Briefly describe the methods and steps of part failure analysis professional IC electronic testing company

Date：2022-03-29 16:53:05 Views：1125

When a mechanical part loses its due function, it is said to be invalid. There are many reasons for failure, such as fracture, deformation, surface wear and so on. The so-called failure mainly refers to the phenomenon that the part loses its specified function due to the change of its size, shape or material structure and performance for some reason. In order to help you have an in-depth understanding, this paper will summarize the relevant knowledge of part failure analysis. If you are interested in what this article will cover, read on.

The failure of mechanical parts generally includes the following situations.

(1) The parts are completely damaged and cannot continue to work.

(2) Although it can still work safely, it can not play the expected role satisfactorily.

(3) The parts are seriously damaged and it is unsafe to continue working.

1. Failure form of parts

According to the damage characteristics of parts, the failure forms can be divided into three basic types: deformation, fracture and surface damage.

1) Deformation failure and material selection

There are two kinds of deformation failure, elastic deformation failure and plastic deformation failure.

(1) Elastic deformation failure.Elastic deformation failure refers to the failure of parts caused by excessive elastic deformation. For example, the stiffness of the motor rotor shaft is insufficient, resulting in excessive elastic deformation. As a result, the rotor collides with the stator, and finally the main shaft bends or even breaks.

The size of elastic deformation depends on the geometric size of the part and the elastic modulus of the material. Diamond and ceramics have the highest elastic modulus, followed by refractory metals and steel, non-ferrous metals are lower, and organic polymer materials have the lowest elastic modulus. Therefore, as a structural member, it is more appropriate to choose steel from the perspective of stiffness and economy.

(2) Plastic deformation failure.Plastic deformation failure refers to the failure of parts due to excessive plastic deformation. Plastic deformation failure is the result that the working stress in the part exceeds the yield strength of the material. Plastic deformation is a kind of permanent deformation, which can be expressed in the shape and size of parts. Under a given load condition, whether plastic deformation occurs or not depends on the geometric size of the part and the yield strength of the material.

Generally, the yield strength of ceramic materials is very high, but the brittleness is very large. In the tensile test, brittle fracture occurs when the yield stress is far from being reached, and the characteristics of high strength cannot be brought into play. Therefore, it can not be used to manufacture high-strength structural parts. The strength of organic polymer materials is very low, and the plastic with the highest strength is no more than key alloy. Therefore, at present, the main material used for high-strength structure is steel.

2) Fracture failure

Fracture failure is the main failure form of mechanical parts. According to the nature and causes of fracture, it can be divided into the following four types.

(1) Plastic fracture.Plastic fracture refers to the fracture that occurs after the stress on a certain section exceeds the yield strength of the material and produces great plastic deformation when the part is subjected to external load. For example, in the tensile test of smooth sample of low carbon steel. Because a large amount of plastic deformation has occurred before fracture and entered the failure state, it can only make the parts inoperable, but it will not cause great danger.

(2) Brittle fracture.When brittle fracture occurs, there is no obvious plastic deformation in advance, and the working stress is usually far lower than the yield strength of the material, so it is also called low stress brittle fracture. This fracture often occurs in parts with sharp notches or cracks. In addition, structural mutations such as edges, steps, grooves and corners in the part structure are also easy to occur, especially under the action of low temperature or impact load.

(3) Fatigue fracture.The fracture that occurs under the repeated action of alternating stress lower than the yield strength of the material is called fatigue fracture. The final fracture due to fatigue is instantaneous, so it is more harmful, which often occurs in gears, springs, shafts, molds and blade parts. Fatigue fracture is a very harmful and common failure form. According to statistics, more than 80% ~ 90% of the damage of parts subjected to alternating stress is caused by fatigue. Using various strengthening methods to improve the strength of materials, especially the surface strength, and form residual compressive stress on the surface can significantly improve the fatigue strength. In addition, reducing various defects, knife marks, sharp corners and abrupt changes in section that can cause stress concentration on the parts can improve the fatigue resistance of the parts.

(4) Creep fracture.Creep fracture refers to the failure caused by excessive deformation or fracture when the stress remains unchanged and the deformation increases with the extension of time. For example, the slow deflection deformation of overhead polyethylene gas wire pipe under the action of wire and self weight is a typical material creep phenomenon. Metal materials generally produce obvious creep at high temperature, while polymers will produce significant creep under load at room temperature. When the creep deformation exceeds a certain range, cracks will appear in the parts and break quickly.

3) Surface damage

During the working process of parts, due to mechanical and chemical effects, the surface of the workpiece and the materials near the surface are seriously damaged, resulting in failure, which is called surface damage failure. Surface damage failure can be divided into three categories: wear failure, surface fatigue failure and corrosion failure.

(1) Wear failure.Under the action of mechanical force, it produces relative motion (sliding, rolling, etc.) to gradually wear the materials on the contact surface in the form of wear debris, so that the shape and size of parts change and become invalid, which is called wear failure. After the parts are worn, their accuracy will be reduced or lost, and even they can not operate normally. The ability of a material to resist wear is called wear resistance, which is expressed by the amount of wear per unit time. The smaller the amount of wear, the better the wear resistance.

Wear mainly includes abrasive wear and adhesive (gluing) wear.

① Abrasive wear.Abrasive wear is the wear caused by hard particles embedded into the material surface to form many chip grooves when the part surface is subjected to friction. This kind of wear often occurs in agricultural machinery, mining machinery, vehicles, machine tools and other machinery due to embedded hard debris (hard particles) and wear.

② Adhesive wear.Adhesive wear, also known as gluing wear, is the wear caused by local welding or adhesion between relatively moving friction surfaces in the friction process, tearing small pieces of materials at the adhesion during separation and forming wear debris. This kind of wear will occur in all friction pairs, such as worm gear and worm, piston ring and cylinder liner of internal combustion engine, bearing bush and journal, etc.

In order to reduce adhesive wear, the selected material and the matched friction pair should be materials of different properties, and the friction coefficient should be as small as possible, preferably with self-lubricating ability or conducive to the preservation of lubricant. For example, in recent years, nylon, polyformaldehyde, polycarbonate and powder metallurgy materials have been used in many equipment to manufacture bearings and shaft sleeves.

(2) Surface fatigue.The two moving surfaces in contact with each other (especially rolling contact) bear the action of alternating contact stress in the working process, which makes the surface material fall off due to fatigue failure, resulting in part failure, which is called surface fatigue failure. In order to improve the surface fatigue resistance of the material, the material should have high enough hardness and certain plasticity and toughness; The material shall contain as few inclusions as possible, and the material shall be subject to surface strengthening treatment. The depth of the strengthening layer shall be large enough to avoid the formation of small cracks in the matrix under the strong pressure layer and the spalling of large blocks of the strengthening layer.

(3) Corrosion failure.The failure of parts caused by surface damage due to chemical and electrochemical corrosion is called corrosion failure. Corrosion failure is not only related to the composition and structure of the material, but also related to the surrounding medium. Materials should be selected according to the composition and properties of the medium.

2. Causes of parts failure

What kind of failure will happen to parts is related to many factors. To sum up, there are four reasons for failure.

1) Part design

Unreasonable design of structure, shape and size of parts is most likely to cause failure. If the keyway, hole or section changes violently, the sharp corner or sharp lack of u is easy to produce stress concentration and cracks. It is caused by the unreasonable estimation of the load-bearing capacity of the parts or the unreasonable reduction of the actual load-bearing capacity in the design environment.

2) Material selection

Unreasonable material selection means that the performance of the selected material cannot meet the requirements of working conditions, or the nominal performance index of the selected material cannot reflect the resistance of the material to the actual failure form. Unreasonable chemical composition and structure of the materials used and poor quality will also lead to the failure of parts, such as excessive inclusions, impurity elements and other defects. Therefore, strict inspection of raw materials is an important step to avoid part failure.

3) Processing technology

In the process of machining and forming, the failure will be caused by unreasonable process methods, process parameters and incorrect operation. Such as overheating, overburning, oxidation and decarburization caused by excessive temperature in the hot forming process; Deformation, cracks and structural defects caused by unreasonable process parameters during heat treatment, and tensile stress at edges, corners and steps of parts due to uneven quenching stress.

4) Installation and use

During the installation of the machine, too tight, too loose, poor alignment, loose fixation or unstable center of gravity, poor sealing, and too much or too little force during assembly and tightening are easy to lead to premature failure of parts. Working under overspeed, overload and poor lubrication conditions, corrosive substances in the working environment and untimely or poor repair and maintenance will cause premature failure of parts.

3. Steps and methods of failure analysis

The basic steps and methods of failure analysis for failed parts are as follows:

(1) Conduct on-site investigation to observe the failure position and form of parts, and clarify the working conditions, operation and failure process of parts; Collect and protect the failed parts, and take photos of the site if necessary.

(2) Understand the background information of the part, a series of historical data such as part design, processing and manufacturing, assembly, use and maintenance, and collect relevant data similar to the failure of the part.

(3) The test analysis mainly includes fracture macro analysis, metallographic structure analysis, electron microscope analysis, composition analysis, surface and internal quality analysis, stress analysis, mechanical analysis and mechanical property test. The above items can be selected according to needs.

(4) Comprehensive analysis: comprehensively analyze the above investigation materials and test results, determine the failure causes (especially the main causes, which is the basis for determining the main failure resistance indicators), put forward improvement measures and test the effect in practice.

The above is the relevant content of "part failure analysis" brought by the core test. I hope it can be helpful to you. We will bring more wonderful content in the later stage. The company's testing services cover: electronic component testing and verification, IC authenticity identification, product design and material selection, failure analysis, function testing, factory incoming material inspection, tape braiding and other testing items. Welcome to call Chuangxin testing. We will serve you wholeheartedly.