Silks cope with stress by tuning their mechanical properties under load

We compare the nonlinear mechanical properties of silks under load with the quasi-static and isothermal dynamic mechanical properties of nylon as well as human hair. For silk and nylon, the dynamic storage modulus increases with increasing static load, while the quasi-static modulus decreases considerably through yield. However, the modulus of hair decreases irreversibly for both loading conditions. For silk, the increase in storage modulus is only partially reversible after high-loading and is accompanied by a non-recoverable strain. For nylon, the dynamic modulus increase is largely reversible after increased static loading up to a second high stress yield point, where modulus then decreases. Taken together, our data suggest that the dynamic modulus increases with increasing order in the silk and nylon structures under static load, whereas the morphology of hair is gradually degraded under load.