Creep and Relaxation
Creep and relaxation are closely related from a physical point of view and occur in almost all materials, such as metals, concrete, plastics, and timber. Both cause an initial state (stress \(\sigma\) and strain \(\varepsilon\)), without any change in external conditions, to continuously change over time depending on the structural system and the applied load. It is necessary to consider these effects in a time-dependent analysis by incorporating viscoelastic effects using a rheological model.
Creep
If a structural component is subjected to a constant load (stress \(\sigma = \text{constant}\)) over a long period of time, 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 timber.
Relaxation
If a structural component is stretched to a specific length (strain \(\varepsilon = \text{constant}\)) and fixed in that position, the material’s internal microstructural transformation causes the applied stress \(\sigma(t)\) to decrease over time. This behavior is particularly relevant for prestressed structural components, such as prestressing steel, preloaded bolts, cables, and membranes.
Shrinkage
Structural components made of certain materials, such as concrete and timber, tend to undergo volume contraction over time. This is caused by moisture loss to the environment and a reduction in volume due to chemical reactions and microstructural changes during hardening. The approach involves applying a time-dependent shrinkage strain. Depending on external conditions, shrinkage leads to confined stresses if the shrinkage strain is restricted, or to free strain.
Aging
Another time-dependent effect is aging, which involves changes in material properties over time. This particularly affects the modulus of elasticity and strength. In the case of concrete, it is worth noting the increase in these values over time due to progressive hardening. Another example is the brittle fracture and loss of strength in plastics due to degradation and decomposition.