A method for determining fatigue test load for a typical overlap joint test specimen

Abstract

The present application provides a method for determining the fatigue test load of a typical lap joint test piece, the method comprising: selecting a plurality of test pieces; wherein the test pieces include a test piece A and a test piece B; obtaining the fatigue limit load P of each test piece in the test piece B according to the fatigue limit trial and error method; jxs ; Based on the fatigue limit load P of each test piece jxs , determine the average fatigue limit load P jxa ; Wherein, the number of rivet connections of the test piece B is recorded as n1; Based on the average fatigue limit load P jxa , the number of rivet connections and the proportional coefficient of the maximum rivet load are used to obtain the maximum extrusion load on the rivet and the bypass load at the rivet cross section; the stress severity coefficient method is used to calculate the maximum stress at the edge of the rivet hole, wherein the maximum stress at the edge of the rivet hole represents the fatigue limit of the structure; the yield limit of the structure is obtained, and the fatigue loading load of the test piece A is obtained based on the fatigue limit of the structure and the yield limit of the structure.

Description

Technical Field

[0001] The present application belongs to the field of comprehensive strength design technology, and in particular relates to a method for determining the fatigue test load of a typical lap joint test piece. Background Art

[0002] The main purpose of the typical connection fatigue test is to obtain the structural fatigue limit taking into account structural characteristics such as friction, surface roughness, and surface state through a simple connection test piece, laying the foundation for the design of the structural connection area. Therefore, the typical connection fatigue test is of great significance.

[0003] At present, the fatigue test load of a typical lap joint test piece is generally determined by summarizing the load data of previous tests for analogy. The fatigue characteristic load of this test is obtained by analyzing the fatigue test data of similar structures (including similar structural features such as materials and connection methods). However, if the test piece fails in the low-cycle segment, this method has great limitations. For fatigue performance tests, it is hoped that the test piece will fail in the high-cycle segment, thereby ensuring that the obtained fatigue performance has higher accuracy.

[0004] Moreover, even if the failure of the test piece occurs in the high-cycle range, using this load as the characteristic load for similar structures will still result in large errors, because the fatigue limit of the structure is greatly affected by the structural details. In summary, the original method has great limitations. Summary of the Invention

[0005] Purpose of the present invention: The present invention uses hole edge stress as a fatigue characteristic characterization value, and on this basis obtains the corresponding characteristic load that causes the test piece to fail in the high cycle segment, effectively overcoming the limitations brought by the background technology.

[0006] The present application provides a method for determining the fatigue test load of a typical lap joint test piece, the method comprising:

[0007] Selecting a plurality of test pieces; wherein the test pieces include test piece A and test piece B;

[0008] The fatigue limit load P of each test piece in the test piece B is obtained according to the fatigue limit trial and error method. jxs ;

[0009] Based on the fatigue limit load P of each test piece jxs , determine the average fatigue limit load P jxa ; Wherein, the number of rivet connections of the test piece B is recorded as n1;

[0010] Based on the average fatigue limit load P jxa , the number of rivet connections and the proportional coefficient of the maximum rivet load, and obtain the maximum extrusion load on the rivet and the bypass load at the rivet section;

[0011] The stress severity coefficient method is used to calculate the rivet hole edge maximum stress, wherein the rivet hole edge maximum stress represents the fatigue limit of the structure.

[0012] The yield limit of the structure is obtained, and the fatigue loading load of the test piece A is obtained based on the fatigue limit of the structure and the yield limit of the structure.

[0013] Preferably, the yield limit of the structure is obtained, comprising:

[0014] The ratio of the yield limit σ 0.2 of the material to the fatigue limit σ -1 of the material is K, that is, K = σ 0.2 / σ -1 , and the yield limit of the structure is σ jg0.2 = K × σ maxjx-1 .

[0015] Preferably, the test piece A represents a test piece that has not undergone fatigue test, and the test piece B represents a test piece that has completed fatigue test.

[0016] Preferably, the test piece A and the test piece B are the same structure.

[0017] Preferably, the failure mode of the test piece B is all the fracture caused by crack propagation after cracks appear on the hole edge.

[0018] Preferably, the fatigue limit load P jxs corresponds to a load ratio R = -1, and the load ratio R is the ratio of the minimum load to the maximum load in the cyclic load.

[0019] Preferably, the stress severity coefficient method is used to calculate the rivet hole edge maximum stress, comprising:

[0020] The stress severity coefficient method is used to calculate the rivet hole edge maximum stress based on the maximum extrusion load and the bypass load at the rivet section.

[0021] Preferably, the test piece is formed by connecting two plates through rivets.

[0022] The beneficial effects of the present application are:

[0023] The method has been applied in the early fatigue test of a helicopter, effectively improves the fatigue characteristic analysis precision of a typical fatigue test piece, reduces the dispersion of test results, and brings great economic benefits. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 A typical lap joint test piece schematic diagram provided by the embodiment of the present application. DETAILED DESCRIPTION

[0025] It should be noted that the fatigue failure of the connection zone structure often occurs at the hole edge, mainly because the hole edge is a high stress area, and the previous approach often does not analyze the details of the hole edge stress, but only uses the load to characterize the structure, especially the fatigue limit. The core idea of the present application is to use the hole edge stress to characterize the structure, especially the fatigue limit of the structure, based on the test data of the test pieces that have completed the fatigue test, and to use the load to characterize the structure, especially the fatigue limit of the structure. According to the fatigue limit of the structure characterized by the hole edge stress, and combined with the full-range S-N curve, a high-precision test load can be given to ensure that the failure cycle number of the subsequent test pieces falls in the high-cycle region.

[0026] Please refer to Figure 1 The typical lap joint test piece is shown in the figure (2-rivet lap joint), and the notes are as follows: W is the width of the test piece, e is the rivet edge distance, f is the rivet spacing, d is the rivet diameter, t1 and t2 are the thicknesses of the lap joint connecting plates, P is the test loading load (cyclic load), and the shaded areas M and N are the test clamping sections. The multi-rivet lap joint is the same as the Figure 1 way.

[0027] In the embodiments of the present application, the specific steps provided are as follows:

[0028] Step 1, A test piece and B test piece are the same structure, that is, A and B test pieces are the same batch of test pieces, which are typical lap joint test pieces, as shown in the attached Figure 1 , the test pieces are all aluminum alloy metal materials, and the materials of the upper and lower plates of the lap joint structure are consistent.

[0029] Among them, A represents a test piece that has not undergone fatigue test; B represents a test piece that has completed fatigue test.

[0030] Step 2, the typical lap joint test pieces A and B are subjected to unidirectional cyclic load, and the unidirectional cyclic load ratio is R=0.1 (R is the ratio of the minimum load to the maximum load in the cyclic load), and the load frequency is ≤20Hz.

[0031] Step 3, B is a test piece that has completed n (n≥3) fatigue tests, and the failure forms are all cracks at the hole edge causing fracture after crack propagation. If the failure form is not the crack at the hole edge, the test piece is not counted in n.

[0032] Step 4, the test cycle number of the failure of the B test piece is less than 10 5 .

[0033] Step 5, according to the fatigue limit trial-and-error method, the fatigue limit load P jxs of each test piece in the B test piece is obtained, and the load ratio R=-1 corresponding to the fatigue limit load (R is the ratio of the minimum load to the maximum load in the cyclic load).

[0034] Step 6, according to step 5, each B test piece gets a corresponding fatigue limit load, that is, P jxs1 , P jxs2 …P jxn .

[0035] On this basis, the average fatigue limit load of B test piece is obtained

[0036] Step 7, the rivet connection number of B test piece is n1, under the action of average fatigue limit load P jxa , the average extrusion load of rivet is P jxa / n1, the maximum load proportion coefficient of rivet is λ (λ≤1), then the maximum extrusion load of rivet is P jy =λ×P jxa / n1, the bypass load at the rivet section is P pl =(1-λ)×P jxa / n1.

[0037] Step 8, the connecting plate with thinner thickness in the upper and lower plates of B test piece is taken to calculate the hole edge stress, if the thickness of the two is the same, any plate can be taken to calculate the hole edge stress.

[0038] Step 9, when calculating the hole edge stress, the stress severity coefficient method (SSF method) is adopted, and the maximum stress at the hole edge is:

[0039]

[0040] Wherein, K tb is the extrusion stress concentration coefficient, θ is the extrusion uneven coefficient, K tg is the bypass load stress concentration coefficient, d is the rivet diameter, W is the width of test piece, t1 is the thickness of connecting plate with smaller thickness or the thickness of connecting plate when the thickness is equal.

[0041] Step 9, σ maxjx (R=-1) calculated from step 9 is the structure fatigue limit represented by hole edge stress, which is denoted as σ maxjx-1 .

[0042] Step 10, the ratio of yield limit σ 0.2 of material to fatigue limit σ -1 (R=-1) of material is K, that is, K=σ 0.2 / σ -1 , then the yield limit of structure is σ jg0.2 =K×σ maxjx-1 .

[0043] Step 11, the number of times of breaking of the A test piece under the action of the unidirectional cyclic load (R=0.1) is required to be 0.5Mc (Mc represents million times).

[0044] Step 12, according to σ maxjx-1 , σ jg0.2 , the full-range S-N curve of the structure is obtained, and the average stress σ pj corresponding to 0.5Mc is obtained from the S-N curve.

[0045] Step 13, the cyclic load applied to the A test piece is required to be R=0.1, that is, the maximum load (P max ) and the minimum load (P min ) applied meet the requirement that P min =0.1×P max .

[0046] Step 14, the purpose of the present application is to determine P max , P min . Assuming that the maximum load P max , P min , the maximum stresses at the hole edge are σ max , 0.1σ max , respectively.

[0047] Step 15, the relationship between σ pj and σ max can be obtained by the Goodman correction, that is:

[0048]

[0049] The solving formula of σ max is obtained from formula (1):

[0050]

[0051] Step 15, σ max calculated according to formula 2 is solved by using the SSF method to solve P max , and the calculation process is as follows:

[0052] The number of rivet connections of the A test piece is the same as that of the B test piece, which is n1. Under the action of the maximum load P max , the average extrusion load borne by the rivet is P max / n1, the maximum load proportion coefficient of the rivet is λ (λ≤1), then the maximum extrusion load borne by the rivet is P jymax =λ×P max / n1, the bypass load at the rivet section is P plmax =(1-λ)×P max / n1, then:

[0053]

[0054] K tb is the extrusion stress concentration coefficient, θ is the extrusion uneven coefficient, K tg is the bypass load stress concentration coefficient, d is the rivet diameter, W is the test piece width, t1 is the smaller thickness of the connecting plate or the connecting plate thickness when the thicknesses are equal;

[0055] From equation (3), it can be seen that P jymax , P plmax are unknown numbers, but both of them are related to the maximum load P max applied, so there is only one unknown number P max in equation (3), which can be solved by σ max to obtain P max ;

[0056] Step 16, P max is obtained from step 15, and P min can be obtained according to P max = 0.1 × P min .

[0057] The core point of the present application is to use the rivet hole edge stress to represent the structure characteristics, especially the fatigue limit of the structure. According to the structure fatigue limit represented by the rivet hole edge stress, the structure characteristics represented by the hole edge stress are more accurate because the structure details such as the rivet connection form, the rivet filling method, the structure surface state and other factors are fully considered. When the load is used to represent the structure fatigue limit, especially when the fatigue failure cycle number of the test piece falls in the low cycle section, the accuracy of the structure fatigue limit obtained is lower, and the load adjustment for the subsequent test piece (the fatigue failure cycle number falls in the high cycle section through load adjustment) has a smaller effect.

[0058] In other embodiments of the present application, the method provided by the present application has the following specific steps:

[0059] A: represents a test piece without fatigue test; B: represents a test piece with completed fatigue test

[0060] a) A and B contain the same lap joint plate material;

[0061] b) According to the information of the material and heat treatment state of the connecting plate of A, the rivet brand, etc., the fatigue limit (the corresponding load ratio R = -1, R is the ratio of the minimum load to the maximum load in the cyclic load) of B with similar structure characteristics is calculated. The fatigue limit is represented by the load, which is denoted as P jx ;

[0062] c) The number of rivets of B is denoted as n, and the load P jxThe average extrusion load borne by the rivet is P jx / n, the maximum load proportion coefficient of the rivet is λ (λ≤1), and the maximum extrusion load borne by the rivet is P jy =λ×P jx / n, the bypass load at the rivet section is P pl =(1-λ)×P jx / n;

[0063] d) The connecting plate with smaller thickness is selected to calculate the hole edge stress. The stress calculation adopts the stress severity coefficient method (SSF method), and the maximum stress at the hole edge is:

[0064]

[0065] Wherein, K tb is an extrusion stress concentration coefficient, θ is an extrusion uneven coefficient, K tg is a bypass load stress concentration coefficient, d is the diameter of the rivet, W is the width of the test piece, and t1 is the thickness of the connecting plate with smaller thickness;

[0066] e) The σ maxjx (R=-1) obtained by the calculation of c) is the structural fatigue limit characterized by the hole edge stress, and is denoted as σ maxjx-1 . The ratio of the yield limit σ 0.2 of the material to the fatigue limit σ -1 (R=-1) of the material is K, that is, K=σ 0.2 / σ -1 , and the yield limit of the structure is σ jg0.2 =K×σ maxjx-1 ;

[0067] f) The cycle number of a single test piece of A is required to be 0.5Mc (Mc represents million times);

[0068] g) The full-range S-N curve of the structure is obtained according to σ maxjx-1 and σ jg0.2 , and the average stress σ pj corresponding to 0.5Mc is obtained from the S-N curve;

[0069] h) The cycle load applied in A test is required to be R=0.1, that is, the maximum load (P max ) and the minimum load (P min ) applied meet the requirement that P min =0.1×P max ;

[0070] i) The purpose of the present application is to determine P max and P min . It is assumed that the maximum load P max and P minThe maximum stress at the hole edge is σ max , 0.1σ max .

[0071] j) The σ max is obtained by the Goodman correction, that is According to the formula, the σ max is obtained;

[0072] According to the σ max , the P is obtained by the SSF method, that is max , and the maximum and minimum loads corresponding to the cyclic load applied in the A fatigue test are determined.

[0073] The present application fully considers the problems caused by the load representation, effectively solves the above problems by using the stress representation method, can reduce the test piece number and test time, and further brings obvious economic benefits.

Claims

1. A method of determining a fatigue test load for a typical overlap joint test specimen, characterized by, The method comprises: selecting a plurality of test pieces; wherein the test pieces comprise a test piece A and a test piece B; The fatigue limit load P of each of the test pieces B is obtained according to the trial and error method of fatigue limit jxs ; determining an average fatigue limit load P jxs based on the fatigue limit load P jxa of each of the test pieces; wherein the number of rivet connections of the test piece B is denoted as n1; based on the average fatigue limit load P jxa rivet connection number and rivet maximum load proportionality coefficient, to obtain the maximum extrusion load and bypass load at the rivet section. calculating a rivet hole edge maximum stress by using a stress severity coefficient method, wherein the rivet hole edge maximum stress represents a fatigue limit of a structure; obtaining a yield limit of the structure, and obtaining a fatigue loading load of the test piece A based on the fatigue limit of the structure and the yield limit of the structure.

2. The method of claim 1, wherein, The obtaining of the yield limit of the structure comprises: The yield strength of the material σ 0.2 and the fatigue limit σ of the material -1 The ratio is K, that is, K = σ 0.2 / σ -1 , then the yield limit of the structure is σ jg0.2 =K×σ maxjx-1 .

3. The method of claim 1, wherein, The test piece A represents a test piece without fatigue test, and the test piece B represents a test piece with completed fatigue test.

4. The method of claim 1, wherein, The test piece A and the test piece B are of the same structure.

5. The method of claim 1, wherein, The failure forms of the test piece B are all fractures caused by crack propagation after cracks appear on hole edges.

6. The method of claim 1, wherein, said fatigue limit load P jxs a corresponding load ratio R = -1, said load ratio R being the ratio of the minimum load to the maximum load in the cyclic load.

7. The method of claim 1, wherein, The calculating of the rivet hole edge maximum stress by using the stress severity coefficient method comprises: calculating the rivet hole edge maximum stress by using the stress severity coefficient method based on a maximum extrusion load borne by the rivet and a bypass load at a rivet section.

8. The method of claim 1, wherein, The test piece is formed by connecting two plates through a rivet.

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