New regime in the mechanical behavior of skin: strain-softening occurring before strain-hardening

We report linear and non-linear shear tests on rat skin, evidencing a strain-softening regime, from 1% to 50% strain, followed by a strong strain-hardening regime, leading to a 'deck chair-shaped' stress-strain curve. The strain-softening regime was never reported as such in the literature, possibly mistaken for the linear regime in experiments starting above 1% deformation. The time-dependent response is akin to that of a gel, with a power law frequency-dependent dynamic shear modulus ranging from similar to 5.6 kPa to similar to 10 kPa between 0.1 Hz and 10 Hz. We present an analytical non-linear viscoelastic model that accounts for both time-dependent and strain dependent features of the skin. This eight-parameter model extends the one we proposed for parenchymatous organs by including strain-softening.