Cohesive zone modeling of crack nucleation at bimaterial corners

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The prediction of crack nucleation from bimaterial corners that have the same corner angle (and therefore singularity) via stress intensity factors is fairly well established. The objective of this work was to examine crack nucleation from a range of corner angles and see if a more general crack nucleation criterion could be formulated. A series of experiments was conducted using an aluminum-epoxy bimaterial specimen loaded under 4-point bending. In addition to the usual measurements of load and an associated displacement, the displacements near the corner were measured using moiré interferometry. Numerical analyses were first conducted assuming a rigid interface. However, the resulting displacements differed from the measured ones, especially near the corner and along the interface. The interface was then modeled as a separate constitutive entity by incorporating a cohesive zone model in the numerical analysis. Following calibration via an interface crack configuration (zero corner angle), the cohesive zone model yielded displacements that were in good agreement with the measured values for all the other corner angles that were considered. The predicted failure loads were also in good agreement with the experimental results. Thus the consistent nucleation criterion was that the area under the traction-separation curve and its maximum traction (the dominant cohesive zone model parameters) remain the same. The numerical solutions indicated that the plastic deformation in the epoxy was small and that failure was predominantly in opening mode. In addition, the critical vectorial crack opening displacement and mode-mix were independent of the corner angle. Finally, a simple design parameter was proposed for predicting the failure load of a bimaterial specimen with an arbitrary corner angle, based on the failure load of a bimaterial specimen with an interface crack.