Load cycling of metal components leads to fatigue and ultimately failure through the propagation of cracks. Koyama et al. took inspiration from bone to develop a steel with a laminated substructure that arrests cracks. The resulting hierarchical material has much better fatigue resistance properties than other iron alloys. The strategy need not be limited to steel; other metal alloys should also benefit from this type of microstructural engineering.
Getting close to the bone is sometimes exactly the right strategy. Mimicking the crack-resistant properties of bone has delivered two new types of steel, which could improve safety in construction and transport applications. Steel is ubiquitous: we use it in everything from cars and aircraft to power plants and bridges. It’s affordable and its alloys can be easily tailored for specific applications. But it is also vulnerable to scratching, which can lead to the development of microcracks that spread over time until the material fails. The changes in air pressure that an airplane is subjected to over its lifetime, for example, can lead to metal fatigue, with potentially catastrophic consequences. An international team of researchers bent on combatting such weaknesses turned to nature for inspiration – in particular, bones. They might seem simple at first glance, but they are lightweight and their multilayered structure gives them good crack resistance.
A typical long bone has a thin outer layer of dense connective tissue covering a lattice-like matrix of cortical bone. Beneath that is a layer of spongy, porous bone, on top of a hollow centre filled with red and yellow bone marrow. Together, this zigzagging hierarchical structure increases the material’s resistance to the proliferation of cracks beyond that of each material on its own, says Motomichi Koyama at Kyushu University in Japan. Koyama and his colleagues altered the nanostructure of two types of steel to mimic the multilayered structure of bone. When subjected to standard stress patterns, the new materials showed better resistance to fatigue.
Even when cracks form, they don’t spread as easily, because it takes more energy to find a path through the complex structure. “The insights into biological strategies to build crack-resistant materials… is an outstanding source of inspiration for the design of advanced materials, including steels,” says Admir Masic at the Massachusetts Institute of Technology. How long will it be before these new steels start appearing in airplanes and bridges ? The biggest barrier is scaling up to commercial production, although Koyama says that conventional steel-making techniques can be used. “So the scaling-up problem can be solved with a small effort.”