Summary of NDT testing technique

Nondestructive testing technology has experienced three stages of the development process:Nondestructive flaw detection, nondestructive testing and nondestructive evaluation phase。The first stage is nondestructive flaw detection, the main is to find the defect detection and, in the second stage is a nondestructive testing, is not only to detect defects, also including the detection of some other information of the specimens, such as structure, property, state, and tried to pass the test, to master more information, nondestructive evaluation is the third stage,It not only requires finding defects, detecting the structure and properties of the specimens, status, also asked for a more comprehensive, more accurate comprehensive information，Defects such as shape, size, location, orientation, contents, organization, residual stress, etc., of defective parts of the combination of imaging technology, automation technology, computer data analysis and processing, such as technology, material mechanics, fracture mechanics and so on comprehensive application of knowledge, the specimen or product quality and performance of comprehensive and accurate evaluation。

Nondestructive testing methods：Radiographic testing, ultrasonic testing, magnetic particle testing, penetrant testing, eddy current testing, acoustic emission testing，To meet the needs of production, and with the progress of modern science and technology, the nondestructive testing methods and increasingly wide range，In addition to the several methods mentioned above，Laser, infrared, microwave, liquid crystal are used for NDT,The generation of nondestructive testing technology is the foundation of modern science and technology development. Used for detection of industrial product defects, for example, X-ray radiography is only after German physicist roentgen discovered x-rays, ultrasound is a rapid development in the two world wars sonar and radar technology developed on the basis of, magnetic particle testing based on the theory of electromagnetism, penetrant and thanks to the progress of physics and chemistry, etc。

With the development of modern industry, the quality of product and structure security, use reliability higher and higher demands are proposed, due to destruction of nondestructive testing technology has not specimen, high detection sensitivity, etc. So its application is increasingly wide. At present, the nondestructive testing technology in the domestic many industries and departments, such as machinery, metallurgy, petroleum, petrochemical, chemical, aerospace, shipbuilding, railways, electric power, nuclear industry, weapons, coal, nonferrous metals, building, etc., are widely used.

Advantages of utilizing NDT 

一、 instant find flaw,improve product quality。

nondestructive testing technology can be applied to detect defects to the naked eye can't see the inside of the specimen, the test specimen surface quality, through NDT methods can detect many visible to the naked eye it is difficult to see the tiny flaws. Due to the application of nondestructive testing technology of defect detection range, high sensitivity, good reliability test results, therefore made in container and other products widely used in process inspection and final inspection。

二、Effective guarantee the safe operation of the equipment

Even the design and manufacturing quality fully meet the requirements of the specification container，After a period of using,It is also possible occur damage accident，This is because of the harsh operating conditions and equipment status changes。For example due to high temperature and stress the role of material creep, alternating stress due to fluctuations in temperature, pressure, fatigue stress concentration parts of the equipment, by corrosion, degrade thickness thinning or material and so on. These factors may make existed in the original equipment, manufacturing specification allows small flaw extending cracking, or make the equipment in the original place without defect sample or as new defects, eventually lead to equipment failure. In order to ensure the use of safe, for the boiler pressure vessel, should be tested on a regular basis, defects found in time, avoid the accident.。

三、promote improvement of manufacturing technology

In production, in order to understand the manufacturing process whether appropriate, must process experiment was carried out in advance. In process test, often to nondestructive testing of process samples, and improved manufacturing processes, according to the results of the test eventually determine the ideal manufacturing process. For example, in order to determine the welding procedure specification, when welding experiment of welding specimen, radiography. Then correct welding parameters according to the test result, finally be able to meet the quality requirements of welding technology. Another example, when casting process design, what is happening by radiographic detection of defects of the specimens, and accordingly improve the location of the gate and riser, ultimately determine the optimal casting process。

四、Save money, reduce the production cost

Nondestructive testing in the product manufacturing process, are often considered to increase the test fee, so that the manufacturing costs have gone up. But, if properly in the middle of the manufacturing process of link correctly nondestructive testing, is to prevent waste after process, reduce rework, reduce rejection rate, thereby reducing the manufacturing cost。For example, in the thick plate welding, if nondestructive testing after welding is complete, excessive defects need repair, it takes a lot of working hours, or it is difficult to repair. So it can be welded to the middle of a nondestructive testing first, make sure no overweight and then continue to welding defects, so although nondestructive testing cost increased, but the total manufacturing cost decreased. As another example, to machining of castings, sometimes do not allow the machine after add appears on the surface of the defects such as slag inclusion, porosity, crack, choose to carry on the processing of parts before machining implementation of nondestructive testing, to found defective parts no longer processing, which reduces the rejection rate, save the machining time。

When applying nondestructive detection ,the problems should be noticed

1.To match the destructive testing

The biggest characteristic of nondestructive testing is not damage the material, the artifacts and structure under the premise of testing, so after nondestructive testing, product inspection rate can reach . However, not all need to test items and index can for nondestructive testing, nondestructive testing technology itself and its limitations. Some test only USES the destructive testing, therefore, in the current nondestructive testing can not completely replace destructive testing. That is to say, to the evaluation of an artifact, materials, machinery and equipment, the nondestructive testing result must be compared with the result of destructive testing each other and cooperate, to make an accurate evaluation. Such as liquefied petroleum gas cylinder blasting experiment was carried out in addition to the nondestructive testing. Boiler pipe weld, sometimes taking metallographic specimens and the fracture to the test。

2.Correct selection of testing time

When doing NDT,Must be based on the purpose of nondestructive testing,Correct selection of timing on the implementation of nondestructive testing,such as ,Ultrasonic testing of forgings,General in the forging and after rough machining,Drilling, milling slot, fine grinding, such as before the final machining, because the scan surface is smooth, coupling, is likely to interfere with the detection of holes, grooves, Taiwan has not been processed and processing also relatively easy to find quality problem,Losses are smaller, and, for example, to have high strength steel weld inspection without delay crack, the timing of the nondestructive testing implementation, should be arranged in 24 h after welding is complete. To check whether the heat treatment process is correct, it should be nondestructive testing implementation time after the heat treatment. Only the right to choose the timing of the implementation of nondestructive testing, can successfully complete detection, correct evaluation of the quality of the product。

3.Choose the nondestructive testing methods

In the application of nondestructive testing (NDT) methods, the test method of itself has limitations, may not apply to all artifacts and all defects，In order to improve the reliability of test results, must be before the test, according to the detected content of material, structure, shape, size, there could be what kind of is what shape defects, in what place, what direction, according to the above situation analysis, and then according to the characteristics of the nondestructive testing methods to choose the most suitable test methods。Such as Steel plate delamination defects because of its extending direction parallel to the plate, is not suitable for the X-ray testing and should choose ultrasonic testing,Check the workpiece surface and the crack should not choose ray and ultrasonic testing, and should choose magnetic powder and penetration testing. In addition, when choosing nondestructive testing methods and the application should also fully realize that the purpose of testing is not a one-sided pursuit of the high requirements of the "high quality" products, but at the same time of guarantee sufficient security to ensure that the product of the economy. Only in this way, the choice of the nondestructive testing methods and the application is correct and reasonable。

4. A variety of integrated application of nondestructive testing method

In nondestructive testing applications，Must be aware of any kind of nondestructive test method is not everything. There's mastercard & each NDT methods has its own advantages, also has its disadvantages. Therefore, in the application of nondestructive testing, if possible, don't just use a nondestructive testing method, and as much as possible at the same time adopt method, in order to ensure various detection methods complement each other, each other to get more information. In addition, you should use the nondestructive testing of the test information, other than using the knowledge of the relevant material, welding, machining process and the knowledge of the product structure, combination of judgment, for example, ultrasonic on crack defects detection sensitivity is higher, but no is the lack of qualitative, and ray is the qualitative defects more accurately, the advantages of both, can guarantee the detection result is reliable and accurate。