Engines of Our Ingenuity

No. 2688
SPECIFIC STRENGTH

by John H. Lienhard

Today, wood or metal? The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Let me ask you this: How strong is a material? We might answer by reporting how many pounds per square inch of stress it'll take before it breaks. But what about weight? Steel is very strong, but it's also very dense. So we have to ask how many tons of structure it takes to carry a particular load.

I thought about that when I found a short article in the 1914 Scientific American magazine about "The All-steel Aeroplane." The opening sentence says, "It is a curious anomaly that in this age of steel construction [so much wood is used in] building ... aeroplanes." Early aeroplanes had to be light and strong, of course. But they also had to be stiff enough to hold their aerodynamic shape. The issues of weight and strength were being brought into sharp focus by these new flying structures.

Thomas Morse S-4 fuselage frame
The wooden frame of the 1917 American-built Thomas Morse S-4 "Scout" at the Pioneer Flight Museum in Kingsbury, TX. This was typical airplane construction of the time.

So consider something called the specific strength of a material -- its breaking stress divided by its density. Balsa wood has a modest tensile strength, but it's also very light. So its specific strength is actually twice that of steel!

Specific strength has a clear physical meaning. Within Earth's gravity field it's exactly the length of the longest hanging rod that won't break off where it's attached under its own weight. Imagine unreeling a steel wire from high above the ground. That wire will finally break off when it reaches a length of 16 miles. A hanging balsa rod would have to be 33 miles long before it failed under its own weight.

Aluminum, or good paper, would break a little before steel. A rod made of carbon nanotubes might reach five thousand miles in length before it broke. And so on.

So we're back to that 1914 article suggesting a steel airplane. The German airplane builder Junkers made an all-steel airplane the next year. It was only a curiosity, but others were soon building airplanes on tubular steel frames.

Fokker Triplane structure
Model of the Fokker Dr-1 Triplane at the Lone Star Flight Museum in Galveston. This model shows the aeroplane's avant garde mixed steel and wood structure. Fokker's design was influenced by Junker's ideas.

Now consider this: a wooden member of equal strength weighs more than a steel member, but how do we make that aeroplane structure stiff? That was easier to do with wood. For an all-metal airplane, we had to invent more creative metal structures. And, of course we were helped by the increasing availability of cheap aluminum -- still something of a rare wonder-metal back in 1914.

When I recently did a program about concrete ships, a friend asked, how in the world could a concrete ship float? Of course the same question might be asked of a steel airplane. Airplanes and ships take us off our familiar earth. Both do so by making full use of the attributes of materials from which we form them.

So one last example: Glass has a breaking length of 83 miles. It is very, very strong. Yet we'd only have to tap that hanging rod with a teaspoon to destroy it. That's a fine reminder of the way clever engineers need to capitalize on strength and walk around weakness at the same time. When we do that, we can make the strangest materials serve us in most unexpected ways.

I'm John Lienhard at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)


See the Wikipedia article on Specific Strength. See also this government account of aluminum production over the past century.

The All-steel Aeroplane. Scientific American, Sat., May 16, 1914. Pg. 408.

Very germane to the issue of specific strength are the current considerations of the possibility of builing a "space elevator" to carry people and goods up to geosynchronous orbit. see, e.g., the 1993 view in Episode 859 or the more recent Wikipedia article on the subject.

Structural engineers will appreciate the extent to which I've had to dodge a host of issues in my limited time frame -- the mechanics of tensile failure, the roles of bending stress, truss layouts, and more, in structural design, a proper examination of stiffness in design, the role of inclusions and flaws and their relation to size in materials failure. And so on.

Thanks to Mech. Engr. Colleagues Haleh Ardebili and Keith Hollingsworth for their counsel. Photos by J. Lienhard



The Engines of Our Ingenuity is Copyright © 1988-2011 by John H. Lienhard.