Temperature dependence of yield and fracture in polymethylmethacrylate

Abstract

The effect of temperature on the deformation and fracture of polymethylmethacrylate has been determined by constant strain rate compressive and tensile tests from 78°K to 350°K. In compression, the yield stress increased continuously with decreasing temperature and, except at 78°K, substantial macroscopic plastic flow occurred at all temperatures. At 78°K, only a small amount of plastic flow occurred before fracture. A similar type of temperature dependence was found in tension between 230° and 350°K for both the yield and fracture stresses, but the tensile yield stress was always lower than the compressive yield stress by about 1500 p.s.i. In this temperature range, fracture is controlled by macroscopic yielding in that defects (crazes) produced during the yielding process serve as crack nuclei. It is deduced that the temperature dependence of fracture in this temperature range is a reflection of the temperature dependence of the yield stress. Below about 200°K, PMMA fractures in tension with little or no plastic flow at an approximately constant stress level well below the general yield stress determined by the compression tests. The consistency of the fracture data suggests that the critical mechanism which nucleates fracture reflects a basic bulk property of the material and is not due to extraneous physical discontinuities such as inclusions or other stress raisers.