Lateral vibration and axial superposition fatigue test method

Abstract

The invention belongs to the field of fatigue test of plate test pieces, and particularly relates to a lateral vibration and axial superposition fatigue test method. The method comprises the following steps: preparing a test piece; providing an axial actuation mode and a lateral vibration actuation mode for the test piece, setting a first test parameter, carrying out a superposition fatigue test of a first test type to a set stop condition, and obtaining an axial fatigue life and a lateral fatigue life of the first test type; after the fitting constants k, m and n are solved, a multi-axial fatigue life prediction equation is formed, the axial actuation mode and the lateral vibration actuation mode are reset as Nth test parameters, and the axial life and the lateral life of the Nth test type are solved through the multi-axial fatigue life prediction equation, namely, the multi-axial fatigue life is obtained. Through the method provided by the invention, the multi-axial fatigue state under non-proportional loading can be predicted.

Description

Technical Field

[0001] The invention belongs to the field of fatigue testing of plate test pieces, and in particular relates to a lateral vibration and axial superposition fatigue testing method. Background Art

[0002] During the service life of civil helicopters, some critical components are subject to the combined effects of local vibration fatigue loading and conventional fatigue loading, i.e., multiaxial fatigue conditions within these components. Multiaxial fatigue estimation methods are primarily based on three criteria: the maximum principal stress / strain criterion. This criterion states that even under multiaxial stress conditions, material damage is primarily caused by the maximum principal stress (principal strain), independent of other factors. The second is the von Mises equivalent stress / strain criterion, which states that under multiaxial stress / strain conditions, material damage is primarily controlled by the von Mises equivalent stress and strain. The third is the Tresca maximum shear stress / strain criterion, which simply attributes damage to being solely controlled by the maximum shear stress / strain. These criteria are effective, simple, and practical for proportional loading. However, under non-proportional loading conditions, particularly when hydrostatic pressure influences fatigue life, these criteria fail to provide reliable predictions. Therefore, these equivalent strain / stress theories are not well-suited for multiaxial life estimation under non-proportional loading.

[0003] In actual engineering applications, the fatigue life of key helicopter components is often subjected to multiaxial fatigue under non-proportional loading. Therefore, in order to systematically study the fatigue life of key helicopter components under both vibration and conventional fatigue loading, a lateral vibration and axial conventional superposition fatigue test method is required. Summary of the Invention

[0004] In order to estimate the multi-axial life of a plate test piece under non-proportional loading with vibration fatigue load and conventional fatigue load superposition working condition, the present invention provides a lateral vibration and axial conventional superposition fatigue test method.

[0005] The purpose of the present invention is achieved through the following technical solutions.

[0006] A lateral vibration and axial superposition fatigue test method is used to measure or predict the multiaxial fatigue life of a test piece, including the following steps:

[0007] Step 1: Prepare a test piece of a set pattern, wherein the middle area of ​​the test piece is a working section, the two end areas of the test piece are clamping sections, and the working section is a stress concentration area;

[0008] Step 2: providing axial actuation modes of equal magnitude and opposite directions to the two supporting sections, and providing lateral vibration actuation mode to the working section, setting the axial actuation mode and the lateral vibration actuation mode as first test parameters, conducting a superimposed fatigue test of the first test type until a set stop condition is set, and obtaining the axial fatigue life and lateral fatigue life of the first test type;

[0009] Step 3: Using the material parameters of the test piece, the maximum axial stress, and the corresponding mean axial fatigue life and mean lateral fatigue life of the first test type as basic data, substitute them into the following formula (1) to solve the fitting constants k, m, and n;

[0010] ;

[0011] Where:

[0012] is the maximum normal stress on the plane of maximum shear strain;

[0013] is the yield strength of the material;

[0014] and are fatigue plasticity coefficient and strength coefficient;

[0015] c is the fatigue plasticity index;

[0016] b is the fatigue strength index;

[0017] N c is the lateral fatigue life;

[0018] N f is the axial fatigue life;

[0019] k, m, and n are fitting constants.

[0020] Step 4: Substitute the fitting constants k, m, and n solved in step 3 into formula (1) to form a multi-axial fatigue life prediction equation. Reset the axial actuation mode and the lateral vibration actuation mode as the Nth test parameters, and use the multi-axial fatigue life prediction equation to solve the axial life and lateral life of the Nth test type, that is, the multi-axial fatigue life.

[0021] In the above-mentioned lateral vibration and axial superposition fatigue test method, the maximum normal stress The equivalent stress calculation formula of the maximum normal stress is the superimposed stress of the axial stress provided by the axial actuation mode and the lateral stress provided by the lateral actuation mode: ,in, is the equivalent stress of the maximum normal stress, is the axial stress, is the lateral stress.

[0022] In the above-mentioned lateral vibration and axial superposition fatigue test method, the action position of the lateral vibration is the maximum stress of the working section, and the action position of the lateral vibration is obtained through finite element simulation analysis;

[0023] In the above-mentioned lateral vibration and axial superposition fatigue test method, the lateral vibration action position is set at the center point of the working section, and during the superposition fatigue test process, the strain values ​​on both sides of the center point are collected in real time;

[0024] In the above-mentioned lateral vibration and axial superposition fatigue test method, a mounting hole is provided at the action position of the lateral vibration, and a vibration rod for providing vibration is installed at the mounting hole.

[0025] In the above-mentioned lateral vibration and axial superimposed fatigue test method, in steps 2 and 4, when conducting the superimposed fatigue test, the two supporting sections are first provided with an axial actuation mode and operated until the load waveform is stable, and then the working section is provided with a lateral vibration actuation mode.

[0026] In the above-mentioned lateral vibration and axial superposition fatigue test method, when preparing a test piece of a set style in step 1, a finite element simulation is performed on the test piece to determine the form of the test piece and the natural frequency of the test piece;

[0027] The parameters of the axial actuation mode include the maximum axial stress and the axial test frequency. The parameters of the axial actuation are set according to the yield strength and natural frequency of the test piece material. The maximum axial stress is 0.7 to 0.9 times the yield strength, and the axial test frequency is 0.5 to 0.7 times the natural frequency.

[0028] In the above-mentioned lateral vibration and axial superposition fatigue test method, the axial actuation mode adopts stress control, and the stress ratio R=0.8; the lateral actuation mode adopts force value control, and the stress ratio R=-1.

[0029] In the above-mentioned lateral vibration and axial superposition fatigue test method, the test piece is 7050-T7451 plate, the value of the axial force of the axial actuation mode is selected as 360~550Mpa, 10~70Hz; the value of the lateral force of the lateral actuation mode is selected as 0~120N, 50~70Hz.

[0030] In the above-mentioned lateral vibration and axial superposition fatigue test method, the fatigue test is high-cycle fatigue, and the fatigue test is conducted using a group method: a three-level group method is used for testing, with expected fatigue lives of 50,000, 100,000 to 200,000, and 300,000 to 500,000 respectively;

[0031] In the above-mentioned lateral vibration and axial superimposed fatigue test method, during the superimposed fatigue test, the temperature of the working section is controlled at 0-60°C.

[0032] In the above-mentioned lateral vibration and axial superposition fatigue test method, the set stop conditions are: the test piece breaks; or the test frequency changes by more than ±5%; or the number of cycles reaches 1×10 6 , the test piece is intact.

[0033] In the above-mentioned lateral vibration and axial superposition fatigue test method, if the set stop condition is that the test piece is broken, it is necessary to judge whether the fracture position is valid by observing the fracture position, wherein the fracture position is valid within the working section area of ​​the test piece, and the obtained axial fatigue life and lateral fatigue life are valid data.

[0034] By means of the above technical solution, the present invention has at least the following advantages:

[0035] 1) The present invention obtains the axial fatigue life and lateral fatigue life of the first test type through the first test type, and forms a fitting formula after solving the fitting constants k, m, and n through formula (1) for multiaxial life estimation under non-proportional loading of axial stress and lateral stress;

[0036] 2) The present invention concentrates axial stress on the working section of the test piece and superimposes lateral vibration on the working section, thereby enabling the working section to simultaneously withstand the combined effects of axial conventional fatigue load and lateral vibration fatigue load. Under the combined effects of the two loads, the maximum stress point of the test piece should be located in the working section;

[0037] 3) The present invention can avoid the test piece from breaking at the beginning of the superimposed fatigue test by further limiting the parameters of the axial actuation and the lateral actuation and applying the lateral actuation after the axial actuation is stabilized.

[0038] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Schematic diagram of the structure of a test piece with a strain gauge attached to it according to the present invention;

[0040] Figure 2 It is a structural schematic diagram of the lateral vibration and axial superposition fatigue test device of the present invention;

[0041] Figure 3-1This is the lateral excitation force-strain calibration relationship (axial force 360 ​​MPa, 10 Hz, lateral force 0~120 N, 50 Hz);

[0042] Figure 3-2 is the other lateral excitation force-strain calibration relationship (axial force 400 MPa, 10 Hz, lateral force 0~120 N, 50 Hz);

[0043] Figure 3-3 This is the other lateral excitation force-strain calibration relationship (axial force 430 MPa, 10 Hz, lateral force 0~120 N, 50 Hz);

[0044] Figure 3-4 This is the other lateral excitation force-strain calibration relationship (axial force 520 MPa, 10 Hz, lateral force 0~120 N, 50 Hz);

[0045] Figure 3-5 is the other lateral excitation force-strain calibration relationship (axial force 550 MPa, 10 Hz, lateral force 0~120 N, 50 Hz);

[0046] Figure 3-6 This is the other lateral excitation force-strain calibration relationship (axial force 520 MPa, 10 Hz, lateral force 0~120 N, 70 Hz);

[0047] Figure 3-7 is the other lateral excitation force-strain calibration relationship (axial force 400 MPa, 10 Hz, lateral force 0~120 N, 70 Hz);

[0048] Figure 3-8 This is the other lateral excitation force-strain calibration relationship (axial force 360 ​​MPa, 10 Hz, lateral force 0~120 N, 70 Hz);

[0049] Figure 4-1 It is the SN curve of type 1 of the lateral vibration and axial fatigue superposition test;

[0050] Figure 4-2 It is the SN curve of type 2 of the lateral vibration and axial fatigue superposition test;

[0051] Figure 4-3 It is the SN curve of type 3 of the lateral vibration and axial fatigue superposition test;

[0052] Figure 4-4 Comparison chart of SN curves of lateral vibration and axial fatigue superposition tests. DETAILED DESCRIPTION

[0053] In order to further illustrate the technical means and effects adopted by the present invention to achieve the predetermined purpose of the invention, the specific implementation methods, structures, features and effects of the present invention are described in detail below in conjunction with the accompanying drawings and preferred embodiments.

[0054] The present invention discloses a lateral vibration and axial superposition fatigue test method for measuring or pre-testing the multi-axial fatigue life of a test piece, comprising the following steps:

[0055] Step 1: Prepare a test piece of a set pattern, wherein the middle area of ​​the test piece is a working section, the two end areas of the test piece are clamping sections, and the working section is a stress concentration area;

[0056] Step 2: providing axial actuation modes of equal magnitude and opposite directions to the two supporting sections, and providing lateral vibration actuation mode to the working section, setting the axial actuation mode and the lateral vibration actuation mode as first test parameters, conducting a superimposed fatigue test of the first test type until a set stop condition is set, and obtaining the axial fatigue life and lateral fatigue life of the first test type;

[0057] Step 3: Using the material parameters of the test piece, the maximum axial stress, and the corresponding mean axial fatigue life and mean lateral fatigue life of the first test type as basic data, substitute them into the following formula (1) to solve the fitting constants k, m, and n;

[0058] ;

[0059] Where:

[0060] is the maximum normal stress on the plane of maximum shear strain;

[0061] is the yield strength of the material;

[0062] and are fatigue plasticity coefficient and strength coefficient;

[0063] c is the fatigue plasticity index;

[0064] b is the fatigue strength index;

[0065] N c is the lateral fatigue life;

[0066] N f is the axial fatigue life;

[0067] k, m, and n are fitting constants.

[0068] Step 4: Substitute the fitting constants k, m, and n solved in step 3 into formula (1) to form a multi-axial fatigue life prediction equation. Reset the axial actuation mode and the lateral vibration actuation mode as the Nth test parameters, and use the multi-axial fatigue life prediction equation to solve the axial life and lateral life of the Nth test type, that is, the multi-axial fatigue life.

[0069] In specific implementation, the maximum normal stress The equivalent stress calculation formula of the maximum normal stress is the superimposed stress of the axial stress provided by the axial actuation mode and the lateral stress provided by the lateral actuation mode: ,in, is the equivalent stress of the maximum normal stress, is the axial stress, is the lateral stress.

[0070] Wherein, the axial stress Much greater than the lateral stress .

[0071] Preferably, the axial stress ≥90 times the lateral stress .

[0072] Preferably, the axial stress ≥100 times the lateral stress .

[0073] In actual implementation, if the lateral force is too large, it will cause the multi-axial fatigue system to become unstable (in this case, the test piece will not necessarily be damaged), resulting in the inability to carry out the test normally.

[0074] Through the lateral vibration and axial superposition fatigue test method provided by the present invention, the obtained axial fatigue life mean value and lateral fatigue life mean value are substituted into formula (1) as basic data to obtain the fitting constants k, m, and n. Then, the specific values ​​of the fitting constants obtained by solving the solution are substituted into formula (1) again to form a multi-axial fatigue life prediction equation.

[0075] The present invention obtains the axial fatigue life and lateral fatigue life of the first test type through the first test type. After solving the fitting constants k, m, and n using formula (1), a fitting formula is formed for multiaxial life estimation under non-proportional loading of axial stress and lateral stress. In this way, it is possible to obtain the fitting formula through the test of one test type and perform multiaxial fatigue life prediction for other test types such as lateral vibration and axial superposition fatigue tests. This reduces the time required to obtain different types of multiaxial fatigue life.

[0076] The most common method for predicting multiaxial fatigue life is to equate the stresses and strains under multiaxial stress conditions, assuming that equivalent uniaxial stress amplitudes and multiaxial stresses will produce the same fatigue damage. Ultimately, this equivalent stress (strain) is considered the controlling parameter of the damage process, and the coefficients in the uniaxial Manson-Coffin equation are modified to estimate the fatigue life of the component under multiaxial conditions. The most commonly used method is to use the Von Mises or Tresca yield criteria to equate multiaxial stresses and strains.

[0077] The Von Mises yield criterion states that under certain deformation conditions, when the second invariant J2 of the stress-tension ratio at a point in a stressed object reaches a certain value, the point begins to enter a plastic state, that is:

[0078] ;

[0079] In the formula is the stress in the x-axis direction, is the stress in the y-axis direction, for Axial stress, is the xy plane shear stress, is the shear stress on the yz plane, is the zx plane shear stress.

[0080] The principal stress is expressed as

[0081] ;

[0082] In the formula is the yield point of the material, is the shear yield strength of the material. is the first principal stress, is the second principal stress, is the third principal stress.

[0083] Comparing with the equivalent stress, we can get:

[0084] ;

[0085] Under certain deformation conditions, when the elastic potential energy (elastic deformation energy) per unit volume of the material reaches a certain constant, the material will yield.

[0086] Non-proportional loading can lead to additional cyclic strengthening and cross-hardening effects, ultimately reducing fatigue life. Current approaches to non-proportional fatigue loading primarily utilize the critical plane method, which considers the changes in quantities related to crack growth at the critical plane. The critical plane method for predicting multiaxial fatigue life first identifies the critical damage plane, then constructs the multiaxial fatigue damage parameters by combining various shear and normal stresses on the plane to establish a fatigue life prediction equation.

[0087] In the present invention, 、 、 and The loads are all 0, and the stress in the x-axis direction That is the axial principal stress of the present invention, the xy plane shear stress is the lateral force of the present invention, and thus, the equivalent stress calculation formula of the maximum normal stress is: .

[0088] Considering that the normal stress acting perpendicularly on the maximum shear plane will affect crack formation and propagation, the critical plane method proposed by Fatemi takes into account the influence of the average stress under non-proportional conditions:

[0089] ;

[0090] Where: is the maximum normal stress on the plane of maximum shear strain;

[0091] is the yield strength of the material;

[0092] is the maximum shear strain;

[0093] is the shear modulus;

[0094] Its multiaxial fatigue life equation is:

[0095] ;

[0096] Where: is the maximum normal stress on the plane of maximum shear strain;

[0097] is the yield strength of the material;

[0098] and are fatigue plasticity coefficient and strength coefficient;

[0099] c is the fatigue plasticity index;

[0100] b is the fatigue strength index;

[0101] k is the fitting constant.

[0102] Therefore, considering the influence of lateral fatigue life, the multiaxial fatigue life equation of the present invention is:

[0103] ;

[0104] Where: is the lateral fatigue life;

[0105] k, m, and n are fitting constants.

[0106] The present invention is based on multi-axial fatigue theory, conducts lateral vibration and axial fatigue superposition tests, compares and analyzes the fatigue performance of different test types, and uses the Von Mises equivalent stress life prediction method and the critical plane method to predict the test life. Specifically, the critical plane method requires two strain parameters in describing the fatigue process. The cyclic shear strain and normal normal strain on the maximum shear strain plane must be considered because the cyclic shear strain contributes to crack nucleation, while the normal strain contributes to crack propagation. The critical plane theory not only considers the magnitude of stress and strain in the selection of parameters, but also considers the direction of stress and strain. Therefore, the damage parameters are more meaningful, and it also makes the critical plane theory closer to reality, providing a basis for accurately predicting the life of fatigue components.

[0107] In a specific embodiment of the present invention, lateral vibration and axial conventional fatigue superposition tests are carried out on an aluminum alloy cantilever beam test piece to obtain relevant test data, establish a theoretical life prediction model, and verify the applicability of the theoretical life prediction model based on the test results.

[0108] The present invention predicts the superposition life of the cantilever beam's lateral vibration and axial conventional fatigue based on the test results of the first test type, combined with a multi-axis theoretical model, and establishes a life prediction model that meets the accuracy requirements.

[0109] In a specific implementation, to achieve the lateral vibration action position being set at the maximum stress of the working section, the lateral vibration action position is determined through finite element simulation analysis. Alternatively, the lateral vibration action position is directly set at the center point of the working section, and during the superimposed fatigue test, the strain values ​​on both sides of the center point are collected in real time. In a specific implementation, the strain peak and valley values ​​are collected in real time. After the load is determined, the strain peak and valley values ​​are fixed within the elastic range. In order to provide lateral vibration to the working section, a mounting hole is provided at the lateral vibration action position, and a vibration rod for providing vibration is installed at the mounting hole.

[0110] Because the lateral vibration fatigue load is subject to the overall stiffness of the test system, the axial conventional fatigue load must be applied first before the lateral vibration fatigue load can be applied. Conversely, the lateral vibration fatigue load may be affected by the axial conventional fatigue load, and the entire test will be abnormally shut down. In order to ensure that the test piece can vibrate smoothly and does not break, in steps 2 and 4, when conducting the superimposed fatigue test, the two supporting sections are first provided with an axial actuation mode and run until the load waveform stabilizes, and then the working section is provided with a lateral vibration actuation mode.

[0111] The present invention controls the amount of axial force by designing and finite element simulation of the test piece, so that the lateral test of the superimposed fatigue test can be carried out within a reasonable range. Specifically, when preparing a test piece of a set style in step 1, the test piece is subjected to finite element simulation to determine the form of the test piece and the natural frequency of the test piece; the parameters of the axial actuation method include the maximum axial stress and the axial test frequency, and the parameters of the axial actuation are set according to the yield strength and natural frequency of the test piece material. The maximum axial stress is 0.7 to 0.9 times the yield strength, and the axial test frequency is 0.5 to 0.7 times the natural frequency. If the axial force is too large, the specimen will be damaged prematurely. If the axial force is too small, the specimen will not be damaged until it reaches the specified fatigue life (such as 10 million times). The test data obtained in both cases cannot be used in engineering.

[0112] During the experiment, the axial test frequency will affect the vibration state of the test piece. If the frequency is too high, the test piece will resonate, which will prevent the test from proceeding normally. At the same time, if the frequency is too low, the test cycle will be extended, wasting test time. Therefore, it is necessary to limit the axial test frequency to 0.5 to 0.7 times the natural frequency. If the frequency is too high, the test piece will resonate and the test will not proceed normally. If the frequency is too low, the test cycle will be extended, wasting test time.

[0113] The lateral test frequency should be far away from the resonance frequency of the test piece, so that the test can be carried out smoothly and the fracture of the test piece due to resonance can be avoided.

[0114] The lateral test frequency must be ≤ the resonant frequency - 30. The test frequency must not exceed the resonant frequency, otherwise the system will resonate at the resonant frequency. For example, at the resonant frequency (sweep resonant frequency of approximately 100 Hz) of a specific embodiment of the present invention, when the lateral frequency of test type 3 is 70 Hz, the fatigue performance is worse than that of test type 1 and test type 2 (lateral test frequency of 50 Hz).

[0115] The axial actuation mode uses stress control with a stress ratio of R = 0.8; the lateral actuation mode uses force control with a stress ratio of R = -1. The stress ratio refers to the ratio of maximum stress to minimum stress.

[0116] The axial test adopts stress control with stress ratio R=0.8, and the lateral test adopts force value control with stress ratio R=-1. The test waveform is a sine wave. One piece of each test type is selected for lateral excitation force-strain relationship calibration to obtain the minimum cross-section excitation force-strain relationship curve of the test piece. The nominal stress (i.e., maximum stress) at the minimum cross-section can be calculated through the calibration relationship. The strain gauge is attached near the minimum cross-section mounting hole of the test piece, such as Figure 1 shown.

[0117] Specifically, the test piece is a 7050-T7451 plate, and the value of the axial force of the axial actuation mode is selected to be 360~550Mpa, 10~70Hz; the value of the lateral force of the lateral actuation mode is selected to be 0~120N, 50~70Hz.

[0118] The material used for the lateral vibration and axial conventional superposition fatigue test is 7050-T7451 aluminum alloy. The test matrix is ​​shown in Table 1 below.

[0119] Table 1 Lateral vibration and axial fatigue superposition test matrix

[0120]

[0121] The raw material was 7050-T7451 sheet (L*T*S: 1250×355×88.9mm). The sheet was cut using a bandsaw and wire cut to produce sheets (L*T*S: 122×32×5mm) for test piece fabrication. The design of the superimposed lateral vibration and axial fatigue test piece was based on the standard HB 5277-84, "Test Methods for Vibration Fatigue of Engine Blades and Materials." To accommodate lateral loading, a mounting hole was added to the center of the cantilever beam, and the test piece width was increased accordingly. After fabrication, all test pieces were inspected using a universal tool microscope. Optical instruments were used to inspect the working surfaces and measure key dimensions. Test specimens were strictly inspected according to the drawing specifications. The actual dimensions of the test pieces were within the drawing tolerances, and the surface roughness met the drawing requirements, with no scratches, detectable cracks, or obvious defects.

[0122] Specifically, the length of the test piece is 120±0.2 mm, the width of the test piece is 30±0.2 mm, the thickness of the test piece is 3±0.05 mm, the diameter of the mounting hole is 5.5, two strain gauges are arranged on both sides of the mounting hole, and the distance between the two strain gauges is 14±1 mm.

[0123] The lateral vibration and axial fatigue superposition test mainly uses an electro-hydraulic servo fatigue testing machine to apply axial conventional fatigue load, and the lateral vibration load is applied through an exciter. The test system consists of the following components: Figure 2 The test equipment and measuring tools are shown in Table 2.

[0124] Table 2 List of equipment for lateral vibration and axial fatigue superposition test

[0125]

[0126] In practice, the test piece is aligned with a centering line and clamped vertically to ensure the axial load passes through the center of the test piece. Lateral loads are applied using a vibrator, which is adjusted to a horizontal direction. The vibrator rod is bolted to the center mounting hole of the test piece. Silicone gaskets are added to the contact surface between the bolt and the test piece to prevent fretting fatigue.

[0127] Specifically, the process of the superimposed fatigue test is:

[0128] Before the test, check the key dimensions and appearance quality of the test piece and fill in the original record sheet;

[0129] Install the test piece and fixture as required;

[0130] First, set the axial test parameters and start the electro-hydraulic servo fatigue testing machine;

[0131] Set the lateral vibration fatigue parameters and start the lateral exciter after the axial test runs stably.

[0132] During the test, an infrared thermometer is used to measure the temperature of the working section of the test piece. If the temperature exceeds 60°C, the cooling air volume needs to be increased to keep the measured temperature of the working section of the test piece within 60°C (i.e., the temperature of the working section is controlled between room temperature and 60°C).

[0133] Formal testing is conducted using a three-level grouping method, with life expectancies of approximately 50,000, 100,000 to 200,000, and 300,000 to 500,000 years, respectively. A maximum of five units are tested at each level.

[0134] After the test piece breaks, observe the fracture location and determine whether the fracture location is valid (fracture within the working section is valid data); fill in the original record sheet;

[0135] After the set stop condition is reached, the test piece is removed, photographed, and then stored in a drying dish to preserve the fracture morphology of the test piece.

[0136] The set stop conditions are: the test piece breaks; or the test frequency changes by more than ±5%; or the number of cycles reaches 1×10 6 , the test piece is intact.

[0137] During the specific test process, the test frequency is monitored and displayed in real time. By setting a set frequency setting value, if the test frequency exceeds the set value, the device will automatically stop testing.

[0138] If the set stop condition is that the test piece is broken, it is necessary to observe the fracture position to determine whether the fracture position is valid, wherein the fracture position is valid within the working section area of ​​the test piece, and the obtained axial fatigue life and lateral fatigue life are valid data.

[0139] The processing method of the present invention for the experimental data obtained in the superposition fatigue experiment is as follows:

[0140] 1) Lateral excitation force and strain calibration

[0141] For each group of test pieces, 1 to 2 pieces need to be selected for lateral excitation force and strain calibration. The collected lateral excitation force and strain values ​​are linearly fitted using the least squares method to obtain the fitting formula shown below.

[0142] (1)

[0143] Where: y is the mean of the absolute values ​​of the maximum strain (corresponding to strain gauges No. 1 and No. 3);

[0144] x is the target lateral exciting force value;

[0145] a and b are fitting coefficients.

[0146] 2) Stress-strain conversion

[0147] During the test, the strain gauge collects the strain value of the working section of the test piece, and then the measured stress value on the test piece is calculated using the following formula.

[0148] (2)

[0149] Where: is the stress value measured by the strain gauge;

[0150] is the elastic modulus;

[0151] is the strain value.

[0152] 3) Group data processing

[0153] Group method median logarithmic fatigue life Calculate according to formula (3)

[0154] (3)

[0155] Where: N i is the fatigue life of the i-th specimen in each test group;

[0156] n is the number of test pieces in each group;

[0157] N 50 is the fatigue life with a 50% survival rate.

[0158] The standard deviation S of fatigue life is calculated as:

[0159] (4)

[0160] Coefficient of variation C v for: .

[0161] 4) Shear stress calculation

[0162] The calculation formula of the shear stress on the test piece is:

[0163] (5)

[0164] Among them, F s is the test shear force, b is the cross-sectional width, and h is the cross-sectional thickness.

[0165] The test results of the superposition fatigue test of the present invention are as follows:

[0166] 1) Calibration results

[0167] The three curves were used to perform lateral excitation force-strain calibration for eight working conditions. For detailed results, see Figure 3-1 to Figure 3-8 The calibration results show that the lateral excitation force-strain relationship is linear and the system is stable during the test.

[0168] 2) SN curve

[0169] Since the axial stress in the lateral vibration and axial fatigue tests is much greater than the lateral shear force, the fatigue SN curve is drawn with the axial stress and life, with a total of 3 curves. The specific results are shown in Figure 4-1 to Figure 4-3 :

[0170] The data of experimental type 1 are shown in Table 3-1. Fitting equation , regression coefficient of determination r 2 is 0.926, and the SN curve fits well. Figure 4-1 shown.

[0171] Table 3-1 Test data of test type 1

[0172]

[0173] The data of experimental type 2 are shown in Table 3-2. Fitting equation , regression coefficient of determination r 2 is 0.918, and the SN curve fits well. Figure 4-2 shown.

[0174] Table 3-2 Test data of test type 2

[0175]

[0176] The data of experimental type 3 are shown in Table 3-3. Fitting equation , regression coefficient of determination r 2 The SN curve is 0.768, and the fitting degree of the SN curve is average. Figure 4-3 The comparison of SN curves of three test types is shown in Figure 4-4 As shown in the figure, we can see that the fatigue performance of test type 2 is better than that of test type 1. This is because the lateral exciting force of test type 2 is smaller than that of test type 1, which is in line with the test law; the fatigue resistance of test type 2 is higher than that of test type 3. The lateral exciting force of the two is consistent, and test type 3 is closer to the lateral resonance frequency (the sweep resonance frequency is about 100 Hz), which may be one of the reasons for its worse fatigue performance.

[0177] Table 3-3 Test data of test type 3

[0178]

[0179] 3) Von Mises equivalent stress life prediction

[0180] The maximum normal stress The equivalent stress calculation formula of the maximum normal stress is the superimposed stress of the axial stress provided by the axial actuation mode and the lateral stress provided by the lateral actuation mode: ,in, is the equivalent stress of the maximum normal stress, is the axial stress, is the lateral stress.

[0181] In this embodiment, due to the force-value control used in the lateral actuation method, the equivalent stress is calculated by first calculating the lateral force value to obtain the corresponding stress. In this embodiment, the lateral force for test type 1 / 3 is 120N, corresponding to a stress of 4MPa; the corresponding stress for test type 2 is 2MPa.

[0182] Calculation of the equivalent stress of test types 1 and 3 according to Von Mises:

[0183] ;

[0184] ;

[0185] ;

[0186] Calculation of test type 2 equivalent stress according to Von Mises:

[0187] ;

[0188] ;

[0189] ;

[0190] Combined with the uniaxial stress fatigue SN curve, the conventional fatigue life prediction method is used to predict the multiaxial fatigue life. 7050-T7451 axial conventional fatigue test SN curve fitting equation , regression coefficient of determination r 2 The SN curve fit is 0.999, indicating a good fit (see the cantilever beam and axial element fatigue test summary report). The axial life prediction results are shown in Table 3-4. The results show that the life prediction method based on multiaxial stress equivalent to uniaxial stress differs significantly from the actual results. This is because uniaxial fatigue theory is effective for dealing with complex multiaxial fatigue problems under proportional loading. However, fatigue behavior under non-proportional loading differs significantly from that under uniaxial or proportional multiaxial fatigue loading. This is especially true under non-proportional variable amplitude loading, where simple cycle counting, as with uniaxial loading, is not feasible. Therefore, simply using traditional uniaxial fatigue theory to predict fatigue damage can result in significant errors.

[0191] Table 3-4 Axial life prediction results

[0192]

[0193] 4) Life prediction results of critical plane method

[0194] Due to the limited accuracy of the Von Mises equivalent stress life prediction method, the present invention uses formula (1) for prediction. The lateral vibration and axial fatigue test results of the 7050-T7451 aluminum alloy cantilever beam are detailed in Tables 3-5 to 3-10.

[0195] Table 3-5 Test Type 1 Raw Data

[0196]

[0197] Table 3-6 Test type 1 test processing data

[0198]

[0199] Table 3-7 Test Type 2 Raw Data

[0200]

[0201] Table 3-8 Test type 2 test processing data

[0202]

[0203] Table 3-9 Raw data of test type 3

[0204]

[0205] Table 3-10 Test type 3 test processing data

[0206]

[0207] Based on the lateral vibration and axial fatigue test data, the axial life of the test type is predicted. First, the mean axial life and mean lateral life after test type 1 are used as basic data and substituted into formula (1) to solve the fitting constants k, m, and n.

[0208] According to the tensile test, the elastic modulus of the 7050-T7451 material is 70.55 GPa and the yield strength of the material is 466.5 MPa; according to the data, the shear modulus of the material is 26.9 GPa.

[0209] Referring to the LY12CZ material, the fatigue plasticity index c is -0.654, the plasticity coefficient is 0.137; the fatigue strength index b is -0.063, and the strength coefficient is 724MPa.

[0210] Substituting the known parameters into formula (1) yields the fitting constants k = -1, m = -0.013, and n = -0.0712. The multiaxial fatigue life prediction equation is then written as:

[0211] ;

[0212] The multiaxial fatigue life prediction equation is used to predict the axial life of test type 2 and test type 3. The results are shown in Table 3-11.

[0213] Table 3-11 Axial life prediction results

[0214]

[0215] Summary: Fatigue test data is generally highly dispersed. This patent successfully predicts the multi-axial fatigue life of other working conditions through a set of basic test data. The prediction accuracy is no more than 15%, and the prediction results can meet the engineering design requirements.

[0216] Fracture analysis of representative specimens from the three test types revealed typical fatigue fracture characteristics, including a crack source, a propagation zone, and a transient fracture zone. The propagation zone exhibited a radial pattern centered on the source zone. Cracks primarily originated at the corners of the specimens, and the crack source zone was free of material defects, inclusions, or pores.

[0217] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Any simple modifications, equivalent changes and modifications made to the above embodiment based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A lateral vibration and axial superposition fatigue test method, characterized in that: Used to measure or pre-test the multi-axial fatigue life of the test piece, including the following steps: Step 1, preparing a test piece of a set pattern, wherein the middle area of ​​the test piece is a working section, the two end areas of the test piece are clamping sections, and the working section is a stress concentration area; Step 2, respectively providing axial actuation modes of the same size and opposite directions to the two supporting sections, providing a lateral vibration actuation mode to the working section, setting the axial actuation mode and the lateral vibration actuation mode as first test parameters, carrying out a superimposed fatigue test of the first test type until a set stop condition is set, and obtaining the axial fatigue life and lateral fatigue life of the first test type; Step 3, using the material parameters of the test piece, the maximum axial stress, and the corresponding axial fatigue life mean and lateral fatigue life mean of the first test type as basic data, substitute into the following formula (1) to solve the fitting constants k, m, and n; ； Where: is the maximum normal stress on the plane of maximum shear strain; is the yield strength of the material; and are fatigue plasticity coefficient and strength coefficient; c is the fatigue plasticity index; b is the fatigue strength index; N c is the lateral fatigue life; N f is the axial fatigue life; k, m, and n are fitting constants; Step 4, substituting the fitting constants k, m, and n solved in step 3 into formula (1) to obtain a multi-axial fatigue life prediction equation, resetting the axial actuation mode and the lateral vibration actuation mode as the Nth test parameters, and using the multi-axial fatigue life prediction equation to solve the axial life and lateral life of the Nth test type, that is, the multi-axial fatigue life.

2. The lateral vibration and axial superposition fatigue test method according to claim 1 is characterized in that: The maximum normal stress The equivalent stress calculation formula of the maximum normal stress is the superimposed stress of the axial stress provided by the axial actuation mode and the lateral stress provided by the lateral actuation mode: ,in, is the equivalent stress of the maximum normal stress, is the axial stress, is the lateral stress.

3. The lateral vibration and axial superposition fatigue test method according to claim 1 is characterized in that: The action position of the lateral vibration is the maximum stress position of the working section, and the action position of the lateral vibration is obtained through finite element simulation analysis; or The action position of the lateral vibration is set at the center point of the working section, and during the superposition fatigue test, the strain values ​​on both sides of the center point are collected in real time; A mounting hole is provided at the action position of the lateral vibration, and a vibration rod for providing vibration is installed at the mounting hole.

4. The lateral vibration and axial superposition fatigue test method according to claim 1 is characterized in that: In step 2 and step 4, when carrying out the superimposed fatigue test, the two supporting sections are firstly provided with an axial actuation mode until the load waveform is stable, and then the working section is provided with a lateral vibration actuation mode.

5. The lateral vibration and axial superposition fatigue test method according to claim 1 is characterized in that: When preparing a test piece of a set style in step 1, a finite element simulation is performed on the test piece to determine the form of the test piece and the natural frequency of the test piece; The parameters of the axial actuation mode include axial maximum stress and axial test frequency. The parameters of the axial actuation are set according to the yield strength and natural frequency of the test piece material. The axial maximum stress is 0.7 to 0.9 times the yield strength, and the axial test frequency is 0.5 to 0.7 times the natural frequency.

6. The lateral vibration and axial superposition fatigue test method according to claim 1 is characterized in that: The axial actuation mode adopts stress control, and the stress ratio R=0.8; the lateral actuation mode adopts force value control, and the stress ratio R=-1.

7. The lateral vibration and axial superposition fatigue test method according to claim 1 is characterized in that: The test piece is a 7050-T7451 plate, and the value of the axial force of the axial actuation mode is selected to be 360~550Mpa, 10~70Hz; the value of the lateral force of the lateral actuation mode is selected to be 0~120N, 50~70Hz.

8. The lateral vibration and axial superposition fatigue test method according to claim 1 is characterized in that: The fatigue test is high cycle fatigue, and the fatigue test is carried out by group method: the test is carried out by 3-level group method, and the expected fatigue life is 50,000, 100,000 to 200,000, and 300,000 to 500,000 respectively; During the superposition fatigue test, the temperature of the working section is controlled to be between room temperature and 60°C.

9. The lateral vibration and axial superposition fatigue test method according to claim 1 is characterized in that: The setting stop condition is: The test piece breaks; or The test frequency varies by more than ±5%; or The number of cycles reached 1×10 6 , the test piece is intact.

10. The lateral vibration and axial superposition fatigue test method according to claim 9, characterized in that: If the set stop condition is that the test piece breaks, it is necessary to determine whether the fracture position is valid by observing the fracture position, wherein the fracture position is valid within the working section of the test piece, and the obtained axial fatigue life and lateral fatigue life are valid data.

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