5 Werkstofftechnik
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- 5 Werkstofftechnik (42)
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- 4 Material und Umwelt (2)
- 4.4 Thermochemische Reststoffbehandlung und Wertstoffrückgewinnung (2)
- 7 Bauwerkssicherheit (2)
- 8.6 Faseroptische Sensorik (2)
Paper des Monats
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Advanced light weight applications like aircrafts and wind turbine blades are made of fibre reinforced plastics (FRP) with continuous fibre reinforcement and must withstand a high thermo-mechanical cyclic loading. The quality of the fibre matrix interface has a high impact on the fatigue life and was continuously improved over the years since the 50th. The fatigue life of glass fibre reinforced plastics (GFRP) used in aircraft industry is 10 to 100 times higher compared to glass fibre non crimp fabrics used for wind turbine blades.
To assure a constant and reliable high quality and strength of reinforcement fibres, synthetic fibre production is state of the art (CF, GF). There is a need for recycling GFRP and CFRP waste due to the upcoming use. Pyrolysis and solvolysis are more expensive than the mechanical route however enable a more sustainable recycling. Natural fibres and recycled synthetic fibres have a high scatter in quality and strength. Hence it is a challenge to optimize the production / recycling processes to get a reliable quality for any demanding (second life) application.
Chemical routes for using renewables resources and recycling, is going to be a good approach especially for polymer-matrix systems to get 100% quality (back) compared to the state of the art.
Finally, a proper design, life-time extension and repair is preferable to recycling to keep the carbon footprint as low as possible.
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.
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.
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.
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.
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.
Glass-fibre-reinforced plastics (GFRP) are commonly used for the construction of pressure vessels and tanks for example in automotive and aerospace industries. Especially for pressure vessels used for the storage of natural gas or hydrogen with operation pressures up to 700 bar, an early material fatigue detection is of great interest. Measuring the distributed strain profile of GFRP structures helps to understand and detect material fatigue. In this article, we demonstrate the great potential of swept wavelength interferometry (SWI) based distributed strain sensing for the monitoring of pressure vessels made from GFRP. A resin transfer molding (RTM) process was used to manufacture GFRP-tube specimens. Commercial optical fibres with polyimid coating were glued to the surface externally in circumferential and axial direction. A cyclic load of up to 150 bar was applied to the samples using a servo-hydraulic test bench. Comparing the loaded and unloaded test conditions, we determined up to 2 % elongation in circumferential direction. We demonstrate reliable distributed strain measurements with sub-centimetre spatial resolution. By monitoring these high-resolution strain profiles, we were able to detect local material degradation that manifested itself as localized strain changes. Crucially, the material Degradation could be detected already after 75 % of dat the fatigue life before a crack appeared that led to leakage.
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.
Cyclic fatigue behavior of glass fiber reinforced epoxy resin at ambient and elevated temperatures
(2018)
The fatigue behavior of ±45° glass fiber reinforced epoxy resin under cyclic mechanical and constant thermal loading is investigated in this study. Tests at three different temperature levels in the range 296 K to 343 K have been performed in order to create S-N curves for each temperature level. The specimen damage is measured in-situ using optical grayscale analysis. The characteristic damage state (CDS) is evaluated for each specimen. It is shown that the point of CDS is suitable as a failure criterion to compare the resulting S-N curves. With micromechanical formulations, the temperature-dependent matrix effort is calculated for each stress-temperature level. In terms of matrix effort, the longest fatigue life is reached at high temperatures, while, in terms of stress, the lowest fatigue life is reached at the highest temperatures.
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.
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.
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.
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.
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.
Self-healing polymers have been developed to improve durability and reduce costs associated with maintenance during service. The addition of thermoplastics to thermosets to produce mendable polymers appears as a promising selfhealing technique. In this study, poly (ethylene-co-methacrylic acid) (EMAA) was added to epoxy resin and the effects of EMAA addition on epoxy properties were evaluated. Specimens with two different contents of thermoplastic and particles sizes were manufactured. A two-level full factorial experimental design was used to evaluate the effect of particle size and particle content on properties of epoxy modified with addition of EMAA. Tensile tests and dynamic mechanical analysis (DMA) were used and the evaluated responses were tensile strength, modulus of elasticity, and glass transition temperature (Tg). X-ray computed tomography (XCT) was used to investigate particle size and concentration after manufacturing. It was found that the particle concentration has greater effects on stress–strain behavior of epoxy while Tg was not significantly affected by neither of the analyzed entrance variables.
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.
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.
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.
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.