JP7172784B2 - Measuring method of plastic strain ratio of metal plate - Google Patents

Description

The present invention relates to a method for measuring the plastic strain ratio of a metal plate.

The plastic strain ratio of a metal plate is the ratio _{( εw} / _{ε t} ), also called the r-value or Lankford value. The plastic strain ratio is a characteristic value that indicates the anisotropy of a metal plate, and is used as an index of workability centering on deep drawing. In addition, it is an important characteristic value related to calculation accuracy in FEM deformation analysis simulation, which has become common in recent years. As a plastic strain ratio test method for metal sheets, a plastic strain ratio test method for thin sheet metal materials described in JIS Z 2254 (hereinafter referred to as the JIS method) is known.

According to the JIS method, when a uniform plastic strain is applied to a test piece by a tensile test method, the plastic strain ratio is calculated from the width and thickness of the test piece before and after tensile deformation. However, in practice, it is easier to measure the length direction of the test piece than to _measure it in the thickness direction. When the plastic strain ratio is calculated by measuring the width of the test piece before and after deformation and the length between gauge points using a calculation formula derived from the law of constant volume, which assumes that the volume is constant before and after plastic deformation. There are many. That is, the width direction true strain _εw and the longitudinal direction true strain _εL of the test piece are determined, the thickness direction true strain _εt is calculated from _εw and _εL , and the plastic strain ratio is determined.

In addition, in the JIS method, in principle, a uniaxial tensile test is performed in a strain range where ε _L is 10% to 20% to obtain the true strain. This is because the larger the deformation, the higher the measurement accuracy.

JIS Z 2254 (2008) Plastic Strain Ratio Test Method for Sheet Metal Materials, JIS Handbook Iron and Steel I, edited by Japanese Standards Association, published on January 23, 2012

In the JIS method, the method of determining the plastic strain ratio from the true strain _εw in the width direction and the true strain _εL in the longitudinal direction of the test piece is premised on accurately calculating the true strain _εL in the longitudinal direction, and the uniform elongation range measurement within the Therefore, for a material that causes necking immediately after the start of the tensile test, the measurement must be performed in a low strain region before the start of necking. Therefore, the measurement error becomes large, and the calculation result of the plastic strain ratio becomes unreliable.

Also, a material with a true strain ε _t in the thickness direction close to 0 (zero) has a large plastic strain ratio, but strain measurement errors greatly affect the calculation results of the plastic strain ratio. Therefore, for example, in the case of pure titanium, which has a small uniform elongation and a large plastic strain ratio, when the rolling width direction is set as the tensile direction of the uniaxial tensile test, the accuracy of measuring the plastic strain ratio is significantly lowered.

Furthermore, with the recent progress in metal sheet forming technology, there has been a demand to know the plastic strain ratio in an arbitrary strain region. For example, materials undergoing deformation-induced transformation may have different plastic strain ratios in different strain regions. However, the plastic strain ratio obtained by the conventional JIS method is an average value in the range of the strain range in which the test piece is deformed, and the plastic strain ratio cannot be obtained in a narrower strain range. If the JIS method were to obtain a plastic strain ratio in an arbitrary strain range, it would be necessary to prepare a large number of test pieces and perform tensile tests, and the amount of work involved would be enormous.

The present invention has been made in view of the above circumstances, and even if it is a material that causes necking immediately after the start of the tensile test, or a material that causes pure titanium or deformation-induced transformation, the plastic strain ratio can be accurately measured. It is an object of the present invention to provide a method for measuring the plastic strain ratio of a metal plate, which is possible and which can easily obtain the plastic strain ratio in a narrower strain range.

In order to solve the above problems, the present invention employs the following configuration.
[1] A plate-shaped test piece formed from a metal plate to be measured, wherein a grid line pattern including a plurality of square patterns is provided on the surface of the parallel part, and one side of each square pattern is the parallel A first step of performing a uniaxial tensile test that applies a uniaxial tensile stress in the longitudinal direction of the parallel portion using a plate-shaped test piece provided so as to be parallel to the longitudinal direction of the portion;
From the grid line pattern provided in the parallel portion after the uniaxial tensile test, a figure pattern corresponding to the square pattern before the uniaxial tensile test, and the parallel portion was equally divided into three in the width direction. a second step of selecting two or more of the figure patterns whose area is included in the central region by 50% or more as evaluation targets;
a third step of obtaining the longitudinal true strain ε _Li and the width direction true strain ε _Wi in the figure pattern to be evaluated selected in the second step for each figure pattern to be evaluated;
Corresponding to the longitudinal true strain _εLi and the widthwise true strain _εWi of the figure pattern to be evaluated on an orthogonal coordinate axis plane having the y-axis as the longitudinal true strain _εL and the x-axis as the widthwise true strain _εw A fourth step of placing a coordinate point to
An approximate straight line is derived by the method of least squares for the coordinate points included in the predetermined ε _L section of the orthogonal coordinate axis plane, and the slope a of the approximate straight line is substituted into the following formula (1) to obtain the plastic strain ratio r. A fifth step of calculating;
A method for measuring the plastic strain ratio of a metal plate, which is characterized in that the
r=−1/(1+a) … (1)
[2] The method for measuring the plastic strain ratio of a metal plate according to [1], wherein the grid line pattern is provided on the parallel portion by a printing method or an electrolytic etching method.
[3] [1] or [2], wherein the square pattern has a side length S of 2 mm or more and (W/5) mm or less when the total width W of the parallel portion is 12 mm or more. The method for measuring the plastic strain ratio of the metal plate according to .
[4] In the third step,
The reference points are the vertices where the four sides formed by the grid lines that partition the figure pattern to be evaluated are in contact with each other. When the total is L _i total (mm) and the total of the components parallel to the width direction of the parallel portion is W _i total (mm),
According to any one of [1] to [3], wherein the true longitudinal strain ε _Li and the true transverse strain ε _Wi in the figure pattern are determined by the following equations (2) and (3). A method for measuring the plastic strain ratio of the metal plate described.
ε _{Li =} ln (Li total/2S) (2)
ε _{Wi =} ln (Wi total/2S) (3)
However, S in Formula (2) and Formula (3) is one side length (mm) of the said square pattern.
[5] In the case where the grid lines forming the sides of the graphic pattern are line patterns having widths, the reference point is an intersection point of diagonal lines at an intersection where the line patterns intersect [4] ] The method for measuring the plastic strain ratio of the metal plate according to .

According to the present invention, it is possible to accurately measure the plastic strain ratio even for materials that cause necking immediately from the start of the tensile test, pure titanium, or materials that cause deformation-induced transformation, and narrower strain It is possible to provide a method for measuring the plastic strain ratio of a metal plate that can easily obtain the plastic strain ratio in the region.

FIG. 1 is a diagram showing an example of a plate-shaped test piece used in a method for measuring the plastic strain ratio of a metal plate according to an embodiment of the present invention, and is a schematic plan view showing the plate-shaped test piece before the first step. FIG. 2 is a schematic plan view showing a plate-shaped test piece after the first step; FIG. 3 is a schematic diagram illustrating an example of a method of selecting a graphic pattern to be evaluated from a plate-shaped test piece in the second step; FIG. 4 is a schematic diagram illustrating an example of a method of selecting a graphic pattern to be evaluated from a plate-shaped test piece in the second step; FIG. 5 is a schematic diagram for explaining an example of a method of obtaining longitudinal true strain and width true strain in a figure pattern to be evaluated in the third step; FIG. 6 is a schematic diagram for explaining an example of a method of determining a reference point in the third step when the grid lines dividing the figure pattern to be evaluated are line patterns; FIG. 7 is a schematic diagram for explaining an example of a method of obtaining a reference point in the third step when the grid lines dividing the figure pattern to be evaluated are line patterns; FIG. 8 is a diagram showing an example of placing coordinate points corresponding to the longitudinal true strain ε _Li and width true strain ε _Wi of the figure pattern on the orthogonal coordinate axis plane in the fourth step. FIG. 9 is a diagram showing an example of deriving an approximate straight line for coordinate points by the method of least squares in the fifth step. FIG. 10 is a graph showing the results of evaluating a stainless steel plate (SUS304). FIG. 11 is a graph showing the results of evaluating a JIS class 1 pure titanium plate. FIG. 12 is a graph showing the results of evaluating the steel plate (JAC270D).

The present inventors have found that it is possible to accurately measure the plastic strain ratio even for materials that cause necking immediately after the start of the tensile test, pure titanium or materials that cause deformation-induced transformation, and more We investigated a method for measuring the plastic strain ratio of a metal plate that can easily obtain the plastic strain ratio in a narrow strain range. In the conventional measurement method represented by the JIS method, strain distribution occurs in the longitudinal direction in the parallel part of the test piece after necking occurs, so there is a problem that the true longitudinal direction strain _εL after necking cannot be accurately calculated. there were.

Therefore, the present inventors divided the parallel portion of the test piece into microregions and measured the true strain in the longitudinal direction and the true strain in the width direction for each microregion. The longitudinal true strain _εL and the widthwise true strain _εW are obtained accurately for each time, thereby making it possible to calculate the plastic strain ratio r. In addition, by utilizing the distribution of the longitudinal true strain _εL that occurs after necking occurs, it is possible to obtain measurement data for a large number of strain regions by performing a single tensile test on one test piece. I found out. Furthermore, it was found that the instantaneous plastic strain ratio r can be easily calculated from transitions of the longitudinal true strain _εL and the widthwise true strain _εW .

Hereinafter, a method for measuring the plastic strain ratio of a metal plate, which is an embodiment of the present invention, will be described.
In the method for measuring the plastic strain ratio of a metal plate according to the present embodiment, the plastic strain ratio is obtained by sequentially performing the following first to fifth steps.

First step: A plate-shaped test piece formed from a metal plate to be measured, wherein a grid line pattern including a plurality of square patterns is provided on the surface of the parallel part, and one side of each square pattern is the parallel part A uniaxial tensile test is performed by applying a uniaxial tensile stress in the longitudinal direction of the parallel portion using a plate-shaped test piece provided so as to be parallel to the longitudinal direction.

Second step: From the grid line pattern provided in the parallel part after the uniaxial tensile test, a central region that is a figure pattern corresponding to the square pattern before the uniaxial tensile test and is evenly divided into three in the width direction Two or more figure patterns are selected, with figure patterns having 50% or more of the area included therein being evaluated.

Third step: Find the true longitudinal strain ε _Li and the true transverse true strain ε _Wi in the figure pattern to be evaluated selected in the second step for each figure pattern to be evaluated.

Fourth step: On the orthogonal coordinate axis plane with the y-axis as the longitudinal true strain ε _L and the x-axis as the width direction true strain ε _w , the figure pattern to be evaluated is placed on the longitudinal direction true strain ε _Li and the width direction true strain ε. Put a coordinate point corresponding to _Wi .

Fifth step: Deriving an approximate straight line by the method of least squares for the coordinate points included in the predetermined ε _L section of the orthogonal coordinate axis plane, substituting the slope a of the approximate straight line into the following formula (A) and plastic strain ratio r Calculate

r=−1/(1+a) … (A)

Each step will be described below.

(First step)
In the first step, a plate-shaped test piece is created from a metal plate to be measured. The thickness of the metal plate to be measured is preferably 0.1 mm or more and 3 mm or less, more preferably 2 mm or less. If it is a metal plate exceeding 3 mm, the plastic strain ratio can be accurately measured by the method of the present embodiment.

The shape of the plate-shaped test piece shall be the shape of No. 13B test piece specified in Annex B of JIS Z2241 (2011) Metal Material Tensile Test Method. For example, a No. 13B specimen is made by shearing or pressing a metal plate. However, if there is a portion hardened by shearing or pressing, it is preferably removed by machining. The thickness of the plate-shaped test piece is the thickness of the original metal plate to be measured. In addition, the test piece preparation method conforms to Annex B of JIS Z2241 (2011) and JIS Z2254 (2008).

FIG. 1(a) shows a schematic plan view of the plate-shaped test piece. As shown in FIG. 1(a), a plate-shaped test piece 1 according to this embodiment has a parallel portion 2 and a pair of grip portions 3 formed on both sides of the parallel portion 2 in the longitudinal direction. Reference points H, H are provided on both sides of the parallel portion 2 in the longitudinal direction. The original gauge length _L0 is set to 50 mm. Further, the total width W of the parallel portion 2 before the uniaxial tensile test is in the range of 12 to 13 mm.

Further, as shown in FIGS. 1(a) and 1(b), a grid line pattern K is provided on the parallel portion 2 of the plate-shaped test piece 1. As shown in FIG. FIG. 1(b) is a partial enlarged view of the grid line pattern K, which is an enlarged view of a region A surrounded by a dashed line in FIG. 1(a). The grid line pattern K is a pattern formed by grid lines composed of a _plurality of parallel lines _NL along the longitudinal direction of the parallel portion 2 and a plurality of parallel lines NW along the width direction. The interval between the parallel lines _NL along the longitudinal direction and the interval between the parallel lines _NW along the width direction are the same. As a result, the grid line pattern K includes a square pattern SQ that is a plurality of minute regions. In FIG. 1(b), one of the square patterns SQ is shown surrounded by a thick line. Each square pattern SQ is provided so that one side thereof is parallel to the longitudinal direction of the parallel portion 2 . The grid line pattern K is preferably provided at least over the entire area between the reference points H, H of the parallel portion 2, and is preferably provided over the entire width direction of the parallel portion 2.

The length S of one side of the square pattern SQ included in the lattice line pattern K is preferably in the range of 2 mm or more and (W/5) mm or less when the total width W of the parallel portion 2 is 12 mm or more. By setting the side length S of the square pattern SQ to 2 mm or more, it is possible to accurately measure the true longitudinal strain ε _Li and the true transverse true strain ε _Wi of the deformed figure pattern in the third step described later. Further, by setting the side length S of the square pattern SQ to (W/5) mm or less, at least five or more square patterns SQ can be arranged along the width direction of the parallel portion 2. In the process, it becomes possible to select the required number of graphic patterns to accurately measure the plastic strain ratio.

The grid line pattern K is preferably provided on the surface of the parallel portion 2 by a printing method or an electrolytic etching method. As for the printing method, any means may be used as long as it is a method capable of printing the grid line pattern K. FIG. For example, a method of drawing a grid pattern K by transferring ink from a printing plate to which ink has been applied to the surface of the parallel portions 2, or a method of drawing a grid pattern K by ejecting droplets of ink onto the surfaces of the parallel portions. method (so-called inkjet method) can be exemplified.

When the grid line pattern K is provided by a printing method or an electrolytic etching method, the grid lines forming the grid line pattern K are drawn with ink or minute etching portions, and unevenness in the thickness direction of the parallel portion 2 is prevented. Therefore, the true longitudinal strain ε _L and the true transverse true strain ε _W of the square pattern SQ after the uniaxial tensile test can be obtained accurately. On the other hand, in a method other than the printing method or the electrolytic etching method, for example, a method of forming the grid line pattern K by providing unevenness on the surface of the parallel portion 2, the square pattern SQ is formed by the unevenness on the surface of the parallel portion 2. It is not preferable because the true longitudinal strain _εL and the true transverse true strain _εW of the minute regions partitioned by cannot be obtained accurately.

When the plate-shaped test piece 1 is prepared, a uniaxial tensile test is performed by applying a uniaxial tensile stress in the longitudinal direction of the parallel portion 2 . The uniaxial tensile test is performed according to the plastic strain ratio test method for sheet metal materials of JIS Z2254 (2008). In the test method of this embodiment, the strain amount in the uniaxial tensile test need not be limited to the range of uniform elongation, and the maximum strain amount may be, for example, 50%.

(Second step)
Next, in the second step, a figure pattern to be evaluated is selected from the lattice line pattern K' of the parallel portion 2 after the uniaxial tensile test. FIG. 2(a) shows a schematic plan view of the plate-shaped test piece after the uniaxial tensile test. Further, FIG. 2(b) is a partial enlarged view of the grid line pattern K' after the tensile test, showing an enlarged view of a region B surrounded by a dashed line in FIG. 2(a). In FIG. 2A, illustration of the grid line pattern K' is omitted.

As shown in FIG. 2(b), when the uniaxial tensile test is performed, most of the square pattern SQ included in the grid line pattern K before the tensile test is stretched in the longitudinal direction of the parallel portion 2, and the square shape can be maintained. There are many things that are deformed. Therefore, the square pattern after the uniaxial tensile test is called a figure pattern P in this embodiment. The square patterns SQ before the uniaxial tensile test have the same shape, but the figure patterns P after the tensile test have different shapes.

In the second step, from the grid line pattern K' after the tensile test, the figure pattern P corresponding to the square pattern SQ before the uniaxial tensile test and the parallel part 2 is equally divided into three in the width direction. Two or more figure patterns P including 50% or more of the area are selected as evaluation targets.

FIG. 3(a) is a schematic plan view of the plate-shaped test piece 1 after the uniaxial tensile test, showing a state in which an auxiliary line _NH for equally dividing the parallel portion 2 into three in the width direction is added. show. A central region C is the region between the reference points H and _H sandwiched between the two auxiliary lines NH. Since the graphic pattern P in the central region C is constrained from both sides in the width direction by regions other than the central region C, the plastic strain ratio can be measured with relatively high accuracy. On the other hand, the figure patterns P in the regions on both sides in the width direction of the central region C are constrained by the central region C from one side in the width direction, but are weakly constrained from the opposite side. Therefore, if the figure pattern P in the area other than the central area C is included in the evaluation target, there is a possibility that the accuracy of the plastic strain ratio is lowered.

In addition, the figure pattern P whose area is 50% or more included in the central region C is to be evaluated. This is because the plastic strain ratio can be measured with relatively high accuracy. In the figure pattern P that occupies less than 50% of the area in the central region C, the constraint on the opposite side is weaker than the constraint from the central region C side. The accuracy of the strain ratio may deteriorate. FIG. 4 is an enlarged view in which FIG. 3(b) is further enlarged. In FIG. 4, the figure pattern whose area is 50% or more is included in the central region C is denoted by P1 and surrounded by _a thick line. The figure pattern to be evaluated is hereinafter denoted by P _i .

Also, the number of graphic patterns to be selected is at least two, and the larger the number, the better. This is because, as the number of figure patterns increases, the precision of the approximate straight line derived in the fourth step increases, and the precision of the plastic strain ratio improves.

(Third step)
Next, in the third step, the longitudinal true strain ε _Li and the width true strain ε _Wi in the figure pattern P ₁ to be evaluated are determined for each figure pattern P ₁ to be evaluated. _An example of a method for measuring the true longitudinal strain ε _Li and the true transverse true strain ε _Wi in the figure pattern P1 will be described below with reference to FIG. FIG. 5 shows a graphic pattern P _i which is one of the graphic patterns P after the uniaxial tensile test. A square pattern having a side length S before the tensile test is transformed into a figure pattern P1 as shown in FIG. ₅ by the tensile test.

_First , attention is focused on _four sides formed by grid lines that partition the graphic pattern P ₁ to be evaluated, and these are _{defined as sides J 1 to} J ₄ . ₄ . Among the distances between the reference points T ₁ to T ₄ at both ends of each side J ₁ to J ₄ , the total of the components L _i1 and L _i2 parallel to the longitudinal direction of the parallel portion 2 is L _i total (mm), The sum of the components W _i1 and W _i2 parallel to the width direction of the parallel portion 2 is W _i total (mm). The component L _i1 is the component of the distance between the reference points T ₁ and T ₂ that is parallel to the longitudinal direction of the parallel portion 2 , and the component L _i2 is the component of the distance between the reference points T ₃ and T ₄ that is parallel to the parallel portion 2 . is the component parallel to the longitudinal direction of Further, the component W _i1 is the component of the distance between the reference points T ₂ and T ₃ that is parallel to the width direction of the parallel portion 2, and the component W _i2 is the component of the distance between the reference points T ₄ and T ₁ It is a component parallel to the width direction of the parallel portion 2 .

Then, the true longitudinal strain ε _Li and the true transverse true strain ε _Wi of the graphic pattern P1 are determined by the following equations (B) and ( _C ). However, S in the formulas (B) and (C) is the length (mm) of one side of the square pattern SQ before the uniaxial tensile test.

ε _{Li =} ln (Li total/2S) (B)
ε _{Wi =} ln (Wi total/2S) (C)

The above operation is performed for all the figure patterns P1 to be evaluated, and the true longitudinal strain _εLi and the true _widthwise strain _εWi of _each figure pattern P1 are obtained.

When measuring L _i total and W _i total in figure pattern P _i , it is preferable to measure figure pattern P _i while observing figure pattern P i with a microscope, for example. The microscope may be an optical microscope or a scanning electron microscope.

_In the example shown in FIG. ₅ , the sides J ₁ to J ₄ of the figure pattern P _i are indicated by straight _lines . is identified as a line pattern with Then, there is a possibility that the positions of the reference points T ₁ to T ₄ cannot be uniquely determined. FIG. 6 shows a state in which sides J ₁ to J ₄ of figure pattern P _i are line patterns having a predetermined width. When the figure pattern P _i is observed with a microscope when a grid pattern is formed by a printing method or an electrolytic etching method, each side J ₁ to J ₄ of the figure pattern P _i is observed as a line pattern as shown in FIG. In this case, unless the positions of the reference points T ₁ to T ₄ are defined, the true longitudinal true strain ε _Li and the true width true strain ε _Wi of the figure pattern P _i cannot be obtained accurately.

Therefore, in the present embodiment, when the grid lines forming the sides J ₁ to J ₄ of the graphic pattern are line patterns having widths, the reference point is the intersection point of the diagonal lines at the intersection of the line patterns. FIG. 7 shows an enlarged view of a region D surrounded by a dashed line shown in FIG. At the crossing portion X of the side J3 and the side J4 _, since the side J3 and the side _{J4 are} substantially perpendicular to _each other, the shape of the crossing portion X in plan view is a quadrangle. The intersection point of the diagonal lines of the intersection X is defined as a reference point _T4 . Although reference point _T4 has been described in detail, other reference points may be determined in the same manner.

So far, an example of a method for determining the longitudinal true strain _εLi and the widthwise true strain _εWi in the figure pattern P _i to be evaluated has been described, but in the present embodiment, determination methods other than the above may be adopted. . For example, a lattice pattern after a uniaxial tensile test is photographed with a camera to obtain imaging data, image analysis is performed on the imaging data, the shape of the figure pattern contained in the lattice pattern is analyzed, and each figure is analyzed from the analysis results. The true longitudinal strain ε _Li and the true transverse true strain ε _Wi of the pattern P _i may be determined. A non-contact strain measuring device can be used as a device for realizing such an analysis. For example, the AutoGrid comsmart system manufactured by Tokyo Boeki Techno System Co., Ltd. can be used.

(Fourth step)
In the fourth step, on the orthogonal coordinate axis plane with the y-axis as the longitudinal true strain ε _L and the x-axis as the width direction true strain ε _w , the figure pattern P _i to be evaluated is the longitudinal true strain ε _Li and the width direction This is the step of placing a coordinate point corresponding to the true strain ε _Wi . In the third step, the true longitudinal strain ε _Li and the true transverse true strain ε _Wi are obtained for each figure pattern P _i to be evaluated. Therefore, in order to grasp the strain distribution of the figure pattern P _i to be evaluated, all of the evaluation objects are placed on an orthogonal coordinate axis plane with the y-axis as the longitudinal true strain ε _L and the x-axis as the width direction true strain ε _w Plot the strain amount of the figure pattern P _i of . An example is shown in FIG. FIG. 8 shows the results of a uniaxial tensile test in which a tensile stress was applied in the rolling width direction to a pure titanium sheet of JIS Class 1 defined in JIS H4600 (2007).

(Fifth step)
In the fifth step, an approximate straight line is derived by the method of least squares for the coordinate points included in the predetermined ε _L section of the orthogonal coordinate axis plane, and the slope a of the approximate straight line is substituted into the following formula (D) to obtain the plastic strain ratio Calculate r.

r=−1/(1+a) … (D)

This step will be described in detail with reference to FIG. FIG. 9 is a diagram in which approximated curves are added to FIG. As shown in FIG. 9, since the distribution of the longitudinal true strain _εL of the figure pattern is distributed in the interval from 0 to _εLi , the approximate straight line is derived by the least squares method in the interval from 0 to _εLi . . Let y=ax+b be the regression equation representing the approximate curve. The approximation curve of the JIS class 1 pure titanium plate is represented by a regression equation of y=-1.5669x. x is the transverse true strain _εw and y is the longitudinal true strain _εL . The slope a of the regression formula is "-1.5669".

Then, the plastic strain ratio r is obtained by substituting the slope a of the regression equation into the above equation (D).

Any interval can be selected as the _εL interval when performing the least squares method. However, from the viewpoint of improving the accuracy of the plastic strain ratio, it is preferable to include at least two or more coordinate points in the selected section.

As described above, the method for measuring the plastic strain ratio of the metal plate of the present embodiment performs a uniaxial tensile test on the plate-shaped test piece 1 provided with the grid line pattern K in the parallel portion 2, and performs a tensile test. From the shape of the figure pattern P _i included in the later grid line pattern K′, the longitudinal true strain ε _Li and the width direction true strain ε _Wi are obtained for each figure pattern P _i , and each figure pattern P _i on the orthogonal coordinate axis plane The longitudinal true strain ε _Li and the width true strain ε _Wi are plotted, an approximate straight line is determined by the least squares method for coordinate points included in a predetermined ε _L interval, and the plastic strain ratio is determined from the slope of the approximate curve. In the present embodiment, since the ε _L interval when obtaining the approximate curve can be arbitrarily determined, for example, the plastic strain ratio in the ε _L interval immediately after the start of the tensile test or the plastic strain ratio in the ε _L interval immediately before the start of necking Alternatively, the plastic strain ratio in the _εL section from the start to the end of the tensile test can be arbitrarily obtained. As a result, the plastic strain ratio can be measured with high accuracy even for a material that causes necking immediately after the start of the tensile test, pure titanium, or a material that causes deformation-induced transformation. A plastic strain ratio in a narrower strain range can be easily obtained.

In addition, in the present embodiment, since two or more of the figure patterns P whose area is 50% or more in the central region are used as the figure patterns _Pi to be evaluated, the accuracy of the plastic strain ratio can be further improved.

Further, according to the method for measuring the plastic strain ratio of the metal plate of the present embodiment, since the grid line pattern K is provided by the printing method or the electrolytic etching method, the formation location of the grid line pattern K does not become uneven, and uniaxial tension The grid line pattern K is less likely to become the starting point of destruction during testing. Thereby, the deformation of the square pattern SQ during the tensile test is not affected by the grid line pattern K, and the plastic strain ratio can be accurately measured.

Further, according to the method for measuring the plastic strain ratio of the metal plate of the present embodiment, the square pattern SQ is set to have a side length S of 2 mm or more and (W / 5) mm or less, so that the square pattern before and after the tensile test The shape change from the SQ to the figure pattern can be easily observed, and the number of figure patterns P _i to be evaluated can be increased, so that the plastic strain ratio can be measured with high accuracy.

Further, according to the method for measuring the plastic strain ratio of the metal plate of the present embodiment, based on the distance between the reference points of the graphic pattern to be evaluated, the longitudinal true strain ε _Li and the width direction true strain ε _Wi , the true longitudinal strain ε _Li and the true transverse strain ε _Wi can be determined relatively easily, and the plastic strain ratio can be easily determined.

Furthermore, according to the method for measuring the plastic strain ratio of the metal plate of the present embodiment, the reference point when the side of the figure pattern is a line pattern is the intersection of the diagonal lines of the intersection, so the position of the reference point is determined. It is possible to improve the accuracy of the plastic strain ratio.

Examples of the present invention will be described below. The examples described below are intended to illustrate one aspect of the present invention, and are not intended to limit the present invention.

(Example 1)
As metal plates to be measured, SUS304, JIS class 1 pure titanium specified in JIS H4600 (2007), and JAC270D steel plate (IF steel for press) were prepared. The thickness of these metal plates was 0.5 mm. As a first step, a JIS13B plate-shaped test piece was prepared from each metal plate. The test piece preparation method conformed to Annex B of JIS Z2241 (2011) and JIS Z 2254 (2008). The original gauge length was set to 50 mm. The longitudinal direction of the parallel portion was the rolling direction of each metal plate. However, with respect to JIS class 1 pure titanium, the longitudinal direction of the parallel portion was set in two directions, the rolling direction and the rolling width direction. Further, as shown in FIG. 1(a), a grid line pattern was provided on the parallel portion of the plate-shaped test piece by stamp printing. The length of one side of the square pattern forming the lattice line pattern was set to 2 mm. Also, the line width of the grid lines was set to 0.2 mm. Then, a uniaxial tensile test was performed by applying a uniaxial tensile stress in the longitudinal direction of the parallel portion. The uniaxial tensile test was performed according to the plastic strain ratio test method for sheet metal materials of JIS Z2254 (2008). The tensile speed was 30%/min. Also, the upper limit of the amount of strain to be applied was set to 50%.

Next, as the second step, among the grid line patterns of the parallel part 2 after the uniaxial tensile test, evaluate a figure pattern in which 50% or more of the area is included in the central region obtained by equally dividing the parallel part into three in the width direction. Selected as a target. Two figure patterns were selected for evaluation.

Next, as a third step, the longitudinal true strain ε _Li and the width true strain ε _Wi of the figure pattern were obtained for each figure pattern to be evaluated. Of the distances between the reference points at both ends of each side of the figure pattern, the total L _i total (mm) of the components parallel to the longitudinal direction of the parallel parts and the total W _i total of the components parallel to the width direction of the parallel parts (mm), and by substituting these into the formulas (E) and (F), the longitudinal true strain ε _Li and the width true strain ε _Wi in the figure pattern were determined for each figure pattern to be evaluated. .

ε _{Li =} ln (Li total/4) … (E)
ε _{Wi =} ln (Wi total/4) … (F)

In addition, in Example 1, the second step and the third step were performed while observing the parallel portion with an optical microscope.

Next, as a fourth step, the _{longitudinal true strain ε Li and the} width Coordinate points corresponding to the directional true strain ε _Wi were plotted.
Furthermore, as a fifth step, an approximate straight line is derived by the method of least squares for the coordinate points included in the predetermined ε _L section of the orthogonal coordinate axis plane, and the slope a of the approximate straight line is substituted into the following formula (G) to obtain plasticity. A strain ratio r was calculated.

r=−1/(1+a) … (G)

The ε _L intervals for obtaining the approximate straight line were five intervals of 0 to 5%, 0 to 10%, 0 to 15%, 0 to 40%, and 0 to 50%. An approximate straight line was determined for each section by the method of least squares, and the plastic strain ratio was determined from the above formula (G). Table 1 shows the results. The evaluation results of Example 1 are those of measurement category B in Table 1.

(Example 2)
In Example 2, the same metal plate as in Example 1 was selected as the object to be measured. The first step was performed in the same manner as in Example 1 above.

Next, the second step and the third step were carried out using the AutoGrid comsmart system manufactured by Tokyo Boeki Techno System Co., Ltd. Specifically, the lattice pattern K' of the parallel part after the uniaxial tensile test is photographed by the camera attached to the AutoGrid comsmart system, the figure pattern to be evaluated is extracted from the image data, and the longitudinal true strain in each figure pattern is measured. ε _Li and transverse true strain ε _Wi were obtained. As for the figure patterns to be evaluated, all figure patterns in which 50% or more of the area is included in the central region obtained by equally dividing the parallel portion into three in the width direction were selected as objects to be evaluated.

Next, in the same manner as in Example 1, the plastic strain ratio r was calculated for each predetermined _εL section by performing the fourth step and the fifth step. Table 1 shows the results. The evaluation results of Example 2 are those of measurement category C in Table 1. 10 to 12 show graphs created in the fourth and fifth steps. Each graph had the longitudinal true strain ε _L on the y-axis and the transverse true strain ε _w on the x-axis.

(Comparative example 1)
In Comparative Example 1, the same metal plate as in Example 1 was selected as the object to be measured. As a first step, a JIS13B plate-shaped test piece was prepared from each metal plate. The test piece preparation method conformed to Annex B of JIS Z2241 (2011) and JIS Z 2254 (2008). The original gauge length was set to 50 mm. In Comparative Example 1, no grid line pattern was provided on the parallel portion of the plate-shaped test piece. Then, a uniaxial tensile test was performed by applying a uniaxial tensile stress in the longitudinal direction of the parallel portion. The uniaxial tensile test was performed according to the plastic strain ratio test method for sheet metal materials of JIS Z2254 (2008). The tensile speed was 30%/min. Moreover, the upper limit of the strain amount to be applied was set to 5%, 10%, 15%, 40%, and 50%, and a uniaxial tensile test was performed for each upper limit value.

Then, the plastic strain ratio of each plate-shaped test piece was measured according to the plastic strain ratio test method for thin sheet metal materials of JIS Z2254 (2008). Table 1 shows the results. The evaluation result of Comparative Example 1 is measurement classification A in Table 1.

As shown in Table 1, for SUS304, in Examples 1 and 2, the plastic strain ratio could be measured up to a maximum strain amount of 50%, but in Comparative Example 1, the strain amount could only be measured up to a maximum of 40%. Moreover, the plastic strain ratios of Examples 1 and 2 were in good agreement with the plastic strain ratio of Comparative Example 1.

In addition, when tensile stress was applied in the rolling direction of JIS class 1 pure titanium, the plastic strain ratio could be measured up to a maximum strain amount of 40% in Examples 1 and 2, but in Comparative Example 1, the strain amount was 30%. % could only be measured. Moreover, the plastic strain ratios of Examples 1 and 2 were in good agreement with the plastic strain ratio of Comparative Example 1.
Further, when tensile stress was applied in the rolling width direction of JIS type 1 pure titanium, the plastic strain ratio could be measured up to a maximum strain amount of 40% in Examples 1 and 2, but in Comparative Example 1, the strain amount was 5%. But I couldn't measure it.

Furthermore, with respect to the JAC270D steel sheet, in Examples 1 and 2, the plastic strain ratio could be measured up to a maximum strain amount of 40%, but in Comparative Example 1, the strain amount could only be measured up to a maximum of 30%. Moreover, the plastic strain ratios of Examples 1 and 2 were in good agreement with the plastic strain ratio of Comparative Example 1.

Furthermore, as shown in FIG. 10, in SUS304, the rows of coordinate points are not straight but meandering, and it was confirmed that the plastic strain ratio changed for each _εL section. On the other hand, as shown in FIGS. 11 and 12, in the JIS class 1 pure titanium and the JAC270D steel plate, the rows of coordinate points are relatively straight, and the plastic strain ratio does not change significantly for each _εL interval. was confirmed.

Further, in Examples 1 and 2, it was possible to obtain the plastic strain ratio for each of various ε _L sections using one plate-shaped test piece, but in the case of Comparative Example 1, which is a conventional example, the maximum It was necessary to prepare a plate-shaped test piece for each strain amount and perform a tensile test, which required a large number of test pieces and required a long time for measurement.

Next, in Example 2, for SUS304 in which the plastic strain ratio varied greatly in each ε _L interval, the ε _L interval when obtaining an approximate straight line was changed from 0 to 4%, 4 to 8%, and 8 to 15 %, 15-20% and 20-50%. This plastic strain ratio was calculated from the slope of the straight line determined for each predetermined ε _L section in FIG. Table 2 shows the results. As shown in Table 2, it can be seen that the plastic strain ratio of SUS304 differs greatly for each _εL section. It is known that deformation-induced martensite precipitates in SUS304 by applying tensile stress, and it is presumed that the variation in the plastic strain ratio is due to the formation of deformation-induced martensite.

Moreover, in Example 2, even when the ε _L interval was changed, the plastic strain ratio in the ε _L interval after the change could be obtained immediately. Furthermore, in Example 2, it was found that by narrowing the ε _L interval, it is also possible to obtain the instantaneous plastic strain ratio at an arbitrary strain amount ε _L .

1... Plate-shaped test piece, 2... Parallel part, K, K'... Grid line pattern, L _i total... Total of components parallel to the longitudinal direction of the parallel part among the distances between the reference points, P... Figure pattern, P _i ... figure pattern to be evaluated, SQ ... square pattern, T ₁ to T ₄ ... reference point, W _i total ... sum of components parallel to the width direction of the parallel portion of the distance between the reference points, X ... intersection, ε _Li . . . longitudinal true strain in the figure pattern to be evaluated, ε _Wi .

Claims (5)

A plate-shaped test piece formed from a metal plate to be measured, wherein a grid line pattern including a plurality of square patterns is provided on the surface of the parallel portion, and one side of each square pattern is the longitudinal direction of the parallel portion A first step of performing a uniaxial tensile test that applies a uniaxial tensile stress in the longitudinal direction of the parallel portion using a plate-shaped test piece provided so as to be parallel to the direction;
From the grid line pattern provided in the parallel portion after the uniaxial tensile test, a figure pattern corresponding to the square pattern before the uniaxial tensile test, and the parallel portion was equally divided into three in the width direction. a second step of selecting two or more of the figure patterns whose area is included in the central region by 50% or more as evaluation targets;
a third step of obtaining the longitudinal true strain ε _Li and the width direction true strain ε _Wi in the figure pattern to be evaluated selected in the second step for each figure pattern to be evaluated;
Corresponding to the longitudinal true strain _εLi and the widthwise true strain _εWi of the figure pattern to be evaluated on an orthogonal coordinate axis plane having the y-axis as the longitudinal true strain _εL and the x-axis as the widthwise true strain _εw A fourth step of placing a coordinate point to
An approximate straight line is derived by the method of least squares for the coordinate points included in the predetermined ε _L section of the orthogonal coordinate axis plane, and the slope a of the approximate straight line is substituted into the following formula (1) to obtain the plastic strain ratio r. A fifth step of calculating;
A method for measuring the plastic strain ratio of a metal plate, which is characterized in that the
r=−1/(1+a) … (1) 2. The method of measuring the plastic strain ratio of a metal plate according to claim 1, wherein the grid line pattern is provided on the parallel portion by a printing method or an electrolytic etching method. 3. The square pattern according to claim 1, wherein when the total width W of the parallel portion is 12 mm or more, the length S of one side of the square pattern is in the range of 2 mm or more and (W/5) mm or less. A method for measuring the plastic strain ratio of a metal plate. In the third step,
The reference points are the vertices where the four sides formed by the grid lines that partition the figure pattern to be evaluated are in contact with each other. When the total is L _i total (mm) and the total of the components parallel to the width direction of the parallel portion is W _i total (mm),
4. The method according to any one of claims 1 to 3, wherein the true longitudinal strain _εLi and the true widthwise strain _εWi in the figure pattern are obtained by the following equations (2) and (3). A method for measuring the plastic strain ratio of the metal plate described.
ε _{Li =} ln (Li total/2S) (2)
ε _{Wi =} ln (Wi total/2S) (3)
However, S in Formula (2) and Formula (3) is one side length (mm) of the said square pattern. 5. The method according to claim 4, wherein when the grid lines forming the sides of the figure pattern are line patterns having a width, the reference point is an intersection point of diagonal lines at an intersection where the line patterns intersect. A method for measuring the plastic strain ratio of a metal plate.

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