Determine compressive modulus from compressive test?

[hootrpootr]

Hello,

Is it possible to determine the compressive modulus of a polyethylene foam using the following spec sheet?

https://www.sekisuivoltek.com/wp-content/uploads/2016/03/Minicel_ProductSheet_Type_M_Data.pdf

It doesn’t contain any information regarding the compressive modulus, but does have compression strength in terms of a compressive stress at 25% and 50% compression. Could i just determine the strain at 25% compression (delta L/L) and then simply divide the stress at 25% by that?

Thanks

 

[3DDave]

The compression of foam is non-linear over anything more than a tiny portion of the deflection. It is low at the initial compression and rises. I expect the curve varies with the exact cell shape and ratio of voids to solid. It is also going to be very sensitive to geometry. Anything other than a flat sheet will behave much differently. Recall that ordinary elastic theory is based on infinitesimal, like parts per million, amounts of deflection.

In the document doubling the deflection from 25% to 50% the load more than doubled. Also at the same 25% condition doubling the density more than doubled the load required.

You could do just exactly what you said, but the precision will be low. It may very well be close enough. If precision matters, you'll need to get a representative sample and have testing done.

Were it up to me, I'd do an analysis with +100% and -50% of the rate shown in that brochure. If the design works right with that then it's good enough. If that is too much, get a better curve.

 

[SWComposites]

Concur with Dave.

If you use the two points in the data sheet, that modulus value is only valid for loading between those exact two points. Not between any other points.

 

[hootrpootr]

Thanks Dave and SWComposites! That makes a lot of sense to me. Luckily only rectangular blocks of the foam are required. We’re using them as impact surfaces for a drop test, and the deflection (or compression) relative to the thickness of the blocks should be minimal. Unfortunately we don’t have the capacity nor time to run our own tests. I reached out to the manufacturer, hoping that they’d actually have that data.

So let’s say I want to determine the compressive modulus at 25% compression, or the total length L has been compressed .25L. So then the strain should be .25L/L, or .25, right? And lastly, E = stress /strain = 9/.25 = 36psi (that’s incredibly low for an elastic modulus in my experience).

 

[3DDave]

I prefer to work in PPM, so it would be 9 psi/250,0000 ppm and yeah, it's going to be really low.
The low modulus is usually very much offset by the typical section area used.

 

[IRstuff]

Isn't the compressive modulus supposed to be low; isn't that why one might use such a material for absorbing impacts? Something with a high modulus would resist the impact and result in a higher impact shock.

TTFN (ta ta for now)
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