Subsonic and intersonic crack growth along weak planes and bimaterial interfaces

Citation

Samudrala, Omprakash (2001) Subsonic and intersonic crack growth along weak planes and bimaterial interfaces. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/51b0-1c87. https://resolver.caltech.edu/CaltechTHESIS:10122010-134248309

Abstract

A combined experimental and analytical study has been conducted to investigate the phenomena of intersonic crack propagation along weak planes in homogeneous solids and 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 impacting the specimen with a high velocity projectile fired from a gas gun. Homalite-100 or PMMA was chosen for the polymer and 6061 Aluminum or 4340 steel was chosen for the metal. The stress field information around the propagating crack tip was recorded in real time by two different optical techniques which provide complimentary information - photoelasticity and coherent gradient sensing (CGS), in conjunction with high speed photography. Along weak planes in Homalite-100, dynamic shear cracks were observed to initiate and propagate at speeds exceeding the shear wave speed (c_s) of the polymer. The isochromatic fringe patterns reveal two distinct lines of strong stress field discontinuity (Mach waves) emanating from the crack tip. Intersonic cracks were observed to initially accelerate up to the longitudinal wave speed (c_l) of Homalite and thereafter slow down to propagate at a near constant velocity slightly above √2c_s . 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 an intersonically propagating steady state mode II crack with a finite sized shear cohesive zone in front of it. The cohesive shear stress was chosen to be either a constant or to depend linearly on the magnitude of the local slip rate. Decohesion was chosen to occur when the relative slip between the two cohesive surfaces reaches a material/interface specific critical value. Unlike the case of a point sized dissipative region, it is shown that with a finite cohesive zone, the dynamic energy release rate is finite through out the intersonic regime. The influence of crack plane shear strength and of the rate parameter on the crack propagation behavior is investigated. Isochromatic fringe patterns were constructed using the cohesive crack tip fields, which compare favorably with the experimentally observed fringe patterns, and an attempt is made to extract the relevant analytical parameters. Unlike for a mode-I crack, a cohesive stress distribution that decreases with the local slip rate is found to match the experimental observations. The rate parameter was extracted by fitting the secondary crack angle observed in the experiments to that predicted by the analytical solution based on a maximum principal stress fracture criterion. Edge notches/cracks on polymer/metal interfaces were loaded under different impact configurations and the conditions governing the attainment of intersonic crack growth along a bimaterial interface were investigated. High resolution isochromatic fringe patterns were obtained to study the nature of the crack tip fields during subsonic/intersonic transition. Careful observations of the transition of an interface crack into the intersonic regime showed the formation of crack face contact at speeds beyond c_R of the polymer. Subsequently, the contact zone is observed to expand in size, detach from the intersonic crack tip and finally vanish. The recorded isochromatic fringe patterns showed multiple Mach wave formation associated with such a scenario. Along PMMA/ Al and PMMA/steel bimaterial interfaces, dynamic cracks initiating from edge notches were observed to accelerate to speeds higher than c_l of PMMA (supersonic), almost reaching c_R of aluminum. The resulting crack growth was observed to be highly transient and the gradients of in-plane normal stress components were recorded using CGS interferometry. Motivated by the aforementioned experimental observations, an asymptotic analysis was performed to obtain the stress and deformation fields around a steadily propagating intersonic crack on an elastic-rigid interface with a finite zone of crack face frictional sliding contact located a finite distance behind the tip. A linear frictional contact model is adopted, wherein the shear stress is proportional to the normal stress through a constant, the coefficient of dynamic friction. Isochromatic fringe patterns predicted by the near-tip fields exhibit the essential features observed during the experiments. Frictional sliding contact is shown to be possible only for velocities between c_s and √2c_s of the polymer. The relevant analytical parameters were predicted by comparing the model to the experimental isochromatic fringe patterns and comments are made on the merits of the model presented.

Thesis Files