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8.4 - Viscous flow converts the elastic strain into...

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Stress Relaxation is a related phenomenon to creep and important for polymers. It is the “relaxation” of stress over a long period of time and a corresponding permanent strain. Whereas creep involves increasing strain over time for materials under constant stresses, stress relaxation involves decreasing stress over time for materials under constant strains. The mechanism of stress relaxation is viscous flow (i.e. molecules gradually sliding past each other over an extended period of time.)
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Unformatted text preview: Viscous flow converts the elastic strain into non-recoverable plastic deformation. The relationship between the stress and time is given by σ = σ o e –t/ τ where τ is the relaxation time and characterizes the phenomenon of stress relaxation. τ is the time necessary for the stress to fall to 37% of the initial stress. Since viscous flow is a thermally activated process it should not be surprising that the relaxation time has an Arrhenius form: RT Q Ce-= τ 1...
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