Delamination damage of carbon fiber-reinforced polymer composite laminates under cyclic shear-induced loading conditions

Interface delamination is a major failure mode induced by in-plane shear stress frequently encountered in carbon fiber-reinforce polymer (CFRP) composite laminates structures. This failure process under monotonic loading has been successfully described using cohesive zone model (CZM). Many previous CZM approaches for cyclic case were considering damage parameters based on a crack growth relation which has some disadvantages for predicting a non-linear crack growth. In addition, the previous CZM approach lacks ground in understanding the physics underlying the delamination process and effect of stress ratio. The objective of the study is to extend the existing CZM to account for the delamination damage evolution of CFRP composite laminates under cyclic shear-induced loading conditions named as cyclic cohesive zone model (CCZM). In this respect, the fatigue damage response and the residual interfacial properties associated with the development of the CCZM are established under cyclic shear-induced loading condition. A series of Mode-II-type tests were performed on prefatigued end-notched flexural (ENF) beams of CFRP composite laminates, [0]8 for different applied load ratio conditions (R = 0.1, 0.15 and 0.25) to induce only interlaminar damage at the pre-existing delamination interface crack front. Subsequent quasi-static test to catastrophic failure establishes the characteristic residual strength responses of the damaged specimen. A hybrid experimental-computational approach was introduced to obtain the residual interlaminar properties for all the loading cases. A normalized gradual degradation rule was used to present the degradation for interlaminar shear strength ( ), penalty stiffness ( ) and the critical Mode-II energy release rate ( ) which cover the wide range of interlaminar failure mode from wear PTTA out to sudden death. This interlaminar properties degradation model can describe the characteristic evolution of the interlaminar damage response and the degradation of CCZM properties under cyclic shear-induced loading case. The interlaminar properties degradation model together with the CCZM model is coded by using user-define material model (UMAT) subroutine to implement in ABAQUS finite element analysis (FEA) software. This model had been used to simulate under 3-point bending cases and compared with the experiment results. Result had shown that the comparison between the FE simulation and the experiment fatigue load-life cycles for CFRP composite laminate interfaces are close with the difference of less than 1% and shows a very successful verification of the modified CCZM model to simulate the interlaminar damage evolution and failure response. Besides that, an independent validation had been run to validate the performance of interlaminar properties which were obtained in the study. A load-deflection response under 3-point bending case was simulated based on [0]16 ENF specimen under identical load cycle parameters and compared with the measured experiment results. Result shows that the peak load differences between the experiment and simulation is less than 6%. From the study, the capability of CCZM model for cyclic case has been demonstrated by linking interlaminar properties degradation with damage mechanics approach. This will help in understanding the physics underlying the delamination process and effect of stress ratio