Helicopter flange bolt load analysis and low-cycle fatigue life prediction method

Abstract

The present invention provides a method for load analysis and low-cycle fatigue life prediction of helicopter flange bolts. The method comprises: performing load analysis based on the distribution form of the helicopter flange bolts to calculate the maximum load value of the flange bolts; calculating the maximum local stress of the helicopter flange bolts based on the maximum load value of the flange bolts, the first thread circle of the helicopter flange bolts, and the minimum cross-section of the bolt head; obtaining a method for expressing the low-cycle fatigue static stress and dynamic stress of the flange bolts based on the maximum local stress of the helicopter flange bolts; obtaining an expression for the low-cycle fatigue equivalent dynamic stress of the helicopter flange bolts based on a life model, the low-cycle fatigue static stress and dynamic stress of the flange bolts, and evaluating the low-cycle fatigue life using a full-range S-N curve.

Description

Technical Field

[0001] The invention belongs to the technical field of helicopter strength design, and in particular relates to a method for analyzing helicopter flange bolt loads and predicting low-cycle fatigue life under complex working conditions. Background Art

[0002] Flange connections are a key connection method used in structural design and are widely used in the development of critical components in aviation, aerospace, and automotive fields. Compared to fixed-wing aircraft, helicopters experience significant vibration during service, complex load bearings, and are more susceptible to fatigue issues. Currently, flange connections are used for helicopter rotors and fuselages. The low-cycle fatigue life of helicopter flange bolts under complex operating conditions is crucial to the safety of the entire aircraft. Summary of the Invention

[0003] In response to the above technical problems, the present invention provides a method for helicopter flange bolt load analysis and low-cycle fatigue life prediction, the method comprising:

[0004] Perform load analysis based on the distribution of helicopter flange bolts to calculate the maximum load value of the flange bolts;

[0005] The maximum local stress of the helicopter flange bolt is calculated based on the maximum load value of the flange bolt, the first thread circle of the helicopter flange bolt and the minimum cross-section of the bolt head.

[0006] Preferably, the method further comprises:

[0007] A method for expressing low-cycle fatigue static stress and dynamic stress of the helicopter flange bolts based on the maximum local stress of the flange bolts;

[0008] Based on the life model, the low-cycle fatigue static stress and dynamic stress of the flange bolts, the low-cycle fatigue equivalent dynamic stress expression of the helicopter flange bolts was obtained, and the full-range SN curve was used to evaluate the low-cycle fatigue life.

[0009] Preferably, the use of the full range SN curve to evaluate the low cycle fatigue life includes:

[0010] Obtain the low cycle fatigue cycle number N of the dangerous area between the first thread of the flange bolt and the transition zone of the bolt head low,首圈螺纹 and N low,螺栓头最小截面 ;

[0011] Based on the flight load spectrum, the number of "ground-air-ground" low-cycle load cycles per hour is selected as p and the dispersion of fatigue behavior. Combined with the test results and according to the reliability theory, the low-cycle fatigue dispersion coefficient f is selected. n , then the low cycle fatigue life T of flange bolts under composite working conditions can be expressed as:

[0012]

[0013] Preferably, the load analysis based on the distribution of the helicopter flange bolts to calculate the maximum load value of the flange bolts includes:

[0014] Determining the most dangerous load situation of the flange bolts based on the distribution pattern of the flange bolts; wherein the distribution pattern includes odd number distribution and even number distribution along the circumference of the flange;

[0015] Based on the most dangerous load situation of the flange bolts, a flange bolt load analysis method is obtained;

[0016] Based on the bolt axial load distribution coefficient, the unevenness coefficient and the flange bolt load analysis method, the maximum load value of the flange bolt under the composite working condition is calculated; wherein the composite working condition includes axial tension, bending and / or bolt self-tightening.

[0017] Preferably, the flange bolts bear the most dangerous conditions, including

[0018] The flange bolts are distributed in an even number, and the bolt load is analyzed by taking the most dangerous case where the bolt holes are located on the symmetry axis;

[0019] The flange bolts are distributed in odd numbers, and the bolt load is analyzed by taking the most dangerous case where the bolt holes pass through the symmetry axis.

[0020] Preferably, the flange bolt load analysis method includes:

[0021] The axial tensile load F1 borne by the flange bolts under bending load can be expressed as:

[0022]

[0023] Preferably, the calculation of the maximum load value of the flange bolt under the composite working condition based on the bolt axial load distribution coefficient, the unevenness coefficient and the flange bolt load analysis method includes:

[0024] Maximum axial load F borne by helicopter flange bolts under combined working conditions total It can be expressed as:

[0025]

[0026] Where F0 is the bolt preload; ξ is the bolt axial load distribution coefficient; and φ is the load unevenness coefficient in a set of flange bolts.

[0027] Preferably, the calculating of the maximum local stress of the helicopter flange bolt based on the maximum load value of the flange bolt, the first thread of the helicopter flange bolt, and the minimum cross-section of the bolt head includes:

[0028] The dangerous parts of the first thread circle and the minimum cross-section of the bolt head transition zone in the flange bolt are analyzed respectively, and the maximum local stress σ of the flange bolt under the composite working condition is obtained according to the following formula: max ;

[0029]

[0030] Where, γ is the load factor of the first thread; Sc is the thread shear area; A is the minimum cross-sectional area of ​​the bolt head transition zone.

[0031] Preferably, the method for expressing the low-cycle fatigue static stress and dynamic stress of the helicopter flange bolt based on the maximum local stress of the helicopter flange bolt comprises:

[0032] According to the following formula, the low-cycle fatigue static stress σ borne by the flange bolts can be obtained S，low and dynamic stress σ d,low :

[0033]

[0034]

[0035] Among them, K t is the bolt stress concentration factor.

[0036] Beneficial technical effects of the present invention:

[0037] The present invention calculates the loads borne by helicopter flange bolts under complex working conditions, analyzes the maximum stress and low-cycle stress in the hazardous area, and proposes a method for expressing the low-cycle fatigue equivalent dynamic stress of helicopter flange bolts based on the Gerber life model. The full-range SN curve is used to simply and effectively predict the low-cycle fatigue life, providing technical support for helicopter design. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 The circumferential distribution of flange bolts when the number of bolts is an even number provided by an embodiment of the present invention;

[0039] Figure 2 The circumferential distribution of flange bolts when the number of bolts is an odd number provided by an embodiment of the present invention;

[0040] Figure 3 The present invention provides a load distribution of a circular cross section under a bending load;

[0041] Figure 4 This is a flow chart provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0042] See also Figure 1-4The present invention calculates the load borne by helicopter flange bolts under complex working conditions through the proposed analysis method, analyzes the stress in the dangerous area by considering the first circle of thread and the minimum cross-section of the bolt head, and can simply and practically predict the low-cycle fatigue life, providing technical support for helicopter structural design.

[0043] The purpose of the present invention is to propose a method for load analysis and low-cycle fatigue life prediction of helicopter flange bolts under composite working conditions to meet the requirements of engineering design and application.

[0044] The technical solution of the present invention is: a method for load analysis and low-cycle fatigue life prediction of helicopter flange bolts under complex working conditions, and the specific steps of the method are as follows.

[0045] Step 1: Analysis of helicopter flange bolt loads under composite working conditions

[0046] The helicopter flange bolts are evenly distributed along the circumference of the flange. The number of flange bolts is n (n≥2, n is an integer), and the distance from the flange center to the bolt center is R.

[0047] Figure 1 The figure shows the structural distribution of the flange with an even number of bolts n. The flange is subjected to the in-plane bending moment Mf. The bolt load is analyzed by taking the most dangerous case where the bolt holes are located on the symmetry axis. The axial load of each bolt under the bending moment Mf is F i (i=1,2,...n), the vertical distance between each bolt and the axis of symmetry is L i (i=1,2,...n), the flange bolts are symmetrically distributed. In this state, there are:

[0048] F1=F n / 2+1 (1)

[0049] L1=L n / 2+1 =R (2)

[0050] When n≥4:

[0051] F i =F n+2-i (i=2,3,...n / 2) (3)

[0052] L i =L n+2-i (i=2,3,...n / 2) (4)

[0053] The load balance is performed about the symmetry axis. Under the action of bending moment Mf, the load relationship of each flange bolt (n is an even number) can be expressed as follows according to equations (1) to (4):

[0054]

[0055] According to the plane bending theory of material mechanics, the cross-sectional load F under plane bending moment is linearly positively correlated with the vertical distance L from the neutral plane (e.g. Figure 2 As shown), then:

[0056]

[0057] Where θ is the axial angle between two adjacent bolts, and θ = (2π) / n.

[0058] Substitute equation (6) into equation (5):

[0059]

[0060] Figure 2 The structural distribution of the flange with an odd number of bolts n is shown. The flange is subjected to the in-plane bending moment Mf. The bolt dangerous load is evaluated in the most dangerous case where the bolt holes pass through the symmetry axis. The flange bolts are symmetrically distributed. In this state,

[0061] L1=R (8)

[0062] F i =F n+2-i (i=2,3,...(n+1) / 2) (9)

[0063] L i =L n+2-i (i=2,3,...(n+1) / 2) (10)

[0064] The load balance is performed about the symmetry axis. Under the action of bending moment Mf, according to equations (8) to (10), the load relationship of flange bolts (n is an odd number) can be expressed as follows:

[0065]

[0066] Similarly, according to the plane bending theory of material mechanics, the relationship between the cross-sectional load F under plane bending moment and the vertical distance L from the neutral plane can be expressed as:

[0067]

[0068] Substitute equation (12) into equation (11):

[0069]

[0070] According to equations (7) and (13), the axial tensile load F1 borne by the flange bolts under bending load can be expressed as:

[0071]

[0072] Helicopter flange connections usually operate under a composite working condition including axial tensile load Fx, bending load Mf and bolt preload F0. The maximum axial load F that helicopter flange bolts can withstand under composite working conditions is total It can be expressed as:

[0073]

[0074] Where F0 is calculated based on the designed bolt tightening torque; ξ is the bolt axial load sharing factor, determined by the stiffness relationship between the bolt and the connected structure; and φ is the load unevenness factor within a set of flange bolts, reflecting the fit between the bolt and the connected component. F0, ξ, and φ can be determined by consulting a mechanical design manual.

[0075] Step 2: Calculation of low-cycle fatigue stress of helicopter flange bolts under composite working conditions

[0076] The dangerous parts of the first thread and the minimum cross section of the bolt head transition zone in the flange bolt are analyzed respectively. According to formula (15), the maximum stress σ of the flange bolt under the composite working condition can be obtained. max :

[0077]

[0078] Where γ is the load factor of the first thread turn, which is determined by referring to the mechanical design manual; Sc is the thread shear area; A is the minimum cross-sectional area of ​​the bolt head transition zone, and Sc and A are calculated based on the bolt size.

[0079] Formula (16) gives the maximum stress σ borne by the helicopter flange bolt under the composite working condition: max Considering the “ground-air-ground” load condition to analyze the low-cycle fatigue load, the low-cycle fatigue static stress σ borne by the flange bolt can be given according to formula (16): S，low and dynamic stress σ d,low :

[0080]

[0081]

[0082] Where K t is the stress concentration factor of the bolt, which is calculated based on the structural dimensions of the transition zone of the bolt head.

[0083] Step 3: Prediction of low-cycle fatigue life of helicopter flange bolts under composite working conditions

[0084] The low-cycle dynamic stress is corrected by considering the influence of stress ratio effect. The Gerber life model is adopted. The low-cycle fatigue equivalent dynamic stress σ borne by the flange bolt under the composite working condition is eq,low for:

[0085]

[0086] Where, σ 0.2 is the yield strength of the material.

[0087] The full range SN curve can be used to describe the fatigue performance of the structure. The low-cycle fatigue equivalent dynamic stress σ is obtained according to formula (20): eq,low , using the full range SN curve, the low cycle life cycle number N of the flange bolt low It can be expressed as:

[0088]

[0089] Where S ∞ is the fatigue limit of the material; S q For the point (10 3 ,σ 0.2 ) is the fatigue load corresponding to the tangent point of the SN curve, B0 and B1 are the intercept and slope of the tangent line; C0 and C1 are the fatigue performance material constants of the bolt, which can be determined by referring to the material performance manual.

[0090] Substituting equations (17) and (18) into equations (19) and (20), we can obtain the low-cycle fatigue cycle number N of the dangerous area between the first thread of the flange bolt and the transition zone of the bolt head: low,首圈螺纹 and N low,螺栓头最小截面 According to the flight load spectrum, the number of "ground-air-ground" low-cycle load cycles per hour is selected as p. Considering the dispersion of fatigue behavior, combined with the test results and based on reliability theory, the low-cycle fatigue dispersion coefficient f is selected. n , then the low cycle fatigue life T of flange bolts under composite working conditions can be expressed as:

[0091]

[0092] The key points of the present invention are:

[0093] (1) Considering the different distribution forms of flange bolts along the circumference, a load analysis method for helicopter flange bolts under composite working conditions is proposed, and a universal maximum load F under composite working conditions is given. total Calculation method.

[0094] (2) Considering the first thread and the minimum cross section of the bolt head, the maximum stress σ of the helicopter flange bolt under the composite working condition is proposed. max The general calculation method of flange bolt low cycle fatigue stress σ is further given. S，low and σ d,low Representation method.

[0095] (3) Calculation of helicopter flange bolt fatigue equivalent dynamic stress σ based on Gerber life model eq,low, considering the first thread and the minimum cross-section of the bolt head, the full range SN curve is used to effectively predict the low-cycle fatigue life T, providing assistance for helicopter component design and life assessment.

[0096] in, Figure 1 and Figure 2 Where n is the number of bolts, R is the distance from the center of the flange to the center of the bolt, and L is the distance from the center of the flange to the center of the bolt. i (i=1,2,...n) is the vertical distance between each bolt and the axis of symmetry, and θ is the axial angle between two adjacent bolts.

[0097] Figure 3 Where Mf is the in-plane bending moment borne by the flange, F n and F m is the cross-sectional load under plane bending moment, L n and L m is the vertical distance from the neutral plane.

Claims

1. A method for load analysis and low-cycle fatigue life prediction of helicopter flange bolts, characterized in that: The method comprises: Performing load analysis based on the distribution of helicopter flange bolts to calculate the maximum load value of the flange bolts; wherein the distribution includes odd-number distribution and even-number distribution along the flange circumference; Calculating the maximum local stress of the helicopter flange bolt based on the maximum load value of the flange bolt, the first thread of the helicopter flange bolt, and the minimum cross-section of the bolt head; The method further comprises: A method for expressing low-cycle fatigue static stress and dynamic stress of the helicopter flange bolts based on the maximum local stress of the flange bolts; Based on the life model, the low-cycle fatigue static stress and dynamic stress of the flange bolts, the low-cycle fatigue equivalent dynamic stress expression of the helicopter flange bolts is obtained, and the full-range SN curve is used to evaluate the low-cycle fatigue life; The calculating of the maximum local stress of the helicopter flange bolt based on the maximum load value of the flange bolt, the first thread of the helicopter flange bolt, and the minimum cross-section of the bolt head includes: The dangerous parts of the first thread circle and the minimum cross-section of the bolt head transition zone in the flange bolt are analyzed respectively, and the maximum local stress of the flange bolt under the composite working condition is obtained according to the following formula: ; in, is the load factor of the first thread; is the thread shear area; A is the minimum cross-sectional area of ​​the bolt head transition zone; F0 is the bolt's own preload; is the bolt axial load sharing factor; is the load unevenness coefficient in a set of flange bolts; Fx is the axial tensile load; n is the number of bolts; R is the distance from the flange center to the bolt center; Mf is the in-plane bending moment borne by the flange.

2. The helicopter flange bolt load analysis and low cycle fatigue life prediction method according to claim 1, characterized in that: The full range SN curve is used to evaluate the low cycle fatigue life, including: Obtain the number of low-cycle fatigue cycles in the dangerous area of ​​the transition zone between the first thread of the flange bolt and the bolt head and ; Based on the flight load spectrum, the number of "ground-air-ground" low-cycle load cycles per hour is selected as p and the dispersion of fatigue behavior, combined with the test results and based on reliability theory, the low cycle fatigue dispersion coefficient is selected f n , then the low cycle fatigue life T of flange bolts under composite working conditions can be expressed as: 。 3. The helicopter flange bolt load analysis and low cycle fatigue life prediction method according to claim 1, characterized in that: The load analysis based on the distribution form of the helicopter flange bolts and the calculation of the maximum load value of the flange bolts include: Determining the most dangerous load situation of the flange bolts based on the distribution pattern of the flange bolts; Based on the most dangerous load situation of the flange bolts, a flange bolt load analysis method is obtained; Based on the bolt axial load distribution coefficient, the unevenness coefficient and the flange bolt load analysis method, the maximum load value of the flange bolt under the composite working condition is calculated; wherein the composite working condition includes axial tension, bending and / or bolt self-tightening.

4. The helicopter flange bolt load analysis and low cycle fatigue life prediction method according to claim 3, characterized in that: The flange bolts are subject to the most hazardous conditions, including The flange bolts are distributed in an even number, and the bolt load is analyzed by taking the most dangerous case where the bolt holes are located on the symmetry axis; The flange bolts are distributed in odd numbers, and the bolt load is analyzed by taking the most dangerous case where the bolt holes pass through the symmetry axis.

5. The helicopter flange bolt load analysis and low cycle fatigue life prediction method according to claim 3, characterized in that: The flange bolt load analysis method includes: Axial tensile load on flange bolts under bending load F 1 can be expressed as: 。 6. The helicopter flange bolt load analysis and low cycle fatigue life prediction method according to claim 3, characterized in that: The calculation of the maximum load value of the flange bolt under the composite working condition based on the bolt axial load distribution coefficient, the unevenness coefficient and the flange bolt load analysis method includes: Maximum axial load borne by helicopter flange bolts under combined working conditions F total It can be expressed as: in, F 0 is the bolt preload; is the bolt axial load sharing factor; is the load unevenness coefficient in a set of flange bolts; F1 is the axial tensile load borne by the flange bolts under the action of bending load.

7. The helicopter flange bolt load analysis and low cycle fatigue life prediction method according to claim 1, characterized in that: The method for expressing the low-cycle fatigue static stress and dynamic stress of the helicopter flange bolt based on the maximum local stress of the helicopter flange bolt includes: According to the following formula, the low-cycle fatigue static stress borne by the flange bolts can be obtained: and dynamic stress : in, K t is the bolt stress concentration factor.

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