Two splitting options are available:

For both the SPLIT and DMP options, if there are disconnected regions in a contact pair, the program first divides that pair into separate pairs based on the regions and then continues to split each of those pairs with respect to the original pair size or the user-specified input. See Example: Contact pair-splitting for Disconnected Pairs.

The split contact pairs are stored in the database (Jobname.db file, as well as Jobname.rdb). You can monitor all split pairs after resuming the database files.

If desired, you can issue CNCHECK,MERGE to merge split contact pairs back to their original contact pairs (see Merging Split Contact Pairs for more information).

The split contact pairs are never stored in the results file or the contact diagnostics file (Jobname.rst, Jobname.cnd). Therefore, if the contact pair-splitting logic is used in the Mechanical Application, contact results can only be viewed for the original contact pairs, and the split pairs cannot be viewed.

Example: Contact Pair-Splitting at the Solution Level

The following example demonstrates contact splitting for an engine block model. There are originally 10 symmetric pairs (real constant set IDs 14 to 23). The largest contact pair has real constant set ID = 18.

Figure 3.46: Engine Block Model With 10 Contact Pairs

The command CNCHECK,DMP,,,,,9 activates the solution level splitting logic, which first removes the 5 companion pairs of the symmetric contact. Two large contact pairs among the remaining 5 are split. The largest contact pair (real constant set ID = 18) is split into nine smaller sub-pairs which have real constant IDs 18, 48, 49, 50, 51, 52, 53, 54, and 55.

Figure 3.47: Largest Pair Split Into Nine Sub-Pairs

Since small sliding contact pairs are defined, the split contact pairs are automatically trimmed by the CNCHECK,DMP command.

Example: Contact Pair-Splitting for Pairs with Disconnected Regions

This tire is modeled as two disconnected regions (cut down the middle) that are included in one contact pair. Issuing the command CNCHECK,SPLIT or CNCHECK,DMP splits the two regions into two separate pairs first, and then does further splitting into 34 pairs total.

Figure 3.48: Splitting of Disconnected Regions in the Same Pair

Use the TRlevel argument on the CNCHECK command to define the maximum number of sub-pairs for splitting the largest contact pair in the model. All smaller contact pairs will be split proportionally based on this number.

The higher the number of CPU cores, the higher the TRlevel value should be used. However, every additional splitting attempt will add an extra target surface duplicated from the original target surface. A higher sub-pair number increases overhead due to extra communication among the split contact pairs.

Ansys, Inc. recommends a sub-pair number between 4 and 20. The general guideline is to use the smallest number to achieve an element load balance ratio below 1.5. The element load balance ratio is output right before the solution starts, for example:

     D I S T R I B U T E D   D O M A I N   D E C O M P O S E R
  ...Number of elements: 69763269
  ...Number of nodes:    281275836
  ...Decompose to 128 CPU domains
  ...Element load balance ratio =     1.002

If TRlevel is not specified, the program automatically determines the maximum number of sub-pairs based on the ratio of contact elements of the largest contact pair over the total number of elements in the model, and the number of CPU cores used.

For contact splitting at the preprocessing level (Option = SPLIT only), you can either define specific contact pairs through RID1, RID2, RINC and/or InterType as follows:

For contact splitting at the solution level (Option = DMP only), you can either define specific contact pairs through InterType. Allowed contact pair types are:

Two contact element real constants, BSRL and KSYM, play an important role in managing real constant set IDs for split contact pairs. Their meaning and usage are described here.

Depending on which contact detection setting is used (KEYOPT(4) of the contact element), each contact surface of split sub-pairs may have overlapping layers of contact elements along the split boundaries. These overlapped contact elements are internally created during the splitting process and are used during solution to ensure the same overall solution results are obtained with or without contact pair-splitting. No actual contact results are computed for the overlapped elements.

If you decide to switch to a different contact detection method after CNCHECK,SPLIT has been issued, you should merge the contact pairs (CNCHECK,MERGE) and issue CNCHECK,SPLIT again.

If you add new elements after the CNCHECK,SPLIT command has been processed, make sure the newly created element numbers do not overwrite the numbers of any existing overlapped contact elements. To avoid this problem, it is strongly recommended that you issue the preprocessor-based splitting command right before the very first SOLVE command (that is, after the entire finite element model has been set up).

Use Item = OVER on the element selection command (ESEL) to manage the overlapped contact elements created by the splitting logic. For example, you may need to unselect the overlapped contact elements during postprocessing to get averaged contact results based on the total number of contact elements of the entire pair:

ESEL,U,OVER

The merging process (CNCHECK,MERGE) is the reverse of contact splitting. It simply combines the split contact pairs back to their original pairs.

You should always execute CNCHECK,MERGE before re-issuing the splitting logic for these cases:

The merge process only works on prior split pairs and performs the following steps:

CNCHECK,MERGE is useful for postprocessing the contact results based on the original contact pair geometry. However, use caution when performing a downstream analysis since the merge operation may result in conflicts due to changes in the database.

The splitting logic subdivides large contact pairs into smaller split contact pairs and duplicates the original (base) target surface for every newly split pair, which adds many target elements. In many cases, trimming contact pairs is needed to reduce the overhead introduced by these additional target elements.

For the solver level splitting option (CNCHECK,DMP), the program automatically deletes all inactive contact pairs defined from auto-asymmetric selection (KEYOPT(8) = 2 or 3). It also trims target elements for the split contact pairs that are associated with bonded contact (KEYOPT(12) = 5 or 6) or small sliding contact (KEYOPT(18) = 1). By setting Val1 = TRIM on CNCHECK, the program automatically trims all split contact pairs, regardless of the small or large deflection setting (NLGEOM).

When splitting is invoked at the preprocessor level (CNCHECK,SPLIT), the program only deletes inactive contact pairs. By setting Val1 = TRIM on CNCHECK, the program automatically trims all split contact pairs, regardless of the small or large deflection setting (NLGEOM). You still have full control over which contact pairs are trimmed after the splitting operation. To obtain the best solution performance, you might consider using one or more of the following trim operations:

CNCHECK,TRIM,,,,SMALL    ! Trim off all small sliding contact pairs
CNCHECK,TRIM,,,,BOND     ! Trim off all bonded contact pairs
CNCHECK,TRIM,,,,SELF     ! Trim off all self-contact pairs

In order to obtain the same results with and without splitting logic, the KEYOPT(8) = 3 setting for auto-asymmetric selection is preferred for the run with no splitting. With this setting the contact stiffness and other contact characteristics used for the active contact pair are completely independent of the inactive pair. It is as if the inactive pair does not exist.

The contact pair-splitting logic is applicable for most contact types. However, it automatically bypasses contact pairs in these categories:

In addition, fluid penetration loading is not supported. Therefore, you need to set real constant BSRL = -1 for contact pairs that will have a fluid penetration load at some point it the analysis.

The splitting logic does not support the following analysis types:

Solution Consistency

You may see differences in the solution results due to the following factors: