Analysis of local stress ratio for delamination in composites under fatigue loads

An approach based on the cohesive zone model for analyzing delamination in composite laminates under cyclic fatigue loading is presented. The proposed technique, called “min-max load approach,” is able to dynamically capture the local stress ratio during the progression of delamination. The possibility to know the local stress ratio is relevant in all the situations where its value is different from the applied load ratio and cannot be determined a priori. The methodology analyzes in a single finite element analysis two identical models with two different constant loads, the minimum and the maximum load of the fatigue cycle. The two models interact with each other, exchanging information to calculate the crack growth rate. At first, the approach has been validated in simulations of mode I and mixed-mode propagation using double cantilever beam and mixed-mode bending tests. Then, to prove the effectiveness of the developed methodology, a modified version of the mixed-mode bending test has been analyzed. Mode I and mode II components of the load are decoupled and applied independently, resulting in a local stress ratio different from the applied load ratio. The results obtained from the simulations, compared with the analytical model obtained using the corrected beam theory, show that the proposed approach is able to predict the local stress ratio and thereby to correctly evaluate the crack growth rate during the propagation of the damage.