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Initial stress values from stationary study not working as initial conditions for time dependent study

Posted Apr 20, 2024, 12:30 p.m. EDT Studies & Solvers, Modeling Workflow Version 6.1 1 Reply

Mariana Blanco-Rojas
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Hello!

I am currently using the Solid Mechanics module to simulate deformation of a shell over time. I have an initial stationary step were my material is originally very rigid, followed by a time dependent study of a much less rigid material (I use a smaller Young's Modulus).

I want to use the solution of the stationary study as an initial condition for my time dependent study. For this, I am setting the 'Initial values for the variables solved for' as the solution to my stationary study. However, when I plot the Von Mises stress for the first time step of my time dependent study, I'm not getting the same result as my stationary study.

Does anyone have any insights on why this might be, or any way to solve this problem?

Thank you so much!

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1 Reply Last Post Apr 23, 2024, 4:50 a.m. EDT
Henrik Sönnerlind COMSOL Employee

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Posted: 2 weeks ago Apr 23, 2024, 4:50 a.m. EDT

This is the expected behavior. The initial conditions are the displacements from the first study. That will give a set of corresponding strains. If you lower Young's modulus, the stresses will be proportionally lower for that given set of strains.

In case you need to explicitly prescribe initial stresses, you need to add an Initial Stress and Strain or External Stress node, and pick up stress values from the previous solution using an expression involving a withsol() operator. Note, however, that such stresses will be added to what you get from the strain field.

In either case, you need to consider the physical feasibility carefully. Is it possible that Young's modulus drops fast without affecting the displacement field? What happens when the imposed stress field is redistributed for maintaining equilibrium? Is stresses or strains that you should prescribe?

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Henrik Sönnerlind
COMSOL
This is the expected behavior. The initial conditions are the displacements from the first study. That will give a set of corresponding strains. If you lower Young's modulus, the stresses will be proportionally lower for that given set of strains. In case you need to explicitly prescribe initial stresses, you need to add an *Initial Stress and Strain* or *External Stress* node, and pick up stress values from the previous solution using an expression involving a *withsol()* operator. Note, however, that such stresses will be added to what you get from the strain field. In either case, you need to consider the physical feasibility carefully. Is it possible that Young's modulus drops fast without affecting the displacement field? What happens when the imposed stress field is redistributed for maintaining equilibrium? Is stresses or strains that you should prescribe?

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