The energy consumption by dynamic shear fracture

The behaviour of cracks loaded dynamically under shear by impact was investigated for two high strength steels using the shadow optical technique and thermocouples. The cracks propagate nearly parallel to the direction of impact loading and not under an angle of about 70 degree as known from quasistatic shear experiments with brittle materials. The fracture surfaces are smooth and shiny which suggests that the material was molten during crack extension. The energy consumption of dynamic shear cracks is calculated using the measured temperature histories. It is found to exceed the energy release rate for crack propagation under mode I loading for these steels about five times. Qualitative measurements of microstresses with X-rays, hardness measurements and fractographic investigations indicate that most of the energy for dynamic shear crack propagation is dissipated in small strips at the fracture surfaces leading to the melting temperature there. The results are discussed with respect to the failure of materials by a fast moving heat source with remaining adiabatic shear bands at the fracture surfaces.