Pre-fatigue training technique doubles the performance of high-strength steel

A NIMS research team has discovered that the fatigue limit of steel is improved by prior cyclic deformation (fatigue) training. Based on this finding, the research team developed a novel pre-fatigue training technique, which successfully doubled the fatigue limit of high-strength steel by suppressing crack initiation.

This strategy offers a versatile approach to improving the fatigue limit in general steels, providing an effective alternative to tempering heat treatment that inevitably sacrifices tensile strength. The research is published in Advanced Science.

Fatigue limit—the stress level below which a material can endure for an infinite or sufficiently large number of loading cycles without failure—increases proportionally with tensile strength in steels. However, when the tensile strength exceeds 1.4 GPa (gigapascals), further increases in the tensile strength do not improve or rather decrease the fatigue limit, that is, the fatigue limit ceiling. In addition, martensitic steel, a representative high-strength steel, generally exhibits a low fatigue limit in the as-quenched state with the highest strength level.

As a result, before practical applications, martensitic steels are typically tempered to improve fatigue performance, sacrificing the strength level. The detailed mechanism behind the fatigue limit ceiling remains unclear, and there has been a strong demand for materials design strategies to overcome the ceiling.

Key findings

The research team successfully doubled the fatigue limit of as-quenched martensitic steel with a tensile strength of 1.6 GPa, thereby overcoming the fatigue limit ceiling. This was achieved through pre-fatigue training, which was performed under the loading condition that did not cause crack initiation.

In-depth analysis revealed that the predominant factor of fatigue crack initiation in high-strength steels is the elastic misfit—i.e., the elastic strain mismatch in the loading direction—at grain boundaries. This study is the first in the world to demonstrate that fatigue deformation, conventionally considered harmful, can suppress the above crack initiation mechanism.

Future outlook

Unlike tempering heat treatment, the pre-fatigue training improves the fatigue limit with minimal reduction in tensile strength, making it a promising approach applicable to general high-strength steels. In addition, this study demonstrated that suppressing crack initiation, rather than the conventionally focused crack termination, is the key to improving the fatigue limit of high-strength steels.

The research team further aims to develop this “microstructural design strategy for crack-initiation-resistant materials” and apply it to fracture phenomena in a wide range of materials, including steels, which significantly contributes to making the social implementation of ultra-high-strength materials become more feasible.

This study was conducted by a research team consisting of Kazuho Okada (Senior Researcher, Steel Research Group (SRG), Research Center for Structural Materials (RCSM), NIMS), Kaneaki Tsuzaki (former Research Fellow, SRG, RCSM, NIMS), Eri Nakagawa (Ph.D. student, SRG, RCSM, NIMS), and Akinobu Shibata (Distinguished Leader, SRG, RCSM, NIMS).