Uninvertible matrix:

Error

Solution diverged as the matrix is singular. This may be caused by an unconstrained rigid body motion, incoherent material parameters or a body is held only by a contact.

What happened?

The solver failed due to illconditionally defined boundary condition, mismatched material units compared to geometry or a structure hold only by the contact in case of contact analysis.

What could be the possible reason?

This error may occur due to several reasons. One reason could be that boundary conditions are not properly defined e.g. applying only a load without constraining the geometry. Other reason could be that the units used for material properties such as Young’s modulus are not consistant with the dimensions of the geometry. Another reason which may trigger this error is while using linear sliding or nonlinear physical contact between two bodies among which one is only hold by the contact.

What can I do now?

You can do the following to get rid of this error:

1. Check if the applied boundary conditions are right or not. An example below elaborates this case:

  • Let’s consider a beam which has been given a force load on the right face in along positive x-direction (highlighted red in figure below) in order to simply stretch it in that direction.
Beam with only load BC

  • Since to stretch it, we need to constrain it from the other end. But running it without constraining that end may lead to this error.

  1. Check if the material parameters provided by you are consistant with the geometry dimensions or not.

  1. If you have a linear sliding or nonlinear physical contact involved, check if there any structure in contact which is unconstrained in all directions. Try to constrain the free structure so that it is hold by the constrained boundary condition but not only by the contact (either linear sliding or nonlinear physical contact). An example below elaborates this case:

  • Let’s consider two cubes such that one of the cube is sitting over the other as shown in the figure below.
Two cubes in contact

  • We than define a linear sliding or a nonlinear physical contact (Penalty or Augmented Lagrange) between two cubes (highlighted red in above figure). If we now move the lower cube while keeping upper one free to move in all directions, we may get this error message stating that one of the structure is just hold by a contact. To avoid this error we can apply the fixation constraint on the top face of the upper cube as shown in figure below (highlighted in red).
Two cubes in contact restriction

Note

If none of the above suggestions did solve your problem, then please post the issue on our forum or contact us.