The wear models calculate how much and in what direction a contact node is to be moved to simulate wear based on the contact results at the contact nodes. A generalized form of the Archard wear model is available. In addition, you can specify a user-defined wear model using the USERWEAR subroutine.

TB,WEAR,1,,,ARCD
TBDATA,1,K,H,m,n
TBDATA,6,nx,ny,nz

TB,WEAR,1,,4,USER
TBDATA,1,C1,C2,C3,C4

The wear increment (the rate of wear times the time increment: x Δt) calculated by the wear model is used to move the contact node along the direction opposite to the contact normal at that node. During the nonlinear solution process, at the iteration in which the force and displacement convergence criteria set by the user are met for a particular substep, the wear increment and direction at that iteration are used to move the contact nodes. Since this repositioning of contact nodes results in a loss of equilibrium, additional iterations are required to achieve convergence. If the solution fails to converge after the wear is applied, the usual procedure of bisection is used. The wear increment is discarded and the solution increment is repeated, applying the bisection.

Since wear is a material removal process, the underlying solid element does not experience any strain or stress due to the movement of the contact nodes undergoing wear. A large wear increment can result in the opening of an initially closed contact pair. This may result in convergence problems, especially if rigid body motion occurs. Thus, it is highly recommended that you use very small time increments when modeling wear.

Because wear involves repositioning of the surface nodes to simulate material loss, the element quality of the solid elements underlying the contact elements becomes progressively worse with increasing wear. The analysis may ultimately terminate due to element distortions. If this happens, you can use manual rezoning or nonlinear mesh adaptivity to improve the mesh and continue the analysis.

Manual rezoning and nonlinear mesh adaptivity are available for elements CONTA172 and CONTA174.

For either method, the total accumulated wear thus far is applied to update the nodal positions, and then the quality of the mesh is improved. Next, the analysis is restarted. For rezoning this is a manual step (see Understanding the Rezoning Process). For nonlinear mesh adaptivity the restart is performed internally by the program (see Understanding the Nonlinear Mesh Adaptivity Process).

Since the geometry is updated with wear during the mesh updating stage (for both manual rezoning and nonlinear mesh adaptivity), the accumulated wear and volume lost due to wear are initialized to zero upon restarting the analysis. They start accumulating again as the analysis is continued.

Use caution when defining time-dependent wear with any analysis involving a restart, including manual rezoning and nonlinear mesh adaptivity. To ensure that the restart uses the correct wear data at the restart point, you should define time-dependent wear instead of reissuing TB,WEAR between load steps. See Assumptions and Restrictions for more information.

In the nonlinear mesh adaptivity procedure, you must define a wear-based criterion for triggering mesh adaptivity. For more information, see the NLADAPTIVE command and the discussion of Contact Surface Wear in the Nonlinear Adaptivity Analysis Guide. Assuming “contwearel” is a component containing contact elements undergoing wear, following is a typical example of commands used to define the wear criterion for nonlinear mesh adaptivity:

nladaptive,contwearel,add,contact,wear,0.75   ! Morph the mesh after 75% is lost in wear.
nladaptive,contwearel,on,,,5,1,7              ! Criterion defined for component contwearel 
                                              ! becomes active from time = 1 to 7 and is
                                              ! checked once every 5 substeps.

Nonlinear mesh adaptivity uses the mesh morphing method to improve the mesh. The morphing algorithm adjusts the nodes of all the contact and target elements and moves the nodes of underlying solid elements to improve mesh quality. In a 2D model, triangular and quadrilateral solid elements can be morphed. In a 3D model, tetrahedral and brick solid elements can be morphed.

Note that the wear criterion for nonlinear mesh adaptivity cannot be combined with any other mesh adaptivity criterion.

Wear is a slow phenomenon, and simulating the entire process of wear is often not possible. However, for most cases the rate of wear is very low and changes slowly. Based on this assumption, automatic scaling of the wear increment can speed up the wear simulation.

Use the TB,WEAR command with TBOPT = AUTS to activate automatic scaling of the wear increment. This automatic scaling feature is used in conjunction with the Archard wear model or the user-defined wear model (TB,WEAR command with TBOPT = ARCD or USER).

The wear increment is scaled to represent wear over the long time-period of the physical wear process. Each time increment in the simulation represents a long interval over which the wear rate can be approximated as a constant. The program uses an adaptive, automated scaling of the wear increment to facilitate this process.

The assumption of a constant wear rate is only valid when the contact interface experiencing wear does not undergo major changes such as the surfaces going out of contact or large changes in contact pressure. This is ensured by scaling the wear increment such that it is always within a factor of penetration (which is controlled by a factor of safety).

The automatic scaling process outlined below ensures that the wear scaling is not too large to cause convergence difficulties or inaccurate results, but is large enough to substantially speed up the simulation.

The following example input activates automatic scaling of the wear increment for a specific wear material model identified by MATID:

TB,WEAR,MATID,,,AUTS
TBDATA,STLOC,C1,C2

If TBDATA is not issued, the default C1 and C2 values are used.

If you have more than one Archard or user-defined wear material model defined in your analysis, be aware that the wear rate is scaled for all TB,WEAR materials, even if you specified scaling for only one of those wear materials. In this case, the C1 and C2 constants specified on TBDATA are used for the MATID wear material, and the default C1 and C2 values are used for other wear materials.

If automatic scaling is defined for multiple wear materials in your model, then the final wear scale factor used will be the minimum scale factor calculated among all contact detection points that undergo wear.

A correspondence between simulation time and the actual physical time of wear is provided in the Jobname.cnd output file (which is written only if you issue NLDIAG,CONT before starting the solution). These quantities are shown as "TIME=" and " PHYSICAL TIME FOR WEAR=" in the header for each set of output corresponding to the output frequency requested on NLDIAG.

Automatic scaling of the wear increment assumes that the contact surface undergoing wear is subjected to a constant wear rate and that wear is happening due to reciprocating motion at the interface. However, if the applied load or sliding velocity changes over a period of time, then the automatic scaling will not scale the different time points of the cycle with the same scale factor and will not produce correct results. This limitation is not too restrictive because most parts undergoing wear are usually subjected to constant load with the reciprocating motion at the interface causing the wear.

We can overcome the limitation of constant wear rate during wear for auto-scaling wear model (TBOPT = AUTS) by using a cycle by cycle wear scaling (CBCS) model. In this wear scaling scheme, the wear is scaled at the end of the load cycle. The wear at each contact point at the end of the cycle is scaled similarly to the auto-scaling scheme (AUTS). Since, the scaling is done only once per cycle, at the end of the cycle, this scaling scheme can be applied to a time-varying load where the wear rate is not constant over a simulation cycle.

Apply the CBCS scheme when the time varying load is applied. The CBCS scaling could be applied using the CLOAD command, however, if CLOAD is not used for a load step, that load step is treated as a cycle. Since the whole contact surface that is undergoing wear in the cycle is updated at once, at the end of the cycle, the typical scaling factor will be less than that calculated by the auto-scaling scheme. This method can be activated by specifying the TBOPT = CBCS for TB,WEAR. The command and defaults are similar to the TBOPT = AUTS autoscaling scheme.

TB,WEAR,MATID,CBCS
TBDATA,STLOC,C1,C2

The wear scale factor at the end of the cycle is calculated as:

Wear scale time within the cycle is updated as in auto scaling:

You should keep the following assumptions and restrictions in mind when modeling wear.

The total wear vector components at a node of a contact element are available as NMISC data. To help in the visualization of the surface that has undergone wear, the wear is also added to the nodal displacements in the Jobname.rst file.

The total volume lost due to wear for a contact pair can be tracked via the NLHIST command. NLHIST must be issued before the solution starts.

As wear accumulates, the contact results at the interface change with time. You can automatically terminate the analysis when a desired contact result has been reached (for example, when contact pressure falls below a critical value, or when wear volume exceeds a critical value). Use the STOP_VAUE and STOP_COND arguments of the NLHIST command to specify the criterion by which the analysis should terminate.

For more information on how to review results in a contact analysis, see Reviewing the Results in Surface-to-Surface Contact (Pair-Based).