Steel with a laminated nanosubstructure, like that seen in bones, is much more resistant to cracking that occurs from repeated stress, a new study reports. Development of steel and other alloys with this structure has potential to improve the safety of buildings and components that experience cyclic, weighty loads. Steel is widely used in many important engineered systems, such as trains, bridges, or power plants, because of its high tensile strength and low cost. As steel undergoes repeated stress, however, it becomes susceptible to cracks. Inspired by the structure of bones, which are endowed with superior crack resistance because of their hierarchical and laminated substructure, Motomichi Koyama et al. suspected that designing a similar substructure in steel could result in a more resilient material. They identified two types of steel with structures comparable to that of bone, ferrite-cementite pearlitic steel and martensite-austenite transformation-induced plasticity steel. The researchers made further enhancements to both types, so they would feature 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, finding that the development of microscopic cracks was delayed until 107 cycles. They say this resistance to cracking is due to something called roughness-induced crack termination; as well, the nature of this material means that, as it undergoes different phases (soft and hard), pressure occurs in a way that prevents cracking. While the laminated steel was resistant to repeated stress, it still cracked under higher initial amplitudes of stress. The authors propose several mechanisms that could address this issue.