A re-evaluation of mixed-mode cohesive zone modeling based on strength concepts instead of traction laws

New methods for implementing cohesive zones in computational mechanics were derived by re-interpreting cohesive-zone "traction laws" as "strength models" and using concepts from damage mechanics. When applied to pure mode I or mode II loading, this approach is identical to prior methods. But, when applied to mixed-mode loading, it defines a new approach called strength cohesive zone modeling (or "strength CZM"). Compared to previous methods, strength CZM provides improved modeling for problems with changing mode mixity, allows independent selection of normal and tangential strength models, and reveals limitations in any method based on effective displacements. New modeling shows that mode mixity, which changes during crack growth, depends on cohesive zone properties and on which end of the cohesive process zone is interpreted as the crack tip. These observations suggest many prior experimental results need reanalysis. By considering a range of independent strength models, strength CZM predicts that all GI-GII failure envelopes are convex. Precracking steps recommended in delamination testing protocols are considered. Strength CZM can realistically model precracking while prior methods generate unrealistic predictions.