Notes

The CNKMOD command has the same syntax as the KEYOPT command; however, it is valid only in the SOLUTION processor. The command is intended only for use in a linear perturbation analysis, and can only be used to modify certain contact element KEYOPT values as described below.

Modifying KEYOPT(12)

A common use for this command is to modify contact interface behavior between load steps in a linear perturbation analysis; it allows you to control the contact status locally per contact pair. For this application, the command is limited to changing the contact interface behavior key option: KEYOPT(12) of CONTA171, CONTA172, CONTA173, CONTA174, CONTA175, CONTA176, and CONTA177; and KEYOPT(10) of CONTA178.

The 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 CONTA171, CONTA172, CONTA173, 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).

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