Chapter 2: Building Blocks
42 - Resilience (Part Two)
Vincent’s preferred approach to learning from nature is abstraction, borrowing structures or mechanisms from biology and using them to create new materials. An example is the discovery at the University of Reading (his former base), by James Gordon and George Jeronimidis, of what makes wood so tough.
As preface, Vincent tells a story about the attempt by Rolls-Royce in the 1960s to design exceptionally strong jet engine blades using high-tech carbon fiber. The company spent untold sums and the engine was fully developed before they discovered that while the new blades held up well under the intense vibration and fierce heat conditions normal inside a jet engine, they couldn’t withstand collisions with birds—something that can happen to any aircraft. That kind of impact shattered the blades to pieces. The reason was that while carbon fiber is very strong, it’s also brittle. Wood, on the other hand, Vincent points out approvingly, is tough.
He describes how miners prefer wooden beams in their tunnels because the wood “talks to them.” That peculiar characteristic—the creaking and groaning of wood long before it reaches a breaking point—comes from its internal structure.
Wood is basically a thick bundle of hollow tubes carrying water through the body of the plant. The walls of these long, narrow tubes—more properly, tubules—are made of microscopic fibers of cellulose wound spirally, like a compressed spring, and then solidified with a gluey substance called lignin. When wood grain is stressed—for instance, in a support beam of a mine shaft—those coils are gradually pulled open. With that, the lignin begins to fail and miners hear the familiar groaning and cracking. By that mechanism, instead of breaking abruptly—as carbon fiber does—wood gives way only gradually, with the coils unwinding lengthwise for some time before they snap. Miners say that for as long as the wood is talking to them, they know they still have time.
Based on its discovery, the Reading team developed a wood analogue using glass fibers embedded in resin. First they made large flat sheets of that material, with the fibers in each sheet running in a single direction just as fibers in wood grain do. They then layered three sheets together into something resembling corrugated cardboard: a sandwich composed of top and bottom layers separated by a rippled center layer.
Their trick was to angle each of the three layers a little differently, so the direction of the fibers in one layer was shifted alternately some fifteen degrees from the next. Once those were cemented in place, the corrugated channels in the center layer were like tubules. And the slightly varying directions of the grain in each layer roughly mirrored the circular spiraling that creates the tubules found in actual wood. The result? Says Vincent, “Under impact, weight for weight, it is the toughest artificial material known.”
In a footnote to this tale Bill Clegg, an engineer at the University of Cambridge, developed a new generation of super-tough jet engine fuel burners based on his studies of mother-of-pearl, the nacre commonly found inside mollusk shells. Although mother-of-pearl is 95 percent chalk, he says, due to its peculiar composition (millions of chalk micro-platelets randomly layered, then cemented with small amounts of protein glue) it is three thousand times tougher than chalk.
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