Tension-compression and fatigue deformation local strain measurement method and system suitable for brittle metal material

Abstract

The invention belongs to the field of material mechanical property testing and strain measurement, and particularly relates to a local strain measurement method and system based on a four-point bending and digital image correlation (DIC) technology, and the method specifically comprises the following steps: cutting to prepare a sample, carrying out surface electrolytic polishing, preparing speckles on the surface of the sample to be tested, and carrying out a four-point bending experiment; speckle images of the lower surface of the sample are collected, displacement correction is conducted in combination with bending deflection calculation, the bending load and the strain of the upper surface of the sample are obtained in real time, the stress-strain relation and local strain distribution of the lower surface of the sample are obtained through calculation, and finally deformation and failure mechanism analysis is conducted in combination with surface deformation and crack morphology. The method provided by the invention combines four-point bending and DIC plane strain measurement technologies, can realize stretching, compression and dynamic fatigue loading on the surface of the sample, is suitable for performance test and failure analysis of tension, compression and fatigue deformation of brittle metal materials such as intermetallic compounds and amorphous materials, and has the characteristics of convenience, rapidness and low cost.

Description

Technical Field

[0001] The present invention relates to the field of material mechanical property testing and strain measurement, and in particular to a local strain measurement method and system based on four-point bending and digital image correlation (DIC) technology, which is suitable for performance testing and failure analysis of tension, compression and fatigue deformation of brittle metal materials such as intermetallic compounds and amorphous materials. Background Art

[0002] With the continuous development of materials science and processing technology, various types of metal materials with excellent performance have emerged, and the mechanical properties are constantly improving. For example, intermetallic compounds represented by TiAl alloys have excellent high-temperature performance and specific strength, and the long-term service temperature can reach about 750°C; zirconium-based amorphous alloys such as Zr-Cu-Ni-Al, etc., can reach a yield strength of more than 1800MPa, and have a low elastic modulus and high elastic deformation capacity, which can meet the deformation requirements under special working conditions. However, while the above materials have excellent performance, they are also accompanied by obvious room temperature brittleness, and often the higher the strength, the lower the plasticity. Low room temperature plasticity or even brittleness not only easily leads to destructive cracking of metal materials, affecting the safety of the service process, but also leads to high dispersion of test results such as material strength and fatigue performance, greatly increasing performance testing requirements and R&D costs.

[0003] In recent years, machine vision-based measurement methods, including video extensometer technology, DIC and speckle interferometry technology, have gradually been applied to material deformation / strain measurement. Among them, DIC technology can achieve full-field, high-precision, non-contact and non-destructive measurement, with significant advantages, and has been widely studied and applied in the field of machine vision mechanical measurement. The DIC method is used to measure the displacement and strain information of an object. Generally, the two-dimensional full-field deformation image of the surface speckle of the object before and during loading is captured, and then the displacement and local strain of the surface speckle are obtained by "matching" correlation calculation of the acquired image sequence. Since the size of the splashing speckle is adjustable and the range is very large, the DIC method can be used to measure strains at different scales, macro and micro, and has been widely used in strain measurement in engineering and scientific research. In addition, compared with other visual measurement methods, the DIC method is easy to realize the automation and digitization of the entire measurement process, and can give full play to the advantages and potential of computer technology in digital image processing. However, the DIC strain measurement method is mainly suitable for plane strain analysis, and the resolution and accuracy are greatly affected by experimental conditions. For rod-shaped specimens used in conventional mechanical tests, such as room temperature tension, compression and fatigue specimens, since the surface of the specimen gauge section is a curved surface, speckle preparation, image acquisition and strain calculation all require targeted design, and there is currently no effective measurement method.

[0004] Therefore, the present invention develops a local strain measurement method and system based on four-point bending and DIC technology. This method uses four-point bending to stretch, compress and dynamically fatigue load the surface of the specimen, and uses DIC to perform plane strain measurement. This method is suitable for brittle metal materials such as intermetallic compounds and amorphous materials, and can greatly reduce the cost of sample preparation and experiments, and realize accurate strain measurement of tension, compression and fatigue deformation and failure mechanism research. Summary of the invention

[0005] The present invention is to solve the problems existing in the above-mentioned prior art and provide a method for measuring local strain of tension, compression and fatigue deformation of brittle metal materials, which is characterized by comprising the following steps:

[0006] S1 cuts out the test sample with regular shape, grinds the surface of the test sample and then performs electrolyte polishing to obtain the test sample with smooth surface and no defects;

[0007] S2: After cleaning and drying the test sample obtained in step S1, speckle preparation is performed to obtain a test sample with speckle particles on the surface;

[0008] S3: affixing a strain gauge at the center of the upper surface of the test sample obtained in step S2, generating tensile, compressive or dynamic fatigue stress on the surface of the test sample by a mechanical testing machine using a four-point bending method, collecting bending load and strain in real time through the strain gauge and converting them into surface stress-strain data;

[0009] S4 acquires the speckle image of the lower surface of the sample through the DIC image acquisition device, obtains the local strain distribution on the surface of the sample to be tested, and analyzes the deformation and failure mechanism in combination with the surface deformation and crack morphology.

[0010] Furthermore, the regular shape in step S1 is a long strip with a rectangular cross section, and its surface is finely polished by using multiple types of sandpaper in stages to ensure that the surface is flat and defect-free.

[0011] Furthermore, the method for preparing the speckle particles in step S2 includes a spray painting method or a method for treating a metal film with steam, and the size of the speckle particles ranges from 50 nm to 10 μm.

[0012] Furthermore, the method for obtaining the local strain distribution on the surface of the sample to be tested in step S4 comprises the following steps:

[0013] S41 image preprocessing, after converting the image format to TIFF format, the image is smoothed using the bilateral filtering method, and the image contrast is enhanced using the histogram equalization method;

[0014] S42 Integer pixel displacement search, based on the fast Fourier transform method, the integer pixel displacement search is performed. The integer pixel displacement (u1, v1) of each point in the search area is obtained through the Fourier transform of the cross-correlation function of the target image and the reference image;

[0015] S43 sub-pixel displacement calculation, using the cubic Hermite interpolation algorithm to reconstruct the sub-pixel grayscale, and combined with the Newton-Raphson iterative algorithm to obtain the optimal matching sub-pixel displacement (u2, v2);

[0016] S44 strain calculation, using Gaussian filtering method to smooth the displacement field results, and using the least squares fitting method to calculate the strain component ε corresponding to each point x , ε y and γ xy .

[0017] Furthermore, before the strain calculation in step S44, the displacement correction in the length X direction is performed based on the deflection calculation result of the four-point bending to eliminate the influence of the surface bending on the displacement measurement result. The displacement increment in the X direction after correction is:

[0018]

[0019] Where x, the center point of the bending beam, is 0, Δw is the deflection increment at x, P is the tensile and compressive loads during the measured deformation process, S1 is the span of the indenter for applying the dynamic compression load, E is the elastic modulus of the material, and I is the moment of inertia of the section.

[0020] Furthermore, the calculation formula for strain calculation in step S44 is:

[0021]

[0022] Where σ is the tensile stress on the lower surface, l = (S2-S1) / 2, S1 is the span of the indenter for applying dynamic compression load, the loading frequency is 1 Hz, S2 is the span of the lower indenter, P is the tensile and compressive loads during the measured deformation process; ε1 is the compressive strain, ε2 is the lower surface strain measured by DIC, b is the specimen thickness, and h is the specimen height.

[0023] Furthermore, the mechanism analysis is carried out by comparing the DIC local strain measurement results with the microstructure morphology to study the deformation, crack initiation and failure mechanism of metal materials; wherein the microstructure morphology is obtained by SEM electron microscopy, including the phase composition, phase morphology, phase distribution and crystal orientation before deformation, and the surface slip traces, shear bands, local orientation differences and microcrack morphology and distribution after deformation.

[0024] The present invention also provides a local strain measurement system suitable for tension, compression and fatigue deformation of brittle metal materials, which is used to implement the above strain measurement method, including:

[0025] Sample preparation module, used to prepare four-point bending specimens;

[0026] Four-point bending test module, used for four-point bending test;

[0027] DIC surface strain measurement module, used to collect and process the speckle morphology information of the analysis surface and calculate the local strain distribution of the analysis surface;

[0028] The mechanism analysis module is used to study the deformation and failure mechanism of materials based on strain measurement and microstructure analysis results.

[0029] Furthermore, the DIC surface strain measurement module includes a DIC image acquisition device, which is composed of an LED lighting source, a CCD camera and a high-resolution lens.

[0030] The present invention has the following beneficial effects:

[0031] The present invention provides a convenient method for measuring local strains of tension, compression and fatigue deformation. The DIC strain measurement method is mainly suitable for plane strain analysis. For the curved surface of a rod-shaped specimen used in conventional mechanical tests, such as room temperature tension, compression and fatigue specimens, strain testing is extremely difficult. The four-point bending method can realize different forms of loading methods such as tension, compression and dynamic fatigue on the specimen analysis surface. At the same time, the analysis surface of the four-point bending specimen is a plane, which is convenient for DIC strain measurement. The combination of the two can realize the testing and strain measurement of different properties of metal materials.

[0032] The present invention provides a local strain measurement method suitable for brittle metal materials. During the mechanical property test, brittle metal materials are prone to premature brittle fracture due to defects and microcracks, and the performance test results fluctuate greatly, and the test is difficult. After the four-point bending specimen is subjected to force, one side is compressed and the other side is tensile, with the neutral plane as the boundary. Compared with the overall uniform tensile stress, it shows higher damage tolerance and performance stability in practice, and is more suitable for local strain measurement of brittle metal materials.

[0033] The local strain measurement method provided by the present invention has low cost. The local strain measurement method based on four-point bending and DIC technology uses a small-sized rectangular strip specimen and can be directly combined with a static and dynamic compression testing machine. The sample preparation cost and the experimental cost are very low, and low-cost measurement of the mechanical properties and local strain of brittle metal materials can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is an application scenario diagram of a local strain measurement method and system based on four-point bending and DIC technology according to some embodiments of the present invention.

[0035] Figure 2 is a module diagram of a local strain measurement system based on four-point bending and DIC technology according to some embodiments of the present invention.

[0036] Figure 3 This is a diagram of the DIC strain measurement results of the TiAl alloy four-point bending fatigue loading obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with the accompanying drawings.

[0038] Embodiment 1:

[0039] This embodiment provides a local strain measurement system suitable for tension, compression and fatigue deformation of brittle metal materials. Figure 1 The figure shows a specific local strain measurement scenario, including a mechanical testing machine 1, a tension and compression load sensor 2, a four-point bending specimen 3 and a DIC image acquisition device. In the four-point bending-DIC local strain test process, static tension, compression and dynamic fatigue stress loading of the analysis surface are achieved through four-point bending, and various parameters such as load and strain are collected in real time. The stress-strain curve of the analysis surface is generated by calculation, and the digital information of the speckle is obtained by the DIC image acquisition device, and the local strain distribution of the analysis surface is obtained by calculation.

[0040] The four-point bending specimen 3 is arranged between the tension and compression load sensor 2 and the DIC image acquisition device. The upper and lower surfaces of the specimen correspond to two pressure heads respectively. The lower surface of the specimen is the analysis surface. The compression of the testing machine corresponds to the tensile deformation of the analysis surface, and the tensile deformation of the testing machine corresponds to the compressive deformation of the analysis surface. The dynamic tensile and compressive loads applied by the testing machine can generate dynamic fatigue stress on the analysis surface. In addition to the upper and lower surfaces of the four-point bending specimen being in contact with the pressure heads, elastic limit devices are required to be added to the left and right ends and the front and back sides to prevent the specimen from moving during the experiment, especially during dynamic loading. The real-time acquisition parameters include the tension and compression load P, the upper surface strain ε1 and the lower surface strain ε2, where the load P is measured by Figure 1 The tensile and compressive load sensors shown in the figure are used for measurement. The strain ε1 is measured by the strain gauge attached to the upper surface of the specimen, and the strain ε2 is measured and calculated by DIC.

[0041] Embodiment 2:

[0042] This embodiment provides a module structure example of a local strain measurement system for tension, compression and fatigue deformation of brittle metal materials. Figure 2 The system shown includes a sample preparation module 210 , a four-point bending test module 220 , a DIC surface strain measurement module 230 , and a mechanism analysis module 240 .

[0043] The sample preparation module 210 is used to prepare a four-point bending specimen. A long strip specimen with a rectangular cross section is processed and polished to obtain a flat and defect-free analysis surface, and speckle particles are prepared on the sample analysis surface for digital information acquisition and strain measurement. In some embodiments, the polishing adopts overall electrolytic polishing to obtain a flat and defect-free analysis surface while eliminating corners and other surface sample preparation defects to prevent brittle fracture due to surface defects during the experiment. The speckle particle preparation adopts a spray painting method or a method of steam treating a metal film to obtain speckles of different particle sizes to achieve strain measurement of multiple scales.

[0044] The four-point bending test module 220 is used for four-point bending testing. Through static tension and compression and dynamic loading, tensile, compressive or dynamic fatigue stress is generated on the four-point bending analysis surface, the bending load and surface strain are collected in real time, and the stress-strain data of the analysis surface are calculated. In some embodiments, the four-point bending experiment is carried out on a conventional mechanical testing machine, and loading and parameter measurement are achieved through a four-point bending device. The four-point bending device includes a four-point bending specimen, pressure heads on the upper and lower surfaces, elastic limit devices on the left and right ends and the front and rear sides, tension and compression load sensors, etc. The real-time collection parameters include tension and compression loads P, upper surface strain ε1 and lower surface strain ε2, where the load P is measured by Figure 1 The tensile and compressive load sensors shown in the figure are used for measurement. The strain ε1 is measured by the strain gauge attached to the upper surface of the specimen, and the strain ε2 is measured and calculated by DIC.

[0045] The DIC surface strain measurement module 230 is used to collect and process the speckle morphology information of the analysis surface and calculate the local strain distribution of the analysis surface. In some embodiments, the speckle morphology information is obtained by Figure 1 The DIC image acquisition device shown in the figure is obtained, including an LED lighting source, a CCD camera and a high-resolution lens. The local strain calculation steps include image preprocessing, integer pixel displacement search, sub-pixel displacement calculation and strain calculation. Among them, image preprocessing includes image format conversion, converting the camera-collected image into TIFF format, smoothing the image using bilateral filtering method, and enhancing the image contrast using histogram equalization method; performing integer pixel displacement search based on fast Fourier transform method, and obtaining the integer pixel displacement (u1, v1) of each point in the search area through Fourier transform of the cross-correlation function of the target image and the reference image; using cubic Hermite interpolation algorithm for sub-pixel grayscale reconstruction, and combining with Newton-Raphson iterative algorithm to obtain the optimal matching sub-pixel displacement (u2, v2); using Gaussian filtering method to smooth the displacement field results, and using the least squares fitting method to calculate the strain component ε corresponding to each point x , ε y and γ xyIn addition, before strain calculation, the displacement in the length X direction is corrected based on the deflection calculation results of four-point bending to eliminate the influence of surface bending on the displacement measurement results. The displacement increment in the X direction after correction is:

[0046]

[0047] Where x, the center point of the bending beam, is 0, Δw is the deflection increment at x, P is the tensile and compressive loads during the measured deformation process, S1 is the span of the indenter for applying the dynamic compression load, E is the elastic modulus of the material, and I is the moment of inertia of the section.

[0048] The mechanism analysis module 240 is used to study the deformation and failure mechanism of the material based on the strain measurement and tissue morphology analysis results. In some embodiments, the tissue morphology analysis is obtained using a SEM electron microscope, including the phase composition, phase morphology, phase distribution and crystal orientation before deformation, and the surface slip traces, shear bands, local orientation differences and microcrack morphology and distribution after deformation. By comparing and analyzing the local strain measurement results of DIC, the deformation, crack initiation and failure mechanism of the metal material are studied.

[0049] Embodiment 3:

[0050] In combination with the above embodiments, this embodiment provides a specific step for measuring the tension, compression and fatigue deformation local strain of brittle metal materials:

[0051] (1) Wire cutting is used to cut the size of 2×4×35mm 3 The TiAl alloy four-point bending specimens were ground with sandpapers of type 240#, 600#, 800#, and 2000# in sequence. The specimens were electropolished as a whole, with the polishing liquid made of perchloric acid, n-butanol, and methanol in a ratio of 1:7:12, a voltage of 35V, and a polishing time of 30-60s.

[0052] (2) Prepare speckle particles on the surface of the four-point bending specimen. First, the polished specimen is ultrasonically cleaned for 15s-30s, and the ultrasonically cleaned specimen is blown dry with high-pressure gas. Then, an ion sputtering instrument is used to spray a gold coating on the surface of the specimen with a thickness of about 20-50nm. The gold-coated specimen is placed in a steam environment and heated at 300℃ for 30 minutes to obtain uniformly and discretely distributed gold speckle particles with a particle size of about 500nm-5μm.

[0053] (3) A four-point bending test was conducted on an MTS Landmark 370 fatigue testing machine. The upper indenter was used for loading, and a dynamic compressive load was applied. The loading frequency was 1 Hz, and the indenter span S1 was 20 mm. The lower indenter was used for support, and the span S2 was 30 mm. Strain gauges were attached to the center of the upper surface of the specimen to measure the tensile and compressive loads P and the compressive strain ε1 during the deformation process. The lower surface strain ε2 was obtained by the DIC strain measurement system, which corresponds to the average strain of the entire analysis area. The tensile stress σ on the lower surface was calculated according to the following formula:

[0054]

[0055] Where, l=(S2-S1) / 2, b is the sample thickness, and h is the sample height.

[0056] (4) The speckle image of the lower surface of the sample was obtained using a DIC image acquisition device with an acquisition frequency of 1 frame / s. A program written in Python was used for image processing, sub-pixel positioning and strain calculation. The strain analysis area size was approximately 2 mm × 8 mm. The equivalent strain ε corresponding to different cycles N was eq The distribution results are as follows Figure 3 shown.

[0057] The above generally describes the present invention in detail, but it is obvious to a person skilled in the art that some modifications or improvements can be made to the present invention. Therefore, modifications or improvements that do not depart from the spirit of the present invention are within the scope of protection of the present invention.

Claims

1. A local strain measurement method for tension, compression and fatigue deformation of brittle metal materials, characterized in that , including the following steps: S1: Cut out the test sample with regular shape, grind the surface of the test sample and then perform electrolyte polishing to obtain the test sample with smooth surface and no defects; S2: After cleaning and drying the test sample obtained in step S1, speckle preparation is performed to obtain a test sample with speckle particles on the surface; S3: Paste a strain gauge at the center of the upper surface of the test sample obtained in step S2, generate tensile, compressive or dynamic fatigue stress on the surface of the test sample by a mechanical testing machine using a four-point bending method, collect bending load and strain in real time through the strain gauge and convert them into surface stress-strain data; S4: The speckle image of the lower surface of the sample is obtained by the DIC image acquisition device to obtain the local strain distribution on the surface of the sample to be tested, and the deformation and failure mechanism analysis is performed in combination with the surface deformation and crack morphology.

2. The method for measuring local strain of tension, compression and fatigue deformation of brittle metal materials according to claim 1, characterized in that: The regular shape in step S1 is a long strip with a rectangular cross section, and its surface is finely polished by using multiple types of sandpaper in stages to ensure that the surface is flat and free of defects.

3. The method for measuring local strain of tension, compression and fatigue deformation of brittle metal materials according to claim 1, characterized in that: The method for preparing the speckle particles in step S2 includes a spray painting method or a method for treating a metal film with steam, and the size of the speckle particles ranges from 50 nm to 10 μm.

4. The method for measuring local strain of tension, compression and fatigue deformation of brittle metal materials according to claim 1, characterized in that: The method for obtaining the local strain distribution on the surface of the sample to be tested in step S4 comprises the following steps: S41: Image preprocessing, converting the image format into TIFF format, smoothing the image using bilateral filtering method, and enhancing the image contrast using histogram equalization method; S42: Integer pixel displacement search, based on the fast Fourier transform method, the integer pixel displacement search is performed, and the integer pixel displacement (u1, v1) of each point in the search area is obtained by Fourier transform of the cross-correlation function of the target image and the reference image; S43: Sub-pixel displacement calculation, using the cubic Hermite interpolation algorithm to reconstruct the sub-pixel grayscale, and combining it with the Newton-Raphson iterative algorithm to obtain the optimal matching sub-pixel displacement (u2, v2); S44: Strain calculation, using Gaussian filtering to smooth the displacement field results, and using the least squares fitting method to calculate the strain component ε corresponding to each point x , ε y and γ xy And calculate the tensile stress σ on the lower surface.

5. The method for measuring local strain of tension, compression and fatigue deformation of brittle metal materials according to claim 4, characterized in that: Before the strain calculation in step S44, the displacement in the length X direction is corrected based on the deflection calculation result of the four-point bending to eliminate the influence of the surface bending on the displacement measurement result. The displacement increment in the X direction after correction is: ； Where x, the center point of the bending beam, is 0, Δw is the deflection increment at x, P is the tensile and compressive loads during the measured deformation process, S1 is the span of the indenter for applying the dynamic compression load, E is the elastic modulus of the material, and I is the moment of inertia of the section.

6. The method for measuring local strain of tension, compression and fatigue deformation of brittle metal materials according to claim 4, characterized in that: The calculation formula for strain calculation in step S44 is: ； Where σ is the tensile stress on the lower surface, l = (S2-S1) / 2, S1 is the span of the indenter for applying dynamic compression load, the loading frequency is 1 Hz, S2 is the span of the lower indenter, P is the tensile and compressive loads during the measured deformation process; ε1 is the compressive strain, ε2 is the lower surface strain measured by DIC, b is the specimen thickness, and h is the specimen height.

7. The method for measuring local strain of tension, compression and fatigue deformation of brittle metal materials according to claim 1, characterized in that: The mechanism analysis is based on the DIC local strain measurement results and the organizational morphology for comparative analysis, and is used to study the deformation, crack initiation and failure mechanism of metal materials; The microstructure is obtained through the SEM electron microscope using the mechanism analysis module, including the phase composition, phase morphology, phase distribution and crystal orientation before deformation, as well as the surface slip traces, shear bands, local orientation differences and microcrack morphology and distribution after deformation.

8. A local strain measurement system for tension, compression and fatigue deformation of brittle metal materials, used to implement the method described in any one of claims 1 to 7, characterized in that: include: Sample preparation module, used to prepare four-point bending specimens; Four-point bending test module, used for four-point bending test; DIC surface strain measurement module, used to collect and process the speckle morphology information of the analysis surface and calculate the local strain distribution of the analysis surface; The mechanism analysis module is used to study the deformation and failure mechanism of materials based on strain measurement and microstructure analysis results.

9. The local strain measurement system for tension, compression and fatigue deformation of brittle metal materials according to claim 8, characterized in that: The DIC surface strain measurement module includes a DIC image acquisition device, which is composed of an LED lighting source, a CCD camera and a high-resolution lens.

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