Fracture accident in Engineering
(1) In 1969, the left wing of the U.S. F-111 aircraft fell off and crashed when it was performing bomb dropping recovery during flight training.
At that time, the flight speed, total weight, overload and other indicators were far lower than the design indicators.
After inspection and analysis, it was found that the wing pivot was defective due to improper heat treatment during manufacturing.
After missing inspection, the crack continued to expand under fatigue load, resulting in low stress damage.
(2) During World War II, 2500 free ships of the United States, 700 were seriously damaged, and 145 were broken into two sections.
Some of them occurred in calm sea, all of which were made of high-strength materials.
After the analysis, it was found that the stress at the welded joint was concentrated, and the impact toughness was reduced when the temperature was reduced, and the fracture was easy to occur.
(3) In 1938, the feline bridge in Belgium was broken into three sections.
What is fracture mechanics?
A large number of studies have pointed out that brittle fracture is always caused by macro cracks, and for most parts and structures, the existence of macro cracks is inevitable.
The strength of a material with cracks depends on the resistance of the material to crack propagation, which is determined by the internal properties of the material.
The stress and strain field near the crack tip and the crack propagation law are studied by using elastic and plastic theory and new test technology, which is called fracture mechanics.
Process of component fracture
(1) Crack generation:
① Under the influence of environment (fatigue, corrosion, stress, temperature, etc.), macro micro cracks are generated at the stress concentration of the structure;
② Defects of the material itself;
③ Cracks during production and processing.
(2) Subcritical crack propagation:
Under the influence of the environment, the macro micro cracks gradually and slowly expand during the service of the component.
(3) Crack initiation:
Under the action of stress, the crack gradually expands to reach the critical length, and the component suddenly loses stability.
(4) Crack propagation:
Unstable cracks propagate at a high speed, which can reach 1 / 4 of the sound speed in the material.
(5) Crack stop:
After the crack is unstable, it can pass through the whole structure and destroy the structural parts;
Or the crack 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
From the perspective of fracture mechanics, it studies the various stages of the fracture process.
On the basis of a large number of experiments, it studies the fracture toughness of materials with cracks and the laws of crack generation, propagation, instability and crack arrest, so as to provide safer design for products.