I am performing a Linear Static analysis on an assembly with some surface contact and am getting "Warning: your model may not be tied down enough or you may have a change in stiffness somewhere..." The analysis will eventually converge and a solution is found but does this effect the results in any way. If so, how can I fix this because the DOF it says to check is fixed and shouldn't be moving. I would appreciate any advice.
Have you reviewed the displacements of your model? Sometimes when this warning happens you do get stress results but the displacements are very very large. You then find some other method of applying constraints that ties things down better for the solver.
Yes I have tryed the Sparse solver. I think that it is the default solver used when surface contact is involved. Would the mesh size have any effect on this?
Do you have contact elements defined in the model? In linear stress static analysis model, contacts often servers as supports or constraints of whole structure parts besides connectors. Therefore, during the process of nonlinear iteration of searching for active contact elements for the solution, at certain state in the iteration matrix solver could report the whole structure is not well constraint ... something like that as you saw because some contacts are open (not contribute stiffness to whole structure). But model may eventually end up tied down enough at the end of that iteration. There is other cases, Algor simulation has some automatic stabilizing scheme in contact iteration that might make structure solvable and event reach to a solution.