KR20120040981A - A mesuring method for stress-strain curve and a apparatus for the same - Google Patents

Abstract

PURPOSE: A method for acquiring stress-strain curve is provided to stress of each grid and to obtain accurate stress-strain curve. CONSTITUTION: A method for acquiring stress-strain curve comprises: a step of applying tension to a sample; a step of measuring stress in real time; a step of measuring each grid shape using image processing; a step of calculating strain curve of each grid; a step of measuring cross-sectional area corresponding to the grid in real time and calculating stress applied to the grid; and a step of acquiring stress-strain curve.

Description

A method of obtaining a stress-strain curve and a device therefor {A MESURING METHOD FOR STRESS-STRAIN CURVE AND A APPARATUS FOR THE SAME}

The present invention relates to a method and an apparatus for obtaining a stress-strain curve, and more particularly, to provide a plurality of grids on a specimen, and to measure the strain and stress of each grid in real time, and then superimpose them as a whole. A method of obtaining a stress-strain curve.

As the plastic processing process analysis technology is generalized, true strain-true stress relations or curves of metal materials are required for process design technicians, but they are not easy to obtain experimentally, so many field technicians and researchers do not know accurate material information. Most technicians use limited information from the literature.

Few methods are as useful and convenient as tensile testing in materials testing. Acquiring the true stress-strain curve is one of the main purposes of the tensile test. Tensile tests are relatively easy to perform and provide the technician with a great deal of information about the material properties.

However, there is a limit in obtaining the correlation between strain and stress, which is essential in plastic flow analysis because of the necking phenomenon in tensile test. Until necking, true stress-strain curves or functions can be obtained from tensile test results under the assumption of incompressibility, but no further information can be obtained from this condition if necking occurs.

In the case of steel, necking usually occurs at a strain of 0.15 or less, but in plastic processing such as forging, the strain often exceeds 1.0. Therefore, the true stress-strain curve before the necking cannot be a major concern in terms of plastic processing involving large strains, and the stress-strain curve after the necking is extrapolated by extrapolating the stress-strain curve before the necking. Use may be very inappropriate in some situations.

If strain and stress information at the minimum cross section of the necking area is available, a tensile test can provide reliable flow stress for high strain. Already, a method of inferring true strain-true stress curves by measuring the deformation shape of the necking area has been developed.

In addition, in a tensile test of a ductile material, a necking phenomenon often occurs before the fracture, which causes a problem that the specimen within the actual gage distance cannot be uniformly deformed. In this case, if the nominal stress-nominal strain curve is converted to the true stress-true strain curve without special consideration, there is a problem that the maximum value of the flow stress remains as it is before and during fracture. This problem is not due to work softening of the material during deformation, but because the true stress-strain curve is actually a 'average' true stress-strain curve within the gauge distance.

As a representative technique for correcting the necking problem of tensile test data, the Bridgman equation is applied. Bridgman's equations can be used to correct tensile test data that is distorted due to necking, resulting in near-real true-strain curves. However, these methods of converting data based on test results and necking shapes can vary depending on the shape of the specimen and require verification of their applicability depending on the material properties.

The present invention provides a tensile test method and apparatus for effectively and more accurately obtaining a true stress-strain curve only by a tensile test, without performing a separate process for correcting the problem of necking, unlike the conventional method.

In particular, in order to solve the problem of necking, a method and apparatus for providing an accurate stress-strain curve by providing a plurality of grids on a specimen, measuring the strain and stress of each grid in real time, and finally superimposing them Doing.

The present invention provides the following problem solving means to achieve the above object.

The method for obtaining a stress-strain curve using a specimen marked with N grids on the surface of the present invention includes applying a tensile force to the specimen, measuring a tensile force applied to the specimen in real time, and Measuring the shape of each grid displayed on the surface of in real time using image processing, calculating the strain of each grid therefrom, and measuring the cross-sectional area corresponding to each grid in real time from each Computing the stress applied to the grid of, and superposition the relationship of the stress and strain corresponding to the N grids to obtain a stress-strain curve.

In another embodiment of the present invention, a method of obtaining a stress-strain curve using a specimen marked with N grids on a surface includes: a first step of applying a tensile force to the specimen by a predetermined elongation; and in the first step. A second step of measuring the tensile force, a third step of measuring the shape of each grid displayed on the surface of the specimen deformed by the predetermined elongation using image processing, and calculating the strain of each grid therefrom; And measuring a cross-sectional area corresponding to each of the deformed grids, calculating a stress applied to each of the deformed grids, and superposing a relationship between stresses and strains corresponding to the N grids. And a fifth step of obtaining a strain curve, and repeating the first to fifth steps.

In still another embodiment of the present invention, a method of obtaining a stress-strain curve using a specimen marked with N grids on a surface may include specifying n grids to be measured among the N grids, Applying a tensile force, measuring the tensile force applied to the specimen in real time, measuring the shape of each specified grid in real time using image processing, and calculating the strain of each specified grid therefrom. And measuring, in real time, the cross-sectional area corresponding to each of the specified grids, calculating a stress applied to each of the specified grids, and calculating the stresses and strains corresponding to the specified n grids. Superposition the relationship to obtain a stress-strain curve.

In another embodiment of the present invention, a method for obtaining a stress-strain curve using a specimen in which N grids are displayed on a surface comprises: a first step of specifying n grids to be measured among the N grids; The second step of applying a tensile force to the specimen by a predetermined elongation, the third step of measuring the tensile force applied in the first step, and the shape of each specified grid deformed by the predetermined elongation using image processing. A fourth step of measuring and calculating a strain of each specified grid therefrom, and a fifth step of measuring a cross-sectional area corresponding to each specified grid and calculating a stress applied to each specified grid therefrom And a sixth step of obtaining a stress-strain curve by superpositioning a relationship between stresses and strains corresponding to the specified n grids. And repeating the second to sixth steps.

Among the above methods, the method for measuring the cross-sectional area of the grid is characterized by using a laser technique or a moire technique.

The present invention provides an apparatus for obtaining a stress-strain curve using a specimen marked with N grids on a surface, the apparatus comprising: a tension member for providing a tensile force to the specimen and a tension force provided to the tension member. A tensile force measurement unit for measurement, a CCD camera for measuring the shape of each grid displayed on the surface of the specimen, an image processor unit for calculating the strain of each grid by image processing the shape of the measured grid, and A cross-sectional area measuring unit for measuring the cross-sectional area of each grid, a stress calculation unit for calculating the stress applied to each grid through the measured tensile force and the cross-sectional area, and a data processing unit for storing or measuring the calculated stress and strain Characterized in that it comprises a.

Unlike the conventional method, the present invention has an effect of effectively and more accurately obtaining a true stress-strain curve by only a tensile test without performing a separate process for correcting a problem of necking.

In particular, in order to solve the problem of necking, by providing a plurality of grids on the specimen, measuring the strain and stress of each grid in real time, and finally overlapping it has the effect of obtaining an accurate stress-strain curve.

1 is a true stress-strain curve measured while varying the gauge distance from 50 mm to 6 mm.
2 shows stress-strain data measured at five grids.
3 is a photograph of a specimen that was subjected to a tensile test to obtain the data of FIG.
4 is a stress-strain curve obtained by integrating data from all grids.
5 is a stress-strain curve obtained by integrating data from all grids.
6 is a block diagram of a stress-strain measuring device according to the present invention.
7 is a flowchart of an embodiment of a stress-strain measurement method according to the present invention.
8 is a flowchart of an embodiment of a stress-strain measurement method according to the present invention.
9 is a flowchart of an embodiment of a stress-strain measurement method according to the present invention.
10 is a flowchart of an embodiment of a stress-strain measurement method according to the present invention.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear. Even if the terms are the same, if the displayed portions are different, it is to be noted that the reference numerals do not match.

The terms to be described below are terms set in consideration of functions in the present invention, and may be changed according to a user's intention or custom such as an experimenter and a measurer, and the definitions should be made based on the contents throughout the present specification.

First, the principle to which the present invention was devised will be described.

In general, when the gauge length is changed during the tensile test, the stress-strain curve changes according to the specific gravity of the necking area. FIG. 1 shows the true stress-strain curve measured with varying gage distances from 50 mm to 6 mm (with decreasing point).

Due to the non-uniform deformation behavior of the necking site within the gauge distance, there is often a problem of instability in the nominal stress-nominal strain as well as the true stress-true strain curve obtained from the tensile test. In this case, as shown in FIG. 1, as the gage distance decreases so that the necking portion occupies a large portion in the measurement region, a true stress-strain curve reflecting the characteristics of the necking region is obtained and observed from the previous stress-strain curve. Instability is reduced considerably.

In general, when necking of a local specimen occurs, the stress-strain curve computed as the mean concept for the gage distance actually yields invalid stress-strain data not only at the necking but also at the non-necking area. It is necessary. If a sufficiently small gage distance is used instead of the conventional gage distance, it can be expected to obtain more reliable data on specimen behavior at least within the gage distance. In other words, if the gage distance becomes small enough around the necking area, stress-strain data can be obtained that can fully express the necking area characteristics. It is possible to obtain stress-strain data that can be expressed fully.

If the material properties, ie true stress-strain curves, exist with the same data, regardless of whether necking has occurred, the true stress-strain curve obtained by applying a sufficiently small gauge distance near the necking area as above It will appear the same as the true stress-strain curve obtained by applying a sufficiently small gage distance to the vicinity. Since the material property problem is a problem outside the range that can be dealt with in the present invention, only the true stress-strain curve acquisition method that can be prepared in case this assumption is satisfied will be described.

As mentioned above, by reducing the gage distance for each part of the specimen, it is possible to obtain true stress-strain curves that sufficiently reflect the material properties of the necking area as well as the necking area. The present invention is to use such a gage distance effect, characterized in that to form and minimize the gongjeom distance so as to reflect the characteristics of the area other than the necking as well as the necking during the tensile test.

However, the gage distance cannot be reduced below the grid shown in FIG. 1. Therefore, if the stress-strain data can be obtained for one grid instead of reducing the gauge distance of the specimen, a flow stress curve can be obtained that eliminates the gauge distance effect.

The core of the present invention is to measure the relationship between stress-strain on each of these grid bases, and superimpose them to obtain the final stress-strain curve. Since the material properties at all points of the specimen are the same, the stress-strain relationships obtained at each grid must all be the same. Therefore, the gauge distance can be reduced, that is, measured and calculated in grid units to obtain a more accurate stress-strain relationship than the conventional method, and if all of them overlap, the local value within the gauge distance may be included, although an error value may be included. It is possible to obtain the same value as the actual physical property value to avoid data distortion due to necking.

In the present specification, the thickness of the grid or the cross-sectional area of the grid means the thickness of the specimen or the cross-sectional area of the specimen at a position corresponding to the grid.

The ideal test method for obtaining these grid-based tensile test data is to simultaneously measure the grid change and load of the specimen in real-time during the tensile test. This requires measuring the thickness at the grid position of the specimen. As an alternative to this real-time measurement method, an interrupt method may be used to test a plurality of tensile specimens up to a predetermined elongation.

The interrupt method is to perform a tensile test for a certain length such as 10%, 20%, 30%, ... elongation of the specimen, and measure the strain of the grid on the resulting specimen.

For example, after applying a tensile force by an elongation of 10%, the stress and strain in the situation are calculated. The tensile force is again applied by an elongation of 20%, and then the stress and strain in the situation are calculated. This is done repeatedly until the desired elongation. It is natural that the elongation can be freely set according to the type of specimen to be measured and the measuring method.

Whether in real time or in an interrupt manner, the strain of each grid is calculated from the initial grid size (Li) and the grid size (Lf) of the strained specimen as follows. Since the present invention uses a grid method, after collecting an image of a specimen on which a grid is displayed by a CCD or the like, the strain may be obtained through image processing.

The stress of each grid can be calculated from the load F at each elongation point and the cross-sectional area (Af = width * thickness) of the specimen cross section containing the grid as follows.

The above equation of the stress of the grid is used in the general tensile test, and it is necessary to consider the load distribution in the cross-sectional area in order to accurately determine the stress acting on the grid. In other words, it is important to accurately measure the cross-sectional area. The cross-sectional area can be obtained more accurately by measuring the three-dimensional shape of the specimen. A detailed description thereof will be described later.

FIG. 2 shows the stress-strain data measured at five grids, and FIG. 3 is a photograph of a specimen that has undergone a tensile test to obtain it. Tensile tests were performed on AZ31 specimens under two conditions: 150 ° C and 250 ° C. Considering the number of tests, the elongation at each temperature was set to 20%, 40%, 60%, .. However, the elongation at break was included. At 250 ° C. tested with the grid coated, it could be tested to 20%, 40%, 60%, and fracture. Five grids were selected to measure the stress-strain, but instead of all the grids, a grid near the center was chosen for convenience. However, number 0 selected the center grid. The stress-strain characteristics away from the necking site are expected to be fully reflected in grid 4.

2 shows the stress-strain relationship obtained in the process of 20%, 40%, 60%, and fracture for each grid. Since the data from the fracture specimens were included, the last data value is seen to decrease rapidly. By integrating the data of all the grids except the fracture specimen data, the graphs of FIGS. 4 and 5 can be obtained finally. 4 shows overlap values of data at 150 ° C. and FIG. 5 shows overlap values of data at 250 ° C. FIG. It shows that the ideal stress-strain curve can be obtained without error of necking. Because there are data variances due to measurement errors and simplification assumptions, the fitting curves are compared and shown.

6 shows a schematic diagram of a device for obtaining a stress-strain curve according to the present invention. The stress-strain curve acquiring device according to the present invention includes a tensile member 10 for providing a tensile force to the specimen, and a tensile force measuring unit 11 for measuring the tensile force provided on the tensile member is the same as a conventional tensile tester. Do. The tensile force measuring unit 11 may be a measuring sensor or a load cell. Depending on the type of specimen, a member for fixing the specimen is included.

The key of the present invention is to first provide a grid on the surface of the specimen. In particular, the lines marking the boundaries of the grid and the area inside the grid should be clearly displayed because they must be acquired separately through image processing.

In particular, for image processing of lines or areas, a method of coating a specific material or classifying a specific color to increase the accuracy of image processing may be considered.

The stress-strain measuring device according to the present invention includes a CCD camera 21 for measuring the shape of each grid displayed on the surface of the specimen, and an image processor for calculating the strain of each grid by image processing the measured shape of the grid. And a cross-sectional area measuring unit 23 for measuring the cross-sectional area of each grid.

The CCD camera 21 is a device for obtaining an image of the grid displayed on the surface of the specimen. Therefore, it is desirable to be provided on the upper part of the specimen, and to accurately measure the strain, the image must be acquired exactly vertically.

The image of the grid thus measured is processed by the image processor 22. That is, the image processor unit 22 divides the boundaries of the grid and the area inside and counts them to give the number of grids and a unique number to each grid. Information about this is preferably stored in a separate storage unit (30). For example, from the grid on the left side of the uppermost side to give a unique number according to a predetermined pattern to store in the storage unit 30.

In the image processing method, an image to be measured (grid in the present invention) provided on the surface of an object is projected onto a sensor device including individual sensing elements referred to as "pixels" in a standard system layout. do. These are arranged in the form of a matrix matrix along the X or Y axis direction in the image plane. Pixels are signaled in series and send individual gray scale values assigned to each pixel coordinate stored digitally and processed with the corresponding pixel coordinates.

Since the size of each pixel is predetermined according to the size of the entire image, the size of the corresponding area can be measured by obtaining the number of pixels. The image processor compares each pixel coordinate to distinguish between pixels representing the grid boundary and pixels representing the interior of the grid. The area of the grid may be calculated by adding the number of pixels representing the inside of the connected grid and the appropriate number of pixels representing the boundary of the grid.

However, the purpose of image processing for the grid shape is mainly to obtain the strain for each grid. For convenience in the present invention, the tension direction is defined as the height direction, the direction perpendicular to the tension direction is defined as the width direction. The grid may be composed of two height direction sides and two width direction sides.

The strain of each grid may measure the height magnitude passing through the center of the grid, or may take an average value by dividing the height per pixel in the width direction and dividing by the number of pixels in the width direction. This is about an approximation method and does not limit the scope of the present invention.

The cross-sectional area measuring unit 23 is for measuring the cross-sectional area of each grid. Specimens are typically non-uniform in thickness. In other words, the center side is generally thinner than the outside. Therefore, measuring the cross-sectional area more accurately is very important for accurate stress calculation.

In the present invention, it is possible to calculate the cross-sectional area of each grid by measuring the cross-sectional area in the thickness direction of the specimen and matching it with the grid boundary of the surface of the specimen obtained by image processing.

In general, the moiré technique using a laser technique and a vision system is used to measure the outer cross section of a non-contact object in industrial use.

In the case of using the laser technique, a plurality of cameras are synchronized to acquire a shape of the surface contour of the specimen by scanning laser light on the specimen to be measured and measuring an image of the surface contour of the specimen formed by the laser light. That is, a plurality of laser light sources for emitting radial or linear light is disposed around the specimen, and a plurality of optical sensors for sensing the laser light that is reflected from the laser light source after being emitted from the laser light source are reflected. In some cases, a control means for controlling the position of the laser light source and the optical sensor may be added. The method may further include image signal processing means for obtaining information on an outer cross section of the specimen according to the obtained light sensing information, and synthesizing the extracted cross section to extract the outer cross section. Since this is a general technique of measuring a cross section using a laser light source, a description of a specific structure will be omitted.

The moiré technique has the advantage of obtaining the overall shape of the specimen. The moiré technique for determining the contour of an object is largely known as a shadow moiré technique and a projection moiré technique. The shadow moiré technique places a reference grating in front of the object and then casts a light onto the grid in front of the object to create a shadow of the grid pattern on the object, which is visible when the shadow is viewed through the reference grid from a different angle. This is how moiré patterns are analyzed. In addition, the projection moire technique projects a grid pattern on an object, places another grid in front of an image sensor for image detection, and then analyzes the moire pattern seen through the grid pattern of the other grid. As described above, various devices and algorithms for analyzing 3D images by analyzing moiré patterns have been introduced. Therefore, detailed descriptions thereof will be omitted (M. Kujawinska, "Use of Phasestepping Automatic Fringe Analysis in Moire interferometry," See Applied Optics, 26 (22) 4712-4714, YB Choi, SW Kim, "Phase-shifting Grating Projection Moire Topography," Optical Engineering 37 (3) 1005-1010, SW Kim et al., "Two frequency phase- shifting projection moire topography, "see SPIE Vol. 3520, 36--42)

By the above method, the cross-sectional area of the specimen can be measured, which is divided by each grid and stored in the storage unit 30. The cross-sectional area can be calculated based on the cross section in the width direction passing through the center of each grid, and the average value can be calculated like the strain. Since the mechanical configuration for measuring the cross-sectional area of the specimen is generally used, it is not specifically shown in the drawings.

In this way, the tensile force, strain, and cross-sectional area are measured for each grid. The stress can be calculated from the relationship between the double tensile force and the cross-sectional area. The stress-strain measuring device according to the present invention further includes a stress calculator 31 and a data processor 32.

That is, the stress calculation unit 31 calculates the stress for each grid stored in the storage unit 30 and stores it again. Thus, the storage unit 30 stores all data measured or calculated for each unique number of the grid.

As described above, the data processor 32 overlaps the data between the stress-strain obtained for each grid. This gives the final result.

The key point in the present invention is that all these measurements are made in real time, and these measurements and their calculations are stored in storage according to a defined time.

However, the present invention may be performed in an interrupt manner as described above.

Hereinafter, the stress-strain measurement method according to the present invention through the above devices will be described. 7 to 10 show flowcharts of various embodiments of a stress-strain measurement method according to the present invention.

The present invention is a method of obtaining a stress-strain curve using a specimen marked with N grids on the surface, applying a tensile force to the specimen, measuring the tensile force applied to the specimen in real time, and the surface of the specimen The shape of each displayed grid is measured in real time using image processing, from which the strain of each grid is calculated, and the cross-sectional area corresponding to each grid is measured in real time and applied to each grid. Computing the losing stress, and obtaining a stress-strain curve by superposition the relationship between the stress corresponding to the N grid and the strain. It is obvious that each step may involve storing the measured and calculated values in the storage.

In addition, if the present invention is applied in an interrupt manner, the first step of applying a tensile force to the specimen by a predetermined elongation, the second step of measuring the tensile force applied in the first step, and the surface of the specimen deformed by a predetermined elongation The third step of measuring the shape of each displayed grid using image processing, calculating the strain of each grid therefrom, and measuring the cross-sectional area corresponding to each strained grid, and applying it to each grid And a fourth step of calculating the stress and a fifth step of superpositioning the relationship between the stresses and strains corresponding to the N grids to obtain a stress-strain curve. It may be characterized by performing repeatedly until the final elongation.

However, the present invention does not need to overlap values of all provided grids, and excludes grids in which an error occurs. That is, the present invention can be performed by selectively selecting the grids to measure and overlap.

That is, in a method for obtaining a stress-strain curve using a specimen in which N grids are displayed on a surface, specifying a n grid among N grids to be measured, applying a tensile force to the specimen, and Measuring the tensile force applied to the grid in real time, measuring the shape of each grid specified in real time using image processing, calculating a strain of each grid specified therefrom, and each grid specified Measuring the cross-sectional area corresponding to and calculating the stress applied to each specified grid from the real-time, and superpositioning the stress and strain relationship corresponding to the specified n grids. Obtaining a step.

As another embodiment thereof, a first step of specifying n grids to be measured among N grids, a second step of applying a tensile force to the specimen by a predetermined elongation, and a third step of measuring tensile force applied in the first step And a fourth step of measuring the shape of each specified grid deformed by a predetermined elongation using image processing and calculating the strain of each specified grid therefrom, and corresponding to each specified grid. A fifth step of measuring a cross-sectional area, calculating a stress applied to each specified grid, and superpositioning the relationship between stress and strain corresponding to the specified n grids to obtain a stress-strain curve Including a sixth step, the second to sixth steps can be repeated to a desired elongation.

Hereinafter, the stress-strain measurement method and apparatus thereof according to the present invention have been described based on the principle. The present invention is not limited to the scope of the embodiments by the above embodiments, all having the technical spirit of the present invention can be seen to fall within the scope of the present invention, the present invention is the scope of the claims by the claims Note that is determined.

10: tensile member, 11: tensile force measuring unit, 21: CCD camera, 22: image processor unit, 23: cross-sectional area measuring unit, 30: storage unit, 31: stress calculation unit, 32: data processing unit

Claims (6)

A method of obtaining a stress-strain curve using a specimen marked with N grids on its surface,
Applying a tensile force to the specimen,
Measuring the tensile force applied to the specimen in real time;
Measuring the shape of each grid displayed on the surface of the specimen in real time using image processing and calculating the strain of each grid therefrom;
Measuring the cross-sectional area corresponding to each grid in real time and calculating a stress applied to each grid from the grid;
Superposition the stresses and strains corresponding to the N grids to obtain a stress-strain curve,
Method of obtaining a stress-strain curve. A method of obtaining a stress-strain curve using a specimen marked with N grids on its surface,
A first step of applying a tensile force to the specimen by a predetermined elongation;
A second step of measuring the tensile force applied in the first step,
A third step of measuring the shape of each grid displayed on the surface of the specimen deformed by the predetermined elongation using image processing, and calculating the strain of each grid therefrom;
A fourth step of measuring a cross-sectional area corresponding to each of the deformed grids, and calculating stresses applied to the respective grids therefrom;
A fifth step of superposing a relationship between stresses and strains corresponding to the N grids to obtain a stress-strain curve,
Repeating the first to fifth steps,
Method of obtaining a stress-strain curve. A method of obtaining a stress-strain curve using a specimen marked with N grids on its surface,
Specifying n grids to be measured among the N grids;
Applying a tensile force to the specimen,
Measuring the tensile force applied to the specimen in real time;
Measuring the shape of each specified grid in real time using image processing, and calculating the strain of each specified grid therefrom;
Measuring the cross-sectional area corresponding to each of the specified grids in real time, and calculating a stress applied to each of the specified grids;
Superposing a relationship between stress and strain corresponding to the specified n grids to obtain a stress-strain curve,
Method of obtaining a stress-strain curve. A method of obtaining a stress-strain curve using a specimen marked with N grids on its surface,
A first step of specifying n grids to be measured among the N grids;
A second step of applying a tensile force to the specimen by a predetermined elongation;
A third step of measuring the tensile force applied in the first step,
A fourth step of measuring the shape of each specified grid deformed by the predetermined elongation using image processing, and calculating the strain of each specified grid therefrom;
A fifth step of measuring a cross-sectional area corresponding to each of the specified grids, and calculating a stress applied to each of the specified grids;
A sixth step of obtaining a stress-strain curve by superpositioning a relationship between stress and strain corresponding to the specified n grids,
Repeating the second to sixth steps,
Method of obtaining stress-strain curves. The method according to any one of claims 1 to 4,
Method for measuring the cross-sectional area of the grid, using a laser technique or Moire (Moire) technique,
Method of obtaining a stress-strain curve. An apparatus for obtaining a stress-strain curve using a specimen marked with N grids on its surface,
A tension member for providing a tensile force to the specimen;
A tensile force measuring unit for measuring a tensile force provided to the tension member;
A CCD camera measuring the shape of each grid displayed on the surface of the specimen,
An image processor configured to calculate a strain of each grid by image processing the measured shape of the grid;
A cross-sectional area measuring unit for measuring a cross-sectional area of each grid;
A stress calculation unit calculating a stress applied to each grid through the measured tensile force and the cross-sectional area;
Including a data processor for storing or measuring the calculated stress and strain,
A device for obtaining a stress-strain curve.

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