The stress-strain curves and stress-relaxation curves of polypropylene
are obtained by using a closed-loop, electrohydraulic, servo-controll
ed testing machine. Effects of mean strain changes on deformation beha
viour are examined in a tension-compression mode under strain control
at room temperature (18-23 degrees C). The hysteresis loops of three m
ean strains show a steady-state response from the stress-strain curves
at a strain rate of 1 x 10(-3) s(-1), at a strain width of 5%, at a n
umber of cycles of N = 50 and at three mean strains (epsilon(m) = 0, 1.0 and +2.0%). The drop of stress at the mean strain of epsilon(m) =
-1.0% is larger in magnitude than that at epsilon(m) = +1.0%; this is
caused by the higher stress level at epsilon(m) = -1.0% as compared wi
th the stress level at epsilon(m) = +1.0%. From the results of stress
amplitude and the stress drop behaviour, the magnitude of stress drop
is hardly affected by the mean strain.