A tunnelling crack density evolution model for FRP laminates subjected to cyclic multi-axial strain-controlled loading

A multi-scale stochastic crack density evolution model for tunnelling cracks under multi-directional cyclic loading is presented. The damage model proposed utilizes a multi-scale stress-based criterion for crack initiation and a triple unit cell approach with inputs from the GLOB-LOC model for crack-front growth rate. Biaxial cruciform specimens subjected to strain-controlled cyclic loading are used to calibrate the crack initiation SN curve and the Paris-Erdogan type of law required for crack growth. Additionally, uniaxial force-controlled cyclic tests are used to calibrate the stochastic parameters associated with crack initiation and crack growth. The performance of the damage model in predicting the crack-front growth compared well with experimental data and 3D finite element analyses. The damage model includes a crack element discretisation scheme which allows for multiple collinear cracks to initiate and coalesce. The model also accounts for the growth of damage outside the window of a primary representative volume element. The crack density predictions of the model are compared with measurements from cruciform specimens. It is shown that the damage model captures the trend of the crack density evolution well and provides a conservative prediction of the crack saturation level.