Look at a hinge on the nearest door. Its halves slide smoothly around each other and around their centering pin. Then look at the ears of a cat or dog as it turns to face some rustle or squeak. Muscles pull on cartilage, twisting each structure around its pivot, and skin stretches to accommodate the motion. These ears are hinges, too, but they’re made from flexible materials that bend instead of solid parts that slide.
That distinction illustrates one great difference between our technology and the technology of the natural world. We prefer stiff materials--metals, ceramics, dry wood, and the like--that keep their shape. Nature, by contrast, commonly reserves stiff materials for special applications--bones, teeth, and so forth--where rigidity is crucial for the devices to work properly. Bones that bend lack standing, so to speak, and soft teeth have no bite to them. But such structures are rare, for nature doles out stiffness with a stingy hand. Her normal criterion is sufficient strength, a different matter altogether from stiffness. Strength means resistance not to bending or to other deformations but to actual breakage, however shape may have changed. In the unstiff, living world, putting pressure on a structure normally changes its shape. Push on an ear and it happily gives. Take your hand away, and it springs back.
It’s rather pointless to ask which criterion is better. The matter is far too subtle for some simple choice that pits human hubris against nature worship. We humans have millennia of accumulated knowledge about how to make things out of stiff materials, and only a really self- deceiving misanthrope would claim that we don’t use such materials well. Nonetheless, when we come to think about problems of mechanical design, we could benefit from taking a nonjudgmental and noncompetitive look at the world of the cat’s ear.
Stiffness, or resistance to deformation, and strength, or resistance to breakage, turn out to have a curious relationship. Things deform before they break, and a structure will often bend quite a bit on the way to snapping. Even if it’s in no danger of actually breaking under a given load, a structure might well need extra support to keep it from bending or buckling. Extra support, of course, means extra material, so it is more expensive to build stiff structures than it is to build merely strong ones. Perhaps a little less obviously, stiff materials tend to break easily--they’re all too apt to be brittle, succumbing with a sudden, catastrophic snap when the pressure’s on.
