Ribbed wallboard butt joint structure and design method

Abstract

The invention belongs to the field of aircraft structure design, and particularly relates to a stiffened wall plate butt joint structure and a design method. The structure comprises a first stiffened wall plate and a second stiffened wall plate, wherein the first stiffened wall plate comprises a first skin and first ribs arranged on the first skin; the second stiffened wall plate comprises a second skin and a second rib arranged on the second skin; wherein the first skin and the second skin are connected through bolts; the first rib and the second rib are connected through a butt joint assembly. The rib butt-joint areas and the skin butt-joint areas are arranged in a staggered mode, and the two rib butt-joint areas are located on the two sides of the skin butt-joint areas correspondingly. The multi-path force transmission butt joint mode is adopted, the skin and the ribs are in scattered butt joint, continuous structural rigidity and continuous load transmission are kept, large stress concentration and weight cost are avoided, and fatigue details at the butt joint position are facilitated.

Description

Technical Field

[0001] The present application belongs to the field of aircraft structure design, and in particular relates to a stiffened wall panel butt joint structure and a design method. Background Art

[0002] In the current aircraft structure design process, due to layout requirements and to make up for the shortcomings of smaller material specifications, the design of wing structures often requires the processing and manufacturing of large-size structural components by dividing them into blocks and then docking them. The docking of wing panels is often designed in the form of skin single shear docking, double shear docking, and comb joints. In addition, the ribs of these three docking forms are disconnected at the docking point, the structural stiffness suddenly changes, and the turning of the rigid center and the load axis will bring additional additional loads, resulting in greater stress concentration, which is not conducive to fatigue. The load of the ribs must be transferred to the skin and all of it is borne by the skin. The load magnitude of the skin increases, and the skin connectors bring a greater load-bearing burden. The fasteners of the comb joint docking are mainly subjected to tension and compression, and the life design of the fasteners becomes the key to the design. The above problems are all solved by measures such as thickening the skin and enlarging the fasteners, which will pay a large weight price.

[0003] Therefore, it is desired to have a technical solution to overcome or at least alleviate at least one of the above-mentioned defects of the prior art. Summary of the invention

[0004] The purpose of the present application is to provide a reinforced wall panel butt joint structure and design method to solve at least one problem existing in the prior art.

[0005] The technical solution of this application is:

[0006] The first aspect of the present application provides a reinforced wall panel joint structure, comprising:

[0007] A first stiffened wall panel, the first stiffened wall panel comprising a first skin and a first rib arranged on the first skin;

[0008] A second stiffened wall panel, the second stiffened wall panel comprising a second skin and second ribs arranged on the second skin;

[0009] in,

[0010] The first skin and the second skin are connected by bolts;

[0011] The first rib and the second rib are connected via a docking assembly;

[0012] The rib butt joint area and the skin butt joint area are arranged in a staggered manner, and the two rib butt joint areas are respectively located on both sides of the skin butt joint area.

[0013] In at least one embodiment of the present application, the first skin and the second skin adopt a wedge-shaped overlap structure and are connected by a plurality of bolts.

[0014] In at least one embodiment of the present application, the butt-joint angle between the first skin and the second skin is 1° to 3°.

[0015] In at least one embodiment of the present application,

[0016] The first ribs located in the rib docking area include first vertical ribs and first horizontal ribs;

[0017] The second ribs located in the rib joint area include second vertical ribs and second horizontal ribs.

[0018] In at least one embodiment of the present application, the docking assembly includes: docking ribs, vertical docking strips and horizontal docking strips, wherein:

[0019] The butt joint ribs include vertical butt joint ribs and horizontal butt joint ribs, and the butt joint ribs are arranged in the skin butt joint area;

[0020] One side of the vertical butt joint rib is connected to the first vertical rib through the vertical butt joint strip, and the other side is connected to the second vertical rib through the vertical butt joint strip;

[0021] One side of the horizontal butt joint rib is connected to the first horizontal rib through the horizontal butt joint strip, and the other side of the horizontal butt joint rib is connected to the second horizontal rib through the horizontal butt joint strip.

[0022] In at least one embodiment of the present application, both ends of the horizontal butt ribs adopt a double wedge structure.

[0023] In at least one embodiment of the present application, the vertical butt joint ribs and the vertical butt joint strips adopt a double shear butt joint configuration, and the butt joint angle is 1° to 5°.

[0024] In at least one embodiment of the present application, a supporting element is installed in the docking area, and the supporting element is a frame or a rib.

[0025] A second aspect of the present application provides a method for designing a reinforced wall panel butt joint structure, based on the reinforced wall panel butt joint structure as described above, comprising:

[0026] Step 1, obtaining initial input data of the stiffened wall panel butt joint structure;

[0027] Step 2: constructing a component model of the stiffened wall panel according to the initial input data;

[0028] Step 3, establishing a finite element model of the component model;

[0029] Step 4: Perform finite element calculation according to the finite element model to obtain the load of the bolt;

[0030] Step 5: Determine whether the bearing capacity of the bolt and the extrusion strength of the nail hole can meet the requirements. If not, increase the bolt diameter, change the parameters of the component model, and recalculate until the requirements are met;

[0031] Step 6, determining the bottom angle of the skin depression area in the skin butt joint area according to the stress concentration factor curve;

[0032] Step 7: Output the structural parameters of the reinforced wall panels that meet the requirements.

[0033] In at least one embodiment of the present application, in step 1, the initial input data includes: skin stress distribution, skin basic thickness, nail hole extrusion allowable strength, docking area bolt parameters, unit model size and loading load.

[0034] In at least one embodiment of the present application, in step 2, the component model:

[0035] The model width W is the rib spacing bs;

[0036] The number of bolts between every two ribs is n_c, and the distance between the bolts is S_c;

[0037] The number of rows of bolts between every two ribs is n_r, and the row spacing is S_r;

[0038] The length of the skin butt joint area is L = n_r × S_r;

[0039] The bolt diameter is D;

[0040] The minimum thickness of the skin butt area is t1;

[0041] The total thickness of the skin butt area is T.

[0042] In at least one embodiment of the present application, in step 3, the finite element model:

[0043] According to the loading load F = σ × t × W, the loads F1, F2, ..., Fn of a single bolt are solved, where σ is the stress distribution of the skin and t is the basic thickness of the skin;

[0044] By adjusting F, n_c, n_r, D, t1, T, the load distribution relationship between F and F1, F2, ..., Fn is constructed: Fn = f(F, n_c, n_r, D, t1, T);

[0045] The purpose of adjusting the bolt load is achieved by adjusting the interference between the bolts and the holes. The interference of the two outer rows of bolts is controlled within (-0.06~0.04), and the interference of the remaining bolts is controlled within (0.02~0.12). The difference in interference between the inner and outer bolts is within (0.02~0.1).

[0046] In at least one embodiment of the present application, in step 4, performing finite element calculation according to the finite element model to obtain the load of the bolt includes:

[0047] Take t1 and T as variables;

[0048] Set the parameter variable interval: 0.3t≤t1≤(1+D×n_c / S_c)t, T=t1+(1+D×n_c / S_c)×t;

[0049] The butt surface angle is adjusted between 1° and 3° by changing the size of t1 and T, and the loads F1, F2, ..., Fn_r transmitted by each bolt are obtained by finite element calculation.

[0050] In at least one embodiment of the present application, in step 4, the nail hole compression strength requirement is F1 / t1×D≤σ bru , σ bru It is the allowable strength of nail hole extrusion.

[0051] The invention has at least the following beneficial technical effects:

[0052] The reinforced wall panel docking structure of the present application can realize the connection of integrally reinforced or combined reinforced wall panels that bear axial loads, so as to solve the problem of part segmentation caused by material raw material size limitations and insufficient manufacturing and processing capabilities in practical applications; compared with conventional docking, this scheme adopts a multi-path force transmission docking form, with skin and ribs docked in a dispersed manner, to maintain continuous structural stiffness and load transfer, avoid large stress concentration and weight penalty, and is beneficial to fatigue details at the docking point. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 It is a schematic diagram of a butt joint structure of a reinforced wall panel according to an embodiment of the present application;

[0054] Figure 2 is a schematic diagram of a skin docking area according to an embodiment of the present application;

[0055] Figure 3 is a schematic diagram of a component model of an embodiment of the present application;

[0056] Figure 4 is a schematic diagram of a finite element model of an embodiment of the present application;

[0057] Figure 5 is a stress concentration factor curve at R of one embodiment of the present application;

[0058] Figure 6 It is a schematic diagram of the docking structure of a certain type of aircraft wing stiffened wall panel according to one embodiment of the present application.

[0059] in:

[0060] 1-first stiffened wall panel; 11-first skin; 12-first rib; 2-second stiffened wall panel; 21-second skin; 22-second rib; 3-skin docking area; 4-docking assembly; 5-docking rib; 6-vertical docking strip; 7-horizontal docking strip; 8-rib docking area; 9-support element; 10-bolt. DETAILED DESCRIPTION

[0061] In order to make the purpose, technical scheme and advantages of the implementation of this application clearer, the technical scheme in the embodiment of this application will be described in more detail below in conjunction with the drawings in the embodiment of this application. In the drawings, the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The described embodiments are part of the embodiments of this application, not all of them. The embodiments described below with reference to the drawings are exemplary and are intended to be used to explain this application, and should not be construed as limitations on this application. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application. The embodiments of this application are described in detail below in conjunction with the drawings.

[0062] In the description of the present application, it should be understood that the terms "center", "longitudinal", "lateral", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the scope of protection of the present application.

[0063] The following is combined with Figures 1 to 6 This application is described in further detail.

[0064] A first aspect of the present application provides a reinforced wall panel joint structure, comprising: a first reinforced wall panel 1 and a second reinforced wall panel 2 .

[0065] Specifically, Figure 1As shown, the first stiffened wall panel 1 includes a first skin 11 and a first rib 12 arranged on the first skin 11; the second stiffened wall panel 2 includes a second skin 21 and a second rib 22 arranged on the second skin 21; wherein, the first skin 11 and the second skin 21 are connected by bolts 10; the first rib 12 and the second rib 22 are connected by a docking assembly 4; the rib docking area 8 and the skin docking area 3 are staggered, and the two rib docking areas 8 are respectively located on both sides of the skin docking area 3.

[0066] In a preferred embodiment of the present application, the first skin 11 and the second skin 21 adopt a wedge-shaped overlap structure and are connected by a plurality of bolts 10. The butt joint angle between the first skin 11 and the second skin 21 is 1° to 3°.

[0067] In the preferred embodiment of the present application, the first rib 12 located in the rib docking area 8 includes a first vertical rib and a first horizontal rib; the second rib 22 located in the rib docking area 8 includes a second vertical rib and a second horizontal rib. The docking assembly 4 includes: docking ribs 5, vertical docking strips 6 and horizontal docking strips 7 (the use of horizontal strips can be eliminated for the integrally reinforced wall panels), wherein the docking ribs 5 include vertical docking ribs and horizontal docking ribs, and the docking ribs 5 are arranged in the skin docking area 3; one side of the vertical docking rib is connected to the first vertical rib through the vertical docking strip 6, and the other side is connected to the second vertical rib through the vertical docking strip 6; one side of the horizontal docking rib is connected to the first horizontal rib through the horizontal docking strip 7, and the other side is connected to the second horizontal rib through the horizontal docking strip 7. In this embodiment, double wedge structures are used at both ends of the horizontal docking ribs. The vertical docking ribs and the vertical docking strips 6 adopt a double shear docking configuration, and the docking angle is 1° to 5°.

[0068] In a preferred embodiment of the present application, a supporting element 9 is installed in the docking area, and the supporting element 9 is a frame or a rib.

[0069] The stiffened wall panel butt joint structure of the present application is to achieve the butt joint of the first stiffened wall panel 1 and the second stiffened wall panel 2. The first skin 11 and the second skin 21 are overlapped in a wedge-shaped manner, and the first rib 12 and the second rib 22 are butt jointed by a butt joint assembly 4 on both sides of the skin butt joint area, and all butt joint positions are connected by bolts 10. Out-of-plane support stiffness is provided by arranging frames and ribs in the butt joint area.

[0070] The key points of this application are that the overlapping of the skin and the butt joint of the ribs are each distributed butt jointed, and the butt joint of the ribs avoids the butt joint area of ​​the skin, eliminating the risk of butt jointing at the same position. The butt joint of the skin adopts wedge-shaped single shear butt joint, and the butt joint angle θ is required to be between 1° and 3°. The design of the butt joint rib 5 adopts a double wedge structure at both ends of the rib web, and the vertical butt joint strip 6 is designed as a double convex strip. The butt joint rib 5 and the vertical butt joint strip 6 adopt a double shear butt joint configuration, and the butt angle θ is required to be between 1° and 5°. The application of the butt joint rib 5 is intended to serve as the transfer of the ribs and avoid the butt joint area of ​​the skin.

[0071] A second aspect of the present application provides a method for designing a reinforced wall panel butt joint structure, which is used to design the reinforced wall panel butt joint structure. The design method comprises the following steps:

[0072] Step 1, obtaining initial input data of the stiffened wall panel butt joint structure;

[0073] Step 2: constructing a component model of the stiffened wall panel according to the initial input data;

[0074] Step 3, establishing a finite element model of the component model;

[0075] Step 4: Perform finite element calculation according to the finite element model to obtain the load of the bolt;

[0076] Step 5: Determine whether the bearing capacity of the bolt and the extrusion strength of the nail hole can meet the requirements. If not, increase the bolt diameter, change the parameters of the component model, and recalculate until the requirements are met;

[0077] Step 6, determining the bottom angle of the skin depression area in the skin butt joint area according to the stress concentration factor curve;

[0078] Step 7: Output the structural parameters of the reinforced wall panels that meet the requirements.

[0079] The stiffened wall panel butt joint structure design method of this application is mainly used for parameter design of stiffened wall panel butt joint structure, which is conducive to improving the distribution of multi-row bolt connection load and improving the reliability and fatigue life of the structural connection. Figure 2 Middle skin butt joint parameters: minimum thickness of skin butt joint area t1, total thickness of skin butt joint area T, bottom angle R of skin sinking area, butt joint surface angle θ.

[0080] First, in step 1, the initial input data obtained include: skin stress distribution, skin basic thickness, nail hole extrusion allowable strength, docking area bolt parameters, unit model size and loading load, as shown in Table 1.

[0081] Table 1

[0082]

[0083]

[0084] In step 2, the complex stiffened panel model is simplified into component models. Figure 3 , used to carry out parameter optimization design. Among them, W is the width of the component model, W = bs, bs is the rib spacing, the bolt arrangement is selected according to the typical connection between every two ribs, n_c is the number of bolts between every two ribs, the row spacing is S_c, n_r is the number of bolts between every two ribs, the row spacing is S_r, and the skin docking area L = n_r × S_r. Because the load and thickness of the wall panel in the transverse direction may be different, the bolt diameter, number of columns, and column spacing are different. This method establishes the dynamic correlation between the component model parameters W, L and n_c, n_r, D, t1, T and other parameters, and analyzes the selection of bolts and the parameter requirements of the skin under different load distributions.

[0085] In step 3, the finite element model of the component model is established. Figure 4 , the loading load is F = σ × t × W, and F1, F2, ..., Fn are solved, which are the loads of each row of single bolts. In this analysis technology, the structure and bolts use solid meshes and are given real stiffness data. By changing F, n_c, n_r, D, t1, T, the load distribution relationship between F and F1, F2, ..., Fn is established: Fn = f (F, n_c, n_r, D, t1, T). This method also proposes to adjust the bolt load by adjusting the interference between the bolt and the hole. The interference of the two outer rows of bolts is controlled within (-0.06 ~ 0.04), and the interference of the remaining bolts is controlled within (0.02 ~ 0.12), and the difference between the inner and outer bolts is in the range of (0.02 ~ 0.1). Establishing the definition of nonlinear contact in the analysis model and simulating the nonlinear load distribution caused by the deformation of the skin material and the fastener material can improve the accuracy of bolt load analysis.

[0086] In step 4, t1 and T are set as variables. This method proposes a parameter variable interval of 0.3t≤t1≤(1+D×n_c / S_c)t and T=t1+(1+D×n_c / S_c)×t. The butt joint angle θ is adjusted by changing the size of t1 and T, which is required to be between 1° and 3°, so as to change the load distribution between multiple rows of bolts, and the loads F1, F2, ..., Fn transmitted by each bolt are obtained by finite element calculation.

[0087] In step 5, determine the bearing capacity of the bolt and the extrusion strength of the nail hole (F1 / t1×D≤σ bru ) can meet the requirements. If not, increase the bolt diameter, change the parameters of the unit model, and recalculate until the requirements are met.

[0088] In step 6, the bottom angle R( Figure 2The size of the stress concentration here has a great influence, see Figure 5 The proposed stress concentration factor curve determines the value of R, and a trapezoidal profile design is added to the edge of the structure combined with the selection of large R to ensure that the structure does not interfere with the seam gap requirements.

[0089] Finally, in step 7, the parameter results are output according to Table 2.

[0090] Table 2

[0091]

[0092] The structural design method for butt jointing of reinforced wall panels of the present application should be combined with actual manufacturing requirements to reduce design deviations. The angle tolerance requirement of θ is proposed to be ±0.1°. When two reinforced wall panels are butt jointed, the end face of the overlap should be used as the positioning reference to ensure the assembly accuracy here first, and the position deviation should be controlled within ±0.5mm. For wall panels that are mainly subjected to tensile loads, the bolt holes can be added with a cold extrusion process, and the surface of the parts can be added with a shot peening process to improve the fatigue life of the structure. For the parameter design and analysis of the ribs, the skin parameter design method can be followed. In the selection of the element model, the unit width W is the height hw of the rib web, and the design variables t1 and T are defined as: 0.2t≤t1≤(1+D / hw)t, and T=2×t1+(1+D / hw)×t, where t is the equivalent web thickness of the rib t=A / hw, and A is the rib area. The remaining methods are the same as the technical requirements for skin butt jointing.

[0093] The reinforced wall panel docking structure and design method of the present application, in the design of large structures, due to the lack of material specifications, manufacturing, transportation and other issues, the structure is divided into several small sections for design and manufacturing. For the reinforced wall panels, docking through this solution can improve the flexibility of design, processing and manufacturing. The application of small-sized raw materials is also conducive to saving material costs and improving material utilization. The use of this solution to achieve the docking of reinforced wall panels ensures the continuity of the main load of the structure and eliminates the risk of centralized docking of the wall panels; in terms of details, the structural stiffness is continuous and there is no large stress concentration, which is conducive to improving the service life of the structure; compared with conventional docking solutions, it can reduce the weight of the structure and improve the efficiency of the structure.

[0094] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application shall be based on the protection scope of the claims.

Claims

1. A reinforced wall panel butt joint structure, characterized in that: include: A first stiffened wall panel (1), the first stiffened wall panel (1) comprising a first skin (11) and first ribs (12) arranged on the first skin (11); A second stiffened wall panel (2), the second stiffened wall panel (2) comprising a second skin (21) and second ribs (22) arranged on the second skin (21); in, The first skin (11) and the second skin (21) are connected via bolts (10); The first rib (12) and the second rib (22) are connected via a docking assembly (4); The rib butt joint area (8) and the skin butt joint area (3) are arranged in a staggered manner, and the two rib butt joint areas (8) are respectively located on both sides of the skin butt joint area (3).

2. The reinforced wall panel butt joint structure according to claim 1, characterized in that: The first covering skin (11) and the second covering skin (21) adopt a wedge-shaped overlapping structure and are connected by a plurality of bolts (10).

3. The reinforced wall panel butt joint structure according to claim 2, characterized in that: The butt-jointed angle between the first skin (11) and the second skin (21) is 1° to 3°.

4. The reinforced wall panel butt joint structure according to claim 3, characterized in that: The first rib (12) located in the rib docking area (8) comprises a first vertical rib and a first horizontal rib; The second ribs (22) located in the rib docking area (8) include second vertical ribs and second horizontal ribs.

5. The reinforced wall panel butt joint structure according to claim 4, characterized in that: The butt joint assembly (4) comprises: butt joint ribs (5), vertical butt joint strips (6) and horizontal butt joint strips (7), wherein: The butt joint ribs (5) include vertical butt joint ribs and horizontal butt joint ribs, and the butt joint ribs (5) are arranged in the skin butt joint area (3); One side of the vertical butt joint rib is connected to the first vertical rib via the vertical butt joint strip (6), and the other side is connected to the second vertical rib via the vertical butt joint strip (6); One side of the horizontal butt joint rib is connected to the first horizontal rib via the horizontal butt joint strip (7), and the other side is connected to the second horizontal rib via the horizontal butt joint strip (7).

6. The reinforced wall panel butt joint structure according to claim 5, characterized in that: The two ends of the horizontal butt ribs adopt double wedge structures.

7. The reinforced wall panel butt joint structure according to claim 6, characterized in that: The vertical butt joint ribs and the vertical butt joint strips (6) adopt a double shear butt joint configuration, and the butt joint angle is 1° to 5°.

8. The reinforced wall panel butt joint structure according to claim 7, characterized in that: A supporting element (9) is installed in the docking area, and the supporting element (9) is a frame or a rib.

9. A method for designing a reinforced wall panel butt joint structure, based on the reinforced wall panel butt joint structure according to claim 8, characterized in that: include: Step 1, obtaining initial input data of the stiffened wall panel butt joint structure; Step 2: constructing a component model of the stiffened wall panel according to the initial input data; Step 3, establishing a finite element model of the component model; Step 4: Perform finite element calculation according to the finite element model to obtain the load of the bolt; Step 5: Determine whether the bearing capacity of the bolt and the extrusion strength of the nail hole can meet the requirements. If not, increase the bolt diameter, change the parameters of the component model, and recalculate until the requirements are met; Step 6, determining the bottom angle of the skin depression area in the skin butt joint area according to the stress concentration factor curve; Step 7: Output the structural parameters of the reinforced wall panels that meet the requirements.

10. The method for designing a butt joint structure of a reinforced wall panel according to claim 9, characterized in that: In step 1, the initial input data includes: skin stress distribution, skin basic thickness, nail hole extrusion allowable strength, docking area bolt parameters, unit model size and loading load.

11. The method for designing a butt joint structure of a reinforced wall panel according to claim 10, characterized in that: In step 2, the component model: The model width W is the rib spacing bs; The number of bolts between every two ribs is n_c, and the distance between the bolts is S_c; The number of rows of bolts between every two ribs is n_r, and the row spacing is S_r; The length of the skin butt joint area is L = n_r × S_r; The bolt diameter is D; The minimum thickness of the skin butt area is t1; The total thickness of the skin butt area is T.

12. The method for designing a butt joint structure of a reinforced wall panel according to claim 11, characterized in that: In step 3, the finite element model: According to the loading load F = σ × t × W, the loads F1, F2, ..., Fn of a single bolt are solved, where σ is the stress distribution of the skin and t is the basic thickness of the skin; By adjusting F, n_c, n_r, D, t1, T, the load distribution relationship between F and F1, F2, ..., Fn is constructed: Fn = f(F, n_c, n_r, D, t1, T); The purpose of adjusting the bolt load is achieved by adjusting the interference between the bolts and the holes. The interference of the two outer rows of bolts is controlled within (-0.06~0.04), and the interference of the remaining bolts is controlled within (0.02~0.12). The difference in interference between the inner and outer bolts is within (0.02~0.1).

13. The method for designing a butt joint structure of a reinforced wall panel according to claim 12, characterized in that: In step 4, finite element calculation is performed according to the finite element model to obtain the load of the bolt, including: Take t1 and T as variables; Set the parameter variable interval: 0.3t≤t1≤(1+D×n_c / S_c)t, T=t1+(1+D×n_c / S_c)×t; The butt surface angle is adjusted between 1° and 3° by changing the size of t1 and T, and the loads F1, F2, ..., Fn transmitted by each bolt are obtained by finite element calculation.

14. The method for designing a butt joint structure of a reinforced wall panel according to claim 13, characterized in that: In step 4, the nail hole extrusion strength requirement is F1 / t1×D≤σ bru , σ bru It is the allowable strength of nail hole extrusion.

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