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. Systematically creasing a thin material sheet can produce a variety of bulk metamaterials, that can be naturally divided into disordered, or crumpled, and ordered, origami-like, structures. On the origami side, carefully picking among the infinite number of possible crease patterns allows designing a wide range of physical properties and shapes [1-5]. An archetypal example is the negative apparent Poisson ratio exhibited by the Miura-ori patterns [6-9]. This unnatural behavior is explained through the rigid-face model, where each fold is described as two rigid panels and a hinge setting an angle between them. This description results in tight kinetic constraints on extended foldings, and only a small number of degrees of freedom usually account for all possible geometric deformations [6]. This simple model is functional regardless of the scale of the system, from micro-robots [10] to space engineering [11]. In crumpled systems, the situation is more complex, as single elastic excitations such as developable cones and ridges [12, 13] act as crease precursors. However, once the system has been prepared, a random network of crease-like, plastified objects competes with the elasticity of the sheet to produce a soft elastic solid, though in this case, self-contact plays a major role [14, 15]. Thus, for many material foldings, the modeling must take into account the properties of the material itself [16]. Indeed, for origami structures hidden degrees of freedom [17, 18] appear that combine the elastic deformation of the faces and the mechanical response of the creases. The former is understood as a classic deformation of thin sheets with boundary conditions imposed by the creases. The latter is usually described as an elastic hinge, with a response that is proportional to the departure of the crease angle from a rest configuration [19, 20]. The range * theo.jules@ens-lyon.fr of reachable configurations for such a model is much broader: it allows, for instance, the passage between stable states in bistable origamis [21-23]. Notably, comparing the elasticity of the crease and the flexural rigidity of the faces gives rise to a characteristic length-scale [24] that relates to the spatial extension of the crease [25, 26]. While this approach is enough to explain the elastic behavior of folded structures, it fails to capture the complete quantitative picture. For intermediate deformations , both origamis [21, 27, 28] and crumpled sheets [29, 30] exhibit hysteresis and relaxation. These phenomena drastically limit the experimental domain of validity for simple elastic models: they induce a temporal evolution of the system and a change of reference state during the experiment. Worse, producing precise and reproducible experiments is severely challenging due to induced memory effects. Nevertheless, the ability to produce a crease within a sheet relies on these very effects [28, 31, 32], which are, in turn, unavoidable. It is thus of crucial interest to disentangle the respective roles of elasticity and dissipative phenomena to understand the macroscopic mechanical behavior of real-world foldings. In this paper, we build on the foundations laid by the purely elastic decription of a single fold [24, 25] and extend this framework to take into account the plasticity of the material through the modification of its reference state. This approach produces a mapping of the load-deformation curve to the rest angle of the crease, allowing to read the latter from macroscopic observations on the fly. The corresponding predictions are then compared to experimental measurements of single folds produced from polymeric and steel thin sheets with remarkable success. Finally, we thouroughly investigate the temporal evolution of the single polymeric fold under stress. A constant macroscopic strain imposes a stress relaxation that is well described by a double logarithm. Such a description , based on the aging of glassy polymers [33, 34]