Notes

The CNKMOD command has the same syntax as the KEYOPT command. However, it is valid only in the SOLUTION processor. The command can be used to modify certain contact element KEYOPT values between load steps in any analysis, including restarts and linear perturbation analyses, as shown in the table below.

In a multiframe restart, CNKMOD must be issued again during each subsequent restart run.

Modifying KEYOPT(12)

A common use for the CNKMOD command is to modify contact interface behavior between load steps in a restart analysis, a linear perturbation analysis, or other types of analyses. This enables you to control the contact status locally per contact pair. The key options that control contact interface behavior are: KEYOPT(12) of CONTA172, CONTA174, CONTA175, and CONTA177; and KEYOPT(10) of CONTA178.

You can change KEYOPT(12) to any value. For example, you can change from standard contact to bonded contact or vice-versa. If an open gap exists at the end of the previous load step and the contact status is adjusted to sliding or sticking due to a bonded or no-separation contact behavior definition, the program considers it as near-field contact when executing CNKMOD in the subsequent load steps.

In a linear perturbation analysis, the CNKMOD command adjusts the contact status from the linear perturbation base analysis (at the point of restart) as described in the table below. It enables you to take points in the base analysis that are near contact (within the pinball region) and modify them to be treated as in-contact in the perturbation analysis (see the "1 - near-field" row with KEYOPT(12) values set to 4 or 5). You can also take points that are sliding in the base analysis and treat them as sticking in the perturbation analysis, irrespective of the MU value (see the "2 - sliding" row with KEYOPT(12) values set to 1,3, 5, or 6).

If an open gap exists at the end of the previous load step and the contact status is adjusted as sliding or sticking due to a bonded or no-separation contact behavior definition, the program considers it a near-field contact when executing CNKMOD in the subsequent load steps.

In the linear perturbation analysis procedure, contact status can also be controlled or modified via the PERTURB command. The contact status always follows local controls defined by the CNKMOD command first, and is then adjusted by the global sticking or bonded setting (ContKey = STICKING or BONDED) on the PERTURB command.

Modifying KEYOPT(3)

Another use for the CNKMOD command is to change the units of normal contact stiffness (contact element real constant FKN) in a linear perturbation modal analysis that is used to model brake squeal. For contact elements CONTA172 and CONTA174, KEYOPT(3) controls the units of normal contact stiffness. You can issue the command CNKMOD,ITYPE,3,1 during the first phase of the linear perturbation analysis in order to change the units of normal contact stiffness from FORCE/LENGTH³ (in the base analysis) to FORCE/LENGTH. Note that KEYOPT(3) = 1 is valid only when a penalty-based algorithm is used (KEYOPT(2) = 0 or 1) and the absolute normal contact stiffness value is explicitly specified (that is, a negative value input for real constant FKN).

Modifying KEYOPT(15)

KEYOPT(15) controls the effect of contact stabilization damping for contact elements CONTA172, CONTA174, CONTA175, and CONTA177. You can use CNKMOD to activate or deactivate the contact stabilization damping between load steps. For example, the command CNKMOD,ITYPE,15,1 deactivates the contact stabilization damping.

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