Learning Forum

[oglawal2]

Hi all,

 

Posting this as a main post to hopefully get wider reach and comments. So I have two cases: one where I simulate impacts and the second one where I apply the force reaction from the contact region in case 1 to the same model as input (Instead of simulating the impact) but the results don't match. I show where I apply the force and the girder displacements. The one with the larger magnitude is the case of simulating the collision. Any suggestions on how to make them match. 

[dlooman]

In the 2nd model, was it really possible to specify the forces at all the nodes for every time point from the first (impact) solution?  That sounds a little difficult.  In priniciple, it seems like the two analyses should give the same stress.  Were the exact same time steps used in the 2nd analysis as the first?

[oglawal2]

I try to apply the forces at all the nodes it seems the vehicle in model 1 makes contact with. And the force time history in model 2 is from model 1 so it seems the force values at the nodes should match the first solution at all time points? However, the time steps used in the 2nd are not exactly the same as the first...do you think this is the reason for the discrepancy?

[Chris Quan]

If you want to match the results between these two simulations, you should make sure the total momentum applied to the girder is the same. In other words, the area under the time history of the force should be the same. 

In dynamic events, it is not the single value of the peak force that determines the stress and deformation. It is the impulse of the force (force x time) or the momentum transferred that determines the material deformation and stress. 

[oglawal2]

Yes, I don't input only the peak force in model two but the whole time history of the force reaction obtained in model one, so I assume this should make impact impulse the same?