Local Delaminations Induced by Interaction Between Intralaminar Cracking and Specimen Edge in Quasi-Isotropic CF/EP NCF Composites in Fatigue Loadings

Abstract

Experimental results are presented on the onset and propagation of local delaminations caused by the interaction between specimen edges and intralaminar cracks in fiber bundles of 90° layers in quasi-isotropic [–45/90/45/0]s CF/EP noncrimp fabric (NCF) laminates subjected to tension-tension fatigue loadings. It is confirmed that the first damage mode is intralaminar cracking in 90° layers, which consists of intrabundle cracks and cracks in the matrix between bundles (often beginning from stitches). This damage mode triggers cracking in off-axis layers and local delaminations in positions where the 90° layer crack meets an adjacent layer. The process of local delamination is significantly enhanced at specimen edges, where the out-of-plane edge stresses contribute to the local delamination. During cyclic loadings, delaminations grow and coalesce along the edge and propagate towards the specimen center. These processes are quantified experimentally at different levels of cyclic load. In a low-stress fatigue, a very high number of cycles is required to detect small edge delaminations, and they stay at the edge. In high-stress cyclic tests, delaminations grow faster inside the composite: about 20% of the interface in the central zone can be delaminated. It is found that the reduction in the axial modulus is proportional to the relative delaminated area, proving that delamination is the major stiffness reduction factor in these laminates.