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Structural Mechanics: Periodic Boundary Conditions and Initial stress
Posted Aug 25, 2016, 2:35 p.m. EDT Studies & Solvers, Structural Mechanics Version 5.2 1 Reply
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Hello,
I'm making a model of a cell in a periodic 3D solid made of high stress (Stoichiometric) Si3N4. The model has a linear material -> initial stress and strain node with the initial components of the stress and periodic conditions nodes for the boundaries perpendicular to the x and y axes. My goal is to first complete a stationary study to see how the initial stress redistributes in the cell, and then use the results in an eigenfrequency study.
The problem is that the stationary study with periodic boundaries and an initial stress will not converge to a solution, and the error is much, much larger than the tolerance (~1e3 order errors).
Does anyone know how to handle a stationary study in this situation, i.e., how to have initial stress and periodic boundaries in a model?
Sincerely,
Gabriel
I'm making a model of a cell in a periodic 3D solid made of high stress (Stoichiometric) Si3N4. The model has a linear material -> initial stress and strain node with the initial components of the stress and periodic conditions nodes for the boundaries perpendicular to the x and y axes. My goal is to first complete a stationary study to see how the initial stress redistributes in the cell, and then use the results in an eigenfrequency study.
The problem is that the stationary study with periodic boundaries and an initial stress will not converge to a solution, and the error is much, much larger than the tolerance (~1e3 order errors).
Does anyone know how to handle a stationary study in this situation, i.e., how to have initial stress and periodic boundaries in a model?
Sincerely,
Gabriel
1 Reply Last Post Sep 2, 2016, 9:53 a.m. EDT
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Posted:
8 years ago
Hi,
Periodic boundary conditions are not enough by themselves - they only state that 'displacements on this side are equal to displacement on that side'. There must also be some constraint which fixes your structure in space, say a Fixed Constraint at a point.
This is a variant of a common general case: There must always be enough constraints to suppress all possible rigid body motions. The physical observation that any rigid translation of your structure would give the same stress solution, manifests itself numerically in a singular stiffness matrix.
Regards,
Henrik
Periodic boundary conditions are not enough by themselves - they only state that 'displacements on this side are equal to displacement on that side'. There must also be some constraint which fixes your structure in space, say a Fixed Constraint at a point.
This is a variant of a common general case: There must always be enough constraints to suppress all possible rigid body motions. The physical observation that any rigid translation of your structure would give the same stress solution, manifests itself numerically in a singular stiffness matrix.
Regards,
Henrik