element method (FEM) simulations, with a focus on the effect of fiber orientation mismatch. Damage onset, crack development, and eventual failure are monitored with in-situ non-destructive testing methods to develop a robust understanding of the effects of repair concepts on material stiffness and strength.
At high temperatures and in harsh environments ceramic springs are often superior to metallic springs and allow for innovative solutions. A recently proposed application involves ceramic springs with metallized surfaces as capacitive force sensor. A strictly linear stress-strain characteristic of the spring is a precondition for such a sensor.
Helical ceramic springs with rectangular cross-section have been produced from sintered hollow cylinders. Alumina, ATZ, Y-TZP, and Ce-TZP springs with identical dimensions were characterized and compared regarding deformation behavior.
Spring deformation was investigated under various load scenarios.
Dynamic compression was performed with deformation speeds from 0.3 to 30 mm/min. Spring constants of alumina springs are strain rate independent. By contrast, Y-TZP spring constant increases by approximately 3 % within the experimental framework.
A high-precision test facility was developed to characterize spring displacement in nm range under static tensile load over long periods of time. Spring elongation with asymptotic course was observed for zirconia containing materials at room temperature. This effect is particularly strong in the case of Y-TZP. Up to 0.3 % time-dependent elongation was measured after 24 h under constant load. Deformation is completely reversible after unloading. Alumina springs do not show any time-dependent deformation under identical test conditions.
Contrary to alumina springs, a non-linear stress-strain behavior of TZP springs at room temperature was proved in both test series. It is supposed, that pseudo-elasticity caused by stress-induced phase transformation from tetragonal to monoclinic is responsible for this special behavior of TZP springs.
The aim of this research is to study the influence of moisture absorption at low moisture contents on the creep behaviour of an epoxy adhesive in steel bonded joints. Single lap joints were manufactured using high strength steel adherends and a two-component epoxy adhesive. The single lap joints were tested at load Levels corresponding to average lap shear stresses of±5%, 15%, 30% and 45% of the dry lap shear strength in both 40 °C air and 40 °C distilled water. Specimens were not pre-aged to be able to analyse the coupled effect of moisture and loading. The test results show that an increase in the load level resulted in an increase in the instantaneous strain and in the creep strain rate. The creep strain of single lap joints loaded in water was generally larger than for the ones loaded in air. For joints loaded in water the creep behaviour was found to be dependent on the moisture concentration in the adhesive. At low moisture percentages creep was suppressed, resulting in a lower instantaneous strain. At higher moisture percentages creep was promoted, resulting in a larger strain rate. The suppression of creep at low moisture percentages is attributed to water molecules bonding to the epoxy macromolecules, resulting in a reduction in molecular mobility and a smaller creep strain. At higher moisture percentages the plasticizing effect of the water dominates, resulting in a larger creep strain. The Maxwell threeelement solid model and Kelvin-Voigt three-element solid model were used to simulate the creep behaviour of the single lap joints loaded in air and water. The models gave good representations of the creep Response across the different load levels in both water and air, they were however unable to give a correct representation of the instantaneous strain of the single lap joints loaded in water. This is attributed to the models being unable to account for the present short-term relaxation process that is dependent on the moisture concentration.
Faser-Kunststoff-Verbunde (FKV) werden zunehmend in der Luft- und Raumfahrt, der Automobil- und Windenergieindustrie eingesetzt. Die hier entwickelten Bauteile sind oft nicht nur multiaxialen mechanischen Belastungen, sondern auch hohen klimatischen Beanspruchungen ausgesetzt. Einsatztemperaturen zwischen -60°C und 100°C bei 10-90% relativer Feuchte sind keine Seltenheit, in der Raumfahrt sind die thermischen Belastungen noch extremer. Die Auswirkungen einer Wechselwirkung extremer klimatischer Beanspruchungen mit multiaxialen mechanischen Belastungen wurden bisher kaum betrachtet. Das Ziel des Projektes ist die Untersuchung des Ermüdungsverhaltens der Faser-Kunststoff-Verbunde in Abhängigkeit von multiaxialer mechanischer Belastung, Temperatur und Feuchte. Dazu werden umfangreiche experimentelle Untersuchungen an Flach- und Rohrproben sowie numerische und analytische Berechnungen durchgeführt.
Im Rahmen des Seminarvortrags wird eine mikromechanische Modellierung der thermischen Eigenspannungen innerhalb des FKV vorgestellt, welche anhand eines Finite-Elemente-Modells validiert wird. Zur Bestimmung einer Schädigungsgrenze werden verschiedene Energieansätze diskutiert, welche anschließend mit den experimentell erhaltenen Ergebnissen im Temperaturbereich zwischen -60 °C und 70 °C verglichen werden. Dabei wird gezeigt, dass mithilfe der mikromechanischen Formulierung eine temperaturunabhängige Masterschädigungslinie für quasistatisch belastete Probekörper abgeleitet werden kann. Des Weiteren können Elastizitäten und Festigkeiten in Abhängigkeit der Temperatur mit den mikromechanischen Mischungsregeln bestimmt werden. So kann der experimentelle Aufwand beim Nachweis thermomechanisch belasteter Strukturen reduziert werden. Ein ähnlicher Ansatz wird für schwingende Ermüdungsbelastung bei verschiedenen Umgebungstemperaturen diskutiert und experimentell überprüft.
Fiber reinforced polymers (FRPs) are a well established material in lightweight applications, e.g. in automotive, aerospace or wind energy. The FRP components are subjected to multiaxial mechanical as well as hygrothermal loads. Common operation temperatures are in the range of 213 K and 373 K (-60 °C and 100 °C) at a relative humidity of 10% to 90%. In spacecraft applications, the environmental conditions are even more extreme. However, the correlation between multiaxial mechanical loading and harsh environment conditions have to-date not been investigated in detail. The project aims to investigate the fatigue behavior of FRPs dependent on multiaxial mechanical loading, temperature, and humidity. Extensive experimental testing is performed on flat plate and cylindrical tube specimens, accompanied by numerical and analytical calculations.
The physical and chemical effects of diesel and biodiesel fuels on two high-density polyethylene (PE-HD) types were investigated. Both semi-crystalline PE-HD are common thermoplastic materials for container and storage tank applications. Biodiesel, a composition of unsaturated fatty acid esters from renewable resources, was chosen as it is regarded a possible green alternative to fossil fuels. The study aims at identifying significant differences between biodiesel and conventional diesel fuels based on the differences in the chemical nature of the two. The physical effects of the fuels on the polymer at first comprises the sorption behavior, i.e. kinetics and final equilibrium concentration. Not only are both fuels absorbed by the amorphous phase of the semi-crystalline PE-HD, they also induce a plasticization effect that modifies the molecular mobility and therefore also the characteristic yielding properties, manifest in the obtained stress-strain curves. The chemical effects related to degradation phenomena is investigated by a long-term storage scenario using partially immersed tensile test specimens in diesel and biodiesel. We were able to confirm the proposed co-oxidation mechanism by Richaud et al. for polyethylene-unsaturated penetrant systems on a larger scale based on practical tensile tests. One of the investigated polyethylene grades subjected to tensile drawing showed a significant loss of plastic deformation and the onset of premature failure after 150 days of storage in biodiesel. Further biodiesel storage showed a systematically reduced elongation at break before necking. None of these effects were observed in diesel. Oxidation of fuels and polymer after progressing storage times were analyzed by the evolution of carbonyl species in FT-IR/ATR spectroscopy.
Sandwich-Schalenstrukturen in situ charakterisiert mit Thermographie und Felddehnungsmessungen
(2018)
An der BAM wurde ein Schalenprüfstand entwickelt, mit dem typische Sandwichschalenstrukturen unter simulierter Betriebsbeanspruchung belastet werden können. Im mechanischen Ermüdungsversuch wurden in situ die Entstehung von Schäden in Schalenprüfkörpern mit Test-Imperfektionen mittels der passiven Thermographie und Felddehnungsmessungen verfolgt. Damit konnten der Zeitpunkt, der Schadensursprung und der Schadensfortschritt zerstörungsfrei bestimmt werden. Zudem wurden vergleichend Schalenprüfkörper mit Luftultraschall und der Röntgen-Laminographie untersucht. Im Rahmen eines BMWi-Forschungsvorhabens wurden die Schädigungsmechanismen evaluiert und Empfehlungen für die Praxis der Fertigung von Rotorblättern abgeleitet. In diesem Beitrag wird sich auf die Darstellung der ZfP-Verfahren und der ZfP-Ergebnisse konzentriert.
Assessing the structural integrity of carbon-fibre sandwich panels in fire: Bench-scale approach
(2019)
The fire resistance of lightweight sandwich panels (SW) with carbon fibre/epoxy skins and a poly(methacryl imide) (PMI) foam core is investigated in compression under direct application of a severe flame (heat flux=200 kW m−2). A bench-scale test procedure was used, with the sample held vertically. The epoxy decomposition temperature was quickly exceeded, with rapid flash-over and progressive core softening and decomposition.
There is a change in failure mode depending on whether the load is greater or less than 50% of the unexposed failure load, or in other words if one or two skins carry the load. At high loads, failure involved both skins with a single clear linear separation across each face. There is an inflection in the failure time relationship in the ∼50% load region, corresponding to the time taken for heat to be transmitted to the rear face, along with a change in the rear skin failure mode from separation to the formation of a plastic hinge. The integrity of the carbon front face, even with the resin burnt out, and the low thermal diffusivity of the core, both play key roles in prolonging rear face integrity, something to be borne in mind for future panel design. Intumescent coatings prolong the period before failure occurs. The ratio of times to failure with and without protection is proposed as a measure of their effectiveness. Apart from insulation properties, their adhesion and stability under severe fire impact play a key role.
In der vorliegenden Arbeit wurden verschiedene Epoxidharzsysteme charakterisiert. Zwei Systeme mit großen bruchmechanischen Unterschieden wurden zur Fertigung äquivalenter GFK- und CFK-Laminate mit Faserausrichtungen in 0/90° und ±45° ausgewählt. In quasi-statischen Zugversuchen und Einstufenschwingversuchen mit einem Beanspruchungsverhältnis von R= 0,1 wurden diese Laminate hinsichtlich ihres Schädigungsbeginns und ihrer Schädigungsentwicklung untersucht. Die Detektion der Schädigungen sowie die Dokumentation der Schädigungsentwicklung wurde anhand der Lichtabsorptionsanalyse an GFK-Laminaten und anhand der Röntgenrefraktionsanalyse an CFK-Laminaten umgesetzt. Auf diese Weise konnten Einflüsse der bruchmechanischen Eigenschaften der Matrix auf die Schädigungsentwicklung im Verbund aufgezeigt werden. Zudem wurden für die untersuchten Laminate die Schädigungsgrenzen bei schwingender Beanspruchung ermittelt. Anhand durchgeführter Schwingversuche an CFK-Laminaten im Very High Cycle Fatigue-(VHCF)-Lastwechselbereich bis 108 konnten Rückschlüsse vom Schädigungsverhalten im High Cycle Fatigue-(HCF)-Lastwechselbereich bis 106 auf die Dauerfestigkeit im VHCF-Bereich gezogen werden und damit VHCF-Dauerfestigkeitsgrenzen bestimmt werden. Mit dem Ziel die Ermüdung der Laminate auf die Beanspruchung der Matrix zurückzuführen, wurden die Erweiterte Inverse Laminattheorie, mikromechanikbasierte Mischungsregeln sowie eine Vergleichsspannungshypothese auf die untersuchten Laminate angewendet. Die Schädigungsgrenzen konnten damit in Form der Matrixbeanspruchung wiedergegeben werden. Die Abbildung der Ermüdung verschiedener Laminate anhand einer matrixspezifischen normierten Masterschädigungslinie ist für die behandelten CFK- und GFK-Laminate gelungen.
Common air-coupled transducers for non-destructive testing consist of a piezocomposite material and several matching layers. Better acoustical matching to air is achieved by transducers based on charged cellular polypropylene (PP). This material has about hundred times lower acoustic impedance than any piezocomposite, having about the same piezoelectric coefficient. The piezoelectric properties of cellular PP are caused by the polarization of air cells. Alternatively, a ferroelectret receiver can be understood as a capacitive microphone with internal polarization creating permanent internal voltage. The sensitivity of the receiver can be increased by applying additional bias voltage. We present an ultrasonic receiver based on cellular PP including a high-voltage module providing bias voltage up to 2 kV. The application of bias voltage increased the signal by 12 to 15 dB with only 1 dB increase of the noise.
This receiver was combined with a cellular PP transmitter in through transmission to inspect several test specimens consisting of glass-fiber-reinforced polymer face sheets and a porous closed-cell PVC core. These test specimens were inspected before and after load. Fatigue cracks in the porous PVC core and some fatigue damage in the face sheets were detected. These test specimens were originally developed to emulate a rotor blade segment of a wind power plant. Similar composite materials are used in lightweight aircrafts for the general aviation. The other inspected test specimen was a composite consisted of glass-fiber-reinforced polymer face sheets and a wooden core. The structure of the wooden core could be detected only with cellular PP transducers, while commercial air-coupled transducers lacked the necessary sensitivity. Measured on a 4-mm thick carbon-fiber-reinforced polymer plate, cellular PP transducers with additional bias voltage achieved a 32 dB higher signal-to-noise ratio than commercial air-coupled transducers.
Common air-coupled transducers for non-destructive testing consist of a piezocomposite material and several matching layers. Better acoustical matching to air is achieved by transducers based on charged cellular polypropylene (PP). This material has about hundred times lower acoustic impedance than any piezocomposite, having about the same piezoelectric coefficient. The piezoelectric properties of cellular PP are caused by the polarization of air cells. Alternatively, a ferroelectret receiver can be understood as a capacitive microphone with internal polarization creating permanent internal voltage. The sensitivity of the receiver can be increased by applying additional bias voltage. We present an ultrasonic receiver based on cellular PP including a high-voltage module providing bias voltage up to 2 kV. The application of bias voltage increased the signal by 12 to 15 dB with only 1 dB increase of the noise.
This receiver was combined with a cellular PP transmitter in through transmission to inspect several test specimens consisting of glass-fiber-reinforced polymer face sheets and a porous closed-cell PVC core. These test specimens were inspected before and after load. Fatigue cracks in the porous PVC core and some fatigue damage in the face sheets were detected. These test specimens were originally developed to emulate a rotor blade segment of a wind power plant. Similar composite materials are used in lightweight aircrafts for the general aviation. The other inspected test specimen was a composite consisted of glass-fiber-reinforced polymer face sheets and a wooden core. The structure of the wooden core could be detected only with cellular PP transducers, while commercial air-coupled transducers lacked the necessary sensitivity. Measured on a 4-mm thick carbon-fiber-reinforced polymer plate, cellular PP transducers with additional bias voltage achieved a 32 dB higher signal-to-noise ratio than commercial air-coupled transducers.
High-performance composites, including glass-fiber reinforced plastic (GFRP) materials, are favored as a construction material for wind turbine rotor blades due to their high specific strength and stiffness properties. During the manufacturing process, however, imperfections are often introduced, then further propagated due to harsh environmental conditions and a variety of loads. This leads to failure significantly before their projected 20-year lifespan. As replacement of entire blades can be a costly potential outcome, localized repair of the damaged region to restore structural integrity and thus lengthen its lifespan has become an important issue in recent years. Rotor blades are often repaired using a common technique for composite laminates: adhesively bonded structural repair patches. These methods involve replacing the lost load path with a new material that is joined to the parent structure, and include scarf or plug repairs. However, there currently do not exist any standardized repair procedures for wind turbine rotor blades, as comparisons of blade properties repaired with the existing methods have not been studied in depth. Namely, there is a lack of understanding about the effects of various repair methods on the fatigue life of the shells of rotor blades. This study therefore aims to begin to fill this knowledge gap by testing the influence of different repair patches on the blades’ mechanical properties. Manufactured with the vacuum-assisted resin infusion process, the test specimens are produced as a curved structure with GFRP sandwiching a polyvinyl chloride foam core to best represent a portion of a rotor blade shell. Scarf repairs are then introduced with varying layup techniques, and material properties are examined with cyclical fatigue tests. Crack growth and development is monitored during fatigue testing by various non-destructive testing methods, including passive thermography with an infrared camera system, and a 3D deformation analysis system with ARAMIS. Large deformation fields and detection of in- and out-of-plane deformations is thus possible in-situ. The mechanical behavior and development of defects in the various repaired specimens is compared to each other as well as to reference test specimens with no repair patches. In-situ test data is combined with further non-destructive testing methods, including laminography, and active thermography, to develop a robust understanding of the effects of repair concepts.
Effects of various geometrical and physical factors, as well as the method of data reduction (analysis of
experimental forceedisplacement curves) on the values of local interfacial strength parameters (local IFSS, td, and critical energy release rate, Gic) determined by means of a single fiber pull-out test are discussed. Experimental results of our pull-out tests on several fiberepolymer matrix systems showed that td and Gic weakly depended on geometrical factors. However, the pull-out test appeared to be sensitive to the conditions of specimen formation and testing, such as changing the nature of the contacting surfaces (fiber sizing) and the fiber pull-out rate. Of several methods of td and Gic Determination from a forceedisplacement curve, the most reliable and reproducible one is the approach based on the values of the maximum force recorded in a pull-out test and the interfacial frictional force immediately after fiber debonding.
Hochleistungsverbundwerkstoffe, bzw. Glasfaser-Kunststoff-Verbunde (GFK), haben sich als Leichtbauwerkstoffe für Leichtflugzeuge und Rotorblätter von Windkraftanlagen etabliert. Die hohe spezifische Festigkeit und Steifigkeit qualifizieren sie besonders für diese Anwendung. Während der Fertigung werden Imperfektionen häufig in die Blattschalen eingebracht. Durch Witterungseinflüsse kommt es zum Schadensfortschritt. Infolgedessen treten Schäden in der Blattschale, die den sicheren Betrieb gefährden können, weit vor der projektierten Lebensdauer von 20 Jahren auf. Da der Austausch von ganzen Blättern sehr kostintensiv ist, ist eine lokale Reparatur des geschädigten Bereichs zur Wiederherstellung der strukturellen Integrität, viel preisgünstiger. Die Reparatur von Rotorblättern ist in den letzten Jahren zu einem wichtigen Thema geworden. Derzeit werden verschiedenste Reparaturkonzepte angewendet. Eine einheitliche Reparaturvorgabe gibt es bisher nicht. Die Auswirkungen der angewendeten Reparaturkonzepte auf die Betriebsfestigkeit der Reparaturstelle sind weitestgehend unbekannt und sollen deshalb in diesem Projekt untersucht werden. Gekrümmte Voll-Laminat sowie Sandwich Prüfkörper werden mit dem Vacuum-Assisted-Resin-Infusion-Prozess (VARI) produziert. Um einen Teil einer Rotorblattschale darzustellen, sind sie als GFK aufgebaut, beziehungsweise die Sandwich Strukturen mit einem Polyvinylchlorid-Schaumkern (PVC) Kern, wie im Original. Schalenreparaturen in verschiedenen Layup-Techniken und Geometrie werden eingebracht und die Materialeigenschaften mit zyklischen Ermüdungstests untersucht. Im Projekt werden ideal im Labor erzeugte Reparaturstellen mit in der Praxis angewendeten Ausführungstechniken eines Industriepartners experimentell verglichen. Für die in-situ Detektion der Schadensentwicklung während des Ermüdungsversuches kommt ein kombiniertes Felddehnungs- und Thermografie-Kamerasystem zum Einsatz. Das mechanische Verhalten und die Schadensentwicklung in den verschiedenen reparierten Prüfkörper wird miteinander sowie mit den Referenzproben ohne Reparaturstellen verglichen. Zudem wird in numerischen Modellen für die verschiedenen Reparaturtechniken eine Spannungsanalyse durchgeführt um hinsichtlich der Betriebsbeanspruchung eine Bewertung angeben zu können.
We invented the first non-metallic, self-adhesive and dry biosignalling electrode. The PEDOT polymer electrode changes its aggregate state and conductivity by a light curing procedure. The electrode can be applied as a gel underneath hair without shaving. With the aid of blue light, the electrode can be hardened within a few seconds at the desired location on the scalp. The cured polymer electrode is highly conductive and can be applied on a very small location. Unlike other EEG electrodes, our electrode does not lose conductivity upon drying. Furthermore, our electrode strongly bonds to Skin and does not require any additional adhesive. Short circuits due to an outflow of gel are prevented with this technique. Therefore, the PEDOT polymer electrode is extremely well suited for applications that, up to now, have been challenging, such as non-invasive EEG recordings from awake and freely moving animals, EEG recordings from preterm babies in the neonatal intensive care unit or long-term recordings in the case of sleep monitoring or epilepsy diagnostics. We addressed two technical questions in this work. First, is the EEG recorded with polymer electrodes comparable to a standard EEG? Second, is it possible to record full-band EEGs with our electrodes?
PE-HD as a polymeric fuel storage tank material: Photooxidation, fuel sorption and long-term storage
(2018)
High-density polyethylene (PE-HD) is a commodity thermoplastic polymer which is typically used for packing of dangerous goods. Its good resistance against photooxidation, fuels, chemicals and other environmental factors in addition to low production costs makes PE-HD attractive for fuel storage applications. Typical engine fuels stored in polymer tanks are petrol, diesel and biodiesel that receives increasing attention as proper alternative to fossil fuels. One of the major problems with biodiesel is its susceptibility to oxidize due to its chemical composition of unsaturated fatty acids which also can cause polymer degradation.
The aim of this study is to investigate the influence of different environmental factors, UV radiation and commonly stored fuels, on the mechanical, physical and chemical properties of two types of PE-HD polymers (stabilized and non-stabilized). The influence on the mechanical properties was tested by Charpy and tensile tests, chemical and physical properties were evaluated by Fourier-transform infrared spectroscopy (FTIR) and by dynamical mechanical analysis (DMA) tests. Samples were characterized after varying exposure time of UV radiation and after fully and partially immersion in biodiesel. In addition, similar experiments were conducted using diesel for comparison.
The first documentation of fuel biodeterioration dates back to the late 19th century. However, extensive studies concerning the microbial fuel contamination started in 1980’s. Polymeric fuel storage tanks containing diesel and biodiesel provide environmental conditions for microbial growth. Several studies demonstrated that bacteria, which were found in contaminated fuel systems, can use fuels as macronutrient; but such bacteria can also cause microbiologically influenced corrosion and fouling.
The aim of this study is to investigate the initial attachment behavior of bacteria, isolated from a diesel contamination, on neat and photooxidized high-density polyethylene (PE-HD). Two common PE-HD’s, less- and biodiesel-stabilized, were radiated to UV light representing a tank exposed to sunlight. The effect of photooxidiation on PE-HD’s surface were characterized chemically by Fourier-transform infrared spectroscopy (FTIR). The attached bacteria Pseudomonas aeruginosa and Bacillus subtilis on the polymer surface were evaluated by fluorescence microscopy and colony-forming unit tests (CFU).