Fracture accident in engineering
(1) In 1969, during flight training, the left wing of a US F-111 aircraft collapsed and crashed, while carrying out a bomb drop recovery exercise. Despite the fact that the flight speed, total weight, overload and other indicators were significantly lower than the design indicators, the wing still separated from the aircraft.
An inspection and analysis revealed that the wing pivot was defective due to inadequate heat treatment during the manufacturing process. Despite a lack of inspections, the crack continued to expand under fatigue loading, leading to low-stress damage.
(2) During World War II, the United States produced 2,500 free ships, of which 700 were seriously damaged and 145 were divided into two sections. Surprisingly, some of these incidents occurred in calm seas, despite all ships being made of high-strength materials.
After analyzing the causes of the damage, it was discovered that the stresses in the welded joint were concentrated and the impact resistance was reduced due to the drop in temperature, which made the ship's structure vulnerable to fractures.
(3) The feline bridge in Belgium was divided into three sections in 1938.
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What is fracture mechanics?
A significant number of studies indicate that brittle fractures are always caused by macrocracks and, in most cases, the existence of such cracks in structures is inevitable.
The crack resistance of a material depends on the material's resistance to crack propagation, which is determined by its internal properties.
The stress and strain field around the crack tip, as well as the law of crack propagation, are studied using elastic and plastic theories and a new testing technology known as fracture mechanics.
Component Fracture Process
(1) Crack generation:
- Macro and microcracks are generated at points of stress concentration in a structure due to environmental factors such as fatigue, corrosion, tension and temperature.
- There may be defects in the material itself.
- Cracks can also occur during production and processing.
(2) Subcritical crack propagation:
Under the influence of the environment, macromicrocracks expand gradually and slowly during component maintenance.
(3) Crack initiation:
Under the action of stress, the crack gradually expands until it reaches the critical length and the component suddenly loses stability.
(4) Crack propagation:
Unstable cracks propagate at high speed, reaching 1/4 the speed of sound in the material.
(5) Crack stop:
Once the crack becomes unstable, it can pass through the entire structure and destroy structural parts;
Or the cracking stops under certain conditions.
Classification of fracture mechanics
Fracture mechanics can be divided into macroscopic fracture mechanics and microscopic fracture mechanics, while macroscopic fracture mechanics can be divided into linear elastic fracture mechanics, elastic-plastic fracture mechanics, fracture dynamics, interface fracture mechanics .
Purpose of Fracture Mechanics
Fracture mechanics studies the different stages of the fracture process. Based on numerous experiments, it examines the fracture resistance of crack-containing materials and the laws governing crack generation, propagation, instability, and retention. This field provides valuable information for safer product design.
