Contact Mechanics
How Dr.Q handles contact between parts: Lagrange enforcement, contact types, and practical tips.
Contact is one of the most common sources of nonlinearity in structural FEM. It models the interaction between two surfaces that can press against each other, slide, or separate, without the surfaces overlapping or passing through each other.
Why contact is hard
In a standard FEM model, each part is meshed independently. Without contact, parts have no knowledge of each other, and the solver would let them overlap freely, which is physically wrong. Contact constraints prevent overlap and optionally transmit friction forces.
The challenge: the solver doesn't know in advance which nodes will be in contact. It must iterate, checking and updating the contact state at each step.
Contact acts on solid faces
Contact and bonding in Dr.Q are solid-element features: they act on the faces of solid (volumetric) meshes. Beams and shells are not contact or bond surfaces. Bonded interfaces additionally support only triangular faces today.
Contact types
Bonded (tied)
The two surfaces behave as if glued together: no separation, no sliding, equivalent to merging the nodes of both parts. It behaves linearly, so no convergence iterations are needed and it is much faster to solve. Use it for permanent welds, adhesive bonds, and press fits that don't slip.
Frictionless
Surfaces can slide freely relative to each other but cannot penetrate. Normal forces are transmitted; no tangential forces. Use it for lubricated sliding surfaces, symmetry planes, and initial contact studies.
Rough
Surfaces can separate but do not slide once in contact: no relative tangential motion. Use it when sliding is physically prevented but the surfaces may still lift off.
Frictional
Surfaces resist relative sliding up to a friction limit (Coulomb friction): once the tangential force exceeds μ × normal force, they slide. Use it for clamped joints, bolted connections, and real mechanical contacts.
Frictional contact is the most computationally expensive type. It needs many iterations and can fail to converge if the friction coefficient is high or the contact area changes rapidly.
How Dr.Q enforces contact
Dr.Q enforces contact and bonding with Lagrange multipliers: the non-penetration constraint is satisfied exactly, with no penalty stiffness to tune. There is deliberately no contact-stiffness knob, because there is no penalty to soften. The parameters you do control are the contact type, the friction law, small-sliding, the contact tolerance, and the activation schedule.
The contact tolerance
Contact and bonded surfaces should be touching or near-conformal. The tolerance is a search distance (default 0.1 mm) for pairing candidate nodes and faces, not a required gap. Size it larger than the modelled gap you want captured, and smaller than the smallest real clearance you don't want to bridge. Bonding uses the same idea (bonding_tolerance, same default).
Practical guidelines
- Start with bonded or frictionless contact and switch to frictional only if sliding is physically relevant to the result you need.
- Mesh consistently across the contact interface. Large element-size mismatches cause contact-pressure spikes.
- Check contact status in the results: are the surfaces actually in contact where you expected? Separated surfaces are a common modelling oversight, and produce empty contact results.
- Avoid point contacts: a sharp edge pressing on a flat surface creates a singularity. Add a small fillet or distribute the load over an area.