Non Linear Iterations and convergence doubt!

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

Thank you once again Rahul!
 

Really helpful information!

Thank you for your time.