Contact Modelling in Impact Analysis

Contact is the load transfer mechanism

In an impact analysis the contact interface is where the impactor momentum and energy are converted into a force-time history on the target. It is not merely a boundary condition that prevents interpenetration; it is the physical mechanism through which the impact load is generated. The correctness of the contact model — the surfaces defined, the normal direction, the friction, the stiffness, the thickness offsets, the search logic — directly determines the magnitude, duration and distribution of the load that drives the structural response. A contact model that is too stiff produces artificially high peak forces and short durations; one that is too soft allows excessive penetration and smears the load. Both can produce a plausible-looking animation that misrepresents the physics.

CONTACT IS OFTEN THE MECHANISM THROUGH WHICH THE IMPACT LOAD IS GENERATED — NOT MERELY A BOUNDARY CONDITION.

Contact formulations in impact

Modern explicit solvers offer several contact formulations, each suited to different physical situations. Surface-to-surface contact is used for distinct bodies impacting, sliding and separating. Self-contact is essential when a structure folds onto itself during crushing or tearing, and omitting it allows a panel to pass through itself, producing an absurd deformation mode. Contact thickness offsets account for the physical shell or membrane thickness so that contact occurs at the true surface rather than at the mid-surface of the element. Initial interference — where the mesh already overlaps at time zero — must be resolved carefully, either by ramping the contact or by correcting the geometry, because an unresolved initial penetration produces an artificial burst of contact force at the start of the analysis. The contact search algorithm determines which surfaces can interact; in a large model an inadequate or overly broad search can be both inaccurate and expensive.

Penalty stiffness, friction and penetration

Most explicit contact implementations use a penalty method: a spring-like force proportional to penetration depth pushes the surfaces apart, with a penalty stiffness that balances accuracy against numerical stability. A penalty stiffness that is too low allows large penetrations and smears the contact load; one that is too high introduces high frequencies into the contact force, reduces the stable timestep and can produce contact energy that is not physical. Friction governs the tangential load transfer and can be highly sensitive in sliding-dominated impacts; the coefficient is often poorly known under dynamic conditions and should be treated as a parameter to be studied, not fixed by habit. Contact damping is sometimes added to reduce high-frequency contact ringing, but excessive damping can again distort the force history. Every one of these parameters has a physical meaning and a numerical consequence, and both should be understood.

EXCESSIVE CONTACT PENETRATION IS NOT A CONTACT PROBLEM TO BE TOLERATED — IT IS A SYMPTOM OF INSUFFICIENT CONTACT STIFFNESS, EXCESSIVE TIMESTEP, OR MESH RESOLUTION THAT IS TOO COARSE FOR THE CONTACT INTERFACE.

Contact with damaged and eroding surfaces

In severe impacts the contacting surfaces do not remain intact. Elements fail and are deleted, exposing new interior surfaces that may continue to interact with the impactor or with each other. The contact formulation must handle this evolving topology: a contact pair defined only on the original exterior surface will fail to represent the interaction once the surface has eroded. Some solvers provide erosion-aware contact that automatically includes newly exposed faces; in others the analyst must ensure that the contact definition follows the erosion. The contact energy in such events — the work done by contact forces — can become large and must be tracked as part of the overall energy balance. A model in which the contact energy grows without bound, or in which deleted surfaces continue to generate phantom contact forces, is numerically sick regardless of how the animation looks.

Contact verification checklist

Contact is sufficiently important and sufficiently error-prone that a structured check should be performed before any impact result is accepted. The checklist below is not exhaustive, but every item should have a defensible answer.

• Is initial penetration present? — Check for overlaps at t=0; resolve by geometry correction or ramping rather than tolerating a force spike.
• Is the contact normal direction correct? — Reversed normals produce attraction instead of repulsion.
• Is self-contact required? — Any structure that can fold or tear onto itself needs self-contact; check the deformed shape without it.
• Is the friction coefficient known? — Dynamic friction under impact conditions is often uncertain; treat it as a studied parameter.
• Is friction sensitivity important? — Run a bracket of friction values and confirm the engineering conclusion is robust.
• Is the penalty stiffness physically reasonable? — Not so soft that penetration is large, not so stiff that contact energy or noise dominates.
• Are contact forces stable? — Look for high-frequency ringing or spurious spikes in the contact force history.
• Is the contact energy explainable? — Track it in the energy balance; unbounded growth indicates a problem.
• Are deleted surfaces still interacting appropriately? — Confirm erosion-aware contact is active where elements are expected to fail.
• Are contact thickness offsets correct? — Mid-surface contact without offsets under- or over-represents the physical gap.
• Does the physical contact sequence match expectation? — Review the contact pressure contours over time against the expected impact sequence.

Contact parameters and their significance

The table summarises the principal contact parameters, what they control, how sensitive the result typically is, and how they should be verified. The guidance is qualitative because the appropriate values depend on the solver, the material system and the physical event.