impact analysis

[AnsysNewUser]

Hi all,

(I am re-posting the same questions as in PTCRO/MECHANICA forum)

I have a project like this:

A cylinder block(the mass is significant) drop on to a sheet of plastic from a certain distance, and this process will repeat 100000 times. The plastic sheet is rigid supported on all edges. The objective is to find out if the plastic sheet is strong enough after these cycles.

I am new in FEA analysis. The question is what will be the step to do this project. I think I have to consider impact analysis as well as fatigue analysis, is this right? By the way, I use Pro/Mechanica.

Thanks!

Anny

 

[Ozimandius]

As far as I can tell you have two problems here:

1. Impact (possibly non-linear)
2. Fatigue ( for plastic )

Since this test is performed ten thousand times I will assume that the drop height is known and constant. On the impact side the first question is this: is the energy of the drop sufficient to cause large deflections (such that the non-linear properties become important). If not, I suggest a simple energy analysis ( deflection of a linear spring/mass dropped from some height ) as a starting point.
Approximate the spring stiffness via the modulus of the plastic section.

The fatigue problem has more uncertainty. I would suggest searching for any empirical results for a similar testing setup. I'm sure the plastics industry must have a standards organization that can help.

For something like this I think that experiment data will provide far more guidance than analysis.

Just my two cents.

 

[AnsysNewUser]

Hi,Ozimandius,

Thank you very much for your valuable post. Since the deflection will not be big, I will start from here as you suggested.

Anny

 

[DickButkus]

Is it a cylinder block or actually a cinder block? If you are dropping a cinder block onto a sheet of plastic from a significant height then good luck - you'll need it! You may as well be using MS Paint for your FEA results.

All teasing aside - if the impact speed is significant, then you better take into consideration the strain rate of the material or your plastic will act way too soft.

I would definitely start with the hand calculations mentioned above. Try to examine the limiting cases. If further study is needed, then look at a code which can do dynamic impact such as Dyna or ABAQUS.

Good luck.
-KF9RI

 

[AnsysNewUser]

Hi, kf9ri,

Thank you for your valuable post.

It is a metal cylinder drop down. The mass is 102kg, and the height of drop will be 150mm.

I don't think Pro/Mechanica can do dynamic impact analysis(though I am not 100% sure since I don't have the module yet), so I will make some "simplify" on this project. First I will find the equivalent static load from impact, and then use this load in fatigue analysis. Do you see any problems in this simplification?

Thanks!