  We investigate the dissipative mechanisms exhibited by creased material
sheets when subjected to mechanical loading, which comes in the form of
plasticity and relaxation phenomena within the creases. After demonstrating
that plasticity mostly affects the rest angle of the creases, we devise a
mapping between this quantity and the macroscopic state of the system that
allows us to track its reference configuration along an arbitrary loading path,
resulting in a powerful monitoring and design tool for crease-based
metamaterials. Furthermore, we show that complex relaxation phenomena, in
particular memory effects, can give rise to a non-monotonic response at the
crease level, possibly relating to the similar behavior reported for crumpled
sheets. We describe our observations through a classical double-logarithmic
time evolution and obtain a constitutive behavior compatible with that of the
underlying material. Thus the lever effect provided by the crease allows
magnified access to the material's rheology.
