A lateral vibration and axial superposition fatigue test method

Abstract

The present invention belongs to the field of fatigue testing 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 to the test piece, setting a first test parameter, carrying out a superposition fatigue test of a first test type until a stop condition is set, and obtaining the axial fatigue life and lateral fatigue life of the first test type; solving a fitting constant k、m、 n After that, a multi-axial fatigue life prediction equation is formed, and the axial actuation mode and the lateral vibration actuation mode are reset as the Nth test parameters. The multi-axial fatigue life prediction equation is used to solve the axial life and lateral life of the Nth test type, that is, the multi-axial fatigue life. The method provided by the present invention can realize the prediction of the multi-axial fatigue state under non-proportional loading.

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 of civil helicopters, some important parts and components are subject to superposition of local vibration fatigue loads and conventional fatigue loads, that is, multiaxial fatigue exists in the parts and components. The multiaxial fatigue estimation method is mainly based on three criteria: the first is the maximum principal stress / strain criterion. This criterion holds that even under multiaxial stress state, the damage of the material is mainly caused by the maximum principal stress (principal strain) and has nothing to do with other factors. The second is the von Mises equivalent stress / strain criterion, which holds that under multiaxial stress-strain state, the damage of the material is mainly controlled by the von Mises equivalent stress and strain. The third is the Tresca maximum shear stress / shear strain criterion, which simply attributes the damage to being controlled only by the maximum shear stress / shear strain. For proportional loading conditions, these criteria are effective, simple and practical. However, under non-proportional loading conditions, especially when hydrostatic pressure affects fatigue life, the above criteria cannot give good prediction results. Therefore, these equivalent strain / stress theories cannot be well applied to multiaxial life estimation under non-proportional loading.

[0003] In actual engineering applications, the fatigue life of important helicopter components is often in a multi-axial fatigue state under non-proportional loading. Therefore, in order to systematically study the fatigue life of important helicopter components under vibration fatigue loads and conventional fatigue loads, it is necessary to design a lateral vibration and axial conventional superposition fatigue test method. Summary of the invention

[0004] In order to estimate the multi-axial life of a plate test piece under a condition where vibration fatigue load and conventional fatigue load are superimposed under non-proportional loading, the present invention provides a lateral vibration and axial conventional superimposed 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 pre-test the multi-axial fatigue life of the test piece, including the following steps:

[0007] 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;

[0008] 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;

[0009] 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;

[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 by finite element simulation analysis;

[0023] In the above-mentioned lateral vibration and axial superposition fatigue test method, 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;

[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 step 2 and step 4, when conducting the superimposed fatigue test, the two supporting sections are first 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.

[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 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.

[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 a 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 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;

[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), which is used for multi-axial 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, so that the working section can simultaneously bear the superposition of axial conventional fatigue load and lateral vibration fatigue load. Under the superposition of the two loads, the maximum stress of the test piece should be located in the working section;

[0037] 3) The present invention can avoid the situation where the test piece breaks at the beginning of the superposition fatigue test by further limiting the parameters of the axial action and the lateral action and applying the lateral action after the axial action is stable.

[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 in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is a schematic structural diagram of a test piece with a strain gauge attached thereto 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 3-1It is the lateral exciting force-strain calibration relationship (axial force 360MPa, 10Hz, lateral force 0~120N, 50Hz);

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

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

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

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

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

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

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

[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 It is a comparison chart of SN curves of lateral vibration and axial fatigue superposition test. DETAILED DESCRIPTION

[0053] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the specific implementation method, structure, characteristics and effects of the present invention application are described in detail below in combination with the accompanying drawings and preferred embodiments.

[0054] The present invention discloses a lateral vibration and axial superposition fatigue test method, which is used to measure or pre-test the multi-axial fatigue life of a test piece, and comprises the following steps:

[0055] 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;

[0056] 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;

[0057] 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;

[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] Among them, 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, the multi-axial fatigue system will become unstable (at this time, 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 average axial fatigue life and the average lateral fatigue life are obtained as basic data, which are substituted into formula (1) to obtain fitting constants k, m, and n, and then the specific values ​​of the fitting constants obtained by solving the solution are re-substituted into formula (1) 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, and forms a fitting formula after solving the fitting constants k, m, and n through formula (1), which is used for multi-axial life estimation under non-proportional loading of axial stress and lateral stress. In this way, it is possible to obtain the fitting formula through a test of one test type to predict the multi-axial fatigue life of lateral vibration and axial superposition fatigue tests of other test types. Thereby reducing the time for obtaining different types of multi-axial fatigue life.

[0076] The most common method for predicting multiaxial fatigue life is to equate stress and strain under multiaxial stress state, assuming that equivalent uniaxial stress amplitude and multiaxial stress will produce the same fatigue damage. Finally, the equivalent stress (strain) is regarded as the control parameter of the damage process, and the coefficient of the Manson-Coffin equation under uniaxial state is corrected, and finally the fatigue life of the component under multiaxial state is estimated. Among them, the most commonly used method is to use Von Mises or Tresca yield criterion to equate multiaxial stress and strain.

[0077] The Von Mises yield criterion means that under certain deformation conditions, when the second invariant J2 of the stress partial tension at a point in the stressed object reaches a certain value, the point begins to enter the 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 shear stress in the xy plane, 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] Compared 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 will cause additional cyclic strengthening and cross-hardening effects, which will eventually reduce fatigue life. The current method for dealing with non-proportional fatigue loading mainly considers the critical plane method, that is, considering the changes in quantities related to crack extension on the critical plane. The critical plane method for predicting multi-axial fatigue life first finds the critical damage plane, then combines the shear and normal stresses on the plane in various ways to construct multi-axial fatigue damage parameters and establish fatigue life prediction equations.

[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, therefore, 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 multi-axial 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-axis 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, and 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, and 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 lateral vibration and axial conventional fatigue of the cantilever beam based on the test results of the first test type, combined with the multi-axis theoretical model, and establishes a life prediction model that meets the accuracy requirements.

[0109] In a specific implementation, in order to set the action position of the lateral vibration at the maximum stress of the working section, the action position of the lateral vibration is obtained through finite element simulation analysis. Alternatively, the action position of the lateral vibration is directly 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. 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 action position of the lateral vibration, 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 lateral vibration fatigue load can only be applied after the axial conventional fatigue load. On the contrary, 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 enable the test piece to vibrate smoothly and not break, in steps 2 and 4, when conducting the superimposed fatigue test, first provide the two supporting sections with axial actuation until the load waveform is stable, and then provide the working section with lateral vibration actuation.

[0111] The present invention controls the amount of axial force by designing and finite element simulating 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, 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 the maximum axial stress and the axial test frequency, and the parameters of the axial actuation are set according to the yield strength and the 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 these two 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, resulting in the inability to conduct the test normally. At the same time, if the frequency is too small, 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 cannot be conducted normally. If the frequency is too small, 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 value of the lateral test frequency is ≤ the value of the resonance frequency - 30, and the test frequency cannot exceed the resonance frequency, otherwise the system will resonate at the resonance frequency. For example, the resonance frequency of the specific embodiment of the present invention (sweep resonance frequency of about 100Hz), when the lateral frequency of test type 3 is 70Hz, the fatigue performance is worse than that of test type 1 and test type (lateral test frequency is 50Hz).

[0115] 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. The stress ratio refers to the ratio of the maximum stress to the minimum stress.

[0116] The axial test adopts stress control, and the stress ratio is R=0.8. The lateral test adopts force value control, and the stress ratio is 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 pasted 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] Among them, the raw material is 7050-T7451 plate (L*T*S: 1250×355×88.9mm), which is cut by band saw and wire cutting to prepare plate (L*T*S: 122×32×5mm) for test piece processing. In the specific implementation, the design of the lateral vibration and axial fatigue superposition test piece is designed with reference to the standard HB 5277-84 "Vibration fatigue test method for engine blades and materials". In order to realize lateral loading, a mounting hole is added to the center of the cantilever beam, and the width of the test piece is also increased accordingly. After the test piece is processed, a universal tool microscope is used to inspect all test pieces, and optical instruments are used to inspect the working surface of the test piece, and the key dimensions are measured. The test piece inspection is strictly carried out in accordance with the drawing requirements. The actual size of the test piece is within the tolerance range of the drawing, and the surface roughness meets the requirements of the drawing. There are 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 loads, 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 the specific implementation, the centering line is drawn on the test piece, and the upper and lower centering is clamped to ensure that the axial load passes through the center of the test piece. The lateral load is applied through the exciter, and the load direction of the exciter is adjusted to the horizontal direction. The excitation rod is connected to the center mounting hole of the test piece with bolts, and a silicone gasket is added to the contact surface between the bolt and the test piece at the connection to prevent micro-motion 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 vibration 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 to be room temperature ~ 60°C);

[0133] The formal test is conducted by group method: a three-level group method is used for testing, with expected lifespans of 50,000, 100,000 to 200,000, and 300,000 to 500,000 respectively, and a maximum of 5 items are tested at each level;

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

[0135] After reaching the set stop condition, 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 equipment will automatically stop the test.

[0138] If the set stop condition is that the test piece breaks, 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 of the test piece, and the obtained axial fatigue life and lateral fatigue life are valid data.

[0139] For the experimental data obtained in the superposition fatigue experiment, the processing method of the present invention is:

[0140] 1) Lateral excitation force and strain calibration

[0141] From each group of test pieces, 1 to 2 pieces need to be selected for calibration of lateral exciting force and strain. The collected lateral exciting 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 by 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) Grouping 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 group of tests;

[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 shear stress calculation formula of 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 for 8 working conditions of lateral excitation force-strain calibration. For details, see Figure 3-1 to Figure 3-8 The calibration results show that the lateral exciting force-strain is in a linear relationship 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. There are 3 curves in total. The specific results are shown in Figure 4-1 to Figure 4-3 :

[0170] The processed data of test 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 processed data of test type 3 are shown in Table 3-3. Fitting equation , regression coefficient of determination r 2 is 0.768, and the SN curve fit 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, it can be seen 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 anti-fatigue performance of test type 2 is higher than that of test type 3. The lateral exciting forces of the two are consistent, and test type 3 is closer to the lateral resonance frequency (the sweep resonance frequency is about 100Hz), 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, in the specific calculation process, since the lateral actuation mode adopts force value control, when calculating the equivalent stress, the force value of the lateral force is first calculated to obtain the corresponding stress. The lateral force of test type 1 / 3 of this embodiment is 120N, and the corresponding stress is 4MPa; the corresponding stress of test type 2 is 2MPa.

[0182] Calculation of equivalent stress for 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 is 0.999, and the SN curve fits well (see the cantilever beam and axial element fatigue test summary report). The axial life prediction results are shown in Table 3-4. From the results, it can be seen that the life prediction method of multi-axial stress equivalent to uniaxial stress is quite different from the actual results. This is because the use of uniaxial fatigue theory to deal with complex multi-axial fatigue problems is effective under proportional loading. However, the fatigue behavior under non-proportional loading is far different from the uniaxial or proportional multi-axial fatigue loading characteristics, especially under non-proportional variable amplitude loading. Simple cycle counting cannot be performed like uniaxial loading. Therefore, simply using traditional uniaxial fatigue theory to predict its fatigue damage will result in large 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 shown in Tables 3-5 to 3-10.

[0195] Table 3-5 Test type 1 test raw data

[0196]

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

[0198]

[0199] Table 3-7 Raw data of test type 2

[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 the 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 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, and 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), we can obtain the fitting constants k=-1, m=-0.013, n=-0.0712. Then the multiaxial fatigue life prediction equation can be written as:

[0211] ;

[0212] The multi-axial 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. The patent of this invention 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] The fracture analysis of typical specimens of the three test types showed that the specimens all showed typical fatigue fracture characteristics, including crack source, expansion zone and instantaneous fracture zone. With the source zone as the center, the expansion zone showed radiating characteristics. Cracks mostly originated from the corners of the test pieces, and the materials in the crack source zone had no material defects, inclusions, pores, etc.

[0217] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention still falls 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 clamping 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 clamping 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 judge 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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