Non-destructive testing technology and application

2023-10-22 No Comments Epower Metals Non-Destructive Testing (NDT)

1. Development history of non-destructive testing technology

Nondestructive testing technology has gone through three stages of development, namely, nondestructive flaw detection (NDI), nondestructive testing (NDT) and nondestructive evaluation (NDE). At present, they are generally referred to collectively as non-destructive testing (NDT) rather than specifically to the second stage mentioned above. NDT is the use of acoustic, optical, magnetic, electrical and other characteristics without harming or affecting the use of the object under the premise of the performance of the object to be examined, the detection of the object to be examined for the presence of defects or inhomogeneities, to give the size of the defects, the location of the defects, the nature of the number of information and so on, and then determine the object to be examined by the state of the technology (eg, qualified or unqualified, the remaining life, etc.) of the general term for all technical means. Not only in product quality control play an irreplaceable role and have been recognized by many scientific and technological personnel and the business community, but it also plays an important role in the operation of the equipment in-service inspection.

Non-destructive testing technology mainly began to be applied in the 1950s and 1960s as the primary stage of non-destructive testing, which is characterized by simpler technology and tasks. The technical means to choose could be richer, mainly using ultrasound, rays and other technologies. The task is mainly to detect the presence of defects or abnormalities in the specimen; its basic task is to find the defects in the parts or components without destroying the product, to meet the needs of the project, and its detection conclusions are mainly divided into two categories of defective and non-defective.
With the continuous development of science and technology, especially the production of non-destructive testing, technology needs to continue to improve; detecting whether there are defects obviously cannot meet the actual needs of people. In this stage of development nondestructive testing not only detects whether the specimen contains defects but also includes the detection of some other information about the specimen, such as defects in the structure, nature, location, etc., and tries to grasp more information through the test, for the international industrialized countries, this stage roughly began in the end of the 20th century in the 1970s or the early 1980s.
Although the second stage of nondestructive testing technology has been able to meet most of the needs of industrial production, the quality of materials, components and other requirements continue to improve, especially for the safety of in-service equipment and economic needs more and more prominent, nondestructive testing technology has entered the third stage, that is, the stage of nondestructive evaluation. A landmark event of this stage is the 14th World Conference on Nondestructive Testing (Word conference on NDT, WCNDT) held in New Delhi in 1996, which put forward the important point of view of turning NDT into the nondestructive evaluation and was soon accepted by the nondestructive testing community in various countries. At this stage, people not only want to grasp the presence or absence of defects, attributes, location, size and other information but also to further assess and analyze the degree of influence of these characteristics of defects on the comprehensive performance indicators (such as life, strength, stability, etc.) of the component being inspected, and ultimately to give some conclusions about the comprehensive indicators. At present, industrialized countries are already in this stage of development; the second stage of technology still dominates some other countries, while some are already in the transition from the second to the third stage of development.

2. Introduction to common NDT techniques

NDT mainly has six methods: ultrasonic testing UT (Ultrasonic Testing), ray testing RT (Radiographic Testing), magnetic particle testing MT (Magnetic Particle Testing), penetration testing PT (Penetrate Testing), eddy current testing ET (Eddy Current Testing). (Eddy Current Testing) and Visual Testing (VT).
In the new standard NB/T 47013 “Non-destructive testing of pressurized equipment” issued by the NEA in 2015, in addition to these six commonly used methods, leakage testing (using air, inert gases, such as bicycle tire patching), acoustic emission testing, TOFD, X-ray digital imaging testing, magnetic leakage testing, and pulsed eddy current testing (compared to the conventional eddy current testing with a wider range, higher efficiency, and greater thickness) are also standardized. larger) and other methods are standardized, of which TOFD has more practical applications. In addition, not included in the standard industrial CT and real-time imaging also has a wide range of applications. The following is an introduction to the commonly used methods.

2.1 Ultrasonic detection UT

An ultrasonic transducer generates sound waves that are emitted into the material being tested. As the sound waves penetrate the material, reflections or echoes occur on the backside; any internal discontinuities reflect the sound waves and generate a signal that is sent to the receiver. The time at which the different echoes are received is recorded to determine the thickness of the material and the distance of the discontinuity in the product.

2.2 Radiographic Inspection RT

2.2.1 Conventional Radiographic Inspection

Radiation is very useful for detecting hidden defects in materials and products. It is worth noting that radiographic inspection is particularly effective in detecting volumetric defects in articles, 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 nature and size (length and width) of the defect. Another advantage of this method of inspection is that the defect can be permanently recorded by placing the item under test on a negative. To do this, a source of ionizing radiation is placed on one side of the product being tested and the negative, which is contained in a dark bag, is placed very close to the other side of the product. The radiation is partially absorbed during transmission, and differences in material thickness or absorption characteristics result in different degrees of radiation absorption being 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 known as industrial NDT film.
Table.1 Industrial Flaw Detection Film Categories, Characteristics and Company Models

EU standard EN584-1 Film category	ISO sensitivity S	Radiation dose K_S, mGy D=2.0	Kodak Film Model	Aikefa film model	Fuji Film Model	Lekai film model
C1	32	29	DR50	D2	JX25	–
C2	64	14	M100	D3	50	L3
C3	100	8.7	MX125	D4	5080	L4
C4	200	4.6	T200	D5	80	L5
C5	320	3.2	AA400	D7	100	L7，LA400
C6	400	2.5	CX	D8	150	–

In addition, sunlight film has a wide range of applications in developed countries in Europe, the United States, Saudi Arabia and other energy countries; sunlight film, as its name suggests, is able to see sunlight film, which is relative to the traditional concept of the photographic film cannot see the light in terms of the manufacturer will be wrapped up beforehand in the dark bag containing a sensing screen, in the construction site for direct use, eliminating the need for the film in the darkroom to load the dark bag of this step.

2.2.2 Digital radiography

Computer radiography CR (computer radiography) refers to the radiation through the workpiece after the information is recorded on the imaging plate, read by the scanning device, and then by the computer to generate a digital image of the technology. Because the IP plate is expensive, the number of times the imaging plate is irradiated is limited, and it is not suitable for field operation, so it cannot be popularized.
DR (digital radiography) usually refers to the use of electronic imaging plate technology – flat panel detector technology (FPD Technique). Flat panel technology is generally derived from medical digital imaging.

2.3 Magnetic Particle Inspection MT

Magnetic particle testing can be used to detect defects on and near the surface of ferromagnetic materials. To perform such an inspection, a permanent magnet, electromagnet or electromagnetic coil is used to generate a magnetic field on the sample being inspected. If there are defects in the inspected product, the magnetic flux will be distorted and “leak”. Fine particles of magnetic powder (usually suspended in a carrier fluid and sprayed as a mist) are applied to the surface of the specimen, where they are attracted to the area of the magnetic flux leakage, creating a visible indication of the defect.

2.4 Penetration testing PT

Penetration testing is a widely used and inexpensive inspection method. It is used to localize cracks on the surface of all non-porous materials (e.g., metals and plastics). The inspection method applies a visible or fluorescent dye to the surface of the product under test, which can be washed away with solvent or water. After the dyes are applied to the surface of the product under test in the form of impregnation or spraying, they are able to enter any discontinuity defects by capillary action. The time taken for the dyes to enter the discontinuity is known as the residence time and usually takes at least 20 min.

2.5 Eddy current detection ET

A coil is energized with an alternating current and the current passing through it is constant under certain conditions. If the coil is brought close to the workpiece under test, like a boat in water, eddy currents are induced within the workpiece, and the coil current changes under the influence of the eddy currents. Since the size of the eddy current varies with the presence or absence of defects in the workpiece, the size of the change in coil current reflects the presence or absence of defects. The type of coil used to detect pipe, bar and wire, its inner diameter is slightly larger than the object to be inspected, can find cracks, inclusions, pits and other defects. A probe coil is used for localized detection of the specimen; the application of the coil is placed on the metal plate, tube or other parts; fatigue cracks can be detected; the inserted coil, also known as the internal probe, is placed in the tube or parts of the hole for the inner wall detection, can be used to check the inner wall of various pipeline It can be used to check the degree of corrosion of the inner wall of various pipelines, etc.

2.6 TOFD detection

TOFD (time off light diffraction) is a diffraction time difference method of ultrasonic inspection technology, using a transmitter and a receiver of two broadband narrow pulse probes for detection; the probe is symmetrically arranged relative to the centerline of the weld. The transmitting probe generates unfocused longitudinal wave beams, which are incident at a certain angle to the inspected workpiece, part of which is propagated along the near surface and received by the receiving probe, and part of which is reflected by the bottom 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.
Features:

(1) One sweep can cover almost the entire weld area (except for the blind area on the upper and lower surfaces), and a very high detection speed can be realized;
(2) Good reliability and high detection rate for defects in the middle of the weld;
(3) Capable of detecting various types of defects and insensitive to the direction of defects;
(4) Capable of recognizing defects extending toward the surface;
(5) Using D-scan imaging, defect interpretation is more intuitive;
(6) Very accurate quantification and localization of defects in the vertical direction, with an accuracy error of less than 1mm;
(7) Better detection effect when combined with pulse reflection method, with 100% coverage;
(8) Is not suitable for T-shaped weld detection.

2.7 Ultrasonic phased array technology

The development of ultrasonic technology in addition to TOFD, there is PAUT (phased array ultrasonic testing), that is, ultrasonic phased array detection technology; the development of this new technology is very fast but has yet to establish a national standard. Ultrasonic phased array detection technology uses different shapes of multi-array transducers to generate and receive ultrasonic beams through the control of the transducer array of each array of transmitting (or receiving) pulses of different delay times, change the acoustic wave arrives at (or comes from) a certain point in the object when the phase relationship to realize the focus and the direction of the beam changes, so as to realize the ultrasonic beam scanning, deflection and focusing. Then, use a combination of mechanical scanning and electronic scanning methods to realize the image imaging.
Features: Compared with the traditional manual ultrasonic detection and ray detection, the phased array has the following advantages:

(1) High detection flexibility, speed, on-site inspection only needs a simple sweep of the ring weld without moving back and forth to complete the full weld inspection;
(2) The detection results are intuitive, repeatable and can be displayed in real-time. The weld seam can be analyzed and judged at the same time as scanning. It can be printed and stored on the disk to realize the permanent maintenance of the detection results;
(3) It can detect complex shapes and surfaces or parts that are difficult to access;
(4) Accurate defect localization and high detection sensitivity;
(5) Low operating intensity, no radiation and no dirt.

2.8 Industrial CT

Industrial CT is the abbreviation of industrial computerized tomography technology, which can be used in the detection of objects without damage to the conditions of two-dimensional tomographic images or three-dimensional stereoscopic images in the form of a clear, accurate, intuitive display of the internal structure of the object to be detected, the composition of the material and defective condition, known as the best non-destructive testing and non-destructive evaluation technology. Industrial CT technology involves nuclear physics, microelectronics, optoelectronics technology, instrumentation, precision machinery and control, computer image processing and pattern recognition and other multidisciplinary fields and is a technology-intensive high-tech product. Industrial CT is widely used in automotive, materials, aerospace, aviation, military, national defense and other industrial fields. A space launch vehicle, spacecraft aviation engines, large weapons, geological structure analysis, as well as the quality of mechanical products is an important means of testing.

2.9 Real-time imaging

Real-time imaging is an X-ray non-destructive testing method. In the early days, because the image obtained was analog, it was called real-time imaging, also known as industrial TV. It is a method of real-time display of test results through the screen image, the use of the image of the detection of the object material qualitative and quantitative analysis, judgment and evaluation, so as to obtain the uniformity and consistency of the material of the test object, or to obtain the structure of the object, the assembly, the density of the material, the thickness of the information, to achieve the purpose of non-destructive testing. The real-time imaging method is highly concerned by the industry and rapid development due to its advantages of intuitive and clear detection image, fast detection speed and low cost.

3. Workpiece defects and nondestructive testing comparison table

According to the characteristics of the workpiece defects, choosing the appropriate nondestructive testing methods is the relevant practitioner of the necessary technology. In practice, there are often complex, multiple defects together, then it is necessary to analyze the specific situation, develop appropriate detection methods, and sometimes design and customize some special equipment. For ordinary workpiece defects and non-destructive testing methods, please refer to Table 2.
Table.2 Workpiece defects and nondestructive testing methods against the table

	Surface a		Surface b		Surface c
	VT	PT	MT	ET	RT	DR	UTA	UTS	TOFD
Defects occur during the use of the test piece
Defects occur during the use of the test piece Pitting corrosion Localized corrosion Crack	●	●	●		●	●		◎
	●	●						●
	◎	●	●	◎	◎	◎	●		●
Defects caused by welding
Defects caused by welding Burn through Crack Slag inclusion Unfused Lack of penetration Weld beading Stoma Undercut	●				●	●	◎		◎
	◎	●	●	◎	◎	◎	●	○	●
			◎	◎	●	●	◎	○	●
	◎		◎	◎	◎	◎	●	◎	●
	◎	●	●	◎	●	●	●	◎	●
	●	●	●	○	●	●	○
	●	●	○		●	●	◎	○	●
	●	●	●	○	●	●	◎	○
Defects caused by product forming
Defects caused by product forming Cracks (all product forming) Inclusion (all product forming) Interlayer (plate, pipe) Heavy leather (forging) Air hole (casting)	○	●	●	◎	◎	◎	◎	○
					●	●
	◎	◎	◎					●
	○	●	●	○	◎	◎
	●	●	○		●	●	○	○

Note:

VT – visual inspection, PT – penetration inspection, MT – magnetic particle inspection, ET – eddy current inspection, RT – radiographic inspection, DR – X-ray digital imaging inspection, UTA – ultrasonic inspection (oblique incidence), UTS – ultrasonic inspection (direct injection), TOFD – diffraction time difference ultrasonic inspection; Under normal circumstances, this non-destructive testing technology can detect such defects;
● – Under normal circumstances, this non-destructive testing technology can detect this defect ; ◎ – Under special conditions, this non-destructive testing technology will be able to detect this defect ; ○ – Detection of this defect requires special technology and conditions

4. Application of non-destructive testing technology

4.1 Aerospace

Aerospace is still the most frequently used field for NDT. Aerospace parts are inspected before they are assembled into an aircraft and then periodically during their service life. Aircraft components need to be designed to be as lightweight as possible and to be able to perform high-strength functions, which means that they are subjected to high loads, and their inherent thin weight makes it possible for a small defect to cause damage to the device. The continuous flight, landing, taxiing and pressurization of the cabin of an aircraft will cause fatigue cracks in many devices, which will gradually grow and grow with time and will result in the breakage of the spacecraft and bring about great potential safety hazards. Therefore, regular inspection of spacecraft has become a necessary measure for the safe use of spacecraft.
The main areas of NDT application in the aviation field are:

(1) Inspection of turbofan blades: blade edge and other inspections;
(2) Inspection of composite bonded parts: many applications in the field of aerospace vehicles utilize adhesive-bonded parts. Utilizing NDT for its inspection will increase its reliability;
(3) Inspection of composite skin material structure: In the spacecraft and aviation field, a large number of carbon fiber/epoxy resin skins and aluminum honeycomb composites are used, which is mainly for weight reduction, thus reducing fuel consumption and lowering operating costs. These materials are likewise subject to periodic inspections;
(4) Inspection of aeronautical parts: a large number of shaped parts for aeronautical use need to be inspected using NDT technology;
(5) Inspection of multilayer aluminum structural devices: monitoring of airframe structures is very important for safety. NDT can play an important role in preventing catastrophic structural damage by monitoring aluminum multilayer riveted parts.

There is no doubt that the success of the aircraft industry depends on NDT. Without NDT, the maintenance and flying of airplanes would add huge costs and the safety of flying would be reduced.

4.2 Railroad Track Inspection

In the early days of railroad development, many accidents and even derailments were caused by rail defects. Therefore, the United States, in the 1920s, set up a special railroad inspection company to maintain the railroad. Of course, manual inspection has become a routine system for railroads for many years.
Nondestructive testing of railroad tracks, the early use of magnetic field detection. Such as the U.S. Sperry Company, in 1928, around the establishment of the magnetic field detection method, the detection equipment was put into an inspection car on the railroad track traveling inspection. Magnetic field detection for rail transverse cracks is more sensitive, while other defects contained in the defects, 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 to use ultrasonic flaw detection facilities and testing equipment was also built to detect the car to 6.5 miles and 13 miles per hour running patrol.
At present, China’s railroad mileage is getting longer and longer, and the mileage of high-speed rail is also more than 10,000 kilometers; high-speed rail vehicles and tracks with nondestructive testing requirements are also getting higher and higher. In addition to the system to improve the daily “window period” of manual inspection, there are more NDT facilities are also put into use; these facilities include track inspection (dynamic inspection) car, on-board alarm instruments, portable ride-on instruments, manual ride-on, line fine measurement trolley, electronic track inspection instrument, etc., which are capable of carrying out precise operations.

4.3 Bridge Inspection

According to the construction specification of the bridge, the service life of the bridge is longer than that of the road, and the longest required service life is 100 years, so it is more necessary for the maintenance and inspection of the bridge. Bridge inspection is generally equipped with a certain number of high-performance maintenance equipment to strengthen the monitoring of the bridge. High-performance non-destructive testing equipment, such as: resistance probe acoustic emission probe to detect the corrosion of reinforcing steel in the concrete; advanced ultrasonic ray equipment to detect cracks in the steel structure defects; ultrasonic back to the probe to measure the strength of the concrete and so on.
In China, more and more bridges are built on canyons, which require the use of a large number of cable-stayed structures, and the nondestructive testing of the use of cable-stayed steel is becoming more and more important. At the same time, the current transportation of overloading and other illegal phenomena occurs from time to time, easily causing fatigue cracks in the bridge structure, etc., and the bearing of road bridges is also subjected to test.
In addition, non-destructive testing is also being used in large numbers in shipbuilding, steel cable testing, testing of oil transmission pipelines, and in the field of national defense. The development of NDT and the improvement of technology will bring great convenience to many industrial fields and greatly reduce the operating costs of many industries. Therefore, the study of NDT has extremely important significance.
Author: Ma Chao

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Non-Destructive Testing (NDT)