A model for the mixed mode crack propagation in composite laminates under cyclic loadings

Composite laminates under fatigue loadings undergo a stiffness degradation due to the initiation and propagation of off-axis cracks. Therefore, predicting the initiation and propagation of such cracks is a matter of extreme importance in the design of composite structures. Recently, Carraro and Quaresimin proposed a criterion for ply crack initiation. In this work, a model for the off-axis crack propagation in laminated composites subjected to multiaxial fatigue loadings is presented. On the basis of several observations reported in the literature, the crack propagation phenomenon can be seen as the result of a series of micro-scale events occurring ahead of the crack tip within a process zone. It is also clear that a change in the damage mode occurs when moving from mode I to mode II conditions. Based on these evidences and by means of a multiscale approach to determine the micro-scale stress fields in the matrix, two simple parameters are defined to be used for predicting the crack growth rate through a Paris-like law. The application to several experimental data from the literature shows the capability of the proposed parameters to summarise all the crack propagation data into two scatter bands covering the whole mode mixity range.