Shear dominated transonic interfacial crack growth in a bimaterial-I. Experimental observations

In this work we describe a series of impact experiments performed on PMMA/4340 steel edge cracked bimaterial plates. Specimens were impacted at 20 m s^-1 in a one point bend configuration using a high speed gas gun. Dynamic interfacial crack propagation was observed using the optical method of Coherent Gradient Sensing and high speed photography. Very high crack tip accelerations (10⁸ m s^-2) and very high crack tip speeds (up to 1.5c_R ^PMMA) were measured and are reported. Quantitative measurements show that in experiments in which the crack tip speed entered the intersonic range for PMMA, the stress field surrounding the crack tip was shear dominated. The observation of high shear around the crack tip can also be explained using wave propagation arguments. It is found that the reason for attainment of intersonic (with respect to PMMA) crack tip speeds is directly related to the large amounts of energy necessary to initiate the crack tip under shear dominated conditions. A comparison with the theoretical results of Part II in this study is also made. There seems to be an unfavorable region of stable crack propagation velocities in the intersonic regime. This region is c_s ^pmma < v < √2c_s ^PMMA. In all experiments performed, the propagating crack accelerated quickly out of this region. In the few interferograms that do actually correspond to crack propagation in this unfavorable velocity range, crack face contact was observed. This observation is also in agreement with the findings of Part II of this investigation.