Significance of morphology changes at a propagating crack edge

During dynamic crack propagation, several distinct changes in the morphology of the dissipative region near the crack edge occur, and they have a pronounced influence on the main propagation mechanism. There are also distinct morphological differences between mode I and mode II. For mode I crack expansion, four successive generations of localization may be observed: micro-separations coalescing with the main crack, protruding clusters of micro-separations, micro-branches, and finally (macro-)branches. The region of localizations is increasing laterally from the main crack plane with crack growth and velocity, as it appears, because of high normal stresses in planes normal to the crack direction. If sufficient space is available, an expanding mode I crack accelerates to a constant velocity, which appears to prevail even after branching and multiple-branching. This indicates an amazing self-similarity over the four generations of localization. The morphology changes during crack propagation depend both on the magnitude of the applied load and on the travelled length of the crack edge. For mode II, the energy dissipation seems generally to be much more concentrated to the crack plane than for mode I. A main reason appears to be that normal stresses in planes normal to the crack direction are comparatively small in front of the crack. Therefore, strong micro-separation localizations seem to appear mainly in shear planes parallel with the crack plane. The appearance of such localizations may be analogous to the remarkable flow velocity gradient discontinuity discovered in turbulent shear flow near a wall.As a consequence of the apparently stronger concentration of the dissipative region to the crack plane, a mode II crack can reach higher velocities than a mode I crack, and it may even reach intersonic velocities.