A calculation method for the low-cycle acceleration test spectrum of helicopter components

Abstract

The present invention provides a calculation method for a low-cycle acceleration test spectrum of helicopter components, including: equivalenting the low-cycle fatigue test spectrum characterized by the combined loading of a helicopter structure into a low-cycle fatigue stress spectrum characterized by a single stress, and establishing a relationship curve (life curve) between the structural fatigue life and the structural fatigue stress limit through normalization. At the same time, based on the life curve and the cumulative damage principle, etc., the optimal solutions of the load acceleration coefficient and the cycle number reduction coefficient acting on the low-cycle fatigue test spectrum are calculated cyclically, and finally a low-cycle fatigue acceleration test load spectrum that not only meets the acceleration requirements but also has a fatigue life equivalent to that of the un-accelerated equivalent is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of helicopter structural strength tests, and particularly relates to a calculation method for a low-cycle acceleration test spectrum of helicopter components. Background Art

[0002] During the process of a helicopter performing various tasks, it bears large load cycles from the ground to the air and back, and also bears the aerodynamic loads of the high-speed rotation of the rotor blades. The loads are very complex. The low-frequency and high-amplitude vibration loads caused by the ground-air-ground and flight state transitions are structural low-cycle fatigue. Conducting structural low-cycle fatigue tests with various loads in the mission profile at a 1:1 ratio, in order to enable the loads to be fully transmitted in the structure, the test loading frequency is generally low. Under the long fatigue life index, the fatigue test cycle will be greatly increased.

[0003] For a single-load structure, the low-cycle load spectrum can be accelerated through damage equivalence, while for a composite-load structure, involving the size ratios and phases of various loads, it is difficult to achieve equivalent acceleration of the low-cycle fatigue test spectrum. Summary of the Invention

[0004] In view of the above technical problems, the present invention provides a calculation method for a low-cycle acceleration test spectrum of helicopter components, and the method includes:

[0005] Based on the helicopter mission profile and the ground-air-ground loads and cycle numbers in the mission profile, compile a low-cycle fatigue test load spectrum corresponding to the structural low-cycle fatigue life;

[0006] Calculate the helicopter structural stress corresponding to the load cycle under each mission profile in the low-cycle fatigue test load spectrum, and convert the low-cycle fatigue test load spectrum into a low-cycle fatigue test stress spectrum;

[0007] Use the mission profile corresponding to the maximum stress amplitude as the normalization factor to normalize the stress amplitudes of all mission profiles, and generate a normalized structural low-cycle fatigue spectrum;

[0008] Based on the normalized structural low-cycle fatigue spectrum, the full-range safe fatigue characteristics curve of the structural material and the cumulative damage theory, calculate the life curve of the structure.

[0009] Preferably, the method further includes:

[0010] Determine the safe fatigue stress limit corresponding to the structural low-cycle fatigue life based on the life curve ;

[0011] Multiply the stress amplitudes corresponding to each profile in the normalized structural low-cycle fatigue spectrum by the acceleration factor , and adopt the full-range average fatigue characteristics of the structural material Curve, structural low-cycle fatigue life Corresponding safe fatigue stress limit , and According to the cumulative damage theory, calculate the corresponding number of structural failure cycles after amplifying the stress amplitude of each flight profile;

[0012] Let be the expected value of the acceleration of the low-cycle fatigue test, and the expected value is less than 1, calculate the low-cycle fatigue stress cycle reduction coefficient , if , then return to the previous iterative calculation until the optimal solution is obtained.

[0013] Preferably, the calculation formula of the low-cycle fatigue test stress cycle reduction coefficient is:

[0014]

[0015] Among them, is the corresponding number of failure cycles of the structure, .

[0016] Preferably, the load corresponding to each mission profile is multiplied by the acceleration coefficient , and the corresponding number of cycles is multiplied by to obtain a structural low-cycle test load acceleration spectrum that meets the acceleration requirements and has an equivalent life.

[0017] Preferably, the low-cycle fatigue test load spectrum corresponding to the structural low-cycle fatigue life is compiled based on the helicopter mission profile and the ground-air-ground load and number of cycles in the mission profile, including:

[0018] Based on the flight measurement results or calculation results, determine the ground-air-ground load cycle corresponding to the th mission profile of the structure ;

[0019] The ground-air-ground load cycle is compiled into a low-cycle fatigue test load spectrum according to the order of appearance of mission profiles during the helicopter life cycle .

[0020] Preferably, calculating the helicopter structure stress corresponding to the load cycle under each mission profile in the low-cycle fatigue test spectrum, and converting the low-cycle fatigue test load spectrum into a low-cycle fatigue test stress spectrum, includes:

[0021] Calculating the combined stress under the action of types of load cycles corresponding to mission profiles ( ) in the low-cycle fatigue test spectrum ​ ;

[0022] Calculate the normal stress corresponding to a single load cycle, and superimpose the normal stresses corresponding to the load cycles in m to obtain the combined stress corresponding to task profiles.

[0023] Convert the low-cycle fatigue test load spectrum into the low-cycle fatigue test stress spectrum.

[0024] Preferably, using the task profile corresponding to the maximum stress amplitude as the normalization factor, normalize the stress amplitudes of all task profiles to generate a normalized structural low-cycle fatigue spectrum, including:

[0025] Select from task profiles () the maximum stress amplitude , and normalize the stress amplitude of the th task profile, , ;

[0026] wherein, the normalized structural low-cycle fatigue spectrum is , , .

[0027] Preferably, the task profile represents a certain type of task performed by the helicopter, , being the number of task profiles during the helicopter's service life.

[0028] Advantageous technical effects of the present invention:

[0029] The low-cycle fatigue test acceleration spectrum determined by the method proposed by the present invention avoids the disadvantages of the mismatch of the magnitudes and phases of various loads in the low-cycle working conditions of complex loaded structures, and shortens the low-cycle fatigue test cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a flowchart of the method provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0031] Please refer to Figure 1 , this application provides a calculation method for a helicopter structural low-cycle acceleration test. Based on the types of tasks performed by the helicopter, determine the structural low-cycle load test spectrum, perform load normalization calculations, establish the relationship between the structural normalized fatigue limit and fatigue life, and cyclically calculate the load acceleration coefficient and cycle number reduction coefficient acting on the low-cycle fatigue test spectrum to achieve the acceleration of the low-cycle test spectrum.

[0032] Step 1) According to the helicopter mission profile, as well as the ground-air-ground loads and the number of cycles in the mission profile, compile the low-cycle fatigue life of the structure. The corresponding low-cycle fatigue test load spectrum.

[0033] Among them, the mission profile represents a certain type of mission performed by the helicopter. , is the number of mission profiles during the life cycle of this type of helicopter.

[0034] In the embodiment of the present application, based on the flight measurement results or calculation results, determine the ground-air-ground load cycle corresponding to the th mission profile of the structure, 0-( , )-0. is the maximum value corresponding to the th type of load borne by the structure during the flight of the th mission profile. , the load type value is specifically determined according to the actual bending, tension, compression, shear, and torsion conditions borne by the structure. is the number of times executed per unit hour of the th mission profile , is the low-cycle fatigue test assessment hour number.

[0035] Furthermore, the ground-air-ground load cycle 0-( , )-0 is compiled into a low-cycle fatigue test load spectrum according to the mission profiles appearing in the life cycle of the helicopter in the order of appearance.

[0036] Step 2) Calculate the helicopter structure stress corresponding to each load cycle under each mission profile in the low-cycle fatigue test load spectrum, and convert the low-cycle fatigue test load spectrum into a low-cycle fatigue test stress spectrum.

[0037] Among them, for complex helicopter structures, the finite element modeling and analysis method is used to calculate the combined stress under the ( ) corresponding load cycles of the mission profiles in the low-cycle fatigue test spectrum .

[0038] Among them, for simple helicopter structures, the engineering method can be used to calculate the normal stress corresponding to a single load cycle, and the normal stresses corresponding to the load cycles are superimposed to obtain the combined stress corresponding to the .

[0039] Among them, the low-cycle fatigue test load spectrum 0—( , )—0 is converted into a low-cycle fatigue test stress spectrum 0—( , )—0, .

[0040] Step 3) Use the mission profile corresponding to the maximum stress amplitude as the normalization factor to normalize the stress amplitudes of all mission profiles, and generate a normalized structural low-cycle fatigue spectrum.

[0041] In the embodiment of the present application, select the maximum stress amplitude from mission profiles ), and normalize the stress amplitude of the th mission profile. , , .

[0042] Among them, the normalized low-cycle fatigue spectrum of the helicopter structure is , , .

[0043] Step 4) According to the normalized structural low-cycle fatigue spectrum, use the full-range safe fatigue characteristics curve of the structural material and cumulative damage theory to calculate the life curve of the structure, and determine the safe fatigue stress limit corresponding to the low-cycle fatigue life of the structure .

[0044] It should be noted that the life curve of the helicopter structure is the relationship curve between the fatigue life and the structural fatigue limit , and the fatigue limit is characterized by the normalized stress.

[0045] Step 5) Let be the acceleration coefficient of the stress amplitude in the low-cycle fatigue stress spectrum, and the initial value is 1, and set the step size .

[0046] Step 6) Calculate , multiply the stress amplitude corresponding to each profile in the normalized structural low-cycle fatigue spectrum by the acceleration coefficient , and use the full-range average fatigue characteristics curve of the structural material, the safe fatigue stress limit corresponding to the low-cycle fatigue life of the structure , and Based on the cumulative damage theory, calculate the corresponding number of structural failure cycles after amplifying the stress amplitude of each flight profile.

[0047] Among them, for the nth mission profile after amplifying the stress amplitude , the corresponding number of structural failure cycles is , .

[0048] Step 7) Let be the expected value of low-cycle fatigue test acceleration, where the expected value is less than 1 (the specific value is determined according to the needs of test acceleration), and calculate the low-cycle fatigue stress cycle reduction coefficient . If , then return to the sixth step for iterative calculation until the optimal solution is obtained and then proceed to the eighth step.

[0049] Among them, the calculation formula for the low-cycle fatigue test stress cycle reduction coefficient is:

[0050]

[0051] It should be noted that the load corresponding to each mission profile is multiplied by the acceleration coefficient , and the corresponding number of cycles is multiplied by to obtain the low-cycle test load acceleration spectrum of the structure that meets the acceleration requirements and has equivalent life.

[0052] The present invention has been applied to the airworthiness certification fatigue test design of the rotor system. The low-cycle fatigue test acceleration spectrum determined by this method avoids the disadvantages of mismatched load magnitudes and phases in the low-cycle conditions of complex loaded structures, and shortens the low-cycle fatigue test cycle. The calculation method of the low-cycle fatigue test acceleration spectrum is applicable to re-evaluating the low-cycle fatigue life of the structure after adjusting the low-cycle measured load spectrum of the structure, and has good generality.

Claims

1. A calculation method for a low-cycle acceleration test spectrum of a helicopter component, characterized in that, The method includes: Based on the helicopter mission profile and the ground-air-ground loads and cycle numbers in the mission profile, compiling a low-cycle fatigue test load spectrum corresponding to the structural low-cycle fatigue life; Calculating the helicopter structural stress corresponding to the load cycles under each mission profile in the low-cycle fatigue test load spectrum, and converting the low-cycle fatigue test load spectrum into a low-cycle fatigue test stress spectrum; Using the mission profile corresponding to the maximum stress amplitude as a normalization factor to normalize the stress amplitudes of all mission profiles, and generating a normalized structural low-cycle fatigue spectrum; Based on the normalized structural low-cycle fatigue spectrum and the full-range safe fatigue characteristics of the structural material curve and the cumulative damage theory, calculate the life curve of the structure; Determine the safety fatigue stress limit corresponding to the structural low-cycle fatigue life based on the life curve ; Multiply the stress amplitude corresponding to each section in the normalized structural low-cycle fatigue spectrum by the acceleration factor , and adopt the full-range average fatigue characteristics of the structural material curve, the structural low-cycle fatigue life corresponding safety fatigue stress limit , and the cumulative damage theory to calculate the corresponding number of structural failure cycles after the stress amplitude of each flight section is amplified; Let be the expected value for accelerating the low-cycle fatigue test, and the expected value is less than 1. Calculate the stress cycle reduction coefficient of the low-cycle fatigue test. If , then return to the previous iterative calculation until the optimal solution is obtained; Multiply the load corresponding to each task profile by the acceleration factor , multiply the corresponding number of cycles by , to obtain a structural low-cycle test load acceleration spectrum that meets the acceleration requirements and has an equivalent life wherein, the stress cycle reduction coefficient of the low-cycle fatigue test is calculated by the formula: Among them, is the number of damage cycles corresponding to the structure, , is the number of mission profiles during the helicopter life cycle.

2. The method according to claim 1, characterized in that, The compiling a low-cycle fatigue test load spectrum corresponding to the structural low-cycle fatigue life based on the helicopter mission profile and the ground-air-ground loads and cycle numbers in the mission profile includes: Based on the actual flight test results or calculation results, determine the mission profile of the corresponding ground-air-ground load cycle The ground-air-ground load cycle is compiled into a low-cycle fatigue test load spectrum according to the sequence of occurrence within the mission profiles during the helicopter's service life. The resulting low-cycle fatigue test load spectrum is based on the order in which these mission profiles occur.

3. The method according to claim 1, characterized in that The calculating the helicopter structural stress corresponding to the load cycles under each mission profile in the low-cycle fatigue test spectrum and converting the low-cycle fatigue test load spectrum into a low-cycle fatigue test stress spectrum includes: Calculate the mission profiles corresponding to the combined stress under various load cycles ; Calculate the normal stress corresponding to a single load cycle, and superimpose the normal stresses corresponding to the load cycles in m to obtain the combined stress corresponding to the task profiles; Converting the low-cycle fatigue test load spectrum into the low-cycle fatigue test stress spectrum.

4. The method according to claim 1, wherein The using the mission profile corresponding to the maximum stress amplitude as a normalization factor to normalize the stress amplitudes of all mission profiles and generating a normalized structural low-cycle fatigue spectrum includes: From task profiles select the maximum stress amplitude , and normalize the stress amplitude of the th task profile, , ; Among them, the normalized structural low-cycle fatigue spectrum is , , ; The mission profile represents a certain type of mission performed by the helicopter.

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