CN113203638A - Method and system for testing shearing large strain of metal sheet under microscopic scale - Google Patents

Abstract

The invention provides a method and a system for measuring large shear strain of a metal sheet at a microscopic scale, and relates to the technical field of experimental solid mechanics. Clean, polish, polish and re-clean in sequence; mark point preparation steps: determine the distribution and spacing of mark points according to test requirements, and prepare mark points in the pretreated sample test area; sample deformation process acquisition and recording steps: Collect and record the relative positions of the marked points during the deformation process of the sample; strain calculation step: calculate the strain of the test area according to the change of the relative positions of the marked points. The invention can make the strain result closer to the real situation and avoid the problems existing in the indirect calculation method.

Description

A method and system for measuring large shear strain of metal sheet at microscopic scale

technical field

The invention relates to the technical field of experimental solid mechanics, in particular to a method and a system for testing large shear strain of a metal sheet at a microscopic scale.

Background technique

Compared with uniaxial tensile test, sheet metal shear test has attracted attention because of its advantages of no necking and large strain. Combined with Digital Image Correlation (DIC), an advanced strain test method, shear It can be used to characterize the hardening, yielding and fracture behavior of metal sheets, which greatly deepens people's understanding of the deformation behavior of sheet materials. However, some recent studies have found that the DIC strain calculation results in the shear test are related to the Virtual Strain Gauge Size (LVGS) parameter settings, especially in the large strain stage, the deformation is concentrated in the micro area, and the micro area strain calculation results More sensitive to LVGS. Generally speaking, the DIC strain calculation results will converge with the decrease of LVGS, and the convergence value is regarded as the value obtained from the test. It cannot be set too small, so that the strain calculation results may not be fully converged in the stage of large shear deformation. According to the research data, it is found that in the shear DIC test conducted by most researchers, the minimum LVGS value used can only reach about 0.1mm, and the convergence of the calculation results is difficult to evaluate.

In recent years, scholars have carried out a series of studies and measurements on the shear large deformation strain of metal sheets at the microscopic scale. Ghahremaninezhad and Scales et al. in 2013 and 2016 respectively (A. Ghahremaninezhad, et al., Int. J. Fracture, 2013; M. Scales et al., Int. J. Solids Struct., 2016.) on aluminum alloy test The direction of the grain orientation in the concentrated area of shear deformation of the sample was measured, and the micro-strain was calculated. It was found that the micro-strain was larger than the strain obtained by the DIC test, that is, the DIC underestimated the strain of the material. The model of shear deformation makes certain assumptions, and whether the assumed model is consistent with the actual model remains to be considered. From 2017 to 2020, Rahmaan and Pathak et al. carried out a series of indirect measurement and calculation work by means of material microstructure orientation measurement and hardness test before and after deformation (Rahmaan, T., et al., Int.J.Impact Eng. , 2017; Pathak, N., et al., Exp.Mech., 2019; Pathak, N., et al., J.Mech.Phys.Solids, 2020), found that in the large shear strain stage, the traditional DIC method The calculated strain is smaller than the real strain, that is, the DIC test results may have been distorted. However, the problem with the work of Rahmaan and Pathak et al. The influence of strain and theoretical assumptions on the calculation results is difficult to evaluate. At the same time, the proposed method for measuring the orientation of the material microstructure cannot eliminate the influence of the rigid rotation of the sample on the test results, which further limits its application.

In summary, there are problems in the current DIC strain calculation of shear test, such as difficulty in further reducing LVGS, and difficulty in evaluating the reliability of indirect calculation results of strain.

SUMMARY OF THE INVENTION

Aiming at the defects in the prior art, the purpose of the present invention is to provide a method and system for measuring large shear strain of a metal sheet at a microscopic scale, so as to solve the above problems.

According to a method and system for measuring large shear strain of a metal sheet at a microscopic scale provided by the present invention, the scheme is as follows:

In a first aspect, a method for testing large shear strain of a metal sheet at a microscopic scale is provided, the method comprising:

Sample pretreatment steps: The area of the surface of the sample to be sheared that needs to be subjected to strain testing is cleaned, polished, polished and re-cleaned in sequence;

Marking point preparation steps: Determine the distribution and spacing of the marking points according to the test requirements, and prepare the marking points in the test area of the sample;

The step of collecting and recording the deformation process of the sample: collecting and recording the relative positions of the marked points during the deformation process of the sample;

Strain calculation step: Calculate the strain of the test area according to the change of the relative position of the marked points.

Preferably, the cleaning and re-cleaning treatments in the sample pretreatment step are performed by alcohol and acetone.

Preferably, the polishing treatment in the sample pretreatment step is sequentially performed with 400-mesh, 800-mesh, and 1200-mesh sandpaper, and the polishing time for each pass is 1-3 minutes.

Preferably, the polishing treatment in the sample pretreatment step is sequentially performed by 9 μm diamond suspension, 3 μm diamond suspension and 0.03 μm silica suspension, and the polishing time for each pass is 3-10 minutes.

Preferably, the marking point preparation method in the marking point preparation step is a microhardness point marking method or a nanoindentation marking method.

Preferably, the acquisition and recording of the sample deformation process in the sample deformation process acquisition and recording step is performed by an in-situ method or a quasi-in-situ method.

Preferably, in the step of collecting and recording the deformation process of the sample, according to the size and distribution characteristics of the marked points, the recording device for collecting and recording the deformation process of the sample is an optical microscope, a laser confocal microscope or a scanning electron microscope.

Preferably, the strain calculation method in the strain calculation step is a nine-point method or a seven-point method.

In a second aspect, there is provided a large-strain testing system for sheet metal shearing at a microscopic scale, the system comprising:

Sample pretreatment module: The area of the sample surface to be sheared that needs to be strain tested is cleaned, polished, polished and re-cleaned in sequence;

Marking point preparation module: Determine the distribution and spacing of the marking points according to the test requirements, and prepare the marking points in the test area of the sample;

Sample deformation process acquisition and recording module: collect and record the relative positions of the marked points during the sample deformation process;

Strain calculation module: Calculate the strain of the test area according to the change of the relative position of the marked points.

Preferably, the strain calculation method in the strain calculation module is a nine-point method or a seven-point method.

Compared with the prior art, the present invention has the following beneficial effects:

1. Compared with the traditional DIC method, the present invention can achieve a smaller LVGS, that is, less than 0.05mm, obtain the strain of the material at the microscopic scale, and the obtained strain result is closer to the real situation;

2. The marking point preparation method of the present invention is simple, stable, reliable, and low in cost;

3. The strain calculation method of the present invention is simple and direct, does not need to make theoretical assumptions on the shear deformation model, and avoids the problems existing in the indirect calculation method.

Description of drawings

Other features, objects and advantages of the present invention will become more apparent by reading the detailed description of non-limiting embodiments with reference to the following drawings:

Fig. 1 is the flow chart of the present invention;

Fig. 2 is the schematic diagram of the marking point prepared in the embodiment of the present invention;

Fig. 3 is the marking point and its deformation physical map prepared by the embodiment of the present invention;

FIG. 4 is a schematic diagram of a nine-point method strain calculation principle according to an embodiment of the present invention;

Fig. 5 is the strain calculation result of the nine-point method according to the embodiment of the present invention;

6 is a schematic diagram of a marking point according to an embodiment of the present invention;

FIG. 7 is a schematic diagram of the principle of seven-point method strain calculation according to an embodiment of the present invention.

Detailed ways

The present invention will be described in detail below with reference to specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that, for those skilled in the art, several changes and improvements can be made without departing from the inventive concept. These all belong to the protection scope of the present invention.

The embodiment of the present invention provides a method for testing large shear strain of a metal thin plate on a microscopic scale, and describes a quasi-in-situ testing method for large shearing strain on a 1.6 mm thick DP980 plate on a microscopic scale.

Example 1:

Referring to Fig. 1, it specifically includes: sample pretreatment steps: cleaning, polishing, polishing and re-cleaning are performed in sequence on the area of the DP980 sheared sample surface that needs to be subjected to strain testing, wherein cleaning and re-cleaning are performed with alcohol, The polishing treatment was carried out with 400-mesh, 800-mesh and 1200-mesh sandpaper in sequence, and the polishing time was 2min per pass; the polishing treatment was successively carried out through 9 μm diamond suspension, 3 μm diamond suspension and 0.03 μm silica suspension, in which 9 μm diamond suspension was used. The diamond suspension was polished for 3 min, then polished with 3 μm diamond suspension for 3 min, and finally polished with 0.03 μm silica suspension for 5 min. After polishing, there should be no visible scratches on the surface of the sample.

Referring to Figures 2 and 3, marking point preparation steps: determine the distribution and spacing of marking points according to test requirements, prepare marking points in the test area of the sample, and the marking points should be clearly visible during the deformation process of the sample. The marking point preparation method may adopt the microhardness point marking method or the nanoindentation marking method. In this example, the micro-hardness point marking method is used, and a Vickers hardness tester is used to prepare marking points on the surface of the sample. The loading force is 98.07 mN, and the holding time is 10 s. The size of the prepared marking points is about 0.01 mm, and the LVSG value is about 0.04. mm.

The sample deformation process acquisition and recording steps: In this example, the quasi-in situ method is used to collect and record the relative position changes of the marked points during the deformation process of the sample, that is, the sample is loaded on the loading device to a specific displacement. According to the size and distribution characteristics of the marked points, The recording device for collecting and recording the deformation process of the sample is an optical microscope, a laser confocal microscope or a scanning electron microscope. In this example, after unloading, take out the sample and place it under an optical microscope for observation, adjust the appropriate magnification, record the topographic picture of the marked point under the loading step, repeat the above process, and record a series of marked points under the loading step. Topographical picture.

Strain calculation step: Calculate the strain of the test area according to the change of the relative position of the marked point, among which, the method of strain calculation can be the nine-point method.

Referring to Figure 4, the nine-point method is used to calculate the strain. Figure 4 shows the relative positional relationship between the marked point P _{(i, j)} and its adjacent marked points after loading in the initial state and the kth step. Measure each point on the picture. coordinates, the following two matrices can be obtained:

表示根据A、B两点坐标差得到的列向量，根据上述两个矩阵，可以得到标记点P_(i，j)在第k步加载后的变形梯度：in,

Represents the column vector obtained according to the coordinate difference of the two points A and B. According to the above two matrices, the deformation gradient of the marked point P _{(i, j)} after loading in the kth step can be obtained:

Among them, (A) ⁺ represents the Moore-Penrose inverse of matrix A. According to the deformation gradient, the corresponding strain tensor of the marked point can be further calculated, and the deformation gradient and strain tensor corresponding to other marked points can be obtained by repeating the above process.

Referring to FIG. 5 , the Mises strain distribution of the test area of the sample under a certain loading step calculated in this embodiment is obtained.

Example 2:

Sample pretreatment steps: Clean, polish, polish and re-clean the area of the surface of the DP980 sheared sample that needs to be subjected to strain testing in sequence. The cleaning and re-cleaning are carried out with alcohol, and the polishing treatment is performed through 400 mesh, 800 mesh in turn. mesh and 1200-grit sandpaper, and the polishing time is 2min per pass; the polishing treatment is carried out sequentially through 9μm diamond suspension, 3μm diamond suspension and 0.03μm silica suspension, of which the 9μm diamond suspension is used for polishing for 3min, and then 3μm is used. The diamond suspension was polished for 3 min, and finally a 0.03 μm silica suspension was used for 5 min.

Marking point preparation steps: Determine the distribution and spacing of marking points according to the test requirements, and prepare marking points in the test area of the sample. In this example, a nano-indenter is used to prepare marking points on the surface of the sample. The loading force is less than 9.807mN, and the loading speed is less than 20nm/s, the obtained marking points are shown in Figure 6. The purpose of using the nanoindenter is to prepare marking points with smaller size and spacing, which can further reduce the LVSG value.

Collection and record of sample deformation process: In this example, the in-situ method is used to collect and record the relative position changes of the marked points during the deformation process of the sample, that is, load the sample on the scanning electron microscope in-situ loading device, and adjust the scanning electron microscope to a suitable magnification. The main purpose of using the in situ method is to have a more comprehensive understanding of the deformation history of the sample.

Strain calculation: In this embodiment, the seven-point method is used to calculate the strain. Referring to Figure 7, it is the relative positional relationship between the marked point P _{(i, j)} and its adjacent marked points after the initial state and the k-th step after loading. Taking the coordinates of each point, the following two matrices can be obtained:

表示根据A、B两点坐标差得到的列向量，根据上述两个矩阵，可以得到标记点P_(i，j)在第k步加载后的变形梯度：in

Represents the column vector obtained according to the coordinate difference of the two points A and B. According to the above two matrices, the deformation gradient of the marked point P _{(i, j)} after loading in the kth step can be obtained:

Among them, (A) ⁺ represents the Moore-Penrose inverse of matrix A. According to the deformation gradient, the corresponding strain tensor of the marked point can be further calculated, and the deformation gradient and strain tensor corresponding to other marked points can be obtained by repeating the above process. The purpose of using the seven-point method is to better adapt to the complex shape boundary of the specimen.

The invention provides a method for measuring large shear strain of metal sheet at microscopic scale. Compared with the traditional DIC method, the present invention can realize a smaller LVGS, that is, less than 0.05mm, and obtain the strain of the material at the microscopic scale. The obtained strain result is closer to the real situation; and the marking point preparation method of the present invention is simple, stable, reliable, and low in cost; the strain calculation method is simple and direct, and does not need to make theoretical assumptions on the shear deformation model, and avoids the problems existing in the indirect calculation method .

Those skilled in the art know that, in addition to implementing the system provided by the present invention and its various devices, modules and units in the form of purely computer-readable program codes, the system provided by the present invention and its various devices can be implemented by logically programming the method steps. , modules, and units realize the same function in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers. Therefore, the system provided by the present invention and its various devices, modules and units can be regarded as a kind of hardware components, and the devices, modules and units included in it for realizing various functions can also be regarded as hardware components. The device, module and unit for realizing various functions can also be regarded as both a software module for realizing the method and a structure within a hardware component.

Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above-mentioned specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essential content of the present invention. The embodiments of the present application and features in the embodiments may be combined with each other arbitrarily, provided that there is no conflict.

Claims (10)

1. A method for testing large shearing strain of a metal sheet under a microscopic scale is characterized by comprising the following steps:

sample pretreatment: sequentially cleaning, polishing and cleaning again an area needing strain testing on the surface of a sample to be sheared;

marking point preparation: determining the distribution and the spacing of the marking points according to the test requirements, and preparing the marking points in the test area of the pretreated sample;

collecting and recording the sample deformation process: collecting and recording the relative position of a marking point in the sample deformation process;

and strain calculation: and calculating the strain of the test area according to the change of the relative positions of the marking points.

2. The method for testing shear large strain of a metal sheet under microscopic scale according to claim 1, wherein the cleaning and re-cleaning processes in the sample pretreatment step are performed by alcohol or acetone.

3. The method for testing the large shearing strain of the metal sheet under the microscopic scale according to claim 1, wherein the polishing treatment in the sample pretreatment step is sequentially performed by 400-mesh, 800-mesh and 1200-mesh sandpaper, and the polishing time of each pass is 1-3 min.

4. The method for testing the shearing large strain of the metal sheet under the microscopic scale according to claim 1, wherein the polishing treatment in the sample pretreatment step is sequentially carried out by using a 9 μm diamond suspension, a 3 μm diamond suspension and a 0.03 μm silicon dioxide suspension, and the polishing time of each pass is 3-10 min.

5. The method for testing the shear large strain of the metal sheet under the microscale of claim 1, wherein the method for preparing the mark points in the step for preparing the mark points is a microhardness point marking method or a nanoindentation marking method.

6. The method for testing the shearing large strain of the metal sheet under the microscopic scale according to claim 1, wherein the step of acquiring and recording the deformation process of the sample is carried out by an in-situ method or a quasi-in-situ method.

7. The method for testing the large shearing strain of the metal sheet under the microscale according to claim 1, wherein in the step of collecting and recording the deformation process of the sample, the recording device for collecting and recording the deformation process of the sample is an optical microscope, a laser confocal microscope or a scanning electron microscope according to the size and distribution characteristics of the marking points.

8. The method for testing large shear strain of a metal sheet under the microscopic scale according to claim 1, wherein the method for calculating the strain in the strain calculating step is nine-point method or seven-point method.

9. A large shear strain test system for a metal sheet under a microscopic scale is characterized by comprising:

a sample pretreatment module: sequentially cleaning, polishing and cleaning again an area needing strain testing on the surface of a sample to be sheared;

a marking point preparation module: determining the distribution and the spacing of the marking points according to the test requirements, and preparing the marking points in the test area of the pretreated sample;

the sample deformation process acquisition and recording module: collecting and recording the relative position of a marking point in the sample deformation process;

a strain calculation module: and calculating the strain of the test area according to the change of the relative positions of the marking points.

10. The micro-scale sheet metal shear large strain test system according to claim 6, wherein the strain calculation method in the strain calculation module is nine-point method or seven-point method.

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