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Stress relaxation due to oscillating strain

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Posts: 1
Topic starter
(@angushsu)
New Member
Joined: 10 years ago

Hi All,

I am trying to model stress relaxation of a viscoelastic material due to oscillating strain.

To do so I modelled a simple beam (1D elements) with definition of viscoelasticity as single prony series in time domain (g=0.5, k=0, t=variable). To check if the beam responses in expected manner I fixed it at one end and enforced displacement in vertical direction at the other end. When I keep displacement constant at some arbitrary value I am seeing a gradual decrease in reaction force at constrained nodes (as well as stress decrease). I presume it is initially OK so I go to next step and prescribe oscillating displacement in a form of sin(wt) at one end and keep it fixed at the other. This time reaction force at constrained nodes has constant amplitude with no sign of relaxation. I tried with different values of relaxation times (from like 0.01 to 10 s) and solution times (1 to 600 s) with no difference in the response. The only difference I noted is that amplitude of reaction force is initially smaller for shorter relaxation times but then still it is constant throughout analysis time. Am I trying to see here something that will not ever happen or there are flaws in my analysis? The sources I found mention oscillating strain when speaking of storage and loss moduli but no mention about relaxation. Does the Boltzmann Superposition Principle apply here and should I see a gradual decrease in amplitude of reaction force due to oscillating strain?

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Posts: 3998
(@jorgen)
Member
Joined: 5 years ago

Yes, if I understand you right, I would expect a gradual reduction in the reaction force during oscillating strain. The amount of reduction will depend on the Prony series parameters.

-Jorgeb

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