Method for determining fatigue test load of typical lap joint test piece

Abstract

The invention provides a method for determining fatigue test loads of typical lap joint test pieces. The method comprises the following steps: selecting a plurality of test pieces; wherein the test piece comprises a test piece A and a test piece B; obtaining a fatigue limit load Pjxs of each test piece in the test pieces B according to a fatigue limit cut-and-try method; determining an average fatigue limit load Pjxa based on the fatigue limit load Pjxs of each test piece; wherein the rivet connection number of the test piece B is recorded as n1; on the basis of the average fatigue limit load Pjxa, the number of connected rivets and the maximum load proportionality coefficient of the rivets, the maximum extrusion load borne by the rivets and the bypass load at the sections of the rivets are obtained; a stress severity coefficient method is adopted, the maximum stress of the rivet hole edge is calculated, and the maximum stress of the rivet hole edge represents the fatigue limit of the structure; obtaining the yield limit of the structure, and obtaining the fatigue loading load of the test piece A 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 technical field of comprehensive strength design, and in particular relates to a method for determining fatigue test loads of typical lap joint test pieces. Background Art

[0002] The main purpose of typical connection fatigue test is to obtain the structural fatigue limit taking into account structural characteristics such as friction, surface roughness, surface state, etc. through simple connection test pieces, laying the foundation for the design of structural connection areas. Therefore, 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, and 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, because 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 falls in the high-cycle section, using this load as the characteristic load of similar structures will still have a large error, 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 a 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 a test piece A and a 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 n 1 ;

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

[0011] The maximum stress at the edge of the rivet hole is calculated by using the stress severity coefficient method, wherein the maximum stress at the edge of the rivet hole 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, obtaining the yield limit of the structure includes:

[0014] The yield limit σ of the material 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 .

[0015] Preferably, the test piece A represents a test piece that has not been subjected to 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 have the same structure.

[0017] Preferably, the failure form of the test piece B is fracture caused by cracks appearing at the hole edge and then expanding.

[0018] Preferably, the fatigue limit load P jxs The corresponding 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 method of using the stress severity coefficient method to calculate the maximum stress at the edge of the rivet hole includes:

[0020] The maximum stress at the edge of the rivet hole is calculated by using the stress severity coefficient method based on the maximum extrusion load on the rivet and the bypass load at the rivet cross section.

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

[0022] Beneficial effects of the present invention:

[0023] This method has been applied in the early fatigue tests of helicopters, effectively improving the fatigue characteristics analysis accuracy of typical fatigue test pieces, reducing the dispersion of test results, and bringing great economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of a typical overlap test piece provided in an embodiment of the present application. DETAILED DESCRIPTION

[0025] It should be noted that fatigue failure of the connection zone structure often occurs at the hole edge, mainly because the hole edge is a high stress area. The previous practice often did not conduct a detailed analysis of the hole edge stress, but only used the load to characterize the characteristics of the structure, especially the fatigue limit. The core idea of ​​the present invention is based on the test data of the test piece that has completed the fatigue test, and the structural fatigue limit previously characterized by the load is replaced by the rivet hole edge stress to characterize the structural characteristics, especially the fatigue limit of the structure. According to the structural fatigue limit characterized by the hole edge stress, combined with the full range SN curve, a high-precision test piece applied load can be given to ensure that the number of subsequent test piece failure cycles falls in the high cycle area.

[0026] See also Figure 1 Schematic diagram of a typical lap joint test piece (2-nail lap joint), Note: W is the width of the test piece, e is the rivet margin, f is the rivet spacing, d is the rivet diameter, t1 and t2 are the lap joint plate thicknesses, P is the test loading load (cyclic load), and the shaded areas M and N are the test clamping sections. Figure 1 Same way.

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

[0028] Step 1: Test piece A and test piece B have the same structure, that is, test pieces A and B are from the same batch of test pieces, both of which are typical lap joint test pieces, see attached. Figure 1 As shown, the test pieces are all made of aluminum alloy, and the materials of the upper and lower plates of the overlapped structure are consistent.

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

[0030] Step 2: Typical lap joint test pieces A and B are subjected to unidirectional cyclic loads, 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 ≤20 Hz.

[0031] Step 3, B is a test piece that has completed n pieces (n≥3) of fatigue tests, and the failure mode is fracture caused by cracks appearing at the hole edge and then cracks extending. If the failure mode is not fracture in the form of cracks appearing at the hole edge, the test piece is not included in n.

[0032] Step 4: The number of test cycles for the destruction of specimen B is less than 10 5 .

[0033] Step 5: Obtain the fatigue limit load P of each specimen in the B specimen according to the fatigue limit trial and error method. jxsThe load ratio corresponding to the fatigue limit load is R=-1 (R is the ratio of the minimum load to the maximum load in the cyclic load).

[0034] Step 6: According to step 5, each test piece of the B test piece obtains the corresponding fatigue limit load, that is: P jxs1 , P jxs2 …P jxn .

[0035] On this basis, the average fatigue limit load is obtained

[0036] Step 7: The number of rivet connections of test piece B is recorded as n 1 , under the average fatigue limit load P jxa Under this condition, the average extrusion load on the rivet is P jxa / n 1 , the maximum load proportionality coefficient of the rivet is λ (λ≤1), then the maximum extrusion load on the rivet is P jy =λ×P jxa / n 1 , the bypass load at the nail section is P pl =(1-λ)×P jxa / n 1 ;

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

[0038] Step 9: The stress severity factor method (SSF method) is used to calculate the hole edge stress. The maximum hole edge stress is:

[0039]

[0040] Among them, K tb is the extrusion stress concentration factor, θ is the extrusion non-uniformity coefficient, K tg is the bypass load stress concentration factor, d is the rivet diameter, W is the test piece width, and t1 is the thickness of the connecting plate when the thickness is smaller or the thickness of the connecting plate when the thickness is equal.

[0041] Step 9: σ calculated in step 9 maxjx (R = -1) is the fatigue limit of the structure characterized by the hole edge stress, denoted by σ maxjx-1 .

[0042] Step 10: The yield limit of the material 0.2 and the fatigue limit σ of the material -1 The ratio of (R = -1) is K, that is, K = σ 0.2 / σ -1 , then the yield limit of the structure is σjg0.2 =K×σ maxjx-1 .

[0043] Step 11: The number of times the A test piece is destroyed under a unidirectional cyclic load (R=0.1) is required to be 0.5Mc (Mc means million times).

[0044] Step 12: According to σ maxjx-1 , σ jg0.2 Get the full range SN curve of the structure, and get the average stress σ corresponding to 0.5Mc from the SN curve pj .

[0045] Step 13: The cyclic load requirement for the A test piece is R = 0.1, that is, the maximum load (P max ), minimum load (P min ) meets the following requirements: P min =0.1×P max .

[0046] Step 14: The purpose of the present invention is to determine P max , P min Assuming the maximum load P max , P min The maximum stresses at the hole edges are calculated to be σ max , 0.1σ max .

[0047] Step 15: Through Goodman correction, we can get σ pj With σ max The relationship between them is:

[0048]

[0049] From formula (1), we can get σ max The solution is:

[0050]

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

[0052] The number of rivet connections in test piece A is the same as that in test piece B, both are n 1 . Under the maximum applied load P max Under this condition, the average extrusion load on the rivet is P max / n 1 , the maximum load proportionality coefficient of the rivet is λ (λ≤1), then the maximum extrusion load on the rivet is P jymax =λ×P max / n1 , the bypass load at the nail section is P plmax =(1-λ)×P max / n 1 ,but:

[0053]

[0054] Among them, K tb is the extrusion stress concentration factor, θ is the extrusion non-uniformity coefficient, K tg is the stress concentration factor of the bypass load, d is the rivet diameter, W is the width of the test piece, t1 is the thickness of the connecting plate when the thickness is smaller or the thickness of the connecting plate when the thickness is equal;

[0055] From formula (3), we can see that P jymax , P plmax is an unknown number, but both are related to the maximum applied load P max There is a correlation, so there is only one unknown number P in formula (3) max , by σ max We can solve for P max ;

[0056] Step 16: Obtain P from step 15 max , according to P min =0.1×P max , we can get P min .

[0057] The core point of the present application is to use the rivet hole edge stress to characterize the structural characteristics, especially the fatigue limit of the structure. The structural fatigue limit characterized by the rivet hole edge stress takes full account of the structural details, such as the rivet connection form, the rivet filling method, the surface state of the structure, etc., so that the structural characteristics characterized by the hole edge stress have high accuracy. Moreover, when the load is used to characterize the structural fatigue limit, especially when the number of fatigue failure cycles of the test piece falls in the low cycle segment, the obtained structural fatigue limit has low accuracy, and is less effective in guiding the subsequent test pieces to adjust the load (adjusting the load to make the number of fatigue failure cycles fall in the high cycle segment).

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

[0059] A: represents the test piece that has not been subjected to fatigue test; B: represents the test piece that has completed fatigue test

[0060] a) The materials of the lap joint plates included in A and B are the same;

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

[0062] c) The number of rivet connections in B is n, and the load P jx Under this condition, the average extrusion load on the rivet is P jx / n, the maximum load proportionality coefficient of the rivet is λ (λ≤1), then the maximum extrusion load on the rivet is P jy =λ×P jx / n, the bypass load at the nail section is P pl =(1-λ)×P jx / n;

[0063] d) Select a connecting plate with a smaller thickness to calculate the stress at the hole edge. The stress severity factor method (SSF method) is used for stress calculation, and the maximum stress at the hole edge is:

[0064]

[0065] Among them, K tb is the extrusion stress concentration factor, θ is the extrusion non-uniformity coefficient, K tg is the stress concentration factor of bypass load, d is the rivet diameter, W is the width of the test piece, and t1 is the thickness of the connecting plate with the smaller thickness;

[0066] e) σ calculated from c) maxjx (R = -1) is the fatigue limit of the structure characterized by the hole edge stress, denoted by σ maxjx-1 . The yield limit of the material σ 0.2 and the fatigue limit σ of the material -1 The ratio of (R = -1) 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 ;

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

[0068] g) According to σ maxjx-1 , σ jg0.2 Get the full range SN curve of the structure, and get the average stress σ corresponding to 0.5Mc from the SN curve pj ;

[0069] h) The cyclic load applied in test A is required to be R = 0.1, that is, the maximum load applied (P max), minimum load (P min ) meets the following requirements: P min =0.1×P max ;

[0070] i) The purpose of the present invention is to determine P max , P min Assuming the maximum load P max , P min The maximum stresses at the hole edges are calculated to be σ max , 0.1σ max .

[0071] j) Obtain σ by Goodman correction max ,Right now According to this formula, we can get σ max ;

[0072] According to σ max The SSF method can be used to calculate P max , thereby determining the maximum and minimum loads corresponding to the cyclic load applied in fatigue test A.

[0073] The present application fully considers the problems caused by the above-mentioned load characterization, and effectively solves the above-mentioned problems by adopting the stress characterization method, which can reduce the number of test pieces and the test time, thereby bringing obvious economic benefits.

Claims

1. A method for determining fatigue test loads for typical lap joint test pieces, It is characterized in that The method comprises: Selecting a plurality of test pieces; wherein the test pieces include a test piece A and a test piece B; 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 ; 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 n 1 ; Based on the average fatigue limit load P jxa , the number of rivet connections and the proportional coefficient of the maximum load of the rivet, and obtain the maximum extrusion load on the rivet and the bypass load at the rivet section; The maximum stress at the edge of the rivet hole is calculated by using the stress severity coefficient method, 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.

2. The method according to claim 1, It is characterized in that The obtaining the yield limit of the structure comprises: The yield limit σ of the material 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 according to claim 1, It is characterized in that The test piece A represents a test piece that has not been subjected to a fatigue test, and the test piece B represents a test piece that has been subjected to a fatigue test.

4. The method according to claim 1, It is characterized in that The test piece A and the test piece B have the same structure.

5. The method according to claim 1, It is characterized in that The failure mode of the test piece B is that the cracks appear at the hole edge and then expand and break.

6. The method according to claim 1, It is characterized in that The fatigue limit load P jxs The corresponding load ratio R=-1, and the load ratio R is the ratio of the minimum load to the maximum load in the cyclic load.

7. The method according to claim 1, It is characterized in that The stress severity coefficient method is used to calculate the maximum stress at the edge of the rivet hole, including: The maximum stress at the edge of the rivet hole is calculated by using the stress severity coefficient method based on the maximum extrusion load on the rivet and the bypass load at the rivet cross section.

8. The method according to claim 1, It is characterized in that The test piece is formed by connecting two plates via rivets.

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