Experimental investigation of transient thermoplastic effects in dynamic fracture

In a recent paper, we investigated the extent to which transient thermoelastic e€ects could a€ect dynamic crack initiation [Rittel, D., 1998a. Experimental investigation of transient thermoelastic e€ects in dynamic fracture. Int. J. Solids and Structures 35(22), 2959±2973]. In the present paper, we pursue this line of investigation by investigating transient thermoplastic e€ects. Three characteristic experiments are described which address the assumptions of the isothermal nature of dynamic crack initiation in two representative materials: `ductileʹ polycarbonate and `brittleʹ polymethylmethacrylate. First, by varying the global (baseline) temperature of dynamic fracture experiments, it is shown that these two materials react quite di€erently. The crack-tip temperature of the PC specimen rises signi®cantly as evidenced from the failure mode at both the macroscopic and microscopic scales. By contrast, the failure mode of PMMA is apparently una€ected by temperatures below its Tg. This experiment demonstrates further that the local crack-tip temperature is likely to be di€erent from the baseline temperature. Next, we investigate the bulk thermomechanical behavior of these two materials to quantitatively assess the temperature rise which accompanies transient adiabatic deformations. This behavior is deemed to be characteristic of that of the process zone material ahead of the crack-tip. Finally, we present thermal and fractographic results about transient temperature recordings ahead of an adiabatic shear band in a PC specimen loaded in mode II. These results show that thermomechanical couplings must be taken into account in dynamic fracture investigations. These couplings are related to the loading mode, to the strain levels reached in a typical process zone and also to the thermomechanical response of the speci®c material at such strain levels