The temperature rise at the tip of fast-moving cracks in glassy polymers

Abstract

The temperature rise at the tip of fast-moving cracks in polymethyl-methacrylate (PMMA) has been determined from the results of two experiments. In the first, thermocouples and a temperature sensitive liquid crystal film were used to measure the total heat evolved at points along the path of the crack. The values show a continuous increase with crack speed, the figure at 650 m s-1reaching a value of 2.3 x 103J m-2. In the second experiment an infrared detector monitored the radiation emitted from the heated zone at the crack tip. The results combined to give a temperature rise of approximately 500 K throughout the velocity range studied (200 - 650 m s-1). As the rise is constant, the increase in the quantity of heat evolved implies that the plastic deformation at the tip becomes more extensive at higher crack speeds. Some preliminary experiments on polystyrene (PS) show that a temperature rise of about 400 K occurs with this polymer, but, as the heat evolution is much smaller than for PMMA, accurate measurement is difficult. The effect of such large temperature rises on the mechanism of fracture in these materials and, in particular, the possibility of significant thermal decomposition within the heated region are considered.