Creep and Relaxation
Creep and relaxation are closely physically linked and occur in almost all materials such as metals, concrete, plastics, and wood. Both cause an initial state (stress \(\sigma\) and strain \(\varepsilon\)), without any change in the external conditions, to change continuously over time depending on the system and loading. To consider these in a time-dependent analysis, the viscoelastic effects must be taken into account using a rheological model.
Creep
If a structural component is subjected to a permanent 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, such as prestressing steel, prestressed bolts, cables, and membranes.
Shrinkage
Structural components made of certain materials such as concrete and wood tend to contract in volume over time. This is caused by moisture loss to the environment and a volume reduction due to chemical reactions as well as structural transformations during hardening. The approach is carried out by applying a time-dependent shrinkage strain. Depending on the external conditions, shrinkage leads to restraint stresses if the shrinkage strain is restrained, or to a free strain.
Aging
Another time-dependent effect is aging, which encompasses the change in material properties over time. This particularly affects the modulus of elasticity and the strength. Using concrete as an example, the increase in the aforementioned values over time due to progressive hardening is worth mentioning. Another example is the embrittlement and loss of strength in plastics due to degradation and decay.