A three-dimensional Finite Fracture Mechanics model for predicting free edge delamination

The free edges in composite laminates can lead to interlaminar stresses that are locally significant and theoretically infinite due to material mismatch at the interface. These stresses can result in interlaminar crack onset between the differently oriented plies and cannot be accurately predicted using traditional strength-of-materials or fracture mechanics approaches. To address this issue, this study employs finite fracture mechanics (FFM) [1] to investigate delamination onset in composite laminates subjected to remote extension. Dimensional analysis is first performed to determine the relevant parameters for interlaminar stresses and energy release rate (ERR). A finite element model is used to obtain the necessary quantities, as there is no exact solution available for the free-edge effect. A new stress criterion is proposed that incorporates the normal and two-shear interlaminar stresses at the interface, considering that delamination is caused by interlaminar stresses. Additionally, a mixed-mode energy- based criterion is also proposed. The unknown variables (failure load and crack area) are determined by solving an optimization problem to find the minimum load corresponding to a specific crack and satisfying both the stress and energy criterion. Finally, the results obtained by FFM are compared to experimental results found in [2] to validate the approach. This approach can accurately predict delamination onset provided the interlaminar strengths and the interfacial fracture toughness are available.