Effect of residual stress on interlaminar fracture toughness of CFRP laminates

In the present study, an effect of residual stress in CFRP laminates for the interlaminar fracture toughness was investigated in detail. The boundary element method based on the Laplace transform were employed to calculate the residual stress and interlaminar fracture toughness of the CFRP laminates. The relaxation moduli of the CFRP laminates were estimated by unidirectional compression creep test experimentally. Using CFRP specimens of orthogonal stacking sequence [0 degrees/90 degrees/0 degrees], double cantilever beam (DCB) test for opening mode crack and end notch flexure (ENF) test for shear mode crack were introduced to obtain the actual fracture toughness of CFRP laminates, where the effect of the residual stress in the cooling sequence of the making process was taken into account. From the experimental results and numerical demonstrations by BEM, it was shown that there is a possibility of underestimation of the interlaminar fracture toughness if it is calculated by elastostatic analysis neglecting the residual stress. On the other hand, there is a possibility of overestimation of the toughness when thermoelastic analysis is applied without thermo-viscoelastic properties.