A CONSTITUTIVE THEORY FOR POLYPROPYLENE IN CYCLIC DEFORMATION

The shape of the hysteresis loop has been studied for polypropylene su bjected to different cyclic loadings. Cyclic tests were performed unde r different sets of cyclic strain amplitude and number of cycles, usin g a closed-loop, electrohydraulic, servocontrolled testing machine. A constitutive theory is developed for the interpretation of loop shape behavior in terms of elastic, perfectly plastic materials of different yield strengths. The constitutive equation is based on a simple overl ay model of bar elements. The calculated results agree well with exper iment, which indicates the distinctive shape of the hysteresis loop, t ermed a propeller-like shape. The truss model reasonably explains the cyclic deformation behavior in polypropylene.