# What could be the reason for for the residual error in post-iteration to high....?

Recently I started having this error in my simulations in AceFEM. For me it is quite strange because it happens in a particular step when the convergence has already achieved and then in the post-iteration call the analysis collapses and automatically cuts the step for the next trial. This is the part of the output file:

15:35:19 2261.21 12.80 Standard iteration

15:35:21 2263.26 2.05 K and R loop PARDISO : number of perturbed pivots: 405

15:35:29 2271.71 8.45 Linear solution (650403) Step=27 Iteration=5 Mode(l/g)=0/0 Events=0 |R|=2.37669e-012 |da|=5.37286e-013 Time(t/dt)=0.0630077/9.42236e-005 Multiplier(l/dl)=0.0630077/9.42236e-005

15:35:29 2271.71 0.00 Modified iteration: Residual: 0 Tangent: 1 Solver: 1

15:35:30 2272.32 0.61 K&R assembly failed in element: 1295 SMTConvergence - residul error in post-iteration to high (see SMTConvergence) |R|:1.*^50 Events=0 SMTConvergence-divergence:{"Residul error in post-iteration to high.", "ErrorStatus-2"}

Warning - step cut. Divergence in sub-iterative process. Events=1

Warning - step cut. Solver: near zero pivots. Events=405

As you can see in the penultimate iteration it has converged already but I don't know why it fails in the post-iteration call. Then if I continue the analysis, I will end up with a really small multiplier that makes the analysis extremely slow to proceed.

Have you ever had the same problem? Do you have any suggestions?

• From experience I can tell that it is always a good idea to have a look into the software's documentation could be of interest to you. Look up SMTSetSolver at page 84. Check out SMTSetSolver[1] or SMTSetSolver[2] (I cannot try it myself). May 25, 2018 at 11:38
• I read that. But apparently they have some other LU-solver available with SMTSetSolver[1] (and I would not be surprised if it called the built-in solver "Multifrontal" which is UMFPACK). May 25, 2018 at 11:51