A Japanese research team has found a surprising way to make steel much tougher against repeated stress.
Instead of weakening the metal, carefully applied “fatigue training” before use actually makes it stronger, doubling its resistance to cracks.
The discovery could change how high-strength steels are prepared for use in cars, airplanes, and other demanding industries.
The study was led by scientists at the National Institute for Materials Science (NIMS) and published in Advanced Science.
Their work focuses on a long-standing problem in metallurgy known as the “fatigue limit ceiling.”
The fatigue limit is the maximum stress level that a material can endure again and again without breaking.
For most steels, this limit increases as their tensile strength (resistance to stretching) goes up. But once tensile strength passes about 1.4 gigapascals, the fatigue limit stops improving. In fact, some steels with extremely high tensile strength actually become more likely to crack under repeated stress.
This has forced manufacturers to use tempering heat treatments, which improve fatigue performance but lower the overall strength of the steel.
The NIMS team wanted a way around this trade-off. They turned to martensitic steel, a widely used high-strength steel, which is usually tempered before practical use.
Instead of tempering, they tried something new: putting the steel through a controlled series of stress cycles that were strong enough to “train” the metal, but not so strong that they caused cracks. This pre-fatigue training had a surprising effect.
The researchers discovered that the main reason cracks form in very strong steels is the mismatch in elastic strain at grain boundaries—the tiny edges between crystal structures inside the metal.
Normally, repeated stress makes this worse and eventually starts cracks. But under controlled conditions, the pre-fatigue process actually relieved this internal mismatch. As a result, the steel became far more resistant to crack initiation, which is the very first step of failure.
The results were dramatic. Martensitic steel with a tensile strength of 1.6 gigapascals—normally sitting right at the fatigue limit ceiling—showed a fatigue limit twice as high after pre-fatigue training.
This is the first time researchers have shown that fatigue itself, usually thought of as harmful, can be harnessed to prevent damage.
Because the new method strengthens steel without significantly reducing its tensile strength, it offers a promising alternative to traditional tempering. The team hopes to refine this “microstructural design strategy” and apply it to a wide range of materials, not just steel.
If successful, the approach could pave the way for safer, longer-lasting, and lighter components in many industries, helping bring ultra-high-strength materials into everyday use.
