A Method for Compiling Random Load Spectra under a Combined Force-Thermal-Vibration Environment

Abstract

This application belongs to the field of flight structure fatigue life analysis, and particularly relates to a method for compiling a random load spectrum under a combined force, heat, and vibration environment. It includes: Step 1, select the fatigue critical parts according to the stress level of the aircraft structure; Step 2, obtain the flight-by-flight fatigue load spectrum of the typical mission profile of the fatigue critical parts, and the fatigue load spectrum includes a vibration fatigue load spectrum, an aerodynamic fatigue load spectrum, and a thermal fatigue load spectrum; Step 3, linearly superimpose the aerodynamic fatigue load spectrum and the thermal fatigue load spectrum to obtain a combined static fatigue load spectrum; Step 4, process the vibration fatigue load spectrum to obtain a processed vibration fatigue load spectrum; Step 5, perform random interpolation on the processed vibration fatigue load spectrum and the combined static fatigue load spectrum to obtain a new load spectrum. This application can realize the compilation of the load spectrum under the combined force, heat, and vibration environment, and form a fatigue analysis load spectrum that comprehensively considers the damage of the flight load spectrum and the environmental spectrum.

Description

Technical Field

[0001] This application belongs to the field of fatigue life analysis of flight structures, and particularly relates to a method for compiling a random load spectrum under a force-thermal-vibration composite environment. Background Art

[0002] The load spectrum used in aircraft design refers to the load spectrum compiled during the aircraft design stage for fatigue / durability / damage tolerance analysis and corresponding tests. For parts mainly designed for aircraft maneuvering overload, the flight-by-flight fatigue load spectrum is the mainstream compilation method. For areas affected by temperature and vibration environments, corresponding environmental spectra should be compiled. However, for parts affected by heat environment and high-magnitude vibration environment, the flight-by-flight load spectrum alone cannot truly reflect the damage of this part. A harsh heat environment can generate thermal stress no less than mechanical stress. Random vibration fatigue is the fatigue failure caused by superimposing vibration stress on the basis of quasi-static stress. For the dynamic stress caused by a high-magnitude vibration environment, although its own amplitude is not large, after being superimposed with the quasi-static stress, the combined stress level cannot be ignored. Especially when the vibration load frequency is much higher than the flight load, it usually aggravates the damage of the structure. For areas affected by aircraft maneuvering overload, temperature and vibration environments simultaneously, simplify its load / environment history, compile a load spectrum under a force-thermal-vibration composite environment, represent the predetermined average design usage of the aircraft, and be used for the fatigue life and durability / damage tolerance analysis of the structure.

[0003] The traditional flight-by-flight fatigue load spectrum cannot truly reflect the actual damage of structures affected by temperature and vibration environments. The method of direct correction is too rough to accurately reflect the damage of the structure. The correction method brings greater design risks. For example, if the corrected damage is too small and the result is on the dangerous side, it may cause serious property losses or even personal injuries; if the corrected damage is too large and the result is on the safe side, it will increase the structural weight and reduce the aircraft performance.

[0004] Therefore, it is hoped that there is a technical solution to overcome or at least mitigate at least one of the above defects of the prior art. Summary of the Invention

[0005] The purpose of this application is to provide a method for compiling a random load spectrum under a force-thermal-vibration composite environment to solve at least one problem existing in the prior art.

[0006] The technical solution of this application is as follows:

[0007] A method for compiling a random load spectrum under a force-thermal-vibration composite environment includes:

[0008] Step 1: Select fatigue critical parts according to the stress level of the aircraft structure;

[0009] Step 2: Obtain the flight - continuous - flight fatigue load spectrum of the typical mission profile of the fatigue - dangerous parts, where the fatigue load spectrum includes a vibration fatigue load spectrum, an aerodynamic fatigue load spectrum, and a thermal fatigue load spectrum;

[0010] Step 3: Linearly superpose the aerodynamic fatigue load spectrum and the thermal fatigue load spectrum to obtain a composite static fatigue load spectrum;

[0011] Step 4: Process the vibration fatigue load spectrum to obtain a processed vibration fatigue load spectrum;

[0012] Step 5: Randomly interpolate the processed vibration fatigue load spectrum and the composite static fatigue load spectrum to obtain a new load spectrum.

[0013] In at least one embodiment of the present application, in Step 4, the processing of the vibration fatigue load spectrum to obtain a processed vibration fatigue load spectrum includes:

[0014] S401: Count the vibration fatigue load spectrum by the Dirlik counting method to obtain the probability density function of the vibration stress amplitude;

[0015] S402: Discretize the probability density function of the vibration stress amplitude to obtain the probabilities of different vibration stress amplitudes;

[0016] S403: Perform high - load truncation and low - load removal on the vibration stress amplitude according to the most dangerous working conditions;

[0017] S404: Re - divide the vibration stress levels;

[0018] S405: Calculate the frequencies of different stress amplitudes for n flights according to the loading time and the average vibration frequency;

[0019] S406: Evenly distribute the load frequencies according to the action time of different working conditions to obtain the vibration load frequencies for one flight of each working condition.

[0020] In at least one embodiment of the present application, in S403, it includes:

[0021] Perform high - load truncation on the peak stress, remove the high - loads that appear less than 1 time within one life cycle, and ensure that the stress level does not exceed the strength limit;

[0022] Perform low - load removal on the valley stress, and define that the stress below 60% of the fatigue limit does not cause damage.

[0023] In at least one embodiment of the present application, in S405, calculate the frequencies of different stress amplitudes for 50 flights according to the loading time and the average vibration frequency.

[0024] In at least one embodiment of the present application, in step five, the mixed multiplicative congruential method is used to generate random numbers, and the shuffle algorithm is used to randomly interpolate the processed vibration fatigue load spectrum and the composite static fatigue load spectrum.

[0025] In at least one embodiment of the present application, in step five, the use of the mixed multiplicative congruential method to generate random numbers includes:

[0026] The iterative formula is:

[0027]

[0028] M = 2 N

[0029] a = 2 c +1

[0030] b = 2 k +1

[0031]

[0032] where X i represents the i-th random number, X i+1 represents the (i + 1)-th random number, M represents the period of the random number sequence, N is the number of digits for generating the random sequence, a is the multiplier, b is the increment, and c is the smallest odd number greater than or equal to k.

[0033] The invention has at least the following beneficial technical effects:

[0034] The method for compiling a random load spectrum in a force-thermal-vibration composite environment of the present application can realize the compilation of the load spectrum in the force-thermal-vibration composite environment, form a fatigue analysis load spectrum that comprehensively considers the damage of the flight load spectrum and the environmental spectrum, reflect the true stress state and damage level of the aircraft structure, and lay a foundation for accurately analyzing the structural life and evaluating the aircraft life index. Description of the Drawings

[0035] Figure 1 is a flowchart of the method for compiling a random load spectrum in a force-thermal-vibration composite environment according to an embodiment of the present application. Detailed Embodiments

[0036] To make the purpose, technical solution and advantages of the present application more clear, the following will describe the technical solution in the embodiments of the present application in more detail in conjunction with the accompanying drawings in the embodiments of the present application. In the drawings, the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The described embodiments are some, but not all, of the embodiments of the present application. The embodiments described below by referring to the drawings are exemplary and are intended to explain the present application and should not be construed as a limitation of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts fall within the scope of protection of the present application. The following will explain the embodiments of the present application in detail in conjunction with the drawings.

[0037] In the description of the present application, it should be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on the scope of protection of the present application.

[0038] The following will further explain the present application in conjunction with the attached Figure 1 Make a more detailed description of the present application.

[0039] The present application provides a method for compiling a random load spectrum under a force-thermal-vibration composite environment, including:

[0040] Step 1: Select the fatigue critical parts according to the stress level of the aircraft structure;

[0041] Step 2: Obtain the flight-by-flight fatigue load spectrum of the typical mission profile of the fatigue critical parts, and the fatigue load spectrum includes a vibration fatigue load spectrum, an aerodynamic fatigue load spectrum, and a thermal fatigue load spectrum;

[0042] Step 3: Linearly superimpose the aerodynamic fatigue load spectrum and the thermal fatigue load spectrum to obtain a composite static fatigue load spectrum;

[0043] Step 4: Process the vibration fatigue load spectrum to obtain a processed vibration fatigue load spectrum;

[0044] Step 5: Randomly interpolate the processed vibration fatigue load spectrum and the composite static fatigue load spectrum to obtain a new load spectrum.

[0045] The method for compiling a random load spectrum under a combined force, heat and vibration environment of the present application first selects fatigue-critical parts according to the stress level of the aircraft structure, and obtains the flight-by-flight fatigue load spectrum of the typical mission profile of the fatigue-critical parts. By analyzing the stress level of the aircraft structure, it is determined that the aircraft structure is mainly subjected to the action of three types of stresses: mechanical stress, thermal stress and vibration stress. Among them, the mechanical stress is the stress caused by the aircraft being subjected to mechanical loads such as aerodynamic loads and inertial loads; the thermal stress is the stress generated by the structure due to factors such as aerodynamic heating and heat source radiation; the vibration stress is the dynamic response formed by the structure after being subjected to external excitation. For the same part, the mechanical stress and the thermal stress can be directly linearly superimposed according to the corresponding load conditions and used as the combined static stress. In the present application, the superimposed result of the mechanical stress and the thermal stress is used as the mean stress of fatigue, which has a certain duration and different magnitudes under different working conditions during a single flight. In this embodiment, the most dangerous working condition (the maximum mean stress) during a single flight is selected as the basis for spectrum compilation during load spectrum compilation, so as to avoid deleting low vibration stress amplitudes that may cause damage under certain working conditions when performing low-load truncation.

[0046] The method for compiling a random load spectrum under a combined force, heat and vibration environment of the present application further needs to process the vibration fatigue load spectrum to obtain the processed vibration fatigue load spectrum, including:

[0047] S401. Count the vibration fatigue load spectrum by the Dirlik counting method to obtain the probability density function of the vibration stress amplitude;

[0048] S402. Discretize the probability density function of the vibration stress amplitude to obtain the probabilities of different vibration stress amplitudes;

[0049] S403. Perform high-load truncation and low-load truncation on the vibration stress amplitude according to the most dangerous working condition;

[0050] S404. Re-divide the vibration stress levels;

[0051] S405. Calculate the frequencies of different stress amplitudes during n flights according to the loading time and the average vibration frequency;

[0052] S406. Evenly distribute the load frequencies according to the action time of different working conditions to obtain the vibration load frequencies of each working condition during a single flight.

[0053] The method for compiling a random load spectrum under the combined force, heat and vibration environment of the present application. Finally, after obtaining the load frequencies for one flight under different working conditions, it is necessary to randomly interpolate the vibration stress amplitude on the mean values of the aerodynamic and thermal stresses. In the present application, the mixed congruential method can be used to generate random numbers, and the shuffle algorithm is used to achieve the random interpolation of the vibration stress, so as to obtain the load spectrum for one flight under one working condition. Then, it is arranged according to the order of the flight working conditions to obtain the load spectrum for one flight, and according to the arrangement of different flight profiles, the load spectrum for the entire life cycle is obtained.

[0054] In the preferred implementation of the present application, during the processing of the vibration fatigue load spectrum, the amplitude probability density function is discretized to obtain the probability density at different amplitudes. According to the total flight time and the average loading frequency, the total number of vibration cycles is obtained, as shown in the following table:

[0055] Table 1

[0056] Stress range / MPa Stress amplitude / MPa Probability Frequency 0-10 5 P1 P1Tf 10-20 15 P2 P2Tf …… -- -- -- 100-110 105 Pi PiTf …… -- -- --

[0057] The stress amplitude levels are divided for different regions to obtain the stress amplitude distribution frequencies:

[0058] Table 2

[0059]

[0060] The Goodman formula is used for conversion to obtain the approximate fatigue limit of the orifice plate corresponding to R = -1. The vibration stress with the composite stress as the mean stress superposed as the amplitude is used to obtain the peak stress and the valley stress.

[0061] High-load truncation is performed on the peak stress, that is, the high loads that appear less than 1 time within one life cycle are removed, and it is ensured that the stress level does not exceed the strength limit;

[0062] Low-load removal is performed on the valley stress. It is defined that the stress below 60% of the fatigue limit does not cause damage. The amplitude lower limit is calculated:

[0063]

[0064] Among them, the frequency of the vibration stress amplitude above σ6 is less than 10 times. According to the principles of high-load truncation and low-load removal, the low-load amplitudes below σ1 are removed, and the load amplitudes above σ6 are intercepted, and the load amplitude range is taken as [σ1, σ6].

[0065] The stress amplitudes in the σ1 - σ6 interval are divided into n levels, and the following table shows the flight stress amplitude distribution frequencies:

[0066] Table 3

[0067]

[0068] Determine the loading period of a load spectrum block according to the number of flights. The length of the loading period will have a certain impact on the fatigue test or life estimation results, mainly reflected in the impact of the loading sequence caused by the length of the loading period. In this embodiment, a loading period is tentatively set to 50 flights.

[0069] After obtaining the frequency table with at least one flight, calculate the remaining frequency of the vibration stress amplitude less than one time per flight within one loading period (50 flights), and make a remaining frequency table.

[0070] Δf′ ij =P j TfR i -Δf ij N i

[0071] Among them, P j TfR i The total frequency of the j - th level load in the i - th flight condition within one loading period of 50 flights is calculated, and N i represents the total number of times of the i - th condition in 50 flights.

[0072] For the peak frequency of the vibration load less than one time in one condition during one flight, calculate the position index PI of each stress level according to the number of flights in one loading period, and list it in a position index table. The position index calculation formula is as follows:

[0073]

[0074] Among them, PI ij means adding the j - th load of the i - th condition once every PI ij flights.

[0075] After completing the stress amplitude frequency statistics, these amplitudes need to be randomly interpolated. In this embodiment, the mixed multiplicative congruential method is used to generate random numbers. The following introduces how to generate random numbers using the mixed multiplicative congruential method. Its iterative formula is as follows:

[0076]

[0077] M = 2 N

[0078] a = 2 c +1

[0079] b = 2 k +1

[0080]

[0081] Among them, X i represents the i - th random number, Xi+1 It represents the (i + 1)-th random number, M represents the period of the random number sequence, N is the number of digits for generating the random sequence, a is the multiplier, b is the increment, and c is the smallest odd number greater than or equal to k.

[0082] In this embodiment, the random numbers are calculated according to the mixed multiplicative congruential method formula. When randomly arranging, the shuffle algorithm is used to achieve the random arrangement of the payload. N is taken as 16, and the initial values for generating random numbers are shown in the following table:

[0083] Table 4

[0084] <![CDATA[X0]]> N M k a b c System time 16 65536 8 129 257 7

[0085] According to the value of M being 65536, the remainder range is [0, 65535], and the maximum number of non-repeating random numbers generated is 65536, which fully meets the usage requirements. In addition, it should be noted that the initial value X0 is determined by the current system time. When obtaining random numbers, according to the current system time, the three parameters of minutes (min), seconds (s), and milliseconds (ms) are taken as the initial values, and the calculation formula is as follows:

[0086] X0 = 60000 * min + 1000 * s + ms

[0087] The random pairing of peak and valley values of the flight payload once: The implementation process of the random pairing of peak and valley values of the flight payload once is as follows:

[0088] (1) Adopt the "shuffle algorithm" to randomly pair the peak and valley values of the payload in each working condition during one flight.

[0089] (2) During one flight, if the number of occurrences of the peak and valley values of the payload under a certain working condition is less than once, it needs to be regularized to an integer greater than once. According to the index position of the payload remainder table, add the j-th level payload of the i-th working condition every n flights, so that it is randomly distributed in this flight working condition.

[0090] (3) Arrange the stress spectra of different working conditions in subsequent flights according to the above process until a loading cycle is completed.

[0091] Random arrangement of each working condition and flight: In this embodiment, a loading cycle includes 50 flights. After obtaining the random spectra of each working condition in 50 flights, first randomly arrange different working conditions according to the aircraft usage mission profile to obtain a complete 50-flight spectrum.

[0092] Random arrangement of 50 flights: The load spectrum numbers of each working condition in 50 flights obtained are shown in the following table. Form one flight according to LC2 - LC1 - LC3 - …… LC N, randomly select numbers for each working condition, and form 50 flights to complete the load spectrum of a loading cycle. See Table 5.

[0093] Table 5

[0094] Operating condition Code number of one flight landing spectrum LC1 LC1-1, LC1-2, LC1-3 …… LC1-49, LC1-50 LC2 LC2-1, LC2-2, LC2-3 …… LC2-49, LC2-50 LC3 LC3-1, LC3-2, LC3-3 …… LC3-49, LC3-50 …… …… LC N LC N -1, LC N -2, LC N -3 …… LC N -49, LC N -50

[0095] The method for compiling a random load spectrum under a combined force-thermal-vibration environment of the present application is based on the traditional flight-by-flight fatigue load spectrum. A combined static fatigue load spectrum is formed by linearly superposing an aerodynamic fatigue load spectrum and a thermal fatigue load spectrum, and the vibration fatigue load spectrum is randomly interpolated with the combined static fatigue load spectrum to simulate the structural damage under the combined force-thermal-vibration load during the entire life cycle, thereby completing the compilation of the random load spectrum under the combined force-thermal-vibration environment. The present application can realize the compilation of the load spectrum under the combined force-thermal-vibration environment, form a fatigue analysis load spectrum that comprehensively considers the damage of the flight load spectrum and the environmental spectrum, reflect the true stress state and damage level of the aircraft structure, and lay a foundation for accurately analyzing the structural life and evaluating the aircraft life index.

[0096] As described above, it is only the specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in the present application should be covered within the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.

Claims

1. A method for compiling a random load spectrum under a combined force, heat, and vibration environment, characterized in that Including: Step 1: Select fatigue critical parts according to the aircraft structural stress level; Step 2: Obtain the flight - by - flight fatigue load spectra of the typical mission profiles of the fatigue critical parts, where the fatigue load spectra include vibration fatigue load spectra, aerodynamic fatigue load spectra, and thermal fatigue load spectra; Step 3: Linearly superpose the aerodynamic fatigue load spectra and the thermal fatigue load spectra to obtain a composite static fatigue load spectrum; Step 4: Process the vibration fatigue load spectra to obtain the processed vibration fatigue load spectra, including: S401: Count the vibration fatigue load spectra by the Dirlik counting method to obtain the probability density function of the vibration stress amplitude; S402: Discretize the probability density function of the vibration stress amplitude to obtain the probabilities of different vibration stress amplitudes; S403: Perform high - load truncation and low - load removal on the vibration stress amplitude according to the most dangerous working conditions; S404: Re - divide the vibration stress levels; S405: Calculate the frequencies of different stress amplitudes for n flights according to the loading time and the average vibration frequency; S406: Evenly distribute the load frequencies according to the action time of different working conditions to obtain the vibration load frequencies for one flight of each working condition; Step 5: Randomly interpolate the processed vibration fatigue load spectra and the composite static fatigue load spectra to obtain a new load spectrum.

2. The method for compiling a random load spectrum under a force-thermal-vibration composite environment according to claim 1, wherein In S403, it includes: Perform high - load truncation on the peak stress, remove the high - loads that occur less than 1 time within one life cycle, and ensure that the stress level does not exceed the strength limit; Perform low - load removal on the valley stress, and define that 60% lower than the fatigue limit does not cause damage.

3. The method for compiling a random load spectrum under a force-thermal-vibration composite environment according to claim 2, wherein In S405, calculate the frequencies of different stress amplitudes for 50 flights according to the loading time and the average vibration frequency.

4. The method for compiling a random load spectrum under a force-thermal-vibration composite environment according to claim 3, wherein In Step 5, use the mixed multiplicative congruential method to generate random numbers, and use the shuffling algorithm to realize the random interpolation of the processed vibration fatigue load spectra and the composite static fatigue load spectra.

5. The method for compiling a random load spectrum under a combined force-thermal-vibration environment according to claim 4, characterized in that, In Step 5, the use of the mixed multiplicative congruential method to generate random numbers includes: The iterative formula is: M＝2 N a＝2 c +1 b＝2 k +1 Among them, X i represents the i-th random number, and X i+1 represents the (i + 1)-th random number. M represents the period of the random number sequence, N is the number of bits for generating the random sequence, a is the multiplier, b is the increment, and c is the smallest odd number greater than or equal to k.

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