Aging of Elastic Bodies

Crumpled Mylar sheets exhibit logarithmic relaxation under both force and displacement-controlled conditions. We perform cyclic relaxation experiments and reveal a continuous family of force-displacement curves that evolve toward equilibrium, bounded by instantaneous and fully relaxed limits. By generalizing the Amir-Oreg-Imry model to include nonlinear elasticity, we predict and experimentally confirm an intrinsic asymmetry in the relaxation response: while force relaxes independently of fluctuations in the elastic response, displacement relaxation inherits its temporal evolution from them. This asymmetry implies that, regardless of the imposed boundary condition, relaxation in crumpled sheets occurs solely via a spectrum of force-relaxation modes.