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Significance of IGAP Parameter in *CONTACT in Implicit Analyses


In traditional explicit analysis, there is no convergence criteria that is checked for each time step. However, in Implicit calculations, incremental displacements are evaluated such that a displacement norm and the energy norm are within a pre-defined tolerance before convergence is assumed to be obtained. This is the so-called iterative scheme in Implicit Non-Linear Analyses (Static and Dynamic).
Any non-linear problem will eventually involve contact between surfaces for large deformation problems. LS-DYNA handles these contact much the same as in Explicit analysis where a penalty-force is imposed if a node is found to be inside a volume of another part. In the case of the Implicit a penalty is imposed by adding a stiffness matrix when the contact is detected and the penalty force is added to the right hand side.
In certain cases, the sudden detection of contact penetration and subsequent addition of the stiffness causes some difficulty in convergence for Implicit analyses. To overcome this, LS-DYNA introduced a parameter called IGAP where the stiffness is not added instantaneously but rather ramped up to 100% over a certain number of iterations. Also, no force vector is added to the right hand side. This has a tendency to cause a ‘sticky’ condition which seems to help overcome convergence issues. IGAP is now set to ON by default. Care must be taken to ensure no excessive penetration is present in the model even with IGNORE=1.

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