Description: Abaqus/Standard now considers step-dependent modifications to friction coefficients in the logic associated with the default selection of a symmetric or unsymmetric solution scheme. Unsymmetric stiffness contributions associated with friction tend to become more significant as the friction coefficient increases. Use of an unsymmetric solution scheme approximately doubles the computational cost of solving a linearized system of equations. Therefore, the friction coefficient value may influence whether a symmetric approximation of the stiffness is more appropriate (based on trade-offs between CPU time per iteration versus convergence behavior in nonlinear steps and CPU time versus solution accuracy in linear perturbation steps).

One of several criteria for invoking the unsymmetric solution scheme by default in past versions of Abaqus/Standard was a Coulomb friction coefficient exceeding 0.2 in the model definition; however, changes to the friction coefficient during an analysis were not considered in this logic. Now if a friction coefficient exceeds 0.2 for a portion of a step, all increments of the step use the unsymmetric solution scheme by default. As in past versions, you can explicitly specify use of a symmetric or unsymmetric solution scheme rather than relying on the default setting.