## LS Dyna

#### Which tool should I use?

• Catacaldos
Subscriber
Hi all.nI have to simulate an impact of a pendulum on a test piece. Normally, it wouldn't have any problem, but the impact has to be repeated. That is, the pendulum strikes the specimen, returns to the initial position, and strikes again. This process is repeated for X times. Which tool can I use to perform the test? Explicit dynamics? Static structural? I really don't know...nThank you very much.n
• Missy Ji
Ansys Employee
Generally impact analysis are handled by explicit dynamics, you can use either Ansys Explicit Dynamics or Ansys Ls-Dyna, both can accomplish this goal.nn
• peteroznewman
Subscriber
nI have seen this question asked several times in the last couple of years, but I have not yet seen a detailed answer on the workflow to achieve repeated impacts.nOne idea, that is very constrained in its implementation details, is ball impact where X balls are lined up in a row and given an initial velocity toward the test piece. The test piece is angled so that each ball bounces off in a direction that clears the way for the next ball to impact. The balls have to be spaced in the setup so they don't collide with each other. Requiring an angle of impact probably is a significant change from the physical test.nnI don't know how to have a single ball make the first impact with the test piece, then have the simulation restarted with the single ball returned to its starting position for the next impact while the test piece remains deformed and retains the stress of the last impact for the next impact, and to repeat that process X times. Please provide details on how this might be done in Explicit Dynamics.n
• Missy Ji
Ansys Employee
One can define multiple steps in Ansys explicit Dynamics, nstep 1, with a very short duration, prescribe a velocity. nstep 2, deactivate the velocity BC, let impact happen, solver calculate the part velocitynstep 3, prescribe the velocity BC to the second impact velocitynStep 4, deactivate the velocity BC and second impactnnWith Ls-Dyna, you can define the velocity with keyword.n
• peteroznewman
Subscriber
ArraynThanks for this idea.nStep 1 can use a Velocity Initial Condition and have a step end time long enough to allow the deformation of the part and some rebound of the hammer.nYou need a velocity BC to activate and deactivate to get the Initial Velocity effect on subsequent steps for a very brief time so it is not part of the impact, because you can't set an Initial Condition for the second and subsequent steps.nThis would only work well for the case when there are constraints keeping the hammer from having only 1 DOF, the impact direction.nThe End Time of each Step would need to be carefully selected so the hammer has either stopped moving forward or is rebounding.n
• Catacaldos
Subscriber
Thanks for your answers.nBut, if I make this, the computer is going to be thinking for days, don't it?n
• peteroznewman
Subscriber
Yes it will.nThere is a problem using this approach of assigning a velocity BC over a very short time during a simulation. It is not the same as an Initial Condition on the body, which assigns that velocity to every node in the body.nWhen I have tried to assign a velocity BC in liu of an initial condition, I could not pick a Solid, only a Face. Even if I pick every bounding face of a body, the nodes on the inside of the body did not get assigned the velocity BC. I tried using a Nodal Selection that included all the nodes in the body, but I could not use a Nodal Selection as a Velocity Boundary condition.n
• kaitova
Subscriber
sorry to disturb you. In this sentence do you mean that we have only one DOF for the part? That it for example moves back and forth along y axis only? nThis would only work well for the case when there are constraints keeping the hammer from having only 1 DOF, the impact direction.nThank youn
• kaitova
Subscriber
,is it possible to use ls dyna, as there you can for sure choose solid body to assign velocity. I have tried very simple simulation on this, that I got help with from college. For the velocity set up,is it possible to set it up as tabular data? Then velocity can be set up as zero several times, but I don't know if it can be written for initial velocity. Then it's possible to set up analysis duration to make an object hit several times (but in this case it doesn't bounce (?)). nThank youn
• peteroznewman
Subscriber
Yes, LS-DYNA is a far more capable and fully-featured solver compared with AUTODYN, but the free Student license provides AUTODYN and not LS-DYNA. I haven't used LS-DYNA.n
• kaitova
Subscriber
sorry, I didn't know that. Seems I am lucky to have it. Thank youn
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