Bone-inspired steel

Load cycling of metal components leads to fatigue and ultimately failure through the propagation of cracks. Taking inspiration from bone structure, an international team of researchers led by Motomichi Koyama of Kyushu University in Fukuoka, Japan, developed steel with a laminated substructure that does not crack in spite of repeated application of pressure.

The research found that steel with a hierarchical laminated nanosubstructure, like that seen in bones, is much more resistant to cracking that occurs from repeated stress. The resulting “hierarchical material” has much better fatigue resistance properties than other iron alloys. This kind of microstructural engineering is applicable to other metal alloys too, the authors said. The research was published in a recent issue of the journal Science.

The research group identified two types of steel with structures comparable to that of bone: ferrite-cementite pearlitic steel and martensite-austenite transformation-induced plasticity steel. Making further enhancements to both types, the team introduced additional bone-like characteristics that resist crack propagation. In subsequent experiments, both were found to be significantly more resistant to cracking than a type of steel typically used in automotive systems.

The authors subjected the laminated steel to repeated cycles of stress and found that the development of microscopic cracks was delayed until 107 cycles. They say this resistance to cracking is owing to something called roughness-induced crack termination. Development of steel and other alloys with this structure has the potential to improve the safety of buildings and components that experience cyclic, weighty loads.

However, while the laminated steel was resistant to repeated stress, it still cracked under higher initial amplitudes of stress.

The authors have proposed several mechanisms that could address this issue.