Isothermal relaxation tests

Relaxation is characterized by the decrease in stress as a function of time when a constant strain is applied to a material after loading.

The relaxation characteristics of a material can be used directly to determine, after a bolt has been tightened, the remanent fastening stress after a given time at a given operating temperature. To maintain and guarantee a minimum level of strength for parts assembled by hot-work bolting, it is necessary to retighten them periodically, as the initial tightening stress relaxes over time. Relaxation tests can be used to determine the time leading up to a stress level at which re-torquing is required, or the periodicity of re-torquing.

More generally, a material's relaxation properties provide information on its ability to accommodate stress concentrations. These stress concentrations can be found, for example, in certain geometries of structural parts (recesses, holes, etc.), crack tips, inclusions and precipitates.

In metals such as steels and nickel-based superalloys, relaxation, like creep, is significant at temperatures above 0.2 T _f (where T _f is the melting temperature of the alloy in question).

As described below, the relaxation test is very different from the creep test. It should be noted that very few studies have been carried out on the relaxation test, so it is important to describe this type of test first.