Understand various fracture failure analysis types of metals

Date：2022-05-18 15:39:45Views：1270

The failure modes of metal materials in various engineering applications are mainly composed of fracture, corrosion, wear and deformation. Fracture failure analysis starts with the macro and micro characteristics of cracks and fractures, and studies the relationship between fracture process and morphological characteristics and material properties, microstructure, stress state of parts and environmental conditions, so as to reveal the causes and laws of fracture failure. The following is a brief analysis of various fracture failure analysis types of metals for your reference.

Ductile fracture

Ductile fracture is also called ductile fracture and plastic fracture, that is, before the fracture of parts, there is obvious plastic deformation at the fracture part. In engineering structures, ductile fracture is generally characterized by overload fracture, that is, the fracture occurs when the actual stress borne by the dangerous section of the part exceeds the yield strength or strength limit of the material.

Under normal circumstances, the design of airborne parts will control the actual stress at the dangerous section of the parts below the yield strength of the material, and generally there will be no ductile fracture failure. However, because there are many links and various complex factors in the whole process of mechanical products from design, material use, processing and manufacturing, assembly to use and maintenance, the ductile fracture failure of mechanical parts is still difficult to completely avoid.

Ductile fracture mechanism:

The microstructure of engineering materials is complex. The specific microstructure has specific fracture mechanism and micro morphology characteristics under specific external conditions (such as load type and size, ambient temperature and medium). The mechanism of ductile fracture of metal parts is mainly slip separation and dimple fracture.

Dimple fracture dimple is the main feature of metal ductile fracture. Dimples are also called overlapping waves, pits, micropores or micro pits. Dimple is the micro cavity produced by plastic deformation of materials in the micro region. After nucleation, growth and aggregation, it is connected with each other, resulting in the trace left on the fracture surface after fracture.

Although dimples are the microscopic characteristics of ductile fracture, the conclusion of ductile fracture cannot be made only on this basis, because the main difference between ductile fracture and brittle fracture lies in whether perceptible plastic deformation occurs before fracture. Even on the fracture surface of brittle fracture, dimples may be formed in some areas due to micro zone plastic deformation.

(1) The formation mechanism of dimple is complex, which can be roughly divided into three stages: the nucleation of micro cavity, the growth of micro cavity and the aggregation of cavity. D. Broek established the dimple nucleation and growth model according to the experimental results.

Under the action of ductile dimple or shear stress, the dimple can be formed under the action of ductile dimple or shear stress.

(2) The shape of dimple depends on the shape of dimple and tearing stress.

Equiaxed dimples are formed under normal stress. Under the action of normal stress, the periphery of the micro cavity grows evenly, and an approximately circular equiaxed dimple is formed after fracture.

Shear dimple is formed under the action of shear stress. It usually appears on the shear lip of tensile or impact fracture. Its shape is parabola, and the convex direction of parabola on the matching section is opposite.

Tear dimples are formed under the action of tear stress, which are common at the front end of sharp cracks and on the low-energy tear fracture under plane strain conditions, and also in parabola shape. However, on the matching fracture, the shape of tear dimples is not only similar, but also the convex direction of parabola is the same.

In the actual fracture, the equiaxed dimple and elongated dimple often coexist, or there are a small number of equiaxed dimples around the elongated dimple.

(3) The size of the dimple includes the average diameter and depth. The depth is often measured by the distance from the section to the bottom of the dimple. The main factors affecting the size of dimple are the size and density of second phase particles, plastic deformation capacity of matrix, hardening index, size and state of stress and loading speed. Generally, for the same material, when the fracture conditions are the same, the larger the dimple size, the better the plasticity of the material.

Brittle fracture

summary

Engineering components have little or no macroscopic plastic deformation (generally according to the requirements of smooth tensile specimens ψ< 5%) is called brittle fracture, which is also called low stress fracture because its fracture stress is lower than the yield strength of the material. Because most brittle fractures have no prior warning and are sudden, they often have extremely serious consequences for engineering components, equipment and personal safety. Therefore, brittle fracture is a fracture failure mode that people try to avoid. Although the engineering circles of various countries attach great importance to the analysis and prevention of brittle fracture, and have taken various measures in the whole process of engineering component design, material use, manufacturing, use and maintenance, however, due to the complexity of brittle fracture, catastrophic accidents caused by brittle fracture failure still occur from time to time.

form

The main forms of brittle fracture failure of metal components are:

(1) Brittle fracture caused by the change of material properties, such as blue brittleness, temper brittleness, brittleness caused by overheating and overburning, 475 ℃ brittleness and σ Phase brittleness, etc.

(2) Brittle fracture caused by ambient temperature and medium, such as cold embrittlement, hydrogen embrittlement, stress corrosion embrittlement, liquid metal embrittlement and irradiation embrittlement.

(3) Brittle fracture caused by loading rate and notch effect, such as high-speed embrittlement, stress concentration and three stress state embrittlement.

fatigue fracture

summary

According to the classification of macroscopic plastic deformation before fracture, fatigue fracture belongs to brittle fracture. However, due to the high proportion of fatigue fracture, great harm, and the fracture under alternating load, the engineering circles at home and abroad focus on it as a fracture form.

Definition of fatigue fracture

The fracture of engineering components under alternating stress after a certain cycle is called fatigue fracture.

Characteristics of fatigue fracture

(1) The stress of most engineering components changes periodically, which is called cyclic alternating stress. Such as crankshaft of piston engine, transmission gear, main shaft of turbine engine, turbine disc and blade, aircraft propeller and various bearings. According to statistics, 60% ~ 80% of the failures of these parts belong to fatigue fracture failure.

(2) The fatigue failure is characterized by sudden fracture and no obvious deformation before fracture. Without special flaw detection equipment, damage traces cannot be detected. In addition to regular inspection, it is difficult to prevent accidental accidents.

(3) The cyclic alternating stress causing fatigue failure is generally lower than the yield limit of the material, and some are even lower than the elastic limit.

(4) The fatigue fracture failure of parts is related to many factors, such as material performance, quality, shape, size, surface state, service conditions, external environment and so on.

(5) A large part of engineering components bear bending or torsional loads, and their stress distribution is large on the surface, so the surface conditions (such as notch, knife mark, roughness, oxidation, corrosion and decarburization) have a great impact on the fatigue resistance.

I hope the above article will help you. Shenzhen Chuangxin Online Testing Technology Co., Ltd. is a well-known professional testing organization for electronic components in China. It has three standardized laboratories with an area of more than 1000 square meters. The scope of testing services covers: testing and verification of electronic components, identification of IC authenticity, product design, material selection, failure analysis, function testing, factory incoming material inspection, X-ray testing of components, tape braiding and other testing items.