Hey everyone,

I hope you're doing well!

I have a question regarding my approach to my final school project. I'm simulating the impact of a bike frame falling from a height of 300 mm. To predict stress flow and strain, I'm using the Johnson-Cook plasticity model. I haven't included any damage model yet, though I'm concerned about whether cracks might appear in the frame. I plan to compare the simulation results with physical tests, focusing on plastic strain, and I’d love to hear your thoughts on this approach.

My approach so far has been to model the ground as close as possible to the roller (the area where the frame impacts the ground) and apply some initial velocity to the frame, bypassing the free fall time to avoid unnecessary calculations. In reality, after the first impact (where the highest stresses occur and transfer through the dummy fork to the roller), the frame rebounds and moves back and forth several times before reaching its steady state. Energy dissipates through plastic deformation, air resistance, and material damping, I assume.

What I've observed in some simulations is that stress takes time to propagate through the frame (which is expected) after the impact. So, even after the highest stress occurs in the impact zones, I still need to analyze several more frames to identify the regions with maximum plastic strain.

My question is this: after the first and possibly second impact (rebound), the stresses should not exceed the values reached on those impacts, correct? This means that the stresses should not cause additional plastic strain in the frame from that moment on. The Johnson-Cook model updates the flow stress (the stress required to continue plastic deformation) based on the equivalent plastic strain, which is updated in each iteration according to the current strain rate. However, if the stresses are not increasing, there should be no further plastic deformation, right?  This way, I can avoid running the simulation until it reaches a steady state and instead focus on the results from the earlier frames.

I've also tried some simulations with mass scaling, but I'm not entirely sure how this affects the values, given that gravity (inertia) is incorporated into the model. Scaling the mass might impact those values, and I'm trying to avoid this. On the other hand, I'm struggling to run the simulation without mass scaling due to long run times. If anyone has any tips or recommendations for optimizing this, I'd really appreciate it.

This is the frame falling test so you can have a better perspective on what I'm talking about.

Key
1 wheelbase
2 permanent deformation
3 mass 1 (m1)
4 mass 2 (m2)
5 mass 3 (m3)
6 rigid mounting for rear-axle attachment point
7 steel anvil - (we are not worried about the deformation on this body)

If necessary, I can create a simplified model to better illustrate the issue.

Thanks for your help!