How to judge brittle fracture and ductile fracture in failure analysis of metal materials?

Date：2022-09-09 15:57:15Views：1319

Failure analysis of metal materials is a technology to analyze and study the damage causes of metal components or equipment that have lost their original functions. Technical analysis of major accidents caused by failure of metal materials is a complex process, which not only involves macro analysis, microstructure analysis, metallographic structure analysis, chemical composition analysis, hardness test, mechanical property test, stress test and other analysis and testing technologies, but also needs to combine a large number of test data and comprehensive information, including design scheme, heat treatment situation and use environment, Carry out a comprehensive and systematic discussion and analysis, and finally conclude and deduce the main causes of failure. Next, we mainly introduce the judgment basis of brittle fracture and ductile fracture. Let's have a look.

Fracture failure analysis is to determine the fracture failure mode by analyzing the macro and micro characteristics of the fracture, analyze and study the relationship between the fracture morphology characteristics and the material structure and performance, the stress state of parts and environmental conditions (such as temperature and medium), reveal the fracture failure mechanism, causes and laws, and then take improvement measures and preventive countermeasures.

Ductile fracture failure analysis 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 position. In engineering structures, ductile fracture is generally manifested as 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. The microstructure of engineering materials is complex, and the specific microstructure has specific fracture mechanism and micro morphology characteristics under specific external conditions (such as load type and size, environmental temperature and medium). The mechanism of ductile fracture of metal parts is mainly slip separation and dimple fracture.

Slip separation: the most prominent feature of ductile fracture is accompanied by a large amount of plastic deformation, and the general mechanism of plastic deformation is slip, that is, a large amount of crystal slip occurs before ductile fracture. Excessive slip deformation will lead to slip separation, and its micro morphology includes slip steps, serpentine patterns and ripples.

Dimple fracture is the main feature of metal ductile fracture. Dimple is also called stack wave, hole pit, micro hole or micro pit. Dimples are micro cavities formed by plastic deformation of materials in micro region, which are nucleated, grown and aggregated; Finally, they are connected to each other, resulting in the marks left on the fracture surface after fracture.

Dimple morphology (SEM)

Analysis basis for ductile fracture failure of metal parts:

(1) The macro appearance of the fracture is rough, the color is dark and fibrous; The edge has a shear lip at 45 ° to the surface of the part; There are obvious plastic deformation near the fracture, such as residual torsion angle, deflection, coarsening, necking and bulging.

(2) The microscopic features on the fracture surface are mainly dimples.

Failure analysis of brittle fracture

When the engineering member has little or no macroscopic plastic deformation (generally, the tensile strength of the smooth tensile specimen is ψ< 5%) is called brittle fracture. Because its fracture stress is lower than the yield strength of the material, it is also called low stress fracture. The manifestations of brittle fracture failure of metal components mainly include:

(1) Brittle fracture caused by the change of material properties, such as blue embrittlement, temper embrittlement, overheating and overburning embrittlement, 475 ℃ embrittlement of stainless steel 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 radiation 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 failure

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

(2) Fatigue failure is characterized by sudden fracture without 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 that causes 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 the performance and quality of materials, the shape, size, surface state, service conditions and external environment of parts.

(5) A large part of engineering components bear bending or torsional loads, and the stress distribution is the largest 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.

The above is related to the specific distinction between brittle fracture and ductile fracture of Chuangxin detection Xiaobian. I hope it will be helpful to you. Chuangxin testing is a professional testing institution for electronic components. At present, it mainly provides testing services for integrated circuits such as capacitors, resistors, connectors, MCU, CPLD, FPGA and DSP. Specializing in functional testing of electronic components, appearance testing of incoming electronic components, anatomy testing of electronic components, acetone testing, X-ray scanning testing of electronic components, ROHS composition analysis testing. Welcome to call, we will serve you wholeheartedly!