help interpreting event simulation results

[prefetch]

hi,

i have a couple of questions about interpreting event simulation results, and so i made a simple event.  

i have a bar fixed on one end made of 6061 aluminum, and a "bullet" made from steel hitting it at 100 mph.  the only settings i change are the "Maximum Elemental Strain for Deletion" which is set to 8.7% which is what 6061 calls for, and the duration i set to .003 seconds.

what i'm trying to figure out is how big of a bar do i have to make to prevent this 100 mph bullet from permanently deforming it.

so i run the simulation, and i'm trying to figure out what the results mean.

at face value, it seems like the bullet bounces off the bar, the bar temporarily deforms but goes back into shape.

this doesn't feel right.

but if i look at the data, it shows max values of strain over 2000 MPa, and if i lookup the properties of 6061 i will find that the yield strength is in the range of 200-300 MPa.

so does this mean that the simulation is showing that this bar would permanently deform if hit by this bullet at 100 mph?

because obviously the simulation, at face value, seems to show that the bar goes back to the original shape and it's a-okay.

looking at another example, if i change the model to make a very thick bar, and run it again and i see the max strain is 635 MPa, does this mean that this very thick bar will also permanently deform?

sorry if these seem like dumb and obvious questions.  🙂

[Anonymous · Accepted solution]

That stress value is a stress concentration and may represent localized yielding but nothing you'd see at the macro scale. 

Your mesh is coarse so dont expect too much fidelity in concentration areas. 

If large scale yielding occurs you will literally see this reflected in the results as only the elastic portion of the deformation rebounds. 

A bullet is traveling closer to 700 mph or much greater depending on the round. 

[Anonymous]

Heres a few videos i made that cover almost exactly what you're doing. 

https://youtu.be/XnZvSMJ1YOk

https://youtu.be/xsSzDIU21KI

[prefetch]

thank you!  this is exactly what i am trying to understand.  i really appreciate the time you took to reply and also thanks for sharing the links to your youtube videos.  those videos are awesome!

with respect to my specific questions:

>so does this mean that the simulation is showing that this bar would permanently deform if hit by this bullet at 100 mph?

>if i change the model to make a very thick bar, and run it again and i see the max strain is 635 MPa, does this mean that this very thick bar will also permanently deform?

it sounds like your answer is "you really need to get your mesh finer to really see what's going on, and if you do then true macro level deformation (if applicable) will be made apparent in the simulation."

is that right?  i'll play around with the mesh values and see what i can come up with.