A method and device for testing composite bending test based on digital image correlation method

Abstract

The application discloses a composite material bending test testing method and device based on a digital image correlation (Digital Image Correlation, DIC) method. The device comprises a mechanical test module conforming to international standards, a double-channel digital image acquisition module and a synchronous control and analysis module. The method realizes double-view field synchronous imaging of the whole bending loading process by arranging three-dimensional and two-dimensional digital image correlation systems on the bottom tension surface and the free side surface of a composite material sample respectively, and utilizes a unified time base to register load-displacement and image data, so that full-field strain, crack initiation and delamination expansion time-space evolution are obtained. Through integration of a mechanical loading device and a digital image correlation system, the application realizes accurate measurement of damage initiation, expansion and delamination evolution of the composite material under bending conditions, has the advantages of strong structure universality, high test precision and high visualization degree, and is suitable for mechanical property evaluation, fracture mechanism research and model / algorithm verification of advanced composite materials.

Description

Technical Field

[0001] This invention belongs to the field of composite material mechanical property testing and non-destructive testing technology, specifically relating to a composite material bending test method and device based on digital image correlation, which can be used for bending tests and progressive damage visualization analysis of carbon fiber reinforced composite materials and other polymer-based, metal-based, ceramic-based, asphalt-based and other structures. Background Technology

[0002] With the widespread application of lightweight, high-strength composite materials in aerospace, automotive, and energy equipment, their reliability evaluation under complex loads is receiving increasing attention. Progressive damage in composite materials plays a decisive role in their structural reliability. Bending tests, as one of the most commonly used mechanical property testing methods, can be used to obtain the bending strength and modulus of composite materials. However, traditional three-point or four-point bending tests typically only measure the overall deflection using contact displacement gauges, making it difficult to capture strain concentration and interlaminar damage evolution. Although in-situ X-ray microcomputed tomography can provide three-dimensional internal structural information, its limitations in specimen size, compatibility with loading stages, and scanning costs hinder its routine application.

[0003] Digital image correlation (DIC) technology, with its non-contact, full-field, and high-resolution characteristics, has become an important means of measuring the mechanical properties of materials. Existing research is mostly based on single-view DIC systems, which can only acquire deformation information on a single surface, making it difficult to simultaneously observe the strain distribution at different locations during the bending process of composite materials. For multilayer, orthogonal layup, or oriented composite materials, damage often originates first from interfacial delamination and interlayer separation at the tension surface or free edges. Existing single-view systems cannot fully reveal this multi-regional failure mechanism.

[0004] On the other hand, most existing bending fixtures are closed structures lacking optical channels, preventing the DIC camera from simultaneously imaging the tensioned surface and the free edge. Furthermore, while some adaptive fixtures offer span adjustment, they cannot simultaneously balance optical path arrangement and standard geometric accuracy. There is an urgent need for a composite material bending testing device that combines standardization, visualization, and multi-channel synchronous measurement capabilities to enhance damage identification. Therefore, a composite material bending test method and device based on digital image correlation is proposed. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of limited test field of view, incomplete damage monitoring, low system integration, high cost of in-situ XCT and incompatibility with standard specimens in the prior art, and to propose a composite material bending test method and device based on digital image correlation method.

[0006] The core idea of ​​this invention is:

[0007] By integrating a standardized four-point bending loading structure with a dual-channel digital image acquisition module, synchronous strain measurement and crack capture of the tensile surface and free edge of the specimen are achieved throughout the loading process. Furthermore, a 3D-printed magnetic camera bracket ensures a stable field of view and convenient reproduction. The camera angle, height, and spacing can be customized, thus constructing a bending performance testing method and device that is compatible with international standards and different composite materials.

[0008] A method for testing the bending of composite materials based on digital image correlation, characterized in that the method includes the following steps:

[0009] Step A: Determine the geometric dimensions and span-to-thickness ratio of the composite rectangular beam specimen according to ASTM D7264 standard, and place it centered in the mechanical testing module of the bending test apparatus;

[0010] Step B: Random speckle patterns are formed on the tension surface and free side (thickness direction) at the bottom of the sample to serve as feature textures for digital image correlation analysis. The area between the bottom and side support rollers is selected as the deformation measurement region (ROI).

[0011] Step C: A three-dimensional digital image acquisition system is arranged at the bottom of the sample, and a two-dimensional digital image acquisition system is arranged on the free side. The three-dimensional DIC system is fixed on a 3D-printed camera bracket, which is connected to the support component by magnets to ensure a stable field of view, quickly positioning the camera so that the tension surface and the key area of ​​the free side are in the center of the field of view, and adjusting the focal length and aperture to meet the depth of field requirements. LED lights are arranged at the bottom of the sample for supplementary illumination. The two-dimensional DIC system is fixed on a tripod camera bracket, and LED lights are arranged on the tripod camera bracket for supplementary illumination.

[0012] Step D: Apply bending load to the specimen at a constant loading rate using a testing machine and simultaneously trigger the image acquisition system. Record the image sequence and load data of the entire loading process at a set frequency until the end of the experiment. Save the image timestamps and load samples with a unified time base.

[0013] Step E: Use digital image correlation software to calculate the full-field strain distribution and displacement of the free side at different times, extract the strain evolution curves of the tensioned surface and the free side, and identify the crack initiation and delamination propagation locations.

[0014] Furthermore, in step C, square or ring-shaped LED lights are used for illumination and a light shield is configured to suppress glare, with an illuminance variance of ≤10% within the field of view.

[0015] Furthermore, in step C, the 3D DIC is bi-targeted using a dot plate, and the load time series is linearly interpolated and registered with the image timestamp.

[0016] Furthermore, in step D, the sampling frequency range of the three-dimensional DIC is 0.2 ~ 2 Hz, the sampling frequency of the two-dimensional DIC is 0.2–5 Hz, the image resolution is not less than 2048×2048 pixels, the size of the DIC subset is 17 ~ 41 pixels, the step size is 3 ~ 7 pixels, and the correlation criterion adopts normalized cross-correlation.

[0017] Furthermore, after the test in step E, the sample can be directly transferred to an X-ray computed tomography (XCT) device to perform spatial verification and matching of the internal crack / delamination morphology and surface strain results, realize the correlation analysis of surface-internal damage information, and further correlate it with stress-strain curves to realize progressive damage analysis.

[0018] The present invention also proposes a composite material bending test device based on digital image correlation method, characterized in that the bending test device includes a mechanical testing module, a dual-channel digital image acquisition module, and a synchronous control and analysis module;

[0019] The mechanical testing module includes a rigid frame structure consisting of an upper loading beam, a loading block with loading rollers, a support structure with support rollers, and a connecting base plate. The loading block is connected to the upper loading beam, the upper loading beam is connected to the loading head of the testing machine by bolts, the support crossbeam is connected to the support vertical beam, the lower support crossbeam is connected to the connecting base plate, and the connecting base plate is fixed to the machine base by bolts. The support span can be adjusted within the range of 60~160 mm, and the loading roller spacing is L / 2.

[0020] The dual-channel digital image acquisition module includes a two-dimensional digital image acquisition system, three LED lights, a camera tripod, a three-dimensional digital image acquisition system, and a pair of 3D-printed camera brackets. The two-dimensional digital image acquisition system is connected to the camera tripod, a set of LED lights is connected to the camera tripod, the three-dimensional digital image acquisition system is connected to the 3D-printed camera bracket set, the 3D-printed camera bracket set is connected to the support beam of the mechanical testing module via magnets, and one LED light is connected to the connecting base plate of the mechanical testing module.

[0021] The synchronization control and analysis module includes a synchronization controller for the two-dimensional and three-dimensional digital image acquisition systems, and two computers containing software related to digital image acquisition and DIC analysis.

[0022] The mechanical testing module comprises a rigid frame structure consisting of an upper loading beam, a loading block with loading rollers, a support structure with support rollers, and a connecting base plate. The loading block can be selected as a three-point bending test loading block or a four-point bending test loading block, depending on the experiment. Both the three-point bending test loading block and the four-point bending loading block can be used in combination with the upper loading beam to achieve three-point bending tests and four-point bending tests.

[0023] The three-dimensional digital image acquisition module includes a three-dimensional digital image acquisition system, a pair of 3D printed camera brackets, and an LED light. The 3D printed brackets can realize customized camera angles, heights, and spacings. The brackets are quickly connected and fixed to the support components via magnets.

[0024] Furthermore, the mechanical testing module includes an upper loading beam, an upper loading beam fixing stud, two loading blocks containing loading rollers, loading block fastening bolts, loading block fastening adjustment blocks, four supporting vertical beams, two upper supporting horizontal beams containing supporting rollers, two lower supporting horizontal beams, a connecting base plate, supporting beam fastening bolts, lower supporting horizontal beam fastening bolts, and connecting base plate fixing bolts. The loading rollers and supporting rollers can rotate freely in the loading blocks and supporting vertical beams, respectively. The upper loading beam fixing stud connects the upper loading beam to the testing machine loading head through a threaded hole in the middle of the upper loading beam. The loading block fastening bolts pass through the threaded holes on the L-shaped loading blocks, through a pre-reserved groove in the middle of the upper loading beam, and are fastened to the loading blocks. The loading block and the loading block fastening adjustment block form a sliding pair that can slide in the groove reserved in the middle of the upper loading beam to adjust the distance between the loading blocks. The upper loading beam has a scale on its side for measuring the distance between the loading rollers. The support beam fastening bolts connect the lower support beam to the connecting base plate through the threaded holes reserved in the connecting base plate. The lower support beam can slide in the groove reserved in the middle of the connecting base plate to adjust the distance between the support components. The connecting base plate has a scale on its side for measuring the distance between the support rollers. The connecting base plate fixing bolts connect the connecting base plate to the testing machine base through the threaded holes reserved in the connecting base plate.

[0025] Furthermore, the mechanical testing module includes an upper loading beam, a loading block with loading rollers, a support structure with support rollers, and a connecting base plate. A scale is set on the surface of the upper support beam to assist in sample centering and size confirmation.

[0026] Furthermore, the dual-channel digital image acquisition module includes a three-dimensional digital image acquisition system and a two-dimensional digital image acquisition system. The three-dimensional digital image acquisition system includes two high-resolution industrial cameras, two industrial camera lenses, an LED light, a 3D-printed camera bracket, a 3D-printed camera limiting and fixing block, and a camera bracket fixing magnet. The industrial camera lens is connected to the high-resolution industrial camera via a thread, and its focal length and aperture are adjustable. The industrial camera and the 3D-printed camera bracket are connected through a positioning groove reserved in the bracket and are limited by the 3D-printed camera limiting and fixing block to prevent the camera from falling off. The camera bracket fixing magnet is connected to the support beam through a groove reserved in the 3D-printed camera bracket to limit the change of the camera's field of view. The two-dimensional digital image acquisition system includes a high-resolution industrial camera, an industrial camera lens, an LED light, and a camera tripod. The industrial camera lens is connected to the high-resolution industrial camera via a thread, and its focal length and aperture are adjustable. The industrial camera and the camera tripod are fixed by a fixing platform.

[0027] Furthermore, the 3D printed camera bracket is arranged symmetrically to form a stereoscopic visual baseline, with a preset three-dimensional DIC tilt angle of 18 degrees, and the optical axis of the two-dimensional DIC camera is perpendicular to the sample cross-section and parallel to the ground.

[0028] Furthermore, the mechanical testing device is compatible with composite material samples of different thicknesses and widths, and can be adapted through an adjustable span structure between the support roller and the loading roller.

[0029] The beneficial effects of this invention include:

[0030] 1. Dual-field synchronous measurement: It can simultaneously capture the longitudinal strain of the tensioned surface and the shear strain between the free edge layers, enabling early identification of cracks and delamination, and realizing multi-region damage visualization;

[0031] 2. Optical path - load geometry compatible; provides unobstructed access for cameras and light sources while preserving the curved standard contact geometry;

[0032] 3. Magnetic 3D printing stand: Baseline, tilt angle and working distance can be quickly reproduced, significantly reducing field of view drift;

[0033] 4. Unified time base and post-hoc verification: The load-displacement and image timestamps are unified, and the surface strain can be aligned after the test;

[0034] 5. Standardization and low cost: Based on ASTM D7264 standard geometry, it can be achieved with common materials / lenses / cameras, resulting in low cost and high repeatability, making it suitable for scientific research and engineering testing.

[0035] Based on the above reasons, this invention can be applied to carbon fiber composites and other polymer-based, metal-based, ceramic-based, and asphalt-based composites. This invention belongs to the field of composite material mechanical property testing and non-destructive testing technology, specifically relating to a composite material bending test method and apparatus based on digital image correlation, which can be used for bending tests and progressive damage visualization analysis of carbon fiber reinforced composites and other polymer-based, metal-based, ceramic-based, and asphalt-based structures. Attached Figure Description

[0036] The present invention will be further described below with reference to the accompanying drawings and examples.

[0037] Figure 1 This is a schematic diagram of the overall structural layout of the device of the present invention;

[0038] Figure 2 This is a schematic diagram of the substructure of the device of the present invention;

[0039] Figure 3 This is a schematic diagram of the mechanical testing module structure of the device of the present invention;

[0040] Figure 4 This is a schematic diagram of the loading block structure, which includes a loading roller, fastening bolts, and fastening adjustment block, in the mechanical testing module of the device of the present invention.

[0041] Figure 5 This is a perspective view showing the structural layout of the mechanical testing module, the sample, and the three-dimensional digital image acquisition module of the device of the present invention.

[0042] Figure 6 This is a schematic diagram of the connection structure between the digital image acquisition system and the 3D printed camera bracket in the three-dimensional digital image acquisition module of the device of the present invention;

[0043] Figure 7 This is a schematic diagram of the structure of the two-dimensional digital image acquisition module of the device of the present invention;

[0044] Figure 8 This is a schematic diagram of the test results of an embodiment of the present invention. Detailed Implementation

[0045] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are merely illustrative and are not intended to limit the present invention or its application or use in any way. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0046] In the description of this invention, it should be understood that the orientation descriptions, such as up, down, front, back, left, right, etc., are based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0047] In the description of this invention, "several" means one or more, "more than" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. The use of "first" and "second" in the description is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0048] In the description of this invention, unless otherwise explicitly defined, terms such as "set up," "install," and "connect" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of the above terms in this invention in conjunction with the specific content of the technical solution.

[0049] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0050] Unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps set forth in these embodiments do not limit the scope of the invention. It should also be understood that the dimensions of the various parts shown in the drawings are not drawn to actual scale. Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of the specification. In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters in the following drawings denote similar items; therefore, once an item is defined in one drawing, it need not be further discussed in subsequent drawings.

[0051] Example 1: Method Implementation

[0052] This embodiment provides a composite material bending test method based on digital image correlation, including the following steps:

[0053] Step A: According to ASTM D7264, determine the length and width of the carbon fiber / epoxy resin composite rectangular beam specimen based on its thickness. In this example, a span-to-thickness ratio of 20:1 is used, with a thickness of 3.8 mm and a width of 13 mm, resulting in a support span of 76 mm. The specimen length must be at least 1.2 times the support span; in this example, the specimen length is 146 mm. Figure 3 As shown, adjust the support span between the support rollers 131 to 76mm, i.e., slide the support member 13, and align the support roller positioning groove 139 with the scale 38 on the scale 143. Similarly, adjust the loading span between the loading rollers 121 to 38mm, i.e., slide the loading member 12, and align the loading roller positioning groove 123 with the scale 19 on the scale 111. Place the sample centered in the mechanical testing module of the bending test apparatus, and proceed according to... Figure 5 As shown in the arrangement, sample 5 is placed on support roller 131, and the centering and size confirmation are assisted by the scale 132 set on the upper support beam 133.

[0054] Step B: Before placing the specimen in the testing apparatus, spray a matte white paint with black dots in a random speckle pattern onto the bottom tension surface and free side (thickness direction) of the specimen. This speckle pattern serves as a feature texture for digital image correlation analysis. The area between the bottom and side support rollers is selected as the deformation measurement region (ROI). The location of the deformation measurement area for specimen 5 is shown below. Figure 5 As shown, the three-dimensional digital image acquisition system 21 acquires the bottom tension surface of the sample 5, with the ROI region being between the two support rollers 131. Figure 3 As shown, the two-dimensional digital image acquisition system 41 acquires the free side of the sample 5, and the length of the ROI region is consistent with that of the three-dimensional digital image acquisition system.

[0055] When spraying speckle, cover the area outside the ROI with tape, and remove the tape after the speckle is made. Align the edge of the ROI area with the center line of the support roller 131 to ensure that the sample is placed between the two support rollers, thereby assisting in the sample centering in step A.

[0056] Step C: As Figure 1 As shown, a three-dimensional digital image acquisition module 2 is arranged at the bottom of sample 5, and a two-dimensional digital image acquisition module 4 is arranged on the free side. A square LED light equipped with a light shield is used to illuminate the sample, and the illuminance variance within the field of view is ≤10%. Figure 5As shown in the diagram, the 3D digital image acquisition system 21 is fixed to a 3D-printed camera bracket 22 made of polylactic acid. The bracket is connected to the experimental fixture via magnets 224 to ensure a stable field of view. The camera 212 is quickly positioned using the bracket's shoulder platform 221, ensuring the key area of ​​the tension surface (ROI) is centered in the field of view and adjusting the focal length and aperture to meet depth-of-field requirements. LED lights 22 are used for supplementary illumination, and a dot plate is used for dual-target positioning. Figure 7 As shown in the diagram, the two-dimensional digital image acquisition system 41 is fixed to the tripod camera bracket 43 via the camera bracket platform 42. The two-dimensional digital image acquisition system 41 is adjusted using the camera bracket platform 42 and the tripod bracket 43 so that the key area ROI on the free side is positioned slightly above the center of the field of view, allowing for a portion of the sample to bend downwards under load. Additionally, LED lights 422 are provided for supplementary illumination.

[0057] Step D: Apply a downward bending load to the specimen using a testing machine at a constant loading rate of 1 mm / min, and then... Figure 2 The synchronous controller 32 shown synchronously triggers the three-dimensional and two-dimensional image acquisition systems to record the image sequence and load data of the entire loading process at a set frequency of 1Hz until the end of the test. The image timestamps and load samples are saved with a unified time base.

[0058] Step E: Using digital image correlation software VIC-3D and VIC-2D, correlation calculations were performed on the acquired images to obtain the full-field strain distribution and displacement of the free side at different loading stages. The size of the 3D and 2D DIC subsets was 17–41 pixels, with a step size of 3–7 pixels. Normalized cross-correlation was used as the correlation criterion. The strain evolution process of the tensioned surface and the free side was extracted and combined with the stress-strain curve of the specimen to identify the crack initiation and delamination propagation locations, thereby comprehensively analyzing the progressive damage mechanism of the specimen.

[0059] Step F: After the experiment, X-ray CT scanning is combined to spatially verify and match the internal crack / delamination morphology of the sample with the dual-channel surface strain results, realize multi-scale correlation analysis of surface-internal damage information fusion, and further correlate with stress-strain curves to realize progressive damage analysis.

[0060] Experimental results show that the bottom tension surface can accurately capture the initiation and propagation of tensile cracks, and the strain field on the free side reveals interlaminar shear bands and delamination propagation paths. After the experiment, X-ray CT was used to scan the fracture surface of the sample, and the internal crack distribution was found to be highly consistent with the surface DIC observation results, verifying the reliability of the method.

[0061] Example 1: Device Structure

[0062] like Figure 1As shown, the present invention discloses a composite material bending test device based on digital image correlation method. The device is generally composed of a mechanical testing module 1, a three-dimensional digital image acquisition module 2, a synchronous control and analysis module 3, a two-dimensional digital image acquisition module 4, and a sample 5.

[0063] like Figure 2 As shown, the mechanical testing module 1 is a rigid frame structure consisting of an upper loading beam 11, two loading blocks 12 with loading rollers, two support structures 13 with support rollers, and a connecting base plate 14. The three-dimensional digital image acquisition module 2 consists of two symmetrically arranged three-dimensional digital image acquisition systems 21, a 3D-printed camera bracket 22, and LED lights 23. The two-dimensional digital image acquisition module 4 consists of a two-dimensional digital image acquisition system 41, a camera bracket platform 42, and a tripod 43. The synchronous control and analysis module 3 consists of two computers. One computer 31 is connected to the three-dimensional digital image acquisition system 21 at one end and to the synchronous controller 32 at the other end. The other computer 33 is connected to the two-dimensional digital image acquisition system 41 at one end and to the synchronous controller 32 at the other end.

[0064] The mechanical testing module 1 is as follows Figure 3 As shown, the mechanical testing module includes an upper loading beam 11, two loading blocks 12 with loading rollers, two support structures 13 with support rollers, and a connecting base plate 14.

[0065] The loading block 12 is connected to the upper loading beam 11, and the main body of the loading block 12 is an L-shaped loading nose 122. For example... Figure 4 As shown, a freely rotatable loading roller 121 is provided on the loading nose 122, and a groove 123 is provided on the side of the loading nose 122. Figure 3 As shown, the groove 123 indicates the center point of the loading roller 121 when adjusting the distance of the loading block 12. When adjusting the distance, simply align the groove 123 with the scale 111 on the upper loading beam 11. Figure 4 As shown, the loading block fastening bolt 125 is connected to the loading block fastening adjusting block 124 on the other side through the threaded hole on the L-shaped loading nose 122. The specific arrangement is as follows... Figure 3 As shown, the loading block fastening bolt 125 passes through the threaded hole on the L-shaped loading nose 122 and passes through the groove 112 reserved in the middle of the upper loading beam 11 to form a moving pair with the loading block fastening adjustment block 124. The distance between the loading blocks 12 is adjusted by aligning the groove 123 with the scale 111 on the upper loading beam 11.

[0066] like Figure 3As shown, the upper loading beam 11 is connected to the loading head of the testing machine via bolts through pre-drilled threaded holes 113. The upper support beam 133, the lower support beam 138, and the support vertical beams 135 and 136 are connected via support beam fastening bolts 134 to form the support member 13. A freely rotatable support roller 131 is fixed between the support vertical beams 135 and 136. The height of the support roller 131 is slightly higher than the plane of the upper support beam 133. A scale 132 is provided on the plane of the upper support beam 133 to assist in the centering and dimensional confirmation of the sample 5. The lower support beam 138 is connected to the pre-drilled threaded holes on the connecting base plate 14 via support beam fastening bolts 137. The lower support beam 138 can slide through the pre-drilled grooves 142 on the connecting base plate 14. A groove 139 is provided on the surface of the support beam 136. The groove 139 indicates the center point of the support roller 131 when adjusting the distance of the support member 13. When adjusting the distance, simply align the groove 139 with the scale 143 on the connecting plate 13. The connecting base plate 14 is fixed to the machine base by bolts 141. In this embodiment, the support span is 76mm and the loading span is L / 2 = 38mm. That is, align the grooves 123 with the reading of 38mm on the scale 111 on the upper loading beam 11, and align the grooves 139 with the reading of 19mm on the scale 143 on the connecting base plate 14.

[0067] like Figure 5 As shown, the 3D digital image acquisition module 2 includes a 3D digital image acquisition system 21, a 3D printed camera bracket 22, and a square LED light 22. The 3D digital image acquisition system includes two high-resolution industrial cameras 212 and two camera lenses 211. The industrial camera lenses 211 are connected to the high-resolution industrial cameras 212 via threads, and their focal length and aperture are adjustable. Figure 6 As shown, the industrial camera 212 and the 3D printed camera bracket 22 are connected by a positioning nose 221 reserved on the bracket body 222, and are limited by a positioning groove reserved on the bracket 22 and a 3D printed camera fiber fixing block 223 to prevent the camera from falling off. Figure 5 As shown, the camera bracket 22 has a groove reserved on the bracket body 222, and the camera fixing magnet 224 is connected to the support beams 135 and 136 through the groove to limit the change of the camera's field of view.

[0068] like Figure 7As shown, the two-dimensional digital image acquisition module 4 comprises a two-dimensional digital image acquisition system 41, a camera support platform 42, and a tripod 43. Similar to the three-dimensional digital image acquisition system 21, the two-dimensional digital image acquisition system 41 includes a high-resolution industrial camera 412 and a camera lens 411. The industrial camera lens 411 is connected to the high-resolution camera 412 via threads, and its focal length and aperture are adjustable. The industrial camera 412 is connected to the camera support platform 421, and a set of square LED lights 422 are also configured on the camera support platform 421. The bottom of the camera support platform 421 is connected to a sleeve 431 on the tripod 43, allowing the camera support platform 422 to rotate and move up and down. The extension leg 432 at the lower end of the tripod 43 can further raise the field of view of the camera 412.

[0069] Example 1: Application Effect

[0070] This invention discloses a composite material bending test method and apparatus based on digital image correlation, which has been validated in the bending performance testing of carbon fiber / epoxy composite materials. Results show that the apparatus exhibits stable loading, good data synchronization, low repeatability error, and convenient reproducibility. The customized 3D-printed camera bracket and its magnetic design significantly improve optical axis consistency, ensuring the accuracy of strain field measurements.

[0071] like Figure 8 As shown, the bottom tension surface accurately captures the initiation and propagation of tensile cracks, while the strain field on the free side reveals interlaminar shear bands and delamination propagation paths. Post-failure XCT verification shows that the abrupt change points in the stress-strain curves correspond to hotspots observed in the digital image, and the internal crack distribution is highly consistent with the surface DIC observations, confirming that combining the stress-strain curve with local surface strain changes can reliably indicate the internal damage path and type of the specimen. Compared to in-situ XCT, this invention's workflow significantly reduces scanning costs while capturing the main damage paths and types by deploying DICs on the tension surface and sides.

[0072] The apparatus and method of this invention can efficiently obtain deformation information throughout the bending test of composite materials, providing an experimental basis for material fracture mechanism analysis, finite element model verification, and machine learning damage prediction.

[0073] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for testing the bending of composite materials based on digital image correlation, characterized in that, The method includes the following steps: Step A: Determine the geometric dimensions and span-to-thickness ratio of the composite rectangular beam specimen according to ASTM D7264 standard, and place it centered in the mechanical testing module of the bending test apparatus; Step B: Random speckle patterns are formed on the tensioned surface and free side (thickness direction) at the bottom of the sample as feature textures for digital image correlation analysis. The area between the bottom and side support rollers is selected as the deformation measurement area (ROI). Step C: A three-dimensional digital image acquisition system is arranged at the bottom of the sample, and a two-dimensional digital image acquisition system is arranged on the free side. The three-dimensional DIC system is fixed on a 3D-printed camera bracket, which is connected to the support component by magnets to ensure a stable field of view, quickly positioning the camera so that the tension surface and the key area of ​​the free side are in the center of the field of view, and adjusting the focal length and aperture to meet the depth of field requirements. LED lights are arranged at the bottom of the sample for supplementary illumination. The two-dimensional DIC system is fixed on a tripod camera bracket, and LED lights are arranged on the tripod camera bracket for supplementary illumination. Step D: Apply bending load to the specimen at a constant loading rate using a testing machine and simultaneously trigger the image acquisition system. Record the image sequence and load data of the entire loading process at a set frequency until the end of the experiment. Save the image timestamps and load samples with a unified time base. Step E: Use digital image correlation software to calculate the full-field strain distribution and displacement of the free side at different times, extract the strain evolution curves of the tensioned surface and the free side, and identify the crack initiation and delamination propagation locations.

2. The composite material bending test method based on digital image correlation according to claim 1, characterized in that, After the test, the sample can be directly transferred to an X-ray computed tomography (XCT) device to perform spatial verification and matching of internal crack / delamination morphology and surface strain results, realize the correlation analysis of surface-internal damage information, and further correlate it with stress-strain curves to realize progressive damage analysis.

3. The composite material bending test method based on digital image correlation according to claim 1, characterized in that, The bending test device includes a mechanical testing module, a dual-channel digital image acquisition module, and a synchronous control and analysis module. The mechanical testing module includes a rigid frame structure consisting of an upper loading beam, a loading block with loading rollers, a support structure with support rollers, and a connecting base plate. The loading block is connected to the upper loading beam, the upper loading beam is connected to the loading head of the testing machine by bolts, the upper and lower support beams are connected to the support vertical beams, the support structure is connected to the connecting base plate, and the connecting base plate is fixed to the machine base by bolts. The support span can be adjusted within the range of 60~160 mm, and the loading roller spacing is L / 2. The dual-channel digital image acquisition module includes a two-dimensional digital image acquisition system, three LED lights, a camera tripod, a three-dimensional digital image acquisition system, and a pair of 3D printed camera brackets. The two-dimensional digital image acquisition system is connected to the camera tripod, a set of LED lights is connected to the camera tripod, the three-dimensional digital image acquisition system is connected to the 3D printed camera bracket set, the 3D printed camera bracket set is connected to the lower support beam of the mechanical testing module via magnets, and one LED light is connected to the connecting base plate of the mechanical testing module. The synchronization control and analysis module includes a synchronization controller for the two-dimensional and three-dimensional digital image acquisition systems, and two computers containing software related to digital image acquisition and DIC analysis.

4. The composite material bending test device based on digital image correlation method according to claim 3, characterized in that, The loading block can be selected as a three-point bending test loading block or a four-point bending test loading block, depending on the experiment.

5. The composite material bending test device based on digital image correlation method according to claim 3, characterized in that, The 3D-printed camera bracket allows for customized camera angles, heights, and spacing. The bracket is quickly connected and fixed to the support components via magnets.

6. The composite material bending test device based on digital image correlation method according to claim 5, characterized in that, The 3D printed camera bracket is arranged symmetrically to form a stereoscopic vision baseline, and the optical axis of the two-dimensional DIC camera is perpendicular to the sample cross-section and parallel to the ground.

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