NDT and SHM in aircraft structure maintenance

Transcription

1 NDT and SHM in aircraft structure maintenance Ning NING More info about this article: Aircraft Strength Research Institute of AVIC, Xi'an, China Corresponding author: Ning Ning, Abstract Based on the development of civil aircraft components reliability and maintain idea, the maintain mode of civil aircraft structure was introduced. The advantage and disadvantage of NDT and that of SHM were compared. It is proposed that SHM should be devoted to early alarming the abnormal state and reducing the NDT scope, combined with NDT to provide the quantitative damage information. Key words: Civil aircraft, Structural maintenance, Reliability, Damage, Nondestructive testing, Structural health monitoring 1. Introduction Airline enterprises make money by transporting the passengers, goods, mail by making use of airplane. Operating costs are an important part of the revenue which affected the income. Maintain cost account for 10% to 20% of the direct operating costs and are an important part of the life cycle cost. Civil aircraft maintenance is to ensure that aircraft, engines and components in the design of the reliability and safety level of the implementation of the scheduled function of all the work carried out. Repair is to restore the aircraft, the engine and components of a good working condition of all the work done, including inspection, fault diagnosis, troubleshooting, troubleshooting after the test and renovation. Inspection is to find the fault, and repair is to restore the structure function. Damage discovery ability, discovery time, and the method of access to damage information, on the one hand, affect the aircraft attendance (whether to stop flight to inspection and inspection frequency),on the other hand, determine the structure maintenance strategy. Non-destructive testing and structural health monitoring technology, the purpose of these two types of work is to obtain the structure of the abnormal information or status, and provide input information to the subsequent maintenance, maintenance work. As a prerequisite for structural maintenance work, the input information should be guided by structural repair needs, with early alarm and early detection as the goal, to provide quantitative structural damage information for the selection of structural maintenance strategies and the establishment of maintenance programs. This paper will explore the requirement and strategies of structural damage test and monitoring from the perspective of aircraft structural maintenance. 2. Aviation structure maintenance Civil aircraft maintenance is a general term for maintenance and repair of aircraft, engines and components. Maintenance refers to all work to maintain aircraft, engines and components work well, including cleaning, lubrication, inspection and replenishment of fuel. Repair refers to all work to restore the good working conditions of aircraft, engines and components, including inspection, Fault diagnosis, troubleshooting, troubleshooting after the test and renovation and so on. 2.1 The concept of fault and its classification The fault is defined as that the device cannot perform the specified function, the content as followed: a destructive failure that causes the system to immediately lose its function; and a 1

2 performance failure related to the performance degradation of the device. Fault can be divided into four categories according to technical reasons: wear fault, corrosive fault, fracture fault and aging fault. 2.2 aviation maintenance ideas Early aircrafts are in the event of many accidents. Through a period of experience accumulated and research, people concluded that the aircraft's reliability and the use time have a certain relationship. The failure rate is low when the aircraft start to use, and remain unchanged for a period of time, and the failure rate will suddenly rise after a certain time. Thus, the traditional maintenance ideas of the safety first, prevention-oriented formed. That is, the aircraft's timing maintenance ideas. Reliability-centric maintenance (RCM) originated in the 1960s. In 1958, after the jet aircraft entered the civil aviation service, the aircraft size became larger, the equipment became complicated, and the accident rate also increased. In accordance with the traditional maintenance theory to ensure safety, the number of detection increase and the inspection cycle shorten, which makes a substantial increase in maintenance costs, and sometimes accidental rate may rise instead of decline. Maintenance idea with focus on reliability is formed from a lot of experience and fault analysis thinking that: (1) The integration reliability of the aircraft is determined by the integration function of each system, and is not necessarily associated with the reliability of each component. Some components are damaged and do not directly affect flight safety, or only affect the economic benefits, and are compensated by the function of other components or systems, which is not necessary to periodically refurbish. Therefore, for each system or components, the reliability analysis of which should be connect to the integration reliability. We should differentiate between which things have serious impact and which things have not. (2) Through the analysis of the component failure, we found that the component failure curve is not a sudden increase at a certain period of time, but there are multiple situations, of which there are three main types: The first is called the tub curve, this kind of component part rate is very high in the early stage because of running-in, with the extension of time, the fault rate tends to be stable, after a certain period of time, the fault rate suddenly rises, as shown in figure 1 (a). This curve is consistent with the traditional analysis curve. (a) (b) (c) Fig 1 Components failure curve The second failure curve has an initial run-in period, but when the fault rate is stable, the fault has been random, so the fault rate remains unchanged, as shown in Figure 1 (b). The third type of fault is after the run-in period, the fault is growing over time, as shown in 2

3 Figure 1 (c), this result is surprising. Statistics results show that 68% of the aircraft components are the second type of fault, the first type account for 12%, the rest belong to other types. This indicates that 68% of the components do not apply to periodic maintenance theory. The core of the reliability-centric maintenance theory is to analyze the reliability of each component from the integration reliability of the aircraft, and to develop specific maintenance methods for the specific situation. This maintenance idea regards the concept of timing maintenance as part of the component maintenance methods which are also included. 2.3 aircraft maintenance methods (1) Hard time maintenance This is the traditional way of maintenance, for some important components with first type fault curve, a certain period of use should to be given, to be replaced or scrapped later, which determine the maintenance period. That is, before the part reaches a certain life, the probability of failure is much larger than before, regardless of his actual state, and we should restore or replace the part. (2) On condition maintenance For components with the second type of fault curve, there is no obvious period of fault growth. According to regular maintenance, the effectiveness of this component cannot be given full play and the occurrence of fault cannot be reduced. The use of these components are monitored and observed. According to the specific use, the repair or replacement is determined. This maintenance method is known as on condition maintenance. Simply, on condition maintenance task is a periodic diagnostic test or inspection, these tests or inspection compared a current material conditions or performance with the known criteria and accordingly take further action. The purpose of on condition maintenance is to find a potential fault before the fault actually occurs. To carry out on condition maintenance, the first step is to make judgment of the components technical status, so as to avoid a sudden occurrence of security accidents. This require to decide the detection method, testing standards and the corresponding detection time. On condition maintenance is the same time as the hard time maintenance which needs regular period testing, the fundamental difference is that On condition maintenance determine whether the components continue to use by non-destructive test, rather than expired one by one scrapped replacement. At the same time, on condition maintenance uses of a variety of means to monitor, multi-parameter to determine the technical status of components, do not need to decomposition inspection. Therefore, on condition maintenances reduce the workload, shorten the maintenance time and make full use of the components. The decompose consumption decrease, and the accident rate declines. (3) Condition Monitoring From the fault analysis of the components, we can seen that some components of the aircraft does not affect flight safety. Even if the fault of these components occur, the aircraft is still able to safely fly, then we do not have to make great efforts to prevent these fault, just need to eliminate the fault when they happened, which save a lot of manpower and resources. But we must monitor these components efficiently, in the event of failure can be found in time for troubleshooting, this maintenance is Condition Monitoring. Condition Monitoring is a way of looking for failure to repair, commonly used in projects and equipment which are only occasionally used. In normal operation, the failures of off-line and intermittent use of project functions are not significant to the operator, and their functions must be tested or inspected to ensure that they are available when they are needed. 3

4 3. Nondestructive testing and structural health monitoring Non-destructive test (NDT) is based on the physical properties of materials, which changes due to defects in fact, without changing and breaking the state of materials and use performance. Nondestructive testing measures the change of the materials physical properties and determines whether the material and components have defective. Nondestructive testing is a relatively independent technology that distinguishes itself from design, materials, processes and products and is a comprehensive technology that runs through the whole process of product design, manufacturing and use. In the design stage, NDT is used to support the damage tolerance design; in the manufacturing stage, NDT is used to remove the substandard raw materials, rough and unqualified products. NDT improves the manufacturing process, identify the product acceptance criteria compliance, and determine the eligibility. In-service testing, NDT is used to monitor product structure and state changes, to ensure safe and reliable operation of the product. Nondestructive testing technology has the following characteristics: 1) Mature technology, with a complete detection and implementation, the results of evaluation technology system; 2) Defects and damage quantitatively description, high confidence of the test results; 3) Widely used in various industrial fields; 4) Inspection work can only be carried out afterwards; 5) In-service detection required structural decomposition. Structure Health Monitoring (SHM) makes use of advanced sensor / driver network integrated in the structure, accesses to structural health information (such as stress, strain, temperature, etc.) online real-time, combined with advanced information processing methods and mechanics modeling methods, to extract structural feature parameters. Then the state and fault of the structure are identified. We achieve the assessment of the structural state and ensure structural integrity and maintenance costs decline. Structural Health Monitoring (SHM) is an innovative NDT method that is characterized by pasting / embedding the sensor into a structure that provides structural events or status information at any time. According to the correlation between the information obtained by monitoring and structural damage, structural health monitoring technology can be divided into two categories: one is state monitoring technology, including acoustic emission technology, piezoelectric detection technology, fiber grating detection technology, strain monitoring technology. This type of technology can indicate the abnormal state of the structural state, but cannot give the exact information of structural damage, such as location, size and damage character. The other monitoring technology such as comparison vacuum technology (CVM), intelligent Coating technology (ICMS), etc can give the indication of the occurrence and expansion of concerned cracks but the actual size of the crack cannot be given. Structural health monitoring technology has the following characteristics: 1) Immature technology, technical specifications and system imperfect; 2) Information access in time, real-time, online work, continuous tracking of the structural state; 3) Monitoring without decomposition structure; 4) Give the damage or status indication information of the structure, cannot give the quantitative information of damage. 4. Damage confirmation before repair Damage inspection and monitoring are the necessary means of confirming the damage before repair which are part of the maintenance work. For the operation aircraft, only by finding the fault and abnormalities, can the subsequent repair work carry out. For the aircraft structure, the first is to find damage, then there will be the repair damage classification, maintenance strategy, program and implementation and latter the components test and a series of work. Therefore, 4

5 through the inspection and monitoring to attain the structure damage information is the early work and input conditions of fault repair work. The aircraft structural repair project selection is in accordance with the aircraft repair manual. If the damage is found, first step is to confirm the location of the structural damage; second step is to confirm the character of the damage may be corrosion damage, broken nails or cracks. The third step is to confirm the size and extend of damage, such as corrosion depth, area and other information. According to the above information, in contrast with the manual, we can look up the table, and assort the classification of structural damage. After confirming the damage classification, we combine with the scope of the manual to determine the repair range to determine whether the damage is repairable damage or repair injury. For repaired damage, in accordance with the manual repair requirements we can determine a maintenance strategy and develop specific maintenance programs. For irreversible damage, we will replace the components directly. Therefore, repair of aircraft structural damage requires confirmation of the location, nature, size information and the reason (available) of the damage. 5. Conclusion The failure analysis of the components shows that 68% of the components failure is the failure mode of the second fault curve, a variety of monitor means, multi-parameter are needed to determine the technical status of components. This shows that aircraft structural repair has a huge demand for structural health monitoring technology. Aircraft structure repair project selection needs damage location, character, size and other quantitative information. Structural health monitoring technology is combined with nondestructive testing technology. Monitoring technology gives fault warning and instructions to reduce the scope of damage inspection, saving the inspection time, to avoid the frequent decomposition of the structure, and nondestructive testing technology attains the damage Location, nature, size and other quantitative information through careful inspection. Structural health monitoring technology should devote to development the structure abnormal state monitoring and traditional non-destructive tests are used to complete quantitative damage inspection. With the precondition to ensure the safety and reliability of the structure, we should reduce structural maintenance decompose and time consumption, Improve work efficiency, save maintenance costs. References 1. Xu Chaoqun Yan Guohua, "Aviation Maintenance Management", China Civil Aviation Press, 2012; 2. Yuan Shenfang, Structural Health Monitoring [M], Beijing, National Defense Industry Press, 2007; 3. Zhang Jiazhong, Zhang Liguo, "Aeronautical Equipment Fault Prediction and Health Management Equipment" Aviation Manufacturing Technology ; 4. Jiang Shaoxin, Civil Aircraft Maintenance Thinking in the Aircraft Structure Corrosion and Protection of the Development of China Civil Aviation Network, September 3, Wu Jingmin, Civil Aircraft Life Maintenance Cost Control and Analysis of Key Technology Research, "Nanjing University of Aeronautics and Astronautics", Wang Ziming, "Aviation Nondestructive Testing Comprehensive Knowledge", National Defense Industry Press, [7] Ningning, Aircraft Structure Damage in Service Detection Technology New Progress, 2016 China non-destructive testing annual report 5