Explicit Dynamic Contact Analysis
An explicit dynamics analysis is particularity well-suited for solving complex and large-scale contact problems. This applies not only to truly dynamic phenomena such as impact but also to slower processes like metal forming. The advantages stem from some of the inherent characteristics of explicit time-stepping, including:
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Small time steps, which improve the time resolution of the contact state
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Direct evaluation of the contact state from known information, eliminating the need to iteratively find the contact state for each time step
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No nonlinear iterations required to establish equilibrium, and therefore no need to assemble or factorize a stiffness matrix
The last two items ares often the main sources of poor convergence in contact problems when using implicit methods, making explicit dynamics a compelling alternative.
In explicit dynamics, contact conditions are mainly implemented using the The Penalty Method, where an additional stiffness is introduced between the contacting boundaries. The trade-off between accuracy and stability when selecting the penalty factor becomes perhaps more apparent when using explicit time-stepping since stiffness added by contact forces also impact the stable time step. This effect is not automatically accounted for, but the default penalty factors definitions are chosen not to impact the stability of the solver. If needed, the contact stiffness can be adjusted by manually tuning the penalty factor multiplier.
The option in General Contact to estimate the penalty stiffness form the mass scales the penalty factor based on the current stable time step of the used by the solver.
The default settings for contact are in most case sufficient to produce stable results for the contact analysis, however, the accuracy and stability of the contact analysis are also affected by other parameters such as the time step and the mesh. If the contact part of a model appears to be the cause of inaccurate results or instabilities, consider the following:
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A simple remedy if instabilities are observed is to reduce the penalty factor, but be aware that a too small penalty factor can cause unphysical results.
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If excessive penetration or even loss of contact occurs, consider using a higher penalty factor. However, do so carefully as a too high value will cause instabilities.
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Evaluating changes to contact state and forces at smaller intervals often improves the accuracy and stability, but comes at a cost in solution time. Reducing the time step is most readily done by the Time step safety factor available for the Explicit Dynamics study step. The safety factor can be a function of time which allows using a stricter time step for selected intervals of the time history.
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Refining the mesh in problematic regions can help to smooth the contact forces as they are distributed over more evaluation points. An improved mesh refinement also improves the resolution of possibly problematic features of the geometry such as fillets. Keep in mind that a refined mesh can cause a reduction in the stable time step.
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An alternative to refining the mesh in problematic contact regions is to increase the quadrature order.
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Adding contact damping can help stabilize the contact state and remove high frequency oscillations from the contact forces. However, too much damping can quickly lead unrealistic contact forces and a significantly altered dynamics behavior. Monitor the energy dissipation by contact damping to verify that effects on the model response is acceptable.
Add contact damping by
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Selecting the Penalty, dynamic method in a Contact node.
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Adding a Damping subnode to a Contact Model node for general contact.
See also Performing an Explicit Dynamics Analysis.