Prediction for stiffness reduction and progressive damage of composite laminate including ply cracks

Fiber-reinforced laminates have a specific fracture mode such as ply cracking (cracking in a direction parallel to the fiber), delamination, and breakage of the fiber. Of these fracture modes, ply cracking occurs in the earliest stage, and it can result in significant degradation in material stiffness. Furthermore, the initiation of ply cracking causes stress redistribution, resulting in more severe forms of damage such as delamination and breakage of the fiber due to the stress concentration at the crack tip. Therefore, it is important to clarify the mechanical behavior of laminates including ply cracking. This study focuses on the stiffness reduction of laminate due to ply cracking and the ply cracking progression in a ply. First, the continuum damage mechanics (CDM) based model for predicting the stiffness reduction of composite laminates is established. The effective compliance of laminate based on CDM and the classical laminate plate theory (CLPT) was used to formulate the thermo-elastic properties of laminate of arbitrary lay-up configurations as a function of ply crack density. The damage variable d₂ is formulated analytically by three-dimensional local stress field (3-D LSF) model in a ply including ply cracking subjected to tensile loading. Next, ply cracking progression in laminates including 90° plies was investigated. The energy release rate associated with ply cracking is calculated using the 3-D LSF model, and then ply cracking progression in 90° plies of CFRP cross-ply laminates based on both strength and energy criteria is predicted using the Monte Carlo method. The models proposed in this study can predict the experiment results and finite element analysis results.