Thermomechanical aspects of dynamic crack initiation

Dynamic fracture is generally addressed as an isothermal phenomenon. When specific phenomena such as shear band formation and propagation are involved, the analyses include thermomechanical conversion of strain and/or fracture energy into heat (thermoplasticity). In this case, it has been shown that very significant temperature rises can develop which cause softening of the crack-tip material. In these works, thermoelastic temperature changes at the tip of the crack are implicitly neglected. In a recent work, we have questioned this issue and shown that for a stationary crack subjected to transient loading, adiabatic thermoelastic effects were noticeable, thus causing a large temperature drop in the elastic zone surrounding the crack-tip (Rittel, 1998). In the present work, we pursue this line of investigation by presenting additional experimental results about temperature changes ahead of a dynamically loaded crack in commercial polymethylmethacrylate. We investigate mode I and mode II loading configurations. We observe, as expected, that the temperature drops for mode I loading while it rises for the mode II case. In each case, the crack initiates during the phase where the temperature changes (drop or rise). While showing that thermoelastic aspects of fracture should certainly be taken into account, the present results indicate that thermomechanical aspects in general should not be overlooked when addressing dynamic crack initiation.