Fatigue Certification Of Metals


At 10:43 am on April 17, 2018, Eastern Time, Southwest Airlines Flight 1380 took off from New York LaGuardia Airport (LGA) for Dallas. However, I am afraid that the five crew members and 143 passengers on the entire plane will not expect that in just 20 minutes, they will experience a life-and-death shock at a height of 10,000 meters.

Above the town of Hershey, Pennsylvania, less than 250 kilometers from New York, the left engine of Southwest Airlines 1380 exploded. The wreckage from the explosion penetrated the hood and broke a window in the cabin. Due to the rapid loss of pressure in the cabin, a female passenger sitting next to this window was “pushed” out of the aircraft. Although several passengers rescued her, she was not sucked out of the cabin, but she ended up in the hospital because of her injuries He died, and 7 other passengers were hospitalized due to injuries.

The air crash caused a major earthquake in the US civil aviation industry. Be aware that in the past few years, the safety record of American civil aviation has been almost perfect, and it has been 9 years since the last air crash that killed passengers. Air crashes that caused passengers to die due to engine debris are even more rare.

Picture: The engine of Southwest Airlines Flight 1380 exploded in this accident
Picture: The engine of Southwest Airlines Flight 1380 exploded in this accident

When investigators from the US National Transportation Safety Board (NTSB) rushed to the scene, they immediately found one of the 24 blades on their left engine.

“This fan blade was broken directly at the blade hub, and after preliminary inspection, we found traces of metal fatigue at the blade blade separation,” said NTSB chairman Robert Sumwalk.

Traces of metal fatigue at fan blade separation
Traces of metal fatigue at fan blade separation

Metal fatigue refers to the formation of micro cracks in the local high-stress area due to repeated use, and then the micro cracks gradually expand to break. Metal fatigue may occur on the blades of turbofan engines, on aluminum skins on airplanes, and on any other metal parts.

What is striking is that this air crash is not the first accident caused by an engine failure of Southwest Airlines. In August 2016, the same model engine (CFM56-7B) of the same model of Southwest Airlines (Boeing 737-700) also produced an engine explosion at cruise altitude, exploding the front of the hood. Judging from the photos taken by passengers, the similarity of the engines in these two crashes is striking.

Fortunately, unlike this time, although the plane crash also produced pressure loss in the cabin two years ago, it was because a large hole was penetrated in the fuselage above the left wing, and no debris was shot into the cabin, so it did not happen. Casualties. However, the accident directly led to the engine manufacturer and regulators to jointly require airlines to conduct ultrasonic testing of the engine fan blades to find out possible metal fatigue.

Picture: Southwest Airlines Flight 3472, which also produced an engine explosion at a height of 10,000 meters in August 2016, is shown in the red circle above the wing on the left.
Picture: Southwest Airlines Flight 3472, which also produced an engine explosion at a height of 10,000 meters in August 2016, is shown in the red circle above the wing on the left.

In fact, the cases of accidents caused by metal fatigue can be traced back for a long time:

Figure: Fighter crash due to metal fatigue
Figure: Fighter crash due to metal fatigue

After the first industrial revolution, steam locomotives and other equipment and equipment were invented one after another, but a large number of fracture accidents followed. It was found that under the action of cyclic load, the service life of components is much less than the design life, or even less than half of the design life. In this way, people gradually realized the destructive power of fatigue, but at first people did not understand this phenomenon, then people carried out some targeted research, and the veil of fatigue fracture of metal components was gradually lifted.

People’s initial understanding of the fatigue problem of metal components began in the 19th century. During the first industrial revolution, key components of heavy trucks, automobiles, ships, and other mechanical equipment often broke and failed under cyclic loading. At first, it is difficult to understand why the service life of metal components under cyclic load or alternating load is much shorter than the design life.

metal fracture
metal fracture

During the Second World War, some serious accidents directly stimulated the research and overcoming of this problem. During the war, large-scale and rapid shipbuilding was required. These ship frames are welded instead of traditional riveted.

In the waters of the Atlantic Ocean, these ships quickly cracked in cold water. In fact, some ships split directly into several parts, and the impact of the waves caused the initiation of fatigue cracks, and then these cracks quickly expanded in cold environments with catastrophic consequences.

When the temperature drops to below zero, it will significantly reduce the plasticity of the weld and the base metal, so it will become brittle. Because the fracture energy of brittle materials is much less than that of plastic materials, this will result in the reduction of the critical crack size inside them, and the fracture occurs under load conditions that seem to be safe at room temperature.

At that time, people discovered the damage caused by metal fatigue in various aspects. However, due to the backwardness of technology, the cause of fatigue damage cannot be identified. Until the emergence of microscopes and electron microscopes in succession, human beings have continuously made new achievements on the road of uncovering the secret of metal fatigue.

Metal fatigue refers to a phenomenon in which metallic materials break down under the action of alternating stress. Under the action of alternating pressure, after a period of time, the mechanical parts form micro cracks in the local high stress area, and then the micro cracks gradually expand to break. Fatigue damage has the characteristics of suddenness in time, locality in location and sensitivity to environment and defects. Therefore, fatigue damage is often not easy to be found in time and it is easy to cause accidents. Stress amplitude, average stress size and number of cycles are the three main factors affecting metal fatigue.

The internal structure of the metal is not uniform, resulting in an imbalance in stress transfer, and some areas will become stress concentration areas. At the same time, there are many tiny cracks in the defects inside the metal. Under the continuous action of force, the crack will become larger and larger, and the part that can transmit stress in the material will become less and less. When the remaining part cannot continue to transmit the load, the metal components will be completely destroyed.

Many mechanical parts and engineering components work under alternating load. Micro cracks will occur when metal materials are subjected to alternating stress or repeated cycling stress. After a certain number of cycles, the cracks will expand to the critical point, resulting in the sudden fracture of custom synthetic metal materials when the working stress is less than the yield strength. It is called metal fatigue fracture. But in fact, this phenomenon is not limited to solid metals. Many solid materials also have this phenomenon, such as plastic, carbon fiber, ceramics, glass, etc.
The characteristics of fatigue fracture of metal materials are:

⑴ The load stress is alternating;

⑵ The action time of the load is longer;

⑶ The fracture occurs instantaneously;

⑷Whether it is a plastic material or a brittle material, it is brittle in the fatigue fracture zone.

Therefore, fatigue fracture is the most common and dangerous form of fracture in engineering.

The fatigue phenomenon of metal materials can be divided into the following types according to different conditions:

(1) High cycle fatigue: refers to fatigue with a stress cycle of more than 100,000 under low stress (working stress is lower than the yield limit of the material or even below the elastic limit). It is the most common type of fatigue damage. High cycle fatigue is generally referred to as fatigue.

⑵ Low cycle fatigue: refers to fatigue under high stress (working stress is close to the yield limit of the material) or high strain conditions, the stress cycle number is less than 10000 ~ 100,000 Since alternating plastic strain plays a major role in this fatigue failure, it is also called plastic fatigue or strain fatigue.

(3) Thermal fatigue: refers to the fatigue damage caused by the repeated action of thermal stress due to temperature changes.

⑷Corrosion fatigue: refers to the fatigue damage of machine parts under the combined action of alternating load and corrosive medium (such as acid, alkali, seawater, active gas, etc.).

⑸ Contact fatigue: This refers to the contact surface of machine parts. Under the repeated action of contact stress, pitting peeling or surface crushing and peeling occur, which causes the failure of the machine parts.

Today’s large-scale equipment such as automobiles, ships, and airplanes have anti-fatigue designs for their components before production. This design ensures the safety of these designs to a large extent within the design life span.

In order to enhance the metal’s anti-fatigue performance, people will add rare earth elements to the metal or further improve the microstructure of the metal. With the development of science and technology, a new technology of “metal immunotherapy” has emerged, which strengthens the fatigue strength of metals through methods introduced in advance to resist fatigue damage. Shock-proof measures should be taken for the mechanical equipment that generates vibration to reduce the possibility of metal fatigue. When necessary, it is necessary to carry out the inspection of the internal structure of the metal, which is also very beneficial to prevent metal fatigue. However, these methods can only improve performance but cannot solve the fundamental problem.

With the higher, faster and more demanding functional requirements of mechanical equipment, fatigue is still inevitable. We often describe a person’s steely will. This shows that steel is very strong in people’s minds, but if it is “tired”, it will also shatter, and the damage caused is fatal and catastrophic.

Cracks caused by metal fatigue can cause disaster. However, there are other magical uses. Now, a stress-cutting machine made with the fatigue fracture characteristics of metals has been born. It can process metal and nonmetal with various properties to produce fatigue fracture in a certain incision. This process takes only 1-2 seconds, and the more difficult it is to cut the material, the easier it is to meet the needs through this cnc machining.

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