[sort out] 10 questions and answers about material failure analysis

Date：2021-09-18 14:50:59 Views：1566

1. What are the types of fractures? How to distinguish ductile fracture from brittle fracture?

A: (1) according to the macro deformation before material fracture, the fracture is divided into ductile fracture and brittle fracture. (2) Ductile fracture is an obvious macroscopic plastic deformation before fracture. Brittle fracture is that plastic deformation does not occur before fracture, and its fracture is flush after fracture, which is composed of countless shiny small planes.

2. How many stages are there in the fracture process? What is the difference between ductile fracture and brittle fracture?

A: whether it is ductile fracture or brittle fracture, the fracture process includes two stages of crack formation and propagation. The process from crack formation to crack propagation to critical length is called the metastable crack propagation stage, in which the crack propagation resistance is large and the propagation speed is slow; The propagation stage after the crack reaches the critical length is called the unstable propagation stage. In this stage, the crack propagation resistance is small and the propagation speed is very fast. For ductile fracture, after the crack is formed, it goes through a long metastable crack propagation stage. Crack propagation and plastic deformation proceed at the same time until it reaches the critical crack length. Finally, it goes through the unstable propagation stage and breaks instantaneously. Therefore, there is obvious plastic deformation before ductile fracture.

For brittle fracture, the critical length is reached soon after the crack is formed, and it enters the unstable crack propagation stage almost without going through the metastable crack propagation stage. The crack propagation speed is very fast, so there is no obvious plastic deformation before brittle fracture.

3. What is the toughness of materials? What are the mechanical property indexes to evaluate the toughness of materials?

A: (1) toughness is the ability of a material to absorb energy in the process of plastic deformation and fracture. It is the comprehensive performance of material strength and plasticity. If the material has good toughness, the tendency of brittle fracture is small.

(2) The mechanical properties to evaluate the toughness of materials are impact toughness and fracture toughness. Impact toughness is the ability of a material to absorb plastic deformation work and fracture work under impact load, which is commonly expressed by the impact absorption work AK of standard samples. Fracture toughness KIC is a mechanical property index to evaluate the resistance of materials to brittle fracture, which refers to the ability of materials to resist the unstable propagation of cracks.

4. Meaning and application of material toughness index?

A: (1) impact absorption energy AK is the mechanical property index to measure the impact toughness of materials. The impact absorption energy is measured by the impact test. It is to put the standard sample with U-shaped or V-shaped notch on the impact testing machine and punch the sample with a pendulum. The work consumed by punching the sample is the impact absorption work AK, and its unit is J. The higher AK and the lower ductile brittle transition temperature TK, the better the impact toughness of the material.

(2) Fracture toughness KIC is a mechanical property index to evaluate the resistance of materials to brittle fracture, which refers to the ability of materials to resist the unstable propagation of cracks, and its unit is MPa · M1 / 2 or Mn · M-3 / 2. Crack tip stress field intensity factor Ki, part crack half length a and part working stress σ There is a relationship between ki = y σ In A1 / 2, y = 1 ~ 2, which is the geometric factor of the crack in the part. When ki ≥ KIC, low stress brittle fracture occurs; When ki ＜ KIC, the parts are safe and reliable. Therefore, Ki = KIC is the critical condition for low stress brittle fracture of parts, i.e. ki = y σ A1 / 2 = KIC from this formula, in order to prevent brittle fracture of parts, three parameters can be controlled, namely KIC σ And a.

5. What is fatigue fracture?

Answer: fatigue fracture refers to the phenomenon that parts break after working for a long time under alternating load. The so-called alternating load refers to the load whose magnitude and direction change periodically with time.

6. What are the components of fatigue fracture?

A: the typical fatigue fracture morphology consists of three parts: fatigue source area, fatigue crack propagation area and final fracture area.

7. What is corrosion? Can be divided into several categories?

Answer: (1) corrosion is the phenomenon of surface damage caused by chemical reaction or electrochemical reaction between the material surface and the surrounding medium. (2) Classification: chemical corrosion and electrochemical corrosion.

8. What are the conditions for electrochemical corrosion?

A: there are electrode potential differences between different metals or between various parts of the same metal, and they are in contact with each other and in interconnected electrolyte solution to form a micro battery.

9. What are the main measures to improve the corrosion resistance of parts?

Answer: for chemical corrosion: choose anti-oxidation materials, such as heat-resistant steel, superalloy and ceramic materials, and the surface coating of parts. For electrochemical corrosion: select corrosion-resistant materials; Surface coating; Electrochemical protection; Add corrosion inhibitor. For stress corrosion: reduce tensile stress; Stress relief annealing; Select materials with high ISCC; Improve media.

10. What are the characteristics of mechanical behavior of metal materials at high temperature?

A: (1) the strength of the material decreases with the increase of temperature. (2) The strength of the material at high temperature decreases with the extension of time. (3) The deformation of the material at high temperature increases with the extension of time.