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Numerical and Experimental Investigations on the Micro Damage Accumulation and Its Effects on the Longitudinal Compressive Strength of Carbon Fibre Reinforced Plastics (CFRP) Under Fatigue Loading



The prediction of the fatigue life of components made of fibre reinforced plastics (FRP) under arbitrary cyclical load is a challenge, which is still not met. For a reliable design of such components, an elaborate understanding of the damage process in the material and its effects on the load carrying capacity of the structure is inevitable. Depending on their loading conditions, laminates made of FRP fail by one of the macroscopic failure modes named fibre fracture, inter-fibre fracture or delamination. It is commonly assumed that prior to these macroscopic failure phenomena micro cracks occur and accumulate under cyclic load. Micro damage here denotes as matrix cracks as well as fibre/matrix debonding with crack lengths in the order of magnitude of the fibre diameter. Micro damage results in the degradation of transverse stiffness and strength properties but also affects the longitudinal strength properties of FRP. In this paper, the micro damage process due to fibre transverse preloading and its effects on the fibre longitudinal compressive strength of carbon fibre reinforced plastic will be discussed. For the investigations a combined approach of experimental investigations , d e p o l e v e d n e e b s a h l e d o m l a c i r e m u n A . d e u s r u p n e e b s a h s n o i t a l u m i s l a c i r e m u n d n aw hich allows for the investigation of the effect of fibre/matrix debonding on the longitudinal compressive strength. A 3D unit-cell model has been set up to investigate the effect of location and extent of fibre/matrix debonding on the longitudinal compressive strength. In the experimental part, a specimen concept has been developed. The specimens consist of one testing layer, on which the desired effects have been investigated and a supporting layer to create the desired stress path. The testing layers have been preloaded at different loading paths in the σ2/τ21 stress plane for a predefined amount of cycles at constant stress amplitudes and mean stresses. After this preloading, the fibre longitudinal compressive strength has been evaluated and is compared to the strength without preloading. The comparison between the numerical and experimental investigations show that the damage process of transversely loaded layers of FRP is highly dependent on the loading path and that the micro damage process strongly depends upon the characteristics of the matrix material.

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