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In general, the engineer is familiar with the kinds of tests, on materials, which involve an instantaneous response (deformation, polarization, etc.) to the given action (mechanical loading, electric field, etc.). For elastic materials that respond to Hooke’s law, such as steel, the deformation at a given instant is directly related to the applied stress at that instant. This relationship is a result of the specific structure of this type of material. There are other groups of materials that, when stressed for a period of time in the past, still exhibit deformation as a consequence of being stressed. In other words, these materials ‘remember’ their previous history. A characteristic of this behaviour is that the memory ‘fades’ in time. |
