Comparison of two methods of measuring the rapid temperature rises in split Hopkinson bar specimens

Abstract

The temperature rise produced in iron and copper specimens by high strain deformation in a compression split Hopkinson pressure bar was measured simultaneously using two independent techniques: (i) small thermocouples (0.2 mm junction size) and (ii) an infrared (IR) camera system based on mercury cadmium telluride (spectral response between 8 and 12 μm). The response time of the thermocouple system was limited by the time taken for heat to diffuse into the junction. In order to obtain useful data with the IR camera system, the emissivity of the specimen surface needed to be modified by depositing a layer of soot. Even so, the measuredemissivity was low (0.4), suggesting that the soot layer was semitransparent to IR radiation. The thermocouples, however, yielded temperature measurements that were consistent with all the mechanical work performed on the specimens being converted to heat. The main positive result obtained with the IR camera system is that heating of the iron specimens was spatially nonuniform.