1. History of the development of non-destructive testing technology
Non-destructive testing technology has gone through three stages of development, namely non-destructive defect detection (NDI), non-destructive testing (NDT) and non-destructive evaluation (NDE). Currently they are generally referred to collectively as non-destructive testing (NDT) and not specifically the second stage mentioned above. END is the use of acoustic, optical, magnetic, electrical and other properties without damaging or affecting the use of the object, subject to the performance of the object under test, detecting the object under test for the presence of defects or heterogeneities, to determine the size of defects, the location of defects, the nature of the number of information, etc., and then select the object to be inspected based on the state of the art (such as qualified or unqualified, remaining life, etc.) of the object to be tested . The general term for all technical means is not only product quality control, which plays an irreplaceable role and is recognized by many scientific and technological workers and the business world, but also the operational inspection function of equipment during operation.
Non-destructive testing technology was primarily used in the 1950s and 1960s as the first stage of non-destructive testing. It is characterized by technology and simpler tasks. The technical means available could be more extensive and mainly use ultrasound, radiation and other technologies. The task is mainly to detect the presence of defects or anomalies in the sample. The basic task is to find defects in the parts or components without destroying the product to meet the project requirements. Findings on detection are mainly divided into two categories: defective and non-defective.
With the continuous development of science and technology, especially the production of non-destructive testing methods, technology needs to be constantly improved. Defect detection obviously cannot meet people's real needs. At this stage of development, non-destructive testing not only detects defects in the sample, but also other information about the sample, such as: B. Defects in structure, type, location, etc., and attempts are made to obtain more information through tests. For international industrialized countries, this phase began around the end of the 20th century, in the 1970s or early 1980s.
Although the second phase of non-destructive testing technology has already managed to satisfy most of the requirements of industrial production, the quality requirements of materials and components continue to improve, especially in terms of the safety of equipment in use and the increasingly important economic requirements. Therefore, non-destructive testing technology has now reached the third phase, the non-destructive testing phase. A milestone in this phase was the 14th World Conference on NDT, WCNDT, which took place in New Delhi in 1996. At this conference the important position was taken to convert NDT into non-destructive testing and this position was quickly accepted by the non-destructive testing community in numerous countries. At this stage, the aim is not only to obtain information about the presence or absence of defects, as well as their characteristics, location and size, but also the influence of these defect properties on comprehensive performance indicators (such as useful life, resistance, stability , etc.) of the inspected component to further evaluate and analyze and finally draw some conclusions about these comprehensive indicators. Industrialized countries are already at this stage of development. In some other countries, the second phase of technology still dominates, while some are already in the transition from the second to the third phase of development.
2. Introduction to common NDT techniques
Non-destructive testing essentially includes six procedures: ultrasonic testing UT (ultrasound testing), radiation testing RT (X-ray testing), magnetic particle testing MT (magnetic particle testing), penetration testing PT (penetration testing) , ET eddy current test ( eddy current test) and visual test (VT).
In the new standard NB/T 47013 “Non-destructive testing of pressure equipment”, issued by NEA in 2015, in addition to these six commonly used methods, leak testing (with air, inert gases, such as bicycle tire patches), acoustic emission, TOFD, digital X-ray imaging test, magnetic leakage test and pulsed eddy current test (compared with traditional eddy current test with longer range, higher efficiency and thicker thickness) and other standardized methods, of which TOFD has more practical applications. In addition, non-standard industrial computed tomography and real-time imaging methods also have a wide range of applications. Below is an introduction to commonly used methods.
2.1 UT ultrasonic detection
An ultrasonic transducer generates sound waves that are emitted into the material being tested. When sound waves penetrate the material, reflections or echoes occur on the back. Any internal discontinuities reflect sound waves and create a signal that is sent to the receiver. The time at which the various echoes are received is recorded to determine the thickness of the material and the distance from the discontinuity in the product.
2.2 RT radiographic examination
2.2.1 Conventional X-ray test
Radiation is very useful for detecting hidden defects in materials and products. It is worth noting that X-ray inspection is particularly effective in detecting volumetric defects in items such as pores, air holes and solid inclusions. Although it cannot be used to measure the thickness of a defect, it is easy to confirm the type and size (length and width) of the defect. Another advantage of this testing method is that the defect can be recorded permanently by placing the item to be tested in a negative. To do this, a source of ionizing radiation is placed on one side of the product to be tested and the negative, contained in a dark bag, is placed very close to the other side of the product. Radiation is partially absorbed during transfer, and differences in material thickness or absorption properties result in different degrees of radiation absorption recorded on the negative. In fact, the negative is sensitive to visible light, not X-rays, and it is the metal sensitizing screens on both sides of the negative in the dark bag that convert the rays into visible light. This type of negative is also called industrial END film.
Table 1: Categories, characteristics and business models for industrial defect detection films
| EU standard EN584-1 film category | ISO S sensitivity | Radiation dose K S mGy D = 2.0 | Kodak Film Model | Aikefa Film Template | Fuji film model | Lekai Film Template |
| C1 | 32 | 29 | DR50 | T2 – The second day | JX25 | – |
| C2 | 64 | 14 | M100 | T3 – The wonderful world of madness | 50 | level 3 |
| C3 | 100 | 8.7 | MX125 | T4 – The big dream | 5080 | L4 |
| C4 | 200 | 4.6 | T200 | T5 – The big success | 80 | L5 |
| C5 | 320 | 3.2 | AA400 | T7 – The big dream | 100 | L7 , LA400 |
| C6 | 400 | 2.5 | Customer experience | T8 – The big dream | 150 | – |
In addition, solar film is widely used in developed countries in Europe, the United States, Saudi Arabia and other energy countries. Sunlight films, as the name suggests, are translucent films that cannot see light compared to traditional photographic films. They are pre-packaged by the manufacturer in a darkroom bag with sensor film and can be used directly on the construction site. This eliminates the need to load film into the darkroom's darkroom.
2.2.2 Digital radiography
CR computed radiography (computed radiography) refers to the radiation through the part after the information is recorded on the imaging plate, read by the scanning device, and then a digital technology image is generated by the computer. Because the IP card is expensive, the irradiation frequency of the imaging card is limited and is not suitable for field operation, so it cannot become popular.
DR (digital radiography) generally refers to the use of electronic imaging board technology – flat panel detector technology (FPD technique). Flat panel technology is often derived from medical digital imaging.
2.3 MT magnetic particle test
Magnetic particle testing can be used to detect defects on and near the surface of ferromagnetic materials. This test uses a permanent magnet, electromagnet, or electromagnetic coil to generate a magnetic field in the sample being tested. If the tested product is defective, the magnetic flux will be distorted and a “leakage” will occur. Fine particles of magnetic powder (usually suspended in a carrier liquid and sprayed as a mist) are applied to the surface of the sample, where they are attracted to the area of magnetic flux leakage, creating a visual indication of the defect.
2.4 PT penetration testing
Penetration testing is a widely used and cost-effective testing method. They are used to locate cracks on the surface of all non-porous materials (e.g. metals and plastics). This testing method involves applying a visible or fluorescent dye to the surface of the tested product, which can be washed off with solvent or water. After dyes have been applied to the surface of the product to be tested in the form of impregnation or spraying, they can penetrate any discontinuity defects by capillary action. The time it takes for dyes to penetrate the discontinuity is called residence time and is usually at least 20 minutes.
2.5 ET eddy current detection
A coil is supplied with alternating current and the current flowing through it is constant under certain conditions. When the coil is brought close to the workpiece to be tested, such as a boat on water, eddy currents are induced in the workpiece and the coil current changes under the influence of the eddy currents. Since the magnitude of eddy current varies with the presence or absence of defects in the workpiece, the magnitude of change in coil current reflects the presence or absence of defects. The type of coil used to detect pipes, rods and wires whose internal diameter is slightly larger than the object being inspected can detect cracks, inclusions, holes and other defects. A probe coil is used for localized detection of the sample; the coil application is placed on the metal plate, tube or other parts; Fatigue cracks can be detected; The inserted coil, also known as an internal probe, is placed in the pipe or parts of the hole to detect the internal wall, and can be used to check the internal wall of various pipes. It can be used to check the corrosion degree of the inner wall of various pipelines, etc.
2.6 TOFD detection
TOFD (Time Off Light Diffraction) is a time difference diffraction method of ultrasonic testing technology in which a broadband narrow-pulse two-probe transmitter and receiver are used for detection. The probe is positioned symmetrically to the center line of the weld. The transmitter probe generates defocused beams of longitudinal waves that fall on the part to be tested at a specific angle. Part of it propagates along the near surface and is received by the receiving probe, part is reflected from the lower surface and received by the probe. The receiving probe determines the position of the defect and its height by receiving the diffraction signal from the defect tip and its time difference.
Characteristics:
- (1) One sweep can cover almost the entire welding area (except the blind area at the top and bottom), and a very high detection speed can be achieved.
- (2) Good reliability and high detection rate of defects in the middle of welding;
- (3) It is capable of detecting various types of defects and is insensitive to the direction of defects.
- (4) ability to detect defects that extend to the surface;
- (5) When using D-scan images, the interpretation of defects is more intuitive.
- (6) Very accurate quantification and location of defects in the vertical direction with an accuracy error of less than 1 mm;
- (7) Better detection effect combined with 100% coverage pulse reflection method;
- (8) Not suitable for detecting T-shaped welds.
2.7 Ultrasonic Phased Array Technology
In addition to TOFD, ultrasonic technology also includes PAUT (Phased Array Ultrasonic Testing), i.e. phased array ultrasonic detection technology. This new technology is developing very quickly, but has not yet become a national standard. Phased array ultrasonic detection technology uses multi-array transducers of different shapes to generate and receive ultrasound beams. By controlling the array of transducers in each array, transmit (or receive) pulses are sent with different delay times. When the sound wave reaches (or comes from) a certain point on the object, the phase relationship changes to change the focus and direction of the beam, thus realizing the scanning, deflection and focusing of the ultrasonic beam. A combination of mechanical and electronic scanning methods is then used to perform the imaging.
Features: Compared with traditional manual ultrasonic detection and beam detection, phased array has the following advantages:
- (1) High flexibility and detection speed. During on-site inspection, only a simple sweep of the weld ring without back and forth movement is required to complete the complete weld inspection.
- (2) Detection results are intuitive, repeatable, and can be viewed in real time. The weld seam can be analyzed and evaluated at the same time as scanning. It can be printed and saved to the hard drive to ensure long-term retention of recognition results.
- (3) Can recognize complex shapes and surfaces or difficult-to-reach parts;
- (4) Accurate defect location and high detection sensitivity;
- (5) Low operating intensity, no radiation and no dirt.
2.8 Industrial TC
Industrial CT is the abbreviation for industrial computed tomography technology, which can be used to detect objects non-destructively using two-dimensional tomographic images or three-dimensional stereoscopic images. The internal structure of the object to be recognized, the material composition and the state of the defect are presented clearly, precisely and intuitively. This is considered the best non-destructive testing and evaluation technology. Industrial CT technology includes nuclear physics, microelectronics, optoelectronics, instrumentation, machinery and precision control, computer vision and pattern recognition and other multidisciplinary fields, and is a high-tech and technology-intensive product. Industrial CT is widely used in automotive, materials, aerospace, aviation, military, national defense and other industrial areas. It is an important testing tool for launch vehicles, spacecraft engines, large weapons, geological structural analysis and quality of mechanical products.
2.9 Real-time images
Real-time imaging is a non-destructive testing method that uses X-rays. It used to be called real-time imaging or industrial television because the image obtained was analog. This is a method of displaying test results on the screen in real time. The object material detection image is used for qualitative and quantitative analysis, evaluation and evaluation to determine the uniformity and consistency of the test object material or to obtain information about the structure, composition, density and thickness of the object, thereby achieving the purpose of non-destructive to achieve testing. The real-time imaging method is highly valued by the industry and is developing rapidly because it has the advantages of intuitive and clear detection image, fast detection speed and low cost.
3. Part defect comparison table and non-destructive testing
According to the characteristics of the part defects, the selection of appropriate non-destructive testing methods is a relevant professional of the necessary technology. In practice, complex and multiple errors often occur together. It is then necessary to analyze the specific situation, develop appropriate detection methods and, sometimes, design and adapt special devices. For common part defects and non-destructive testing methods, see Table 2.
Table 2: Part defects and non-destructive testing methods compared to the table
| Surface one | Surface b | surface c | |||||||
| TV | PT | MT | ET | TR | DR | UTA | UTS | TOFD | |
| Defects occur when using the test sample | |||||||||
|
Defects occur when using the test sample Localized corrosion Local corrosion Crack |
● | ● | ● | ● | ● | ◎ | |||
| ● | ● | ● | |||||||
| ◎ | ● | ● | ◎ | ◎ | ◎ | ● | ● | ||
| Defects caused by welding | |||||||||
|
Defects caused by welding To escape Crack Slag inclusion Not fused Lack of penetration Weld stoma Undercut |
● | ● | ● | ◎ | ◎ | ||||
| ◎ | ● | ● | ◎ | ◎ | ◎ | ● | ○ | ● | |
| ◎ | ◎ | ● | ● | ◎ | ○ | ● | |||
| ◎ | ◎ | ◎ | ◎ | ◎ | ● | ◎ | ● | ||
| ◎ | ● | ● | ◎ | ● | ● | ● | ◎ | ● | |
| ● | ● | ● | ○ | ● | ● | ○ | |||
| ● | ● | ○ | ● | ● | ◎ | ○ | ● | ||
| ● | ● | ● | ○ | ● | ● | ◎ | ○ | ||
| Defects caused by product deformation | |||||||||
|
Defects caused by product deformation Cracks (on all products) Inclusion (all product forms) Intermediate layer (plate, tube) Heavy Leather (Forging) Air hole (casting) |
○ | ● | ● | ◎ | ◎ | ◎ | ◎ | ○ | |
| ● | ● | ||||||||
| ◎ | ◎ | ◎ | ● | ||||||
| ○ | ● | ● | ○ | ◎ | ◎ | ||||
| ● | ● | ○ | ● | ● | ○ | ○ | |||
observation :
- VT – visual inspection, PT – penetration test, MT – magnetic particle test, ET – eddy current test, RT – radiographic test, DR – digital X-ray imaging test, UTA – ultrasonic test (oblique incidence), UTS – ultrasonic test (direct injection), TOFD – ultrasonic test with difference in diffraction time; Under normal circumstances, this non-destructive testing technology can detect such defects;
- ● – Under normal circumstances, this defect can be detected using this non-destructive testing technology; ◎ – Under special conditions, this non-destructive testing technology can detect this defect; ○ – Detection of this defect requires special technology and conditions
4. Application of non-destructive testing technology
4.1 Aerospace
The aerospace industry remains the most widely used field for non-destructive testing. Aerospace parts are tested before installation on an aircraft and then regularly throughout their service life. Aircraft components must be designed to be as light as possible and capable of performing high-strength functions, meaning they are subject to high loads and their inherently light weight makes it possible for a small defect to damage the device. The continuous flight, landing, taxiing, and cabin pressurization of an aircraft results in fatigue cracks in many devices, which gradually increase over time and result in spacecraft rupture, posing major potential safety risks. Therefore, regular inspection of spacecraft has become a necessary measure for the safe use of spacecraft.
The main areas of application of END in the aviation sector are:
- (1) Turbofan blade inspection: blade edges and other inspections;
- (2) Testing of composite parts: Bonded parts are used in many aerospace applications. Testing using non-destructive testing (NDT) increases your reliability.
- (3) Testing the structure of coating composite materials: In the field of spacecraft and aviation, many carbon fiber/epoxy resin and aluminum honeycomb composite materials are used, which are mainly for weight reduction, thus reducing fuel consumption and reducing operating costs. These materials are also subject to regular testing.
- (4) Testing of aerospace parts: A large number of aerospace molded parts need to be inspected using NDT technology.
- (5) Inspection of multi-layer aluminum structural components: Inspection of aircraft structures is very important for safety. Non-destructive testing can play an important role in preventing catastrophic structural damage by monitoring multi-layer riveted aluminum parts.
There is no doubt that the success of the aviation industry depends on non-destructive testing. Without non-destructive testing, aircraft maintenance and operating costs would increase enormously and flight safety would decrease.
4.2 Track inspection
In the early days of railway development, many accidents and even derailments were due to railway defects. Therefore, in the 1920s, the United States established a special railroad inspection company to maintain the railroad. Of course, manual inspection has been a routine system on railways for many years.
Magnetic field detection has previously been used in non-destructive testing of railroad tracks. For example, the magnetic field detection method was introduced by the American company Sperry in 1928. The detection equipment was installed on an inspection car that ran along the tracks. Magnetic field detection is most sensitive to transverse cracks in rails, while other defects such as seams, delamination, corrosion and other defects are not sensitive. These defects are also the cause of fatigue cracks. Therefore, in the 1960s, the United States began using ultrasonic flaw detection equipment, and test equipment was also built to detect cars patrolling at speeds of 6.5 miles and 13 miles per hour.
At present, China's railway network is becoming longer and longer, and the route length of high-speed trains is also more than 10,000 kilometers. The requirements for high-speed vehicles and rails, which must undergo non-destructive testing, are also becoming increasingly high. In addition to the system to improve the daily “window” for manual checks, more NDT facilities are also being used. These facilities include track inspection cars (dynamic testing), on-board alarm devices, portable devices, hand-held devices, track fine measuring cars, electronic track inspection devices, etc., which can carry out precise operations.
