  Skin is a highly non-linear, anisotropic, rate dependent inelastic, and
nearly incompressible material which exhibits substantial hysteresis even under
very slow (quasistatic) loading conditions. In this paper, a series of uniaxial
cyclic loading tests of porcine and rat skin at different strain rates and with
samples oriented in different directions (with respect to the spine) were
conducted to study the effect of strain rate and samples orientations with
respect to spine on mullins effect and skin inelastic response. A noteworthy
feature of skin is that, similar to certain filled rubbers, its mechanical
response shifts after the first extension and exhibits softening and hysteresis
when loaded under cyclic tension and mullins effect is observed. The results of
these strain-controlled cyclic loading tests also indicated that the extent of
softening is different for different strain rates and orientations. Also a
substantial hysteresis persists even at very low strain rates indicating
inelastic behavior beyond the rate sensitive viscoelastic response. Through
this series of experiments, by investigating the effect of strain rate on pig
skin and rat skin, we conclude that the skin response is rate dependent but
inelastic and shows irreversible changes in fiber orientation which are
observed in histology results. Also, skin shows persistent deformation that is
only partially recovered even after a long period of unloading.
