A test device and test method for the threshold value of metal fatigue crack propagation

Abstract

The present invention discloses a test device for the threshold value of metal fatigue crack propagation, which includes a low-frequency fatigue testing machine, two sets of pulsed laser thermographic measurement systems, a computer processing control system, and an equipment communication system. The low-frequency fatigue testing machine includes a main machine and a control cabinet, and a fixture and a crack propagation specimen are installed on the main machine. The two sets of pulsed laser thermographic measurement systems are arranged on both sides of the crack propagation specimen. The present invention also discloses a test method based on the above test device for the threshold value of metal fatigue crack propagation. The present invention can achieve the goal of loading operation, data recording and calculation of the test for the threshold value of metal fatigue crack propagation under the condition of less manual intervention. In addition, through the pulsed laser thermographic technology, the crack propagation length can be read in real time, and the load magnitude and amplitude can be changed in time through computer processing, so as to save the time of measurement reading, judgment and loading operation, avoid wasting a large amount of human and time costs, and ensure the accuracy of the test results.

Description

Technical Field

[0001] The present invention relates to the technical field of metal fatigue crack experiments, and in particular to a test device and a test method for the threshold value of metal fatigue crack propagation. Background Art

[0002] With the increasing attention to the fatigue life of metal materials and structures, fatigue tests of metal materials have received extensive attention. By using a metal fatigue testing machine, the fatigue characteristics, fatigue life, and fatigue crack propagation rate of metals and their fabricated structural components under tensile, compressive, or tensile-compressive alternating loads at room temperature can be measured, and thus applied to damage tolerance design of aircraft structures and structural fatigue-resistant design such as pipeline structure life design. For structural fatigue-resistant design, the fatigue crack propagation threshold value ΔK th (hereinafter referred to as the threshold value ΔK th ) is an important performance index, which is defined as the asymptotic value of the stress intensity factor change range ΔK when the crack propagation rate da / dN approaches 0. In test measurements, it is specified that under air medium and plane strain conditions, the stress intensity factor change range corresponding to the metal material crack propagation rate close to 10 -7 mm / cycle (or lower), and the threshold value ΔK th is of great significance for predicting fatigue crack propagation. For a structural component with cracks generated during manufacturing, if the stress intensity factor range ΔK under the service load is less than the threshold value ΔK th , the structural component can work safely for a long time without crack propagation.

[0003] The existing tests for the threshold value of metal fatigue crack propagation can be mainly divided into the decreasing-K test and the increasing-K test. Among them, the increasing-K test will cause a large instantaneous effect, which is not conducive to the precision control of test results. In the decreasing-K test, there are usually two ways to measure the crack propagation amount. One way is to manually measure the crack propagation length by visual inspection using a low-power microscope with a magnification of 20 to 50 times. However, during the measurement process, the test usually needs to be paused to ensure a more stable imaging effect. Pausing the test will cause problems such as crack closure, resulting in insufficient measurement accuracy. At the same time, multiple pauses in the test will also cause damage to the test piece that affects the test accuracy. The other way is to apply image processing technology to the test for the threshold value of metal fatigue crack propagation, which can replace the human eye to measure the crack propagation length, thus achieving the beneficial effects of improving the test accuracy and test efficiency. For example, a crack detection system using a three-axis movable electron microscope and crack recognition technology is disclosed in the prior art. However, this system uses a high-brightness light source, so the visibility of the crack tip is not high. This system uses one-time calibration, and large errors may occur due to the three-axis movement of the electron microscope during the test, thus affecting the test accuracy. This system can only take pictures of the crack and cannot record videos to obtain a complete record of the crack propagation process. Similarly, the image processing part of this system cannot perform further analysis on the crack symmetry, etc. At the same time, the relevant parameters of its test system also need to be manually calculated and input, and automated operation cannot be achieved, which is not universal for the test of the threshold value of fatigue crack propagation.

[0004] However, currently, no matter which crack length measurement method is used, it is necessary to calculate and input the next test load according to the crack length. This step is usually completed manually, and there are problems such as data omission and misrecording caused by the negligence of the operator. In addition, the fatigue crack image recognition technology does not perform further analysis on the fatigue crack such as crack offset, so that the subsequent processing of the crack length obtained after image recognition has not been fully automated. At the same time, laser thermal imaging technology has also been applied to crack detection, but generally, it uses laser for long-term irradiation and static observation and cannot be directly applied to the dynamic observation of the crack length in the test for the threshold value of fatigue crack propagation. Summary of the Invention

[0005] The purpose of the present invention is to provide a test device and test method for the threshold value of metal fatigue crack propagation to solve the problems in the above background technology.

[0006] The technical solution of the present invention is: a test device for the threshold value of metal fatigue crack propagation, which includes a low-frequency fatigue testing machine, two sets of pulsed laser thermal imaging measurement systems, a computer processing control system, and an equipment communication system. The low-frequency fatigue testing machine includes a main machine and a control cabinet. A fixture and a crack propagation specimen are installed on the main machine. The two sets of pulsed laser thermal imaging measurement systems are arranged on both sides of the crack propagation specimen. Each set of pulsed laser thermal imaging measurement systems includes a laser emitter, an infrared thermal imager, a high-speed camera, a lifting bracket, and a rotating bracket. A first mounting seat is connected to the lifting bracket for driving the first mounting seat to move up and down. The laser emitter and a displacement sensor are provided on the first mounting seat. The infrared thermal imager and the high-speed camera are respectively provided on the rotating bracket for driving them to rotate horizontally. The computer processing control system is used to control each system of the test device, record test data, calculate the current crack propagation rate and the load amplitude, maximum load, and minimum load required for the next test stage, and output relevant parameters to the low-frequency fatigue testing machine as the load data for the next test stage. The equipment communication system includes a test end and a monitoring end, which are used to transmit the captured images and the crack propagation size measured by the computer processing control system to the monitoring end of the test operator, and transmit the instructions input by the test operator at the monitoring end to the computer processing control system.

[0007] Preferably, calibration stickers are pasted on both sides of the crack propagation specimen, and there is a color difference between the calibration stickers and the surface color of the fatigue crack propagation specimen.

[0008] Preferably, the lifting bracket includes a first support base, a transmission component, a servo motor, and three sets of high-speed ball screw feed components arranged in parallel. The first support base is set on a horizontal support plane. The transmission component and the servo motor are both set on the top of the first support base, and the servo motor is electrically connected to the computer processing control system. Each set of high-speed ball screw feed components includes a nut and a screw. The first mounting seat is arranged between the three sets of high-speed ball screw feed components, and the side wall of the first mounting seat is fixedly connected to the three nuts. The output ends of the transmission component are respectively connected to the three screws to drive the three screws to rotate synchronously.

[0009] Preferably, the transmission assembly includes a box body fixed to the top of the first support base. A planetary gear set is arranged inside the box body. The planetary gear set includes a sun gear, a sun gear shaft, a first planetary gear, a first planetary gear shaft, a second planetary gear, a second planetary gear shaft, a third planetary gear, a third planetary gear shaft, a connecting frame, a ring gear and an outer ring bearing sleeve. The sun gear shaft is connected to the output shaft of the servo motor. The sun gear is sleeved and fixed on the sun gear shaft. A connecting frame is installed between the sun gear shaft, the first planetary gear shaft, the second planetary gear shaft and the third planetary gear shaft. The first planetary gear is correspondingly sleeved and fixed on the first planetary gear shaft. The second planetary gear is correspondingly sleeved and fixed on the second planetary gear shaft. The third planetary gear is correspondingly sleeved and fixed on the third planetary gear shaft. The inner sides of the first planetary gear, the second planetary gear and the third planetary gear are all meshed with the sun gear. The first planetary gear shaft, the second planetary gear shaft and the third planetary gear shaft are respectively connected to the lead screws in three groups of high-speed ball screw feeding assemblies. The outer ring bearing sleeve is fixed inside the box body. The ring gear is in sliding friction contact with the outer ring bearing sleeve. The outer sides of the first planetary gear, the second planetary gear and the third planetary gear are all meshed with the ring gear.

[0010] Preferably, the rotary bracket includes a rotary base and two telescopic struts. The rotary base is arranged on the second support base. The second support base is arranged on a horizontal support plane. The rotary base is disc-shaped and is controlled to rotate by a servo motor. The two telescopic struts are fixed at the edge of the rotary base. The included angle between the geometric centers of the bottom surfaces of the two telescopic struts and the connection line of the center of the rotary base is 90°. A second mounting seat is fixed at the top of each telescopic strut. The infrared thermal imager and the high-speed camera are respectively arranged on the two second mounting seats and are fixed to the second mounting seats by bolts. The telescopic struts can adjust the height positions of the two second mounting seats manually.

[0011] Preferably, the control cabinet of the low-frequency fatigue testing machine is connected to the computer processing control system through a data transmission system. The equipment communication system is connected to the computer processing control system through a data transmission system.

[0012] A test method based on the above-mentioned test device for the threshold value of metal fatigue crack propagation includes the following steps:

[0013] Step 1, measure the size of the crack propagation specimen, select a suitable fixture according to the size of the crack propagation specimen, and install the fixture on the testing machine;

[0014] Step 2, turn on the low-frequency fatigue testing machine and zero it, and install the crack propagation specimen on the fixture;

[0015] Step 3: Place two sets of pulsed laser thermography measurement systems on the support planes on both sides of the testing machine. Start the pulsed laser thermography measurement system, and adjust the positions of the infrared thermal imager, high-speed camera, and laser emitter by adjusting the lifting bracket and rotating bracket to ensure that the infrared thermal imager and high-speed camera meet the position, imaging, and field of view requirements when facing the specimen, and the laser emitter meets the irradiation position requirements;

[0016] Step 4: Input the geometric dimensions of the crack propagation specimen measured in Step 1 into the human-machine interface of the computer processing control system to calculate the initial test parameters, and then input the initial test parameters into the control cabinet of the low-frequency fatigue testing machine;

[0017] Step 5: The testing machine starts the test, obtains the thermal images through the pulsed laser thermography system, and extracts the crack characteristics;

[0018] Step 6: The computer processing control system performs the fault judgment of the low-frequency fatigue testing machine and the test failure judgment. If there is a fault or failure, directly execute Step 11; otherwise, continue the test until Step 8;

[0019] Step 7: The test operator can remotely check the infrared thermal image with a virtual scale with a scale interval of 0.5 mm and the existing crack length L output by the computer processing control system at any time through the monitoring end of the equipment communication system; k If there is a large deviation between the existing crack length L k and the visual measurement value of the test operator, the test operator stops the test through the monitoring end; otherwise, continue the test until Step 8;

[0020] Step 8: When the crack propagation length L k reaches 0.25 mm, that is, the test pause condition is met, the computer processing control system performs the relevant test pause operations, and calculates the crack propagation rate and the parameters for the next test stage;

[0021] Step 9: The computer processing control system determines whether there are 6 or more da / dN-ΔK data pairs evenly distributed between 10 -7 ~10 -6 mm / cycle. If there are, enter Step 11; if not, enter Step 5;

[0022] Step 10: The computer processing control system automatically performs the control operations related to stopping the test on the testing machine and other systems, and calculates the fatigue threshold value;

[0023] Step 11: The test operator records the test data, removes the specimen and saves it, and shuts down all systems of the metal fatigue crack propagation threshold test device to end the test.

[0024] Preferably, the steps of obtaining a thermal image by a pulsed laser thermography system and extracting crack features include:

[0025] The computer processing and control system controls the rotation of the rotating bracket 9 to make the high-speed camera face the specimen;

[0026] The high-speed camera takes pictures of the vibrating specimen. The computer processing and control system obtains the displacement vibration frequency and amplitude information of the calibration sticker according to the captured images. At the same time, the displacement sensor sends the vibration frequency and amplitude information of the laser emitter to the computer processing and control system;

[0027] Based on the above information, the computer processing and control system uses a sliding mode control algorithm to close-loop control the movement of the laser emitter so that the relative position relationship between the target position irradiated by the laser of the laser emitter and the position where the calibration sticker is pasted remains stable;

[0028] The laser emitter emits short-pulse high-power laser, irradiates on the specimen, and cools for a period of time;

[0029] The computer processing and control system controls the rotation of the rotating bracket to make the infrared thermal imager face the specimen;

[0030] The infrared thermal imager takes a thermal image of the specimen and performs crack measurement and further analysis.

[0031] Preferably, the extracted crack features include crack propagation length, crack offset, crack symmetry, and crack bifurcation.

[0032] Preferably, the judgment of test failure includes the following steps:

[0033] In each crack identification process, it is judged whether the crack meets the requirement of crack offset. Specifically, it is judged whether any point of the crack feature deviates from the theoretically crack propagation direction by more than 0.05 times the width W of the crack propagation specimen. If so, it is said that the offset is too large. If not, it is said that the offset is satisfied;

[0034] In each crack identification process, it is necessary to judge whether the crack meets the requirement of crack symmetry. Specifically, it is judged whether the difference L c in the existing crack length of the infrared thermal images taken by the two infrared thermal imagers exceeds 0.25 times the thickness B of the crack propagation specimen. If so, it is said that the crack symmetry requirement is not met. If not, it is said that the crack symmetry requirement is met;

[0035] Preferably, in each crack identification process, it is necessary to judge whether the crack bifurcates. If it bifurcates, the longer crack bifurcation is selected as the main crack feature for crack length and crack propagation length measurement and calculation. All bifurcated cracks need to be judged whether they meet the requirements of crack offset and crack symmetry.

[0036] Compared with the prior art, the beneficial effects of the present invention are:

[0037] 1. The present invention can achieve the goal of loading operation, data recording and calculation in the metal fatigue crack growth threshold test under less manual intervention; in addition, through the pulsed laser thermography technology, the present invention can read the crack growth length in real time, and timely change the load magnitude and amplitude through computer processing, so as to save the measurement reading, judgment and loading operation time, avoid wasting a large amount of human and time costs and ensure the accuracy of the test results.

[0038] 2. The present invention can drive the laser emitter to rise and fall through the lifting bracket to achieve the function of synchronous vibration of the laser emitter and the specimen, so as to be able to conduct dynamic observation, reduce laser scattering, concentrate heat, effectively improve the test accuracy, and at the same time add a rotating bracket for installing a high-speed camera and an infrared thermal imager. The rotating bracket can rotate automatically. The high-speed camera is used for vibration calibration of the laser emitter, and the infrared thermal imager is used for crack identification.

[0039] 3. Compared with the traditional infrared thermography method, the present invention also has low sensitivity to the crack tip under no excitation conditions and cannot well reflect the thermal resistance effect of the crack under normal temperature conditions. The present invention uses the pulsed laser thermography method. A short-pulse high-power laser is emitted by the laser emitter and irradiated on the specimen. After cooling for a period of time, the thermal image of the specimen is taken by the infrared thermal imager. Therefore, it has obvious high-precision characteristics compared with the traditional method of using infrared thermography for crack identification. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 is a schematic structural diagram of the test device in the present invention;

[0041] Figure 2 is a schematic diagram of the installation position of the pulsed laser thermography measurement system in the present invention;

[0042] Figure 3 is a schematic structural diagram of the lifting bracket in the present invention;

[0043] Figure 4 is a schematic structural diagram of the rotating bracket in the present invention;

[0044] Figure 5 is a schematic diagram of the test method flow in the present invention;

[0045] Figure 6 is a schematic structural diagram of the transmission component in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0046] The following is combined with the attached Figures 1 to 6, a detailed description of the specific embodiments of the present invention will be given. In the description of the invention, it should be understood that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present invention.

[0047] The terms "first" and "second" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of such features; in the description of the invention, unless otherwise specified, the meaning of "a plurality" is two or more.

[0048] Example 1

[0049] As Figures 1 to 5As shown in the figure, an embodiment of the present invention provides a test device for the threshold value of metal fatigue crack propagation, which includes a low-frequency fatigue testing machine, two sets of pulsed laser thermographic measurement systems, a computer processing and control system, and an equipment communication system; the model of the low-frequency fatigue testing machine is Instron 8803, and the test frequency of the low-frequency fatigue testing machine is between 10 - 20 Hz. To protect the testing machine and ensure the camera shooting resolution, the maximum test frequency does not exceed 25 Hz, and there is an accurate digital device to record the number of load cycles. The low-frequency fatigue testing machine includes a main machine, a cooling device, and a control cabinet. The cooling device cools the main machine, and the control cabinet controls the main machine and the cooling device. A fixture and a crack propagation specimen 4 are installed on the main machine. Among them, the fixture is a friction clamping fixture, which should cooperate well with the crack propagation specimen 4 and have sufficient stiffness to avoid the specimen from translating or rotating. At the same time, the fixture is divided into an upper fixture 1 and a lower fixture 8, and they are respectively installed on the upper and lower fixture mounting seats. Among them, the crack propagation specimen 4 is a clamped center crack SENT single-edge notch tensile specimen, made of metal or alloy, and the surface is ensured to be smooth and pollution-free through polishing and wiping processes. And a sufficiently long and sharp straight prefabricated crack is prefabricated at the single-edge notch. Define the distance between the two edges of the specimen along the crack propagation direction as the specimen width W, and define the distance between the two edges of the specimen in the direction perpendicular to the tensile direction and in the plane formed by the width direction as the specimen thickness B. At the same time, calibration stickers 11 with an obvious color difference from the specimen are pasted on both sides of the crack propagation specimen away from the crack propagation area; the two sets of pulsed laser thermographic measurement systems are arranged on both sides of the crack propagation specimen 4. Each set of pulsed laser thermographic measurement systems includes a laser emitter 2, an infrared thermal imager 6, a high-speed camera 7, a lifting bracket 3, and a rotating bracket 9. Among them, the power of the laser emitter is 21 W, and the pulse duration is 50 ms. The infrared thermal imager has a high resolution and takes multiple infrared thermal images after a certain cooling time after the pulsed laser is emitted. The high-speed camera 7 is a color high-definition camera with a resolution of 60H; the lifting bracket 3 is connected to a first mounting seat 3002, which is used to drive the first mounting seat 3002 to lift. The laser emitter 2 and a displacement sensor 3003 are arranged on the first mounting seat 3002. The infrared thermal imager 6 and the high-speed camera 7 are respectively arranged on the rotating bracket 9, which is used to drive them to rotate horizontally. During the test, when the infrared thermal imager 6 faces the specimen, the axis is as perpendicular as possible to the surface of the fatigue crack propagation specimen 2, and the field of view can cover the two edges of the specimen on this side along the crack propagation direction of the fatigue crack propagation specimen 4 to ensure the measurement accuracy. The high-speed camera can observe the pasting position of the calibration sticker 11 when facing the specimen during the test. The laser irradiation position of the laser emitter 2 is at the crack propagation direction; the computer processing and control system is used to control each system of the test device, record the test data, calculate the current crack propagation rate and the load amplitude, maximum load, and minimum load required for the next test stage, and output the relevant parameters to the low-frequency fatigue testing machine as the load data for the next test stage;The device communication system includes a test end and a monitoring end, which are used to transmit the images obtained by shooting and the crack propagation sizes measured by the computer processing control system to the monitoring end of the test operator, and to transmit the instructions input by the test operator at the monitoring end to the computer processing control system. Among them, the test end of the device communication system is a small computer, and the monitoring end is a handheld device. High-power WIFI modules are installed on both the test end and the monitoring end and are paired with each other to achieve an effective communication distance of at least 500m under unobstructed conditions. The monitoring end of the device communication system has a screen with a high resolution, which can display the pictures and text information sent by the test end and display the signal connection status between the monitoring end and the test end.;

[0050] Specifically, as Figure 3 shown, the lifting bracket 3 includes a first support base 3005, a transmission component 3004, a servo motor, and three groups of parallel high-speed ball screw feed components 3001; the first support base 3005 is arranged on the horizontal support plane 10; the transmission component 3004 and the servo motor are both arranged on the top of the first support base 3005, and the servo motor is electrically connected to the computer processing control system; for the three groups of parallel high-speed ball screw feed components 3001, each group of high-speed ball screw feed components 3001 includes a nut and a screw. The first mounting seat 3002 is arranged between the three groups of high-speed ball screw feed components 3001, and the side wall of the first mounting seat 3002 is fixedly connected to the three nuts. The output ends of the transmission component 3004 are respectively connected to the three screws to drive the three screws to rotate synchronously. The servo motor receives the instructions of the computer processing control system and can provide torques in both forward and reverse directions to enable the parallel high-speed ball screw feed components 3001 to drive the first mounting seat 3002, the displacement sensor 3003, and the laser emitter 2 to perform high-speed linear motion in both forward and reverse directions.

[0051] Furthermore, as Figure 6As shown in the figure, the specific structure of the transmission assembly 3004 includes a box body fixed on the top of the first support base 3005. A planetary gear set is arranged inside the box body. The planetary gear set includes a sun gear 301, a sun gear shaft 302, a first planetary gear 303, a first planetary gear shaft 304, a second planetary gear 305, a second planetary gear shaft 306, a third planetary gear 307, a third planetary gear shaft 308, a connecting frame 309, a ring gear 3010 and an outer ring bearing sleeve 3011. The sun gear shaft 302 is connected to the output shaft of the servo motor. The sun gear 301 is sleeved and fixed on the sun gear shaft 302. A connecting frame 309 is installed between the sun gear shaft 302, the first planetary gear shaft 304, the second planetary gear shaft 306 and the third planetary gear shaft 308. The first planetary gear 303 is correspondingly sleeved and fixed on the first planetary gear shaft 304. The second planetary gear 305 is correspondingly sleeved and fixed on the second planetary gear shaft 306. The third planetary gear 307 is correspondingly sleeved and fixed on the third planetary gear shaft 308. The inner sides of the first planetary gear 303, the second planetary gear 305 and the third planetary gear 307 are all meshed with the sun gear 301. The first planetary gear shaft 304, the second planetary gear shaft 306 and the third planetary gear shaft 308 are respectively connected to the lead screws in the three high-speed ball screw feed assemblies 3001. The outer ring bearing sleeve 3011 is fixed inside the box body. The ring gear 3010 is in sliding friction contact with the outer ring bearing sleeve 3011. The outer sides of the first planetary gear 303, the second planetary gear 305 and the third planetary gear 307 are all meshed with the ring gear 3010.

[0052] When driving the transmission assembly through the servo motor to drive the three lead screws to rotate synchronously, the output shaft of the servo motor drives the sun gear 301 to rotate. When the sun gear 301 rotates, it will drive the first planetary gear 303, the second planetary gear 305 and the third planetary gear 307 meshed with it to rotate synchronously and in the opposite direction, and then drive the three lead screws to rotate synchronously through the first planetary gear shaft 304, the second planetary gear shaft 306 and the third planetary gear shaft 308.

[0053] Specifically, such as Figure 4As shown in the figure, the rotating bracket 9 includes a rotating base 9003 and two telescopic struts 9002; the rotating base 9003 is arranged on the second support base 9004, and the second support base 9004 is arranged on the horizontal support plane 10. The movement degrees of freedom of the rotating base 9003 in other directions are restricted by the second support base 9004. The rotating base 9003 is disc-shaped and is controlled to rotate by a servo motor; the two telescopic struts 9002 are fixed at the edge of the rotating base 9003, and the included angle between the geometric centers of the bottom surfaces of the two telescopic struts 9002 and the connection line of the center of the rotating base 9003 is 90°. A second mounting seat 9001 is fixed at the top of each telescopic strut 9002. The infrared thermal imager 6 and the high-speed camera 7 are respectively arranged on the two second mounting seats 9001 and are fixed to the second mounting seat 9001 by bolts. The telescopic strut 9002 adjusts the height positions of the two second mounting seats 9001 in a manually adjustable manner.

[0054] Further, the control cabinet of the low-frequency fatigue testing machine is connected to the computer processing control system through the data transmission system, and the equipment communication system is connected to the computer processing control system through the data transmission system. The data transmission system has a relatively wide communication bandwidth and a relatively low communication delay, which can ensure two-way communication.

[0055] Embodiment 2

[0056] This embodiment discloses a test method based on a test device for the threshold value of metal fatigue crack propagation in Embodiment 1, including the following steps:

[0057] Step 1: Polish and wipe the surface of the crack propagation specimen with a prefabricated crack, paste calibration stickers on both sides of the notch of the crack propagation specimen, accurately measure the geometric dimensions of the crack propagation specimen, and select a suitable fixture according to the size of the crack propagation specimen and install the fixture on the testing machine.

[0058] Step 2: Turn on the low-frequency fatigue testing machine and zero it, install the crack propagation specimen on the fixture, and ensure that the crack propagation direction of the specimen is perpendicular to the direction of the test load.

[0059] Step 3: Place two sets of pulsed laser thermographic measurement systems on the support planes on both sides of the testing machine, start the pulsed laser thermographic measurement system, and adjust the positions of the infrared thermal imager, the high-speed camera, and the laser emitter by adjusting the lifting bracket 3 and the rotating bracket 9, so that the infrared thermal imager and the high-speed camera meet the position, imaging, and field of view requirements when facing the specimen, and the laser emitter meets the irradiation position requirements.

[0060] Step 4: Consult literature materials such as national standards for fatigue tests. According to the literature materials and the geometric dimensions of the crack propagation specimen measured in Step 1, input the initial data and calculate the initial test parameters through the man-machine interface of the computer processing control system. Then input the initial test parameters into the control cabinet of the low-frequency fatigue testing machine.

[0061] Since the test method of the present invention is designed based on the K-decreasing method, the input initial data includes specimen number, specimen thickness B, fracture depth a n , prefabricated crack length a p , specimen width W, stress ratio R, loading frequency f, initial maximum stress σ max,ini , K-decreasing gradient C0 and other data. Based on these data, calculate the initial test parameters including initial shape factor g0, initial stress intensity factor K0, initial stress intensity factor range ΔK0, initial maximum stress σ max,0 , initial minimum stress σ min,0 , initial average load F ave,0 , initial load amplitude ΔF0, etc. At the same time, the test operator can consult the test data stored in the computer processing control system through this man-machine interface. The data automatically received by the computer processing control system includes the current load maximum value F max , load minimum value F min , load average value F ave , current load value F r , load amplitude ΔF, loading frequency f, current total number of cycles N, specimen position, information that the low-frequency fatigue testing machine cannot start, information on the low-frequency fatigue testing machine pausing the test, etc., as well as the infrared thermal images obtained by the two infrared thermal imagers of the pulsed laser thermography measurement system and the images obtained by the two high-speed cameras of the pulsed laser thermography measurement system, as well as the data of the movable support displacement sensor, and the manual commands from the equipment communication system. Among them, the test end of the equipment communication system can translate and forward the instructions sent by the computer processing control system. Specifically:

[0062] "1000" instruction: When the test end receives the "1000" instruction, send the text information of "The test has started" to the monitoring end.

[0063] "1001" instruction: When the test end receives the "1001" instruction, send the text information of "The testing machine cannot start" to the monitoring end.

[0064] "1002" instruction: When the test end receives the "1002" instruction, send the text information of "The test has been completed in stages" and the existing crack length L x to the monitoring end, and send the photo image information of the last group of photos taken by the two infrared thermal imagers before pausing.

[0065] "1003" Instruction: When the test end receives the "1003" instruction, it sends the text message "The test has stopped" to the monitoring end.

[0066] "1004" Instruction: When the test end receives the "1004" instruction, it sends the text message "The test has failed" to the monitoring end.

[0067] Step Five, input the "Test Start" instruction into the computer processing control system, the testing machine starts the test, and obtains the thermal image and extracts the crack characteristics through the pulsed laser thermography system.

[0068] Among them, the specific steps of obtaining the thermal image and extracting the crack characteristics through the pulsed laser thermography system include:

[0069] The computer processing control system controls the rotation of the rotating bracket 9 to make the high-speed camera face the specimen;

[0070] The high-speed camera takes pictures of the vibrating specimen. The computer processing control system obtains the displacement vibration frequency and amplitude information of the calibration sticker according to the captured images. At the same time, the displacement sensor sends the vibration frequency and amplitude information of the laser emitter to the computer processing control system;

[0071] The computer processing control system, based on the above information, uses the sliding mode control algorithm to close-loop control the movement of the laser emitter to keep the relative position relationship stable between the target position irradiated by the laser of the laser emitter and the position where the calibration sticker is pasted;

[0072] The laser emitter emits short-pulse high-power laser, irradiates on the specimen, and cools for a period of time;

[0073] The computer processing control system controls the rotation of the rotating bracket 9 to make the infrared thermal imager face the specimen;

[0074] The infrared thermal imager takes pictures of the thermal image of the specimen and conducts crack measurement and further analysis.

[0075] Among them, the extracted crack characteristics include crack propagation length, crack offset, crack symmetry, and crack bifurcation. At the beginning of each stage of the test, first measure the arithmetic mean of the crack lengths in the first group of photos taken by the two cameras received after at least 100 cycles as the old crack length L m To ensure that the test is carried out under a stable crack propagation rate state, and then measure the existing crack length L after each new photo image is input e , and calculate the crack propagation length L through the crack propagation formula k , and the corresponding crack propagation formula is:

[0076] L k = L e - L m (1)

[0077] Step 6: During the test, the computer processing control system performs fault judgment of the low-frequency fatigue testing machine and test failure judgment. If there is a fault or failure, directly execute Step 11; otherwise, continue the test until Step 8.

[0078] Among them, the judgment of test failure includes the following steps:

[0079] During each crack identification process, judge whether the crack meets the requirements of crack offset. Specifically, judge whether any point of the crack characteristics deviates from the theoretical crack propagation direction by more than 0.05 times the crack propagation specimen width W. If so, it is called excessive offset; if not, it is called the offset meets the requirements.

[0080] During each crack identification process, it is necessary to judge whether the crack meets the requirements of crack symmetry. Specifically, judge the difference in the existing crack length L c between the infrared thermal images taken by two infrared thermal imagers. Whether it exceeds 0.25 times the crack propagation specimen thickness B. If so, it is called that the crack symmetry requirements are not met; if not, it is called that the crack symmetry requirements are met.

[0081] During each crack identification process, it is necessary to judge whether the crack bifurcates. If it bifurcates, select the longer crack bifurcation as the main crack characteristic for crack length and crack propagation length measurement and calculation. All bifurcated cracks need to be judged whether they meet the requirements of crack offset and crack symmetry.

[0082] Step 7: The test operator can remotely check the infrared thermal image with a virtual scale with a scale interval of 0.5 mm attached and the existing crack length L output by the computer processing control system through the monitoring end of the equipment communication system at any time. k If there is a large deviation between the existing crack length L k and the visual value of the test operator, the test operator stops the test through the monitoring end; otherwise, continue the test until Step 8.

[0083] Step 8: When the crack propagation length L k reaches 0.25 mm, that is, the test suspension condition is met, the computer processing control system performs relevant test suspension operations and performs crack propagation rate calculation and parameter calculation for the next test stage.

[0084] Among them, the process of parameter calculation for the next test stage in Step 8 is as follows:

[0085] First, calculate the shape factor g for the next stage according to the crack elongation and other geometric dimension information. n According to the stress intensity factor K n-1 for the previous stage, calculate the stress intensity factor K n and the stress intensity factor range ΔK nAfter that, according to the shape factor g in the next stage n , the stress intensity factor K in the next stage n , the stress ratio R, etc., calculate the maximum stress σ in the next stage max,n and the minimum stress σ min,n , and deduce and calculate the average load F in the next stage ave,n and the load amplitude ΔF n and other parameters.

[0086] Specifically, according to the different geometric characteristics of the specimen, referring to the literature formula, the shape factor in the next stage can be calculated. For a single-edge notched clamped specimen with a net distance between the clamps equal to 4W, the formula is as follows:

[0087]

[0088] In the formula, α = (L e +a n +a p ) / W, L e is the crack propagation length, a n is the initial input fracture depth, and a p is the initial input prefabricated crack length.

[0089] The stress intensity factor K in the next stage n is calculated by the following formula

[0090]

[0091] In the formula, K n-1 is the stress intensity factor in the previous stage, C0 is the initial input K reduction gradient, and L e is the crack propagation length. According to the stress intensity factor K in the next stage n , the stress intensity factor range ΔK in the next stage n can be calculated. The formula is as follows:

[0092] ΔK n =(1 - R)·K n (4)

[0093] In the formula, R is the initial input stress ratio.

[0094] Thus, the maximum stress σ in the next stage max,n , and the minimum stress σ min,n can be deduced, and the average load F in the next stage ave,n can be deduced. The formula is as follows;

[0095]

[0096] In the formula, B is the initial input specimen thickness.

[0097] Furthermore, the load amplitude ΔF in the next stage can be calculated n , and the calculation formula is as follows:

[0098]

[0099] Step 9: The computer processing control system determines whether there are 6 or more da / dN-ΔK data pairs evenly distributed between 10 -7 ~10 -6 mm / cycle. If so, go to Step 11; if not, go to Step 5

[0100] Step 10: The computer processing control system automatically performs control operations related to stopping the test on the testing machine and other systems, and calculates the fatigue threshold value

[0101] The calculation of the fatigue threshold value includes the following steps:

[0102] Calculate the logarithmic value lg(ΔK) of the stress intensity factor range according to the stress intensity factor range ΔK calculated from the crack growth rate, and calculate the logarithmic value lg(da / dN) of the crack growth rate according to the crack growth rate da / dN

[0103] Taking lg(da / dN) as the ordinate and lg(ΔK) as the abscissa, plot a scatter diagram, and perform linear fitting using methods such as linear regression. According to the logarithmic form of the Paris equation and the fitted straight-line equation, obtain the material parameter C and the material parameter m. The corresponding logarithmic form of the Paris equation is:

[0104] lg(da / dN) = lgC + mlg(ΔK) (7)

[0105] Based on the above data, calculate the fatigue crack threshold value ΔK corresponding to da / dN ≤ 10 -7 mm / cycle according to the formula th , and the corresponding formula is:

[0106]

[0107] Step 11: The test operator records the test data, removes the specimen and saves it, and shuts down each system of the metal fatigue crack growth threshold test device to end the test

[0108] The above-disclosed are only several specific embodiments of the present invention. However, the embodiments of the present invention are not limited thereto, and any changes that can be thought of by those skilled in the art should fall within the protection scope of the present invention

Claims

1. A test device for the threshold value of metal fatigue crack propagation, comprising a low-frequency fatigue testing machine, the low-frequency fatigue testing machine including a main machine and a control cabinet, with a fixture and a crack propagation specimen (4) installed on the main machine, characterized in that, It further includes: Two sets of pulsed laser thermography measurement systems, which are arranged on both sides of the crack propagation specimen (4). Each set of pulsed laser thermography measurement systems includes a laser emitter (2), an infrared thermal imager (6), a high-speed camera (7), a lifting bracket (3) and a rotating bracket (9). A first mounting seat (3002) is connected to the lifting bracket (3) for driving the first mounting seat (3002) to lift. The laser emitter (2) and a displacement sensor (3003) are provided on the first mounting seat (3002). The infrared thermal imager (6) and the high-speed camera (7) are respectively provided on the rotating bracket (9) for driving them to rotate horizontally; Calibration stickers (11) are pasted on both sides of the crack propagation specimen (4), and there is a color difference between the calibration stickers (11) and the surface color of the crack propagation specimen (4); A computer processing and control system is used to control each system of the test device, record test data, calculate the current crack propagation rate and the required load amplitudes, maximum load and minimum load for the next test stage, and output relevant parameters to the low-frequency fatigue testing machine as the load data for the next test stage. The laser emitter (2) is used to emit short-pulse high-power laser and irradiate the specimen. The high-speed camera (7) is used to photograph the vibrating specimen. The computer processing and control system obtains the displacement vibration frequency and amplitude information of the calibration sticker (11) according to the photographed images. At the same time, the displacement sensor (3003) sends the vibration frequency and amplitude information of the laser emitter (2) to the computer processing and control system; An equipment communication system, which includes a test end and a monitoring end, is used to transfer the photographed images and the crack propagation size measured by the computer processing and control system to the monitoring end of the test operator, and transfer the instructions input by the test operator at the monitoring end to the computer processing and control system.

2. The test device for the threshold value of metal fatigue crack propagation according to claim 1, characterized in that, The lifting bracket (3) includes: A first support base (3005) is arranged on the horizontal support plane (10); A servo motor and a transmission component (3004) are both arranged on the top of the first support base (3005), and the servo motor is electrically connected to the computer processing and control system; Three groups of high-speed ball screw feed components (3001) arranged in parallel are all connected to the transmission component (3004). The transmission component (3004) is used to drive the three groups of high-speed ball screw feed components (3001) to move synchronously. Each group of high-speed ball screw feed components (3001) includes a nut and a screw. The first mounting seat (3002) is arranged between the three groups of high-speed ball screw feed components (3001), and the side wall of the first mounting seat (3002) is fixedly connected to the three nuts. The output ends of the transmission component (3004) are respectively connected to the three screws to drive the three screws to rotate synchronously.

3. A test device for the threshold value of metal fatigue crack propagation according to claim 2, characterized in that, The transmission assembly (3004) includes a box body fixed on the top of the first support base (3005). A planetary gear set is arranged inside the box body. The planetary gear set includes a sun gear (301), a sun gear shaft (302), a first planetary gear (303), a first planetary gear shaft (304), a second planetary gear (305), a second planetary gear shaft (306), a third planetary gear (307), a third planetary gear shaft (308), a connecting frame (309), a ring gear (3010) and an outer ring bearing sleeve (3011). The sun gear shaft (302) is connected to the output shaft of the servo motor. The sun gear (301) is sleeved and fixed on the sun gear shaft (302). A connecting frame (309) is installed between the sun gear shaft (302), the first planetary gear shaft (304), the second planetary gear shaft (306) and the third planetary gear shaft (308). The first planetary gear (303) is correspondingly sleeved and fixed on the first planetary gear shaft (304). The second planetary gear (305) is correspondingly sleeved and fixed on the second planetary gear shaft (306). The third planetary gear (307) is correspondingly sleeved and fixed on the third planetary gear shaft (308). The inner sides of the first planetary gear (303), the second planetary gear (305) and the third planetary gear (307) are all meshed with the sun gear (301). The first planetary gear shaft (304), the second planetary gear shaft (306) and the third planetary gear shaft (308) are respectively connected to the lead screws in three groups of high-speed ball screw feed assemblies (3001). The outer ring bearing sleeve (3011) is fixed inside the box body. The ring gear (3010) is in sliding friction contact with the outer ring bearing sleeve (3011). The outer sides of the first planetary gear (303), the second planetary gear (305) and the third planetary gear (307) are all meshed with the ring gear (3010).

4. A test device for the threshold value of metal fatigue crack propagation according to claim 2, characterized in that, The rotating bracket (9) includes: A rotating base (9003) is arranged on the second support base (9004). The second support base (9004) is arranged on the horizontal support plane (10). The rotating base (9003) is disc-shaped and is controlled to rotate by a servo motor; Two telescopic struts (9002) are fixed at the edge of the rotating base (9003). The included angle between the geometric centers of the bottom surfaces of the two telescopic struts (9002) and the connecting line of the center of the rotating base (9003) is 90°. A second mounting seat (9001) is fixed at the top of each telescopic strut (9002). The infrared thermal imager (6) and the high-speed camera (7) are respectively arranged on the two second mounting seats (9001) and are fixed through bolts on the second mounting seats (9001). The telescopic struts (9002) are adjusted manually to adjust the height positions of the two second mounting seats (9001).

5. The test device for the threshold value of metal fatigue crack propagation according to claim 2, wherein The control cabinet of the low-frequency fatigue testing machine is connected to the computer processing control system through a data transmission system. The equipment communication system is connected to the computer processing control system through a data transmission system.

6. A test method for a test device of the threshold value of metal fatigue crack propagation according to claim 1, characterized in that, It includes the following steps: Step 1: Measure the dimensions of the crack propagation specimen (4), select a suitable fixture according to the dimensions of the crack propagation specimen (4), and install the fixture on the testing machine; Step 2: Turn on the low-frequency fatigue testing machine and zero it, and install the crack propagation specimen (4) on the fixture; Step 3: Place two sets of pulsed laser thermography measurement systems on the support planes on both sides of the testing machine, start the pulsed laser thermography measurement system, and adjust the positions of the infrared thermal imager (6), high-speed camera (7) and laser emitter (2) by adjusting the lifting bracket (3) and rotating bracket (9) so that the infrared thermal imager (6) and the high-speed camera (7) meet the position, imaging and field of view requirements when facing the specimen, and make the laser emitter (2) meet the irradiation position requirements; Step 4: Input the geometric dimensions of the crack propagation specimen measured in Step 1 into the human-machine interaction interface of the computer processing control system to calculate the initial test parameters, and then input the initial test parameters into the control cabinet of the low-frequency fatigue testing machine; Step 5: The testing machine starts the test, obtains the thermal image through the pulsed laser thermography system and extracts the crack characteristics; Step 6: Execute the fault judgment and test failure judgment of the low-frequency fatigue testing machine through the computer processing control system. If there is a fault or failure, directly execute Step 11. Otherwise, continue the test until Step 8; Step 7: At any time, the test operator remotely checks, through the monitoring end of the equipment communication system, the infrared thermal image with a virtual scale having a scale interval of 0.5 mm and the existing crack length output by the computer processing control system. , if there is a large deviation between the existing crack length and the visual measurement value of the test operator, the test operator stops the test through the monitoring end; otherwise, the test continues to Step 8. Step 8, when the crack propagation length reaches 0.25 mm, that is, the test suspension condition is satisfied, the computer processing control system performs relevant test suspension operations, and calculates the crack propagation rate and the parameters for the next test stage; Step Nine, the computer processing control system determines whether there are six or more data pairs evenly distributed between among If so, proceed to Step Eleven; if not, proceed to Step Five. Step 10: The computer processing control system automatically executes the relevant control operations for stopping the test on the testing machine and other systems, and performs the relevant calculations of the fatigue threshold value; Step 11: The test operator records the test data, removes the specimen and saves it, and closes each system of the metal fatigue crack propagation threshold test device to end the test.

7. The test device and test method for the threshold value of metal fatigue crack propagation according to claim 6, characterized in that, The steps of obtaining the thermal image through the pulsed laser thermography system and extracting the crack characteristics include: The computer processing control system controls the rotation of the rotating bracket (9) to make the high-speed camera face the crack propagation specimen (4); The high-speed camera (7) takes pictures of the vibrating crack propagation specimen (4). The computer processing control system obtains the displacement vibration frequency and amplitude information of the calibration sticker (11) according to the taken images. At the same time, the displacement sensor (3003) sends the vibration frequency and amplitude information of the laser emitter (2) to the computer processing control system; The computer processing control system, according to the above information, closed-loop controls the movement of the laser emitter (2) through the sliding mode control algorithm to keep the relative position relationship between the target position irradiated by the laser of the laser emitter (2) and the pasting position of the calibration sticker (11) stable; The laser emitter (2) emits short-pulse high-power laser, irradiates on the specimen, and cools for a period of time; The computer processing control system controls the rotation of the rotating bracket (9) to make the infrared thermal imager (6) face the crack propagation specimen (4); The infrared thermal imager (6) takes the thermal image of the crack propagation specimen (4), and performs crack measurement and further analysis.

8. The metal fatigue crack growth threshold test device and test method according to claim 6, characterized in that, The extracted crack characteristics include crack propagation length, crack offset, crack symmetry and crack bifurcation.

9. The metal fatigue crack growth threshold value test device and test method according to claim 8, characterized in that, The judgment of the test failure includes the following steps: In each crack recognition process, it is judged whether the crack meets the requirements of crack offset. Specifically, it is judged whether any point of the crack feature deviates from the theoretically crack propagation direction by more than 0.05 times the width of the crack propagation specimen. If so, it is said that the offset is too large; if not, it is said that the offset meets the requirements. In each crack recognition process, it is necessary to determine whether the crack meets the crack symmetry requirement. Specifically, the difference in the existing crack lengths of the infrared thermal images captured by the two infrared thermal imagers is judged whether it exceeds 0.25 times the thickness of the crack propagation specimen , if so, it is said that the crack symmetry requirement is not met; if not, it is said that the crack symmetry requirement is met; In each crack recognition process, it is necessary to determine whether the crack bifurcates. If it bifurcates, the longer crack bifurcation is selected as the main feature of the crack for measuring and calculating the crack length and crack propagation length. All bifurcated cracks need to be judged whether they meet the requirements of crack offset and crack symmetry.

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