Stress relaxation is decay of stored stress after a deformation is imposed and then held fixed. It probes a material's memory through its relaxation modulus. An instantaneous generalized Newtonian fluid has no stored shear stress once the shear rate is zero and cannot describe a nontrivial relaxation tail.
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Stress relaxation is a phenomenon observed in materials, particularly in polymers and metals, where the stress in a material decreases over time when it is held at a constant strain (deformation). This occurs due to the rearrangement of the internal microstructure of the material, allowing it to redistribute stress more evenly or accommodate the deformation.