Error: STIFFNESS MATRIX SINGULAR OR NON-POSITIVE DEFINITE

[Anonymous]

Hello, 

I need help. 

A very simple simulation gives me this error.

I can't find how to solve this to get a good solution.

 Cause : A singularity or non-positive definite has been detected in the stiffness matrix during the
preconditioning phase of the iterative solver.
Action : Investigate the model for a lack of constraint. If using shell elements, either set the
K6ROT model parameter to a value between 1.0 and 100.0, or set the SHELLRNODE model
parameter to ON. Also, check for elements with bending stiffness (line and shell
elements) improperly connected to elements without (solids). If you are unable to
locate the source of the singularity, consider using the sparse direct solver. 

[John_Holtz]

Hi @Anonymous 

Some more information would be helpful, such as

1. What type of analysis are you performing? (Static stress? Thermal? Modal?)
2. How many bodies are in the model?
3. What type of contact did you define between each of the bodies?
4. What constraints did you apply? To what geometry did you apply the constraints?

The error message indicates that the model is not statically stable. The analysis is applying a "load", and the model is free to "move" an infinite distance. A unique solution cannot be calculated, so the analysis stops.

The things that cause this problem are the following:

1. The model is missing constraints or contact so some part is free to move in either X, Y, Z or free to rotate about X, Y, Z. In an assembly, contact between parts connects the pieces together which connects to a constraint which holds everything in place.
2. The material properties are wrong. For example, the shear modulus is 0 or the Poisson's Ratio is 0.5.
3. The mesh is highly distorted.

Items b and c cause problems in the stiffness matrix which are similar to item a. The only information that the analysis has is a giant matrix of numbers, so it cannot distinguish what is causing the problem (a, b, or c). The solver only knows that there is a problem.

I hope this helps. If you are still having problems, feel free to export your model (to a Fusion archive file, .f3d) and attach it to the forum post.