When a crack grows in a ductile material energy is dissipated at the crack tip via plastic deformation. Approximately 90% of the energy of plastic deformation is itself dissipated as heat energy. When a crack is running at high speeds the heat generated at the tip of the crack tip does not have time to conduct away from the crack tip, resulting in a large, local temperature rise at the crack tip. We devised a system for measuring the crack tip temperature rise distribution, using an array of high speed, In-Sb infrared detectors. With this system we are able to measured the crack tip temperature with a spatial resolution of about 0.10mm, and a time resolution of about 0.2 microseconds. A sample of our experimental results, shown as a color mpa of the crack tip temperature is given below. This was for a crack running at 500m/s in a high strength titanium alloy, Beta-C. In the image, the crack is running from left to right, and the crack tip is located in the red region, as x=0mm. The peak crack tip temperature rise is over 400C.
Such a large temperature increase raises the question of what is the effect of this on the dynamic fracture toughness. One might expect a large effect, since the elastic modulus and flow stress will both decrease by around 40% at 400C. To answer this question, as part of his Ph.D. thesis, Jacob Kallivayalil performed a simulation of crack growth, bringing into the calculation the coupling between the thermal fields, and the mechanical fields, and accounting for the decreases in modulus and flow stress with temperature. His results show that for stress controlled crack growth, crack tip temeperature rise has a significant effect, due to the reduction in stress ahead of the crack tip caused by thermal softening. For strain controlled crack growth, crack tip temperature rise appears to have little effect.