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

The temperature rise produced in iron and copper specimens by high strain d eformation in a compression split Hopkinson pressure bar was measured simul taneously using two independent techniques: (i) small thermocouples (0.2 mm junction size) and (ii) an infrared (IR) camera system based on mercury ca dmium telluride (spectral response between 8 and 12 mu m). The response tim e of the thermocouple system was limited by the time taken for heat to diff use into the junction. In order to obtain useful data with the IR camera sy stem, the emissivity of the specimen surface needed to be modified by depos iting a layer of soot. Even so, the measured emissivity was low (0.4), sugg esting that the soot layer was semitransparent to IR radiation. The thermoc ouples, 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 heati ng of the iron specimens was spatially nonuniform. (C) 2000 American Instit ute of Physics. [S0034-6748(00)03604-2].