Applying Enforced Motion
Instead of guessing the ultimate load, I applied Enforced Motion and calculated the reaction force. The primary benefit of this approach is that it minimizes challenges with nonlinear analysis convergence.
Enforced motion is the same approach as used by the stress test machine, where the structure is deformed by X mm to find out its reaction force and material strain. In this model I literally pulled the tow bar by 100mm to see what the result would be.
To better understand the model behavior and prepare accurate Strain-Displacement and Force-Displacement charts I set the result save interval to 1% of the load. So, it gave me loading history with 100 result sets (see video recording).
In addition to the examination of stress plots, I extracted Strain-Displacement and Force-Displacement data from Autodesk Inventor Nastran’s plot tool. Usually, it’s quick and simple to use Inventor Nastran’s built-in graphs for strain and force. But sometimes I prefer to export result values and create charts in MS Excel.
Discovering breaking point
In order to determine a moment when the material starts to break, I extracted Maximum Effective Strain Plastic data from 4 points around the chain ring corner. Then using a MAX function in Excel I created a summary chart and determined a load step when it reaches the material elongation limit (0.2).
As a result we see that the material limit was reached at 45mm displacement. Then using a Force-Displacement chart we can find that the Reaction force at the same moment is approximately 18kN.
Static nonlinear analysis shows that the tow bar breaks at much higher than the design load (18kN vs 5kN). This is a significant insight into design strength that is possible due to allowed large plastic deformations.