A fatigue curve determination device and method based on electromagnetic force and resonance equation

Abstract

The present invention provides a fatigue curve measuring device and method based on electromagnetic force and resonance equation. The fatigue curve measuring device comprises a frame, a three-jaw chuck, a tailstock, an electromagnetic force loading component, a power control module, an oscilloscope, and a strain sensor. The measuring device provided by the present invention is simple and convenient to operate. According to the principles of resonance and electromagnetic force, the current amplitude required to be connected for the initial test can be directly calculated by using computer technology, and the load and frequency of the test loading can be adjusted according to the waveform and frequency of the load borne by the test piece under actual working conditions, so that the natural frequency of the test piece can be controlled, so that the test result is closer to the actual working condition and the obtained data is more in line with the actual situation. A stress diagram is directly obtained by using a strain gauge sensor, and the result is more accurate. The present invention has greatly improved principle control, data processing and result accuracy, and further improves the credibility of the data.

Description

Technical Field

[0001] The invention relates to the field of material fatigue performance testing machines, and in particular to a fatigue curve measuring device and a measuring method based on electromagnetic force and resonance equation. Background Art

[0002] The fatigue phenomenon of metal materials has a great influence on the performance of products. In order to detect the fatigue performance of metal materials, fatigue testing machines are developed. Fatigue testing machines are mainly used to measure the fatigue characteristics, fatigue life and other fatigue performance tests of metals and their alloys under tension, compression, bending or composite alternating loads at room temperature. Existing fatigue testing machines usually use mechanical, hydraulic and motor loading, which have large load impact and strong noise. There are also problems such as the load is not easy to load on dangerous sections due to space limitations. At present, fatigue testing machines that use electromagnetic force as a loading method also have some problems. For example, the measurement process is more cumbersome due to the insufficiency of the control principle, the resonance cannot be controlled, and the unavoidable system errors lead to inaccurate measurement results. Summary of the invention

[0003] In order to improve the use efficiency, accuracy and data credibility of a fatigue testing machine using electromagnetic force as a loading method, the present invention provides a fatigue curve measuring device and method based on electromagnetic force and resonance equation, the fatigue curve measuring device comprises a frame, a three-jaw chuck, a tailstock, an electromagnetic force loading component, a power control module, an oscilloscope and a strain sensor, the upper part of the frame is provided with a tailstock slide rail, and side slide rails are respectively provided on both sides; the three-jaw chuck is arranged at one end of the frame, the tailstock is arranged on the tailstock slide rail and is slidably connected to the tailstock slide rail, the three-jaw chuck and the tailstock are arranged oppositely, and a tested piece is fixed between the three-jaw chuck and the tailstock; two groups of electromagnetic force loading components are respectively arranged on the side slide rails on both sides of the frame and are slidably connected to the side slide rails, and the excitation coils of the two groups of electromagnetic force loading components are arranged oppositely and are located on both sides of the tested piece; the power control module and the oscilloscope are arranged at one end of the frame, and the electromagnetic force loading component is connected to the power control module through a wire; the strain sensor is arranged on the tested piece and is connected to the oscilloscope through a wire.

[0004] The electromagnetic force loading component includes a component body, a slider, a fixed valve, an iron core and an excitation coil. The lower end of the component body is provided with a slider and a fixed valve respectively, which are slidably connected to the side slide rails on both sides of the frame through the slider, and the electromagnetic force loading component is fixed on the side slide rails by screwing the fixed valve; the iron core is arranged on one side of the upper part of the component body, and the excitation coil is wound on the iron core; the excitation coil is connected to the positive and negative poles of the power supply of the power control module through wires.

[0005] The fixing valve has a locking screw inside, which crosses the assembly body and comes out from the end of the slider; the fixing valve is rotated to make the locking screw pass through the slider and abut against the side slide rail, so that the electromagnetic force loading assembly is fixed. The slider of the electromagnetic force loading assembly is provided with a damping hole, and the side slide rail of the frame is provided with a damping screw. The damping hole is sleeved on the damping screw, which is used to give the electromagnetic force loading assembly a certain sliding friction resistance, so that the electromagnetic force loading assembly moves more smoothly.

[0006] A sliding seat is provided at the lower part of the tailstock, and the tailstock slide rail is a T-shaped slide rail. The tailstock is slidably connected with the tailstock slide rail through the sliding seat at the lower part.

[0007] The power control module includes a power supply, a power control chip, a current amplitude display screen, a current waveform display screen, a current amplitude adjustment button, a current waveform adjustment button, a current signal output key and a positive and negative pole of the power supply. The current amplitude display screen, the current waveform display screen, the current amplitude adjustment button, the current waveform adjustment button and the current signal output key are respectively connected to the power control chip, and the power supply output current amplitude and output waveform can be adjusted; the power control chip is connected to the power supply, and the positive and negative poles of the power supply are connected to the power supply.

[0008] The strain sensor comprises four strain gauges with equal resistance values, and the four strain gauges are arranged on the surface of the tested piece by using a bridge circuit (ie, an electric bridge) full bridge connection method.

[0009] The present invention uses the above fatigue curve measuring device based on electromagnetic force and resonance equation to perform the measurement as follows:

[0010] (1) Data preparation before the test: Query the load frequency and load waveform of the test piece's application conditions, measure the specimen's elastic modulus E, and use the formula:

[0011]

[0012] Substitute the natural frequency of the part made of this specimen material under known working conditions into the formula ω r , then we can solve a certain I z The combination of ω and L, or substituting one of the quantities of the existing specimen into the value of the other quantity, select the specimen length L and the specimen cross-sectional dimensions and record them; where ω r is the natural frequency, in rad / s; r is the order of the natural frequency; E is the elastic modulus of the material, in MPa; L is the length of the test piece, in m; m is the mass of the beam per unit length, in kg / m; I Z is the moment of inertia of the specimen cross section, in m 4 ;

[0013] Prepare a certain number of test pieces that meet the above formula and use the formula:

[0014]

[0015] Estimate the required current amplitude I; where σ b is the strength limit under static load (if it is a plastic material, it is the yield limit), the unit is MPa; W is the bending section coefficient, the unit is m 3 ; b is the air gap length; μ0 is the vacuum magnetic permeability; S0 is the air gap area; N is the number of turns of the excitation coil; x is the coordinate value of the force position, the unit is m, here we take

[0016] (2) Installing the test piece: Move the tailstock on the tailstock slide rail, install the test piece between the three-jaw chuck and the tailstock, and install and clamp the test piece by screwing the locking hole of the three-jaw chuck and tightening the locking valve on the tailstock;

[0017] (3) Adjust the position of the electromagnetic force loading assembly: Adjust the position of the electromagnetic force loading assembly on the side rail so that the excitation coil of the electromagnetic force loading assembly is in the position of the test piece. and fix the electromagnetic force loading component by tightening the fixing valve, and connect the wires of the electromagnetic force loading component to the positive and negative terminals of the power control module;

[0018] (4) Arrangement of strain sensor: Application:

[0019]

[0020] Calculate the coordinates of the first peak point, install the four strain gauges of the strain sensor at the first peak point on the surface of the test piece using a bridge circuit (i.e., electric bridge) full bridge method, and connect the terminals at both ends of the strain sensor output voltage to the data interface of the oscilloscope to realize the transmission of the voltage signal; strain sensor bridge balance condition:

[0021] R1=R2=R3=R4=R

[0022] In the formula, R1, R2, R3, and R4 are four strain gauges, and R is the resistance value in Ω;

[0023] (5) Connect the oscilloscope: Connect the oscilloscope to the host computer (computer), turn on the power of the oscilloscope, turn on the host computer (computer), and perform data transmission and storage;

[0024] (6) Output waveform: Adjust the current waveform adjustment knob of the power control module to select the appropriate output waveform, then adjust the current amplitude adjustment knob to adjust the output current amplitude to the estimated value, and press the current signal output key to start the test. At this time, if there is an output voltage U y, the data will be transmitted to the filter chip in the oscilloscope. After filtering out the high-frequency noise, the information will be transmitted to the voltage waveform display chip, and the waveform will be visualized on the oscilloscope display screen. At the same time, the information will be transmitted to the digital signal conversion chip, the signal will be sampled, truncated, and discretized, and then transmitted to the data storage chip for storage. At the same time, the converted digital signal will be transmitted to the host computer (computer); its output voltage is:

[0025]

[0026] Where U y is the output voltage of the sensor, in V; ΔR1, ΔR2, ΔR3, ΔR4 are the resistance changes, in Ω; U0 is the input voltage, in V;

[0027] The image displayed is U y -t graph;

[0028] (7) Drawing fatigue curve:

[0029]

[0030] In the formula, S g is the sensitivity of the resistance strain gauge; dR is the differential of resistance; ε is the axial strain; v is the Poisson's ratio of the material;

[0031] Bridge sensitivity:

[0032]

[0033] According to Hooke's law:

[0034] σ=Eε

[0035] The relationship between stress value and output voltage is obtained:

[0036]

[0037] It can be seen that the measured stress value has a linear relationship with the output voltage, and the σ-t graph can be obtained. Then, the number of stress cycles n experienced from the start of the experiment to the fracture of the test piece can be recorded to obtain a data point of the fatigue curve σ-n graph. By changing the current amplitude to change the stress amplitude and repeating the experiment, the fatigue curve σ-n graph can be drawn.

[0038] (8) Cut off the power supply of the electromagnetic force loading component until the test piece is destroyed, remove the test piece, and save the data;

[0039] (9) Replace the device under test, adjust the current amplitude by adjusting the current amplitude appropriately on the power control module, and repeat the above operation.

[0040] Beneficial effects of the present invention:

[0041] The measuring device and method provided by the present invention are simple and convenient to operate; based on the principles of resonance and electromagnetic force, using computer technology, the entire measurement process is controlled by theory, and there is no need to conduct repeated experiments to find the resonance frequency of the test piece to be tested. The current amplitude required to be connected for the initial test can be directly calculated, which is different from other measurement methods that require multiple changes in the load frequency to find the resonance point; the load and frequency of the test loading can be adjusted according to the waveform and frequency of the load borne by the test piece under actual working conditions, so that the natural frequency of the test piece can be controlled, so that the test results are closer to the actual working conditions, and the data obtained are more in line with the actual situation; the stress diagram is directly obtained by using a strain gauge sensor, and the result is more accurate; it can be compared with the ideal fatigue curve obtained by theory, which is helpful for result analysis. The present invention has greatly improved in principle control, data processing and result accuracy, and can use virtual software technology to further process the obtained data, further improving the credibility of the data. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a schematic diagram of the overall structure of the device of the present invention;

[0043] Figure 2 It is a schematic diagram of the structure of the three-jaw chuck and tailstock of the present invention;

[0044] Figure 3 This is a schematic diagram of the structure of the electromagnetic force loading component of the present invention;

[0045] Figure 4 This is a schematic diagram of the structure of the power control module of the present invention;

[0046] Figure 5 It is a schematic diagram of the arrangement and circuit connection of the strain sensor of the present invention;

[0047] 1. frame 2. three-jaw chuck 201. locking hole;

[0048] 3. Tail seat 301, sliding seat 302, top 303, locking valve;

[0049] 4. Electromagnetic force loading component 401, component body 402, slider 403, fixed valve 404, iron core 405, excitation coil 406, damping hole;

[0050] 5. Power control module 501, current amplitude display screen 502, current waveform display screen 503, current amplitude adjustment button 504, current waveform adjustment button 505, current signal output button 506, positive and negative poles of power supply;

[0051] 6. Oscilloscope 7. Strain sensor 701. Strain gauge 8. Housing 9. Tailstock slide rail 10. Side slide rail 11. Test piece 12. Damping screw. DETAILED DESCRIPTION

[0052] The present invention provides a fatigue curve measuring device based on electromagnetic force and resonance equation, referring to Figure 1-4 As shown: it includes a frame 1, a three-jaw chuck 2, a tailstock 3, an electromagnetic force loading component 4, a power control module 5, an oscilloscope 6, and a strain sensor 7. One end of the frame 1 is provided with a shell 8, and the upper part of the frame 1 body is provided with a tailstock slide rail 9, which is a T-shaped slide rail, and the two sides of the frame 1 body are respectively provided with side slide rails 10; the three-jaw chuck 2 is arranged on the shell 8 at one end of the frame 1, and the lower part of the tailstock 3 is provided with a sliding seat 301, and the tailstock 3 is slidably connected with the tailstock slide rail 9 through the sliding seat 301; the three-jaw chuck 2 and the tailstock 3 are arranged opposite to each other, and the test piece 11 is fixed on the three-jaw chuck 2. The chuck 2 is fixed to the top 302 of the tailstock 3 by rotating the locking valve 303 on the tailstock 3; two sets of electromagnetic force loading components 4 are respectively arranged on the side slide rails 10 on both sides of the frame 1, and are slidably connected to the side slide rails 10, and the excitation coils 405 of the two sets of electromagnetic force loading components 4 are arranged opposite to each other and are located on both sides of the test piece 11; the power control module 5 and the oscilloscope 6 are integrated in the shell 8 at one end of the frame 1, and the electromagnetic force loading component 4 is connected to the power control module 5 through a wire; the strain sensor 7 is arranged on the test piece 11 and is connected to the oscilloscope 6 through a wire.

[0053] The electromagnetic force loading component 4 includes a component body 401, a slider 402, a fixed valve 403, an iron core 404 and an excitation coil 405. The lower end of the component body 401 is provided with a slider 402 and a fixed valve 403, which are slidably connected to the side slide rails 10 on both sides of the frame 1 through the slider 402. The fixed valve 403 has a locking screw inside, which crosses the component body 401 and passes through the slider 402. Rotate the fixed valve 403 to make the locking screw pass through the slider 402 and abut against the side slide rail 1 0, the electromagnetic force loading component 4 is fixed on the side slide rail 10; a damping hole 406 is provided on the slider 402 of the electromagnetic force loading component 4, and a damping screw 12 is provided next to the side slide rail 10, and the damping hole 406 is sleeved on the damping screw 12 to increase the sliding friction resistance; the iron core 404 is provided on the upper part of the component body 401 facing the side of the test piece 11, and the excitation coil 405 is wound on the iron core 404 to form an electromagnet; the excitation coil 405 is connected to the positive and negative power poles 506 of the power control module 5 through a wire.

[0054] The power control module 5 includes a power supply, a power control chip, a current amplitude display screen 501, a current waveform display screen 502, a current amplitude adjustment button 503, a current waveform adjustment button 504, a current signal output key 505 and a positive and negative power supply electrode 506. The current amplitude display screen 501, the current waveform display screen 502, the current amplitude adjustment button 503, the current waveform adjustment button 504, and the current signal output key 505 are respectively connected to the power control chip, and the power supply output current amplitude and output waveform can be adjusted; the power control chip is connected to the power supply, and the positive and negative power supply electrodes 506 are connected to the power supply.

[0055] The strain sensor 7 includes four strain gauges 701 with equal resistance values, and the four strain gauges 701 are arranged on the surface of the test piece 11 using a full bridge connection method of a bridge circuit (ie, an electric bridge).

[0056] Working principle of the present invention:

[0057] Since the tested piece 11 has different materials and sizes, in order to avoid wasting materials in the invalid test group, we first estimate the current that needs to be output:

[0058] Generally, the maximum stress of the first test piece 11 is:

[0059] σ1=0.6σ b (1)

[0060] Where σ1 is the stress value substituted in the first set of tests, σ b is the ultimate strength of the specimen;

[0061] The pure bending stress formula under static load is:

[0062]

[0063] Where W is the bending section coefficient, unit is m 3 ; M is the bending moment, F is the magnitude of the applied lateral load, in N; L is the length of the specimen; x is the coordinate value of the force application position, in m;

[0064] The equation for the magnitude of the force exerted by the electromagnet in the electromagnetic force loading assembly 4 on the test piece 11 (when the sample is iron) is:

[0065]

[0066] Where F is the magnitude of the electromagnetic force; μ0 is the vacuum magnetic permeability; S0 is the air gap area, in m 2 ; N is the number of turns of the excitation coil 405; I is the excitation current, in A; b is the air gap length;

[0067] By combining equations (1), (2) and (3), the amplitude of the excitation current I that needs to pass through the first test piece 11 can be estimated:

[0068]

[0069] Later, the test load decreased continuously and the number of stress cycles increased continuously.

[0070] According to the general working conditions of the specimen material, the frequency of the applied load is f; then the circular frequency of the forced vibration is:

[0071] ω d =2πf

[0072] ω d The unit is rad / s.

[0073] The differential equation of motion for a bending vibrating beam is:

[0074]

[0075] Where E is the elastic modulus of the material, in MPa; I z (x) is the moment of inertia of the cross section at x about the z-axis perpendicular to the x and y axes and passing through the centroid of the cross section, in m 4 ; y(x,t) is the lateral displacement of the beam element dx at position x at time t, in m; f(x,t) is the lateral load per unit length of the beam, in N / m; m(x) is the mass per unit length of the beam, in kg / m; To find the second-order partial derivative of y(x,t).

[0076] Since the strain sensor 7 can directly output the stress diagram, we only need to consider the natural frequency and the normal vibration mode, and use the separation of variables method to solve:

[0077] y(x,t)=Y(x)F(t)

[0078] Substituting the above formula and simplifying it, we get the characteristic problem:

[0079]

[0080] Among them, Y(x) is the part related only to x after the variables are separated, and the time part is a harmonic function with a frequency of ω.

[0081] Considering the specimen as a uniform geometric body with uniform density and cross section, we have:

[0082] I z (x) = I z ,m(x)=m

[0083] I z is the calculated value of the moment of inertia by substituting the cross-sectional dimensions of the specimen.

[0084] The boundary conditions of the characteristic problem with fixed supports at both ends are:

[0085]

[0086] and

[0087]

[0088] Substituting the boundary conditions and solving the characteristic problem, the natural frequency is obtained as:

[0089]

[0090] ω r is the natural frequency; L is the length of the specimen, in m; r is the order of the natural frequency.

[0091] By changing the length L of the test piece 11 and the section inertia moment I of the test piece z To change the natural frequency of the test piece 11, when the circular frequency is close to a certain order natural frequency, resonance will occur. When resonating, the vibration amplitude reaches a maximum value, and the input energy is absorbed to the maximum extent. Ensuring the occurrence of resonance saves the test cost to a great extent. When conducting the test, it is first necessary to understand the angular frequency ω of the load that the test piece material bears under general working conditions. d (generally known as circular frequency), and then design the length L and section inertia moment I of the specimen z , so that:

[0092] ω r =ω d

[0093] That is, a certain order of natural frequency is equal to the circular frequency, ensuring resonance occurs.

[0094] Solving the eigenproblem and obtaining the normalized natural vibration modes at the same time:

[0095]

[0096] It needs to be explained that when the resonance phenomenon occurs, the specimen is performing a synchronous motion, that is, the phase at each point in time is the same, and the vibration mode in space is determined by the natural vibration mode. The vibration mode is independent of time. This motion is also called standing wave motion. The crest and antinode exist at fixed characteristic points. The length L of the specimen is always an integer multiple of the wavelength, and the order of the natural frequency and the normalized natural vibration mode is always one-to-one corresponding. That is, when the specimen resonates near the first-order natural frequency, the corresponding waveform is also a first-order normalized natural vibration mode. The order is reflected in the formula through the value of r. Generally speaking, the crest is the point with the largest amplitude. When there are no obvious defects at other points of the specimen, the crest should be the first to break. This also provides a theoretical basis for the position of the strain sensor 7. The first crest position is:

[0097]

[0098] In x m The strain gauge 701 is arranged at the position, and the arrangement method, i.e., the circuit connection method, is as follows Figure 5 As shown. Using the full bridge connection method of the bridge circuit (i.e., bridge), the bridge balance condition is:

[0099] R1=R2=R3=R4=R

[0100] Wherein, R1, R2, R3, and R4 are four strain gauges 701 respectively, and R is the resistance value in Ω.

[0101] Its output voltage is:

[0102]

[0103] Where U y is the sensor output voltage, in V; ΔR1, ΔR2, ΔR3, ΔR4 are the resistance changes, in Ω; U0 is the input voltage, in V, and the displayed image is U y -t graph.

[0104] Sensitivity S of the resistance strain gauge 701 g :

[0105]

[0106] In the formula, S g is the sensitivity of the resistance strain gauge 701; dR is the differential of resistance; ε is the axial strain; v is the Poisson's ratio of the material;

[0107] Bridge sensitivity:

[0108]

[0109] According to Hooke's law:

[0110] σ=Eε (9)

[0111] Combining equations (7), (8), and (9) yields the relationship between strain and output voltage:

[0112]

[0113] It can be seen that the stress value measured by the present invention has a linear relationship with the output voltage. The output electrical signal is filtered, and sampling is performed after filtering out high-frequency noise. The time domain image from the start of the experiment to the destruction of the test piece 11 is intercepted, and finally discretization processing is performed to obtain a digital signal that can be stored in a computer. Each data point of this signal is multiplied by a constant (Formula 10) using a multiplier to obtain a σ-t graph. Then, the total number of stress cycles n experienced from the start of the experiment to the fracture of the specimen is recorded to obtain a data point of the fatigue curve σ-n graph. By changing the current value to change the stress amplitude and repeating the experiment, the fatigue curve σ-n graph can be drawn.

[0114] The present invention uses the above fatigue curve measuring device based on electromagnetic force and resonance equation to perform the measurement as follows:

[0115] (1) Data preparation before the test: Query the load frequency and load waveform of the test piece 11 under application conditions, measure the elastic modulus E of the test piece, and use the formula:

[0116]

[0117] In the formula, ω r is the natural frequency; r is the order of the natural frequency; E is the elastic modulus of the material, in MPa; L is the length of the test piece 11, in m; m is the mass of the beam per unit length, in kg / m; I z is the moment of inertia of the section at x about the z-axis perpendicular to the x and y axes and passing through the centroid of the section, in m 4 ; Select the specimen length L and the specimen cross-sectional dimensions, calculate the moment of inertia and record it, prepare a certain number of specimens to be tested that meet the above formula, and use the formula:

[0118]

[0119] Estimate the required current amplitude I in A; where σ b is the strength limit under static load (if it is a plastic material, it is the yield limit), the unit is MPA; W is the bending section coefficient, the unit is m 3 ; b is the air gap length; μ0 is the vacuum magnetic permeability; S0 is the air gap area; N is the number of turns of the excitation coil 405; x is the coordinate value of the force application position, in m, where

[0120] (2) Installing the test piece 11: Move the tailstock 3 on the tailstock slide rail 9, install the test piece 11 between the three-jaw chuck 2 and the tailstock 3, and install and clamp the test piece 11 by screwing the locking hole 201 of the three-jaw chuck 2 and tightening the locking valve 303 on the tailstock 3;

[0121] (3) Adjusting the position of the electromagnetic force loading assembly 4: By adjusting the position of the electromagnetic force loading assembly 4 on the side slide rail 10, the excitation coil 405 of the electromagnetic force loading assembly 4 is positioned at the position of the test piece 11. and fix the position of the electromagnetic force loading component 4 by tightening the fixing valve 403, and connect the wires of the electromagnetic force loading component 4 to the positive and negative terminals of the power control module 5;

[0122] (4) Arrangement of strain sensor 7: Application:

[0123]

[0124] Calculate the coordinates of the first peak point and place the strain sensor 7 according to Figure 5 In the arrangement method shown, four strain gauges 701 are installed at the first peak point on the surface of the test piece 11 using a bridge circuit (i.e., an electric bridge) full bridge method, and the terminals at both ends of the output voltage of the strain sensor 7 are connected to the data interface of the oscilloscope 6 to realize the transmission of the voltage signal; the balance condition of the strain sensor 7 bridge is:

[0125] R1=R2=R3=R4=R

[0126] Where, R1, R2, R3, and R4 are four strain gauges 701 respectively, and R is the resistance value in Ω;

[0127] (5) Connecting the oscilloscope 6: Connecting the oscilloscope 6 to the computer, turning on the power of the oscilloscope 6, turning on the computer, and performing data transmission and storage;

[0128] (6) Output waveform: Adjust the current waveform adjustment button 504 of the power control module 5 to select the waveform of the load borne by the part made of the specimen material under the known working conditions, then adjust the current amplitude adjustment button 503 to adjust the output current amplitude to the estimated value, and press the current signal output key 505 to start the test. At this time, if there is an output voltage U on the strain sensor 7 y , the data will be transmitted to the filter chip in the oscilloscope 6, after filtering out the high-frequency noise, the information will be transmitted to the voltage waveform display chip, the waveform will be visualized on the oscilloscope display screen, and at the same time the information will be transmitted to the digital signal conversion chip, the signal will be sampled, truncated, discretized and then transmitted to the data storage chip for storage, and the converted digital signal will be transmitted to the computer; its output voltage is:

[0129]

[0130] Where U y is the sensor output voltage, in V; U0 is the input voltage, in V; ΔR1, ΔR2, ΔR3, ΔR4 are the resistance changes, in Ω

[0131] The image displayed is U y -t graph;

[0132] (7) Drawing fatigue curve:

[0133]

[0134] In the formula, S g is the sensitivity of the resistance strain gauge 701; dR is the differential of resistance; ε is the axial strain; v is the Poisson's ratio of the material;

[0135] Bridge sensitivity:

[0136]

[0137] According to Hooke's law:

[0138] σ=Eε

[0139] The relationship between stress value and output voltage is obtained:

[0140]

[0141] The measured stress value σ and the output voltage U y There is a linear relationship. The above formula can be used to obtain the σ-t diagram. Then, the number of stress cycles n experienced from the start of the experiment to the fracture of the test piece 11 can be recorded to obtain a data point of the fatigue curve σ-n diagram. By changing the current amplitude to change the stress amplitude and repeating the experiment, the fatigue curve σ-n diagram can be drawn.

[0142] (8) until the test piece is destroyed, the power supply of the electromagnetic force loading assembly 4 is cut off, the test piece 11 is removed, and the data is saved;

[0143] (9) Replace the tested device 11, adjust the current amplitude by appropriately adjusting the current amplitude with the current amplitude adjustment knob 503 on the power control module 5, and repeat the above operation.

Claims

1. A fatigue curve determination method based on electromagnetic force and resonance equation, characterized in that: The fatigue curve is measured using a fatigue curve measuring device based on electromagnetic force and resonance equation. The operating steps are as follows: (1) Data preparation before the test: Query the load frequency and load waveform of the test piece's application conditions, measure the elastic modulus E of the test piece, and use the formula: Select the specimen length L and the specimen cross-sectional dimensions, calculate the moment of inertia and record it; where ω r is the natural frequency, in rad / s; r is the order of the natural frequency; E is the elastic modulus of the material, in MPa; L is the length of the test piece, in m; m is the mass of the beam per unit length, in kg / m; I Z is the moment of inertia of the specimen cross section, in m 4 ; Prepare a certain number of test pieces that meet the above formula and use the formula: Estimate the required current amplitude; where σ b is the strength limit under static load, if it is a plastic material, it is the yield limit, the unit is MPa; W is the bending section coefficient, the unit is m 3 ; b is the air gap length; μ0 is the vacuum magnetic permeability; S0 is the air gap area; N is the number of turns of the excitation coil; x is the coordinate value of the force position, the unit is m, here we take (2) Installing the test piece: Move the tailstock on the tailstock slide rail, install the test piece between the three-jaw chuck and the tailstock, and install and clamp the test piece by screwing the locking hole of the three-jaw chuck and tightening the locking valve on the tailstock; (3) Adjust the position of the electromagnetic force loading assembly: Adjust the position of the electromagnetic force loading assembly on the side rail so that the excitation coil of the electromagnetic force loading assembly is in the position of the test piece. and fix the position of the electromagnetic force loading component by tightening the fixing valve, and connect the wires of the electromagnetic force loading component to the positive and negative terminals of the power control module; (4) Arrangement of strain sensor: Application: Calculate the coordinates of the first peak point, install the four strain gauges of the strain sensor at the first peak point on the surface of the test piece using the full bridge method of the bridge circuit, connect the terminals at both ends of the strain sensor output voltage to the data interface of the oscilloscope to realize the transmission of the voltage signal; strain sensor bridge balance condition: R1=R2=R3=R4=R In the formula, R1, R2, R3, and R4 are four strain gauges, and R is the resistance value in Ω; (5) Connect the oscilloscope: Connect the oscilloscope to the host computer, turn on the power of the oscilloscope, turn on the host computer, and perform data transmission and storage; (6) Output waveform: Adjust the current waveform adjustment knob of the power control module to select the output waveform, then adjust the current amplitude adjustment knob to adjust the output current amplitude to the estimated value, and press the current signal output key to start the test. At this time, if there is an output voltage U y , the data will be transmitted to the oscilloscope, and the waveform will be visualized on the oscilloscope display. At the same time, the digital signal of the signal conversion will be sampled, truncated, discretized, transmitted and stored, and the converted digital signal will be transmitted to the host computer; its output voltage is: Where U y is the output voltage of the sensor, in V; ΔR1, ΔR2, ΔR3, ΔR4, are the resistance changes, in Ω; U0 is the input voltage, in V; The image displayed is U y -t graph; (7) Drawing fatigue curve: In the formula, S g is the sensitivity of the resistance strain gauge; dR is the differential of resistance; ε is the axial strain; v is the Poisson's ratio of the material; Bridge sensitivity: According to Hooke's law: σ=Eε The relationship between stress value and output voltage is obtained: According to the measured stress value σ and the output voltage U y The linear relationship between the stress amplitude and the stress amplitude can be obtained, and the σ-t graph can be obtained. Then the number of stress cycles n experienced from the start of the experiment to the fracture of the test piece can be recorded to obtain a data point of the fatigue curve σ-n graph. By changing the current amplitude to change the stress amplitude and repeating the experiment, the fatigue curve σ-n graph can be drawn. (8) Cut off the power supply of the electromagnetic force loading component until the test piece is destroyed, remove the test piece, and save the data; The fatigue curve measuring device based on electromagnetic force and resonance equation comprises a frame, a three-jaw chuck, a tailstock, an electromagnetic force loading component, a power control module, an oscilloscope, and a strain sensor. A tailstock slide rail is provided on the upper part of the frame, and side slide rails are provided on both sides; the three-jaw chuck is fixed at one end of the frame, and the tailstock is arranged on the tailstock slide rail and is slidably connected to the tailstock slide rail; the three-jaw chuck and the tailstock are arranged opposite to each other, and the tested piece is fixed between the three-jaw chuck and the tailstock; two groups of electromagnetic force loading components are respectively arranged on the side slide rails on both sides of the frame and are slidably connected to the side slide rails, and the excitation coils of the two groups of electromagnetic force loading components are arranged opposite to each other and are located on both sides of the tested piece; the power control module and the oscilloscope are arranged at one end of the frame, and the electromagnetic force loading component is connected to the power control module through a wire; the strain sensor is arranged on the tested piece and is connected to the oscilloscope through a wire.

2. According to the fatigue curve determination method based on electromagnetic force and resonance equation described in claim 1, other features include: the electromagnetic force loading component includes a component body, a slider, a fixed valve, an iron core and an excitation coil, and the lower end of the component body is respectively provided with a slider and a fixed valve, which are slidably connected to the side slide rails on both sides of the frame through the slider, and the electromagnetic force loading component is fixed on the side slide rail by screwing the fixed valve; the fixed valve has a locking screw inside, which crosses the component body and comes out from the slider end; a damping hole is provided on the slider, and a damping screw is provided next to the side slide rail of the frame, and the damping hole is sleeved on the damping screw; the iron core is provided on one side of the upper part of the component body, and the excitation coil is wound on the iron core to form an electromagnet; the excitation coil is connected to the positive and negative poles of the power supply of the power control module through wires.

3. According to the fatigue curve determination method based on electromagnetic force and resonance equation described in claim 1, other features are: the power control module includes a power supply, a power control chip, a current amplitude display screen, a current waveform display screen, a current amplitude adjustment button, a current waveform adjustment button, a current signal output key and a positive and negative pole of the power supply, the current amplitude display screen, the current waveform display screen, the current amplitude adjustment button, the current waveform adjustment button, and the current signal output key are respectively connected to the power control chip, and the power supply output current amplitude and output waveform can be adjusted; the power control chip is connected to the power supply, and the positive and negative poles of the power supply are connected to the power supply.

4. According to the fatigue curve measurement method based on electromagnetic force and resonance equation described in claim 1, other features include: the strain sensor includes four strain gauges with equal resistance, and the four strain gauges are arranged on the surface of the test piece using a bridge circuit full bridge method.

Images