Localized Damage in a Viscous Medium (Glass). Appendix A. Variations in Localized Impact Damage Caused by Nominally Similar Impacts. Appendix B. Crack Branching at Hertz Cracks.

摘要

Glass plates were lightly abraded and impacted by glass spheres at various velocities and temperatures. The incoming and rebounding velocities were measured by high speed photography. The coefficients of restitution, kinetic energies and kinetic energy losses were calculated. At room temperatures the coefficient of restitution decreases with increasing velocity. The kinetic energy losses appear to increase linearly with increasing kinetic energy at low velocities but at higher velocities the losses increase at a faster rate. This greater increase may result from additional loss mechanisms associated with fracturing of the surface. At temperatures above 580 C viscous flow permits indentation of the surface. The losses associated with the viscous flow reduce the coefficient of restitution over the entire velocity range. The impact damage was characterized by optical and scanning electron microscopy. Typically, the ring cracks show extensive crack branching which is clearly revealed in etched specimens. The crack branching was used to locate the fracture origin of the Hertz cracks and to estimate the fracture stresses. These fracture stresses were compared with estimates based on the Hertz theory. Forty specimens were subjected to similar impacts at a velocity near the threshold for Hertz crack formation. Hertz cracks were observed in approximately half of these specimens. The penetration of these cracks varied over a wide range. (Author)