Material science, a tale of 4 foams.

Just measured stress-strain curves for 4 foams.

Foam 1. Carpet underlay. Very close to linear elastic, deviates from linear only because foam can’t be compressed 100% because there’s some solid material there to stop that happening. The two curves are two different bearing areas.

Foam 2. Dense bedding foam. Not far from linear elastic but a little memory effect. Memory effect means higher pressure at small deflection at shorter times. Three curves are different bearing areas, the fourth curve failed to wait long enough for memory effect to kick in.

Foam 3. Camera case foam. The foam has an elastic and inelastic component, the inelastic component failed somewhere between 30 and 35 kPa. Thereafter, a relatively low pressure caused a high deflection.

Foam 4. That the hell is happening here? Foam for an exercise mat. Type of foam “Closed cell NBR”. Changing the bearing area has a huge effect on the deflection at given load. So much so that a load of 10 kg causes the same deflection when applied to two different bearing areas that differ by nearly a factor of 2 in area. No memory effect and no failure of an inelastic component.

Material science, a tale of 4 foams.

mollwollfumble said:

… Foam 4. That the hell is happening here? Foam for an exercise mat. Type of foam “Closed cell NBR”. Changing the bearing area has a huge effect on the deflection at given load. So much so that a load of 10 kg causes the same deflection when applied to two different bearing areas that differ by nearly a factor of 2 in area. No memory effect and no failure of an inelastic component.

OK, remeasured the same sample for Foam 4.

This makes a bit more sense, eg. I used a slightly different way of measuring the deflection for the large bearing area, which could perhaps account for the difference in deflection between the large and medium bearing area. Also, bearing area can effectively get larger deeper in the foam, which would also act as a correction in the same direction.

Now, which foam to select?
I’m looking for the minimum foam thickness and cost.
At a thickness of 50 mm:

• Foam 1 costs $45/m^2.
• Foam 2 costs $75/m^2 at 50 mm thick (but also $75/m^2 at 100 mm thick)
• Foam 3 costs $95/m^2
• Foam 4 costs $28.43/m^2 for 46 mm thick.

The foam has to have no less than 20 mm deflection at 50 kPa.
OR no less than 20 mm deflection between loads 20 and 50 kPa.
OR no less than 20 mm deflection between loads 11.25 and 30 kPa.
I’m not sure which, and it makes a difference.

The foam is subject to random high loads so, while it is tempting to take Foam 3 prefail 20 kPa and postfail 50 kPa, that’s not going to happen, the whole foam would have to be overloaded in advance.

So, which foam for minimum thickness?
Which foam for minimum cost?

Oops.

OR no less than 20 mm deflection between loads 11.25 and 50 kPa.

Maybe try some sorbothane foam.

Spiny Norman said:

Maybe try some sorbothane foam.

Looking that up.

“Sorbothane is the brand name of a synthetic viscoelastic urethane polymer used as a shock absorber and vibration damper. … The resulting solid polymer behaves like a quasi-liquid, being readily deformed by an applied force and slow to recover, although in the absence of such a force it takes up a defined shape and volume.”

Is that the same as what some people call “memory foam”? Um, they are both urethane foams, both viscoelastic, i’m not sure.

mollwollfumble said:

Spiny Norman said:

Maybe try some sorbothane foam.

Looking that up.

“Sorbothane is the brand name of a synthetic viscoelastic urethane polymer used as a shock absorber and vibration damper. … The resulting solid polymer behaves like a quasi-liquid, being readily deformed by an applied force and slow to recover, although in the absence of such a force it takes up a defined shape and volume.”

Is that the same as what some people call “memory foam”? Um, they are both urethane foams, both viscoelastic, i’m not sure.

Sorbothane has a really high cost. The most expensive foam of the four is $96 a square metre for 50 mm thichmess. Sorbothane is of the order of $9,500 a square metre for the same thickness.

I need to pre-fail Foam 3 by walking on it and test it with a smaller bearing area to see what happens under a higher pressure. In order to get a realistic comparison.

mollwollfumble said:

Sorbothane has a really high cost. The most expensive foam of the four is $96 a square metre for 50 mm thichmess. Sorbothane is of the order of $9,500 a square metre for the same thickness.

Ouch, I didn’t know it was so expensive sorry.

mollwollfumble said:

Material science, a tale of 4 foams.

Now, which foam to select?
I’m looking for the minimum foam thickness and cost.
At a thickness of 50 mm:

• Foam 1 costs $45/m^2.
• Foam 2 costs $75/m^2 at 50 mm thick (but also $75/m^2 at 100 mm thick)
• Foam 3 costs $95/m^2
• Foam 4 costs $28.43/m^2 for 46 mm thick.

The foam has to have no less than 20 mm deflection at 50 kPa.
OR no less than 20 mm deflection between loads 20 and 50 kPa.
OR no less than 20 mm deflection between loads 11.25 and 30 kPa.
I’m not sure which, and it makes a difference.

The foam is subject to random high loads so, while it is tempting to take Foam 3 prefail 20 kPa and postfail 50 kPa, that’s not going to happen, the whole foam would have to be overloaded in advance.

So, which foam for minimum thickness?
Which foam for minimum cost?

Redone Foam 3 (camera foam) by trampling on it. Apart from 1 point as 23 kPa (oops, didn’t wait long enough for it to compress) it’s an exact match for the previous post-failure curve and extends to higher pressures. New result dashed line.

So Foam 3 (camera foam) gives minimum thickness for two of the three pressure ranges, despite needing to pre-fail it before using it. It’s also the most expensive foam. Don’t you hate it when price is a good indicator of performance.

I think that 0 to 50 kPa range gives a thickness that is unreasonably thin. Different foams come in different standard thicknesses.
Foam 1 can get any thickness above 16 mm to the nearest mm.
Foam 2 basically by manually cutting up 100 mm thick with a hot wire cutter so only 33, 50, 66 or 100 mm thick.
Foam 3 let’s say by manually cutting 50 mm thick with a hot wire cutter so only 25, 50, 75 or 100 mm thick.
Foam 4 by combining units of 12 mm so 12, 24, 36, 48, 60 mm thick.

Foam 2 (dense bedding foam) has the best combination of thickness and price when the 11.25 to 50 kPa range is selected.
Foam 4 (exercise mat foam) has the best combination of thickness and price when the 20 to 50 kPa range is selected.

That’s engineering, folks.

The only foam to lose out totally is Foam 1 (carpet underlay). Uness there’s a cheaper supplier for Foam 1, which is quite possible.

mollwollfumble said:

mollwollfumble said:

Material science, a tale of 4 foams.

Now, which foam to select?
I’m looking for the minimum foam thickness and cost.
At a thickness of 50 mm:

• Foam 1 costs $45/m^2.
• Foam 2 costs $75/m^2 at 50 mm thick (but also $75/m^2 at 100 mm thick)
• Foam 3 costs $95/m^2
• Foam 4 costs $28.43/m^2 for 46 mm thick.

The foam has to have no less than 20 mm deflection at 50 kPa.
OR no less than 20 mm deflection between loads 20 and 50 kPa.
OR no less than 20 mm deflection between loads 11.25 and 30 kPa.
I’m not sure which, and it makes a difference.

The foam is subject to random high loads so, while it is tempting to take Foam 3 prefail 20 kPa and postfail 50 kPa, that’s not going to happen, the whole foam would have to be overloaded in advance.

So, which foam for minimum thickness?
Which foam for minimum cost?

Redone Foam 3 (camera foam) by trampling on it. Apart from 1 point as 23 kPa (oops, didn’t wait long enough for it to compress) it’s an exact match for the previous post-failure curve and extends to higher pressures. New result dashed line.

So Foam 3 (camera foam) gives minimum thickness for two of the three pressure ranges, despite needing to pre-fail it before using it. It’s also the most expensive foam. Don’t you hate it when price is a good indicator of performance.

I think that 0 to 50 kPa range gives a thickness that is unreasonably thin. Different foams come in different standard thicknesses.
Foam 1 can get any thickness above 16 mm to the nearest mm.
Foam 2 basically by manually cutting up 100 mm thick with a hot wire cutter so only 33, 50, 66 or 100 mm thick.
Foam 3 let’s say by manually cutting 50 mm thick with a hot wire cutter so only 25, 50, 75 or 100 mm thick.
Foam 4 by combining units of 12 mm so 12, 24, 36, 48, 60 mm thick.

Foam 2 (dense bedding foam) has the best combination of thickness and price when the 11.25 to 50 kPa range is selected.
Foam 4 (exercise mat foam) has the best combination of thickness and price when the 20 to 50 kPa range is selected.

That’s engineering, folks.

The only foam to lose out totally is Foam 1 (carpet underlay). Uness there’s a cheaper supplier for Foam 1, which is quite possible.

OK, found some cheaper carpet underlay on ebay.

Now
$18 / m^2 for the 11.25 to 50 kPa pressure range (50 mm thick)
$32 / m^2 for the 20 to 50 kPa pressure range (90 mm thick)

Making foam 1 the cheapest overall.
Sigh, does it really matter?

mollwollfumble said:

mollwollfumble said:

mollwollfumble said:

Material science, a tale of 4 foams.

Now, which foam to select?
I’m looking for the minimum foam thickness and cost.
At a thickness of 50 mm:

• Foam 1 costs $45/m^2.
• Foam 2 costs $75/m^2 at 50 mm thick (but also $75/m^2 at 100 mm thick)
• Foam 3 costs $95/m^2
• Foam 4 costs $28.43/m^2 for 46 mm thick.

The foam has to have no less than 20 mm deflection at 50 kPa.
OR no less than 20 mm deflection between loads 20 and 50 kPa.
OR no less than 20 mm deflection between loads 11.25 and 30 kPa.
I’m not sure which, and it makes a difference.

The foam is subject to random high loads so, while it is tempting to take Foam 3 prefail 20 kPa and postfail 50 kPa, that’s not going to happen, the whole foam would have to be overloaded in advance.

So, which foam for minimum thickness?
Which foam for minimum cost?

Redone Foam 3 (camera foam) by trampling on it. Apart from 1 point as 23 kPa (oops, didn’t wait long enough for it to compress) it’s an exact match for the previous post-failure curve and extends to higher pressures. New result dashed line.

So Foam 3 (camera foam) gives minimum thickness for two of the three pressure ranges, despite needing to pre-fail it before using it. It’s also the most expensive foam. Don’t you hate it when price is a good indicator of performance.

I think that 0 to 50 kPa range gives a thickness that is unreasonably thin. Different foams come in different standard thicknesses.
Foam 1 can get any thickness above 16 mm to the nearest mm.
Foam 2 basically by manually cutting up 100 mm thick with a hot wire cutter so only 33, 50, 66 or 100 mm thick.
Foam 3 let’s say by manually cutting 50 mm thick with a hot wire cutter so only 25, 50, 75 or 100 mm thick.
Foam 4 by combining units of 12 mm so 12, 24, 36, 48, 60 mm thick.

Foam 2 (dense bedding foam) has the best combination of thickness and price when the 11.25 to 50 kPa range is selected.
Foam 4 (exercise mat foam) has the best combination of thickness and price when the 20 to 50 kPa range is selected.

That’s engineering, folks.

The only foam to lose out totally is Foam 1 (carpet underlay). Uness there’s a cheaper supplier for Foam 1, which is quite possible.

OK, found some cheaper carpet underlay on ebay.

Now
$18 / m^2 for the 11.25 to 50 kPa pressure range (50 mm thick)
$32 / m^2 for the 20 to 50 kPa pressure range (90 mm thick)

Making foam 1 the cheapest overall.
Sigh, does it really matter?

Where does that leave me? I’m leery of any foam under 40 mm thick or over 60 mm thick.

Lowest price then would be $18/m^2 for 50 mm of underlay foam.

Or would it? That is new underlay foam. There may be a lot of second hand underlay foam floating around at a cost of near zero.

Some new foams are cheaper than that. eg. Jiffy packing foam can be had in bulk (to offset high postage costs from the UK) at $14.4/m^2 for 50 mm thick. NOT to be used without first testing it for elasticity, could be too stiff, like EPR.

So let’s stick to 5 layers summing to 50 mm for underlay foam off ebay – so long as the good price lasts. If the good price doesn’t last, next best for price in the 40 to 60 mm thick bracket would be 48 mm thick of Foam 4 for $30/m^2 at 48 mm thick, quite a jump in price up from $18.

Is Foam 4 available cheaper on Ebay? No.