Dear all, I have a benchmark 2-D problem with a square with linear elastic material under a uniform gravity in the horizontal direction. No boundary condition is given to the structure. I lumped the mass matrix so there is no solver involved in the problem. I expected the results would give me very small values in the y direction; However, I noticed I got significant rotations resulting from a negative displacements on the left side of the structure and the positive displacements on the right side. After debugging, I found that my stiffness matrix is symmetric but not positive definite in this case. I know this comes from the fact that there is not sufficient boundary conditions provided to eliminate some entries, I am just wondering if there is anything in dealii that I can use to improve the stiffness matrix. Any suggestions would be helpful, thank you!
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