Can you imagine the situation where the nose of the plane suddenly breaks off during the flight?
In 2007, an F-15 fighter jet of the U.S. Air Force had such a thrilling scene during a simulated air battle. The accident caused a large-scale grounding of the U.S. F-15 fighter jets, and the investigation results showed that the accident was caused by the fatigue of a metal stringer on the plane.
Coincidentally, in 2002, a Boeing 747 passenger plane flying from Taiwan to Hong Kong disintegrated and crashed in the waters near Penghu, killing a total of 225 people including crew members. Subsequent investigations concluded that severe metal fatigue cracking occurred in a repaired skin on the aircraft, which caused the tail to fall off, and eventually caused the aircraft to disintegrate due to the loss of pressure in the cabin.
Seeing this, many friends will be puzzled: People will get tired when they are tired, so how can metal get tired?
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Separation of the nose and fuselage of the F-15 fighter jet and the process of ejecting the pilot out of the cabin
The process of separating the nose of the F-15 fighter from the fuselage and ejecting the pilot out of the cabin:
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The flight accident of F-15 was caused by the fatigue of the stringer in the picture
Life experience tells us that it is very difficult to break the wire by hand, but it is easy to break if it is folded several times.
This shows that even if the repeatedly changing external force is much smaller than the constant force that can directly pull the metal off, it will gradually weaken its mechanical properties and eventually destroy it.
This phenomenon of metal is very similar to the fatigue of people under long-term work, and scientists vividly call it "metal fatigue".
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Metal Fatigue Example
Although many people have never heard of metal fatigue, it is widely lurking in people's daily life, often causing unexpected and serious accidents. It is estimated that about 90% of mechanical accidents are related to metal fatigue.
Why does seemingly hard metal fatigue?
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As the saying goes, "gold has no color, but white jade has slight flaws." The metals we currently use are not perfect. During processing or use, metals will always have some defects, such as impurities or holes inside, scratches on the surface, etc. mark. These defects are often only on the order of microns, which are difficult to observe with the naked eye. If a constant tension is applied to the metal, they are not prone to cracks.
However, if the external force changes repeatedly, sometimes it is tension and sometimes it is pressure, a part of the energy will be converted into heat and accumulated inside the metal. Once it exceeds a certain limit, the metal will easily break the chemical bonds between atoms at the defect, resulting in structural damage. cracking.
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▲Metal defects observed under a microscope and the metal fatigue cracking process starting from the defects
If a person is overtired, it will often cause disease or even death. If metal is tired, it will bring greater harm and even cause group casualties.
In addition to the flight accidents mentioned above, ships, trains, bridges, cars, etc. are also often caused by metal fatigue disasters. During World War II, nearly 1,000 metal fatigue accidents occurred on 5,000 cargo ships in the United States, and more than 200 cargo ships were completely out of service; in 1998, a train traveling at high speed in Germany derailed due to fatigue and fracture of wheel tires, killing more than 100 people...
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▲In 1998, the most serious train accident in German history was caused by the fatigue fracture of the wheel tire
Because metal fatigue is the result of repeated long-term action of small external forces, the metal basically has no obvious plastic deformation before cracking, so it is often difficult to detect metal fatigue in advance.
Are we helpless against metal fatigue?
Through the unremitting efforts of scientists, there are many methods to detect the fatigue of metals. Ultrasonic, infrared, gamma rays, etc. can all perform physical examination on metals.
Japanese scientists also invented a special paint mixed with lead titanate powder. When the metal is struck, a current will flow through the paint film on the metal surface, and the magnitude of the current is related to the fatigue degree of the metal. By measuring this In order to reduce the occurrence of metal fatigue accidents, scientists have also made great efforts in the preparation and use of metals.
Almost all the machines we come into contact with in life are made of alloys, and rarely use a single metal. This is because several substances in the alloy can fill the gaps between each other, effectively improving the metal's ability to resist fatigue.
When processing and using metal parts, keeping the surface clean and away from corrosive environments can also effectively reduce the occurrence of fatigue.
However, due to the complexity of the influencing factors, it is still impossible to completely avoid metal fatigue, and scientists still have a long way to go.
