Kovacs effect

The Kovacs effect describes a phenomenon observed in certain materials, particularly polymers and glasses, during the process of physical aging. When a material is subject to a temperature change, especially in a glassy state, it can exhibit a non-linear response to stress or strain. More specifically, when a sample is suddenly subjected to a step change in temperature (for example, from below to above its glass transition temperature), it can exhibit a characteristic "overshoot" in its mechanical properties.