Dynamic fatigue testing machine for metal material and testing method

Abstract

The invention discloses a dynamic fatigue testing machine for a metal material and a testing method, aiming at a thin-wall round tube with a hole, the method comprises the following steps: projecting a speckle pattern in a hole area of a sample by adopting an infrared light source, and respectively collecting sample images containing the speckle pattern and without the speckle pattern through a binocular DIC system; continuously recording load-displacement data and an image sequence containing or not containing speckles in the test process; on the basis of difference analysis between the image sequence containing the speckles and the image sequence not containing the speckles, interference areas caused by the speckles are recognized and eliminated; and carrying out DIC strain calculation based on the corrected image sequence containing the speckles. According to the invention, invisible infrared light of a conventional lens is used as a speckle light source to obtain an image sequence containing speckles, an image sequence without speckles is acquired through imaging of a reflector and is used as a net field to correct a speckle field, the interference phenomenon is eliminated, accurate monitoring of the DIC strain at the notch is realized, and the accuracy of the DIC strain at the notch is improved. The accurate analysis on the fatigue characteristic of the thin-wall round pipe with the hole is improved.

Description

Technical Field

[0001] The present invention relates to the field of measurement technology, specifically to the fatigue test of metal materials, and particularly to a dynamic fatigue testing machine and test method for metal materials. Background Art

[0002] After a metal material is subjected to cyclic loading, the local stress level exceeds the yield stress, and plastic strain accumulates. This process is manifested on the material surface as an increase in roughness. By using the optical effects caused by the roughness change, plastic strain is detected and the initiation of fatigue cracks is predicted.

[0003] Many components in engineering practice have notches. The presence of notches leads to stress concentration, which has a very significant impact on the life of the components. Therefore, stress-strain analysis and life prediction of notched components under multiaxial loading are of practical significance for the design and use of actual components. For notched specimens, obtaining the complex stress-strain relationship at the notch and finding the location of crack initiation are the key points.

[0004] In fatigue tests, the laser speckle method is often used, that is, a laser is emitted through a beam expander to form randomly distributed bright and dark spots, namely speckles, on the surface of the specimen. The distribution and intensity of the speckles can reflect the microscopic structure changes on the metal surface, be used for material quality inspection, and by analyzing the distortion of the speckle pattern, the three-dimensional morphology of the metal surface can be reconstructed.

[0005] However, on a thin-walled circular metal tube with holes, when laser speckles are generated in the hole area, part of the laser hits the hole edge or enters the hole and is reflected by the inner wall on the other side, resulting in interference phenomena such as laser reflection and refraction. Among them, the scattering of the laser at the hole edge will cause phase noise in the non-target area to be superimposed in the speckle field, and the laser passing through the hole and reflected by the inner wall of the circular tube will introduce additional optical path difference and polarization state changes, resulting in aliased artifact signals in the speckle field.

[0006] Therefore, it is necessary to provide a dynamic fatigue testing machine and test method for metal materials to solve the above technical problems. Summary of the Invention

[0007] The present invention overcomes the deficiencies of the prior art and provides a dynamic fatigue testing machine and test method for metal materials.

[0008] To achieve the above object, the technical solution adopted by the present invention is: a dynamic fatigue test method for metal materials, including the following steps: S1. Install a thin-walled circular tube specimen with holes in the loading fixture of the fatigue testing machine; S2. Project a speckle pattern on the hole area of the specimen using an infrared light source, and respectively collect specimen images with and without the speckle pattern through a binocular DIC system; S3. Apply an axial tensile-compressive cyclic load to the thin-walled circular tube specimen to conduct a dynamic fatigue test; S4. Continuously record the load-displacement data and the specimen image sequences with and without speckles during the test; S5. Based on the difference analysis between the image sequences with and without speckles obtained in step S4, identify and eliminate the interference areas caused by the speckles; S6. Based on the corrected image sequence with speckles, conduct DIC strain calculation and analyze its fatigue characteristics.

[0009] In a preferred embodiment of the present invention, in S2, the infrared light source is a near-infrared laser with a wavelength of 1550 nm. The near-infrared laser beam is uniformly expanded to the hole area on the specimen surface through a beam expander and a collimator to form a speckle pattern.

[0010] In a preferred embodiment of the present invention, in S2, the binocular DIC system includes: A beam splitter prism for splitting the reflected light on the specimen surface into a first optical path and a second optical path, wherein the second optical path is deflected by 90°; A first camera corresponding to the first optical path for collecting specimen images with the speckle pattern; a lithium niobate thin film is arranged in front of the first camera lens, and the lithium niobate thin film is used to convert the infrared speckles into visible light; A second camera corresponding to the second optical path for collecting specimen images without the speckle pattern.

[0011] In a preferred embodiment of the present invention, in S2, the infrared light source irradiates obliquely from below the specimen; The center of the hole on the specimen, the center of the beam splitter prism, and the optical axis of the first camera are on the same straight line, and the distances between the beam splitter prism and the hole and the first camera are equal; The second camera is arranged directly above the beam splitter prism, and the distance from the second camera to the beam splitter prism is the same as the distance from the first camera to the beam splitter prism.

[0012] In a preferred embodiment of the present invention, the thickness of the lithium niobate thin film is 180 - 220 nm, and a periodic silica grating is arranged on the surface.

[0013] In a preferred embodiment of the present invention, in S4, the test data acquisition method includes the following steps S41. Generate a unified TTL pulse signal to synchronously trigger the acquisition of axial force, axial displacement measurement, and specimen images with and without speckle patterns. S42. During data and image acquisition, automatically adjust the image acquisition frequency according to the load frequency to ensure that at least 10 frames of images are acquired in each load cycle. S43. Record unified timestamps for each frame of image and mechanical data, establish a time-sequence correlation database, and store the test data in blocks according to the number of cycles.

[0014] In a preferred embodiment of the present invention, in the above S5, the difference analysis method includes the following steps: S51. Based on the unified timestamp in S43, perform sub-pixel registration and histogram equalization on the speckle-containing image and the speckle-free image at the same moment. S52. Calculate the absolute difference image between the registered speckle-containing image I_speckle and the speckle-free image I_clean: ΔI = |I_speckle - I_clean|. Perform Otsu adaptive threshold segmentation on ΔI to extract high-difference regions and mark them as potential interference candidate regions. S53. Based on the specimen CAD model or pre-calibrated hole positions, define an annular buffer zone along the hole edge in the image. The width of the annular buffer zone is 5 - 10 pixels. Perform opening operation on the candidate regions of ΔI to remove isolated noise points and retain continuous scattering stripes. S54. According to the incident angle of the infrared light source and the hole geometric parameters, construct a ray tracing model to predict the projection area of the reflected light on the inner wall of the hole in the image. Use the predicted reflection area as a template to perform normalized cross-correlation matching in ΔI to locate the actual reflection interference area. S55. Before the dynamic fatigue test, based on the speckle-containing image I_speckle and the speckle-free image I_clean in the unloaded state, generate an initial interference mask to mark the spatial edge buffer zone, the inner wall reflection area, and the permanent interference area. During the test, update the dynamic mask according to the real-time difference analysis results to add temporary interference areas caused by specimen deformation or speckle shedding. S56. For the interference regions in the speckle-containing image I_speckle, including the regions marked as interference by the static mask or the dynamic mask, replace them with the pixel values at the corresponding positions in the speckle-free image to generate a purified speckle image.

[0015] In a preferred embodiment of the present invention, in the above S1, filler rods are inserted into the clamping positions at both ends of the thin-walled circular tube specimen with holes. The diameter of the filler rods differs from the inner diameter of the specimen by 0 - 1 mm, and the end is at least 5 - 10 mm away from the hole. The filling material is aluminum alloy.

[0016] A dynamic fatigue testing machine for metal materials, comprising: a main frame, an actuator, a test fixture, a hydraulic system, a control system, a light source system, and a binocular DIC system; Wherein, the actuator is connected to the test fixture to apply axial tensile and compressive loads; The light source system includes an infrared laser, a beam expander, and a collimator, and is used to project a speckle pattern in the hole area of the specimen; The binocular DIC system includes a beam splitting prism, a first camera, and a second camera, and the first camera is configured with a lithium niobate thin film for converting infrared speckles into visible light.

[0017] In a preferred embodiment of the present invention, the beam splitting prism divides the reflected light of the specimen into two optical paths. The first optical path directly guides to the first camera, and the second optical path deflects 90° to guide to the second camera; the fields of view of the first camera and the second camera cover the hole area of the specimen, and the shooting positions and angles are the same.

[0018] The present invention solves the defects existing in the background technology, and the present invention has the following beneficial effects: (1) The present invention provides a dynamic fatigue testing machine and a testing method for metal materials. Infrared light invisible to conventional lenses is used as a speckle light source to obtain an image sequence containing speckles, and an image sequence without speckles is collected through mirror imaging as a clean field to correct the speckle field, eliminating the above interference phenomena, realizing accurate monitoring of DIC strain at the notch, and improving the accurate analysis of the fatigue characteristics of thin-walled circular tubes with holes.

[0019] (2) Through the position setting of the hardware in the binocular DIC system of the present invention, the position and angle of the image taken by the second camera are the same as those of the image taken by the first camera, obtaining the shooting effect at the same position. Moreover, a near-infrared laser with a wavelength of 1550 nm is used as the speckle light source, which is invisible under the lens of a conventional camera (the second camera). The surface image of the specimen without a speckle pattern is taken. A lithium niobate thin film is arranged in front of the lens of the first camera. Based on the SHG / SFG effect of the lithium niobate thin film, the infrared speckles with a wavelength of 1550 nm are converted into visible light with a wavelength of 550 nm. Therefore, the first camera takes the surface image of the specimen with a speckle pattern. Through the coordinated setting of the light source system and the binocular DIC system, in the dynamic fatigue test of a thin-walled circular tube specimen with holes, images with and without speckles at the same position can be collected simultaneously, which can be used for subsequent comparative verification to improve the accuracy of fatigue test analysis.

[0020] (3) By combining ray tracing with difference analysis, the present invention accurately locates and eliminates speckle noise caused by interference phenomena such as reflection and refraction of laser light in the hole area. Specifically, an image without speckles is used as the clean field to correct the speckle-containing image, eliminate interference phenomena, and preserve the integrity of the true speckle field, providing a high-fidelity data basis for stress-strain analysis at the notch and crack initiation prediction, and ensuring the accuracy of DIC strain calculation. Brief Description of the Drawings

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings; Figure 1 It is a diagram of the dynamic fatigue test method for metallic materials in the preferred embodiment of the present invention; Figure 2 It is a schematic diagram of the infrared optical path and the imaging optical path in the preferred embodiment of the present invention; Figure 3 It is a flowchart of the test data acquisition method in the preferred embodiment of the present invention; Figure 4 It is a flowchart of the difference analysis method in the preferred embodiment of the present invention; In the figure: 1. Specimen; 2. Infrared laser; 3. Beam expander; 4. Collimator; 5. Beam splitter prism; 6. First camera; 7. Second camera. Detailed Embodiments

[0022] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0023] Many specific details are set forth in the following description in order to provide a thorough understanding of the present invention, but the present invention can also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited by the specific embodiments disclosed below.

[0024] In the description of this application, it should be understood that the orientation or positional relationships indicated by the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. are based on the orientation or positional relationships shown in the drawings. These are only for the convenience of describing this application 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. Therefore, it should not be construed as limiting the protection scope of this application. In addition, terms such as "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Therefore, features defined with "first", "second", etc. may explicitly or implicitly include one or more of such features. In the description of this invention, unless otherwise stated, the meaning of "a plurality" is two or more.

[0025] In the description of this application, it should be noted that unless otherwise clearly specified and limited, the terms "installed", "connected", "coupled" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood through specific circumstances.

[0026] As Figure 1 shown, the present invention provides a dynamic fatigue test method for metal materials, including the following steps: S1. Install the perforated thin-walled circular tube specimen 1 in the loading fixture of the fatigue testing machine; S2. Use an infrared light source to project a speckle pattern in the hole area of the specimen 1, and respectively collect images of the specimen 1 with and without the speckle pattern through a binocular DIC system; S3. Apply an axial tensile-compressive cyclic load to the thin-walled circular tube specimen 1 to conduct a dynamic fatigue test; S4. Continuously record the load-displacement data and the image sequences of the specimen 1 with and without speckles during the test; S5. Based on the difference analysis between the image sequences with and without speckles obtained in step S4, identify and eliminate the interference areas caused by speckles; S6. Based on the corrected image sequence with speckles, conduct DIC strain calculation and analyze its fatigue characteristics.

[0027] For a thin-walled circular metal tube with holes, the present invention proposes a dynamic fatigue test method for metal materials. Infrared light invisible to a conventional lens is used as a speckle light source to obtain an image sequence containing speckles, and an image sequence without speckles is collected through mirror imaging as a net field to correct the speckle field, eliminating the above interference phenomena, achieving accurate monitoring of DIC strain at the notch, and improving the accurate analysis of the fatigue characteristics of the thin-walled circular tube with holes.

[0028] The following elaborates in detail each step of a dynamic fatigue test method for metal materials according to the present invention.

[0029] Step S1: Install the thin-walled circular tube specimen 1 with holes in the loading fixture of the fatigue testing machine.

[0030] Among them, the preparation of the thin-walled circular tube specimen 1 with holes includes: Step S11: Cut the thin-walled circular tube with holes into a length of 50 - 100 mm, with the notch close to the middle of the axial direction of the specimen 1.

[0031] Step S12: Insert a filling round bar into the tube at the positions where both ends of the thin-walled circular tube with holes are clamped by the fixture, and fix the filling round bar to the thin-walled circular tube with holes by pouring glue.

[0032] In addition, refer to GB / T 228.1 - 2021 to prepare the specimen.

[0033] Among them, the diameter of the filling round bar differs from the inner diameter of the thin-walled circular tube with holes by 0 - 1 mm, ensuring that the filling round bar can be inserted into the thin-walled circular tube with holes without providing additional outward support force to the thin-walled circular tube with holes.

[0034] The length of the part of the filling round bar inserted into the thin-walled circular tube with holes is greater than the length of the clamping contact part between the movable clamp block and the thin-walled circular tube with holes; the end of the filling round bar is at least 5 - 10 mm away from the hole to avoid the formation of a reflection path of the laser between the filling round bar and the edge of the hole.

[0035] By inserting a filling round bar into the thin-walled circular tube with holes, there is no stress concentration phenomenon or the stress concentration phenomenon is reduced at the positions where both ends of the thin-walled circular tube with holes are clamped by the fixture during the fatigue test, ensuring that the positions where the fixture clamps are intact when cracks appear at the notch of the specimen 1.

[0036] It should be noted that the material of the filling round bar is aluminum alloy to ensure the test strength requirements.

[0037] The loading fixture includes: The upper fixture, which is used to apply tensile or compressive loads.

[0038] The lower fixture, which cooperates with the upper fixture to jointly clamp the specimen 1.

[0039] The upper fixture and the lower fixture fix the specimen 1 by means of threads, buckles, hydraulic pressure or mechanical locking, etc., to ensure that the specimen 1 does not slip or shift during the loading process.

[0040] Among them, the length of one end of the fixture clamped on the specimen 1 is 8 - 12 mm.

[0041] Step S2: Project a speckle pattern on the hole area of the specimen 1 by using an infrared light source, and respectively collect the surface images of the specimen 1 with and without the speckle pattern through a binocular DIC system.

[0042] Among them, the hole area is a circular area with a radius of 5 - 20 mm extending outward from the hole and its edge.

[0043] The infrared light source is a near-infrared laser with a wavelength of 1550 nm. The near-infrared laser beam is uniformly expanded to the hole area on the surface of the specimen 1 through a beam expander 3 and a collimator 4 to form a speckle pattern. Among them, the beam expander 3 is composed of a spherical lens and a cylindrical lens, and expands the near-infrared laser into a beam with a diameter of 50 - 80 mm.

[0044] The laser power is controlled at 100 - 300 mW to avoid speckle saturation caused by too strong power.

[0045] It should be noted that the near-infrared laser with a wavelength of 1550 nm is invisible light, that is, it does not appear in a conventional camera lens.

[0046] The binocular DIC system includes: A beam splitting prism 5 for splitting the reflected light on the surface of the specimen 1 into a first optical path and a second optical path, among which the second optical path is deflected by 90°. A first camera 6 configured as a high-resolution industrial camera, corresponding to the first optical path, and the field of view covers the hole area of the specimen 1; the lens of the first camera 6 uses a telecentric lens, and a lithium niobate thin film with a thickness of 180 - 220 nm is arranged in front of the lens, and a periodic silica grating is arranged on the surface of the lithium niobate thin film. In this embodiment, the SHG / SFG effect of the lithium niobate thin film is used to convert the infrared speckle with a wavelength of 1550 nm into visible light with a wavelength of 550 nm, so that the camera can simultaneously capture infrared and visible light information in a single image, that is, the surface image of the specimen 1 with the speckle pattern.

[0047] A second camera 7 configured as a high-resolution industrial camera, corresponding to the second optical path, and the field of view covers the hole area of the specimen 1 in the beam splitting prism 5; the second camera 7 uses a telecentric lens and is used to capture the surface image of the specimen 1 without the speckle pattern.

[0048] An image acquisition module for synchronously triggering the first and second cameras 7 to acquire images, and the acquisition frequency is 50 - 200 Hz to capture the transient strain field.

[0049] AsFigure 2 As shown, the thin-walled perforated circular tube specimen 1 is vertically fixed in the fixture, and the hole is located in the middle of the specimen 1. The infrared laser 2 irradiates obliquely from below the specimen 1, and the incident angle is 45°-60°; the center of the hole, the center of the beam splitter prism 5, and the optical axis of the first camera 6 are on the same straight line, and the distances between the beam splitter prism 5 and the hole and the first camera 6 are equal; the second camera 7 is arranged directly above the beam splitter prism 5, and the distance from the second camera 7 to the beam splitter prism 5 is the same as the distance from the first camera 6 to the beam splitter prism 5.

[0050] Through the position setting of the hardware in the binocular DIC system, the positions and angles of the images captured by the second camera 7 and the first camera 6 are made consistent, so as to obtain the shooting effect at the same position. Moreover, a near-infrared laser with a wavelength of 1550 nm is used as the speckle light source, which is invisible under the lens of a conventional camera (the second camera 7), and the surface image of the specimen 1 without the speckle pattern is captured. A lithium niobate thin film is arranged in front of the lens of the first camera 6. Based on the SHG / SFG effect of the lithium niobate thin film, the infrared speckle with a wavelength of 1550 nm is converted into visible light with a wavelength of 550 nm. Therefore, the first camera 6 captures the surface image of the specimen 1 with the speckle pattern. Through the coordinated setting of the light source system and the binocular DIC system, in the dynamic fatigue test of the thin-walled perforated circular tube specimen 1, images with and without speckles at the same position can be simultaneously collected, which can be used for subsequent comparison and verification to improve the accuracy of fatigue test analysis.

[0051] Step S3: Apply an axial tensile-compressive cyclic load to the thin-walled circular tube specimen 1 to conduct a dynamic fatigue test.

[0052] Organize the test according to the test purpose. For high-cycle fatigue, refer to GB / T 3075 or GB / T 37616 for cyclic tests, and for low-cycle fatigue, refer to GB / T 15248 for cyclic tests.

[0053] In one embodiment, the load waveform and amplitude in the dynamic fatigue test include: The loading waveform adopts a sine wave or a triangular wave, and the frequency range is 0.1-5 Hz.

[0054] Stress amplitude: The maximum stress σ_max: Take 0.5-0.8 times of the material yield strength σ_y to avoid premature plastic saturation at the notch; The minimum stress σ_min is selected according to the test objective: Tension-tension cycle: σ_min ≥0 (such as σ_min = 0.2σ_max); Tension-compression cycle: σ_min = -σ_max (symmetric cycle) or set proportionally (such as σ_min = -0.5σ_max).

[0055] The stress ratio R is defined as R = σ_min / σ_max, and typical values include R = 0 (pulsating tension) and R = -1 (symmetric cycle).

[0056] Load control feeds back the strain amplitude (Δε) at the notch in real time through DIC and dynamically adjusts the load to maintain the target strain range.

[0057] Step S4: Continuously record the load-displacement data and the image sequences of Specimen 1 with and without speckles during the test.

[0058] As Figure 3 shown, the specific test data acquisition method includes the following steps: Step S41: Generate a unified TTL pulse signal to synchronously trigger the measurement of axial force, axial displacement, and the image acquisition of the first and second cameras 7.

[0059] Specifically, the fatigue testing machine controller generates a unified TTL pulse signal to synchronously trigger the load sensor (measuring axial force F), the displacement gauge (measuring axial displacement ΔL), the binocular DIC system (image acquisition of the first and second cameras 7), and the infrared light source (timing control of speckle projection), with a time deviation ≤ 10 μs to ensure strict correspondence between the mechanical data and the image frames.

[0060] Step S42: Automatically adjust the image acquisition frequency according to the load frequency during data and image acquisition to ensure that at least 10 frames of images are acquired in each load cycle.

[0061] Step S43: Record a unified timestamp for each frame of image and mechanical data, establish a time series correlation database, and store the test data in blocks according to the number of cycles, such as each 500 cycles as a data block.

[0062] It should be noted that the synchronous imaging of the binocular DIC system includes: Speckle-containing image (the first camera 6): Receive the 550 nm visible speckle light converted by the lithium niobate thin film through the beam splitter prism 5 to record the surface deformation information of Specimen 1; Speckle-free image (the second camera 7): Receive the natural light on the surface of Specimen 1 reflected by the beam splitter prism 5 (without infrared speckle interference) as the "clean field" reference.

[0063] Step S5: Based on the difference analysis between the image sequences with and without speckles obtained in Step S4, identify and eliminate the interference regions caused by speckles.

[0064] As Figure 4 shown, the specific difference analysis method includes the following steps: Step S51: Based on the unified timestamps in Step S43, perform sub-pixel registration and histogram equalization on the speckle-containing image and the non-speckle image at the same moment. Among them, sub-pixel registration uses SIFT feature matching or phase correlation method to eliminate the tiny offsets caused by camera parallax or the displacement of Specimen 1, and histogram equalization is used to eliminate the background gray-scale differences caused by illumination fluctuations, while retaining the local contrast features of the speckles and interferences.

[0065] Step S52: Calculate the absolute difference map of the registered speckle-containing image I_speckle and the non-speckle image I_clean: ΔI = |I_speckle - I_clean|. Perform Otsu adaptive threshold segmentation on ΔI to extract the high-difference regions, which are marked as potential interference candidate regions. The regions where ΔI is higher than the set threshold are high-difference regions.

[0066] Step S53: Based on the CAD model of Specimen 1 or the pre-calibrated hole positions, define an annular buffer zone along the hole edge in the image. The width of the annular buffer zone is 5 - 10 pixels, and perform opening operation on the candidate regions of ΔI to remove isolated noise points and retain continuous scattering stripes. It should be noted that subsequent analysis of scattering interference is only carried out within the annular buffer zone.

[0067] Step S54: According to the incident angle of the infrared light source and the hole geometric parameters, construct a ray-tracing model to predict the projection region of the reflected light on the inner wall of the hole in the image, and use the predicted reflection region as a template to perform normalized cross-correlation matching in ΔI to locate the actual reflection interference region.

[0068] Step S55: Before the dynamic fatigue test, based on the speckle-containing image I_speckle and the non-speckle image I_clean in the unloaded state, generate an initial interference mask to mark the spatial edge buffer zone, the inner wall reflection region, and the permanent interference region. During the test, update the dynamic mask according to the real-time difference analysis results, and add temporary interference regions generated due to the deformation of Specimen 1 or the shedding of speckles.

[0069] Step S56: For the interference regions in the speckle-containing image I_speckle, including the regions marked as interference by the static mask or the dynamic mask, replace them with the pixel values at the corresponding positions in the non-speckle image to generate a purified speckle image, and perform Gaussian blur transition on the boundaries of the replaced regions to avoid introducing false strains due to sudden gray-scale changes during DIC calculation.

[0070] The above-mentioned difference analysis method combines ray tracing with difference analysis to accurately locate and eliminate the speckle noise caused by interference phenomena such as reflection and refraction of laser in the hole area. Specifically, the speckle-free image is used as the clean field to correct the speckle-containing image, eliminate the interference phenomena, and retain the integrity of the true speckle field, providing a high-fidelity data basis for stress-strain analysis at the notch and crack initiation prediction, and ensuring the accuracy of DIC strain calculation.

[0071] Step S6: Based on the corrected speckle image sequence, perform DIC strain calculation and analyze its fatigue characteristics.

[0072] That is to say, through the laser speckle interferometry image processing technology, combined with the digital image correlation (DIC) algorithm, a high-precision strain field is extracted from the purified speckle image, and a quantitative correlation with fatigue damage is established.

[0073] The DIC strain calculation method includes the following steps: Step S61: Perform frequency-domain transformation on the corrected speckle image using the fast Fourier transform (FFT), extract the phase difference information, establish a linear relationship between the phase difference and the surface displacement gradient of the specimen 1 (the laser wavelength is used as the calibration parameter), and combined with the inverse synthetic aperture algorithm, based on the phase difference data, iteratively optimize the displacement field to achieve sub-pixel displacement measurement and obtain the full-field displacement distribution data; among them, the full-field displacement distribution data includes: axial, circumferential and shear components.

[0074] Step S62: Based on the full-field displacement distribution data, calculate the non-linear strain components through the displacement gradient to characterize large plastic deformation, and calculate the maximum principal strain and the equivalent plastic strain amplitude in the pre-defined annular buffer zone at the hole edge. Combine the number of cycles to construct a local strain-life evolution curve.

[0075] Step S63: Combine the fatigue damage model with the machine learning algorithm to analyze the local strain parameters (maximum principal strain, equivalent plastic strain amplitude) and load data (stress ratio, amplitude) output in Step S62, and output the crack initiation life and the fatigue damage distribution cloud map.

[0076] Specifically: Based on the critical plane method, correct the fatigue damage calculation model to quantify the coupling effect of shear strain and normal stress; secondly, use random forest or convolutional neural network, with strain statistical features (mean, gradient) and load parameters as inputs, train a high-precision life prediction model, and dynamically generate the remaining life confidence interval. Update the model parameters every 1000 cycles to correct the prediction results in real time, and finally output the crack initiation life and the fatigue damage distribution cloud map, providing data support for the life assessment of thin-walled structures with holes in the fields of aviation, automotive, etc.

[0077] The present invention also provides a dynamic fatigue testing machine for metal materials, comprising: a main frame, an actuator, a test fixture, a hydraulic system, a control system, a light source system, and a binocular DIC system. Among them, the main frame is a double-column gantry frame structure; the actuator is an electro-hydraulic servo actuator, and the hydraulic system adopts a high-pressure plunger type oil pump motor set, which is installed vertically. The hydraulic system provides power for the electro-hydraulic servo actuator; the test fixture includes an upper fixture and a lower fixture, both of which are connected to the actuator, support axial tensile and compressive loads, and fix the specimen 1 by means of threads, buckles, hydraulic or mechanical locking, etc.

[0078] The control system includes: a controller and a data acquisition module; among them, the controller is a full digital hydraulic servo controller based on the PCI bus, supports force / displacement / strain closed-loop control, according to; the data acquisition module includes: a load sensor (measuring the axial force F), a displacement meter (measuring the axial displacement ΔL), and an image trigger. The controller generates a unified TTL pulse signal to synchronously trigger the load, displacement, and binocular camera to collect data.

[0079] The light source system includes: an infrared laser 2 for emitting near-infrared laser with a wavelength of 1550 nm; a beam expander 3 and a collimator 4 for uniformly expanding the near-infrared laser beam to the hole area on the surface of the specimen 1 to form a speckle field.

[0080] The binocular DIC system includes: The first camera 6 is configured as a high-resolution industrial camera, and its field of view covers the hole area of the specimen 1; the lens of the first camera 6 is a telecentric lens, and a lithium niobate thin film with a thickness of 180 - 220 nm is arranged in front of the lens. A periodic silica grating is arranged on the surface of the lithium niobate thin film. In this embodiment, the SHG / SFG effect of the lithium niobate thin film is used to convert the infrared speckle with a wavelength of 1550 nm into visible light with a wavelength of 550 nm, so that the camera can simultaneously capture infrared and visible light information in a single image, that is, the surface image of the specimen 1 containing the speckle pattern.

[0081] The second camera 7 is configured as a high-resolution industrial camera, and its field of view covers the hole area of the specimen 1 in the reflector; the second camera 7 uses a telecentric lens and is used to capture the surface image of the specimen 1 without the speckle pattern.

[0082] The beam splitter prism 5 is used to divide the reflected light on the surface of the specimen 1 into two optical paths. The first optical path directly guides to the first camera 6, and the second optical path deflects 90° and guides to the second camera 7.

[0083] The image synchronization module is used to synchronously trigger the first and second cameras 7 to collect images, and the acquisition frequency is 50 - 200 Hz to capture the transient strain field.

[0084] Furthermore, a white background board is provided on the side of the specimen 1 away from the first camera 6, enabling the first camera 6 and the second camera 7 to clearly distinguish the edge of the specimen 1 and preventing mismeasurement caused by a cluttered background.

[0085] Through the position setting of the hardware in the binocular DIC system, the position and angle of the image captured by the second camera 7 are made consistent with those of the image captured by the first camera 6, obtaining the shooting effect at the same position.

[0086] Based on the ideal embodiments of the present invention as an inspiration, through the above description, relevant personnel can completely make various changes and modifications without departing from the technical idea of this invention. The technical scope of this invention is not limited to the content in the specification, and the technical scope must be determined according to the scope of the claims.

Claims

1. A dynamic fatigue test method for metallic materials, characterized in that, It includes the following steps: S1. Install the perforated thin-walled circular tube specimen in the loading fixture of the fatigue testing machine; S2. Use an infrared light source to project a speckle pattern in the hole area of the specimen, and respectively collect specimen images with and without the speckle pattern through a binocular DIC system; S3. Apply axial tensile-compressive cyclic loads to the thin-walled circular tube specimen to conduct a dynamic fatigue test; S4. Continuously record the load-displacement data and the specimen image sequences with and without speckles during the test; S5. Based on the difference analysis between the image sequences with and without speckles obtained in step S4, identify and eliminate the interference areas caused by speckles; S6. Based on the corrected image sequence with speckles, conduct DIC strain calculation and analyze its fatigue characteristics.

2. The dynamic fatigue test method for a metallic material according to claim 1, wherein, In S2, the infrared light source is a near-infrared laser with a wavelength of 1550 nm. The near-infrared laser beam is uniformly expanded to the hole area on the specimen surface through a beam expander and a collimator to form a speckle pattern.

3. A dynamic fatigue test method for a metal material according to claim 1, characterized in that In S2, the binocular DIC system includes: A beam splitter prism for splitting the reflected light on the specimen surface into a first optical path and a second optical path, where the second optical path is deflected by 90°; A first camera corresponding to the first optical path for collecting specimen images with a speckle pattern; a lithium niobate thin film is arranged in front of the first camera lens, and the lithium niobate thin film is used to convert the infrared speckles into visible light; A second camera corresponding to the second optical path for collecting specimen images without a speckle pattern.

4. A dynamic fatigue test method for a metal material according to claim 3, characterized in that, In S2, the infrared light source irradiates obliquely from below the specimen; The center of the hole on the specimen, the center of the beam splitter prism, and the optical axis of the first camera are on the same straight line, and the distances between the beam splitter prism and the hole and the first camera are equal; The second camera is arranged directly above the beam splitter prism, and the distance from the second camera to the beam splitter prism is the same as the distance from the first camera to the beam splitter prism.

5. The dynamic fatigue test method for a metal material according to claim 3, wherein: The thickness of the lithium niobate thin film is 180 - 220 nm, and a periodic silica grating is arranged on the surface.

6. A dynamic fatigue test method for a metal material according to claim 1, characterized in that, In S4, the test data acquisition method includes the following steps S41. Generate a unified TTL pulse signal to synchronously trigger the measurement of axial force, axial displacement, and the acquisition of specimen images with and without a speckle pattern; S42. When collecting data and images, automatically adjust the image acquisition frequency according to the load frequency to ensure that at least 10 frames of images are acquired in each load cycle; S43. Record a unified timestamp for each frame of image and mechanical data, establish a time-series correlation database, and store the test data in blocks according to the number of cycles.

7. A dynamic fatigue test method for a metal material according to claim 6, characterized in that, In S5, the difference analysis method includes the following steps: S51. Based on the unified timestamp in S43, perform sub-pixel registration and histogram equalization on the speckle-containing image and the speckle-free image at the same moment; S52. Calculate the absolute difference map of the registered speckle-containing image I_speckle and the speckle-free image I_clean: ΔI = |I_speckle - I_clean|, perform Otsu adaptive threshold segmentation on ΔI, extract the high-difference areas, and mark them as potential interference candidate areas; S53. Define an annular buffer for the hole edge in the image based on the specimen CAD model or the pre-calibrated hole positions. The width of the annular buffer is 5 - 10 pixels, and perform an opening operation on the candidate region of ΔI to remove isolated noise points and retain continuous scattering fringes. S54. Construct a ray tracing model according to the incident angle of the infrared light source and the hole geometric parameters, predict the projection area of the reflected light on the inner wall of the hole in the image, and use the predicted reflection area as a template to perform normalized cross-correlation matching in ΔI to locate the actual reflection interference region. S55. Before the dynamic fatigue test, generate an initial interference mask based on the speckle-containing image I_speckle and the speckle-free image I_clean in the unloaded state, and mark the spatial edge buffer, the inner wall reflection area, and the permanent interference area. During the test, update the dynamic mask according to the results of real-time difference analysis, and add temporary interference areas caused by specimen deformation or speckle shedding. S56. For the interference regions in the speckle-containing image I_speckle, including the regions marked as interference by the static mask or the dynamic mask, replace them with the pixel values at the corresponding positions in the speckle-free image to generate a purified speckle image.

8. A dynamic fatigue test method for a metal material according to claim 1, characterized in that, In S1, filling rods are inserted at the clamping positions at both ends of the thin-walled circular tube specimen with holes. The diameter of the filling rod differs from the inner diameter of the specimen by 0 - 1 mm, and the end is at least 5 - 10 mm away from the hole. The filling material is aluminum alloy.

9. A dynamic fatigue testing machine for metal materials, which is applied to the dynamic fatigue testing method for metal materials according to any one of claims 1-8, and is characterized in that, Including: A main frame, an actuator, a test fixture, a hydraulic system, a control system, a light source system, and a binocular DIC system; Among them, the actuator is connected to the test fixture to apply axial tensile and compressive loads; The light source system includes an infrared laser, a beam expander, and a collimator, and is used to project a speckle pattern in the hole area of the specimen; The binocular DIC system includes a beam splitter prism, a first camera, and a second camera. The first camera is equipped with a lithium niobate thin film for converting infrared speckles into visible light.

10. A dynamic fatigue testing machine for metal materials according to claim 9, characterized in that: The beam splitter prism divides the specimen reflected light into two optical paths. The first optical path directly guides to the first camera, and the second optical path deflects 90° and guides to the second camera. The fields of view of the first camera and the second camera cover the hole area of the specimen, and the shooting positions and angles are the same.

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