#1 2021-10-19 22:32:08

DougAtMcGill

Member

Registered: 2021-10-12

Posts: 3

Multi-step nonlinear dynamic analysis that keeps velocity.. [SOLVED]?

I'm trying to model a mallet head striking a bar using 3D solid elements. The bar is initially at rest and the mallet head is given an initial velocity. Collision between the mallet head and bar is captured using a contact surface and large displacements. After the collision the mallet head bounces away from the bar. At this point in the simulation there are no more forces on the mallet head, so it keeps moving in a straight line until the simulation ends. My goal is to capture the bar's displacement both during the collision and for some time after (where it essentially becomes free vibration with damping).

My question is: is there a way to eliminate the mallet head from the calculation after the collision has finished? At that point I no longer care what happens to the mallet head. However, since it keeps moving in a continuous straight line, its nodal displacements keep growing and growing (which, I imagine, could lead to some numeral problems or otherwise slow down the calculation).

I was thinking perhaps there might be a way to define a multi-step nonlinear dynamic analysis? The first step could capture the collision. Then, after a fixed amount of time, when I can be sure the mallet head is no longer in contact with the bar, I could define a second analysis step. In the second step I could add a new boundary condition that sets the mallet head displacement to zero, while allowing the bar to continue to vibrate. For this to work, the displacements, velocities and accelerations of each node in the bar would need to carry over from Step 1. A document called "Non-linear analysis - Advanced topics" from the Code_Aster website mentioned that displacements, stresses and internal state variables can be carried over between analysis steps, but it doesn't mention velocities or accelerations.

Is it possible to carry over velocities and accelerations between multiple steps of DYNA_NON_LINE analyses and model the mallet/bar impact as I've described? Or, is there another way to model this system so that the solver doesn't need to keep track of the mallet head after the collision?

Last edited by DougAtMcGill (2021-11-01 04:14:43)

Hanbin

Member

From: Shanghai

Registered: 2020-07-19

Posts: 86

Re: Multi-step nonlinear dynamic analysis that keeps velocity.. [SOLVED]?

Maybe you can have a look at ETAT_INIT?  I think ETAT_INIT(EVOL_NOLI=LASTSTEP) can do that with EXIT=(newboundry=xxx) in your new DYNA_NON_LINE.Or I think you can use this with VITE=xxx , maybe xxx= CREA_CHAMP(the velocity you get in the first step)? Hope some other people can give better answers.

sameer21101970

Banned

Registered: 2019-09-06

Posts: 354

Re: Multi-step nonlinear dynamic analysis that keeps velocity.. [SOLVED]?

can you share .med file and give explanation.

DougAtMcGill

Member

Registered: 2021-10-12

Posts: 3

Re: Multi-step nonlinear dynamic analysis that keeps velocity.. [SOLVED]?

Maybe you can have a look at ETAT_INIT?  I think ETAT_INIT(EVOL_NOLI=LASTSTEP) can do that with EXIT=(newboundry=xxx) in your new DYNA_NON_LINE.Or I think you can use this with VITE=xxx , maybe xxx= CREA_CHAMP(the velocity you get in the first step)? Hope some other people can give better answers.

Thank you Hanbin. The EVOL_NOLI seems to work. I had read somewhere else that Code_Aster could only carry displacement between different analysis steps, but it certainly seems to run using ETAT_INIT as you suggested. I presume this is correctly carrying the velocity and acceleration from the end of the first analysis step into the second. I will confirm this at some point after solving other problems with the model.