In the world of materials sciences, metal fatigue describes the weakening of a material that occurs as a result of being exposed to repeated applied work or stress loads. When a material is exposed to a workload that exceeds its acceptable threshold limit, microscopic cracks will form where the stress concentrators are present on the materials such as at the surface, persistent slip bands (PSBs) and grain interfaces1. As these cracks are expressed to a continuous amount of stress, otherwise referred to as cyclic loading, they will increase in size and propagate to a point where the material completely collapses.
The fatigue behavior of each metal is specific, and it can be determined through various different laboratory tests that treat the materials to a specific heat indicating the strength level of the metal2. Several different designs are available in the detection of the fatigue life of the material such as fault-tolerant designs, safe-life designs, etc. While these methods are satisfactory in understanding the potential of these materials to crack, a much better utilization of resources would involve the creation of a material that could prevent these microcracks from occurring altogether.
In following this thought process, a team of researchers at Massachusetts Institute of Technology (MIT) in collaboration with scientists in Japan and Germany have found a way to greatly reduce the effects that metal fatigue pose on materials through their incorporation of a laminated nanostructure into steel3. Inspired by the durability of the human skeleton system and how resistant bones are to cracks while also maintaining their lightweight nature, C. Cem Tasan and his team focused on the hierarchical mechanical structure of bones that accounts for its impressive stress. By manipulating the microstructures of steel to be hierarchical and laminated, the material is able to exhibit a superior crack resistance as compared to any other material that has been used before4.
Three specific aspects of the steel developed by this research group accounts for its combined ability to prevent the spread of microcracks that are typical of this material. The multiple layers present within the steel nanostructure prevent cracks that do form from spreading beyond the layers in which they start3. Similarly, the microstructure of the steel is composed of different degrees of hardness that complement each other. As a result of these varying hardness phases, its response to crack formation requires an energy-intensive path to be followed, therefore allowing for a great reduction in the ability of the crack to spread. The third characteristic of this material involves its metastable composition, which describes the microscopic areas present within the steel that are positioned between different stability states, some of which are more flexible than others. This difference in the phase transitions of the metals allow for an absorption of the energy associated with the spread of microscopic cracks following cyclic loading, which can in fact close the cracks that have been previously formed3.
The advantageous of utilizing the steel material created by this group of researchers are innumerable. While the alloy employed in this material would be more expensive than basic low-carbon steel that may be employed in typical material applications, the benefits associated with its use are exceptional, and could allow for a significant reduction in the amounts of alloying metals required with most materials, which could therefore reduce long-term costs. Automotive and aerospace industries, to name a few, could find particular benefits in the utilization of this material, as its application could prevent many of the failures of these products that are often a result of metal fatigue3. While the metal of choice analyzed in this study was steel, the strategy employed to exhibit its brilliant properties can be applied to a number of other types of metal alloys that could benefit a wide variety of industries in its future application.
- Kane, Jack. "Metal Fatigue." EPI Inc. Web. http://www.epi-eng.com/mechanical_engineering_basics/fatigue_in_metals.htm.
- Kim, W.H.; Laird, C. (1978). Crack Nucleation and State I Propagation in High Strain Fatigue- II Mechanism. Acta Metallurgica. pp. 789–799.
- Chandler, David L. "Conquering Metal Fatigue." MIT News. 09 Mar. 2017. Web. http://news.mit.edu/2017/metal-fatigue-laminated-nanostructure-resistance-fracturing-0309.
- Koyama, Motomichi, Zhao Zhang, Meimei Wang, Dirk Ponge, Dierk Raabe, Kaneaki Tsuzaki, Hiroshi Noguchi, and Cemal Cem Tasan. "Bone-like Crack Resistance in Hierarchical Metastable Nanolaminate Steels." Science. American Association for the Advancement of Science, 10 Mar. 2017. Web. http://science.sciencemag.org/content/355/6329/1055.