Modelize a crash-pad in Abaqus?

[olov]

I'm trying to modelize a climber, falling from 5 meters on a crash pad, using Abaqus. A crash pad is used for outdoor climbing and typically consists of:

a 3cm top layer of polyethylene
a 7cm bottom layer of polyurethane

It is used as crash protecetion in climbing up to 5 meters. Thus far we have a dynamic hyperfoam 3D-model consisting of the two layers (1mx1mx0.09m) with a 0.3mx0.3mx0.3m cube (almost solid to modelize the feet) falling on it.

Unfortunately, our currrent results are not very satisfying. With a speed of impact of only 2m/s the climber (cube) bottoms out, which is not at all realistic. We fear that this might be because of the uniaxal test data we've put into Abaqus. We only have access to testing equipment with a maximal compression rate of 100mm/min (compared to 10*1000*60mm/min at impact when fallling from 5 meters ). Is there any reason to continue our model or can compression test data with higher strain rates be found elsewhere? Could the problem be something else?

I would be very grateful if someone could give me some comments about this.


Thank you in advance,

Olov Isaksson

[sq]

Interesting problem, but isn't a 1 cubic foot "foot" pretty big and heavy?

Anyway, while I don't have a lot of research on foam, Chen, et al, Jo. Appl. Mech., 69, 2002 does have some very high-rate compression data on urethane foam you might find helpful. I'm sure there's more recent stuff by now, too.

[Jorgen]

Foams are certainly strain-rate dependent, but the strain-rate dependence of foams is typically lower than the strain-rate dependence of solid polymers. In your case, I would start by making sure that you have tested your foam to large enough compressive strains. As I am sure you know, the compressive response of foams is highly non-linear due to geometric effects of the microstructure, and at large compressive strains the stiffness of foams increases very rapidly. I suspect that your foam model does not accurately capture the large strain compressive response.

You should also consider performing high strain-rate tests of your material, since the response becomes stiffer at higher rates.

- Jorgen

[olov]

Thank you very much for your answers. We continue working on our model but we've now realized that we need the coefficients of the Prony series. Since polyurethane and polyethylene are such common materials one would think that the coefficients already are available in the literature. Does anyone know where? We've been searching quite a lot, but with no luck.

Somle general information: "The modelization of a crash pad" is a student project at Ecole Centrale Paris at undergraduate level. Hence the somewhat basic questions. However, we really appreciate the help.

Olov Isaksson