Skip to main content

Newton-Raphson Residues

When doing non-linear analysis, it is good practice to indicate you would like to save the Newton-Raphson Residuals before the analysis is started. It can go a long way in troubleshooting when/if the solution doesn't converge later. You may do so by specifying how many youngest residuals you wish to keep. I personally like to keep 3 to see if it is consistent. Also, increase "Identify Element Violations" toggle to 1 to locate any mangled elements.


Number of Residuals & Identify Distorted Elements

ANSYS now spends a bit of time & resource saving the residuals as it solves each iteration (see below). If you specify only 3 (as suggested above), only the youngest 3 residuals are saved with successive overwriting of the older files. The process is highlighted in the output file below:

file.out showing when residual files are saved

If later ANSYS spouts out colorful language disguised as WARNING or ERROR, you would then see the following objects under Solution Information:

Pointers on why it doesn't converge

The nd001_* are a selection of elements that are distorted. They are sometimes referred to as "Error in Element Formulation". In addition, the 3 Newton-Raphson Residual contour plots highlights the last three iteration's out of balanced forces that are causing difficulties in convergence.

Now What?
These "hot spots" highlighted by the Newton-Raphson Residuals are the causing difficulty for convergence. Tweaking the mesh here may offer some relief. Or decrease the Normal Stiffness Factor of the contact in question to 0.01 if resultant penetration is allowed. Decreasing the initial step size may help. Making those few elements linear could be an option if that approximation is acceptable.

Troubleshooting non-converging solutions can be tough. Hopefully by identifying the offending nodes with the above method, this will help debugging the analysis easier. 

Comments

Post a Comment

Popular posts from this blog

ANSYS User Defined Results

There is an abundant of options in ANSYS classic when one wishes to post process results. ANSYS workbench default pull down menu post processing options are more limited but they can still be accessed via the User Defined Results. One way not commonly used but can come in handy is as follows: Zeroth: Under Analysis Settings, there is "Output Controls" where you can toggle to "Yes" what you would like to save before the solution starts. This is like OUTRES in APDL. Output Controls First: After solving the model, click on Solution in the tree to highlight it. Solution Second: Click on Worksheet in the toolbar. Worksheet Third: In the worksheet, you will see list of results that are saved. Right click on it to create the User Defined Results. Create User Defined Results So here we have it. You could of look up the different expressions in the help document but I find this method of accessing the results convenient.  Example: Aspec

Export Stiffness Matrix from Ansys

It is sometimes useful to extract the mass and stiffness matrix from Ansys.     *SMAT, MatK, D, IMPORT, FULL, file.full, STIFF       *PRINT, matk, matk, txt Exporting mass matrix would be similar:       *SMAT, MatM, D, import, full, file.full, MASS The above script uses APDL Math to get the job done. (Please see previous post for another example). The ordering of the matrix is unfortunately not concurrently exported. To verify the sequencing is as expected, we will work to replicate a truss example in the  Finite Element Trusses course notes by Bob Greenlee. Figure 1: Truss Problem Setup Model Creation Script to create model: /prep7 !! Creates Model to reflect course notes ! Properties et ,1,1  mp , ex, 1, 29.5e6 r , 1, 1 ! Geometry n ,1 $  n ,2, 40 $  n ,3, 40, 30 $  n ,4, 0, 30 e ,1,2 $  e ,2,3 $  e ,1,3 $  e ,3,4 ! Boundary Conditions d ,1,ux,0 $  d ,1,uy,0 d ,2,uy,0 d ,4,ux,0 $  d ,4,uy,0 f ,2,fx,20e3 f ,3,fy,-25e3 ! solves /solu eqslv , sparse

ANSYS APDL Syntax Highlighting editor

Notepad++ with APDL User Defined Language The editor of my choice is Notepad++  with the available User Defined Language Files for APDL. You can install it without administrative privileges via the zip file. The best part of it is, it's FREE! After installing Notepad++, go to "Language>Define Your Language..." then "Import" the XML file downloaded from the above link. Remember to restart Notepad++ so that the language changes will take into effect. Opening up any *.inp or *.ans files should automatically switch highlighting to APDL. I made some minor edits. Here's my XML file: LINK . I also heard Sublime Text and  Ultraedit  has more advance features but they aren't (totally) free.