Is aluminum foam strong? Or just really cool looking?
December 5, 2011 5:33 PM   Subscribe

Aluminum foam is said to have a compression strength of ~350 psi. Does this mean that it can support up to 350 pounds of stuff on top of it before it crushes?

So, the internet tells me that aluminum foam has a compression strength of around 350 psi, give or take 10 psi. But one site describes its bubbles as being compressible by one's finger: "The bubbles can be collapsed by the intentional and direct force of a finger, so it is recommended that it be out of the reach of the public or otherwise protected (glass covered)."

A friend and I are debating whether aluminum foam blocks could be used as spacing for shelving. The shelves in question are two-inch thick planks of wood and the shelves would hold books and miscellaneous office supplies, well below 350 pounds in weight.

posted by dfriedman to Home & Garden (17 answers total)
It has high compression strength only in one direction. And in that direction, each square inch of surface area can support 350 pounds (psi = pounds per square inch).

But there are undoubtedly a whole host of restrictions to using a super-material like this, like low resistance to shear, twist, bending, or vibration. There are also elastic deformation properties that would make this material bad for your purpose.
posted by supercres at 5:39 PM on December 5, 2011

They'd be fine for shelves provided that the force was always directed completely downwards. They would withstand 350 pounds of pressure from books and things on the shelf, but if someone was to lean against the shelves, or you were to slide something along the surface of the shelf, it would change how the force is directed. I don't know much about this material, but compression strength alone isn't enough to make it suitable for what you want.
posted by twirlypen at 5:51 PM on December 5, 2011

Response by poster: Yeah I don't think the shelves would be placed anywhere where someone would lean against them. He's envisioning using these blocks to make ad hoc shelves that would rest on his desk....
posted by dfriedman at 5:53 PM on December 5, 2011

Response by poster: And his desk is against a wall....
posted by dfriedman at 5:54 PM on December 5, 2011

Actually it probably can't be used for a bookcase. Forces on a shelf in a bookcase are complex and consist of several combined forces. The following list assumes a shelf being supported along the outside edges only by some kind of vertical support. You have vertical compression between any books and the supports directly under them. In the area directly adjacent to the supports you have shear (think of tearing a piece of paper-thats shear force) in the middle of a shelf you have a bending moment with horizontal shear on the top of the shelf and horizontal tension on the bottom of the shelf (the weight of the books is trying to fold the shelf in half about its midpoint). The photo that wilful shows is a case of compressive force only since the mat is supported fully underneath by a rigid structure.

The 350 psi strength is pounds (a unit of force, not mass) for every square inch of material. So if you have a square foot of material you can support 50,400 pounds of force (144 square inches x 350 lbs) assuming the force is spread equally. If you have any point exceeding the yield strength of material the material starts failing at that point, this increases the force on every additional area of the material and the failure cascades. This is why engineers always have a healthy factor of safety whenever possible.
posted by bartonlong at 6:21 PM on December 5, 2011

sorry, should have proofread, the horizontal force on the upper part of the shelf is not shear buy compression.
posted by bartonlong at 6:22 PM on December 5, 2011

Those two sites are hawking two different products. It's entirely possible that the one is suitable for bookshelf supports and the other isn't. The ERG product looks eminently suitable (IANAE) and I'll wager it's priced much differently from the other one.
posted by bricoleur at 7:21 PM on December 5, 2011

I earned a D- in my Mechanics of Materials course, but I do know that many things that can support a bookcase can presumably have aluminum foam glued to the outside so it looks like it's holding up the bookcase.
posted by Homeboy Trouble at 7:29 PM on December 5, 2011

Looking at the bubble, figure out the area that is being compressed, and it's probably a very small fraction of a square inch. Which means its resistance to crush is that same fraction of 350 pounds.
posted by gjc at 7:48 PM on December 5, 2011

There's a question-answer thingy at the bottom of the page you linked to, have you tried asking the horse's mouth?
posted by rhizome at 8:18 PM on December 5, 2011

Remember that a fingertip isn't all that large. If you assume it is a circle of 1/4" radius which actually exerts the pressure then a total gripping or poking pressure of just under 18 pounds would collapse a 350 psi material. The same principal is why a woman in stiletto heals is "heavier" than an elephant when considering compression on flooring materials.

Whether the material could support book shelves probably depends on the design, whether it will subjected to twisting or sideways forces and whether the shelves themselves will be able to act as levers to magnify the force or localize the pressure at an edge of the material.
posted by meinvt at 8:24 PM on December 5, 2011 [1 favorite]

Just to be clear, bartonlong, the OP is not talking about using the aluminum foam as a material for shelves, but rather as a spacer/support for wooden shelves (basically taking the place of the cinder blocks shown here.) I think it could absolutely work if you find foam in big enough blocks, or stack layers of it to create such blocks. Standing an inch-thick piece on edge at each end is going to be a disaster, but filling half the volume between shelves with foam will totally work. You just need to find the right block size for the level of stability you need. I'm not a mechanical engineer and don't know how to calculate those magic dimensions, but I bet if you got a sample of the stuff in hand you'd be able to eyeball it well enough (erring on the side of too much foam, of course.)

As for the finger-poke thing, think about the hard, formed styrofoam used in electronics packaging. That stuff is certainly strong enough that solid blocks of it could be used to support hundreds of pounds of stuff on shelves (I see it used routinely to ship amplifiers weighing on the order of 100lb, so it's certainly subjected to forces much stronger than that when the package goes through the UPS tilt-a-whirl) and yet if you jab it with your index finger you'll leave a dent. Distributing the weight over a large area makes a huge difference.
posted by contraption at 8:26 PM on December 5, 2011

So a stack of these
F                     F
will likely fail, while a stack of these
will be quite stable but will use lots of expensive foam ('F') and offer little room for book storage ('B').
posted by contraption at 8:47 PM on December 5, 2011


To add to what contraption wrote above, bear in mind that books tend to get shoved right to the back of the shelf, so while you might assume that the mass of the books (times the acceleration gravity exerts on them) would transfer directly & evenly through the shelf & evenly onto the surface of the foam supports, if the centre of gravity is way towards the back of the shelf, then the force exerted there would be greater than the force exerted at the front of the shelf.

A simple analogy is to imagine you & a friend are at the ends of a long plank, and at one end a whole bunch of books are stacked. You'd be fighting over who has to lift the heavy end, right?

There's a similar kind of situation if your friend stacks some heavy textbooks on one side of the shelf, and some origami cranes on the other - combined with shoving the books to the rear of the shelf, suddenly one corner is bearing most of the load. What could potentially happen is that the very corner of the shelf could act like the fingertip, and start compressing the foam, then it's all downhill with a bullet from there.
posted by UbuRoivas at 9:33 PM on December 5, 2011

Come to think of it, the unfinished faces of that foam look pretty abrasive, and I would imagine that any book you slide in and out of a spot adjacent to one of the supports would have its cover shredded in no time. Maybe what you need are bookends, or rather things that appear to be bookends but are actually serving the dual purposes of 1) protecting the foam and the book covers from one another and 2) supporting the shelves by being a fraction of an inch taller than the foam.
posted by contraption at 11:17 PM on December 5, 2011

I think the foam would be fine. These are (some) of the properties indicated:

Aluminium Foam
Compression Strength 367 psi (2.53 MPa)
Tensile Strength* 180 psi (1.24 MPa)
Shear Strength 190 psi (1.31 MPa)

Polystyrene Foam
Density (pcf)4.0
Compression Strength 80 psi
Tensile Strength 108psi
Shear Strength 175psi

Bigger is better when it comes to acheiving the calculated properties, a small 1" x 1" cube may not have the properties due to the bubble size (for example, I'm not 100% sure of bubble size) but a 12' x 12' sample (to exaggerate) would closely meet the properties above.

So yeah, I think it would work fine. You need to watch that the blocks don't tip or roll, as they won't be very heavy, I can't think of an elegant way to fasten them, but you could probably put a layer of suitable epoxy on the bottom of the shelf and the top of the block, so that the maximum surface area is bonded, that will help the stability.
posted by defcom1 at 8:00 AM on December 6, 2011

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