Fatigue strength analysis method based on opposite-pulling angle box stud

Abstract

The invention discloses a fatigue strength analysis method based on opposite-pulling angle box studs. Calculating the working stress of the dangerous section of the stud of the opposite-pulling angle box under each working condition; calculating a detail fatigue rated value DFR of a key structure of the dangerous section of the stud; the maximum allowable working stress is converted through the DFR; and calculating the fatigue margin MS of the dangerous section of the stud of the opposite-pulling angle box, and when the fatigue margin MS is greater than 0, determining that the dangerous section of the stud meets the fatigue strength design requirement. According to the method, the DFR estimation precision of the corner box stud is improved, and the reliability and credibility of structural safe life design are improved.

Description

Technical Field

[0001] This invention belongs to the field of aircraft structural fatigue strength calculation technology, specifically relating to a fatigue strength analysis method based on tie rods in a tie box. Background Technology

[0002] The successful development and widespread application of new materials such as high-strength alloy steel in major aircraft load-bearing components since the 1950s have indeed improved the static strength and stiffness design of aircraft. However, this has also increasingly exposed the weaknesses of static strength design; aircraft designed solely based on static strength and stiffness requirements cannot adequately guarantee operational safety. Initially, aircraft structural fatigue failure did not receive sufficient attention, and the causes of accidents were not properly analyzed. Catastrophic accidents caused by fatigue failure occurred in both military and civilian aircraft, most notably the two consecutive crashes of the Comet I airliner into the sea in 1954. Based on this, the fatigue problem in aircraft gradually attracted attention from all sides. In recent years, serious accidents have still occurred due to inadequate consideration of aircraft structural fatigue. Therefore, modern aircraft structural strength design must consider the gradual structural failure of aircraft under long-term complex load processes and complex environmental conditions—what is known in engineering as fatigue failure.

[0003] Pull-out corner boxes (commonly known as corner joints or corner boxes) play a crucial role in aircraft structures, especially in structural connections, load transfer, stability assurance, and fatigue life management. Although it may seem like a simple connector, its design directly affects the strength, stiffness, and fatigue life safety of the entire structure.

[0004] Commonly used tension corner boxes are mainly of the slotted and angular shapes. Corner boxes are common connecting parts in aircraft structures (such as the connection between wing spars and ribs, fuselage frames and skin, and engine mount connecting joints and wing spars). Vertical stiffeners (usually referring to the vertical stiffeners on the corner box) are key features for improving its stiffness and buckling strength, but they are also often fatigue-sensitive parts.

[0005] The safety and reliability of an aircraft's structural lifespan, in addition to the stress level it withstands, mainly depend on the structural details' ability to resist fatigue failure. This requires structural design engineers to have superb theoretical skills and rich design experience to meticulously design the fatigue performance of important structural units of the aircraft.

[0006] Currently, a rapid stress fatigue strength analysis engineering method, the Detailed Fatigue Rating (DFR) method, is used for fatigue strength design analysis of important structural units in civil aircraft airframes. When performing fatigue strength analysis on key structural details such as the diagonal box ribs, the fatigue strength can be assessed according to the Detailed Fatigue Rating (DFR) method in the "Civil Aircraft Structural Durability and Damage Tolerance Design Manual (Volume 1) - Fatigue Design and Analysis." The calculation formula is as follows: DFR= DFR cutoff However, in engineering practice, it has been found that using this DFR value calculation formula to evaluate the fatigue strength of the corner box reinforcement poses a potential danger to the structural strength design of the corner box. Summary of the Invention

[0007] The objective of this invention is to provide a fatigue strength analysis method based on the uprights of corner box ribs. This invention improves the fatigue strength of the uprights of corner boxes. DFR This improves estimation accuracy and enhances the reliability and credibility of structural safety life design.

[0008] The technical solution of this invention is: a fatigue strength analysis method based on the vertical stiffeners of a tie rod box, comprising the following steps: S1: Calculate the working stress of the critical section of the tie rod of the tie angle box under various working conditions using the following formula. σ :

[0009] In the formula, P For the bolt load at the bottom plate of the tie rod box, A 截面 This represents the critical cross-sectional area of ​​the reinforcing bars. c y This refers to the distance from the center of the bolt hole on the bottom plate of the corner box to the back plate of the corner box. Y The distance from the centroid of the critical section of the vertical reinforcement of the tie rod to the back plate of the tie rod box. H The height of the critical section of the vertical reinforcement. I x The moment of inertia of the cross section; S2: Calculate the fatigue rating of critical structural details at the critical section of the vertical reinforcement using the following formula. DFR : DFR = DFR cutoff ·S WFD ·C In the formula, DFR cutoff For materials DFR Cutoff value; S WFD The fatigue damage sensitivity coefficient is widely distributed in the structure. C For the corrosion effect correction factor; S3: By Calculate the maximum allowable working stress. σ max ]; S4: Calculate the fatigue margin MS of the critical section of the tie rod of the tie corner box according to the following formula:

[0010] In the formula, σ max For working stress under various working conditions σ The maximum working stress in the middle; when the fatigue margin MS>0, the critical section of the vertical reinforcement meets the fatigue strength design requirements.

[0011] In S1 of the aforementioned fatigue strength analysis method based on the tie rod of the corner box, the minimum working stress among the working stresses under each working condition is... σ min and σ max stress ratio When, use the following formula for σ max Correction, to obtain the corrected maximum working stress σ max ':

[0012] In the formula, CSAF This is the negative stress correction factor.

[0013] In the aforementioned fatigue strength analysis method based on the tie rods of the corner box, when R < -2.0, .

[0014] In the aforementioned fatigue strength analysis method based on the tie rods of the corner box, CSAF Find it in the fatigue design manual.

[0015] In the aforementioned fatigue strength analysis method based on the vertical ribs of the tie corner box, the critical section of the vertical rib is selected as the section parallel to the bottom plate of the corner box at the center of the first fastener hole on the back plate connecting the tie corner box to the surrounding structure.

[0016] In S2 of the aforementioned fatigue strength analysis method based on the tie rods of the corner box, S WFD Please confirm according to the table below:

[0017] In S2 of the aforementioned fatigue strength analysis method based on the tie rods of the corner box, C is confirmed according to the following table:

[0018] In the aforementioned fatigue strength analysis method based on the tie rods of the tie rod box, the tie rod box is used for the connection of the aircraft structure.

[0019] The technical effect of this invention is as follows: The inventors discovered that traditional strength assessment methods, when calculating DFR (Damage Reduction Factor) for aircraft corner box connectors, do not consider the complex application environment of these connectors. This leads to calculated DFR values ​​exceeding actual values, posing a significant risk to the structural strength design of the corner box. To overcome this problem, the inventors, through analysis and evaluation of the application environment of the corner box connectors, introduced a widely distributed fatigue damage sensitivity coefficient when designing the fatigue strength analysis method for the tie rods of the corner box. The corrosion-affected correction factor C effectively solves the technical problem of distorted DFR estimation of corner box ribs, avoids significant deviations in structural fatigue strength design from reality, and ensures the accuracy of fatigue margin estimation for corner box ribs.

[0020] This invention, through extensive experimentation and analysis based on practical engineering experience, ultimately yielded a widely distributed fatigue damage sensitivity coefficient. The method not only makes the estimated DFR of the corner box reinforcement closer to the true value, but also reduces the complexity of strength assessment, improves assessment efficiency, and reduces costs.

[0021] As can be seen from the above, this invention addresses the engineering problem of insufficient consideration of important factors such as widespread fatigue damage and the influence of corrosive environments in the traditional formula for calculating the fatigue rating (DFR) of key structural details of corner box ribs. Based on the analysis and research of relevant structural component test results and engineering experience, this invention introduces a widespread fatigue damage sensitivity coefficient into the original formula for calculating the fatigue rating (DFR) of details. Including the corrosion influence correction factor C, a novel DFR value calculation formula based on the tie rods of corner boxes is provided for fatigue strength design analysis of corner box ribs, improving the accuracy of fatigue margin estimation for structural details of corner box ribs and enhancing the reliability and credibility of structural safety life design.

[0022] In summary, the present invention provides a theoretical estimation method for fatigue strength design of important structural components, which has the advantages of low cost, high efficiency, reliable evaluation method, and credible estimation results, and effectively solves the problem of fatigue strength design of aircraft corner box connectors. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the cross-section of the corner box reinforcement in the embodiment; Figure 2 This is a schematic diagram of the cross-sectional dimensions of the corner box reinforcement in the embodiment. Detailed Implementation

[0024] To make the implementation process of this invention clearer, the technical solutions of this invention will be described in more detail below with reference to the accompanying drawings. 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, embodiments of this invention, and the parts not detailed are conventional techniques. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this invention, and should not be construed as limiting this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0025] Example 1. As... Figure 1 , 2 As shown, a fatigue strength analysis method based on the vertical ribs of a tie rod box is implemented as follows: Step 1: Determine the location of the critical section of the corner box's vertical reinforcement; Step 2: Determine the geometric and mechanical parameters of the critical section location, including: the distance from the center of the bolt hole on the corner box bottom plate to the back plate of the corner box. The distance Y from the centroid of the critical section of the corner box's vertical reinforcement to the back panel of the corner box; the height H of the critical section of the corner box; and the area of ​​the critical section. Bolt load P at the bottom plate of the corner box, moment of inertia of the section ; Step 3: Determine the location of the critical section of the corner box reinforcement, i.e., the formula for calculating the working stress of key structural details: ; Step 4: Based on Step 2 and Step 3, calculate the working stress under different working conditions; Step 5, when the stress ratio of the task profile At that time, the maximum working stress needs to be considered. The maximum stress correction formula is as follows: ; Step 6: Negative Stress Correction Factor Find it in the fatigue design manual; Step 7: Determine the DFR value of key structural details. Calculation formula: ; Step 8 Materials for key structural details Cutoff value; Step 9: Determine the widespread fatigue damage sensitivity coefficient based on whether the structure is a widely distributed fatigue damage sensitive structure. The specific value to be taken; Step 10: Combine working stress and Substituting the value into the tensile fatigue check table, we obtain the fatigue margin MS for the corner box rib details:

[0026] In the formula, σ max For working stress under various working conditions σ The maximum working stress in the middle; when the fatigue margin MS>0, the critical section of the vertical reinforcement meets the fatigue strength design requirements.

[0027] S WFD Please confirm according to Table 1: Table 1

[0028] C. Confirm according to Table 2: Table 2

[0029] The critical section of the vertical reinforcement is selected from the section parallel to the bottom plate of the corner box, located at the center of the first fastener hole on the back plate connecting the corner box to the surrounding structure.

[0030] See Table 3 for a comparison of traditional theoretical estimation (theoretical estimation 1), the present invention (theoretical estimation 2), and experimental results. The comparison shows that the estimation accuracy of the present invention is significantly higher than that of the traditional method.

[0031] Table 3 Comparison of Theoretical Estimation and Experimental Data Results for Corner Box Reinforcement

[0032] Note: The pull-out angle box is a Ti-6Al-4V titanium alloy free forging; material cutoff value. DFR cutoff The DFR value for the corner box rib details is obtained from the manual. MPa; Theoretical estimate 1 (original DFR calculation formula - not considering widespread fatigue damage and corrosion effects): ; Theoretical Estimation 2 (Modified DFR Calculation Formula - Considering Widespread Fatigue Damage and Corrosion Effects):

[0033] S WFD =0.9——Sensitivity coefficient for widespread fatigue damage; C=0.87——Corrosion effect correction factor.

[0034] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for analyzing fatigue strength of a tension angle box stud, characterized by, Comprising the steps of: S1: Calculate the working stress of the web angle box under each working condition according to the following formula σ : wherein P is the bolt load at the corner brace bottom plate, A 截面 is the dangerous section area of the vertical brace, c y is the distance from the bolt hole center of the corner brace bottom plate to the corner brace back plate, Y is the distance from the dangerous section center of the vertical brace of the corner brace to the corner brace back plate, H is the height of the dangerous section of the vertical brace, I x is the section moment of inertia; S2: Calculate the key structural detail fatigue rating of the danger section of the stud according to the following formula DFR : DFR = DFR cutoff ·S WFD ·C In the formula, DFR cutoff is the material, DFR cut-off value; S WFD is the structure widely fatigue damage sensitive coefficient, C is the corrosion influence correction coefficient; S3: by Convert the maximum allowable working stress[ σ max ]; S4: Calculate the fatigue margin MS of the dangerous section of the stand for the pull angle box as follows: In the formula, σ max Maximum working stress in each working condition σ When the fatigue margin MS> 0, the dangerous section of the stud meets the fatigue strength design requirements.

2. The method for analyzing fatigue strength of a tension angle box stud according to claim 1, characterized by, In S1, when the minimum working stress among the working stresses under each working condition... σ min and σ max stress ratio When, use the following formula for σ max Correction, to obtain the corrected maximum working stress σ max ': In the formula, σ is a negative stress correction factor.

3. The method for analyzing fatigue strength of a tension angle box stud according to claim 2, characterized by, CSAF 。 4. The fatigue strength analysis method of a tension angle box stud according to claim 2, characterized by, When R < -2.0, Look up in fatigue design handbook.

5. The method for analyzing fatigue strength of a tension angle box stud according to claim 1, wherein CSAF 6. The method for analyzing fatigue strength of a tension angle box stud according to claim 1, wherein In S2, S WFD The following table confirms:

7. The method for analyzing fatigue strength of a tension angle box stud according to claim 1, wherein The dangerous section of the stand is selected as the section parallel to the bottom plate of the angle box at the center of the first fastener hole on the back plate connecting the pull angle box with the surrounding structure.

8. The method for analyzing fatigue strength of a tension angle box stud according to claim 1, wherein In S2, C is confirmed as follows: The pull angle box is used for the connection of the aircraft structure.

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