Tag Archives: strength

Transparent, super wood

As mentioned in a previous post, wood is more among the strongest materials per unit weight, making it ideal for table tops and telephone polls. On a per pound basis, most species of wood are more than twice as strong as aluminum or mild steel. Wood’s structure is is the reason; it’s a natural composite of air-filled, aligned tubes of crystalline cellulose, held together by natural glue, lignin.

In terms of raw strength though, pounds/in2, wood is not particularly strong, only about 7000 psi (45MPa) both in tension and compression, about half the strength of aluminum. It is thus not well suited to supporting heavy structures, like skyscrapers. (I calculate the maximum height of a skyscraper here), but wood can be modified to make it stronger by removing most of the air, and replacing it with plastic. The result is a stronger, denser, flexible composite, that is typically transparent. The flower below is seen behind a sheet of transparent wood.

A picture of a flower taken through a piece of transparent super-wood.

To make a fairly strong, transparent wood, you take ordinary low-density wood (beech or balsa are good) and soak it in alkali (NaOH). This bleaches the wood, softens the cellulose, and dissolves most of the lignin. You next wash off the alkali and soak the wood in a low viscosity epoxy or acrylic. Now, put it in a vacuum chamber to remove the air — you’ll need a brick to hold the wood down in the liquid. You’ll see bubbles in the epoxy as the air leaves. Then, when the vacuum is released, the wood soaks up the epoxy or acrylic. On curing, you get a composite strong and transparent, but not super strong.

To make the wood really strong, super-strong, you need to compress the uncured, epoxy soaked wood. One method is to put it in a vice. This drives off more of the air and further aligns the cellulose fibers. You now cure it as before (you need a really slow cure epoxy or a UV-cure polymer). The resultant product have been found to have tensile strengths as high as 270 MPa in the direction of alignment, over 40,000 psi. This is three times stronger than regular aluminum, 90 MPa, (13,500 psi). It’s about the strength of the strongest normal aluminum alloy, 6061. It’s sort of expensive to make, but it’s flexible and transparent, making it suitable for space windows and solar cells. It’s the lightest flexible transparent material known. It’s biodegradable, and that’s very cool, IMHO. See here for a comparison with other, high strength, transparent composites.

Robert Buxbaum, November 10, 2022. I think further developments along this line would make an excellent high school science fair project, college thesis, or PhD research project. Compare different woods, or epoxies, different alkalis, and temperatures, or other processing ideas. How strong and transparent can you make this material, or look at other uses. Can you use it for roof solar cells, like Musk’s but lighter, or mold it for auto panels, it’s already lighter and stronger, or use it as bullet-proof glass or airplane windows.

Wood, the strongest material for some things, like table-tops

Natural wood has a lower critical strength than most modern materials, and a lower elastic constant, yet it is the strongest material for some applications because it is remarkably light and remarkably cheap on a per-volume or weight. In some important applications, high strength per volume is the important measure, and in virtually every case high strength per dollar is relevant. Consider the table top: it should support a person standing on it, as one might do to change a lightbulb, and it should not weigh too much, or cost too much.

A 250 lb man on a table. The table should not weight too much, nor cost too much, yet it should support the man.

I’ve drawn a 9 foot by 4 foot table at left, with a 250 lb person in the center. Assuming that the thickness of the table is t, the deflection in the center, ∂, is found by the formula ∂ =FL3/4Ewt3. Here, F is the downward force, 250 lbs (a bit higher if we include the weight of the table), L is the length between the supports, 6 feet = 72 inches, E is the elastic constant of the table top, 2,300,000 psi assuming ash wood, w is the width of the table, 48″, and t is the thickness, let’s say 1″.

Using the formula above, we fid that the deflection of this tabletop is 0.211″ for a force of 250 lbs. That’s not bad. The weight of the 9′ table top is 125 lbs, which is not too bad either, and the cost is likely going to be acceptable: ash is a fairly cheap, nice-looking wood.

By comparison, consider using a 1/4″ thick sheet of structural aluminum, alloy 6061. The cost will be much higher and the weight will be the same as for the 1′ thick piece of ash. That’s because the density of aluminum is 2.7 g/cc, more than three times that of ash. Aluminum 6061is four times stiffer than ash, with an elastic constant of 10,000,000 psi, but the resistance to bending is proportional to thickness cubed; and 1/4 cubed is 1/64. We thus find that the 125 lb tabletop of Al alloy will deflect 3.11 inches, about 16 times more than ash, far too much to be acceptable. We could switch to thicker aluminum, 3/8″ for example, but the weight would be 50% higher now, the cost would be yet 50% higher, and the deflection would still be too high, 0.92 inches. Things get even worse with steel since steel is yet-denser, a 1/4″ sheet of steel would deflect about as much as the 3/8″ aluminum, but would weigh about twice as muc. For this application, and many others like it, wood is likely the best choice; its light weight per strength and low cost can’t be beat.

Robert E. Buxbaum, January 11, 2022

An Aesthetic of Mechanical Strength

Back when I taught materials science to chemical engineers, I used the following poem to teach my aesthetic for the strength target for product design:

The secret to design, as the parson explained, is that the weakest part must withstand the strain. And if that part is to withstand the test, then it must be made as strong as all the rest. (by R.E. Buxbaum, based on “The Wonderful, One-hoss Shay, by Oliver Wendell Holmes, 1858).

My thought was, if my students had no idea what good mechanical design looked like, they’d never  be able to it well. I wanted them to realize that there is always a weakest part of any device or process for every type of failure. Good design accepts this and designs everything else around it. You make sure that the device will fail at a part of your choosing, when it fails, preferably one that you can repair easily and cheaply (a fuse, or a door hinge), and which doesn’t cause too much mayhem when it fails. Once this failure part is chosen and in place, I taught that the rest should be stronger, but there is no point in making any other part of that failure chain significantly stronger than the weakest link. Thus for example, once you’ve decided to use a fuse of a certain amperage, there is no point in making the rest of the wiring take more than 2-3 times the amperage of the fuse.

This is an aesthetic argument, of course, but it’s important for a person to know what good work looks like (to me, and perhaps to the student) — beyond just by compliments from the boss or grades from me. Some day, I’ll be gone, and the boss won’t be looking. There are other design issues too: If you don’t know what the failure point is, make a prototype and test it to failure, and if you don’t like what you see, remodel accordingly. If you like the point of failure but decide you really want to make the device stronger or more robust, be aware that this may involve strengthening that part only, or strengthening the entire chain of parts so they are as failure resistant as this part (the former is cheaper).

I also wanted to teach that there are many failure chains to look out for: many ways that things can wrong beyond breaking. Check for failure by fire, melting, explosion, smell, shock, rust, and even color change. Color change should not be ignored, BTW; there are many products that people won’t use as soon as they look bad (cars, for example). Make sure that each failure chain has it’s own known, chosen weak link. In a car, the paint on a car should fade, chip, or peel some (small) time before the metal underneath starts rusting or sagging (at least that’s my aesthetic). And in the DuPont gun-powder mill below, one wall should be weaker so that the walls should blow outward the right way (away from traffic).Be aware that human error is the most common failure mode: design to make things acceptably idiot-proof.

Dupont powder mills had a thinner wall and a stronger wall so that, if there were an explosion it would blow out towards the river. This mill has a second wall to protect workers. The thinner wall should be barely strong enough to stand up to wind and rain; the stronger walls should stand up to explosions that blow out the other wall.

Dupont powder mills had a thinner wall and a stronger wall so that, if there were an explosion, it would blow out ‘safely.’ This mill has a second wall to protect workers. The thinner wall must be strong enough to stand up to wind and rain; the stronger walls should stand up to all likely explosions.

Related to my aesthetic of mechanical strength, I tried to teach an aesthetic of cost, weight, appearance, and green: Choose materials that are cheaper, rather than more expensive; use less weight rather than more if both ways worked equally well. Use materials that look better if you’ve got the choice, and use recyclable materials. These all derive from the well-known axiom, omit needless stuff. Or, as William of Occam put it, “Entia non sunt multiplicanda sine necessitate.” As an aside, I’ve found that, when engineers use Latin, we look smart: “lingua bona lingua motua est.” (a good language is a dead language) — it’s the same with quoting 19th century poets, BTW: dead 19th century poets are far better than undead ones, but I digress.

Use of recyclable materials gets you out of lots of problems relative to materials that must be disposed of. E.g. if you use aluminum insulation (recyclable) instead of ceramic fiber, you will have an easier time getting rid of the scrap. As a result, you are not as likely to expose your workers (or you) to mesothelioma, or similar disease. You should not have to pay someone to haul away excess or damaged product; a scraper will oblige, and he may even pay you for it if you have enough. Recycling helps cash flow with decommissioning too, when money is tight. It’s better to find your $1 worth of scrap is now worth $2 instead of discovering that your $1 worth of garbage now costs $2 to haul away. By the way, most heat loss is from black body radiation, so aluminum foil may actually work better than ceramics of the same thermal conductivity.

Buildings can be recycled too. Buy them and sell them as needed. Shipping containers make for great lab buildings because they are cheap, strong, and movable. You can sell them off-site when you’re done. We have a shipping container lab building, and a shipping container storage building — both worth more now than when I bought them. They are also rather attractive with our advertising on them — attractive according to my design aesthetic. Here’s an insight into why chemical engineers earn more than chemists; and insight into the difference between mechanical engineering and civil engineering. Here’s an architecture aesthetic. Here’s one about the scientific method.

Robert E. Buxbaum, October 31, 2013