Aircraft structure health state evaluation method

Abstract

The invention discloses an aircraft structure health state evaluation method and belongs to the field of aircraft design. The method comprises the following steps: 1, determining an aircraft structure health state evaluation principle; 2, analyzing and determining key indexes for evaluating the health state of the whole aircraft structure of the aircraft; thirdly, analyzing and determining key indexes for evaluating the health state of the local structure of the aircraft; 4, carrying out aircraft structure health key index attenuation hazard evaluation, and determining a structure health state grade; the method can determine the structural health state key index based on the characteristics of each aircraft, monitors the dynamic change process of the structural health state key index in the use process of the aircraft, and gives the attenuation harmfulness to the structural health state key index to determine the structural health state grade. An important basis is provided for formulating a subsequent training use and maintenance guarantee scheme of the aircraft, so that the use efficiency of the aircraft is effectively improved, and the maintenance period and cost are reduced.

Description

Technical Field

[0001] The invention belongs to the field of civil aircraft design and relates to a method for evaluating the health status of aircraft structures. Background Art

[0002] The airframe structure is an important part of the civil aircraft platform, and its safety and reliability directly affect the safe and reliable flight of civil aircraft. If a structural system fails during the flight of a civil aircraft, it will affect the performance of the civil aircraft at the least, and even threaten the flight safety and even cause the aircraft to be destroyed and people to die at the worst. The purpose of the civil aircraft structural health status evaluation is to monitor and evaluate the reliability of the civil aircraft structure during service, determine the structural health status according to the use and maintenance conditions, perform maintenance work scientifically and in a targeted manner, and selectively perform flight missions to give full play to the life potential of civil aircraft.

[0003] The present invention is based on the demand for monitoring and managing the structural health status of civil aircraft in a complex service environment throughout their life cycle, and proposes a method for evaluating the structural health status of civil aircraft. Based on the structural health status of a single aircraft, subsequent training, use and maintenance support plans for civil aircraft are formulated, thereby effectively improving the utilization efficiency of civil aircraft and reducing maintenance cycles and costs. Summary of the invention

[0004] The present invention proposes a method for evaluating the structural health status of an aircraft, which can quantitatively characterize the structural health status of a civil aircraft according to the safety, functionality and economic requirements of the civil aircraft, and determine the structural health status level, thereby providing a basis for the use and maintenance of civil aircraft.

[0005] The technical solution of the present invention is:

[0006] A method for evaluating the health status of aircraft structures, the steps are as follows:

[0007] First, determine the principles for evaluating the structural health of civil aircraft

[0008] The principle of civil aircraft structural health status evaluation is whether the structural integrity requirements are met. Structural integrity refers to the state in which the structure can be used normally and its functions are not impaired under the required structural safety, structural capacity, durability and guarantee level. When the structural integrity of a civil aircraft is weakened or damaged, the structure can be judged to be in an unhealthy state.

[0009] According to the requirements of civil aircraft structural integrity, the structural health status assessment includes three aspects:

[0010] a) Safety: assess whether the current state of the structure affects the flight safety of civil aircraft;

[0011] b) Functionality: assess whether the current state of the structure will affect the normal operation, take-off and landing functions of civil aircraft;

[0012] c) Economic feasibility: Evaluate whether the cost of maintaining the current structure is within an acceptable range.

[0013] Second, analyze and determine the key indicators for evaluating the structural health status of civil aircraft

[0014] Based on the analysis of the structural design parameters that affect the overall safety, functionality and economy of civil aircraft, the key indicators for evaluating the health of the entire aircraft are determined:

[0015] The key indicators that affect the overall safety of civil aircraft include: causing damage to the main load-bearing structure of civil aircraft, and the residual strength of the damage tolerance structure cannot meet the normal use envelope of civil aircraft;

[0016] Key indicators that affect the overall functionality of civil aircraft include: movable control surface deflection function, fuel tank sealing performance, etc.;

[0017] Key indicators that affect the economic efficiency of civil aircraft include: causing a significant increase in the repair costs and cycles of civil aircraft.

[0018] Further,

[0019] 2.1) Probability of damage to the main load-bearing structure of civil aircraft

[0020] The probability of damage to the main load-bearing structure of civil aircraft is evaluated using the structural reliability analysis theory, based on the stress-strength interference model. The stress-strength interference model assumes that the stress and structural strength of the structure are both random variables, so the basic point of structural reliability analysis and design is to first understand the distribution law of stress and strength and to reasonably establish a mathematical model between stress and strength.

[0021] Structural strength reliability analysis process: determine stress distribution through load statistics and probability distribution, and at the same time, determine strength distribution through material property statistics and probability distribution; stress distribution and stress distribution simultaneously consider the distribution of geometric dimensions and other random factors to obtain stress statistics and its probability distribution, strength statistics and its probability distribution respectively; then obtain the strength statistical distribution and its possibility of the structure through stress statistics and its probability distribution, strength statistics and its probability distribution.

[0022] In practice, both stress and strength are time-varying quantities. Usually, the change of stress S is random, while the strength R decays with time. The relationship between the two is constantly changing, and their mutual interference area is also constantly changing with time, thus generating a dynamic stress-strength interference model, that is, a stress-strength interference model that takes the time factor into account.

[0023] The overlapping area of ​​the probability change curve of stress and the probability change curve of strength is the interference area. The overlapping area indicates that the structure may fail. Therefore, the smaller the area of ​​the overlapping area, the higher the reliability of the structure; similarly, the larger the area of ​​the overlapping area, the lower the reliability of the structure. However, the overlapping area is not equal to the numerical quantification of the reliability or failure of the structure, but only represents the interference between stress and strength. The reliability of the structure is quantitatively calculated based on the overlapping area. The calculation theory is the stress-strength interference theory, and the calculation model is the interference model. In the overlapping area, if R>S, the structure is safe. On the contrary, it means that the structure fails.

[0024] 2.2) Evaluation of residual strength of damage tolerant structures

[0025] Damage tolerance analysis and residual strength assessment are carried out for key damage tolerance parts. The basic process is as follows:

[0026] 2.2.1) Screening and determining the locations for damage tolerance analysis and assessment, including screening and determining damage tolerance key parts;

[0027] 2.2.2) Damage tolerance requirements for given damage tolerance assessment locations, including material parameters, structural design type, inspection level and minimum unrepaired service life, crack form, initial crack size, and minimum residual strength requirements;

[0028] 2.2.3) Determine the design load spectrum;

[0029] 2.2.4) Conduct stress analysis on civil aircraft structures based on load spectra, and compile stress spectra of damage tolerance analysis locations based on stress results and practical experience;

[0030] 2.2.5) Crack growth life analysis: Calculate the stress intensity factor according to the crack form, stress spectrum, loading mode and geometric configuration of the damage tolerance analysis part; select the crack growth equation and crack growth model; select the analysis software to perform crack growth analysis; draw the crack growth life curve based on the crack growth analysis results, and finally determine the maintenance cycle;

[0031] 2.2.6) Safety margin analysis of damaged structures: Determine the safety margin of damaged parts in combination with the crack growth life curve.

[0032] 2.3) Structural and functional index evaluation

[0033] Key indicators that affect the overall functionality of civil aircraft include the deflection function of movable control surfaces and the sealing performance of fuel tank structures.

[0034] The structural functional indicators were evaluated using the following methods:

[0035] 2.3.1) Evaluation of fuel tank sealing performance

[0036] The functionality of the fuel tank seal is related to the damage to the fuel tank load-bearing structure. When the fuel tank load-bearing structure is intact and there is no crack damage, the fuel tank seal is considered intact. When cracks appear in the fuel tank load-bearing structure, the fuel tank seal will be reduced, and even cause the overall function of the fuel tank to fail due to structural damage.

[0037] Therefore, the oil tank sealing performance is evaluated by the crack size of the main load-bearing structure of the oil tank. According to the above damage tolerance analysis steps, damage tolerance analysis is carried out on the oil tank load-bearing structure, and combined with experimental verification, the relationship curve between oil tank sealing performance and crack size is obtained. The oil tank sealing performance is evaluated based on whether the crack causes obvious oil leakage, rather than the residual strength.

[0038] 2.3.2) Evaluation of the deflection function of the movable control surface

[0039] The bearings used in the movable control surfaces of civil aircraft are spherical bearings. The decrease in the deflection capacity of the control surfaces is characterized by the wear and damage of the spherical bearings. The wear of the spherical bearings occurs on the lining layer, and the wear amount is calculated according to the following formula:

[0040]

[0041] In the above formula, V is the wear amount, F N is the normal load, x is the sliding distance, σ s is the compressive yield limit of the softer material, K s is called the wear constant.

[0042] The wear process of spherical bearings can be divided into three stages: running-in wear stage (I), stable wear stage (II), and rapid wear stage (III). In the running-in wear stage, the wear rate gradually decreases with time; in the stable wear stage, the friction surface reaches a stable operating state after running-in, and the wear rate remains basically unchanged. This is the normal working period of the friction pair and the most critical period for determining the wear life of the friction pair; in the rapid wear stage, the wear rate increases rapidly with time, causing the working conditions of the friction pair to deteriorate sharply until the parts fail completely.

[0043] The influence on rudder surface deflection appears in the third stage, that is, when the bearing wear exceeds the threshold value, the bearing clearance will increase with wear, the roughness will increase, and the bearing accuracy will also decrease, thus affecting the rudder surface deflection efficiency and even causing the deflection function to fail.

[0044] 2.3.3) Economic evaluation of structural repair

[0045] Structural repair includes structural crack repair, replacement of new parts, etc. Taking the most common and most expensive structural crack repair as an example, the crack repair cost model is expressed as follows:

[0046]

[0047] Where: C repair represents the repair cost corresponding to the crack length;

[0048] a(t) represents the crack growth curve.

[0049] Third, analyze and determine the key indicators for evaluating the local structural health status of civil aircraft

[0050] The fault manifestations and failure modes of key local structures of civil aircraft include local fatigue microcracks, stress corrosion, and local wear. The failure and damage states of these key local structures will evolve into changes in the global structural health characteristic parameters after further expansion, which will in turn affect the changes in key indicators of the health of the entire aircraft, such as the probability of structural damage, the damage residual strength coefficient, and functional indicators.

[0051] The local structural health characteristic parameters include local detail strain parameters, structural modal parameters, crack sensing parameters, and corrosion sensing parameters. The identification of each parameter includes the following methods:

[0052] 3.1) Local detail strain identification method

[0053] Local strain parameters are identified and monitored using strain gauges or fiber optic sensors. Fiber optic sensors can reconstruct local strain distribution based on the fiber optic spectrum characteristics.

[0054] 3.2) Structural modal identification method

[0055] Structural modal parameters or local health assessment indicators extracted from responses can be used to indicate the structural health status and damage occurrence.

[0056] The vibration characteristics-based identification methods used include damage identification methods based on natural frequency, vibration mode and vibration mode curvature, flexibility, frequency response function, and modal strain energy and dynamic parameters.

[0057] 3.3) Local crack identification method

[0058] 3.3.1) Structural crack parameter identification based on eddy current array sensor

[0059] Eddy current monitoring technology uses the principle of electromagnetic field changes and can be used to monitor the local structure of bolted joints and curved transition structures. During crack propagation, the sensor output signal changes with the crack propagation: when the crack tip extends to the corresponding induction channel coil, the sensor output of the corresponding channel reaches the maximum inflection point value, and then according to the position of the induction channel, the crack length can be determined quantitatively in real time.

[0060] 3.3.2) Structural crack parameter identification based on vibration characteristics of civil aircraft

[0061] The vibration characteristic parameters of the typical structure, such as natural frequency, vibration mode, flexibility, and modal strain energy, are extracted. Based on the damage identification theory of vibration characteristics, the structural dynamics equation containing damage is established, and the expression containing damage parameters is derived. Based on the vibration characteristic parameter expression, the influence of damage parameters on vibration characteristic parameters is analyzed. Finally, the effectiveness and sensitivity of each vibration characteristic parameter on structural damage parameter identification are compared and analyzed, and the typical structural damage method of civil aircraft based on vibration characteristics is determined.

[0062] 3.3.3) Structural crack parameter identification based on optical fiber sensor

[0063] It is necessary to analyze the stress of the hole structure to be monitored, determine the dangerous area where cracks may occur, and design and layout the sensor in combination with the strain distribution near the crack and the monitoring requirements. The strain distribution near the crack at the hole edge is obtained through finite element analysis, and the optical fiber is arranged reasonably according to the strain distribution, and then the positioning and quantitative relationship between the spectral characteristic parameters and the crack is established.

[0064] 3.4) Corrosion damage identification method

[0065] Corrosion damage identification is based on the analysis of impedance measurement methods and noise interference current characteristics, establishing a mathematical model of spatial noise current, designing impedance sensing probes and precision measurement circuit modules suitable for high-resistance coating systems in low-conductivity environments, and optimizing electrochemical impedance calculation methods to identify coating damage status. Basic principle: The metal substrate and surface protection of the probe are consistent with the monitoring parts of civil aircraft. The probe does not need to be in direct contact with the metal structure to be measured and its coating during measurement. It is sufficient to ensure that the probe and the monitoring structure coating are in the same environment, which is used to completely reproduce the corrosion process of the coating in the monitoring part of civil aircraft. Using high signal-to-noise ratio signal input and management control hardware precision circuit measurement module, the precision circuit measurement module is connected to the probe. When a thin liquid film gathers on the coating surface to conduct the circuit, an AC excitation voltage signal at a certain frequency is applied to the coating through the electrode, which can accurately measure the failure performance changes of the probe coating, thereby accurately detecting the coating failure, which is suitable for corrosion monitoring of civil aircraft structures.

[0066] Fourth, conduct a critical assessment of the attenuation of key indicators of civil aircraft structure health and determine the structural health status level

[0067] The hazard assessment of the degradation of key indicators of civil aircraft is divided into safety assessment, functional assessment and economic assessment. The hazard of degradation of key indicators of structural damage probability and damage tolerance structural residual strength is to affect the flight safety of civil aircraft. The hazard of degradation of structural functional indicators should be assessed according to the functions affected by the indicators and the consequences of functional loss on the system level or civil aircraft level. The hazard of degradation of structural repair economic indicators is mainly the increase in the life cycle and cost of civil aircraft.

[0068] Beneficial effects of the invention: This patent proposes a method for evaluating the structural health status of an aircraft, which can determine the key indicators of the structural health status based on the characteristics of each civil aircraft, monitor the dynamic changes of the key indicators of the structural health status during the use of the civil aircraft, and determine the structural health status level by giving the attenuation hazard of the key indicators of the structural health status, and then provide an important basis for formulating subsequent training, use and maintenance guarantee plans for civil aircraft, thereby effectively improving the use efficiency of civil aircraft and reducing maintenance cycles and costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Figure 1 Strength reliability analysis flow chart.

[0070] Figure 2 Diagram of the stress-strength interference model.

[0071] Figure 3 Crack length-residual strength curve of typical structures.

[0072] Figure 4 Crack growth life-residual strength curve for typical structures.

[0073] Figure 5 Schematic diagram of a typical wear process. DETAILED DESCRIPTION

[0074] A method for evaluating the health status of aircraft structures, the steps are as follows:

[0075] First, determine the principles for evaluating the structural health of civil aircraft

[0076] The principle of civil aircraft structural health status evaluation is whether the structural integrity requirements are met. Structural integrity refers to the state in which the structure can be used normally and its functions are not weakened under the required structural safety, structural capacity, durability and guarantee level. Its contents include the strength, stiffness, durability, damage tolerance and function of the airframe that affect the safe use and cost of civil aircraft. When the structural integrity of a civil aircraft is weakened or damaged, the structure can be judged to be in an unhealthy state.

[0077] According to the requirements of civil aircraft structural integrity, the structural health status assessment includes three aspects:

[0078] a) Safety: assess whether the current state of the structure affects the flight safety of civil aircraft;

[0079] b) Functionality: assess whether the current state of the structure will affect the normal operation, take-off and landing of civil aircraft;

[0080] c) Economic feasibility: Evaluate whether the cost of maintaining the current structure is within an acceptable range.

[0081] Since the health status of civil aircraft changes dynamically and continuously during use, that is, from the completely healthy state at the time of leaving the factory to the decline of health status indicators after a period of use, until the use of civil aircraft is affected or even the overall structure is damaged or the function is lost, the structural characteristics are different throughout the process. Therefore, the characterization method of the structural health status of civil aircraft at different stages is also different. For example, in the stage of initial cracks in the structure, it only affects the local stress field distribution and has no substantial impact on the overall bearing capacity or functional integrity of the structure. The health status at this time can only be characterized by the characteristic parameters of the local structure. Therefore, to fully describe the change process of the structural state of civil aircraft throughout its life cycle, it is necessary to identify the structural characteristic parameter indicators of the whole aircraft and local levels.

[0082] Second, analyze and determine the key indicators for evaluating the structural health status of civil aircraft

[0083] Based on the analysis of structural design parameters that affect the overall safety, functionality, economy and other key performance of civil aircraft, the key indicators for evaluating the health of the whole aircraft are determined:

[0084] The key indicators that affect the overall safety of civil aircraft mainly include: causing damage to the main load-bearing structure of civil aircraft, and the residual strength of the key damage tolerance structure cannot meet the normal use envelope of civil aircraft;

[0085] The key indicators that affect the overall functionality of civil aircraft mainly include: key functions that affect the normal deflection of control surfaces and the retraction and extension of landing gear;

[0086] The key indicators that affect the economy of civil aircraft mainly include: causing a significant increase in the repair costs and cycles of civil aircraft, etc.

[0087] Further,

[0088] 2.1) Probability of damage to the main load-bearing structure of civil aircraft

[0089] The probability of damage to the main load-bearing structure of civil aircraft is evaluated using the structural reliability analysis theory, based on the stress-strength interference model. The stress-strength interference model assumes that the stress and structural strength of the structure are both random variables, so the basic point of structural reliability analysis and design is to first understand the distribution law of stress and strength and to reasonably establish a mathematical model between stress and strength.

[0090] Figure 1 Jian gave the general process of structural strength reliability analysis: determine stress distribution through load statistics and probability distribution, and determine strength distribution through material performance statistics and probability distribution; stress distribution and stress distribution consider the distribution of geometric dimensions and other random factors at the same time, and obtain stress statistics and its probability distribution, strength statistics and its probability distribution respectively; then obtain the strength statistical distribution and its possibility of the structure through stress statistics and its probability distribution, strength statistics and its probability distribution. The structure is subject to the action of generalized external loads, which include force, temperature, corrosion, etc., which are usually random processes. When calculating the internal forces of each component of the structure from the load, it is necessary to consider the influence of internal factors such as the geometric dimensions of the structure. The strength of the material (in a broad sense, the resistance of the material) is a random variable, which is usually a monotonically decreasing function of time. The resistance of the structure calculated from the resistance of the material must consider the random factors such as the load-bearing mode of the structure and the geometric characteristics of the structure. Then, based on the random distribution of the internal forces of the structure and the random distribution of the resistance of the structure, the reliability of the structure is analyzed using the stress-strength interference model.

[0091] In practice, both stress and strength are time-varying quantities. Usually, the change of stress S is random, while the strength R decays with time. The relationship between the two is constantly changing, and their mutual interference area is also constantly changing with time, thus generating a dynamic stress-strength interference model, that is, a stress-strength interference model that takes the time factor into account.

[0092] The probability change curves of stress S and strength R are as follows: Figure 2 As shown. The overlapping area of ​​the probability change curve of stress and the probability change curve of strength is the interference area. The overlapping area indicates that the structure may fail. Therefore, the smaller the area of ​​the overlapping area, the higher the reliability of the structure; similarly, the larger the area of ​​the overlapping area, the lower the reliability of the structure. However, the overlapping area is not equal to the numerical quantification of the reliability or failure of the structure, but only an indication of the interference between stress and strength. With its existence, we can quantitatively calculate the reliability of the structure based on the overlapping area. The calculation theory is the stress-strength interference theory, and the calculation model is the interference model. In the overlapping area, if R>S, the structure is safe. On the contrary, it means that the structure fails.

[0093] 2.2) Evaluation of residual strength of damage tolerant structures

[0094] Damage tolerance analysis and residual strength assessment are carried out for key damage tolerance parts. The basic process is as follows:

[0095] 2.2.1) Screening and determining the locations for damage tolerance analysis and assessment, including screening and determining damage tolerance key parts;

[0096] 2.2.2) Damage tolerance requirements for given damage tolerance assessment locations, including material parameters, structural design type, inspection level and minimum unrepaired service life, crack form, initial crack size, minimum residual strength requirements, etc.;

[0097] 2.2.3) Determine the design load spectrum;

[0098] 2.2.4) Conduct stress analysis on civil aircraft structures based on load spectra, and compile stress spectra of damage tolerance analysis locations based on stress results and practical experience;

[0099] 2.2.5) Crack growth life analysis: Calculate stress intensity factor according to crack form, loading mode and geometric configuration; select crack growth equation and crack growth model; select analysis software to perform crack growth analysis; draw crack growth life curve according to crack growth analysis results, and finally determine maintenance cycle;

[0100] 2.2.6) Safety margin analysis of damaged structures: Determine the safety margin of damaged parts in combination with the crack growth life curve.

[0101] According to the above steps, the crack growth analysis of typical civil aircraft structures can be carried out to obtain the relationship between the crack length and residual strength, and the crack growth life and residual strength of typical structures. Figure 3 , Figure 4 In actual operation, the residual strength can be judged according to the crack length obtained by inspection, so as to evaluate the health status of the structure.

[0102] 2.3) Structural and functional index evaluation

[0103] The structural capabilities of civil aircraft in service mainly include the deflection capability of movable control surfaces, the sealing and pressure bearing capacity of fuel tank structures, the opening and closing capabilities of hatches, etc. (functional degradation caused by structural failures, excluding systems and accessories). Since the current structural functional design mainly adopts the test verification method, the fuel tank sealing and control surface deflection capability are verified through functional tests, that is, if the functional test and fuel tank fatigue test are passed, it is considered that the design requirements are met, so it is difficult to accurately quantify the structural functionality.

[0104] The present invention uses the following method to evaluate the structural functional indicators:

[0105] 2.3.1) Fuel tank sealing evaluation

[0106] Since the current fuel tank sealant has a high reliability, from the actual use of civil aircraft in the field, no fuel tank sealing problems caused by sealant failure have been found. A large number of civil aircraft use statistics show that the functionality of the fuel tank seal is mainly related to the damage of the fuel tank load-bearing structure. When the fuel tank load-bearing structure is intact and there is no crack damage, the fuel tank sealing is considered to be intact. When cracks appear in the fuel tank load-bearing structure (such as the lower wall plate of the fuel tank, the lower edge strip of the central wing wall, etc.), the fuel tank sealing will be reduced, and even cause the overall function of the fuel tank to fail due to structural damage.

[0107] Therefore, the oil tank sealing performance is evaluated by the crack size of the main load-bearing structure of the oil tank. According to the above damage tolerance analysis steps, damage tolerance analysis is carried out on the oil tank load-bearing structure, and combined with experimental verification, the relationship curve between oil tank sealing performance and crack size is obtained. It should be noted that the oil tank sealing performance is evaluated based on whether the crack causes obvious oil leakage, rather than the residual strength.

[0108] 2.3.2) Evaluation of the deflection capability of movable control surfaces

[0109] The bearings used in active control surfaces such as flapers on civil aircraft are generally spherical bearings. The decline in the deflection capacity of the control surface is mainly characterized by the wear and damage of the spherical bearings. The wear of the spherical bearings mainly occurs on the lining layer, and the wear amount is calculated according to the following formula:

[0110]

[0111] In the above formula, V is the wear amount, F N is the normal load, x is the sliding distance, σ s is the compressive yield limit of the softer material, K s is called the wear constant.

[0112] The typical wear process of spherical bearings can be divided into three processes (see Figure 5 ): running-in wear stage (I), stable wear stage (II), and rapid wear stage (III), as shown in the figure below. In the running-in wear stage, the wear rate gradually decreases with time; in the stable wear stage, the friction surface reaches a stable operating state after running-in, and the wear rate remains basically unchanged. This is the normal working period of the friction pair and the most critical period for determining the wear life of the friction pair; in the rapid wear stage, the wear rate increases rapidly with time, causing the working conditions of the friction pair to deteriorate sharply until the parts fail completely.

[0113] The factors affecting the deflection of the rudder surface mainly occur in the third stage, that is, when the bearing wear exceeds the threshold value, the bearing clearance will increase with wear, the roughness will increase, and the bearing accuracy will also decrease, thereby affecting the deflection efficiency of the rudder surface and even causing the deflection function to fail.

[0114] 2.3.3) Economic evaluation of structural repair

[0115] Structural repair mainly includes the repair of cracks. For typical cracked metal structures of civil aircraft, the repair cost can be expressed by a step function. The crack repair cost model is as follows:

[0116]

[0117] Where: C repair represents the repair cost corresponding to the crack length;

[0118] a(t) represents the crack growth curve.

[0119] Third, analyze and determine the key indicators for evaluating the local structural health status of civil aircraft

[0120] The fault manifestations and failure modes of key local structures of civil aircraft include local fatigue microcracks, stress corrosion, local wear, etc. The failure and damage states of these key local structures will evolve into changes in the global structural health characteristic parameters after further expansion, which will in turn affect the changes in key indicators of the health of the entire aircraft, such as the structural damage probability, damage residual strength coefficient, and functional indicators.

[0121] The local structural health characteristic parameters mainly include local detail strain parameters, structural modal parameters, crack sensing parameters, corrosion sensing parameters, etc. The identification of each parameter mainly includes the following methods:

[0122] 3.1) Local detail strain identification method

[0123] Local strain parameters are mainly identified and monitored by strain gauges or fiber optic sensors. Fiber optic sensors can reconstruct local strain distribution based on the optical fiber spectrum characteristics, overcoming the stress and strain distribution errors obtained by traditional finite element modeling and simulation.

[0124] 3.2) Structural modal identification method

[0125] Structural vibration is a sensitive indicator of the structural integrity of an aircraft. Structural damage will cause changes in the physical parameters of the structure (mass, stiffness, etc.), which in turn will cause changes in the dynamic response and modal parameters of the structure. Therefore, structural modal parameters or local health evaluation indicators extracted from the response can be used to indicate the structural health status and damage occurrence.

[0126] The main vibration characteristics-based identification methods currently used include damage identification methods based on natural frequency, vibration mode and vibration mode curvature, flexibility, frequency response function, and modal strain energy and other dynamic parameters.

[0127] 3.3) Local crack identification method

[0128] Structural crack sensors generally include smart coatings, eddy current arrays, vibration modulus monitoring and other methods, and different aspects have different methods for identifying crack parameters.

[0129] 3.3.1) Structural crack parameter identification based on eddy current array sensor

[0130] Eddy current (flexible eddy current array sensor, SHM bolts, etc.) monitoring technology adopts the principle of electromagnetic field change and can be used to monitor the local structure of bolted joints and curved transition structures. During the crack propagation process, the sensor output signal changes with the crack propagation: when the crack tip extends to the corresponding induction channel coil, the sensor output of the corresponding channel reaches the maximum inflection point value, and then according to the position of the induction channel, the crack length can be determined quantitatively in real time.

[0131] 3.3.2) Structural crack parameter identification based on vibration characteristics of civil aircraft

[0132] Extract the vibration characteristic parameters of typical structures such as natural frequency, vibration mode, flexibility, modal strain energy, etc., establish the structural dynamic equation containing damage based on the damage identification theory of vibration characteristics, and derive the expression containing damage parameters. Based on the expression of vibration characteristic parameters, analyze the influence of damage parameters on vibration characteristic parameters. Finally, compare and analyze the effectiveness and sensitivity of each vibration characteristic parameter for structural damage parameter identification, and determine the typical structural damage method of civil aircraft based on vibration characteristics.

[0133] 3.3.3) Structural crack parameter identification based on optical fiber sensor

[0134] In order to maximize the monitoring efficiency and reduce the number of sensors, it is necessary to analyze the stress of the hole structure to be monitored, determine the dangerous area where cracks occur, and design and layout the sensors in combination with the strain distribution near the cracks and the monitoring requirements. The strain distribution near the cracks at the hole edge is obtained through finite element analysis, and the optical fiber is reasonably arranged according to the strain distribution, and then the positioning and quantitative relationship between the spectral characteristic parameters and the cracks is established.

[0135] 3.4) Corrosion damage identification method

[0136] Corrosion damage identification is generally based on the analysis of impedance measurement methods and noise interference current characteristics, establishing a spatial noise current mathematical model, designing impedance sensing probes and precision measurement circuit modules suitable for high-resistance coating systems in low-conductivity environments, and optimizing electrochemical impedance calculation methods to identify coating damage status. Basic principle: The metal substrate and surface protection of the probe are consistent with the monitoring parts of civil aircraft. The probe does not need to be in direct contact with the metal structure to be measured and its coating during measurement. It is sufficient to ensure that the probe and the monitoring structure coating are in the same environment, which is used to completely reproduce the corrosion process of the coating in the monitoring part of civil aircraft. Using high signal-to-noise ratio signal input and management control hardware precision circuit measurement module, the precision circuit measurement module is connected to the probe. When a thin liquid film gathers on the coating surface to conduct the circuit, an AC excitation voltage signal at a certain frequency is applied to the coating through the electrode, which can accurately measure the failure performance changes of the probe coating, thereby accurately detecting the coating failure, which is suitable for corrosion monitoring of civil aircraft structures.

[0137] Fourth, conduct a critical assessment of the attenuation of key indicators of civil aircraft structure health and determine the structural health status level

[0138] Table 1 Criticality analysis of key indicators for whole-machine structural health status evaluation

[0139]

[0140]

[0141] For key safety indicators, the health status level assessment method is shown in Table 1.

Claims

1. A method for evaluating the health status of an aircraft structure, characterized in that: Here are the steps: The first step is to determine the principles for evaluating the structural health of civil aircraft; The principle of civil aircraft structural health status evaluation is whether the structural integrity requirements are met; structural integrity refers to the state in which the structure can be used normally and its functions are not impaired under the required structural safety, structural capacity, durability and guarantee level; When the structural integrity of a civil aircraft is weakened or damaged, the structure can be judged to be in an unhealthy state; According to the requirements of civil aircraft structural integrity, the structural health status assessment includes three aspects: a) Safety: assess whether the current state of the structure affects the flight safety of civil aircraft; b) Functionality: assess whether the current state of the structure will affect the normal operation, take-off and landing functions of civil aircraft; c) Economic feasibility: assess whether the cost of maintaining the current structure is within an acceptable range; The second step is to analyze and determine the key indicators for evaluating the structural health status of civil aircraft; Based on the analysis of the structural design parameters that affect the overall safety, functionality and economy of civil aircraft, the key indicators for evaluating the health of the entire aircraft are determined: The key indicators that affect the overall safety of civil aircraft include: causing damage to the main load-bearing structure of civil aircraft, and the residual strength of the damage tolerance structure cannot meet the normal use envelope of civil aircraft; Key indicators that affect the overall functionality of civil aircraft include: movable control surface deflection function, fuel tank sealing performance; Key indicators that affect the economic efficiency of civil aircraft include: causing a significant increase in the cost and cycle of repairing civil aircraft; Step 3: Analyze and determine the key indicators for evaluating the local structural health status of civil aircraft; The fault manifestations and failure modes of key local structures of civil aircraft include local fatigue microcracks, stress corrosion, and local wear. The failure and damage states of these key local structures will further expand and evolve into changes in the global structural health characteristic parameters, which will in turn affect the changes in key health indicators of the entire aircraft, such as the probability of structural damage, the residual strength coefficient of damage, and the functional indicators. The local structural health characteristic parameters include local detail strain parameters, structural modal parameters, crack sensing parameters, and corrosion sensing parameters; Step 4: Conduct a hazard assessment of the attenuation of key indicators of civil aircraft structure health and determine the structural health status level; The hazard assessment of the attenuation of key indicators of civil aircraft is divided into safety assessment, functional assessment and economic assessment; the hazard of the attenuation of key indicators of structural damage probability and damage tolerance structural residual strength is to affect the flight safety of civil aircraft, the hazard of the attenuation of structural functional indicators should be assessed based on the functions affected by the indicators and the consequences of functional loss at the system level or civil aircraft level, and the hazard of the attenuation of structural repair economic indicators is mainly the increase in the entire life cycle and cost of civil aircraft.

2. The method for evaluating the health status of an aircraft structure according to claim 1, characterized in that in the second step: 2.1) Probability of damage to the main load-bearing structure of civil aircraft The probability of damage to the main load-bearing structure of civil aircraft is evaluated by structural reliability analysis theory, based on the stress-strength interference model. The stress-strength interference model assumes that the stress and strength of the structure are both random variables. Therefore, the basic point of structural reliability analysis and design is to first understand the distribution law of stress and strength and to reasonably establish a mathematical model between stress and strength. Structural strength reliability analysis process: Determine stress distribution through load statistics and probability distribution, and determine strength distribution through material performance statistics and probability distribution; Stress distribution and stress distribution simultaneously consider the distribution of geometric dimensions and other random factors to obtain stress statistics and its probability distribution, strength statistics and its probability distribution respectively; Then obtain the strength statistical distribution and its possibility of the structure through stress statistics and its probability distribution, strength statistics and its probability distribution; In practice, stress and strength are both time-varying quantities. Usually, stress S changes randomly, while strength R decays over time. The relationship between the two is constantly changing, and their mutual interference area is also constantly changing over time, thus generating a dynamic stress-strength interference model, that is, a stress-strength interference model that takes time factors into account. The overlapping area of ​​the probability change curve of stress and the probability change curve of strength is the interference area. The overlapping area indicates that the structure may fail. Therefore, the smaller the area of ​​the overlapping area, the higher the reliability of the structure. Similarly, the larger the area of ​​the overlapping area, the lower the reliability of the structure. However, the overlapping area is not equal to the numerical quantification of the reliability of the structure or the failure, but only represents the interference between stress and strength. The reliability of the structure is quantitatively calculated based on the overlapping area, and its calculation theory is the stress-strength interference theory, and its calculation model is the interference model. In the overlapping area, if R>S, the structure is safe; otherwise, it means that the structure fails. 2.2) Evaluation of residual strength of damage tolerant structures Damage tolerance analysis and residual strength assessment are carried out for key damage tolerance parts. The basic process is as follows: 2.2.1) Screening and determining the locations for damage tolerance analysis and assessment, including screening and determining damage tolerance key parts; 2.2.2) Damage tolerance requirements for given damage tolerance assessment locations, including material parameters, structural design type, inspection level and minimum unrepaired service life, crack form, initial crack size, and minimum residual strength requirements; 2.2.3) Determine the design load spectrum; 2.2.4) Conduct stress analysis on civil aircraft structures based on load spectra, and compile stress spectra of damage tolerance analysis locations based on stress results and practical experience; 2.2.5) Crack growth life analysis: Calculate the stress intensity factor according to the crack form, stress spectrum, loading mode and geometric configuration of the damage tolerance analysis part; select the crack growth equation and crack growth model; select the analysis software to perform crack growth analysis; draw the crack growth life curve based on the crack growth analysis results, and finally determine the maintenance cycle; 2.2.6) Safety margin analysis of damaged structures: Determine the safety margin of damaged parts in combination with the crack growth life curve; 2.3) Structural and functional index evaluation Key indicators that affect the overall functionality of civil aircraft include the deflection function of movable control surfaces and the sealing performance of fuel tank structures; The structural functional indicators were evaluated using the following methods: 2.3.1) Evaluation of fuel tank sealing performance The functionality of the fuel tank seal is related to the damage of the fuel tank load-bearing structure. When the fuel tank load-bearing structure is intact and there is no crack damage, the fuel tank seal is considered intact. When cracks appear in the fuel tank load-bearing structure, the fuel tank seal will be reduced, and even the overall function of the fuel tank will fail due to structural damage. Therefore, the evaluation of the sealing performance of the fuel tank is carried out by using the crack size of the main load-bearing structure of the fuel tank. According to the above damage tolerance analysis steps, damage tolerance analysis is carried out on the load-bearing structure of the fuel tank, and combined with experimental verification, a relationship curve between the sealing performance of the fuel tank and the crack size is obtained. The evaluation of the sealing performance of the fuel tank is based on whether the crack causes obvious oil leakage, rather than the residual strength. 2.3.2) Evaluation of the deflection function of the movable control surface The bearings used in the movable control surfaces of civil aircraft are spherical bearings. The decrease in the deflection capacity of the control surfaces is characterized by the wear and damage of the spherical bearings. The wear of the spherical bearings occurs on the lining layer, and the wear amount is calculated according to the following formula: In the above formula, V is the wear amount, F N is the normal load, x is the sliding distance, σ s is the compressive yield limit of the softer material, K s is called the wear constant; The wear process of spherical bearings can be divided into three stages: running-in wear stage (I), stable wear stage (II), and rapid wear stage (III). In the running-in wear stage, the wear rate gradually decreases with time. In the stable wear stage, the friction surface reaches a stable operating state after running-in, and the wear rate remains basically unchanged. This is the normal working period of the friction pair and the most critical period for determining the wear life of the friction pair. In the rapid wear stage, the wear rate increases rapidly with time, causing the working conditions of the friction pair to deteriorate sharply until the parts fail completely. The influence on the deflection of the rudder surface appears in the third stage, that is, when the bearing wear exceeds the threshold value, the clearance of the bearing will increase with the wear, the roughness will increase, and the precision of the bearing will also decrease, thus affecting the deflection efficiency of the rudder surface and even causing the deflection function to fail; 2.3.3) Economic evaluation of structural repair Structural repairs include repairing structural cracks and replacing new parts.

3. The method for evaluating the health status of an aircraft structure according to claim 1, characterized in that: The identification of each parameter includes the following methods: 3.1) Local detail strain identification method Local strain parameters are identified and monitored using strain gauges or fiber optic sensors; fiber optic sensors can reconstruct local strain distribution based on fiber optic spectrum characteristics; 3.2) Structural modal identification method Structural modal parameters or local health assessment indicators extracted from responses can be used to indicate the structural health status and damage occurrence; The vibration characteristics-based identification methods used include: damage identification methods based on natural frequency, mode shape and mode shape curvature, flexibility, frequency response function, and modal strain energy dynamic parameters; 3.3) Local crack identification method 3.3.1) Structural crack parameter identification based on eddy current array sensor Eddy current monitoring technology uses the principle of electromagnetic field change and can be used to monitor the local structure of bolted joints and curved transition structures. During the crack propagation process, the sensor output signal changes with the crack propagation: when the crack tip extends to the corresponding induction channel coil, the sensor output of the corresponding channel reaches the maximum inflection point value, and then according to the position of the induction channel, the crack length can be quantitatively determined in real time; 3.3.2) Structural crack parameter identification based on vibration characteristics of civil aircraft Extract the natural frequency, vibration mode, flexibility, modal strain energy vibration characteristic parameters of typical structures, establish the structural dynamics equation containing damage based on the damage identification theory of vibration characteristics, and derive the expression containing damage parameters; analyze the influence of damage parameters on vibration characteristic parameters based on the vibration characteristic parameter expression; finally, compare and analyze the effectiveness and sensitivity of various vibration characteristic parameters for structural damage parameter identification, and determine the typical structural damage method of civil aircraft based on vibration characteristics; 3.3.3) Structural crack parameter identification based on optical fiber sensor It is necessary to conduct stress analysis on the hole structure to be monitored, determine the dangerous area where cracks may occur, and design and layout the sensor in combination with the strain distribution near the crack and the monitoring requirements. The strain distribution near the crack at the hole edge is obtained through finite element analysis, and the optical fiber is reasonably arranged according to the strain distribution, so as to establish the positioning and quantitative relationship between the spectral characteristic parameters and the crack. 3.4) Corrosion damage identification method Corrosion damage identification is based on the analysis of impedance measurement methods and noise interference current characteristics, establishing a spatial noise current mathematical model, designing impedance sensing probes and precision measurement circuit modules suitable for high-resistance coating systems in low-conductivity environments, and optimizing electrochemical impedance calculation methods to identify coating damage status; Basic principle: The metal substrate and surface protection of the probe are consistent with the monitoring parts of civil aircraft. The probe does not need to be in direct contact with the metal structure to be measured and its coating during measurement. It is sufficient to ensure that the probe and the monitoring structure coating are in the same environment, which is used to completely reproduce the corrosion process of the coating in the monitoring part of civil aircraft; using high signal-to-noise ratio signal input and management control hardware precision circuit measurement module, the precision circuit measurement module is connected to the probe, when a thin liquid film gathers on the coating surface to conduct the circuit, an AC excitation voltage signal at a certain frequency is applied to the coating through the electrode, which can accurately measure the failure performance changes of the probe coating, thereby accurately detecting the coating failure, which is suitable for corrosion monitoring of civil aircraft structures.

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