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Einleitung
Bei der Detektion von Mikrorissen und der Charakterisierung spezifischer Schädigungen von Polymer- und Kompositwerkstoffen kommt es neben quantitativen Bewertungen auch auf die Analyse von orientierungsabhängigen Struktureffekten an. Am Beispiel typischer Vertreter ausgewählter Materialien (PP/PE, CFK) wird dargestellt, inwieweit das Potential der Röntgen-Refraktions-Topographie vorteilhaft zur umfassenden Materialcharakterisierung genutzt werden kann. Die korrosiven Eigenschaften nichtmetallischer Werkstoffe werden maßgeblich vom mikrostrukturellen Aufbau der verwendeten Materialien bestimmt. Typische Versagens-mechanismen von Faser-Kompositen sind neben der Faserenthaftung vor allem durch Mikrorisse gekennzeichnet, die sich teils parallel, teils senkrecht zu den Fasern in der Matrix ausbreiten. Eine wichtige Anwendung der Röntgen-Refraktions-Topographie umfaßt die orientierungsselektive Detektion von Mikroriß-Oberflächen. Die Nachweisgrenze derartiger Schädigungen erstreckt sich bis zum Nanometerbereich. Definiert herbeigeführte Ermüdungsschäden an Kompositmaterialien lassen sich mit dieser Methode ausgezeichnet erfassen und gestatten eine quantitative Korrelation zu mechanischen Kenngrößen. Mittels schrittweiser Probenabtastung und der ortskorrelierten Speicherung von simultan erfassten Refraktions- bzw. Absorptionszählraten wird ein zweidimensionales Röntgen-Refraktions-Topogramm rekonstruiert, das u.a. die präzise Information der integralen Rissdichte enthält.
Damage State of Characterized Non-Destructively by X-Ray-Refraction-Topography and Ultrasound
(2003)
Damage state of CFRP characterized non-destructively by X-ray-refraction-topography and ultrasound
(2003)
Non-invasive characterisation of intra laminar fatique of carbon fibre composites Aero NDT Forum
(2004)
Structural health monitoring of wind turbine blades by strain measurement and vibration analysis
(2011)
Wind turbine blades have to withstand a high number of load cycles in mostly hard weather conditions over 20 years. In a research project BAM and several partners have designed, evaluated and tested a condition monitoring system for all parts of a wind turbine. At a rotorblade with a length of 58 m fibre bragg grating sensors were applied for in-service strain measurement. Additionally a complex test rig was designed to enable real biaxial loading conditions at a representative test rotor blade to simulate the mechanical loading conditions in the lab. In this test blade with a length of 8 m also fibre bragg grating sensors were implemented to determine their influences for the structure and the condition monitoring system. Vibration measurements were carried out at different test phases. The results were compared with finite element strain and modal analysis.
Structural health monitoring for wind turbine rotor blades by strain measurement and vibration
(2011)
Hinsichtlich Materialermüdung und Betriebsfestigkeit in Faserkunststoffverbunden besteht - verglichen mit anderen Aspekten des Faserverbundleichtbaues – noch erheblicher Forschungsbedarf. Schon weit vor den globalen Festigkeiten des Laminates treten eine Vielzahl von Zwischenfaserbrüchen und vereinzelte Faserbrüche auf. Die Dichte dieser Mikrorisse soll hier als Indikator für den Zustand des Verbundes genutzt werden. Ihre Ermittlung stellt insbesondere bei Kohlenstofffasern eine Herausforderung dar. Über vergleichende Messungen mit der Schallemissionsanalyse (SEA) und der Röntgenrefraktionsscans (RR) wird die Entwicklung der Rissdichte bei Belastung untersucht. Die SEA zeichnet die Entstehung der Risse auf und kann wegen der kompakten Messtechnik auch gut am Bauteil eingesetzt werden. Die RR beobachtet die Änderung der inneren Grenzflächen, die ebenfalls die Entwicklung der Rissdichte widerspiegelt. Sie ist allerdings zurzeit nur als Laborverfahren geeignet. Der Vortrag berichtet(e) über den aktuellen Stand des vergleichenden Einsatzes beider Verfahren an Flachproben und innendruckbelasteten Rohrproben.
Hinsichtlich Materialermüdung in Faserkunststoffverbunden besteht verglichen mit anderen Aspekten des Faserverbundleichtbaues – noch erheblicher Forschungsbedarf. Schon weit vor den globalen Festigkeiten des Laminates treten eine Vielzahl von Zwischenfaserbrüchen und vereinzelte Faserbrüche auf. Die Dichte dieser Mikrorisse soll als Indikator für den Zustand des Verbundes genutzt werden. Ihre Ermittlung stellt insbesondere bei Kohlenstofffasern eine Herausforderung dar. Über vergleichende Messungen mit der Schallemissionsanalyse (SEA) und der Röntgenrefraktionsanalyse (RR) wird die Entwicklung der Rissdichte bei Belastung untersucht. die SEA zeichnet die Entstehung der Risse auf und kann wegen der kompakten Messtechnik auch gut am Bauteil eingesetzt werden Die RR beobachtet die Änderung der inneren Grenzflächen, die ebenfalls eine Entwicklung der Rissdichte widerspiegelt. Sie ist allerdings zurzeit nur als Laborverfahren geeignet. Der Vortrag berichtet über den aktuellen Stand des vergleichenden Einsatzes beider Verfahren an Zugproben und Modellbehältern.
Die Restlebensdauerabschätzung von Faserkunststoffverbunden ist ein ungelöstes
Problem. Die Beobachtung der Matrixschädigung ist dabei ein vielversprechender Ansatz zur Bewertung des Schädigungszustandes insgesamt. Unter kombinierter Beobachtung mit Röntgenrefraktionsmessungen und Schallemissionsmessung wurden uni- und biaxiale Lasten statisch und zyklisch auf Kohlenstofffaserepoxidharz-Proben aufgebracht. Die uniaxiale Belastung erfolgt mittels einer eigenentwickelten Kompaktprüfmaschine direkt während der Röntgenrefraktions- und Schallemissionsmessung in der Röntgenkammer. Die biaxiale Belastung wird an Modelldruckbehältern ebenfalls mit Schallemissionsmessung durchgeführt. Hier wird der Modelldruckbehälter mit dem Röntgenrefraktionsverfahren periodisch inspiziert.
Festigkeits- und Ermüdungsverhalten von Faserverbundwerkstoffen, betrachtet im µm- und µs-Maßstab
(2006)
Möglichkeiten und Grenzen der Röntgen-Computertomographie zur Schadensdetektion bei Segelflugzeugen
(2008)
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.
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.
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.
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.
Following a modular concept an integrated monitoring system has been developed that includes all components of offshore wind turbines (OWEC). Using commercially available measurement equipments of the involved partners this system was tested at the wind turbine Multibrid M5000_2 in Bremerhaven. Additional results of the research project are findings on the use of embedded FBG sensors in rotor blades that allow condition assessments based on local information. Newly developed algorithms for the identification of external loads and damage characteristics of the structural components have been tested in operation. Developed methods for monitoring based structural assessment were used, which will be suitable to optimize structural maintenance works.
Fibre Reinforced Plastics (e.g. CFRP, GFRP) characteristically show nonlinear stress-strain behaviour due to intralaminar shear loading. The determination of the in-plane shear stiffness and strength for this class of material is difficult and common test standards and methods of analysis are partially inaccurate. The identification of the in-plane shear properties was made with an especial designed “picture frame test device” which enables shear loading up to 950N/mm. Therefore the strength limit can be reached at specimens with adequate thickness and a high safety factor against buckling. Due to the chosen design the maximum of the shear loading and the final failure occur in the centre of the specimen. The experiments match with the numerical analysis.
Mit Hilfe der an der BAM entwickelten Röntgenrefraktionstopographie lassen sich zerstörungsfrei innere Oberflächen insbesondere in CFK messen. Somit gelingt es quantitativ und ortsaufgelöst Mikrorisse im μm-Maßstab (Faser-Matrix-Ablösungen, Matrix-Matrix-Brüche) auch im komplexen Laminat zu bestimmen. Ziel ist es, die Mikrorissbildung an Geweben und Gelegen in-situ zur Schwingbeanspruchung zu ermitteln, um Materialkonstanten und Modelle zu bestimmen, mit Hilfe derer die Zwischenfaserbruchanstrengungen angeben werden kann, bei der die Grenze zum „unendlichen“ Leben liegt, also der Werkstoff als „dauerfest“ bezeichnet werden kann. Die Schädigungsmechanismen konnten aber auch an Schalenstrukturen aus GFK mit optischen Verfahren und mit In-Situ-Messungen mittels Thermographie nachgewiesen werden. Zudem spielt die Faser-Matrix-Grenzfläche, also die Oberflächenchemie auf der Faser sowie der Vernetzungsgrad der Epoxidharzmatrix eine entscheidende Rolle.
Carbon Fibre Reinforced Plastics (CFRP) are more and more used in modern civil aircrafts. These days the whole fuselage is made of this material (B787; A350). Due to strict certification standards the normal in-service loading gives a low stress level compared to the static and even the fatigue strength of the material. Hence CFRP are assumed to have an infinite life. To evaluate this assumption, fatigue tests on CFRP-specimens were performed up to 108 load cycles and the first inter-fibre failure was evaluated non-destructively by accompanying X-ray-refraction topography.
A tensile testing machine was integrated in a small angle X-ray scattering (SAXS) setup. X-ray refraction topography was performed while the CFRP-samples were tensile loaded. This non-destructive technique enables the detection of micro-cracking and inter-fibre failure especially for CFRP. For Glass Fibre Reinforced Plastic (GFRP) X-ray refraction and in-situ loading has already been successfully used. The increase of inner surfaces due to inter fibre failure was measured as a function of the stress state. Fatigue tests were performed at and below the limit of inter-fibre failure strength.
State of the art is to assume the failure of the samples under cyclic loading as the fatigue life. Accompanying non-destructive X-ray refraction measurements reflects the damage state and enables to trace its evolution even if the total failure of the specimens does not occur. This investigation technique is of high interest to give the engineer a design value of infinite life which is practically often reached due to knock down factors of certification standards. Finally the infinite life was found for cyclic fatigue loaded CFRP-samples even under high inter fibre transverse and shear loading investigated up to 108 load cycles.
Fibre Reinforced Plastics (e.g. CFRP, GFRP) characteristically show nonlinear stress-strain behaviour due to intralaminar shear loading. The determination of the in-plane shear stiffness and strength for this class of material is difficult and common test standards and methods of analysis are partially inaccurate. The identification of the in-plane shear properties was made with an especial designed “picture frame test device” which enables shear loading up to 950N/mm. Therefore the strength limit can be reached at specimens with adequate thickness and a high safety factor against buckling. Due to the chosen design the maximum of the shear loading and the final failure occur in the centre of the specimen. The experiments match with the numerical analysis.
Für die Schubkennwertermittlung an faserverstärkten Kunststoffen existieren verschiedene Verfahren, die nur eine Prüfung bis zu einer vorgegebenen maximalen Schubdehnung zulassen. Die Grasse Zur Ingenieurgesellschaft hat auf Basis eines an der BAM entwickelten Schubrahmens ein Schubprüfsystem etabliert, das diese Nachteile aufhebt. Damit ist es nun erstmals möglich, Schubkennwerte ohne Einschränkung einer maximalen Schubdehnung effizient und mit sehr guter Reproduzierbarkeit zu ermitteln.
The following paper focuses on the evolution of micro damage in short fibre reinforced polyamide. Therefore, tube samples are subjected to uni- and biaxial fatigue loadings. The evolution of micro damage is analysed by the non-destructive method of X-ray refraction analysis with consideration of the fibre orientation distribution. For validation of the applied micro damage models, fractographic analyses are performed. Concluding some general results, it has been observed that the load ratio influences the quantitative dominance of micro damage, whereas occurring damage phenomena depend on the type of loading (i.e. tension and torsion). Thus, zones in the Haigh-diagram are detected, where the occurrence of damage mechanisms qualitatively and quantitatively changes. This is a basis for further research regarding anisotropic damage criteria.
The damage process of short glass fibre (30% weight) reinforced polyamide caused by mechanical loading was investigated from the beginning on micro cracking level to the incipient crack of mm-dimension. Based on high resolution computer tomography and the X-ray-refraction technique the inner surface due to micro-cracking at the short fibre ends and the fibre matrix debonding of the skin surface of the filament was determined quantitatively. With the knowledge of the fatigue crack propagation rate and fracture toughness of the material from former research projects, it was derived that the total inner surface due to micro cracks measured by X-ray refraction is much higher than the specimen could have withstand the load, supposed the surface is in a localized crack. Hence, the damage process could be described from micro to macro level. Accompanying fractographic investigations endorse the modelling based on the NDT-techniques.
The damage process of short glass fibre (30% weight) reinforced polyamide caused by mechanical loading was investigated from the beginning on micro cracking level to the incipient crack of mm-dimension. Based on high resolution computer tomography and the X-ray-refraction technique the inner surface due to micro-cracking at the short fibre ends and the fibre matrix debonding of the skin surface of the filament was determined quantitatively. With the knowledge of the fatigue crack propagation rate and fracture toughness of the material from former research projects, it was derived that the total inner surface due to micro cracks measured by X-ray refraction is much higher than the specimen could have withstand the load, supposed the surface is in a localized crack. Hence, the damage process could be described from micro to macro level. Accompanying fractographic investigations endorse the modelling based on the NDT-techniques.
Fatigue life evaluation of spar cap materials by four point bending test on width-tapered specimens
(2015)
The presented research project focuses on an effective method to evaluate the fatigue strength of thick unidirectional laminates to be applied in spar caps. Therefore, a width-tapered bending specimen was developed for a four point loading set up. Static and fatigue loading was performed with the load ratio of R=-1. The complex stress state was investigated numerically with a finite element analysis. Finally, the concept was proven experimentally on specimens made of pultruded fibre rods. The fatigue behaviour of continuous tapered width by water jet cutting was compared to discrete tapering by rod-drop.
Fatigue life evaluation of spar cap materials by four point bending test on width-tapered specimensa
(2015)
The presented research project focuses on an effective method to evaluate the fatigue strength of thick unidirectional laminates to be applied in spar caps. Therefore, a width-tapered bending specimen was developed for a four point loading set up. Static and fatigue loading was performed with the load ratio of R=-1. The complex stress state was investigated numerically with a finite element analysis. Finally, the concept was proven experimentally on specimens made of pultruded fibre rods. The fatigue behaviour of continuous tapered width by water jet cutting was compared to discrete tapering by rod-drop.
The most substantial innovations in radiographic imaging techniques of the last two decades aim at enhanced image contrast of weakly absorbing micro and nano structures by taking advantage of X-ray refraction effects occurring at outer and inner surfaces. The applications range from fibre reinforced plastics to biological tissues. These techniques comprise, among others, X-ray refraction topography, diffraction enhanced imaging, phase contrast imaging, Talbot-Lau grating interferometry, and refraction enhanced imaging. They all make use of selective beam deflections up to a few minutes of arc: the X-ray refraction effect. In contrast to diffraction, this type of interaction has a 100 % scattering cross section, as shown experimentally. Since X-ray refraction is very sensitive to the orientation of interfaces, it is additionally a tool to detect, e.g., fibre or pore orientation. If the detector resolution exceeds the size of (small) individual features, one detects the integral information (of inner surfaces) within the gauge volume. We describe the above-mentioned techniques, and show their experimental implementation in the lab and at a synchrotron source. We also show strategies for data processing and quantitative analysis.