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Development of a dynamic decohesion criterion for subsonic fracture of the interface between two dissimilar materials

We present findings of an experimental study of dynamic decohesion of bimaterial systems composed of constituents with a large material property mismatch. Poly-methylmethacrylate (PMMA)-steel and PMMA-aluminium bimaterial fracture specimens were used. Dynamic one-point bend loading was accomplished with a drop-weight tower device (for low and intermediate loading rates) or a high-speed gas gun (for high loading rates). High-speed interferometric measurements were made using the lateral shearing interferometer of coherent gradient sensing in conjunction with high-speed photography. Very high crack propagation speeds (terminal crack-tip speeds up to 1.5csPMMA, where csPMMA is the shear wave speed of PMMA) and high accelerations (of about 107g, where g is the acceleration of gravity) were observed and are reported. Issues regarding data analysis of the high-speed interferograms are discussed. The effects of near-tip three-dimensionality are also analysed. Dynamic complex stress factor histories are obtained by fitting the experimental data to available asymptotic crack-tip fields. A dynamic crack growth criterion for crack growth along bimaterial interfaces is proposed. In the subsonic regime of crack growth it is seen that the opening and shearing displacements behind the propagating crack tip remain constant and equal to their value at initiation, i.e. the crack retains a self-similar profile during crack growth at any speed. This forms the basis of the proposed dynamic interfacial fracture criterion.