A dynamic fatigue testing machine and testing method for metal materials

Abstract

The present invention discloses a dynamic fatigue testing machine and testing method for metal materials. This method targets thin-walled circular tubes with holes. The method includes: using an infrared light source to project a speckle pattern onto the hole region of the specimen; using a binocular DIC system to capture images of the specimen, both containing and excluding the speckle pattern; continuously recording load-displacement data and image sequences containing and excluding speckle during the test; identifying and eliminating interference areas caused by speckle based on differential analysis between the image sequences containing and excluding speckle; and performing DIC strain calculation based on the corrected image sequences containing speckle. The method uses infrared light, invisible to conventional lenses, as a speckle light source to obtain an image sequence containing speckle. A speckle-free image sequence is then captured using a reflector for imaging. This is used as a net field to correct the speckle field, eliminating the aforementioned interference, enabling precise monitoring of DIC strain at the notch and improving the accuracy of fatigue analysis of thin-walled circular tubes with holes.

Description

Technical Field

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

[0002] When metal materials are subjected to cyclic loading, local stress levels exceed the yield stress, leading to the accumulation of plastic strain. This process manifests itself as an increase in surface roughness. Using the optical effect caused by this roughness change, we can detect plastic strain and predict the initiation of fatigue cracks.

[0003] Many components in engineering applications have notches. The presence of notches leads to stress concentrations, significantly impacting component life. 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, determining the complex stress-strain relationship at the notch and pinpointing the crack initiation location are crucial.

[0004] Laser speckle analysis is often used in fatigue testing. Laser light is emitted through a beam expander, creating randomly distributed bright and dark spots on the specimen surface. The distribution and intensity of speckle patterns can reveal microstructural changes on metal surfaces and are used for material quality testing. By analyzing the distortion of the speckle pattern, the three-dimensional topography of the metal surface can be reconstructed.

[0005] However, when laser light generates speckle in a thin-walled metal tube with a hole, some of the laser light will hit the hole edge or enter the hole and reflect off the inner wall on the other side, causing interference such as laser reflection and refraction. The scattering of the laser light at the hole edge causes phase noise from non-target areas to be superimposed on the speckle field. Furthermore, the laser light that passes through the hole and reflects off the inner wall of the tube introduces additional optical path difference and polarization state changes, resulting in aliasing artifacts in the speckle field.

[0006] Therefore, it is necessary to provide a dynamic fatigue testing machine and testing 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 a testing 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, comprising the following steps:

[0009] S1. Install the thin-walled circular tube specimen with a hole in the loading fixture of the fatigue testing machine;

[0010] S2, projecting a speckle pattern onto the hole area of the sample using an infrared light source, and collecting images of the sample with and without the speckle pattern using a binocular DIC system;

[0011] S3, applying axial tension and compression cyclic loads to the thin-walled circular tube specimen to perform a dynamic fatigue test;

[0012] S4, continuously recording load-displacement data and image sequences of specimens with and without speckles during the test;

[0013] S5. Based on the difference analysis between the image sequences containing speckle and the image sequences without speckle obtained in step S4, identifying and removing the interference area caused by speckle;

[0014] S6. Based on the corrected speckle-containing image sequence, DIC strain calculation is performed to analyze its fatigue characteristics.

[0015] 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 evenly expanded to the hole area on the surface of the sample through a beam expander and a collimator to form a speckle pattern.

[0016] In a preferred embodiment of the present invention, in S2, the binocular DIC system includes:

[0017] A beam splitter prism is used to split the reflected light from the sample surface into a first light path and a second light path, wherein the second light path is deflected by 90°;

[0018] A first camera, corresponding to the first optical path, is used to capture a sample image containing a speckle pattern; a lithium niobate film is provided in front of the lens of the first camera, and the lithium niobate film is used to convert the infrared speckle into visible light;

[0019] The second camera, corresponding to the second optical path, is used to collect a sample image without a speckle pattern.

[0020] In a preferred embodiment of the present invention, in S2, the infrared light source irradiates obliquely from below the sample;

[0021] The center of the hole on the sample, the center of the dichroic prism, and the optical axis of the first camera are on the same straight line, and the distances between the dichroic prism and the hole and the first camera are equal;

[0022] The second camera is arranged directly above the beam splitter prism, and the distance from the second camera to the beam splitter prism is consistent with the distance from the first camera to the beam splitter prism.

[0023] In a preferred embodiment of the present invention, the thickness of the lithium niobate film is 180-220 nm, and a periodic silicon dioxide grating is provided on the surface.

[0024] In a preferred embodiment of the present invention, in said S4, the test data collection method comprises the following steps:

[0025] S41, generating a unified TTL pulse signal to synchronously trigger the measurement of axial force and axial displacement, and the acquisition of images of samples with and without speckle patterns;

[0026] 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 collected per load cycle;

[0027] 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.

[0028] In a preferred embodiment of the present invention, in S5, the difference analysis method comprises the following steps:

[0029] S51, performing sub-pixel registration and histogram equalization on the image containing speckle and the image not containing speckle at the same moment based on the unified timestamp in S43;

[0030] S52, calculating the absolute difference between the registered speckle-containing image I_speckle and the non-speckle-containing image I_clean: ΔI = |I_speckle - I_clean|, performing Otsu adaptive threshold segmentation on ΔI, extracting high-difference areas, and marking them as potential interference candidate areas;

[0031] S53, based on the sample CAD model or the pre-calibrated hole position, define a circular buffer around the hole edge in the image, where the width of the circular buffer is 5-10 pixels, and perform an opening operation on the candidate area of ΔI to remove isolated noise points and retain continuous scattering fringes;

[0032] S54. Construct a ray tracing model based on the incident angle of the infrared light source and the geometric parameters of the hole to predict the projection area of the light reflected from 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.

[0033] S55. Before the dynamic fatigue test, an initial interference mask is generated based on the unloaded speckle image I_speckle and the unloaded speckle image I_clean, marking the spatial buffer zone, the inner wall reflection zone, and the permanent interference zone. During the test, the dynamic mask is updated according to the real-time difference analysis results, and temporary interference zones caused by specimen deformation or speckle shedding are added.

[0034] S56 , replacing the interference areas in the speckle-containing image I_speckle, including the areas marked as interference by the static mask or the dynamic mask, with pixel values of corresponding positions in the image not containing speckle, to generate a purified speckle image.

[0035] In a preferred embodiment of the present invention, in S1, filling round rods are inserted into the clamping positions at both ends of the thin-walled circular tube sample with a hole. The diameter of the filling round rod differs from the inner diameter of the sample by 0-1 mm, and the end is at least 5-10 mm away from the hole. The filling material is aluminum alloy.

[0036] 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;

[0037] wherein the actuator is connected to the test fixture to apply axial tensile and compressive loads;

[0038] The light source system includes an infrared laser, a beam expander, and a collimator, and is used to project a speckle pattern onto the sample hole area;

[0039] The binocular DIC system includes a beam splitter prism, a first camera, and a second camera. The first camera is configured with a lithium niobate film that converts infrared speckle into visible light.

[0040] In a preferred embodiment of the present invention, the dichroic prism divides the sample reflected light into two light paths, the first light path is directly directed to the first camera, and the second light path is deflected 90° to the second camera; the field of view of the first camera and the second camera covers the sample hole area, and the shooting position and angle are consistent.

[0041] The present invention solves the defects existing in the background technology and has the following beneficial effects:

[0042] The present invention provides a dynamic fatigue testing machine and testing method for metal materials. Infrared light, which is invisible to conventional lenses, is used as a speckle light source to obtain an image sequence containing speckle. A speckle-free image sequence is collected through reflector imaging and used as a clean field to correct the speckle field, eliminating the aforementioned interference phenomenon. This allows for precise monitoring of DIC strain at notches and improves the accuracy of fatigue analysis of thin-walled circular tubes with holes.

[0043] The present invention utilizes hardware positioning within the binocular DIC system to ensure that the position and angle of the image captured by the second camera align with those captured by the first camera, resulting in images captured from the same location. Furthermore, a near-infrared laser with a wavelength of 1550nm is used as the speckle light source. This is invisible to conventional cameras (the second camera), and the image captured is of the sample surface without the speckle pattern. A lithium niobate film is placed in front of the first camera lens. The SHG / SFG effect of the lithium niobate film converts the 1550nm infrared speckle into visible light at a wavelength of 550nm, resulting in the first camera capturing an image of the sample surface with the speckle pattern. By combining the light source system and the binocular DIC system, the present invention can simultaneously capture images of the sample surface with and without speckle during dynamic fatigue testing of thin-walled circular tube specimens with holes. This can be used for subsequent comparative verification, improving the accuracy of fatigue test analysis.

[0044] This method combines ray tracing with differential analysis to precisely locate and eliminate speckle noise caused by interference phenomena such as reflection and refraction of the laser light in the hole area. Specifically, the image without speckle is used as the clean field to correct the image containing speckle, eliminating interference while preserving the integrity of the true speckle field. This provides a high-fidelity data foundation for stress and strain analysis at the notch and crack initiation prediction, ensuring the accuracy of DIC strain calculations. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments described in the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive efforts.

[0046] Figure 1 2. It is a diagram of a dynamic fatigue testing method for metal materials according to a preferred embodiment of the present invention;

[0047] Figure 2 Schematic diagram of the infrared optical path and imaging optical path of a preferred embodiment of the present invention;

[0048] Figure 3 is a flow chart of a test data collection method according to a preferred embodiment of the present invention;

[0049] Figure 4 is a flow chart of a difference analysis method according to a preferred embodiment of the present invention;

[0050] In the figure: 1. Thin-walled circular tube sample; 2. Infrared laser; 3. Beam expander; 4. Collimator; 5. Beam splitter; 6. First camera; 7. Second camera. DETAILED DESCRIPTION

[0051] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0052] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below.

[0053] In the description of the present application, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present 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, and therefore cannot be understood as limiting the scope of protection of the present application. In addition, the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

[0054] In the description of this application, it should be noted that, unless otherwise expressly specified or limited, the terms "installed," "connected," and "connected" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to internal connections between two components. Those skilled in the art will understand the specific meanings of the above terms in this application based on specific circumstances.

[0055] like Figure 1 As shown, the present invention provides a dynamic fatigue test method for metal materials, comprising the following steps:

[0056] S1. Install the perforated thin-walled circular tube specimen 1 in the loading fixture of the fatigue testing machine;

[0057] S2, projecting a speckle pattern onto the hole area of the thin-walled circular tube specimen 1 using an infrared light source, and collecting images of the thin-walled circular tube specimen 1 with and without the speckle pattern using a binocular DIC system;

[0058] S3. Apply axial tension and compression cyclic loads to the thin-walled circular tube specimen 1 to perform a dynamic fatigue test;

[0059] S4, continuously recording load-displacement data and image sequences of the thin-walled circular tube specimen 1 with and without speckle during the test;

[0060] S5. Based on the difference analysis between the image sequences containing speckle and the image sequences without speckle obtained in step S4, identifying and removing the interference area caused by speckle;

[0061] S6. Based on the corrected speckle-containing image sequence, DIC strain calculation is performed to analyze its fatigue characteristics.

[0062] For thin-walled metal circular tubes with holes, the present invention proposes a dynamic fatigue testing method for metal materials. Infrared light, which is invisible to conventional lenses, is used as a speckle light source to obtain an image sequence containing speckle. A speckle-free image sequence is then collected through reflector imaging, which serves as a clean field to correct the speckle field, eliminating the aforementioned interference phenomenon. This allows for precise monitoring of DIC strain at the notch, improving the accuracy of fatigue analysis of thin-walled circular tubes with holes.

[0063] The following is a detailed description of each step of a dynamic fatigue testing method for metal materials according to the present invention.

[0064] Step S1: Install the perforated thin-walled circular tube specimen 1 in a loading fixture of a fatigue testing machine.

[0065] The preparation of the thin-walled circular tube sample 1 with a hole includes:

[0066] Step S11: Cut the thin-walled circular tube with a hole into pieces 50-100 mm long, with the notch close to the axial center of the thin-walled circular tube sample 1.

[0067] Step S12: inserting a filling round rod into the tube at the positions where both ends of the thin-walled circular tube with holes are clamped by the clamps, and fixing the filling round rod and the thin-walled circular tube with holes by glue injection.

[0068] In addition, the specimens were prepared according to GB / T 228.1-2021.

[0069] The diameter of the filling rod differs from the inner diameter of the perforated thin-walled round tube by 0-1 mm, ensuring that the filling rod can be inserted into the perforated thin-walled round tube without providing additional outward supporting force to the perforated thin-walled round tube.

[0070] The length of the portion of the filling rod that penetrates into the thin-walled tube with a hole is greater than the length of the clamping contact portion between the movable clamping block and the thin-walled tube with a hole; the end of the filling rod is at least 5-10 mm away from the hole to avoid the laser forming a reflection path between the filling rod and the edge of the hole.

[0071] By inserting a filling round rod into the thin-walled circular tube with a hole, stress concentration does not exist or is reduced at the positions where the clamps at both ends of the thin-walled circular tube with a hole are clamped during the fatigue test, ensuring that the positions where the clamps are clamped are intact when cracks appear at the notch of the thin-walled circular tube specimen 1.

[0072] It is worth mentioning that the filling round rod is made of aluminum alloy to ensure the test strength requirements.

[0073] The loading fixture includes:

[0074] Upper grip, used to apply tensile or compressive loads.

[0075] The lower fixture cooperates with the upper fixture to jointly clamp the thin-walled circular tube specimen 1.

[0076] The upper fixture and the lower fixture fix the thin-walled circular tube specimen 1 by means of threads, buckles, hydraulic or mechanical locking, etc., to ensure that the thin-walled circular tube specimen 1 does not slide or deflect during the loading process.

[0077] The length of one end of the clamp clamped on the thin-walled circular tube sample 1 is 8-12 mm.

[0078] Step S2: projecting a speckle pattern on the hole area of the thin-walled circular tube sample 1 using an infrared light source, and collecting surface images of the thin-walled circular tube sample 1 with and without the speckle pattern using a binocular DIC system.

[0079] The hole area is a circular area extending 5-20 mm outward from the hole and its edges.

[0080] The infrared light source is a near-infrared laser with a wavelength of 1550nm. The near-infrared laser beam is evenly expanded to the hole area on the surface of the thin-walled circular tube sample 1 by a beam expander 3 and a collimator 4, forming a speckle pattern. The beam expander 3 is a combination of a spherical lens and a cylindrical lens, which expands the near-infrared laser into a beam with a diameter of 50-80mm.

[0081] The laser power is controlled within 100-300mW to avoid speckle saturation caused by excessive power.

[0082] It should be noted that near-infrared laser with a wavelength of 1550nm is invisible light, which means it is not displayed in conventional camera lenses.

[0083] Binocular DIC system includes:

[0084] The beam splitter prism 5 is used to split the reflected light from the surface of the thin-walled circular tube sample 1 into a first light path and a second light path, wherein the second light path is deflected by 90 degrees.

[0085] The first camera 6 is configured as a high-resolution industrial camera, corresponding to the first optical path, and its field of view covers the hole area of the thin-walled circular tube specimen 1. The first camera 6 uses a telecentric lens, and a lithium niobate film with a thickness of 180-220nm is set in front of the lens. A periodic silicon dioxide grating is set on the surface of the lithium niobate film. In this embodiment, the SHG / SFG effect of the lithium niobate film is used to convert the infrared speckle with a wavelength of 1550nm into visible light with a wavelength of 550nm. The camera is used to simultaneously capture infrared and visible light information in a single image, that is, the surface image of the thin-walled circular tube specimen 1 containing the speckle pattern.

[0086] The second camera 7 is configured as a high-resolution industrial camera, corresponding to the second optical path, and its field of view covers the hole area of the thin-walled circular tube sample 1 in the dichroic prism 5; the second camera 7 adopts a telecentric lens to capture the surface image of the thin-walled circular tube sample 1 without the speckle pattern.

[0087] The image acquisition module is used to synchronously trigger the first and second cameras 7 to acquire images at a frequency of 50-200 Hz to capture the transient strain field.

[0088] like Figure 2 As shown, the thin-walled circular tube sample 1 is vertically fixed in the fixture, the hole is located in the middle of the thin-walled circular tube sample 1, and the infrared laser 2 is irradiated obliquely from the bottom of the thin-walled circular tube sample 1 with an incident angle of 45°-60°; the center of the hole, the center of the dichroic prism 5, and the optical axis of the first camera 6 are on the same straight line, and the distances between the dichroic prism 5 and the hole and the first camera 6 are equal; the second camera 7 is set directly above the dichroic prism 5, and the distance from the second camera 7 to the dichroic prism 5 is consistent with the distance from the first camera 6 to the dichroic prism 5.

[0089] By positioning the hardware in the binocular DIC system, the image captured by the second camera 7 is aligned with the position and angle of the image captured by the first camera 6, resulting in images captured at the same location. Furthermore, a near-infrared laser with a wavelength of 1550 nm is used as the speckle light source. This is invisible to conventional cameras (second camera 7), and the surface image of the thin-walled circular tube specimen 1 is captured without the speckle pattern. A lithium niobate film is placed in front of the lens of the first camera 6. The SHG / SFG effect of the lithium niobate film converts the 1550 nm infrared speckle into visible light at a wavelength of 550 nm, resulting in the first camera 6 capturing the surface image of the thin-walled circular tube specimen 1 with the speckle pattern. By coordinating the light source system and the binocular DIC system, the present invention can simultaneously capture images with and without speckle at the same location during dynamic fatigue testing of a perforated thin-walled circular tube specimen 1. This can be used for subsequent comparative verification and improve the accuracy of fatigue test analysis.

[0090] Step S3: applying axial tension and compression cyclic loads to the thin-walled circular tube specimen 1 to perform a dynamic fatigue test.

[0091] The test shall be organized according to the test purpose. The high cycle fatigue test shall be carried out in accordance with GB / T 3075 or GB / T 37616, and the low cycle fatigue test shall be carried out in accordance with GB / T 15248.

[0092] In one embodiment, the load waveform and amplitude in the dynamic fatigue test include:

[0093] The loading waveform adopts sine wave or triangle wave with a frequency range of 0.1-5Hz.

[0094] Stress amplitude:

[0095] Maximum stress σ_max: Take 0.5-0.8 times the material yield strength σ_y to avoid premature plastic saturation at the notch;

[0096] The minimum stress σ_min is selected according to the test objectives:

[0097] Pull-pull cycle: σ_min ≥ 0 (e.g. σ_min = 0.2σ_max);

[0098] Tension-compression cycle: σ_min=-σ_max (symmetrical cycle) or set proportionally (e.g. σ_min=-0.5σ_max).

[0099] The stress ratio R is R=σ_min / σ_max, and typical values include R=0 (pulsating stretching) and R=-1 (symmetrical cyclic).

[0100] The load control uses DIC to provide real-time feedback of the strain amplitude (Δε) at the notch and dynamically adjust the load to maintain the target strain range.

[0101] Step S4: continuously record load-displacement data and image sequences of the thin-walled circular tube specimen 1 with and without speckles during the test.

[0102] like Figure 3 As shown, the specific test data collection method includes the following steps:

[0103] Step S41 : Generate a unified TTL pulse signal to synchronously trigger the axial force and axial displacement measurement and the image acquisition of the first and second cameras 7 .

[0104] Specifically, the fatigue testing machine controller generates a unified TTL pulse signal to synchronously trigger the load sensor (measuring the axial force F), the displacement meter (measuring the axial displacement ΔL), the binocular DIC system (image acquisition of the first and second cameras 7) and the infrared light source (speckle projection timing control). The time deviation is ≤10μs, ensuring that the mechanical data strictly corresponds to the image frame.

[0105] Step S42: When collecting data and images, the image collection frequency is automatically adjusted according to the load frequency to ensure that at least 10 frames of images are collected in each load cycle.

[0106] 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 500 cycles as one data block.

[0107] It is worth noting that the binocular DIC system's simultaneous imaging includes:

[0108] Speckle image (first camera 6): Receives the 550nm visible speckle light converted by the lithium niobate thin film through the beam splitter prism 5 and records the surface deformation information of the thin-walled circular tube sample 1;

[0109] Image without speckle (second camera 7): receives natural light (without infrared speckle interference) from the surface of the thin-walled circular tube sample 1 reflected by the beam splitter 5 as a "clean field" reference.

[0110] Step S5: Based on the difference analysis between the image sequences containing speckles and the image sequences without speckles obtained in step S4, the interference areas caused by speckles are identified and eliminated.

[0111] like Figure 4 As shown, the specific difference analysis method includes the following steps:

[0112] Step S51: Based on the unified timestamp in step S43, sub-pixel registration and histogram equalization are performed on the speckle-containing image and the non-speckle-containing image at the same moment. The sub-pixel registration uses SIFT feature matching or phase correlation to eliminate slight offsets caused by camera parallax or displacement of the thin-walled circular tube specimen 1. Histogram equalization is used to eliminate background grayscale differences caused by illumination fluctuations and preserve the local contrast characteristics of speckle and interference.

[0113] Step S52 : Calculate the absolute difference between the registered speckle-containing image I_speckle and the non-speckle-containing image I_clean: ΔI=|I_speckle-I_clean|, perform Otsu adaptive threshold segmentation on ΔI, extract high-difference areas, and mark them as potential interference candidate areas; areas with ΔI higher than the set threshold are high-difference areas.

[0114] Step S53: Based on the CAD model of the thin-walled circular tube specimen 1 or the pre-calibrated hole position, a circular buffer zone is defined along the hole edge in the image. The width of the circular buffer zone is 5-10 pixels, and an opening operation is performed on the candidate area of ΔI to remove isolated noise points and retain continuous scattering fringes. It should be noted that the subsequent analysis of scattering interference is only within the circular buffer zone.

[0115] Step S54: Construct a ray tracing model based on the incident angle of the infrared light source and the geometric parameters of the hole to predict the projection area of the light reflected from 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.

[0116] Step S55: Before the dynamic fatigue test, an initial interference mask is generated based on the unloaded speckle image I_speckle and the unloaded speckle image I_clean. The mask marks the spatial buffer zone, the inner wall reflection zone, and the permanent interference zone. During the test, the dynamic mask is updated based on the real-time difference analysis results, and temporary interference zones caused by deformation of the thin-walled circular tube specimen 1 or speckle shedding are added.

[0117] Step S56: For the interference areas in the speckle-containing image I_speckle, including the areas marked as interference by the static mask or the dynamic mask, the pixel values of the corresponding positions in the non-speckle-containing image are replaced to generate a purified speckle image. Gaussian blur transition is performed on the boundaries of the replaced areas to avoid the introduction of false strain due to grayscale mutations during DIC calculation.

[0118] This difference analysis method combines ray tracing with difference analysis to precisely locate and eliminate speckle noise caused by interference phenomena such as reflection and refraction of the laser light in the hole area. Specifically, the image without speckle is used as the clean field to correct the image containing speckle, eliminating interference while preserving the integrity of the true speckle field. This provides a high-fidelity data foundation for stress and strain analysis at the notch and crack initiation prediction, ensuring the accuracy of DIC strain calculations.

[0119] Step S6: Based on the corrected speckle image sequence, DIC strain calculation is performed to analyze its fatigue characteristics.

[0120] That is to say, through laser speckle interferometry image processing technology combined with digital image correlation (DIC) algorithm, high-precision strain field is extracted from the purified speckle image, and quantitative correlation with fatigue damage is established.

[0121] The DIC strain calculation method includes the following steps:

[0122] Step S61: Use fast Fourier transform (FFT) to perform frequency domain transformation on the corrected speckle image, extract phase difference information, establish a linear relationship between the phase difference and the surface displacement gradient of the thin-walled circular tube specimen 1 (laser wavelength is used as a calibration parameter), and combine with the inverse synthetic aperture algorithm to iteratively optimize the displacement field based on the phase difference data to achieve sub-pixel displacement measurement and obtain full-field displacement distribution data; wherein the full-field displacement distribution data includes: axial, circumferential, and shear components.

[0123] Step S62: Based on the full-field displacement distribution data, the nonlinear strain component is calculated by the displacement gradient to characterize the large plastic deformation, and the maximum principal strain and equivalent plastic strain amplitude are calculated in the predefined annular buffer area at the edge of the hole. Combined with the number of cycles, a local strain-life evolution curve is constructed.

[0124] 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 fatigue damage distribution cloud map.

[0125] Specifically, the fatigue damage calculation model is modified based on the critical plane method to quantify the coupling effect of shear strain and normal stress. Secondly, a high-precision life prediction model is trained using random forest or convolutional neural networks, taking strain statistical characteristics (mean, gradient) and load parameters as input, and dynamically generating remaining life confidence intervals. The model parameters are updated every 1000 cycles, and the prediction results are revised in real time. Ultimately, a cloud map of the crack initiation life and fatigue damage distribution is output, providing data support for life assessment of thin-walled structures with holes in the aviation, automotive and other fields.

[0126] 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. The main frame is a double-column, door-shaped frame structure; the actuator is an electro-hydraulic servo actuator; the hydraulic system utilizes a high-pressure plunger oil pump motor unit, adopts a vertical mounting structure, and the hydraulic system provides power to the electro-hydraulic servo actuator; the test fixture comprises an upper fixture and a lower fixture, both of which are connected to the actuator, supporting axial tensile and compressive loads, and securing a thin-walled circular tube specimen (1) via threads, snaps, hydraulic pressure, or mechanical locking.

[0127] The control system includes: a controller and a data acquisition module; the controller is a fully digital hydraulic servo controller based on the PCI bus, which supports force / displacement / strain closed-loop control. According to the data acquisition module, it includes: a load sensor (measuring axial force F), a displacement meter (measuring 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.

[0128] The light source system includes: an infrared laser 2 for emitting near-infrared laser with a wavelength of 1550nm; 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 thin-walled circular tube sample 1 to form a speckle field.

[0129] Binocular DIC system includes:

[0130] The first camera 6 is configured as a high-resolution industrial camera, and its field of view covers the hole area of the thin-walled circular tube specimen 1. The first camera 6 uses a telecentric lens, and a lithium niobate film with a thickness of 180-220nm is set in front of the lens. A periodic silicon dioxide grating is set on the surface of the lithium niobate film. In this embodiment, the SHG / SFG effect of the lithium niobate film is used to convert infrared speckle with a wavelength of 1550nm into visible light with a wavelength of 550nm. The camera is used to simultaneously capture infrared and visible light information in a single image, that is, an image of the surface of the thin-walled circular tube specimen 1 containing the speckle pattern.

[0131] The second camera 7 is configured as a high-resolution industrial camera, and its field of view covers the hole area of the thin-walled circular tube sample 1 in the reflector; the second camera 7 uses a telecentric lens to capture the surface image of the thin-walled circular tube sample 1 without the speckle pattern.

[0132] The beam splitter prism 5 is used to split the reflected light from the surface of the thin-walled circular tube sample 1 into two light paths. The first light path is directly directed to the first camera 6, and the second light path is deflected by 90 degrees and directed to the second camera 7.

[0133] The image synchronization module is used to synchronously trigger the first and second cameras 7 to capture images at a frequency of 50-200 Hz to capture the transient strain field.

[0134] Furthermore, a white background plate is provided on the side of the thin-walled circular tube sample 1 away from the first camera 6 , so that the first camera 6 and the second camera 7 can clearly distinguish the edge of the thin-walled circular tube sample 1 and prevent mismeasurement caused by a cluttered background.

[0135] By setting the position of the hardware in the binocular DIC system, the position and angle of the image taken by the second camera 7 are consistent with those of the image taken by the first camera 6, so that the shooting effect at the same position is obtained.

[0136] The above description is based on the ideal embodiment of the present invention. Based on the above description, relevant personnel can make various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the content of the specification and must be determined according to the scope of the claims.

Claims

1. A dynamic fatigue test method for metal materials, characterized in that: The following steps are involved: S1. Install the thin-walled circular tube specimen with a hole in the loading fixture of the fatigue testing machine; S2, projecting a speckle pattern onto the hole area of the sample using an infrared light source, and collecting images of the sample with and without the speckle pattern using a binocular DIC system; S3, applying axial tension and compression cyclic loads to the thin-walled circular tube specimen to perform a dynamic fatigue test; S4, continuously recording load-displacement data and image sequences of specimens with and without speckles during the test; S5. Based on the difference analysis between the image sequences containing speckle and the image sequences without speckle obtained in step S4, identifying and removing the interference area caused by speckle; S6. Based on the corrected speckle-containing image sequence, DIC strain calculation is performed to analyze its fatigue characteristics.

2. A dynamic fatigue testing method for metal materials according to claim 1, characterized in that: In S2, the infrared light source is a near-infrared laser with a wavelength of 1550 nm. The near-infrared laser beam is evenly expanded to the hole area on the surface of the sample through a beam expander and a collimator to form a speckle pattern.

3. A dynamic fatigue testing method for metal materials according to claim 1, characterized in that: In S2, the binocular DIC system includes: A beam splitter prism is used to split the reflected light from the sample surface into a first light path and a second light path, wherein the second light path is deflected by 90°; A first camera, corresponding to the first optical path, is used to capture a sample image containing a speckle pattern; a lithium niobate film is provided in front of the lens of the first camera, and the lithium niobate film is used to convert the infrared speckle into visible light; The second camera, corresponding to the second optical path, is used to collect a sample image without a speckle pattern.

4. A dynamic fatigue testing method for metal materials according to claim 3, characterized in that: In S2, the infrared light source irradiates obliquely from below the sample; The center of the hole on the sample, the center of the dichroic prism, and the optical axis of the first camera are on the same straight line, and the distances between the dichroic 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 consistent with the distance from the first camera to the beam splitter prism.

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

6. A dynamic fatigue testing method for metal materials according to claim 1, characterized in that: In said S4, the test data collection method is The following steps are included S41, generating a unified TTL pulse signal to synchronously trigger the measurement of axial force and axial displacement, and the acquisition of images of samples with and without speckle patterns; 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 collected per 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 testing method for metal materials according to claim 6, characterized in that: In S5, the difference analysis method comprises the following steps: S51, performing sub-pixel registration and histogram equalization on the image containing speckle and the image not containing speckle at the same moment based on the unified timestamp in S43; S52, calculating the absolute difference between the registered speckle-containing image I_speckle and the non-speckle-containing image I_clean: ΔI = |I_speckle - I_clean|, performing Otsu adaptive threshold segmentation on ΔI, extracting high-difference areas, and marking them as potential interference candidate areas; S53, based on the sample CAD model or the pre-calibrated hole position, define a circular buffer around the hole edge in the image, where the width of the circular buffer is 5-10 pixels, and perform an opening operation on the candidate area of ΔI to remove isolated noise points and retain continuous scattering fringes; S54. Construct a ray tracing model based on the incident angle of the infrared light source and the geometric parameters of the hole to predict the projection area of the light reflected from 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, an initial interference mask is generated based on the unloaded speckle image I_speckle and the unloaded speckle image I_clean, marking the spatial buffer zone, the inner wall reflection zone, and the permanent interference zone. During the test, the dynamic mask is updated according to the real-time difference analysis results, and temporary interference zones caused by specimen deformation or speckle shedding are added. S56 , replacing the interference areas in the speckle-containing image I_speckle, including the areas marked as interference by the static mask or the dynamic mask, with pixel values of corresponding positions in the image not containing speckle, to generate a purified speckle image.

8. A dynamic fatigue testing method for metal materials according to claim 1, characterized in that: In S1, filling round rods are inserted into the clamping positions at both ends of the thin-walled circular tube sample with a hole. The diameter of the filling round rod differs from the inner diameter of the sample 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, applied to the dynamic fatigue testing method for metal materials according to any one of claims 1 to 8, characterized in that: include: Main frame, actuator, test fixture, hydraulic system, control system, light source system and 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 onto the sample hole area; The binocular DIC system includes a beam splitter prism, a first camera, and a second camera. The first camera is configured with a lithium niobate film that converts infrared speckle 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 sample reflected light into two light paths, the first light path is directly directed to the first camera, and the second light path is deflected 90 degrees to the second camera; the field of view of the first camera and the second camera covers the sample hole area, and the shooting position and angle are consistent.

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