A vibration fatigue design method for helicopter secondary load-bearing structure

Abstract

The present invention provides a vibration fatigue resistance design method for a helicopter secondary load-bearing structure, the method comprising: applying a vertical unit acceleration excitation to an installation platform to obtain a stress frequency response curve of a secondary load-bearing structure member; based on the frequency response curve, determining a first resonance main peak and a first frequency value corresponding to the first resonance main peak, and determining a dangerous part based on the first frequency value; based on an S-N curve of a structural material at the dangerous part, determining a fatigue limit value of the structural material; based on the fatigue limit value and a helicopter vibration environment spectrum, obtaining a maximum allowable transfer function; the present invention proposes a vibration fatigue resistance design process and method for a helicopter secondary load-bearing structure, analyzing the vibration fatigue strength of the secondary load-bearing member under a vibration environment and optimizing the structure to ensure that the structure meets strength and use requirements.

Description

Technical Field

[0001] The invention belongs to the technical field of helicopter structural strength test, and in particular relates to a vibration fatigue resistance design method for a helicopter secondary load-bearing structure. Background Art

[0002] With the increasing frequency of helicopter applications in recent years, fatigue damage accidents caused by vibration of structures such as fairings, fuel pipelines, and equipment brackets on helicopters have occurred frequently during actual use. The durability of secondary load-bearing structures in vibration environments has attracted more and more attention.

[0003] At present, when conducting helicopter strength design, standards and specifications for static strength, fatigue and dynamics have been formed, but they are independent and disconnected from each other. Faced with the common vibration fatigue problem of secondary load-bearing structures on helicopters, it is difficult to predict and verify the structural strength and life using only conventional strength design methods, and there are almost no specifications related to vibration fatigue design. Summary of the invention

[0004] In view of the above technical problems, in a first aspect, the present invention provides a method for designing the vibration fatigue resistance of a secondary load-bearing structure of a helicopter, the method comprising:

[0005] Apply vertical unit acceleration excitation to the installation platform to obtain the stress frequency response curve of the secondary load-bearing structural member;

[0006] Based on the frequency response curve, determining a first resonance main peak and a first frequency value corresponding to the first resonance main peak, and determining a dangerous part based on the first frequency value;

[0007] Determining the fatigue limit value of the structural material based on the SN curve of the structural material of the dangerous part;

[0008] Based on the fatigue limit value and the helicopter vibration environment spectrum, a maximum allowable transfer function is obtained.

[0009] Preferably, before applying vertical unit acceleration excitation to the installation platform to obtain the stress frequency response curve of the secondary load-bearing structure, the method further includes:

[0010] A dynamic model is established based on the secondary load-bearing structural components and installation boundary conditions;

[0011] Based on the installation position of the secondary load-bearing structure, the direction of the foundation excitation is determined.

[0012] Preferably, the dynamic model includes simulation of dangerous part structure and connectors; the direction of the basic excitation includes vertical excitation, lateral excitation and heading excitation.

[0013] Preferably, after obtaining the maximum allowable transfer function based on the fatigue limit value and the helicopter vibration environment spectrum, the method further comprises:

[0014] Based on the maximum allowable transfer function and the node dynamic stress frequency response curve of the dangerous part, a second frequency value and a third frequency value are obtained; wherein the first frequency value is between the second frequency value and the third frequency value.

[0015] Preferably, after obtaining the second frequency value and the third frequency value based on the maximum allowable transfer function and the node dynamic stress frequency response curve of the dangerous part, the method further includes:

[0016] The safety factor of the secondary load-bearing structure is defined based on a minimum frequency value and a fourth frequency value; wherein the minimum frequency value is a smaller value between the second frequency value and the third frequency value, and the fourth frequency value is a frequency value closest to the first frequency value in the periodic excitation in the helicopter vibration environment spectrum.

[0017] Preferably, determining the dangerous part based on the first frequency includes:

[0018] Based on the frequency response stress cloud diagram of the first frequency, the maximum stress position is determined; wherein the maximum stress position corresponds to the dangerous part.

[0019] Preferably, the method further comprises:

[0020] A vertical unit acceleration excitation is applied to the installation platform to obtain the initial acceleration of the secondary load-bearing structural components.

[0021] Preferably, obtaining the maximum allowable transfer function based on the fatigue limit value and the helicopter vibration environment spectrum includes:

[0022] Based on the excitation frequency value in the helicopter vibration environment spectrum, determining a fourth frequency that is smaller than the first frequency value and closest to the first frequency value;

[0023] Assume that the amplitude corresponding to the fourth frequency value in the helicopter vibration environment spectrum is ;

[0024] Based on the material fatigue limit value and the helicopter vibration environment spectrum, the maximum allowable transfer function is calculated by the following formula:

[0025]

[0026] in, are the first four frequency values, is the fatigue limit value of the material.

[0027] Beneficial technical effects of the present invention:

[0028] The present invention proposes a vibration fatigue resistance design process and method for a helicopter secondary load-bearing structure, analyzes the vibration fatigue strength of the secondary load-bearing component under a vibration environment and optimizes the structure to ensure that the structure meets the strength and use requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a flow chart of a design method provided by an embodiment of the present invention;

[0030] Figure 2 is an acceleration frequency response curve provided by an embodiment of the present invention;

[0031] Figure 3 is a schematic diagram of dangerous parts confirmation provided by an embodiment of the present invention;

[0032] Figure 4 is a schematic diagram of solving the minimum natural frequency provided by an embodiment of the present invention;

[0033] Figure 5 is a comparison diagram of changes to the support assembly provided in an embodiment of the present invention;

[0034] Among them, 1-gasket, 2-support. DETAILED DESCRIPTION

[0035] See also Figure 1-4 The present invention proposes a new design method to evaluate the durability life of the helicopter secondary load-bearing structure in a vibration fatigue environment, which can be used for the vibration fatigue strength verification and design basis of the helicopter structure.

[0036] The invention provides a vibration fatigue design method for a helicopter secondary load-bearing structure, so as to solve the problem that there is no design code or standard for the vibration fatigue strength of the helicopter secondary load-bearing structure.

[0037] The technical solution adopted by the present invention is to provide a vibration fatigue design method for a helicopter secondary load-bearing structure, which specifically comprises the following steps:

[0038] Prerequisites and Assumptions:

[0039] 1) Structural damping remains unchanged when the structural natural frequency changes within a small range;

[0040] 2) The vibration of the structure is mainly caused by fixed-frequency excitation rather than random excitation;

[0041] Design input conditions:

[0042] 1) Vibration environment spectrum;

[0043] 2) Initial structural parts;

[0044] 3) Installation boundary conditions;

[0045] The design method includes the following steps:

[0046] Step 1: Dynamic modeling

[0047] Based on the initial secondary load-bearing structural members and installation boundary conditions, a refined dynamic model is established. The model is required to include possible dangerous parts structure and connection simulation. For small connections, connection unit simulation can be used, and then the problem of inconsistent mesh size can be avoided by extracting connection loads and building local detail models of the connections.

[0048] Then, according to the installation position of the structure, the direction of the foundation excitation is determined. The subsequent steps are carried out in three directions. The vertical excitation is used as an example:

[0049] Step 2: Solving the dynamic stress frequency response function

[0050] Using the large mass method, a vertical unit acceleration excitation is applied to the foundation to solve the dynamic response of the secondary load-bearing structure with boundary conditions, and the acceleration or stress frequency response curve of the structure is obtained, as shown in the attached figure. Figure 2 shown.

[0051] Step 3: Identify dangerous areas

[0052] Find the first resonant peak from the frequency response curve in the previous step and set the corresponding frequency to , check the first order natural frequency The stress cloud diagram of all elements in the model can be used to determine the maximum stress position or dangerous parts, such as the attached Figure 3 shown.

[0053] Step 4: Material fatigue limit

[0054] Determine the fatigue limit value of the material based on the SN curve of the unit material in the dangerous part , and this limit value is used as the basis for the design of the infinite life of the structure.

[0055] Step 5: Maximum Allowable Transfer Function

[0056] According to the excitation frequency value in the vibration environment spectrum, find the value less than The frequency closest to , assuming that the amplitude of this frequency in the ambient spectrum is . Based on the material fatigue limit value in the previous step and the vibration environment spectrum, the maximum allowable transfer function can be calculated:

[0057]

[0058] Step 6: Minimum Natural Frequency

[0059] Select the dangerous part or the node near the maximum stress in step 3, read the dynamic stress frequency response curve of the node, and set it as ,make

[0060]

[0061] Then two frequency values ​​can be solved:

[0062]

[0063] Among them, the three frequency values ​​satisfy the relationship: , as attached Figure 4 As shown. So far, the minimum natural frequency required for structural design can be given as .

[0064] Step 7: Safety margin definition

[0065] Safety margin parameters for structural vibration fatigue can be defined:

[0066]

[0067] By setting the minimum margin value, the structural stiffness can be designed. Generally, a minimum safety factor of 1.2 is recommended, that is, When , it indicates that the structural strength meets the vibration fatigue design.

[0068] Step 8: Durability test verification

[0069] Since nonlinear contact boundaries are common in secondary load-bearing structures, the stiffness value will gradually decrease during use, and the natural frequency will also decrease accordingly. The above method can be used to set requirements for the measured natural frequency value of the installed structure for periodic limits on replacement or structural reinforcement. However, since the speed of stiffness decrease has large accidental factors and nonlinearity, this process cannot be simulated, and a vibration fatigue test method is required to ultimately confirm the durability life of such structures.

[0070] The present invention proposes a vibration fatigue design process and method for the secondary load-bearing structure of a helicopter, analyzes the vibration fatigue strength of the secondary load-bearing parts under the vibration environment and optimizes the structure to ensure that the structure meets the strength and use requirements. At the same time, the present invention provides a definition method of dynamic safety margin, which can be used as a basis for determining the structural vibration fatigue durability and maintenance cycle based on the measured modal results of the installed parts.

[0071] In the embodiments of this application, please refer to Figure 5 Taking the operating rod support of a certain type of machine as an example, the above analysis method is used to analyze the vibration fatigue strength and structural optimization of the operating rod support under the vibration environment of the transmission system. The safety margin after optimization is , optimization scheme such as Figure 5 shown.

Claims

1. A method for designing vibration fatigue resistance of a helicopter secondary load-bearing structure, characterized in that: The method comprises: Apply vertical unit acceleration excitation to the installation platform to obtain the stress frequency response curve of the secondary load-bearing structural member; Based on the frequency response curve, determine a first resonance main peak and a first frequency value corresponding to the first resonance main peak , and determining a dangerous part based on the first frequency value; Determine the fatigue limit value of the structural material based on the SN curve of the structural material of the dangerous part ; Based on the fatigue limit value and the helicopter vibration environment spectrum, a maximum allowable transfer function is obtained; The maximum allowable transfer function is obtained based on the fatigue limit value and the helicopter vibration environment spectrum, including: Based on the excitation frequency value in the helicopter vibration environment spectrum, a fourth frequency value smaller than the first frequency value and closest to the first frequency value is determined. ; Assume that the amplitude corresponding to the fourth frequency value in the helicopter vibration environment spectrum is ; Based on the fatigue limit value and the helicopter vibration environment spectrum, the maximum allowable transfer function is calculated by the following formula: ； Based on the maximum allowable transfer function and the node stress frequency response curve of the dangerous part, a second frequency value and a third frequency value are obtained; wherein the first frequency value is between the second frequency value and the third frequency value; defining a safety factor of the secondary load-bearing structure based on the minimum frequency value and the fourth frequency value; Wherein, the minimum frequency value is a smaller value between the second frequency value and the third frequency value; Before applying vertical unit acceleration excitation to the installation platform to obtain the stress frequency response curve of the secondary load-bearing structural member, the method further includes: Based on the secondary load-bearing structural components and the installation boundary conditions, a dynamic model is established, wherein the dynamic model includes simulation of dangerous part structures and connectors; Based on the installation position of the secondary load-bearing structure, the direction of the foundation excitation is determined. The direction of the foundation excitation includes vertical excitation, lateral excitation and heading excitation. Subsequent steps are carried out in sequence in the three directions.

2. The method according to claim 1, characterized in that The determining of the dangerous part based on the first frequency includes: Based on the frequency response stress cloud diagram of the first frequency, the maximum stress position is determined; wherein the maximum stress position corresponds to the dangerous part.

3. The method according to claim 1, characterized in that The method further comprises: A vertical unit acceleration excitation is applied to the installation platform to obtain the initial acceleration of the secondary load-bearing structural components.

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