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We have heard of many disasters caused by metal fatigue. In order to prevent this problem, we have to invest a lot of manpower and material resources every year, but it is still difficult to completely eliminate it. That kind of self-healing metal material seems to appear only in science fiction, or the villains have mastered the material, resulting in repeated setbacks of the protagonists, or coincidental acquisition of this material, so that the protagonists can survive. Now, scientists have turned this sci-fi plot into reality.

Wen Jing | Wen Jing

The "difficulty" and "pain" of metal fatigue in the process of processing or using metals, it is inevitable that there are some defects, such as internal impurities or holes, scratches on the surface, these defects are very subtle and difficult to observe with the naked eye. If a constant pulling force is applied to the metal, it will not cause cracks in the metal, but if a repeatedly changing force is applied to the metal, either pulling force or pressure, part of the energy will be converted into heat and accumulated inside the metal. Once it exceeds a certain limit, it will cause chemical bonds between atoms near the metal defects to break, or even metal components to crack. This is the process of metal fatigue.

Because metal fatigue is the result of repeated long-term action of small external force, there is basically no obvious deformation of metal before cracking, so it is often difficult to detect metal fatigue in advance. Sometimes a small crack can have disastrous consequences and is difficult to prevent. There have been many tragedies caused by metal fatigue in history, and these accidents usually occur in a bizarre form.

In 1998, a high-speed train in Germany derailed due to fatigue fracture of wheels and tyres, killing more than 100 people.

In 2002, a Boeing 747 disintegrated and crashed en route from Taiwan to Hong Kong due to severe metal fatigue cracking on a repaired skin on the plane, killing 225 people, including crew members.

In 2007, the nose of an Fmur15 fighter of the United States Air Force suddenly broke during a simulated air combat due to fatigue of a metal stringer on the plane.

In 2011, a passenger train overturned the track in Taiyuan, Shanxi province due to fatigue cracks on the rail, killing at least 19 people and injuring 141 others.

In 2018, a newly designed and safety-conscious pedestrian bridge collapsed near Florida International University due to cracks and deformation in metal components, killing at least six people.

Metal fatigue can only be prevented but not reversed? When machining and using metal parts, designing the part structure reasonably, reducing the stress concentration as much as possible, improving the surface finish and keeping away from the corrosion environment can effectively reduce the occurrence of fatigue.

Although there are preventive measures for metal fatigue, it is generally believed that the cracks caused by metal fatigue are irreversible and will only get bigger and bigger. Even when scientists formulate some basic equations to describe crack growth, they directly rule out the possibility of metal self-healing process.

But a study published in Nature on July 19 shows that scientists have achieved self-repair of fatigue cracks in metals. This is the first time that scientists have observed metal self-healing, indicating that the design of metal internal structure can not only prevent the occurrence of fatigue cracks, but also promote crack healing.

In this study, scientists repeatedly pulled 40 nm thick platinum foil (200 times per second) to observe the initiation and propagation of fatigue cracks. But surprisingly, after 40 minutes of the experiment, the scientists observed that the fatigue cracks began to heal on their own. Continuing to pull the platinum foil repeatedly, the scientists observed that the initial fatigue cracks gradually disappeared and the cracks began to grow in a new direction-indicating that the previous cracks had indeed healed.

Scientists describe the observed process as crack side cold welding (cold welding) induced by local stress state and grain boundary migration. Cold welding is a metallurgical process in which chemical bonds are reformed when two relatively smooth and clean metal surfaces come into contact.

This discovery is amazing, which is of great significance to the explanation of fatigue response and the design of anti-fatigue materials, and challenges the most basic theory of how engineers design and evaluate the fatigue life of structural materials.

Although this cold welding process is not yet controllable and can only be observed in platinum and copper at this stage, the grain boundary changes observed in this study are instructive. Scientists expect countless other microstructure configurations to have a similar effect.

References:

Autonomous healing of fatigue cracks via cold welding | Nature

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