Short-Time Stress Relaxation Behavior of Plastics

Abstract

An apparatus for measuring stress relaxation of plastics at constant strain in the time range of 0.01 to 2.5 sec after load application is described. Experimental results obtained indicate that rigid plastics at room temperature may be classified as either brittle or ductile. Brittle plastics sustain only low strains without fracture at the testing speed of the apparatus and undergo little relaxation of stress in this time range; ductile plastics sustain higher strains and undergo considerable relaxation of stress. The factors affecting relaxation behavior are briefly discussed. Increasing the strain, temperature, or plasticizer content generally increases the relaxation rate. Many of the results yield a linear plot of stress vs logarithmic time, as has been reported in the literature for various materials tested at longer times. By application of one form of the Eyring absolute rate theory, an average free energy of activation for the relaxation process can be calculated. The value thus calculated for polymethyl methacrylate is in the range reported in the literature for other polymers. An empirical measure of ``toughness'' can be based on these results. For many plastics this measurement is in accord with service performance.