Читать книгу The Rheology Handbook - Thomas Mezger - Страница 241

For VE materials under a constant stress, the resulting deformation can be imagined to be composed of proportions: the first one occurs immediately and the second one delayed. After removing the stress, delayed re-formation takes place either partially or completely. The γ(t)-diagram shows for both creep curve and creep recovery curve, the shape of an increasing or decreasing exponential function, respectively (e-function; see Figure 6.5).

The degree of re-formation depends on the elastic proportion of the VE behavior of the sample.

For VE liquids (1) after releasing the stress and even after a longer rest phase, a certain extent of deformation is remaining permanently. This value represents the viscous proportion of the VE behavior of the sample. This behavior is exhibited by concentrated polymer solutions and polymer melts. Here, a load-and-removal cycle is an irreversible process, since the shape of the sample remains permanently changed after the experiment is finished.

For VE solids (2), delayed but complete re-formation occurs if the period of testing is sufficiently long. This behavior is displayed by chemically crosslinked materials, gels and concentrated dispersions with a gel-like structure at rest. Here, a load-and removal cycle is a reversible process since the shape of the test material will be the same again finally when compared to the initial shape.