FRIC

Relative motion flag. User subroutine FRIC is called only if the contact point is determined to be closed; that is, if the contact pressure is positive (the contact point was closed in the previous iteration) or if the contact point is overclosed (the contact point was open in the previous iteration).

During iterations LM is passed into the subroutine as the value defined during the previous iteration. At the start of an increment or if the contact point opened during the previous iteration, this variable will be passed into the routine depending on the contact condition in the previous increment. If the contact point was slipping, LM is equal to 0; if the contact point was sticking, LM is equal to 1; and if the contact point was open, LM is equal to 2.

Set LM equal to 0 if relative motion is allowed (either due to slip or elastic stick). In this case the subroutine must specify the frictional stress ${\tau }_1$ (and ${\tau }_2$ for three-dimensional analysis) as a function of the relative sliding motion ${\gamma }_1$ (and ${\gamma }_2$ ), the interface contact pressure p, and other predefined or user-defined state variables. In addition, the subroutine must define the derivatives of the frictional stress with respect to ${\gamma }_1$ , ( ${\gamma }_2$ ), and p. For example, in the case of isotropic elastic sticking, $\partial {\tau }_1/\partial {\gamma }_1=\partial {\tau }_2/\partial {\gamma }_2=k_{elas}$ , $\partial {\tau }_1/\partial {\gamma }_2=\partial {\tau }_2/\partial {\gamma }_1=0$ , where $k_{elas}$ is the elastic stiffness of the interface.

Set LM equal to 1 if no relative motion is allowed; a rigid sticking condition at the interface is enforced by a Lagrange multiplier method. In this case no further variables need to be updated. If LM is always set to 1, a “perfectly rough” interface is created. It is not advisable to set LM to 1 when the finite-sliding, surface-to-surface contact formulation is used.

Set LM equal to 2 if friction is ignored (frictionless sliding is assumed). In this case no further variables need to be updated. If LM is always set to 2, a “perfectly smooth” interface is created.

You can make decisions about the stick/slip condition based on incremental slip information and calculated frictional stresses. These quantities are passed in by Abaqus/Standard, as discussed below.

To avoid convergence problems for the general class of frictional contact problems, set LM to 2 and exit this routine if the contact point was open at the end of the previous increment; that is, if Abaqus/Standard sets LM=2 when it calls this routine, exit the routine.

These values are passed in as the values of the frictional stress components, ${\tau }_{\alpha }$, at the beginning of the increment and must be updated to the values at the end of the increment. Here, and in the rest of this description, Greek subscripts ($\alpha$, $\beta$) refer to frictional shear directions. The orientation of these directions on contact surfaces is defined in Contact Formulations in Abaqus/Standard.

$\partial \mathrm{\Delta }{\tau }_{\alpha }$/$\partial \mathrm{\Delta }{\gamma }_{\beta }$, partial derivative of the frictional stress in direction $\alpha$ with respect to the relative motion in direction $\beta$.

$\partial \mathrm{\Delta }{\tau }_{\alpha }$/$\partial \mathrm{\Delta }p$, partial derivative of the frictional stress in direction $\alpha$ with respect to the contact pressure. Since these terms yield an unsymmetric contribution to the stiffness matrix, they are used only if the unsymmetric equation solver is used (see Defining an Analysis).