Friction

In real-world applications, a majority of contact-based systems are affected by friction-induced shear stresses to varying degrees. Frictional effects occur when two surfaces come into contact and try to move tangentially relative to one another.

A variety of factors affect the amount of friction that exist between the surfaces. Friction is a function of the nature of the surfaces in contact (coefficient of static and kinetic friction) and the normal reaction at the contact interface (Normal force).

It is a highly nonlinear problem in finite element analysis, and should be utilized only if the inclusion of frictional effects is essential to the solution of the problem. The MUMPS solver is used in models involving friction. Friction effects generate unsymmetrical terms when surfaces slide relative to one another. These terms may have a strong influence in the overall displacement field. The unsymmetrical solver (MUMPS) will be used by default when friction is specified. This may lead to slower convergence, however the results are accurate. The MUMPS solver can be turned off if frictional effects are not anticipated to be significant using PARAM,UNSYMSLV,NO.

Friction can be incorporated within the OptiStruct contact interface in two ways. The MU1 (and/or MU2) field on the PCONT Bulk Data Entry, or MU1 field on the CONTACT Bulk Data Entry.

OptiStruct uses the Isotropic Coulomb Friction model to solve the friction problem. Based on the Isotropic Coulomb model, friction, in the real world, can be idealized as an increasing force with zero tangential slip up to the static friction limit. Then sliding is initiated and the frictional force immediately switches to the kinetic friction force. However, in the finite element analysis, such idealization can lead to convergence difficulties due to the presence of extreme discontinuities. Therefore, a nonlinear spring model is used, wherein the transverse frictional force increases linearly with the sliding distance until it reaches the static frictional limit, and then switches to the kinetic frictional force which is constant afterward.

The stiffness of this spring is equal to KT=MU1*STIFF (KT=0.1*STIFF in the case of STICK) in the transverse direction until the static limit is reached. This acts as a linear spring in linear solution sequences. For nonlinear solution sequences, the frictional force increases with sliding distance in proportion to KT until it reaches the static frictional limit force (MU1*Fx), where Fx is the normal force in the contact element. With further transverse deformation, friction becomes kinetic and the frictional force is MU2*Fx.


Figure 1.
Note: The nonlinear contact element's force-displacement behavior may produce negative contributions to the compliance of the structure. As an example, when secondary and main bodies have initial overlap and the contact releases elastic energy during the solution.