Material in-situ strain DIC analysis method and system based on CT imaging

Abstract

The invention relates to a material in-situ strain DIC analysis method and system based on CT imaging, and the method comprises the steps: preparing speckles having significant X-ray absorption difference with a matrix, directly tracking speckle displacement through a CT image while carrying out the defect change of a sample in a deformation process through CT, and replacing a conventional camera to achieve speckle collection for DIC strain analysis. According to the method, synchronous analysis of defect expansion and a strain field in a material loading process is realized through staged CT scanning and image post-processing, and an integrated solution is provided for research on mechanical properties of materials. The space limitation of CT equipment is broken through, internal defect evolution and surface full-field strain data of the material are synchronously obtained, and the method is suitable for multi-scale mechanical behavior research of metal, ceramic, polymer and composite materials thereof.

Description

Technical Field

[0001] This invention relates to the field of materials testing technology, and in particular to a method and system for in-situ strain digital image correlation (DIC) analysis of materials based on computed tomography (CT) imaging. Background Technology

[0002] In the study of materials' mechanical properties, combined in-situ deformation testing techniques using CT and DIC (Dissipative Incisional Conversion) are crucial for simultaneously analyzing changes in internal defects and material deformation. CT is used to analyze the initiation and propagation of internal defects, while a camera captures the evolution of DIC speckle patterns on the sample surface to analyze strain distribution changes. The former uses X-ray imaging, and the latter uses camera imaging. Traditionally, DIC typically relies on a separate camera to capture the displacement changes of the speckle pattern on the specimen surface to measure the strain field. Cameras can be categorized based on their light source: visible light cameras adapted to white / monochromatic light, infrared cameras adapted to infrared light sources, ultraviolet cameras adapted to ultraviolet light sources, and high-speed cameras that utilize high-speed pulsed light sources. However, the limited internal space of CT scanning equipment presents the following technical challenges for camera installation: (1) The interior space of the CT scanning room is small, and the additional camera installed is prone to mechanical interference with the CT equipment.

[0003] (2) The camera may experience imaging noise or equipment damage in an X-ray environment.

[0004] (3) When the sample rotates relative to the CT light source, the camera may block the image of the sample, affecting the image quality.

[0005] (4) The timing synchronization control between camera imaging and CT scanning is complex and can easily lead to data matching errors.

[0006] Therefore, the traditional CT-DIC combined in-situ testing method faces problems such as difficult equipment installation and low data acquisition efficiency in practical applications, and there is an urgent need for an integrated solution that does not require an additional camera. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the existing technology by providing a material in-situ strain DIC analysis method and system based on CT imaging. By using CT for speckle imaging, the invention enables simultaneous testing of defects and strain without the need for a camera throughout the entire process, thereby solving or partially solving a series of technical problems that require an additional camera in traditional CT-DIC combined testing.

[0008] The objective of this invention can be achieved through the following technical solutions: One aspect of the present invention provides a method for in-situ strain DIC analysis of materials based on CT imaging, comprising the following steps: The material sample to be tested is processed into standard sheet-shaped specimens to form test pieces; A speckle pattern is prepared on the surface of the specimen, and the material of the speckle has a density difference with the material of the specimen. The specimen with speckle pattern was fixed on the in-situ strain gauge of the CT. Mechanical loads are applied to the fixed specimen. During the loading process, three-dimensional images of the internal structure of the specimen are acquired in stages by CT scanning, and speckle patterns on the surface of the specimen are also acquired. Based on the three-dimensional image, speckle displacement information is extracted, the strain field on the specimen surface is calculated, and digital image correlation analysis is performed.

[0009] As a preferred technical solution, the calculation of the strain field on the specimen surface and the digital image correlation analysis include the following steps: The three-dimensional image reconstructed from two-dimensional projection is color-coded according to the material density difference. Based on the preset grayscale range, the image is divided into the image of the measured material and the speckle image. Based on the image of the tested material, a three-dimensional structural model of the specimen is reconstructed, and defect evolution analysis is performed. For the speckle image, the plane containing the speckle image is rotated to be parallel to the screen, and the three-dimensional displacement coordinates of the speckle at each loading stage are calculated, i.e., the speckle displacement information. The strain distribution on the specimen surface is calculated using a digital image correlation algorithm, and digital image correlation analysis is performed.

[0010] As a preferred technical solution, the defect evolution analysis includes crack propagation identification and pore evolution analysis.

[0011] As a preferred technical solution, the mechanical load is in-situ tension, in-situ compression, or in-situ bending.

[0012] As a preferred technical solution, the specimen is fixed by installing it in the clamping system of the in-situ CT deformation testing device.

[0013] As a preferred technical solution, CT scans are triggered in stages according to the stress-strain curve of the specimen during the loading process.

[0014] As a preferred technical solution, the speckle pattern is prepared using any of the following methods: Spraying method: A speckle pattern is formed by spraying liquid speckle material onto the surface of the specimen; Brushing method: After pretreating the surface of the specimen, apply liquid or paste-like speckle material to the surface to form random speckles; Particle bonding method: Mix AB glue in proportion, add speckled material particles, adjust to a viscous state, and apply to the surface of the specimen to form speckles.

[0015] As a preferred technical solution, the density difference between the speckled material and the tested material sample is greater than the density resolution of the CT scan.

[0016] Another aspect of the present invention provides a material in-situ strain DIC analysis system based on CT imaging, for implementing the aforementioned material in-situ strain DIC analysis method, the system comprising: The speckle pattern preparation module is used to prepare speckle patterns on the surface of a specimen, wherein the material of the speckle has a density difference from the material of the specimen; The CT in-situ loading module is used to apply mechanical loads to the fixed specimen. During the loading process, three-dimensional images of the internal structure of the specimen are acquired in stages by CT scanning. The image analysis module is used to extract speckle displacement information based on the three-dimensional image, calculate the strain field on the specimen surface, and perform digital image correlation analysis.

[0017] As a preferred technical solution, the CT in-situ loading module includes a CT scanning device and a tensile clamp for fixing the specimen.

[0018] As a preferred technical solution, the CT scanning device includes an X-ray source and a detector.

[0019] Compared with the prior art, the present invention has at least one of the following beneficial effects: (1) No additional camera required: This invention prepares a speckle pattern on the surface of the specimen, so that the speckle material and the specimen material have a density difference. The speckle material is selected from substances with significantly different X-ray absorption than the material being tested, ensuring that the two form a clear contrast in CT imaging. Furthermore, a suitable speckle material is selected according to the characteristics of the material being tested. The CT equipment is used to replace the traditional camera, and the specimen is scanned during in-situ loading, simultaneously acquiring CT images containing speckle displacement information and the internal structure of the material, without the need for an additional camera. This overcomes the problems of mechanical interference, X-ray interference, and image occlusion between the camera and the CT equipment, eliminates synchronization errors of multiple devices, and saves the cost of purchasing an additional expensive camera.

[0020] (2) Realize in-situ synchronous coupling analysis of CT defect testing and DIC strain analysis: This invention integrates CT scanning equipment with in-situ loading device. During the loading process, CT scanning is triggered in stages according to the material stress-strain curve. CT scanning records two types of core data at the same time. Through image analysis, defect evolution data and speckle displacement data are processed synchronously. Combined with DIC algorithm, strain field is calculated, and the correlation between defect expansion and strain concentration is established to realize synchronous coupling of data.

[0021] (3) Improve the accuracy of speckle identification and strain calculation: The present invention takes the density difference greater than or equal to the CT density resolution as the core standard, and selects the appropriate speckle material in combination with the characteristics of the material being tested to ensure that the speckle and the matrix are clearly distinguished in CT imaging, while avoiding artifacts. Through CT image color coding and grayscale threshold adjustment, the contrast difference between the speckle and the material being tested is further enhanced to ensure that the speckle can be accurately identified in the three-dimensional reconstructed image.

[0022] (4) Achieving high-resolution strain analysis at the micron to submicron scale: This invention utilizes the submicron-level spatial resolution advantage of the CT system to break through the limitation of the micron-level limit resolution of traditional optical cameras. It performs fine three-dimensional reconstruction of small areas in CT imaging and combines the DIC algorithm to achieve high-precision analysis of local strain fields at the submicron scale, significantly improving the characterization ability of micro-area mechanical response. Attached Figure Description

[0023] Figure 1 This is a flowchart of the in-situ strain DIC analysis method for materials based on CT imaging in the embodiments; Figure 2 This is a schematic diagram of the in-situ strain DIC test procedure in the embodiment; Figure 3 This is a schematic diagram of three-dimensional reconstruction of CT images in an embodiment. Figure 4 This is a schematic diagram of the in-situ strain DIC analysis system for materials based on CT imaging in the embodiment. Detailed Implementation

[0024] 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 only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0025] Example 1 To address the aforementioned problems in the existing technology, this embodiment provides a material in-situ strain DIC analysis method based on CT imaging. It aims to achieve DIC strain analysis without an additional camera by using X-ray absorption differential speckle design and CT image analysis. This method simultaneously realizes in-situ testing of material CT defects and DIC strain analysis, overcoming the installation difficulties of speckle image acquisition cameras in CT environments in traditional DIC testing methods, and improving the feasibility and data accuracy of in-situ testing.

[0026] See Figure 1 The method includes the following steps: Step S1, Sample preparation.

[0027] Sample preparation: Based on the requirements of in-situ testing, the material sample to be tested is prepared into a standard shape to facilitate loading and scanning.

[0028] Sample surface speckle formation: Select a suitable speckle material and create speckles on the surface of the sample to be tested.

[0029] Step S2, speckle pattern preparation step.

[0030] A speckle pattern is prepared on the surface of the specimen. The speckle material is selected from substances that have different X-ray absorption than the material being tested. Generally, the density difference between the two should be greater than the density resolution of CT. The density resolution of CT refers to the smallest density difference that can be displayed. Thus, the two can be distinguished in CT imaging and present contrast in CT images.

[0031] For example, if a CT scanner has a density resolution of 0.2%, then the density difference between the speckle material and the measured material should be ≥0.2%. Generally, the density of the speckle material is chosen to be greater than that of the measured material. The greater the density difference between the speckle material and the measured material, the easier it is to distinguish them. However, the choice of speckle material is not simply a matter of choosing the largest possible density difference with the measured material; an appropriate material should be selected. For example, when measuring a low-density non-metallic material with CT, a high-density metallic material should be avoided as the speckle material may appear around the metal, obscuring the details of the measured material.

[0032] The specific method for preparing speckle patterns is as follows: Spraying method: Liquid speckle material is sprayed onto the sample surface using spraying equipment to form speckle patterns; Brushing method: After pretreating the surface of the material to be tested (removing grease and stains), use a brush to apply liquid or paste-like speckle material to the surface to form random speckles; Particle bonding method: Mix AB glue in proportion, add speckled material particles, adjust to a viscous state, and apply to the surface of the specimen to form speckles.

[0033] The CT in-situ deformation test procedure for metals mainly includes sample preparation, sample mounting, loading and scanning, and data analysis, as detailed below: Step S3, sample installation.

[0034] Sample fixation: The sample is installed in the clamping system of the in-situ CT deformation testing device to ensure that the sample will not shift or loosen during loading. The in-situ CT deformation testing device can be used for various strain tests such as in-situ tension, in-situ compression, and in-situ bending.

[0035] Environmental settings: Adjust environmental conditions, such as temperature and pressure, according to the sample testing requirements.

[0036] Step S4, Loading and Scanning.

[0037] Applying load: Activate the deformation loading system to apply mechanical loads such as tension, compression, or three-point bending to the sample. The loading process must be carried out slowly to ensure that the CT scan can capture the dynamic changes in the internal structure of the sample in real time.

[0038] CT scan: During loading, CT equipment is used to perform in-situ scanning of the sample, generating three-dimensional images of the sample's internal structure. The scan can be performed in stages to record the structural evolution under different loading conditions.

[0039] Step S5: Data Analysis.

[0040] Image processing: The reconstructed image is color-coded according to the material density difference. The matrix of the tested material and the defects correspond to different gray ranges, so that the speckle pattern and the tested material can be distinguished. Thus, the image generated by CT scan is processed into two parts, namely, the tested material and the speckle pattern are processed separately.

[0041] The images of the material under test generated by CT scans are processed using conventional methods to reconstruct the three-dimensional structural model of the sample and analyze its changes during deformation, such as crack propagation and pore evolution. When processing images of speckle patterns, the plane containing the speckle pattern is rotated to be parallel to the computer screen, and then the speckle pattern is recorded.

[0042] Results Interpretation: By combining loading data and CT images, the mechanical properties and structural responses of the samples, including internal microstructure and defects, were evaluated, providing a basis for material performance research. Simultaneously, speckle patterns were analyzed. The speckle displacement data was imported into the DIC system, and pixel-level displacement data was extracted by analyzing the positional changes of metallic speckles in the CT image sequence, using mesh generation and interpolation algorithms to obtain the corresponding sample strain state. The correlation between the propagation of defects such as cracks and pores and the strain field was also analyzed concurrently.

[0043] The following section uses a tensile test of carbon fiber composites as an example to detail the implementation process of this method: (1) Preparation of speckled pattern.

[0044] 1. Specimen pretreatment: Select carbon fiber composite plate, wipe the surface with alcohol to remove oil stains, and ensure the surface is smooth and flat; 2. Implementation of particle bonding method: Mix AB glue in a 1:1 ratio, add micron-sized magnesium particles and stir until viscous, apply to the surface of the specimen with a pipette, and after drying, magnesium particle agglomerates are formed, which serve as speckles of DIC.

[0045] (2) CT in-situ tensile test.

[0046] 1. Simulation test: The specimen is pre-stretched to obtain the force-displacement curve and determine the linear stage (e.g., strain 0.5%), nonlinear stage (strain 2%) and fracture critical point (strain 3.5%). 2. Formal Experiment: The specimen was installed in the tensile fixture of the CT scanning equipment, and the CT scan was started to obtain the initial three-dimensional image. Load at a rate of 0.5 mm / min, pause when the strain reaches 0.5%, and perform a CT scan; Continue loading to 2% strain, pause and scan; Load the specimen until it fractures, and obtain a CT image of the final fracture state.

[0047] (3) Image analysis and strain calculation.

[0048] 1. 3D Reconstruction: CT images at various stages are reconstructed using 3D visualization analysis software. The grayscale threshold is adjusted to make magnesium speckles appear white and the resin matrix dark gray, such as... Figure 3 As shown; 2. Speckle Tracking: Feature speckles are marked in the reconstructed image, and the three-dimensional displacement coordinates of speckles at different loading stages are calculated by software.

[0049] 3. DIC strain analysis: The speckle displacement data is imported into the self-developed DIC algorithm module, and relevant calculations are performed using a 5×5 pixel grid to obtain the axial strain and shear strain distribution cloud maps of the specimen surface.

[0050] 4. Defect-strain correlation analysis: By comparing crack propagation and strain concentration areas in CT images, it was found that the crack initiation location highly coincides with the high strain area.

[0051] Example 2 Based on Example 1, this example provides a material in-situ strain DIC analysis system based on CT imaging. See [link to example]. Figure 4 and Figure 2 ,include: Speckle preparation module: used to prepare high-contrast speckles, including surface pretreatment components and material coating tools; CT in-situ loading module: integrates CT scanning equipment and loading device to enable CT scanning to be triggered at preset strain points during loading; Image analysis module: Includes 3D reconstruction software and DIC analysis algorithm, which calculates the strain field of the specimen through CT image sequence.

[0052] This invention has the following characteristics: 1. Good spatial compatibility: No need to install an additional camera in the CT room, avoiding equipment interference, and suitable for in-situ testing in confined spaces; 2. Strong data consistency: CT images and DIC data originate from the same scanning process, eliminating synchronization errors between multiple devices and improving data matching accuracy; 3. Excellent functional integration: It can simultaneously acquire information on internal material defects (such as cracks and delamination) and surface strain fields, enabling multi-physics field coupled analysis; 4. Good versatility: It is suitable for various combinations of materials with significant differences in X-ray absorption, and the speckle material can be flexibly selected according to the CT scan energy range.

[0053] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for in-situ strain DIC analysis of materials based on CT imaging, characterized in that, Includes the following steps: The material sample to be tested is processed into standard sheet-shaped specimens to form test pieces; A speckle pattern is prepared on the surface of the specimen, and the material of the speckle has a density difference with the material of the specimen. The specimen with speckle pattern was fixed on the in-situ strain gauge of the CT. Mechanical loads are applied to the fixed specimen. During the loading process, three-dimensional images of the internal structure of the specimen are acquired in stages by CT scanning, and speckle patterns on the surface of the specimen are also acquired. Based on the three-dimensional image, speckle displacement information is extracted, the strain field on the specimen surface is calculated, and digital image correlation analysis is performed.

2. The in-situ strain DIC analysis method for materials based on CT imaging according to claim 1, characterized in that, The calculation of the strain field on the specimen surface and the digital image correlation analysis include the following steps: The three-dimensional image reconstructed from two-dimensional projection is color-coded according to the material density difference. Based on the preset grayscale range, the image is divided into the image of the measured material and the speckle image. Based on the image of the tested material, a three-dimensional structural model of the specimen is reconstructed, and defect evolution analysis is performed. For the speckle image, the plane containing the speckle image is rotated to be parallel to the screen, and the three-dimensional displacement coordinates of the speckle at each loading stage are calculated, i.e., the speckle displacement information. The strain distribution on the specimen surface is calculated using a digital image correlation algorithm, and digital image correlation analysis is performed.

3. The in-situ strain DIC analysis method for materials based on CT imaging according to claim 2, characterized in that, The defect evolution analysis includes crack propagation identification and pore evolution analysis.

4. The in-situ strain DIC analysis method for materials based on CT imaging according to claim 1, characterized in that, The mechanical load is in-situ tension, in-situ compression, or in-situ bending.

5. The in-situ strain DIC analysis method for materials based on CT imaging according to claim 1, characterized in that, During the loading process, CT scans are triggered in stages according to the stress-strain curve of the specimen.

6. The in-situ strain DIC analysis method for materials based on CT imaging according to claim 1, characterized in that, The speckle pattern can be prepared using any of the following methods: Spraying method: A speckle pattern is formed by spraying liquid speckle material onto the surface of the specimen; Brushing method: After pretreating the surface of the specimen, apply liquid or paste-like speckle material to the surface to form random speckles; Particle bonding method: Mix AB glue in proportion, add speckled material particles, adjust to a viscous state, and apply to the surface of the specimen to form speckles.

7. The in-situ strain DIC analysis method for materials based on CT imaging according to claim 1, characterized in that, The density difference between the speckled material and the tested material sample is greater than the density resolution of a CT scan.

8. A material in-situ strain DIC analysis system based on CT imaging, characterized in that, For implementing the in-situ strain DIC analysis method for materials as described in any one of claims 1-7, the system comprises: The speckle pattern preparation module is used to prepare speckle patterns on the surface of a specimen, wherein the material of the speckle has a density difference from the material of the specimen; The CT in-situ loading module is used to apply mechanical loads to the fixed specimen. During the loading process, three-dimensional images of the internal structure of the specimen are acquired in stages by CT scanning. The image analysis module is used to extract speckle displacement information based on the three-dimensional image, calculate the strain field on the specimen surface, and perform digital image correlation analysis.

9. The material in-situ strain DIC analysis system based on CT imaging according to claim 8, characterized in that, The CT in-situ loading module includes a CT scanning device and a tensile clamp for fixing the specimen.

10. A material in-situ strain DIC analysis system based on CT imaging according to claim 9, characterized in that, The CT scanning equipment includes a radiation source and a detector.

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