OptiStruct solves nonlinear set of equations with an implicit solution technique
OptiStruct uses the incremental-iterative procedure based on Newton’s method
The applied load is broken down into small increments
For each of the small increments of load an approximate solution is obtained
Several iterations may be required to obtain a sufficiently accurate approximate solution
OptiStruct checks for convergence on three fronts U,P, and W (Displacement, Load and Work)
Measures error in the nonlinear solution against a predetermined tolerance level
The default error tolerances for the convergence criteria are 0.001 for displacement ,load & 1.0E-7 for work/strain
It is not generally recommended to change the default values. However, varying the tolerances or excluding one or more criteria may affect the result accuracy
Nonlinear expert system
PARAM, EXPERTNL, AUTO (default)/YES/CNTSTB/NO
The expert system monitors the convergence of nonlinear processes and tries to improve the convergence for poorly converging cases
“YES” activates an ‘expert system’ that aids in the convergence. Possible actions like performing additional iterations, under-relaxation, automatic adjustment of the load increment, backing off to the last converged solution and retrying. May lead to a large number of nonlinear iterations.
“AUTO” activates a ‘light’ version of the expert system which facilitates converging nonlinear process in reasonably close to minimum number of iterations.
CNTSTB” introduces temporary stabilization on contact interfaces to improve nonlinear convergence
You can model the shell as membrane, by setting the MID2/3=0.0 (basically we are reducing the bending stiffness to zero, it will act like a membrane). which we can do by editing the PSHELL property card.