Stress Wave Radiation From a Crack Tip During Dynamic Initiation

Plate impact experiments are conducted to study the dynamic fracture processes which occur on submicrosecond time scales. These experiments involve the plane strain loading of a plane crack by a square tensile pulse with a duration of approximately one microsecond. The crack-tip loading rates achieved are K₁ ˜ 10⁸$MPams−1$⁠, which are approximately two orders of magnitude higher than those obtained in other dynamic fracture configurations. Motion of the rear surface caused by waves diffracted from the stationary crack and by waves emitted by the running crack is monitored at four points ahead of the crack tip using a laser interferometer system. The measured normal velocity of the rear surface of the specimen agrees very well with the scattered fields computed using an assumed elastic viscoplastic model, except for the appearance of a sharp spike with a duration of less than 80 nanoseconds. This spike, which is not predicted by the inverse square root singular stress fields of linear elastic fracture mechanics, is understood to be related to the onset of crack growth and coincides with the abrupt and unstable ductile growth of a microstructural void to coalescence with the main crack. The crack initiation process is modeled as the sudden formation of a very small hole at the crack tip. This admits the possibility of dynamic crack-tip stress fields with crack-tip singularities stronger (˜r^−3/2) than the inverse square root singular fields of fracture mechanics. The elastodynamic radiation resulting from the formation of a traction free hole at the crack tip is applied first to the case of antiplane shear deformation and then to the corresponding plane strain problem. The radiated fields predicted by the strongly singular solutions are found to be in good agreement with the spikes observed in the experiments. The radius of the hole, which appears as a parameter in the solution for the radiated field, agrees reasonably well with the interparticle spacing.