Cohesive layer models for predicting delamination growth and crack kinking in sandwich structures

There are potentially two types of fracture that sandwich structures with strong and stiff facing sheets and lightweight cores are liable to suffer. These are the delamination growth at the face-sheet core interface and crack kinking into the sandwich core, respectively. The paper proposes computational models to simulate these failure mechanisms. The models employ the cohesive layer concept and are so constructed as to ensure that the crack advance is controlled by the critical value of strain energy release rate in mode I fracture. Of these, the first model can treat only delamination along a predetermined plane and is designated as CLD (cohesive layer delamination model). The performance of this model is thoroughly investigated in the light of experimental results. The influence of the key parameters of the model, viz. the thickness of the cohesive layer and the strength and stiffness of the cohesive layer material, have been studied. It is found that the model, as developed in this study, is fairly robust and is not sensitive to changes in parameters other than the critical strain energy release rate. The second model can track crack growth which is not predetermined in its direction. This it does by identifying the element in which the maximum principal tensile stress exceeds a critical value; once a crack is nucleated, the stress across the crack is relieved so that the right amount of energy is released when the crack is fully developed - much in the same manner as in a cohesive layer model. This model is designated as CLDK (Cohesive Layer Delamination and Kinking) model as it deals with interfacial delamination and crack kinking- whichever is the preferred mode of fracture. Experimental results of three sandwich specimens, viz. bottom restrained beams with 0° and −10° tilt angle, respectively, and a compressed beam, were used for comparison. The results indicate that the both the models are able to capture the initiation and track the growth of the interfacial delamination. The CLDK model is capable in addition to track the crack kinking into the core, and its subsequent return to the face sheet-core interface.