Читать книгу The Rheology Handbook - Thomas Mezger - Страница 258

Ideal-elastic materials are showing as well immediate deformation after applying a step-like stress, as well as immediate re-formation after removing the stress afterwards in the step-like form again. For all VE samples, this elastic behavior occurs with a certain time delay. To evaluate this time-

dependent deformation behavior, two parameters have been defined:

the relaxation time λ [s] and the retardation time Λ [s], both are pronounced lambda.

Typically, the term relaxation time is used in combination with controlled shear strain tests (or controlled shear deformation, CSD, resp.), and controlled strain rate tests (or controlled shear rate, CSR, resp.), e. g. when performing relaxation tests.

Relaxation is a process in the state at rest after a forced deflection or strain and can be described as “delayed elasticity” in the sense of “delayed stress decrease” (see Chapter 7.3.3.2). On the other hand, the term retardation time is used when performing controlled force tests or controlled shear stress tests (CSS), such as creep tests. Retardation is a delayed response to an applied force or stress and can be described as “delayed elasticity” in the sense of “delayed re-formation”.