Creep and Relaxation
Creep and relaxation are physically closely linked and occur in almost all materials such as metals, concrete, plastics, and wood. Both lead to a continuous change over time of an initial state (stress \(\sigma\) and strain \(\varepsilon\)), without a change in external conditions, depending on the system and loading. To consider these in a time-dependent analysis, the viscoelastic effects must be taken into account via a rheological model.
Creep
If a structural component is permanently subjected to a constant load (stress \(\sigma = \text{constant}\)), an additional time-dependent creep strain \(\varepsilon(t)\) occurs in addition to the immediate elastic deformation. This is particularly relevant for structural components made of concrete and wood.
Relaxation
If a structural component is stretched to a certain length (strain \(\varepsilon = \text{constant}\)) and fixed in this position, the internal structural transformation of the material causes the applied stress \(\sigma(t)\) to decrease over time. This behavior is particularly relevant for prestressed structural components, for example prestressing steel, prestressed bolts, cables, and membranes.
Shrinkage
Structural components made of certain materials such as concrete and wood tend to experience volumetric contraction over time. This is caused by moisture release to the environment and a volume reduction due to chemical reactions as well as structural transformations during hardening. The approach is implemented by applying a time-dependent shrinkage strain. Depending on the external conditions, shrinkage leads to restraint stresses, if the shrinkage strain is impeded, or to free strain. The following image shows the schematic progression over time for free (top) and restrained (bottom) shrinkage.
Aging
Another time-dependent effect is aging, which encompasses the change in material parameters over time. This particularly affects the modulus of elasticity and strength. Using concrete as an example, the increase of the aforementioned values over time due to progressive hardening is noteworthy. Another example is the embrittlement and loss of strength in plastics due to degradation and decay.