Common Errors and Warnings in Contact and Convergence
While running the analysis on a model with contacts, a major problem arises i.e. convergence. It is not just because of single reason, that it can be resolved in an easy way. When you come across such problems in different types of analysis, job will terminate by showing an error message or a set of warnings in the analysis. That means the solution is unable to converge.
There are different reasons for why an ABAQUS analysis fails in obtaining the convergence. The main key area we need to look at is errors and warnings. Almost all symptoms of convergence issue are mentioned in the message file. The following are some common set of error and warning messages that arise during the convergence:
- ERROR: TOO MANY INCREMENTS NEEDED TO COMPLETE THE STEP
This error arises mainly because of zero pivot or numerical singularity warnings. Check the message file for any warning message. Check the loads and make sure the model can withstand that amount of load and also increase the limit of maximum number of increments in the step.
- WARNING: ELEMENT 441 IS DISTORTING SO MUCH THAT IT TURNS INSIDE OUT
- Refining the mesh into small element length to improve the convergence.
- By using the complex element type, such as using hybrid formulation, using hourglass enhance technique, etc.
This warning is because of Mesh Convergence and it can be fixed by two methods:
- ERROR: TIME INCREMENT REQUIRED IS LESS THAN MINIMUM SPECIFIED -ANALYSIS ENDS
Analysis terminates because the minimum time increment specified is less to achieve the convergence. In the first step, you need to check the message file to see the warnings and error message. To resolve this error, minimum allowable increment size in the step needs to be reduced to obtain the converged solution.
- WARNING: THE STRAIN INCREMENT HAS EXCEEDED FIFTY TIMES THE STRAIN TO CAUSE FIRST YIELD AT 500 POINTS
It indicates that the analysis is undergoing excessive plastic yielding which leads to solution inaccuracy and convergence problem. This warning is because of unstable material behaviour. The main cause for this warning is insufficient material data with respect to stress-strain data. The other factors that influence strain increment are: insufficient mesh refinement and unstable deformation, such as buckling. It is always better to extrapolate the plasticity data so that the slope is positive over the range of strain.
- WARNING: THE SOLUTION APPEARS TO BE DIVERGING
- Instabilities with respect to contact discontinuity in the analysis directly affect the convergence rate. To overcome local instability due to contact separation, we need to assign the surface wise stabilization in the interaction module by creating the stabilization in contact.
- Contact non-convergence problem relies on the stability of contact. Keeping that in mind, ABAQUS offers contact controls for stabilization in static problems. Apply the contact controls in order to resolve instabilities in the model during analysis.
- Pay attention to warning messages as some of them are specific. If the warning message repeats itself and repeated cutbacks occur, it may indicate a stability issue. This is the most common cause of non-convergence. This can be overcome by specifying the dissipated energy fraction under automatic stabilization in step module.
- One cause for convergence issue is boundary conditions. If the model is assigned with inadequate boundary condition, it can lead to over or under-constrained conditions. Due to unreasonable boundary conditions, warnings will be generated under job monitor.
This warning message is because of a large increment in the step. Automatic time increment resolves this issue by reducing time increment. It is not a cause for convergence problem but such warnings may lead to cutbacks in analysis.
The majority of convergence problems can be resolved with different approaches. Some of the tips needed to be considered while resolving the convergence problems are mentioned below:
The major issue while running contact based problems is convergence and analysis will terminate because of different reasons related to convergence issues. To overcome these problems and to get an accurate output, we need to look at warnings and errors in the message file to judge the aspects responsible for convergence issue in a finite element analysis. Convergence plays an important role in terms of accuracy of simulation problems. So we need to resolve the warnings and errors efficiently to get the required output. I hope this blog has given you the overall solution for convergence problem, considering the most common warnings and error messages.
Ms. Shwetha S is an Engineering Professional in SIMULIA stream with a total experience of 1.5 years. She graduated in Aeronautical Engineering from VTU and is currently working in COE team in EDS Technologies. She has Dassault Systemes’ certifications in SIMULIA and has handled multiple trainings across India for different OEMs, suppliers, engineering service providers and academia. She has managed post-sales role supporting customers across verticals by providing solutions in SIMULIA/Abaqus suite. In her current role, she is working in Abaqus and 3DEXPERIENCE platform.