How do I do an impact analysis if using velocity and weight of an item? (enforced displacement)

[glulambeam]

thread825-204956

I have a cover that I need to analyze for an impact load (in this case, a 1 lb object that is dropped from a height of 20 in). I have an analysis document, but the person who wrote it skipped a bunch of steps and also might be too busy to answer any questions/get involved.

Using equations for energy, it's computed that the velocity of the 1 lbs object is 125 in/s when it hits. this velocity is converted into a set of enforced displacements to apply directly to a FEA model of the cover (which is basically a cylinder; there's a cap on the top surface of the cylinder, and a rectangular plate on the bottom surface of the cylinder, and the rectangular piece bolts down at the four corners):

(a) when the impact occurs on the top of the cover (flat), the enforced displacement is 0.0134"
(b) when the impact occurs on the side of the cover (cylindrical), the enforced displacement is 0.0193"

At least these are the numbers shown on the screenshots.



So the questions are:

(1) How do I do the velocity to enforced displacement conversion? I thought this was some sort of response simulation within NX10 for transient load, but that's just a wild guess. I've seen this analysis in a document floating around on the internet from ATA engineering, but they used I-DEAS and some of the steps were unintelligible.

(2) I'm hoping/assuming that I don't need to bust out abaqus to do any of this? I do not know how to use that software and this thing's due in about two weeks (that sounds like alot of time but I have other tasks I have to do as well).


Please advise. Thanks in advance.

 

[janFEA]

Hello glulambeam,

your task reminded me our older example what we use for training purpose:
heavy object is dropped on the transom of ladder. Weight of the object and height is known, so the velocity is known as well.
The object causes large displacement of the first transom, there is assumption that this first transom will not withstand the impact and after its fracture the object will continue to the secound transom - but with smaller velocity (and energy).

Here is the animation what I mean (model and loading is symmetrical, there is therefor only a half)

(1) How do I do the velocity to enforced displacement conversion?

Yes you can of course do it, convert to displacement or acceleration, you can do it "manually" in f.e. Excel in the form of some tab with some very small increment and then import it to NX.
Or there is a special tool in NX, you can covert it automatically.

But why? It seems to me useless.
Firstly think about strategy, which solver you would like to use?
In my example, we need to consider: large displacements and strains, frictional contacts, nonlinear material model, and possibility to change time step in case of divergence.
Modeling of fracture is just a "bonus", model is definitely reliable for simulation of fracture, keep it as example.

It was therefor chosen implicit transient SOL 601-129. Alternatively explicit SOL 701, typical for wave propagation problems.
Here you can define velocity directly, you don't need any conversion.

Is this what you want?

Main advice: keep it simple as possible, reduce complexity, number of DOFs...this is not fun to manage it compared to traditional static structural analyses.

Good luck and regards,
Jan