Mechanisms and effects of heat generation at the tips of dynamic cracks and notches in metals

It is known that plastic work in metals is converted mostly to heat. Under dynamic plastic deformation conditions there is not enough time for conductive, convective or radiative cooling to prevent the generated heat from raising the temperature of the material. The resulting temperature rise can significantly alter the material properties and influence the failure behavior of the material. Several experiments have been performed using a high speed InSb infrared (IR) detector array to examine the mechanisms and effects of heat generation at the tip of a dynamic notch or crack. First, dynamically propagating cracks are investigated. It is seen that adiabatic conditions exist at the crack tip resulting in a simple relationship between the active plastic work zone and the temperature field and that, consequently, no effects of hyperbolic heat condition are observed. The dependence of the dynamically propagating crack tip temperature field upon crack speed and material parameters is reported. Next, the dependence of the rate of conversion of plastic work to heat upon strain and strain rate are examined. Using the split hopkinson bar several materials were tested and compared to simple theories in the literature. It is seen that the materials do not always behave as expected, and that further investigation is necessary. Last, asymmetrically impact loaded, stationary notches are investigated with special attention paid to the formation of adiabatic shear bands at the notch tip. A new interferometer, the coherent gradient sensor (CGS), is used in conjunction with high speed photography to record the deformation field around the crack tip. The recorded field is shown to agree with a model of the deformation for low velocity impact of PMMA. For high velocity impact of C300 steel, shear bands are formed and a Dugdale model is used to produce measurements of the shear stress on the shear band from the recorded fringe patterns.