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No. 2688:
Specific Strength

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.

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 building 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