An improved description of the exponential Xu and Needleman cohesive zone law for mixed-mode decohesion

Delamination between a coating and a substrate is often an undesired phenomenon, for which predictive numerical schemes are required. The interface between the coating and the substrate is commonly described by cohesive zones, for which several different cohesive zone laws have been proposed in literature. Among the widening class of developed cohesive zone laws this paper focuses on the most frequently adopted law of Xu and Needleman. It is shown that this cohesive zone law needs improvement in order to describe a mixed-mode decohesion process. This conclusion is based on investigations of the influence of the coupling parameters and the assessment of the work-of-separation under mixed-mode conditions. Remedies for this problem with the Xu and Needleman law are not yet available in literature. An alternative exponential cohesive zone law, derived from the law of Xu and Needleman, is presented, which correctly describes mixed-mode decohesion. Attention is also given to the practical determination of all the parameters involved.