Through-thickness fatigue behavior of non-crimp fabrics featuring manufacturing defects

Abstract

The use of cost efficient CFRP materials is growing especially in the automotive applications. Here, a challenge is delamination occurring during service. Out-of-plane loading should be avoided in composite structures, since it is the weakest loading direction. However, this is not always possible in more complex geometries or at joints. Hence, there is a demand for through-thickness fatigue properties, for design and analysis of CFRP structures.Fatigue damage is often initiated by manufacturing imperfections or defects, such as voids, in-plane or out-of-plane ply waviness or folds of plies. In this research work, the influence of a fold defect on the through-thickness mode I fatigue fracture behavior of an automotive non-crimp fabric laminate is assessed, through the means of both experimental testing and numerical simulation. The selected test method is the direct through-thickness tensile load application. This method allows microscopic analysis of the fracture surfaces of the delaminated specimens, which contributes to a better understanding of the influence the manufacturing defect has on the damage behavior and material performances.Fractography shows, that interlaminar fracture is predominant due to stitching yarn concentration and resin rich areas. A knockdown of 10% due to the presence of the fold defect is observed and can be further fed back into the fatigue material properties for simulation and analysis of structural composite parts. In this way, the presence of defects can be quantified and accounted for during the design stage.