Skip to main content

Posts

Showing posts from May, 2017

Defining Ansys Superelement SUB File Manually

Photo by  James Owen  on  Unsplash A surprisingly popular blog-post written here is Exporting Stiffness Matrix from Ansys . A sensible follow up question is what can one do with the exported stiffness matrix? In a recent Xansys Forum post, a question was raised on how we can edit the stiffness matrix of a superelement and use it for our model.  An approach presented below is to first create a superelement that has the same number of DOF and nodal location that will serve as a template. An APDL script can then be written to edit the stiffness matrix entries as desired before exporting to a new superelement *.SUB file for use in future models. The self-contained script below demonstrates this.  /prep7 et ,1, 185 mp , ex, 1, 200e3 mp , prxy, 1, 0.33 w = 0.1 ! single element (note nodal locations) n , 1, w, -w, -w n , 2, w, w, -w n , 3, -w, w, -w n , 4, -w, -w, -w n , 5, w, -w, w n , 6, w, w, w n , 7, -w, w, w n , 8, -w, -w, w e , 1, 2, 3, 4, 5, 6, 7, 8 /solu antype , substr     ! analy

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.

Component Mode Synthesis(CMS) Substructure in ANSYS Workbench

Here is a basic example of how one may use CMS inside Workbench with APDL command snippets. These appropriately named superelements has the advantage of being quick to solve once the initial investment is put in. This saves time especially for multiple iteration of the same parts (condensed once with CMS) while changes are performed on a local full model part. The archived workbench file (R18) can be downloaded here   (download icon at top-right). Original Model in ANSYS Work Bench The model is rather simple with 3 parts. The bodies are in this case to be connected with CEINTF . The "MainBody" part will be condensed into a superelement (Craig-Bampton method) and will be used later when connected to "Leg1" and "Leg2". The system natural frequencies are then evaluated. The results are compared to the old fashion way of creating a full model for verification. Step 1 Suppress all other bodies except for the "MainBody" which a superelement

FEA Stress Singularities

Equivalent Stress - 4 Models of Square Hole in Plate  What are stress singularities?  Stress singularities are artificial stresses computed because of simplification in the FEA model. What are the common causes of stress singularities?  Corners: e.g. sharp corners has theoretically infinite stresses because the radius is zero Stiff Boundary Conditions: e.g. a tension bar with one end surface fixed in all direction.  Point loads Constraint Equations, Coupled Nodes etc How does one overcome stress singularities?  1. Submodeling Creating a submodel at the high stress locations with enough detail (e.g. fillet radius) along with mesh convergence study is the way to go. There is still engineering judgement on deciding what radius to use to best reflect the actual part. 2. Stress Concentration Calculation Roark's formulas for stress and strain is a classic that is often used to determine the appropriate factors relative to the nominal stress. This works great if t

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

Precise Mass Summary

Precise Mass Summary Output One of the many checks one can do in FEA is to compute the mass, inertia and center of gravity. Besides doing this for the normal models, it can quickly extract out the mass and inertia of the superelement ( SE ) to verify the MATRIX50  that you're bringing in  has the same mass and inertia as the genuine article. If the mass comes in too low, try specifying more modes in  CMSOPT  to see if it fixes it. My previous co-worker also came across intermittent issues with using constraints equation ( CE ) inside a superelement. He replaced it with really stiff beams instead before generating the superelement. Personally, I would take a bit of time to select master nodes ( M ) not just at the connections but at various points on the model to properly capture the modal mass. This would be similar to identifying accelerometer locations on your part to properly identify modes of interest for a modal test. Here's the command to calculate it all : AL