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Why getting trouble with Abaqus UMAT subroutine when using interaction features like contact or tie?

Rudraprasad Bhattacharyya's picture

I am implementing a constitutive relation in UMAT subroutine using the commercial software Abaqus. First, I tested the UMAT subroutine with 'one element model' first. Then I tested with comparatively bigger geometry. It worked fine. Now when I am trying to do a models involving contact, i.e. use of the 'Interaction module'. I am getting error of over-constraint warning for 'Split Tension test of concrete cylinder'. The same input file (except the material definition) is running on Abaqus perfectly with Mohr-Coulomb criteria. I can see a reasonable stress and strain contour. But when I am using UMAT, the simulation runs with no warning or error message. The ODB file shows zero stress in the specimen. Interestingly, the Abaqus message file gives following information
1. " ***WARNING: Solver problem. Zero pivot when processing D.O.F. 3 of 2 nodes. The nodes have been identified in node set WarnNodeSolvProbZeroPiv_3_2_1_1_1".
2. "110 SEVERE DISCONTINUITIES OCCURRED DURING THIS ITERATION. 10 POINTS CHANGED FROM CLOSED TO OPEN. 100 POINTS CHANGED FROM SLIPPING TO STICKING"

My questions are:
1. Why the same input file with Abaqus' in-built material definition did not lead to above mentioned warning?
2. Is there a way to numerically assign a unsymmetrical solver to my UMAT material?

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