Ill-conditioned or non-invertible system

This warning means that the model was not fully constrained against movement. There are one or more degrees of freedom that has no stiffness or mass, which means that the equation of motion does not have a well-defined solution.

Reason
A common reason for this is that there is some element that is free to rotate about its own axis (the 1st principal axis, or torsional DOF). If an element is connected with hinges on both ends, there is nothing preventing this motion from large accelerations--which causes the solution to "blow up".

Solution
  • Check that the model does not contain elements that are not connected to other elements.
  • If you model elements connected to hinges, make sure the elements have a non-zero mass. The axial degree of freedom must be constrained by either connecting a rotational spring about its axial axis, or adding a rotational point inertia. See note below.
In some cases, the solver will be able to continue with correct results, even if this warning appears initially. Even so, the recommended practice is to make sure all elements of the model are constrained.

Note that Ashes models all elements with lumped point masses, and no rotational inertia/mass.