An investigation of the heat generated during cyclic loading of two glassy polymers. Part I: Experimental

A comparative study of hysteretic heating was carried out in commercial polymethylmethacrylate (PMMA) and polycarbonate (PC) specimens subjected to cyclic compressive loading. Both materials heat up upon cycling with a pronounced influence of the cycling frequency and the stress amplitude. Commercial PMMA is very sensitive to minor variations in the maximum applied stress, which does not exceed 0.45 times the yield strength of this material. The temperature rise is continuous throughout the test. The maximum temperature reached is of the order of the glass transition temperature (T-g), and often more. Failure is sudden and consists of localized bulging in the central part of the specimen. Commercial PC can be tested at much higher stress levels, of the order of its yield strength. Here, a well-defined temperature peak, which has not been reported previously, develops during the initial stage of the loading. The maximum temperature reached during the test does not exceed 0.8T(g). The sharpness of the peak improves with increasing stress amplitude and testing frequency. Failure of the specimen occurs by diffuse barreling. Annealing heat treatments shorten the fatigue life of PMMA specimens and decrease the sharpness of the thermal peak of PC specimens. A non-uniform temperature distribution is observed to develop in the specimen during cycling. Consequently, care should be paid to the thermal boundary conditions of the problem. The failure mechanism (diffuse vs. localized) of the investigated materials is thus dictated both by the temperature distribution and by the extent of the temperature rise. (C) 2000 Elsevier Science Ltd. All rights reserved.