Current status and development trend of nondestructive testing technology for aviation equipment

　　Nondestructive testing technology has played an important role in ensuring the safe and reliable operation of aviation equipment and providing technical support. Whether in the manufacturing and production process of aviation equipment or in-service testing, daily maintenance and inspection of various types of aircraft, nondestructive testing has played an extremely important role. In order to achieve the sustainable development of nondestructive testing technology, it is necessary to promote some new nondestructive testing concepts and new testing ideas. Green nondestructive testing, which is typically characterized by health monitoring, integrated testing and data fusion, as well as digitization, imaging and informatization, may be a testing technology worth developing in the future.

　　Although in the past 10 to 20 years, especially after entering the 21st century, the traditional manufacturing industry is facing huge challenges from the IT industry, the level of equipment manufacturing, especially the level of large-scale equipment manufacturing, is still the most important indicator of a country's industrial level. As a non-destructive testing technology that is closely related to the safety, quality and reliability of the entire life cycle of equipment, its development level is also an important indicator of a country's equipment manufacturing industry level. The modernization of economic development must be based on reliable modernization and safe modernization. The former must be based on manufacturing, which is the foundation of the country and an important guarantee for the sustainable development of a society. The important guarantee condition for the latter is that the core and key manufacturing technology of major equipment related to the national economy and people's livelihood can be mastered in their own hands. This is particularly prominent in the aviation industry. Aviation manufacturing technology has become one of the top ten key development industries in my country in the future, and its safety and reliability are even more important. Although the overall level is far behind that of developed countries such as the United States and Europe, the launch of commercial large aircraft projects and the establishment of Zhuhai Aviation Industrial City mark the coming of an era of rapid development of my country's aviation manufacturing industry. In connection with this, in the future for a long period of time, aviation non-destructive testing technology will inevitably have a rapid, stable and lasting development period.

　　Extending the safe service life of aviation equipment, including aircraft, is the development trend of modern aviation equipment. Nondestructive testing (NDT) or nondestructive evaluation (NDE) technology plays an extremely important role in the entire life cycle of aviation equipment. It is of great significance to ensure flight safety and extend the life of aircraft. With the use of damage tolerance theory to replace the past safe life design theory in aviation technology, the application view of nondestructive testing technology in aviation equipment has also undergone a fundamental change. That is, nondestructive testing technology should be responsible for the entire life of the equipment (from cradle to grave). It should not only be able to detect existing defects or cracks (in the traditional sense), but also be able to predict the law of crack development to ensure the correct implementation of damage tolerance theory.

　　Nondestructive testing technology is a typical engineering application technology with low input and high output. It is difficult to find any other branch of applied disciplines that can compare with nondestructive testing in terms of the profound technical knowledge, the numerous basic research fields covered, and the wide range of application fields involved. In October 1982, Reagan, then President of the United States, said in a message of congratulations to the American Society for Nondestructive Testing (ASNT) on its 20th anniversary that (nondestructive testing) can bring greater reliability to aircraft and space vehicles, power plants, ships, automobiles and buildings. Without nondestructive testing, (the United States) would not be able to enjoy its current leading position in many fields such as aircraft, ships and automobiles and other fields [3]. As a highly applicable technology, nondestructive testing technology can only be put to good use and have a broad space for development if it is combined with national large-scale engineering projects to solve the safety issues that are urgently needed in national large-scale and key engineering projects.

　　The non-destructive testing of aviation equipment covers a wide range of areas, including the testing of aviation materials, the testing of the manufacturing process of aviation structural parts, and the testing of in-service equipment (aircraft). Although they are all non-destructive testing, their testing focuses and covered areas are very different. When discussing the non-destructive testing technology of aviation equipment, this article will focus on the testing of in-service equipment (or field testing), which may also be the most challenging research and development field.

　　Main features of nondestructive testing of aviation equipment

　　The nondestructive testing of aviation equipment faces some special situations, mainly: (1) The structure is complex. (Except for production lines) in most cases, automatic equipment may not play a big role. (2) The range of materials is wide, including aluminum alloys, titanium alloys, high-strength steel, composite materials, etc., and the detection methods suitable for different materials will also be different. For example, composite materials have special failure phenomena. The most threatening defects are delamination, matrix cracking and fiber breakage. At the same time, due to acoustic anisotropy and a particularly large acoustic wave attenuation coefficient, conventional ultrasonic testing is difficult. Titanium alloy is a non-magnetic material, but it has a coarse-grained structure. Both ultrasonic and eddy current testing face some new problems. (3) The object is complex. Whether it is an old aircraft or a new aircraft, in many cases the accessibility of the inspection is poor, especially in parts such as blades, compressor disks and turbine disks, which also brings great difficulties to in-service inspection work. (4) It is necessary to conduct preliminary research on some special parts, special areas and possible defect detection methods of leading flying aircraft. Due to the lack of relevant information, this work often requires a lot of preliminary research based on existing inspection experience, so the risk is often relatively high.

　　Current status of nondestructive testing of aviation equipment in my country

　　The aviation field has always been the most common and active department in China for conducting research on non-destructive testing theory and applications. The principle that should be followed in non-destructive testing of aviation equipment is to find suitable and effective methods, rather than blindly pursuing high, precise and cutting-edge methods; to use the "right" ones, not blindly pursuing "expensive" ones! For example, in civil aviation, visual inspection (including visual inspection with the help of instruments ) may account for more than 70% of its non-destructive testing workload, and visual inspection plus conventional inspection may account for more than 90%. Of course, due to the particularity of aviation equipment and the demand for high safety and high reliability, the latest and most advanced detection methods are often first applied in the aviation sector (structural health monitoring, pulsed eddy current, laser and infrared imaging, etc.), but that is the demand for some special parts or special occasions in actual detection, rather than using new technologies for the sake of applying new technologies.

The non-destructive testing　　in China's aviation field is generally at the world's advanced level, and its researchers and technicians can solve and overcome various complex technical problems faced by the aviation field. Friction welding is a new solid-phase welding technology with important applications in aviation manufacturing. Because the defects in the weld area are close and subtle, and the metallurgical structure in the weld area is close to the parent material, it is difficult to solve the friction welding detection problem using conventional X-ray or ultrasonic detection methods for molten welds. Liu Songping et al. used the comprehensive information such as reflection/scattering/diffraction generated by the incident sound wave in the weld area to distinguish and identify weld defects, and achieved good detection results. On the basis of using conventional TOFD technology, it is proposed to use L-TOFD and P-scanning technologies to comprehensively detect the quality of stir friction welding. This research method can replace the traditional X-ray detection method and overcome its shortcomings of difficulty in detecting longitudinal cracks in the weld, which plays a role in ensuring the quality of stir friction welding. Liu Songping et al. have also conducted innovative research on the detection of composite material defects. The CUS-6000 composite material high-efficiency ultrasonic automatic scanning imaging detection equipment successfully developed has reached the industrial application level and can directly serve the manufacturing of composite materials. The ultrasonic microscopy technology developed by the research team can accurately determine the defects of each composite material ply, and will play an important role in improving the manufacturing quality of composite materials in the future.

Figure 1 Key cracks in a wall of an aircraft predicted by AE

　　Guo Guangping and others have conducted in-depth research on laser offset speckle interferometry technology for a long time. For the C-sandwich radome peak-cell structure, the weak debonding defects existing in the manufacturing process are difficult to detect using conventional ultrasonic C-scanning technology. The vacuum-loaded laser speckle interferometry system they developed can detect close-fitting weak debonding defects on site. This technology can be used for debonding detection between the upper skin and the peak cell of the radome, and between the middle skin and the upper side peak cell. Its detection capability reaches the industrial application level. [page]

Figure 2 A small crack in a key structure discovered in advance due to AE prediction

　　Geng Rongsheng and others tracked and monitored the damage development of two types of third-generation aircraft in the fatigue test of the whole aircraft for 8 years from 2004 to 2011. For the first time, they established a comprehensive crack monitoring technology centered on acoustic emission technology, which ensured the smooth progress of the test. The life of the two models was increased by more than 50%. Their research group developed a relatively complete set of acoustic emission signal processing and prediction methods. After fully studying and obtaining the statistical average characteristics of the background acoustic emission signal in the absence of cracks, they adopted trend analysis technology based on statistical analysis principles, filtering technology based on time, space, amplitude and energy, signal processing methods based on the study of the amplitude distribution characteristics of acoustic emission signals, and the use of multi-parameter comprehensive identification technology to obtain the acoustic emission characteristics of fatigue crack initiation and expansion. They developed a unique set of splitting the monitored acoustic emission signals into different time series, and then used the correlation of statistical average parameters and other characteristic quantities to implement monitoring, which achieved significant results. Taking the monitoring of a certain type of aircraft from 2004 to 2008 as an example, during the entire test process, 38 key cracks were successfully predicted and discovered, and more than 100 general cracks were discovered with almost no false alarms. This is a miracle in the history of crack monitoring in full-scale aircraft fatigue tests. Due to the implementation of full-process monitoring and detection (and targeted durability repairs), the flight life of the fleet has increased by 50%. The above method was also successfully applied to the full-scale fatigue test of another type of aircraft after 2009. This research has great political, military and social significance, and has created economic benefits in the tens of billions. Figures 1 and 2 are two examples of using acoustic emission (AE) technology to predict key structural cracks (if not predicted, the fatigue test is very likely to fail).

Development direction of nondestructive testing technology for aviation equipment and areas that need attention

　　The author has discussed the future development direction of my country's non-destructive testing technology many times, and proposed to rebuild China's non-destructive testing market on a higher platform. While vigorously developing intelligent, automated and image-based testing devices, it is necessary to actively develop some new application areas. This article explores some issues closely related to aviation equipment.

　　1. Vigorously develop structural health monitoring (SHM) technology

　　The so-called structural health monitoring is to implement a series of monitoring measures on the structural (health) state and conduct (health) state evaluation. When the structure is damaged early or fatigue cracks are initiated, repair measures can be taken to the structure to avoid the occurrence of irreparable damage to the structure. Just as the best "cure" is to strengthen prevention and early diagnosis to "prevent problems before they happen", for aviation equipment, health monitoring is the best way to effectively improve the reliability of equipment and extend its service life in the future. In addition to the acoustic emission monitoring mentioned above, which is a structural health monitoring technology to a certain extent, many "effective" methods have been developed, such as the use of smart sensor technology, smart material technology, fiber optic sensors and strain monitoring technology. Since a single method always has its limitations, the combined use of multiple methods may be the most effective health monitoring method. The essence of health monitoring is the need to make a comprehensive evaluation of the health state. At this level, there is currently no technology that is truly health monitoring. Most of them belong to the category of damage detection (which can detect early damage or early cracks), but some "pre-buried" or preset sensors can implement continuous detection. Regardless of the technology used (note that no one technology is a panacea), understanding of the structure itself and engineering application experience are crucial.

　　2. Promote green NDT - the development of non-destructive testing technology must be consistent with the overall idea of ​​my country's industrial development

　　Green manufacturing, that is, the use of energy-saving and emission-reduction technologies to produce environmentally friendly mechanical manufacturing equipment, is undoubtedly the development direction of the mechanical manufacturing industry [6]. Future non-destructive testing equipment should also be green, that is, environmentally friendly equipment. Therefore, some traditional detection methods that may pollute the environment will gradually be eliminated or replaced by new methods and new detection media. For example, magnetic particle inspection, with the advancement of magnetic flux leakage detection technology and the improvement of detection sensitivity, and the latter may be easier to achieve intelligent detection and visual detection, in many occasions, magnetic particle inspection may be replaced by magnetic flux leakage detection. Another phenomenon that has already occurred and is accelerating is that digital radiographic detection technology will eventually replace traditional film detection methods. In recent years, with the rapid development of new digital image plates and new CR technology that uses image fluorescence storage, the imaging effect of digital radiography is completely comparable to that of traditional film. Therefore, given its environmentally friendly characteristics and the ability to store, reuse and achieve long-distance transmission, it is just around the corner to replace traditional film radiographic detection technology. On the other hand, with the advancement of ultrasonic testing technology, such as the strengthening of the role of TOFD technology in weld detection and the establishment of standards, it has become possible to replace or partially replace radiographic testing (in terms of welds) with ultrasonic TOFD, which is also in line with the concept of green testing. The non-destructive testing of aviation equipment needs to pay attention to the promotion and application of these new technologies, and there is still a long way to go in this regard.

　　3. Smart NDT and information-based NDT - information-based and intelligent non-destructive testing equipment

　　A hot topic in the manufacturing industry at present is information manufacturing and intelligent manufacturing, that is, the mechanical equipment system manufactured can perform intelligent activities such as analysis, reasoning, judgment, conception and decision-making. By using the chip sensor technology that has been developed and combining it with microelectronics and high-performance computers, the conditions for producing integrated (comprehensive) equipment with information system functions and intelligent detection should be met. A direct result of intelligent NDT and informationized NDT is to make flaw detection (non-destructive testing) "more simple". In fact, as early as the 1990s, the Air Force conducted very useful explorations in this regard. Some major field flaw detection processes are stored in digital ultrasonic and eddy current flaw detection equipment, which greatly simplifies the operating steps of field personnel and improves the reliability of field flaw detection. The KK-30 intelligent ultrasonic flaw detector and SMART-97 intelligent eddy current flaw detector, which have solidified the field flaw detection process of major aircraft models, have the prototype of intelligent instruments . Of course, due to the lack of visual detection results and the lack of automatic recognition function, the intelligence level of these instruments is still low.

　　At present, image processing and automatic image recognition technologies are developing rapidly, and some of them are already very mature. We can cooperate with researchers in this field to study non-destructive testing equipment with a high degree of intelligence.

　　4. Integration and data fusion technology

　　Each nondestructive testing method has its scope of application, so it is difficult to achieve a complete evaluation of the object being tested. In order to improve the reliability and efficiency of testing, reduce testing costs, and achieve the integrity of testing, nondestructive testing integration technology that includes multiple testing methods can be used, but the results obtained by different testing methods should not be isolated, but should be organically connected and comprehensively weighted, and then the test object should be evaluated, that is, the data (results) need to be fused. Therefore, one of the most basic requirements for integrated testing instruments is to provide an effective data fusion platform, rather than simply piecing together several different testing functions. At the 2011 Global Chinese Nondestructive Testing Summit Forum, Lin Junming proposed the concept of cloud testing, trying to implant cloud computing into nondestructive testing technology. This idea may subvert some traditional nondestructive testing design concepts. Although it is still under exploration, with the so-called "cloud technology" surging development, what impact will it have on the development of nondestructive testing really needs special attention. [page]

　　The same as the integrated instrument is the special detection instrument for specific objects, which is equivalent to the problem-solving method of "tailoring" and "customizing on demand". For aviation equipment detection, this is also a very promising development direction. Perhaps because of the special needs of the field, the Air Force has relatively rich experience in this regard. For example, when testing the blade slot on the left side of the first-stage disc boss of a certain type of engine low-pressure compressor, the TOFD double probe of one receiving and one transmitting was changed to a double crystal probe. The 5MHz19° left TOFD probe specially developed for the blade slot of the first-stage disc can effectively solve the crack detection problem of the blade slot on the left side of the first-stage disc boss. In addition, in the detection of cracks on the back of the second-stage turbine blade of a certain type of engine, the relevant research units of the Air Force developed a special eddy current probe and distributed it to the troops, which effectively guaranteed the safety of this type of engine.

5. Application of phased array technology in crack detection of complex components such as turbine blades, turbine disks and compressor disks

　　Since the focus position and the direction of the sound beam (deflection angle) can be easily adjusted by the circuit, the application of phase control technology in aviation equipment may first be reflected in the crack detection of complex components such as turbine blades, turbine disks, grate disks and compressor disks. This requires some experimental research in advance based on the geometric dimensions of the specific detection object and the formulation of a detailed detection plan.

　　6. Application of new technologies such as infrared and laser speckle interferometry

　　Composite materials are more widely used in new aircraft, especially fourth- and fifth-generation fighters (including improved third-generation aircraft) and large commercial aircraft. Miniaturized infrared thermal imaging and laser speckle interferometry detectors can quickly and extensively detect major defects such as delamination and debonding of composite materials. Therefore, it can be foreseen that these instruments will have good application prospects in the field in the future.

　　Chinese Society of Nondestructive Testing (ChSNDT) and Overview of Domestic Nondestructive Testing

　　The Society for Non-destructive Testing was founded in 1978. It is an important branch of the Chinese Mechanical Engineering Society (CMES) and accepts the leadership of CMES. However, in foreign relations, it is an independent society. Its external name is the Chinese Society for Non Destructive Testing (ChSNDT-China Society for Non-destructive Testing). ChSNDT is one of the few non-destructive testing societies in the world that was established the earliest, performed the most actively, and had the largest number of members. Over the past 33 years, although the Society for Non-destructive Testing has gone through nine national conferences and personnel changes, its development purpose has always been to unite non-destructive testing personnel across the country, to promote academic exchanges, to improve the level of non-destructive testing technology, to improve product quality, to improve the safe and reliable operation of equipment, to help non-destructive testing personnel actively participate in international cooperation and exchanges, to promote the development and progress of my country's non-destructive testing industry, and to promote international cooperation.

　　ChSNDT consists of 7 professional committees (ultrasound, electromagnetic eddy current, ray, acoustic emission, magnetic powder and penetration, new technology, stress testing), 5 working committees (academic, education and training and popular science, personnel qualification certification, in-service equipment testing, editing and publishing) and the NDT magazine editorial committee. The current 9th committee has a total of 99 members (directors).

　　ChSNDT held the 17th World Conference on Nondestructive Testing (WCNDT) in Shanghai in October 2008. The scale, number of participants and exhibitors were unprecedented. The purpose of the conference was to let (China's) nondestructive testing go to the world and let the world know about China's (nondestructive testing). The convening of this grand event marked that the development and progress of China's nondestructive testing technology level was recognized by the world, and also marked the lofty status and reputation of China's nondestructive testing industry in the world.

　　ChSNDT took advantage of the opportunity of hosting the 17th WCNDT and the fact that China's non-destructive testing technology level has been recognized and highly concerned by the world, and has carried out the mutual recognition of non-destructive testing personnel qualifications with the European Federation of Non-destructive Testing (EFNDT). After the unremitting efforts of the society, this goal was finally achieved. In April 2009, the European Union sent two reviewers to review the qualification certification system of the Society for Non-destructive Testing. On September 1, 2009, at the first working committee meeting of the European Union in Brussels, it was discussed and unanimously passed: the personnel certification system of the China Society for Non-destructive Testing was added to the mutual recognition system, and it was officially approved and issued on September 15 of the same year. Mutual recognition with the European Union is an important progress in the qualification certification work of the society's personnel, and may be a milestone event in this regard.

　　ChSNDT has hosted many large-scale international non-destructive testing conferences. In addition to the 17th WCNDT conference mentioned above, in August 2011, the first World Acoustic Emission Conference (WCAE'2011, Beijing) was held in Beijing, and in November, the Global Chinese Non-destructive Testing Summit Forum was successfully held.

　　The overall level of my country's non-destructive testing technology is consistent with my country's economic development level. Its position in the world's non-destructive testing is more or less consistent with China's economic and industrial level, especially the position of the equipment manufacturing industry in the world. Data show that after 2008, China, as a major machinery manufacturing country in the world and the second largest exporter of machinery, electrical and transportation equipment, has stood among the world's major machinery manufacturing countries [1]. Although non-destructive testing technology itself is not a direct productivity to a certain extent, its technical level can reflect a country's industrial level and is closely related to the country's economic development trend. In line with the world's major machinery manufacturing country, China is a major non-destructive testing country, and its non-destructive testing market is the most dynamic and attractive market, which has been recognized by the world's non-destructive testing community. Based on the number of non-destructive testing practitioners alone, data show that my country currently has about 250,000 people directly engaged in or engaged in technical work related to non-destructive testing, and there are more than 5,000 units involved in non-destructive testing technology application, scientific research, teaching and training, distribution, technical consulting services, engineering services, etc., which is unmatched by any country. China currently has about 2,000 inspection and testing institutions engaged in non-destructive testing services, more than 20,000 machinery manufacturing and installation companies have their own non-destructive testing teams, and there may be nearly 500 manufacturers of non-destructive testing equipment. A number of large-scale production companies or groups have emerged in the field of ultrasonic, eddy current, magnetic powder and conventional X-ray testing instruments, and some cutting-edge instruments (such as phase control, ultrasonic TOFD special instruments, array eddy current) also have market-oriented production scale.

　　The engineering application field of non-destructive testing technology in my country is at an internationally advanced or even leading level. Thanks to the rapid economic development and the extensive development of large-scale engineering construction projects in various fields of national economy and people's livelihood, my country has carried out many leading research, applications and technology development in the field of non-destructive testing engineering applications, and has made remarkable achievements in nuclear power, railways, especially high-speed railways, special equipment (pipelines, pressure vessels, amusement facilities, etc.), oil pipelines, natural gas pipelines, mountain and dam rock stability monitoring, aircraft fatigue damage monitoring and other fields. At present, my country's engineering and technical personnel can basically solve various types of conventional non-destructive testing and various technical problems faced by various large-scale engineering projects on their own. It can be said that there is almost no major industrial sector where non-destructive testing technology is still in a blank position. At the same time, almost all existing NDT methods have been applied or studied in China without exception. On the one hand, it is unrealistic and may not be allowed for a large country like us to rely on foreign companies to contract engineering projects and solve various engineering and technical problems. On the other hand, non-destructive testing technicians in various industrial sectors in China also have the ability to solve these technical problems. For an overview of the development of nondestructive testing technology in various industrial sectors, please refer to the 2008 supplement of Nondestructive Testing Magazine and the 2010 Annual Report on the Development of China's Nondestructive Testing Technology.

　　Conclusion

　　Nondestructive testing technology has played an important role in ensuring the safe and reliable operation of aviation equipment and providing technical support. Whether in the manufacturing and production process of aviation equipment or in-service inspection, daily maintenance and inspection of various types of aircraft, nondestructive testing has played an extremely important role. In order to achieve the sustainable development of nondestructive testing technology, it is necessary to promote some new nondestructive testing concepts and new testing ideas. Green nondestructive testing, which is typically characterized by health monitoring, integrated testing and data fusion, as well as digitization, imaging and informatization, may be a testing technology worth developing in the future. Taking the research team of the author as an example, a comprehensive crack monitoring technology centered on acoustic emission technology was established in the life extension test of two types of third-generation aircraft fleets to ensure the smooth progress of the test. The life of both types of aircraft has increased by more than 50% and has played an important role. This shows that nondestructive testing is a typical engineering application technology with low input and high output. We should make full use of the existing good development trend, make necessary integration of existing resources, and strive to enable nondestructive testing technology in the aviation field to operate on a higher platform in a relatively short period of time.