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- Damage evolution (2)
- Druckbehälter (2)
- Faserverbundwerkstoffe (2)
- Fatigue (2)
- Non-destructive testing (NDT) (2)
- Rotorblatt (2)
- Röntgenrefraktionsmessung (2)
- X-ray refraction (2)
- Aircraft (1)
- Automotive (1)
Organisationseinheit der BAM
Moderne Leichtbaustrukturen (Luftfahrt, Eisenbahn, Automotive) aus Faserverbundwerkstoffen (FVW) sind hohen mechanischen und klimatischen Beanspruchungen ausgesetzt. Aussagen über die Lebensdauer und Funktionssicherheit der FVW-Bauteile können eher aus Untersuchungen der mikromechanischen Eigenschaften gewonnen werden als aus der globalen Festigkeit. Die Röntgenrefraktionstopografie ist ein Verfahren zur zerstörungsfreien Bestimmung von inneren Oberflächen in Werkstoffen. Angewendet auf FVW lassen sich Aussagen über die Faserorientierungen, die Fertigungsqualität sowie über Faser-Matrix-Ablösungen und Matrixrisse infolge mechanischer Beanspruchung gewinnen. Nachfolgend wird ein neues Verfahren zur Bestimmung des Zwischenfaserbruchversagens am komplexen Laminat vorgestellt. Hierbei wird das Röntgenrefraktionsverfahren "online bei Zug-Belastung der Probe angewendet.
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.
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.
Short glass fiber reinforced polyamides are increasingly used in automotive applications. Concepts and models are needed, which enable the prediction of the structural durability. The fiber orientation and the content of moisture have a major influence on the fracture mechanical properties and on the damage mechanisms. The complex crack growth behavior was investigated and described in an empirical model.
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.
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 Xray-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.
CFRP are more and more used in modern civil aircrafts hence the whole fuselage is made of this material (B787; A350). Due to strict certification Standards finally 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. The first inter-fiber failure was evaluated non-destructively by the fatigue tests accompanying X-ray-refraction topography altemately and in-situ mechanical loading. The basic idea of the investigation is represented in fig. 1. At low load levels it could not be assumed the total failure of the samples. Hence accompanying non-destructive testing (NDT) is mandatory to get information about the damage state of the material.
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.
In a research project fatigue tests on two identical wing sections that are used as representative substitute components were performed to estimate lifetime enhancement of light-weight aircraft such as general aviation gliders. Single step fatigue tests at limit load on one component were compared with spectrum loading on the other. A characteristic damage behaviour was observed. Local buckling due to high shear loading of the sandwich core of the wing shell causes the main damage. Even though an intentionally high load level was chosen no increase of micro-cracking could be detected. Additionally no delamination effects in laminates or bondings were observed except the skin-core delamination which causes the buckling effect in the wing shell due to fatigue loading. Finally the concept of using representative substructures and accompanying specimen tests is an effective approach for in-service loading investigations and could also be applied to research on wind turbine blades.