Conclusion
Isothermal relaxation tests

Carrying out a relaxation test is a delicate operation, requiring very precise control of the loading parameters. Indeed, relaxation behavior is highly sensitive to loading conditions. What's more, given the low levels of strain involved during relaxation itself, and the significant effect of machine stiffness on relaxation results, it is preferable to carry out the relaxation test at constant specimen strain: i.e., by controlling stress release with a servo-control based on the strain measured on the specimen. Finally, extreme caution is advised when using data derived from extrapolation methods between relaxation and creep.