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

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

(OP)
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?

### RE: LS Dyna Impact Analysis....object not rebounding after impact

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).

### RE: LS Dyna Impact Analysis....object not rebounding after impact

is your object penetrating the surface it's hitting ?

have you tried running a tutorial, test case ?

### RE: LS Dyna Impact Analysis....object not rebounding after impact

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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### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@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.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@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.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

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 ?

### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@ 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.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

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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### RE: LS Dyna Impact Analysis....object not rebounding after impact

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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### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@IceBreakerSours

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

### RE: LS Dyna Impact Analysis....object not rebounding after impact

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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### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@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.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

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

### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@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.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

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

### RE: LS Dyna Impact Analysis....object not rebounding after impact

"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 ??

### RE: LS Dyna Impact Analysis....object not rebounding after impact

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

### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@ rb1957

1st plot is with rigid ram. 2nd plot is with more denser ram to speed up the run. Tank is the same. I will run the 2nd case to 0.3 sec and see what I get.

I use student version to learn at home. We dont get a laptop.

FYI, I use FEMAP to setup and DYNA solver.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@rb1957

My bad. I guess the ram @ 0.3 sec was a different material. Below plot is the same at the 0.5 second but run till 0.3 sec. Either way initial velocity didn't change the behavior much.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

the "Ram" is, I think, the thing hitting the object ("tank" ?) shown. (oh, I think I see now, the ram is that little prism shape ?)

It makes sense for the ram to be rigid. but if the tank doesn't deform (as in your first pic @ 16:20) is the tank rigid material ?

don't understand your last pic ... is all well now ? I would've expected the ram to maintain constant speed until it contacted the tank, then slow to a stop, then the tank would rebound slightly (releasing the elastic energy from the impact). Possibly this rebound velocity would pull the ram away from the tank ?

### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@rb1957

Yes. Ram is that tiny object. Tank is the cylinder.

Tank is regular steel.

Last pic is same at one before except time is 0.3 sec instead of 0.5 sec.

Still working on it. I do expect a rebound but I do not see any.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
It appears that when I use the initial velocity the ram rebounds. But for some reason the tank is rigid.

Below plot is similar to the one @ 16:20 but run till 0.5 sec. You will notice that it has rebounded more than the 0.3 sec plot.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
Yes. Tank should look like pic @ 14:59 and ram should rebound as pic @ 16:20.

On another note, do you how to do a density scale factor? In the analysis I am trying to replicate, the ram and tank densities were scaled to get the right weight. I know I can adjust the material density to get the right weight but that will affect the stresses. So I am guessing there is another way.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

material density should be the correct value, in the correct units ... mass density, not weight

### RE: LS Dyna Impact Analysis....object not rebounding after impact

Why isn't the tank supported to begin with? The wall has contact with the tank but you'd think the tank would be resting on supports. Is this choice (no SPCs) governed by the physical phenomenon you are trying to model? Your modeling choice may be appropriate; I am simply curious.

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### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@IceBreakerSours

This is how the analysis that I am trying to replicate was setup. In reality the tank (actually a tank car) will be on wheels.

Right now my concern is lack of deformation with rebound or lack of rebound with deformation.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
Hi All

I just realized that in plots @ 16:20 and 2:31 the ram doesn't really impact the tank. The contact doesn't work. It just goes through the shell a bit and retreats.

But for some reason when the ram density is increased contact is fine. See pic @ 17:16.

Merry Xmas guys!

### RE: LS Dyna Impact Analysis....object not rebounding after impact

Try increasing contact stiffness (with the correct density for the ram).

Also, make sure the normals are facing each other on opposing surfaces and the entire ram part is moving and not just a subset of its nodes (as you had set it up previously).

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### RE: LS Dyna Impact Analysis....object not rebounding after impact

See attached job; doesn't do much other than fix a couple of things.

SPCs are not needed for wall which has MAT_RIGID assigned to it; you can simply fix the rigid material in the material card itself.

MSTYPE and SSTYPE in contact are back to 3 (instead of 1) as I had suggested previously.

I am not sure about the physics and modeling choices but the overall set-up looks okay now.

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### RE: LS Dyna Impact Analysis....object not rebounding after impact

When I compared the latest version of the job with the first one you had uploaded, I noticed you increased the density by 6 or 7 orders of magnitude. Firstly, that seems odd. Secondly, if the ram is so massive, why should the (unscaled) initial velocity do anything reasonable? I think this explains why the ram doesn't even move in the latest job.

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### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)

I ran your job. There was no movement at all. See plot below.

I have run multiple versions the same model. It is only when I run with the ram having a higher density that the deformation is good. At steel density the ram passes through the tank as the contact is not recognized.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

I was wrong in my thinking.

Don't know how it happened but the ram ended up getting assigned the rigid material (which I fully constrained in the material card). That is why the ram does not move. Once you switch the material for ram back to rigid steel, the job runs just fine with the correct density. I suggest doing sanity checks with various output requests, including ones related to contact.

Unless you were doing it for debugging reasons, increasing the density of a moving object by 6/7 orders of magnitude seems excessive.

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### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@IceBreakerSours

The results are very similar to plot @ 17:16.

I increased time to 0.5 sec and the ram kept on going. No rebound.

I have this feeling that the fix is something very trivial that I am missing. No reason for the ram to keep going. It has to stop and rebound a bit.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

The job is doing precisely what it is set-up to do; I am afraid it is your interpretation where the gap lies.

Plot the displacement history of a node on the ram alongside the visual results, interpret them together, and you will see it for yourself. Pay attention to the first few time steps before the rebound starts.

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### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)

I do not see any rebound. Please refer to my Y displacement plot for a node on the ram. Maybe I am missing something. Also I did not notice any rebound during plot animation.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

It looks like the simulation is working correctly. My guess is that you have one of two issues:
1. Your assumption of what "should" happen is incorrect.
2. There's something wrong with your geometry, material properties, impactor velocity/mass, etc.

This impact looks like there's going to be a lot more plastic strain than elastic strain. So I wouldn't expect much rebound. But that's assuming that the impactor did eventually come to a stop. Based on your displacement-time plot it looks like you have way too high of an impact energy, or your tank is way too compliant. Also, you would save everyone a lot of time trying to guess what you want to do if you would show us the analysis you are trying to recreate.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@swimfan

At 0.5 sec the velocity went from 211.2 in/sec to 211.02 in/sec. The ram has to slow down a bit more than that in 0.5 sec. My guess is that my material is wrong but I am not sure what.

I just have the report from that analysis. What info do you want? I have deformation plots, material curves etc.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

Here is what I see:

Are you using a single precision or an SMP binary? I am using MPP R12 double precision binary on Windows 10.

Your results make me think the ram is not even seeing the cylinder i.e., contact is not working.

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### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@IceBreakerSours

LS Dyna 4.7 Student Version
Windows 10 64 bit

### RE: LS Dyna Impact Analysis....object not rebounding after impact

I think 4.7 is the version of the graphical user interface (LS-PrePost). It's the binary I was asking about. Open the D3HSP file and paste the name of the binary. For instance, mine says: Revision: mpp d R12-1032-gb1be2fbad8

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### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@IceBreakerSours

Revision: smp d R12-1081-gb2b2774cf7

Just saw your post@ 05:30. So your plot says the ram rebounds 12" after around 0.0125 sec?! The contact does work with high ram density. But at steel density it goes through the tank. ie. contact is ignored.

Thanks for taking the time to help me.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

Try using the MPP binary; some contact cards don't work the same way between SMP and MPP binaries.

#### CODE --> LS-DYNA

*MAT_ELASTIC_TITLE
Rigid Steel
\$#     mid        ro         e        pr        da        db  not used
37.33000E-42.90000E10      0.32       0.0       0.0       0.0 

I am using the density you had specified in the first job you uploaded.

I am not sure if I agree with your interpretation of what my plot is showing. The rebound starts at around 0.0125 sec and continues for the rest of the analysis (i.e., t=0.3 sec).

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### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@IceBreakerSours

Looks like my student version doesn't have a MPP option. Just SMP Double.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

Use mortar contact i.e., try adding MORTAR option to CONTACT_AUTOMATIC_SURFACE_TO_SURFACE. You will take a small penalty in terms of computational expense but it is worth a shot if you are looking into options.

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### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
@IceBreakerSours

I think I am close. I thought I got it to work but then I tweaked something and now the ram doesn't rebound unless I reduce its density. Do you know how to use the airbag simple pressure option? The tank is filed with liquid so after impact the internal pressure in the tank will push against the ram. What about mat damage gissmo? I want to delete elements that are above 1 triaxiality. I have a curve for all 3 materials. Thanks.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

(OP)
I finally got it. I had to fix the contacts and tweak the airbag setup. Thanks everyone!!

It is not perfect but close. My only issue is that for some reason my internal energy increases a bit after impact and then remains there. I would expect it to go down after impact.

Anyone here familiar with mat_damage_gissmo? I have these triaxiality models that I would like to use to delete elements above damage factor of 1.

### RE: LS Dyna Impact Analysis....object not rebounding after impact

Material parameters are used to compute contact penalty stiffness factor. I suspect if you play with the contact penalty stiffness, you will see appropriate behavior. In relation to GISSMO, DYNAmore put a few good videos up on YouTube; start there.

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