Mechanical response test method of high temperature superconducting Roebel single strip

Abstract

The present invention discloses a mechanical response test method for a high-temperature superconducting Roebel single tape, comprising: fixing a Roebel single tape sample to be tested after spraying speckle treatment in a tensile testing machine; applying a mechanical load to the Roebel single tape sample to be tested after spraying speckle treatment, and obtaining a load-displacement curve through a 3D-DIC observation system; determining the macroscopic mechanical properties of the Roebel single tape sample to be tested; obtaining the local deformation characteristics of the Roebel single tape sample to be tested under mechanical load; matching the local deformation characteristics of the Roebel single tape sample to be tested under mechanical load with the macroscopic mechanical properties, and obtaining the mechanical response under different working conditions during displacement-controlled loading. The technical solution of the present invention is adopted to realize full-field, real-time, and accurate observation of the local mechanical behavior characteristics of the structure, while meeting the requirements of high efficiency, simple operation, and not being limited by the mechanical loading form and sample morphology.

Description

Technical Field

[0001] The invention belongs to the technical field of superconducting materials, and in particular relates to a high-temperature superconducting Roebel single-strip mechanical response testing method. Background Art

[0002] Since the discovery of superconductivity in 1911, the types and quantity of superconducting materials have been growing explosively year by year. The advantages of superconducting materials, such as zero resistance, high current carrying capacity, and anti-magnetism, have greatly expanded human cognition of matter and promoted the efficient use of clean energy. At present, the second-generation high-temperature superconducting REBCO tape with a critical temperature higher than the liquid nitrogen temperature (77 K) is regarded as a superconducting material with great commercial prospects. In the field of applied superconductivity, due to the limited current carrying capacity of a single REBCO tape, engineers have developed a variety of cables made of stacked, twisted, and transposed tapes to meet higher power transmission needs while reducing AC losses. Among them, Roebel cable is a typical cable configuration with full transposition characteristics. The cable requires cutting the REBCO tape into Roebel single tapes with transposition shapes, and then assembling multiple single tapes. Roebel cable has the characteristics of high current density, low loss, and small in-plane bending radius, and can be widely used in large current transmission, accelerator magnets, aerospace and other fields. However, REBCO tapes and Roebel tapes face the risk of failure such as fracture when used in extreme, multi-physical field environments during application. Once the structure is damaged, it will not only lead to safety hazards but also cause irreversible degradation of superconductivity, which directly affects the safe and efficient operation of related superconducting magnets. Therefore, necessary stress management and monitoring are extremely important, especially for Roebel tapes with unique shapes. It is very important to understand their mechanical properties and responses, which is also an important basis for the optimal design of Roebel cables. At present, on the one hand, the mechanical properties of Roebel tapes, such as stiffness and ultimate load, are still unclear at room temperature and liquid nitrogen temperature. On the other hand, there is a lack of effective experimental observation methods for the local mechanical deformation of Roebel tapes. This further makes it difficult to investigate the relationship between its mechanical properties and local deformation characteristics under load.

[0003] In the previous mechanical property tests of REBCO strips, (low temperature) strain gauges were often bonded to the surface of the strip to measure its local deformation. This approach is applicable to strips without geometric mutations. However, Roebel strips are different from REBCO strips in that their geometric characteristics (i.e., transposition structure) are unique, which can lead to significant deformation differences at different locations on their surface while their mechanical properties deteriorate relative to REBCO strips. If a limited number of (low temperature) strain gauges are used to measure deformation only at certain locations, not only will the measured data set be difficult to fully reflect the mechanical behavior of the Roebel strip, but the implementation process will also be time-consuming, labor-intensive, and extremely costly. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a method for testing the mechanical response of a high-temperature superconducting Roebel single strip.

[0005] To achieve the above object, the present invention adopts the following technical solution:

[0006] A method for testing the mechanical response of a high temperature superconducting Roebel single tape, comprising:

[0007] Step S1, performing a spraying speckle treatment on the surface of the Roebel single tape to be tested sample, and fixing the Roebel single tape to be tested sample after the spraying speckle treatment in a tensile testing machine;

[0008] Step S2, applying a mechanical load to the Roebel single tape sample to be tested after the spraying speckle treatment, and obtaining a load-displacement curve through a 3D-DIC observation system;

[0009] Step S3, determining the macroscopic mechanical properties of the Roebel single belt sample to be tested through the load-displacement curve;

[0010] Step S4, obtaining the local deformation characteristics of the Roebel single tape to be tested sample under mechanical load according to the speckle position change information of the Roebel single tape to be tested sample after the spraying speckle processing;

[0011] Step S5, matching the local deformation characteristics of the Roebel single belt sample under mechanical load with the macroscopic mechanical properties to obtain the mechanical response under different working conditions during displacement controlled loading;

[0012] Among them, step S4 specifically includes: collecting speckle images during displacement-controlled loading; graying and enhancing the speckle images; selecting a reference area on the grayed and enhanced speckle images, searching for the best matching position in the deformation image of the reference area, calculating the correlation coefficient using a cross-correlation method, and determining the displacement vector; using the camera calibration parameters of the 3D-DIC observation system to convert the pixel displacement into the actual space coordinate displacement; and calculating the strain field using a gradient operator according to the actual space coordinate displacement.

[0013] Preferably, a REBCO superconducting tape is used to prepare a Roebel single tape sample with or without rounded corner transition in the transposition region.

[0014] Preferably, in step S2, a direct tensile or bending load is applied to the Roebel single tape sample to be tested after the spray-speckling treatment at room temperature or liquid nitrogen bath temperature.

[0015] Preferably, for applying direct tensile load, the Roebel single tape sample to be tested after the spraying speckle treatment is installed and fixed together with the clamp on the tensile testing machine.

[0016] Preferably, for applying bending load, the Roebel single-belt test sample to be tested after spraying speckle treatment is installed and fixed to the tensile testing machine together with the balance beam.

[0017] Preferably, in step S1, the speckle pattern is sprayed on the surface of one side of the strip facing the 3D-DIC observation system.

[0018] The present invention realizes full-field, real-time and precise observation of the local mechanical behavior characteristics of the structure, while meeting the requirements of high efficiency, simple operation and not being limited by the mechanical loading form and sample morphology. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0020] Figure 1 This is a schematic diagram of the principle of the high-temperature superconducting Roebel single-strip mechanical response test method;

[0021] Figure 2 Schematic diagram of the mechanical response test of a high-temperature superconducting Roebel strip under different load forms combined with 3D-DIC observation. DETAILED DESCRIPTION

[0022] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0023] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0024] Embodiment 1:

[0025] The embodiment of the present invention provides a method for testing the mechanical response of a high temperature superconducting Roebel single strip. Figure 1As shown in the figure, when investigating the mechanical deformation of the Roebel strip, the three-dimensional digital image correlation (3D-DIC) method is introduced into the 3D-DIC observation system for auxiliary measurement; the main part of the 3D-DIC observation system is the ARAMIS Adjustable stereo camera device, which includes an LED light source and a high-resolution camera. As a non-contact optical measurement method, the 3D-DIC method measures the full-field displacement and strain of the surface of the sample under test by tracking the surface speckle pattern before and after the deformation of the sample under test. Compared with 2D-DIC, which is only suitable for measuring plane deformation, 3D-DIC overcomes its defect of being unable to measure out-of-plane displacement based on the binocular vision model, and is therefore more suitable for capturing the local deformation characteristics of the Roebel strip when it is subjected to mechanical loads. Since 3D-DIC observation is a non-contact optical measurement, the deformation measurement of the Roebel strip is not limited by the load form and sample morphology factors. In addition, in the loading system, the sample to be tested needs to be fixed in the tensile testing machine together with the fixture or balance beam. The tensile and bending mechanical loads can be applied at room temperature or 77K (liquid nitrogen bath) temperature environment, and the load-displacement curve can be recorded at the same time. The macroscopic mechanical properties of the sample to be tested can be determined by the load-displacement curve. The local deformation characteristics (displacement and strain) during the loading process can be obtained simultaneously by using ZEISS INSPECT Correlate software combined with 3D-DIC auxiliary measurement. The collected data can be recorded and sorted to further study the relationship between local deformation and macroscopic mechanical properties.

[0026] like Figure 2 As shown, an embodiment of the present invention provides a method for testing the mechanical response of a high-temperature superconducting Roebel single tape, comprising:

[0027] Step S1, performing a spraying speckle treatment on the surface of the Roebel single tape to be tested sample, and fixing the Roebel single tape to be tested sample after the spraying speckle treatment in a tensile testing machine;

[0028] Step S2, applying a mechanical load to the Roebel single tape sample to be tested after the spraying speckle treatment, and obtaining a load-displacement curve through a 3D-DIC observation system;

[0029] Step S3, determining the macroscopic mechanical properties of the Roebel single belt sample to be tested through the load-displacement curve; wherein the macroscopic mechanical properties include: peak load and stiffness;

[0030] Step S4, obtaining the local deformation characteristics of the Roebel single tape to be tested sample under mechanical load according to the speckle position change information of the Roebel single tape to be tested sample after the spraying speckle processing;

[0031] Step S5, matching the local deformation characteristics of the Roebel single tape sample under mechanical load with the macroscopic mechanical properties to obtain the mechanical response under different working conditions during displacement controlled loading.

[0032] As an implementation method of the present invention, in step S1, REBCO superconducting tape (12 mm wide) is used to prepare transposition regions with / without rounded corners ( The sample to be tested is a Roebel single tape (5.5 mm wide) with a chamfer radius. The clamping area (end) of the sample to be tested is wider than the area to be observed to increase the contact area between the sample to be tested and the fixture to avoid slippage. At the same time, the part connecting the clamping area of ​​the sample to be tested and the area to be observed is chamfered ( The fillet radius is designed to avoid introducing stress concentration at the clamping end and thus affecting the strain distribution in the area to be observed.

[0033] Furthermore, the surface of the sample to be tested is sprayed with speckle treatment, and the speckle is sprayed on the surface of the strip facing the 3D-DIC observation system to maintain uniformity and density. For direct tensile load conditions, no other operations are required; for bending load conditions, the sample to be tested should be welded to the balance beam before spraying the speckle.

[0034] As an implementation method of the present invention, for direct tensile load conditions, the sample to be tested after the spraying speckle treatment is installed and fixed together with the clamp on the tensile testing machine; the distance between the upper and lower clamps of the testing machine is adjusted so that the tensile state of the strip changes from completely relaxed to slightly tensed, and this time is set as the initial state; the sample to be tested after the spraying speckle treatment is placed in a corresponding temperature environment (room temperature or liquid nitrogen bath). For bending load conditions, the sample to be tested after the spraying speckle treatment is installed and fixed together with the balance beam on the tensile testing machine; the sample to be tested after the spraying speckle treatment is placed in a corresponding temperature environment (room temperature or liquid nitrogen bath).

[0035] As an implementation method of an embodiment of the present invention, in step S2, the 3D-DIC observation system is calibrated. After the calibration is successful, the size of the area to be observed of the 3D-DIC observation system in the sample to be tested after the spray speckle treatment is set to be approximately 7cmx5cm (width x height), and the observation area must include the transposition area of ​​the Roebel single band. In addition, the characteristic position for real-time monitoring should be selected as needed. When the 3D-DIC observation system is used for observation, the area to be observed in the lens should always be kept at the center of the image. When the displacement-controlled loading starts, the 3D-DIC observation system is started at the same time and image acquisition is performed; so that the 3D-DIC observation system can always perform synchronous observation during the entire process of loading to the destruction or rated load of the sample to be tested after the spray speckle treatment, and record the corresponding load-displacement curve.

[0036] As an implementation method of an embodiment of the present invention, in step S3, the macroscopic mechanical properties include: peak load and stiffness; wherein the peak load refers to the maximum load in the process of directly stretching the object under test to failure, and the stiffness is determined by the slope of the load-displacement curve in the linear elastic stage.

[0037] As an implementation of the embodiment of the present invention, in step S4, according to the speckle position change information of the sample to be tested after the spraying speckle processing during the displacement control loading process obtained by the 3D-DIC observation system, the local deformation characteristics (displacement and strain) of the sample to be tested under the mechanical load are obtained; specifically, the following steps are included: collecting the speckle image during the displacement control loading process; graying and enhancing the speckle image; selecting a reference area on the grayed and enhanced speckle image, searching for the most matching position in the deformation image of the reference area, using the cross-correlation method to calculate the correlation coefficient, and determining the displacement vector; using the camera calibration parameters of the 3D-DIC observation system to convert the pixel displacement into the actual space coordinate displacement; and using the gradient operator to calculate the strain field according to the actual space coordinate displacement. At the same time, the final damage form of the sample to be tested can also be determined (determined by the photos taken by the 3D-DIC observation system, such as the fracture position and fracture path).

[0038] As an implementation of an embodiment of the present invention, in step S5, since 3D-DIC observation and displacement-controlled loading are performed synchronously, there is a corresponding local deformation image at each point of the load-displacement curve from the beginning to the end of the displacement-controlled loading process. For example: manually set the 3D-DIC observation system to take a frame of photos every 1 second, and align the start time of the shooting with the beginning of the loading, so as to obtain the mechanical response under different working conditions during the displacement-controlled loading process.

[0039] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for testing the mechanical response of a high-temperature superconducting Roebel single tape, characterized in that: include: Step S1, spraying speckle treatment on the surface of the Roebel single tape to be tested, and fixing the Roebel single tape to be tested after the spraying speckle treatment in a tensile testing machine; wherein the speckle spraying is on the surface of one side of the tape facing the 3D-DIC observation system; Step S2, applying a mechanical load to the Roebel single tape sample to be tested after the spraying speckle treatment, and obtaining a load-displacement curve through a 3D-DIC observation system; Step S3, determining the macroscopic mechanical properties of the Roebel single belt sample to be tested through the load-displacement curve; wherein the macroscopic mechanical properties include: peak load and stiffness; Step S4, obtaining the local deformation characteristics of the Roebel single tape to be tested sample under mechanical load according to the speckle position change information of the Roebel single tape to be tested sample after the spraying speckle processing; Step S5, matching the local deformation characteristics of the Roebel single belt sample under mechanical load with the macroscopic mechanical properties to obtain the mechanical response under different working conditions during displacement controlled loading; Among them, step S4 specifically includes: collecting speckle images during displacement-controlled loading; graying and enhancing the speckle images; selecting a reference area on the grayed and enhanced speckle images, searching for the best matching position in the deformation image of the reference area, calculating the correlation coefficient using a cross-correlation method, and determining the displacement vector; using the camera calibration parameters of the 3D-DIC observation system to convert the pixel displacement into the actual space coordinate displacement; and calculating the strain field using a gradient operator according to the actual space coordinate displacement.

2. The high temperature superconducting Roebel single tape mechanical response test method according to claim 1, characterized in that: The Roebel single tape samples with or without rounded corner transition in the transposition region were prepared using REBCO superconducting tape.

3. The high temperature superconducting Roebel single tape mechanical response test method according to claim 2, characterized in that: In step S2, a direct tensile or bending load is applied to the Roebel single tape sample to be tested after the spray-speckling treatment at room temperature or liquid nitrogen bath temperature.

4. The high temperature superconducting Roebel single tape mechanical response test method according to claim 3, characterized in that: For direct tensile loading, the Roebel single strip sample to be tested after spraying speckle treatment is installed and fixed to the tensile testing machine together with the fixture.

5. The high temperature superconducting Roebel single tape mechanical response test method according to claim 3, characterized in that: For the bending load, the Roebel single strip sample to be tested after the spraying speckle treatment was installed and fixed to the tensile testing machine together with the balance beam.

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