Subsonic and intersonic crack growth along weak planes and bimaterial interfaces

A combined experimental and analytical study was conducted to investigate the phenomena of subsonic and intersonic crack propagation along weak planes in homogeneous solids and along dissimilar material interfaces. A single edge notch/crack oriented along a weak plane in a brittle polymer or along a polymer/metal interface was loaded in shear by a projectile fired from a gas gun. The stress field information around the propagating crack tip was recorded by photoelasticity or coherent gradient sensing, in conjunction with high-speed photography. Along weak planes in Homalite, shear cracks were observed to initiate and propagate at speeds exceeding the shear wave speed (c_s) of the polymer (intersonic). The isochromatic fringe patterns reveal two distinct Mach waves emanating from the crack tip. A series of short secondary opening cracks parallel to each other and at a steep angle to the weak plane (≈80°) were also observed to initiate behind the main intersonic crack tip. Motivated by the experimental observations, an asymptotic analysis was performed to obtain the near tip fields for a dynamically propagating steady state mode II crack with a finite shear cohesive zone in front of it. The cohesive shear stress was either a constant or varied linearly with the slip rate. Decohesion occurs when relative slip between the two surfaces reaches a material/interface specific critical value. The predictions of the model compare favorably with experimental observations and relevant analytical parameters were extracted.; Edge notches/cracks on polymer/metal interfaces were loaded under different impact configurations and the conditions governing the attainment of intersonic speeds were investigated. High resolution isochromatic patterns were obtained to study the nature of crack tip fields during subsonic/intersonic transition. Careful observations showed detached contact zones under certain loading configurations. Again, an asymptotic analysis was performed to obtain the near-tip fields around a steadily propagating intersonic crack on an elastic-rigid interface with a finite zone of crack face frictional contact located a finite distance behind the tip. A Coulomb friction model was adopted. Isochromatic fringe patterns predicted by the near-tip fields exhibit the essential features observed during the experiments.