Bolted Joint Assembly

Advanced Solid Body Contact Options Goal – In this workshop, our goal is to study the effect that contact stiffness specification has on convergence and result accuracy. Model Description 3D bolted assembly - 4 parts: –Bracket –Bushing –Nut –Bolt Loads and Boundary Conditions: –One fixed support –45,000 N Bolt preload Contact Stiffness

Advanced Solid Body Contact Options Steps to Follow: Start an ANSYS Workbench session. Browse for and open “Bolted_Joint_ws03A.wbdb” project file. – This project contains a Design Modeler (DM) geometry file “Bolted_Joint_ws03A.agdb” and a Simulation (S) file “Bolted_Joint_ws03A.dsdb”. Highlight the “Bolted_Joint_ws03A” file and open a Simulation Session. At this point, solve the analysis. Contact Stiffness

Advanced Solid Body Contact Options Review the contents of the model Highlight each item in the “Geometry” and “Contact” branches of the Project tree to become familiar with the model. Also, review the specifications in the Details Window for each highlighted item. Contact Stiffness

Advanced Solid Body Contact Options Change Pretension load to: 20,000 Contact Stiffness

Advanced Solid Body Contact Options Review the contents of the model (cont’d): Note especially the “frictionless – bushing to bracket” contact region. It will be used to evaluate the pressure profile at the bushing-bracket interface after the bolt preload closes this gap. This region is initially set up as an asymmetric frictionless pair using the Pure Penalty method. Recall that this algorithm depends on a contact stiffness and a very small penetration to generate forces at the interface to prevent penetration once contact is established. Contact Stiffness

Advanced Solid Body Contact Options Under the model branch, highlight “Frictionless – bushing To bracket” In the Details Window: –Change the normal stiffness specification from “Program Controlled” to “Manual” –Change the normal stiffness factor from the default (1) to 10. Solve the analysis again Contact Stiffness

Advanced Solid Body Contact Options Review the Solution Information Branch The red lighting bolts in the Solution branch indicates an incomplete Solution run. Below the worksheet window, the message window contains a summary of warnings and errors suggesting a problem with convergence. By double clicking on each message with LMB, you can expand a message into a separate window for easier reading. Contact Stiffness

Advanced Solid Body Contact Options In the Details of “Solution Information” Window, switch Solver Output to Force Convergence. This displays the same convergence data in graphical form. Note, the force convergence value oscillates up and down between iterations well above the acceptable convergence criteria. After two automatic bisections, substep 1 converges. However, substep 2 ultimately fails to converge. Review the Solution Information Branch (cont’d) Contact Stiffness

Advanced Solid Body Contact Options Open the Solution Information Folder and highlight the Newton- Raphson Residual Force Clearly the source of highest residual (force imbalance) is located along the bushing-bracket contact region. Review the Solution Information Branch (cont’d) Contact Stiffness

Advanced Solid Body Contact Options Review the Solution Information Branch (cont’d): Return to the Solver Output and scroll to near the top of the solution information worksheet. In a summary listing of all the contact pairs, find “Frictionless – Bushing to Bracket”. We see that this region is associated with Real constant sets 15 &16. Contact Stiffness

Advanced Solid Body Contact Options Review the Solution Information Branch (cont’d): Again, scroll further down the solution information worksheet to find the contact specifications and calculations for this set. Note the large default contact stiffness (0.923e6) being used. Given the relatively low stiffness of the bracket feature in this model, it is possible that the contact stiffness being used is too high for this application. Contact Stiffness

Advanced Solid Body Contact Options We will attempt to achieve a successful convergence by adjusting the normal stiffness of Contact Region 6 downward based on the feedback reviewed in the unconverged output. Without changing any specifications in the current tree, duplicate the Model branch as follows: –In the existing Project tree, highlight “Bolted_Joint_ws03A” model –RMB – Duplicate Rename the new model branch to reflect the change that will be made –“Bolted_Joint_ws03A,Norm Stiff Factor = 1e-3” This will enable us to run a modified analysis without losing the existing information. Contact Stiffness

Advanced Solid Body Contact Options Under the newly created model branch, highlight “Frictionless – bushing To bracket” In the Details Window: –Change the normal stiffness specification from “Program Controlled” to “Manual” –Change the normal stiffness factor from 10 to “1e-3”. Solve the analysis again Contact Stiffness

Advanced Solid Body Contact Options The solution now converges successfully in 11 iterations and no bisections. This is ideal. Bisections are a helpful automatic adjustment to achieve a converged solution, but they are not efficient as all the CPU time from the last successfully converged solution leading up to the bisection is wasted. Contact Stiffness

Advanced Solid Body Contact Options Review the Solution Information of the successful run. – Verify in the Solver Output that the modified contact stiffness was used and that solution converged successfully. Contact Stiffness Note: In this analysis, the first load step calculates the necessary interference needed to generate the prescribed preload. The second load step locks the bolt pretension element into this calculated adjustment to achieve the bolt pretension load.

Advanced Solid Body Contact Options Under Solution Branch, RMB>Insert >Probe>Bolt Pretension On Bolt Pretension > RMB>”Evaluate Results” Contact Stiffness

Advanced Solid Body Contact Options Review Bolt Pretension Results: Contact Stiffness

Advanced Solid Body Contact Options Review Contact Results at the bushing - bracket interface (nut side): – Open the Contact Tool Folder and Select “Frictionless - bushing To bracket” – Review Contact Pressure and Penetration results for this pair Is “1e-3” an acceptable normal stiffness factor for this model? The best way to ensure an accurate result with a standard contact pair is to perform a sensitivity study with different stiffness values, stiffness updating schemes and algorithms until results converge to the same “correct” answer. Too high a stiffness can produce divergence, too low a stiffness can produce convergence but possible over penetration, an excessive bolt pretension adjustment and ultimately an inaccurate prediction of surface contact pressure profile. Contact Stiffness

Advanced Solid Body Contact Options Consider the following sensitivity study on the effects of changes to contact stiffness: For this model, as stiffness increases, contact penetration and the required bolt pretension adjustment decreases while maximum pressure increases. Notice also the general trend toward more iterations and longer run times. It is also worth noting the benefit of using the automatic stiffness updating tool between iterations and turning on Large Deflection to achieve convergence at the default normal stiffness factor =1.0. Specifying the right contact stiffness is highly problem dependent and is always a balance between quality of results (accuracy) and cost (run time). Based on this study, a normal stiffness factor of 0.10 would be satisfactory. The Augmented Lagrange contact algorithm has proven to provide more robust contact solutions with many applications and is recommended for frictionless or frictional contact. Contact Stiffness