LS Dyna Impact Analysis....object not rebounding after impact

[jhp1981]

I am new to LS Dyna. I setup an analysis in FEMAP and ran it using Dyna solver. The results look OK except for the fact that my object doesn't rebound after impact. It keep going. I ran it to 1 sec. What am I doing wrong? Material?

 

[FEA way]

I don’t have access to LS Dyna to open this file but make sure that you properly defined boundary conditions, initial velocity and properties of the materials (elasticity, plasticity, sometimes also damage).

 

[rb1957]

is your object penetrating the surface it's hitting ?

have you tried running a tutorial, test case ?

another day in paradise, or is paradise one day closer ?

 

[IceBreakerSours]

Firstly, you are running an explicit job with termination time = 0.5 sec (which, although not extremely rare, seems like a lot for an explicit run). Also, it looks like you are not displacing the whole part (Ram); just nodes on a subset of the faces with boundary conditions on each DOF of each node. All shell element formulations are type = 2. Finally, in my experience, contact in Dyna seems to work well when the contact entity types are either parts or part sets, instead of segments (element faces). Taken together, I think think defaults generated by FEMAP might be partly to blame.

In my experience, as a beginner in a new tool, it is best to learn it as intended to be used by the designers of the tool (i.e., reading the manuals of the tool, immersing yourself in examples, working with others who use the same tool, etc.). If you try to use the new tool as you imagine (coming from a background with another tool), you may be in for some surprises.

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[jhp1981]

@IceBreakerSours

Thank you for your inputs. Below are my responses.

1. I am trying to replicate another analysis and that went to 0.35 sec. So I setup mine to terminate at 0.5
2. I fixed the velocity on the ram in DYNA. FEMAP doesn't do it properly.
3. 'All shell element formulations are type=2' Is that wrong?
4. I changed contact to parts.

I ran the job again with the changes mentioned above. Also I increased density to speed it up. No difference in deformation. Similar deformation as before with same density. Still no rebounding.

 

[jhp1981]

@rb1957

No penetration. If I increase the time the ram just keeps moving forward while stretching the tank as if it were made up of chewing gum.

 

[rb1957]

I can't open LS-Dyna files. Having trouble understanding your problem ... "my object doesn't rebound after impact. It keep going." and "the ram just keeps moving forward while stretching the tank as if it were made up of chewing gum.". but hopefully others can help.

have you done tutorial problems ?

another day in paradise, or is paradise one day closer ?

 

[jhp1981]

@ rb1957

In reality after impact the ram should stop after around a fraction of a sec and retreat a bit. In the case study I am trying to replicate this happens at 0.35 sec.

I did check out tutorials on YT. I do not see anything different that they did to have that rebound effect.

 

[IceBreakerSours]

2a. My point was that the whole Ram part is not moving; only the faces whose normals are NOT pointing towards the Shell (cylindrical part).
2b. Normals for the element faces on the Ram pointing towards the Shell (cylindrical part) need to be reversed.
3. No, it is not wrong. However, it is unusual to have the cheapest element formulation defined for every component.

5. Normals on the Wall are pointing away from the Shell. Reverse the Wall normals.

This explains why the contact was not being resolved.

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[IceBreakerSours]

Update:

I managed to get the job to run after I took care of the issues mentioned in my previous response (2a and fixing those normal directions). I made a few additional changes; it is not perfect but should be a decent starting point.

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[jhp1981]

@ IceBreakerSours

I reversed the normals for the wall and the ram face. No change.

 

[IceBreakerSours]

A working model is attached in my previous response.

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[jhp1981]

@IceBreakerSours

Your model is very similar to mine. I do not see any change in deformation.

 

[IceBreakerSours]

I did not run the job to completion; I just got it to run in a way that seemed to make sense. Can you upload an animation or two and explain what you think is wrong or what you think ought to happen instead? Can you plot some history variables and see if those make sense?

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[jhp1981]

@IceBreakerSours

This is my results at 0.5 sec. It looks good. Pretty close to what I am trying to replicate. But that analysis had max ram displacement at 0.35 sec. After than the ram rebounded a bit. Mine keeps going even up to 1 sec at which point the tank cylinder is extremely deformed!

Thanks for your help. Much appreciated.

 

[rb1957]

is the ram being driven by an initial velocity or a constant velocity ?

another day in paradise, or is paradise one day closer ?

 

[jhp1981]

@rb1957

This is my plot with initial velocity. The ram appears to rebound after barely touching the tank. Now if I could merge the 1st plot deformation with 2nd plot rebound I would be on track.

 

[jhp1981]

This is my plot @ 0.5 sec with initial velocity. Now the ram doesn't rebound as above. But deformation looks right.

 

[rb1957]

"This is my plot with initial velocity. The ram appears to rebound after barely touching the tank." ... that sounds like a rigid tank ?

all you changed between your two pictures is the time ? that's odd ??



another day in paradise, or is paradise one day closer ?

 

[rb1957]

you're using an educational license for work ... ???

another day in paradise, or is paradise one day closer ?