Influence of Skin-core Effects on Residual Stress/Strain Distribution in Thick CFRTP Laminates

Abstract

Carbon fiber reinforced thermo-plastic (CFRTP) has several advantages such as fast-cycle molding and high toughness. Hence CFRTP is recently expected to be applied to primary structures of aircrafts. When we manufacture a thick CFRTP product, it is difficult to suppress temperature distribution in the through-thickness direction under practical high-cycle processing condition. Since the thermoplastic resin does not show a curing reaction, the start time of solidification differs between surface and central parts of the laminate. This difference leads to the residual stress/strain and material property distribution. This phenomenon is called “skin-core effectsâ€. However, the mechanism of skin-core effects has not been investigated experimentally. Therefore, this study evaluates the residual stress/strain distribution due to the skin-core effects in thick CFRTP laminates. We evaluated residual strain distribution in CF/PPS thick uni-directional (UD) and cross-ply laminates by embedded fiber Bragg grating (FBG) sensors. They were placed near the surface and central of the laminate. In the test, specimens were cooled by three methods which resulted in different temperature and cooling speed distributions. Residual strain depending on the cooling method was successfully characterized. This trend was especially-pronounced in UD laminates. It could be estimated that in the rapidly cooled specimen tensile (compressive) stress existed in the central (surface) part after molding. Furthermore, the residual strain of UD specimens changed after the subsequent annealing when the crystallinity did not reach the maximal level. Finally, the effects of residual stress distribution on the mechanical property were evaluated by tensile tests of cross-ply thick laminates. The density of the transverse cracks in the 90-degree layers was non-uniform in the specimen affected by the skin-core effects. It was confirmed that premature cracks occurred due to the residual tensile stress at the central part.