Fracture failure analysis of metal materials what are the measures to reduce fracture failure?

Date：2022-03-28 14:50:52Views：1397

Under the action of external load, the metal breaks when the stress reaches the breaking strength of the material. The fracture process of metal materials generally has three stages: crack initiation, crack metastable propagation and unstable propagation, and finally fracture. Metal components may crack and initiate cracks under different conditions in material manufacturing, component forming or use stages; Under the influence of different environmental factors and bearing state, the crack propagates until fracture.

After the metal component is broken, there are two matching fracture surfaces at the fracture site, which is called fracture. The fracture and its surroundings leave information closely related to the fracture process. Through fracture analysis, we can judge the type of fracture and the mechanism of fracture process, so as to find out the causes of fracture and the measures to prevent fracture.

1. Type of fracture

According to the plastic deformation of metal materials before fracture, it can be divided into ductile fracture and brittle fracture. Ductile fracture: large plastic deformation occurs before fracture, and the fracture is dark gray fibrous. Brittle fracture: there is no obvious plastic deformation before fracture, and the fracture is flush and in bright crystalline shape. The significant difference between ductile fracture and brittle fracture is the difference of crack diffusion.

Ductile fracture and brittle fracture are only relative concepts. Under actual load, brittle fracture may occur in different materials; The same material will have different fracture due to different temperature, stress, environment and other conditions.

2. Mode of fracture

According to the orientation of the fracture surface, it can be divided into normal fracture and cut-off. Normal fracture: the macro fracture surface of the fracture is perpendicular to the direction of the maximum normal stress, which is generally brittle fracture or ductile fracture. Cutting: the macro fracture surface of the fracture is 45 ° to the direction of the maximum normal stress, which is ductile fracture.

3. Form of fracture

The ways of crack propagation can be divided into transgranular fracture and intergranular fracture. Transgranular fracture: the crack passes through the interior of the grain, and the ductile fracture can also be brittle fracture. Intergranular fracture: the crack passes through the grain itself, brittle fracture.

4. Fracture analysis

Fracture analysis is an important method for fracture failure analysis of metal materials. The causes of fracture, the way of diffusion, the diffusion process and the internal and external factors affecting crack diffusion are recorded. Therefore, through fracture analysis, the causes and influencing factors of fracture can be found out, which provides a basis for improving component design, material properties and technology. Fracture analysis can be divided into macro fracture analysis and micro fracture analysis.

(1) Macro fracture analysis

Three elements of fracture: fiber area, radiation area and shear lip. Fiber area: dark gray, no metallic luster, rough surface, fibrous, located in the center of fracture, which is the source of crack. Radiation area: the macroscopic feature is that the surface is crystalline, metallic luster and has radial lines. The radiation direction of the lines is parallel to the crack diffusion direction, and these lines point inversely to the crack source. Shear lip: the macroscopic feature is that the surface is smooth, the section is 45 ° to the external force, and it is located at the edge of the sample fracture.

(2) Micro fracture analysis (need further study)

5. Brittle failure accident analysis

Brittle fracture has the following characteristics:

(1) Brittle fracture belongs to low stress failure, and its failure stress is often far lower than the yield limit of materials.

(2) It usually occurs at a lower temperature, and the temperature of the material when brittle fracture usually occurs is 20 ℃ below the room temperature.

(3) Before brittle fracture, there is no warning, and the cracking speed is fast, which is 1 / 3 of the sound speed.

(4) The crack source of brittle fracture is the stress concentration in the component.

What are the measures to reduce fracture failure?

(1) Steel with good low-temperature impact toughness shall be selected.

(2) Try to avoid stress concentration in components.

(3) Pay attention to the service temperature.

6. Ductile brittle transition temperature

In order to determine the brittle transition temperature of materials, a lot of experimental research work has been carried out. If a group of notched metal material samples are used, the impact test shall be carried out at each temperature in the whole temperature range.

The typical ductile brittle transition temperature of low carbon steel. With the decrease of temperature, the impact value of the material decreases and the crystalline section on the fracture surface increases, that is, the toughness of the material decreases and the brittleness increases.

There are several ways:

(1) The impact value is reduced to 50 ~ 60% of the normal impact value.

(2) The temperature at which the impact value drops to a specific and allowable minimum impact value.

(3) To produce the corresponding temperature when the maximum and minimum impact values are averaged.

(4) The temperature at which the crystalline section accounts for 50% of the area in the fracture.

For the marine mild steel sheet with a thickness of less than 40mm, the temperature when the Charpy V-notch impact energy is 25.51j/cm2 is taken as the brittle transition temperature of the material.

7. Non plastic temperature

The ductile brittle transition temperature is for low carbon steel, low carbon manganese steel and other steels, so a large number of tests cannot be carried out. Depending on other test methods, determine the "non plastic temperature" NDT of the material

(1) Pile a small section of brittle weld bead on the square sample plate of bulging test, and saw a gap on the weld bead. Above the sample, judge whether it is plastic failure according to the damage of the sample. Flat crack, concave crack, bulging tear

(2) Drop weight test

8. Conditions for brittle fracture of metal materials

(1) Any kind of temperature fracture has two strength indexes, yield strength and critical fracture strength characterizing the unstable diffusion of crack. When the temperature is high, the heat energy of atomic movement is large, and the dislocation source releases dislocation and moves to absorb energy; The temperature is low and vice versa.

(2) The stress at the crack tip of the defective material is large, the toughness is good, yield occurs, plastic deformation occurs, and the crack spreads further. The longer the crack length is, the more brittle fracture is likely to occur. The sharper the defect is, the more prone it is to brittle fracture.

(3) The thicker the steel plate, the lower the impact toughness and the higher the ductile brittle transition temperature.

Reasons: (1) the thicker the thickness, the greater the restraint effect on the shrinkage deformation in the thickness direction, which increases the restraint stress and forms a plane strain state within the thickness range of the steel plate. (2) Due to metallurgical effect, the grains in the thick plate are coarser and there are more internal segregation.

(4) The faster the loading speed of low strength steel, the lower the ductile brittle transition temperature.

The above is related to the fracture failure analysis of metal materials compiled by the core detection team. I hope it will be helpful to you. Chuangxin testing is a professional testing organization for electronic components. At present, it mainly provides integrated circuit testing services such as capacitance, resistance, connector, MCU, CPLD, FPGA, DSP and so on. Specializing in functional testing of electronic components, incoming appearance testing of electronic components, anatomical testing of electronic components, acetone testing, X-ray scanning testing of electronic components and RoHS component analysis testing. Welcome to call, we will serve you wholeheartedly!