Modify the control parameters to work best with Simulation Composite Analysis.
In MSC Nastran, the default settings for the nonlinear solution process are based on the fundamental assumption of the Newton-Raphson algorithm. This states that the nonlinear response of the composite structure is sufficiently smooth at both the global and local levels. However, in a progressive failure simulation of a composite structure, the nonlinear response of the composite structure is not smooth, especially at the local level where material failure results in an instantaneous reduction of material moduli. This non-smooth material response is one of the primary factors responsible for the difficulty in obtaining convergence in progressive failure simulations. Simulation Composite Analysis' method of managing material nonlinearity is specifically designed to handle this localized non-smooth material response. However, the default settings of MSC Nastran's NLSTEP entry must be changed to allow Simulation Composite Analysis to improve the convergence characteristics of the finite element simulation.
Create an NLSTEP entry for each step of the analysis. Consider the example below from a bulk data file:
NLSTEP, 1, 1.0 , GENERAL, 1000, , FIXED, 20, , MECH, PV, , 0.005
In this example we have created a step and given it an identification number of 1 with a total time of 1.0. In the GENERAL field we set the maximum number of iterations allowed for each increment to 1000. This is done to significantly increase the number of equilibrium iterations that MSC Nastran will perform before evaluating the need to reduce the time incrementation. In this example we use the FIXED field to set the number of increments in the step to 20. Since our total time is 1.0, there will be 20 increments of 0.05 each. This number can be adjusted depending on your analysis. Finally, we use the MECH field to set the force convergence to 0.005 (0.5%). This step is required for use with Simulation Composite Analysis. The default level of force convergence (0.1 or 10%) is relatively loose and can result in unacceptable accuracy. Refer to the MSC Nastran Quick Reference Guide for further details surrounding the NLSTEP entry.