Mechanical response of polypropylene under multiple-step loading

Experimental data are reported on isotactic polypropylene in uniaxial multiple-step cyclic tests at room temperature. The deformation programs involve (i) one- and two-step cyclic tests with a stress-controlled program (ratcheting with various maximum stresses and the zero minimum stress), (ii) short-term creep tests on specimens subjected to cyclic deformation, (iii) cyclic tests with a stress-controlled program on specimens subjected to creep flow, and (iv) triple-step tests (ratcheting with various maximum stresses interrupted by creep). A pronounced difference is observed in damage accumulation under creep and cyclic loading. Applicability is discussed of these observations for lifetime prediction under fatigue conditions. (c) 2012 Elsevier Ltd. All rights reserved.