By adding a Wear subnode to a Contact node, it is possible to model adhesive or abrasive wear of the material when the contacting boundaries are sliding along each other. Since wear involves solving evolution equations, the Wear node only adds a contribution for time-dependent studies.

In the Solid Mechanics and the Multibody Dynamics interfaces, the removal of material during the wear process can be modeled with two fundamentally different techniques. The most general technique is to model the removal using the deformed geometry concept. With this approach, the material frame X of the domains adjacent to the contacting boundaries is updated according to the computed wear depth hwear. This means that there is an actual removal of material during the simulation which affects, for example, the contact search, mapping and conditions. When selecting the Deformed geometry formulation, the wear feature adds a (hidden) Deforming Domain feature that controls the material frame through an adaptive mesh smoothing. The removal of material is made through a (hidden) Prescribed Normal Mesh Displacement boundary condition controlled by hwear on the selected contact boundaries. By adding the deformed geometry, an extra dependent variable is added, the material mesh displacement material.disp. This adds a set of extra degrees of freedom to the model that needs to be solved for.

Alternatively, the removal of material can be modeled using an offset-based approach. This formulation offers a simplified approach that is computationally less expensive, but mainly suitable when the amount of worn-off material is small. When selecting the Offset-based formulation, hwear is subtracted from the offset variables doffset,s and doffset,d in Equation 3-174. Hence, the material is considered removed only in the definition of the physical gap, while the contact search and mapping are unaffected. The latter follows from the fact that the actual coordinates and normals of the contacting boundaries essentially remain constant with respect to the wear; they, however, can change due to the deformation induced by the wear and changing contact conditions.

where the rate of the wear depth is given by some source term f that is typically a function of the slip velocity vslip, the contact pressure Tn, and the temperature T. The surface and material properties also play an important role, and are represented by the generic quantity θ in the above equation.

In COMSOL Multiphysics, the wear depth can be computed using a Generalized Archard law for which the wear rate is

Here kwear is a dimensionless wear constant and the exponent n controls the dependence of the wear rate on the contact pressure. The reference contact pressure, Tn,ref, can be chosen arbitrarily, and is used only to obtain consistent units. The classical Archard wear equation is retrieved from Equation 3-197 by setting n = 1 and Tn,ref = 1 Pa. In addition, it is also possible to enter an arbitrary expression for the source term f that defines the wear rate.

It is possible to account for wear on both the source and destination boundaries. However, it is generally more accurate to model wear on the destination side. This follows from the fact that most relevant quantities, such as Tn and vslip, are defined only on the destination boundary in the Contact node and by, for example, the Friction node. Hence, when modeling wear on the source boundary, these quantities are mapped from the destination to the source. For example, when applied to the source side, Equation 3-197 actually reads

The definition of the wear rate thus includes multiple mappings form source to destination, and form destination to source. As described in Contact Search and Kinematics, these mappings are not necessarily one-to-one, which can lead to accumulating errors. The offset-based wear formulation can be especially sensitive to such errors.