EFFECT OF MEAN STRAIN ON THE CYCLIC DEFORMATION AND STRESS-RELAXATIONIN POLYPROPYLENE

The influence of mean strain changes in the range from 0 to 4% on the cyclic deformation and stress-relaxation properties is investigated us ing rod specimens of polypropylene. An extensometer measures and contr ols the axial strain in a closed-loop, electrohydraulic, servocontroll ed mechanical testing machine. The hysteresis loop at different number s of cycles N of N = 35, 45, and 50 are irregular in shape, and the te nsile portion decreases in size as the number of cycles was increased. The stress value of the tensile portion for three mean strains of eps ilon(m) = 1.0%, 2.0%, and 4.0% remains constant, but the value of the compressive portion decreases as the mean strain was increased. The st ress level at a strain width of 3% changes little with mean strain. Fo r the strain widths of 5% and 7%, however, the minimum stress levels a t N = 35, 45, and 50 increase with increasing mean strain, in contrast to the behavior of maximum stress level. The stress-relaxation tests show that the drop of stress decreases with an increase in number of c ycles. The discrepancy in the results of relaxation tests is due to th e effect of the difference in strain rates.