A fatigue stress calculation method based on helicopter structure vibration

Abstract

The application relates to the field of helicopter structure strength design, in particular to a fatigue stress calculation method based on helicopter structure vibration. The method comprises the following steps: performing frequency spectrum analysis on the time domain data of vibration measuring points on the body to obtain the frequency response results at typical vibration measuring points; determining the main damage frequency and acceleration amplitude of the main damage of the structure according to the frequency response results at the typical vibration measuring points; determining the structural natural modal frequency closest to the main damage frequency and the working modal corresponding to the structural natural modal as the working modal under the main damage frequency based on the main damage frequency and in combination with the finite element modal analysis results of the body; taking the measuring point as the reference, performing equivalent displacement analysis according to the acceleration amplitude and the working modal mode, and converting the acceleration amplitude into equivalent displacement distribution and direction under the condition that the phase angle of the working modal mode is kept unchanged; and performing equivalent stress calculation according to the equivalent displacement distribution and direction and the finite element model of the structure to obtain the static stress.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of helicopter structure strength design, in particular to a fatigue stress calculation method based on helicopter structure vibration. BACKGROUND

[0002] The helicopter structure bears complex structural vibration load in the flight process, and the vibration fatigue damage failure caused by the superposition of conventional fatigue and the structure resonance occurs frequently in many types of helicopters in China in recent years, which often occurs in the secondary load-bearing structure with local resonance, seriously affecting the safety of the helicopter platform structure. The helicopter structure fatigue test and structure vibration test verification technology has been relatively mature in engineering, and the universal test method and standardized test procedure have been established. However, the current domestic design analysis method of helicopter vibration fatigue is still immature, especially for engineering structures, it is difficult to obtain accurate structural dynamic stress, and the measurement is difficult, which leads to the inability to guarantee the precision of the structure life assessment in the vibration environment. SUMMARY

[0003] The purpose of the present application is to provide a fatigue stress calculation method based on helicopter structure vibration, which predicts the fatigue dynamic stress of the structure through the vibration measurement data on the machine body, and is used for vibration fatigue life prediction and optimization design of the structure.

[0004] The technical scheme of the present application:

[0005] A fatigue stress calculation method based on helicopter structure vibration, comprising:

[0006] Performing frequency spectrum analysis on the time domain data of the vibration measurement points on the machine body to obtain the frequency response results at the typical vibration measurement points;

[0007] According to the frequency response results at the typical vibration measurement points, determining the main damage frequency and acceleration amplitude constituting the main damage of the structure;

[0008] Based on the main damage frequency, combining the finite element modal analysis results of the machine body, determining the structural natural modal frequency closest to the main damage frequency, and the corresponding structural natural mode as the working mode under the main damage frequency;

[0009] Taking the measurement point as the reference, according to the acceleration amplitude and working mode shape, performing equivalent displacement analysis, and converting the acceleration amplitude into equivalent displacement distribution and direction while keeping the phase angle of the working mode shape unchanged;

[0010] According to the equivalent displacement distribution and direction, and the finite element model of the structure, performing equivalent stress calculation to obtain the static stress, which is approximately the equivalent dynamic stress caused by the working mode vibration of the structure under the excitation of the main damage frequency, and is used for the fatigue damage and life calculation of the structure.

[0011] The time-domain data measured at the vibration measuring points on the body is subjected to frequency spectrum analysis, including:

[0012] The time-domain data measured at the typical vibration measuring points on the body is subjected to Fourier transform to obtain a frequency response curve, i.e. a frequency response result; the time-domain data is selected from the data measured at the typical vibration measuring points and is stable-state data of the helicopter.

[0013] According to the frequency response result at the typical vibration measuring points, main damage frequency and acceleration amplitude constituting main damage of the structure are determined, including:

[0014] The dynamic displacement of all periodic excitations in the frequency response curve is compared, and the frequency and amplitude corresponding to the maximum dynamic displacement are taken as the main damage frequency and acceleration amplitude constituting main damage of the structure; the dynamic displacement is obtained from the frequency and acceleration amplitude according to the following formula:

[0015]

[0016] Wherein, U is the dynamic displacement amplitude, A is the acceleration amplitude, and f is the main damage frequency (Hz).

[0017] Based on the main damage frequency, the closest inherent modal frequency of the structure to the main damage frequency is determined in combination with the finite element modal analysis result of the body, and the corresponding inherent mode of the structure is taken as the working mode under the main damage frequency, including:

[0018] According to the dynamic characteristic test or the finite element modal analysis result of the body, each order inherent frequency of the body structure is obtained;

[0019] The i-th order inherent frequency closest to the main damage frequency is selected from the each order inherent frequency, and i is a positive integer; the inherent mode corresponding to the i-th order inherent frequency is taken as the working mode under the main damage frequency.

[0020] Taking the measuring point as a reference, equivalent displacement analysis is performed according to the acceleration amplitude and the working mode shape, and the acceleration amplitude is converted into equivalent displacement distribution and direction while keeping the phase angle of the working mode shape unchanged, including:

[0021] The dynamic displacement amplitude at the typical vibration measuring point is calculated from the acceleration amplitude;

[0022] Taking the typical vibration measuring point as a reference, the working mode shape of the structure is normalized with the amplitude being set to 1;

[0023] The amplitude of each node of the normalized mode shape corresponding to the finite element model is enlarged by n times while keeping the phase angle of each node unchanged, to obtain the equivalent displacement distribution and direction under the working mode of the structure;

[0024] Wherein, n is the value of the maximum dynamic displacement, and the typical vibration measuring point is one of the nodes.

[0025] The equivalent stress is calculated according to the equivalent displacement distribution and direction and the finite element model of the structure, and the static stress is obtained, including:

[0026] In the finite element model of the structure, the equivalent displacement vector is synchronously applied as a forced displacement load at each node; the equivalent displacement vector is obtained from the equivalent displacement distribution and direction;

[0027] The static stress of the structure under the boundary constraint condition is calculated by using the static force solution of the applied finite element model, and the boundary constraint condition is consistent with the boundary condition of the finite element modal analysis result of the body.

[0028] The typical vibration measuring point is located at the maximum vibration of the body structure.

[0029] The application provides a fatigue stress calculation method based on the vibration of a helicopter structure, and the relationship between the structural fatigue stress and the vibration acceleration result is established by using the vibration acceleration data which is relatively easy to collect in engineering, for the damage of the typical structure of the helicopter under single cycle excitation, so as to solve the key problems of the difficulty in testing the dynamic stress of the related structure and the difficulty in calculating the dynamic stress caused by unknown load source and damping, and provide a design idea and method for similar problems which can be implemented in engineering, and can be used for vibration fatigue analysis and life prediction of most helicopter structures. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 It is a flowchart of the fatigue stress calculation method based on the vibration of the helicopter structure.

[0031] Figure 2 It is a typical measuring point vibration time domain curve diagram.

[0032] Figure 3 It is a vibration frequency response result diagram.

[0033] Figure 4 It is a structural modal analysis result diagram.

[0034] Figure 5 It is a dynamic stress and equivalent stress analysis result comparison diagram. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0036] The typical vibration environment of helicopter structure includes two parts of several periodic excitations and broadband random, for most of the helicopter structure, the response of periodic excitation is generally much larger than that of broadband random excitation, therefore, this patent mainly aims at the structure vibration fatigue problem caused by periodic excitation.

[0037] This patent designs a fatigue stress calculation method based on the vibration of helicopter structure, including vibration data processing, main damage frequency analysis, operational modal analysis, equivalent displacement analysis, equivalent stress calculation and other aspects.

[0038] The following will be combined Figure 1 The vibration fatigue stress calculation method of helicopter structure is further described in detail:

[0039] Step one: vibration data processing

[0040] In order to determine the frequency and amplitude of the periodic excitation of helicopter structure, first, the Fourier transform is carried out on the time domain data of the vibration measuring point on the machine body, the frequency response curve of vibration is obtained, a section of helicopter stable state data is selected, for discrete data, generally DFT method is adopted, thus, the frequency response result of the measuring point on the structure can be obtained.

[0041] Step two: main damage frequency analysis

[0042] Although there are multiple periodic excitations on the helicopter, the vibration problem is often caused by the excessive dynamic response of a certain frequency. Therefore, based on the frequency response result of the above step, the amplitude of all periodic excitations can be compared and analyzed to determine the maximum amplitude or the frequency value and amplitude of the main damage of the structure. For the case of multiple main damage frequency components or broadband response, this patent method is not applicable.

[0043] Step three: operational modal analysis

[0044] According to the dynamics theory, the main reason for the excessive vibration of structure caused by periodic excitation is that the natural frequency is close to the excitation frequency, therefore, based on the main frequency value analyzed in the above step, combined with the dynamic characteristic test or finite element modal analysis result, the corresponding structure natural mode near the main excitation frequency can be determined, that is, the working mode under the frequency;

[0045] Step four: equivalent displacement analysis

[0046] The acceleration amplitude of the main damage frequency obtained from the analysis in step two can be used to calculate the dynamic displacement amplitude at the measuring point on the structure; the working modal shape obtained from the test identification or finite element analysis in step three is selected, and the measuring point is taken as the reference (amplitude is set to 1), and the working modal shape of the structure is normalized, and the normalized modal shape is multiplied by the dynamic displacement amplitude at the measuring point obtained in step two, while the phase angle of the working modal shape is kept unchanged, so as to obtain the equivalent displacement distribution and direction of the working modal of the structure;

[0047] Step five: equivalent stress calculation

[0048] The equivalent displacement distribution and direction of the working modal of the structure obtained in step four are applied to the corresponding nodes in the finite element model of the structure as the forced displacement load, and the static stress of the structure under the corresponding boundary constraint condition is calculated by using the static solution, which can be approximately regarded as the equivalent dynamic stress caused by the working modal vibration of the structure under the excitation of the main damage frequency, and can be used for the fatigue damage and life calculation of the structure.

[0049] The fatigue stress calculated by the above method can realize the damage evaluation and life prediction of the structure to a certain extent, and can be used to guide the design optimization of similar structures. The relationship between the vibration and the fatigue dynamic stress of the structure established based on the method can provide technical support for the vibration and life monitoring of the structure in engineering.

[0050] Example:

[0051] Taking a cantilever thin plate structure made of aluminum alloy as an example, the end of the cantilever thin plate produces forced vibration response under the excitation of random and constant frequency vibration of the base, and the time-domain curve of the vibration is as shown in Figure 2 .

[0052] ①Vibration data processing

[0053] Firstly, the vibration data is analyzed by using the method in step 1, and the result is as shown in Figure 3 .

[0054] ②Main damage frequency analysis

[0055] As can be seen from Figure 2 , the forced vibration response of the structure includes three main frequency components, which are 4.3 Hz, 20 Hz and 80 Hz. By comparing the amplitude analysis, the main damage frequency of the structure is 20 Hz, and the amplitude is 1.44 g.

[0056] ③Working modal analysis

[0057] The dynamic model of the structure is established by using the finite element method, and the dynamic characteristics of the cantilever thin plate are analyzed, and the first three order modal frequency domain and mode shape are obtained, as shown in Figure 4as shown.

[0058] According to the main damage frequency in the second step, compared with the modal frequency, the first order vertical bending modal frequency is close to the main damage frequency, and other modal frequencies are far away from it, so it is determined that the modal can be approximated as the working modal of the structure.

[0059] ④Equivalent displacement analysis

[0060] Based on the first order vertical bending modal vibration mode result of the finite element, the corresponding node vertical vibration mode at the position of the end point measuring point is normalized, and the vertical vibration mode of the end node is set to 1.0 at this time. At the same time, the acceleration-displacement formula is used to calculate the dynamic displacement of the measuring point:

[0061]

[0062] The normalized vibration mode is multiplied by the dynamic displacement to obtain the vertical forced displacement of all nodes, wherein the vertical equivalent displacement of the end node is 0.89mm.

[0063] ⑤Equivalent stress calculation

[0064] The finite element model is used, the end is constrained by the actual boundary condition, and the other end is subjected to the equivalent displacement value calculated in the previous step, and static analysis is carried out, and the result of 9.1Mpa is obtained, which is the vibration fatigue stress of the structure, and can be used for fatigue life analysis.

[0065] Supplementary verification:

[0066] The dynamic stress analysis result calculated by the finite element is compared with the equivalent stress result of the patent, and the stress distribution cloud map and amplitude are as shown. Figure 5

[0067] From the comparison result, the error of the equivalent stress and the dynamic stress result is 14.1%, which can be used for the vibration fatigue design reference of the helicopter structure.

[0068] The above is only a specific embodiment of the present application, which is described in detail, and the part not described is the conventional technology. However, the protection scope of the present application is not limited to this, any change or replacement easily thought by those skilled in the art within the technical range disclosed by the present application should be covered in the protection scope of the present application. The protection scope of the present application should be subject to the protection scope of the claims.​

Claims

1. A method for calculating fatigue stress based on structural vibration of a helicopter, characterized by, The method comprises the following steps: Spectrum analysis is performed on the time domain data measured at the vibration measuring points on the body to obtain the frequency response results at the typical vibration measuring points; According to the frequency response results at the typical vibration measuring points, the main damage frequency and the acceleration amplitude constituting the main damage of the structure are determined; Based on the main damage frequency, the main damage frequency closest to the natural modal frequency of the structure is determined in combination with the finite element modal analysis results of the body, and the corresponding natural modal of the structure is taken as the working modal at the main damage frequency; Equivalent displacement analysis is performed according to the acceleration amplitude and the working modal shape, and the acceleration amplitude is converted into the equivalent displacement distribution and direction while keeping the phase angle of the working modal shape unchanged; Equivalent stress calculation is performed according to the equivalent displacement distribution and direction and the finite element model of the structure to obtain the static stress, which is approximately the equivalent dynamic stress caused by the working modal vibration of the structure under the excitation of the main damage frequency and is used for the fatigue damage and life calculation of the structure.

2. The method of claim 1, wherein, Spectrum analysis is performed on the time domain data measured at the vibration measuring points on the body, including: Fourier transform is performed on the time domain data measured at the typical vibration measuring points on the body to obtain the frequency response curve, i.e. the frequency response result; the time domain data is selected from the data measured at the typical vibration measuring points on the body and is the data of the helicopter in a stable state.

3. The method of claim 2, wherein, According to the frequency response results at the typical vibration measuring points, the main damage frequency and the acceleration amplitude constituting the main damage of the structure are determined, including: The dynamic displacement of all periodic excitations in the frequency response curve is compared, and the frequency and amplitude corresponding to the maximum dynamic displacement are taken as the main damage frequency and the acceleration amplitude constituting the main damage of the structure; the dynamic displacement is obtained from the frequency and the acceleration amplitude according to the following formula: wherein, is the amplitude of the primary displacement, is the amplitude of the acceleration, is the primary loss frequency.

4. The method of claim 3, wherein, Based on the main damage frequency, the main damage frequency closest to the natural modal frequency of the structure is determined in combination with the finite element modal analysis results of the body, and the corresponding natural modal of the structure is taken as the working modal at the main damage frequency, including: The natural frequencies of the body structure are obtained according to the dynamic characteristic test or the finite element modal analysis results of the body; The i-th natural frequency closest to the main damage frequency is selected from the natural frequencies, and i is a positive integer; the natural modal corresponding to the i-th natural frequency is taken as the working modal at the main damage frequency.

5. The method of claim 4, wherein, Equivalent displacement analysis is performed according to the acceleration amplitude and the working modal shape, and the acceleration amplitude is converted into the equivalent displacement distribution and direction while keeping the phase angle of the working modal shape unchanged, including: The dynamic displacement amplitude at the typical vibration measuring point is calculated from the acceleration amplitude; The working modal shape of the structure is normalized with the amplitude being taken as 1 and the typical vibration measuring point being taken as the reference; The equivalent displacement distribution and direction of the structure under the working modal are obtained by amplifying the amplitude of each node of the normalized modal shape by n times while keeping the phase angle of each node unchanged; Wherein, n is the value of the maximum dynamic displacement, and the typical vibration measuring point is one of the nodes.

6. The method of claim 5, wherein, Equivalent stress calculation is performed according to the equivalent displacement distribution and direction and the finite element model of the structure to obtain the static stress, including: The equivalent displacement vector is obtained from the equivalent displacement distribution and direction; The equivalent displacement vector is applied as a forced displacement load at each node in the finite element model of the structure; The finite element model after the application is solved by static force, and the static stress of the structure under the boundary constraint condition is calculated, and the boundary constraint condition is consistent with the boundary condition of the finite element modal analysis result of the body.

7. The method according to any one of claims 1 to 6, characterized in that, The typical vibration measuring point is located at the maximum vibration of the body structure.

8. A computer-readable storage medium having stored thereon a computer program, characterized in that The computer program, when executed by a processor, implements the method of any one of claims 1-7.

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