Chief Delphi - Lignin fiber matrix (wood) as a chassis material

Given the interest in various materials and their suitability, it seems odd to skip over what they are suitable for.

For items that are subject to purely tensile loads, it is desirable to have a material with a high strength to density ratio. It turns out that for common steels and aluminums these are almost the same. If the stretchiness (elasticity) is important, again common steels and aluminums are most the same.

Titanium is sometimes held as a miracle material, but its claim to fame is that its density is about that of aluminum with a strength close to that of steel. Check matweb to see if I got that straight. It's other nicety is that it does not lose strength as drastically as other materials do at higher temperatures.

If steel and aluminum are so similar why the preponderance of aluminum in aircraft?

The first good reason is that aluminum oxidizes so fast and the oxide sticks so well that further corrosion of the metal requires special circumstances - like a nearly closed lap joint with some salt and water. Common steels are either corrosion resistant and relatively weak or strong and tend to develop a flaky rust that does not protect against more rust.

The second good reason is based on this - roughly speaking, the resistance to bending of a flat piece of material is proportional to the cube of its thickness. A piece of aluminum 2 inches thick will deflect 1/8th as much as a piece one inch thick. Aluminum is about 1/3 the density of steel, so an aluminum part can be 3 times thicker for the same weight. It should deflect (1/3)^3 or 1/27th as much

Unfortunately, aluminum is 3 times as elastic as steel. so, in bending, the double thickness of aluminum is only 9 times as stiff as the same weight of steel.

One of the most important tasks a wing skin has is to not wrinkle. One of the best ways to not wrinkle is to have increased bending stiffness.

A similar effect is true when it comes to wood. Its low density can offset its low strength by allowing more thickness for a similar weight. Wood is more complicated because the strength depends on the direction of the load relative to the grain, but its properties in this regard is one of the reasons trees seem to have favored wood, rather than metal. Wood is also much better at avoiding fatigue cracking, a continuous menace in the world of metals.

Additionally, even though wood is combustible, a large wood beam in a building fire is a much safer beam because wood doesn't soften in the heat. The amount of beam remaining is a decent estimator of the amount of strength remaining. That said, fire in not a part of FIRST.

Remember - the largest wingspan aircraft built was built of wood. Yes the A380 wins on some scores, but not all. see en.wikipedia.org/wiki/File:Giant_planes_comparison.svg. Wood construction costs more and, while metal corrodes, wood can rot. Ask Knute Rockne about that.

Some of the most advanced materials today are fibers held together with resins, already mentioned as the way wood is built.