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.