Ductile Fracture And Brittle Fracture Of Mechanical Parts
DATE：2019/9/23 8:52:58 / READ： / SOURCE：This station
Ductile fracture and brittle fracture of mechanical parts
Mechanical parts typically have two forms of fracture under static and impact loads: ductile fracture and brittle fracture.
1. Ductile fracture
The characteristic of ductile fracture is that the macroscopic plastic deformation occurs before the fracture, that is, the fracture is generated after the generation of a sufficient amount of permanent deformation. For example, the fracture that occurs after the low carbon steel is stretched at room temperature is in this state, and the fracture is cup-shaped and is dark gray fiber.
Studies have shown that whether it is ductile fracture or brittle fracture, the fracture process involves two stages of crack formation and expansion. In ductile fracture, after crack formation, it will undergo a long crack propagation stage. Crack propagation and plastic deformation will proceed simultaneously. Once the deformation is stopped and the crack stops, the crack will only expand when the deformation is continued. Continue to expand. The load increases continuously, the plastic deformation continues, the crack continues to expand until the critical crack length is reached, and finally the rapid failure expansion stage is experienced and the instantaneous fracture occurs. Therefore, due to the obvious plastic deformation before the ductile fracture, this can warn people to pay attention in advance, and generally will not cause serious accidents.
2. Brittle fracture
Brittle fracture is characterized by no plastic deformation before fracture. For example, a brittle gray cast iron, which is a state of stretching at room temperature, is a state in which the fracture is flush and consists of a number of shiny facets.
In the case of brittle fracture, when the crack is formed, it reaches the critical length very quickly, and enters the crack instability stage without undergoing the crack propagation stage. Since the crack propagation speed is extremely fast, there is no obvious sign before the brittle fracture, and the damage is extremely great. Historically, accidents due to brittle fractures have been numerous and have caused great harm, such as the sinking of tankers, the breakage of generator rotors, the cracking of turbine blades and main shafts, the explosion of boilers and pressure vessels, and large iron bridges. The break and so on.
It must be pointed out that no matter what kind of fracture form, crack formation is often related to material defects (microcracks, impurities, voids, etc.), and these depend on the quality of the material and the processing technology (casting, pressure processing). , welding, etc.) quality.
Although brittle fracture is very harmful, the brittle fracture and toughness of the material can be transformed into each other under certain conditions. For example, low-carbon steel, as a ductile material, exhibits brittle fracture when stretched at low temperatures, while brittle gray cast iron exhibits a large indentation when subjected to Brinell hardness testing, exhibiting good plasticity.
Mechanical parts typically have two forms of fracture under static and impact loads: ductile fracture and brittle fracture.
1. Ductile fracture
The characteristic of ductile fracture is that the macroscopic plastic deformation occurs before the fracture, that is, the fracture is generated after the generation of a sufficient amount of permanent deformation. For example, the fracture that occurs after the low carbon steel is stretched at room temperature is in this state, and the fracture is cup-shaped and is dark gray fiber.
Studies have shown that whether it is ductile fracture or brittle fracture, the fracture process involves two stages of crack formation and expansion. In ductile fracture, after crack formation, it will undergo a long crack propagation stage. Crack propagation and plastic deformation will proceed simultaneously. Once the deformation is stopped and the crack stops, the crack will only expand when the deformation is continued. Continue to expand. The load increases continuously, the plastic deformation continues, the crack continues to expand until the critical crack length is reached, and finally the rapid failure expansion stage is experienced and the instantaneous fracture occurs. Therefore, due to the obvious plastic deformation before the ductile fracture, this can warn people to pay attention in advance, and generally will not cause serious accidents.
2. Brittle fracture
Brittle fracture is characterized by no plastic deformation before fracture. For example, a brittle gray cast iron, which is a state of stretching at room temperature, is a state in which the fracture is flush and consists of a number of shiny facets.
In the case of brittle fracture, when the crack is formed, it reaches the critical length very quickly, and enters the crack instability stage without undergoing the crack propagation stage. Since the crack propagation speed is extremely fast, there is no obvious sign before the brittle fracture, and the damage is extremely great. Historically, accidents due to brittle fractures have been numerous and have caused great harm, such as the sinking of tankers, the breakage of generator rotors, the cracking of turbine blades and main shafts, the explosion of boilers and pressure vessels, and large iron bridges. The break and so on.
It must be pointed out that no matter what kind of fracture form, crack formation is often related to material defects (microcracks, impurities, voids, etc.), and these depend on the quality of the material and the processing technology (casting, pressure processing). , welding, etc.) quality.
Although brittle fracture is very harmful, the brittle fracture and toughness of the material can be transformed into each other under certain conditions. For example, low-carbon steel, as a ductile material, exhibits brittle fracture when stretched at low temperatures, while brittle gray cast iron exhibits a large indentation when subjected to Brinell hardness testing, exhibiting good plasticity.
Author：admin