JP2004257925A - Material testing machine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To remove an effect of an anteroposterior move of a test piece on an elongation measurement value in material test using a video-type strain measurement means. <P>SOLUTION: A reference scale 22 having a prescribed reference length is stuck on the test piece in the vicinity of a gage line 21 attached thereto. The size of an image of the reference scale taken by a video camera changes as the test piece makes an anteroposterior move. A distance between two gage lines 21 situated one above the other is corrected based on the size of the image of the reference scale 22, thereby obtaining an inter-gage-line distance cleared of the effect of the test piece moving toward or away from the video camera, and thus a correct elongation value is measured. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a material testing machine having a video type strain measuring means, wherein a test camera body is used to measure the elongation of a test piece which is expanded by, for example, being subjected to a tensile load, by using a video camera to mark a line attached to the test piece. The present invention relates to a material testing machine that takes an image and measures the amount of elongation (amount of strain) of a test piece from the amount of movement of the marked line. Further, in the measurement of the strain of the material referred to in the present specification, the amount of compression can be measured only by reversing the direction in which a load is applied to the material and taking the strain in the negative direction opposite to the elongation. It shall also include material testing and compression measurement.
[0002]
[Prior art]
Since the video extensometer measures the distance between the marking lines attached to the test piece based on an image of the test piece, the measured distance between the marking lines varies depending on the magnification at which the image is taken. The imaging magnification is fixed at a fixed value if the type of camera, the lens to be used, and the distance between the camera and the test piece are determined, but the distance between the camera and the test piece is likely to change due to various factors. Among other factors, a slightly bent specimen may become straight as it is loaded, or the specimen may move back and forth with respect to the camera due to non-uniformity of the applied force. It is thought to have the greatest effect.
[0003]
An error when the test piece moves back and forth in the direction of the video camera will be described with reference to FIG. Assuming that the position N is the correct position where the test piece should be placed, the test piece 31 to which the two upper and lower mark lines 32 are attached is located here. A video camera is arranged facing the test piece at a position where the surface of the test piece 31 can be photographed, and the image of the reference line 32 is formed on the CCD image pickup device in the video camera by the camera lens 33. The distance between the two marked lines is L. The distance between the position N and the camera lens 33 is A, and the distance between the camera lens 33 and the CCD element 34 is B. In this state, a case where the test piece 31 is displaced by the distance d in the direction away from the video camera and moved to the position F will be considered. Note that the video camera is adjusted so that the test piece is in focus when the test piece is at the position N. Even when the test piece moves to the position F, it is considered that the test piece is within the depth of focus of the video camera. Here, it is assumed that the image is not blurred.
[0004]
The gauge length to be tied on the CCD element when the test piece 31 is in the position N and L _N, the gauge length to be tied on the CCD element when the test piece 31 in position F When L _F ,
L _N = L × B / A (1)
L _F = L × B / (A + d) (2)
Therefore, L _F ≠ L _N. If the position N is the position of the correct test piece and the coefficient corresponding to the camera magnification, that is, the coefficient for converting the length on the CCD element into the length at the position N is G, the distance L Is determined as follows.
L = G × L _N (3)
When where the test piece 31 is moved to the position F, it is calculated without taking into account the effect of this movement, the formula (3) L _N instead from be using _{L F L '= G × L} F of = L × A / (A + d) (4)
Will be calculated. This is L '≠ L, which is an incorrect value.
[0005]
An example of a method for removing the influence of the displacement of the test piece before and after is described in Patent Document 1. In this document, a displacement meter that measures the distance variation between a camera and a test piece is placed at the camera position, the displacement before and after the test piece is measured, and the elongation measurement value is corrected based on the position information of the test piece. To accurately measure the elongation between marked lines.
[0006]
[Patent Document 1]
Japanese Patent Application Laid-Open No. Hei 8-226888 (FIG. 1)
[0007]
[Problems to be solved by the invention]
Many conventional video extensometers measure the elongation of a test piece ignoring its error, assuming that the test piece moves in the front-back direction is small. In this case, the accuracy of the extensometer is poor, and accurate elongation measurement cannot be performed.
[0008]
In addition, the method described in Patent Document 1 uses a separate device such as a displacement meter, which is originally not required for elongation measurement of a test piece performed in a material test, so that the structure is complicated and the cost is increased.
[0009]
The present invention has been made in view of such circumstances, and has a simple configuration that corrects fluctuations in elongation measurement values due to movement of a test specimen in a forward and backward method, and that can accurately measure the amount of elongation of the test specimen. The purpose is to provide an extensometer.
[0010]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention has a video camera that captures a mark attached to a test piece to which a load is applied, and distorts the test piece based on the amount of movement of the mark taken by the video camera. In a material testing machine provided with a video type strain measuring means for measuring a reference scale having a predetermined size was attached to the surface of a test piece taken by the video camera, and the reference scale was taken by the video camera. The distortion measurement value of the test piece is corrected based on the size of the image.
[0011]
Since the reference scale is attached to the surface of the test piece and follows the movement of the test piece in the front-rear direction, the distance between the marked lines due to the test piece moving back and forth based on the change in the size of this reference scale Fluctuations in measured values can be corrected. Here, the movement of the test piece in the front-back direction means the movement in the direction of the line connecting the test piece and the video camera, that is, the movement of the test piece relatively approaching or moving away from the video camera. ing. This includes the case where the video camera moves in the direction of the test piece without moving the test piece.
[0012]
The above-mentioned reference scale can also serve as a gauge for distortion measurement inherent in the video-type distortion measurement means. In this case, there is no need to specially attach a reference scale, so that there is no useless labor or error factor.
[0013]
Further, the above-described reference scale can be used for calibration of the reference line distance. The measured value of the distance between the marked lines is corrected based on the length of the reference scale whose absolute value is known, and a more accurate elongation amount can be obtained.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic side view of a whole of a tester main body for performing a material test by applying a tensile load to a test piece and a video extensometer attached to the tester to measure the elongation of the test piece. ing.
[0015]
The tester main body 1 applies a tensile load to the test piece to deform the test piece, and measures a test force applied to the test piece at that time. On the other hand, the video-type extensometer 14 is arranged at a position facing the surface of the test piece so that the whole can be photographed, and measures the respective positions by photographing two upper and lower marked lines attached to the surface of the test piece. Then, the distance between the upper and lower two marked lines is calculated from the position of the marked line photographed when a load is applied to the test piece and the test piece is stretched, and the elongation of the test piece is measured. Then, the relation between the test force applied to the test piece and the elongation is plotted by a computing device in a graph, and various characteristic values of the material of the test piece are calculated and evaluated. The controller 12 has a function of controlling the tester main body 1 and the video extensometer 14 and taking in measured values, and is a device for storing and calculating data, and is constituted by a computer and various interfaces. I have. These may be configured as a plurality of units separately for each function.
[0016]
The configurations of the tester main body 1 and the video extensometer 14 will be further described. The test machine main body 1 has a crosshead 3 that can be driven up and down provided in a frame 2 that is erected in a square shape. A lower chuck 4 for gripping one end of the test piece is attached to the lower part of the frame 2 upward, and an upper chuck 5 for holding the other end of the test piece is downwardly attached to the crosshead 3 via a load cell 6 for measuring a test force. Installed. The test piece 7 cut into a shape suitable for the test is set so as to be gripped by the upper and lower chucks 5 and 4 in a straightened state. A lower mark line 8, an upper mark line 9, and a reference scale 10 are attached to the surface of the test piece 7. For example, a mark made in a bar shape or a specific shape is drawn on a sheet (marked line sheet) made of paper or the like, and the mark is attached to the surface of the test piece to make the marked lines 8 and 9. Adhesive is applied to the back side of the marked line sheet, the marked line mark on the front side is at a predetermined position determined by the standard, and the distance between the upper and lower two marked lines is Affixed to the test piece so that the distance is determined by. The reference scale 10 is also obtained by attaching a sheet (reference scale sheet) on which a mark having a specific shape is drawn.
[0017]
On the other hand, the video camera 11 is arranged so as to be fixed to the frame of the tester main body 1 so as to face the surface of the test piece 7 on which the marked line is attached. The video camera 11 is focused so that the surface of the test piece is just focused, and images of the upper and lower marked lines 9 and 8 are formed on the CCD image pickup device inside the video camera. The image processing function included in the controller 12 calculates the positions of the two marked lines connected on the CCD element, and multiplies the distance between the images of the upper and lower marked lines by a predetermined coefficient G corresponding to the camera magnification. The distance between the two marked lines attached to the actual test piece is calculated. The field of view 13 of the video camera includes both the upper and lower marked lines 9 and 8, and is set with a margin in the direction in which the test piece is extended.
[0018]
When the material test is started in this state, the crosshead 3 is displaced upward (in the direction of the arrow U in FIG. 1) with respect to the frame 2 by a drive mechanism such as a motor and a screw rod in response to a command from the controller 12. 7 is pulled in the direction in which it is extended, and the test force applied to the test piece 7 based on the signal from the load cell 6 is fetched and stored in the controller 12 every moment. At the same time, the video extensometer calculates the distance between the lower standard line 8 and the upper standard line 9 from the positions of the two standard lines projected on the CCD image pickup device inside the video camera 11, and calculates the amount of change in the distance. Is measured and stored every moment as the elongation of the test piece. Then, a graph or the like representing the test force with respect to the amount of elongation of the test piece is drawn on the display device, and various calculations are performed to evaluate the tested material.
[0019]
Next, a method of correcting the elongation measurement value when the test piece fluctuates in the front-back direction with respect to the video camera will be described.
[0020]
First, the test piece 7 to which the mark sheet is attached is attached to the tester main body 1, and the distance between the mark lines measured by the video camera 11 at that time is a value determined by the standard of the upper and lower two mark lines. (For example, 50 mm), and a coefficient G for calculating an actual distance from a captured image is calculated and stored. As illustrated in FIG. 3A, a reference scale sheet is attached to the test piece 7 in addition to the marked line sheet. These are sheets on which marks representing the marking lines 21 or the reference scales 22 are drawn, and are made of paper or other thin materials. In the first stage of the test execution, the reference scale is also photographed at the same time as the reference line, and the size of the image is stored as the reference length (for example, 20 mm). The reference length serves as a reference, and serves as data for correcting a change due to the subsequent movement of the test piece in the forward and backward directions.
[0021]
The correction of the effect of moving the test piece back and forth will be described with reference to FIG. The lower line 8 and the upper line 9 and the reference scale 10 are photographed by a video camera. Suppose that the reference length of the reference scale is stored as K at the first time of the measurement. As shown as a measurement value before correction in FIG. 2A, the reference length at a certain point in time when the elongation of the test piece is measured is measured as K ′, and the distance between the marked lines at that time is L ′. Assume that it was measured. This is a calculated value assuming that the test piece does not move in the front-rear direction. Therefore, as shown in FIG. 2B, the true distance L between the reference marks after correcting the effect of moving the test piece in the front-rear direction is calculated as follows.
L = L '× K / K' (5)
The true amount of elongation of the test piece can be calculated from the value of the true distance L between the reference marks obtained here.
[0022]
It is desirable that the length K serving as the reference of the reference scale has a known size in absolute value in advance, but it is not always necessary to know the absolute value accurately. This is because, as described above, the distance between the marked lines may be corrected based on the length stored at the time of the first setting.
[0023]
As another concept, the following configuration can be adopted. In the above description, at the time of the first setting to start the test, the marked line seal was affixed to the test piece so that the distance between the marked lines became a predetermined value determined by the standard, but in reality, for example, the standardized It is not always easy to affix the upper and lower marked lines to the test piece with high precision so as to have a predetermined interval. In this case, if the reference length of the reference scale is a known value that is accurately known, or if the reference length is accurately measured in advance by another measuring device and the value is stored, the value of the reference scale can be obtained. It is also possible to calculate the exact absolute value of the distance between the marked lines affixed to the test piece based on the reference length, that is, to calibrate. Based on this value, the amount of elongation or the ratio of the amount of elongation to the test force applied to the test piece can be measured more accurately.
[0024]
That is, a reference scale of the reference length is absolute value correct K _0, reference length of reference scale on the CCD is measured as S, when the inter-marked line distance is measured as H, gauge length The absolute value L ₀ of the absolute value of is calculated as follows.
K ₀ = G × S (6)
L ₀ = K ₀ × H / S = G × H (7)
Here, G is a coefficient for converting the distance measured on the CCD image sensor into the actual distance on the test piece, and is defined by G = K ₀ / S. The above equation (6) is a modified version of this definition equation. In terms of relationship with the above equation (5), the distance L between the true target line, as a replacement for K in equation (5) _{K 0, L '= G ×} H' and K '= G × S' You just have to calculate. Here, H 'is the distance value between the reference lines on the CCD element measured at that time, and S' is the reference length value on the CCD element measured at that time. That is, the true distance L between the reference marks after correcting the effect of moving the test piece in the front-rear direction is expressed by the following equation.
L = L ′ × K ₀ / K ′ = K ₀ × H ′ / S ′ (8)
[0025]
As shown in FIG. 3, there are three methods for attaching the reference scale to the test piece, but any method may be used in principle. FIG. 3A shows a case where the reference scale is attached as a sheet completely different from the reference line. One marking line 21 is drawn on one sheet, and two sheets are attached to the test piece 7 as upper and lower marking lines. The reference scale 22 is drawn on a sheet different from the reference line, and is attached near the reference line 21. The attaching position of the reference scale is not particularly limited, but is preferably a position near one of the two marked lines and not between the upper and lower two marked lines (a position close to the chuck). This is because, in this case, the affixed state of the sheet is hardly affected by the elongation of the test piece and has no effect.
[0026]
In FIG. 3B, the reference scale 22 is also written in advance on the sheet on which the marking lines 21 are written, so that the marking lines 21 and the reference scale 22 can be attached to the test piece 7 only once. Conveniently, the reference scale is necessarily located close to the reference line. However, in this case, the position of the reference scale may be a position between two upper and lower marked lines. Because the sheet is stuck to the test piece with the adhesive at the position behind the mark line 21, the reference scale 22 is drawn at any position of the sheet without being affected by the elongation of the test piece, and It has no effect.
[0027]
FIG. 3 (c) shows that the sheet on which the reference scale / marker line 23 is drawn is affixed to the test piece 7 and the reference scale is also used as the reference line, and it is not necessary to attach the reference scale as a separate body. In addition, since the conventional mark line can be used as it is as the reference scale, it is the least time-consuming. At this time, either of the two upper and lower marked lines may be used as the reference scale.
[0028]
FIG. 3 shows an example in which one reference scale is attached near one of the two marking lines, but the reference scale is separately attached near each of the upper and lower marking lines, and the test specimen is separately attached. If the front-back effect is corrected, more accurate correction can be performed corresponding to the tilt of the test piece in the front-back direction. In this case, the average value of the two elongation amounts obtained by correcting using the data from the upper and lower reference scales can be used as the true test piece elongation amount.
[0029]
FIG. 4 shows an example of the shape of the reference scale. FIG. 4A shows the reference scale 22 drawn on a single sheet. The symbol K in the figure represents the reference length. The shape of the reference scale may be any shape as long as it can serve as a reference for the length, but when photographed by a video camera using a CCD element, the pitch between the CCD elements can be accurately obtained by interpolation. Shape is desirable. For example, a shape in which the reference position is the vertex of the triangle is desirable. FIG. 4B is an example in which the reference scale 22 is drawn on the same sheet as the reference line 21, and an example in which two triangles are arranged as the reference scale. FIGS. 4C and 4D show the reference scale and mark line 23 in which the reference scale also serves as the mark line on one sheet. As shown in FIG. 4 (c), the end to end of the bar-shaped mark may be set as the reference length K. However, as shown in FIG. 4 (d), a mark having a shape in which a plurality of rhombic marks are arranged also serves as a reference scale. In this way, the reference length K can be determined with high accuracy. The shape of the reference scale is not limited to the above example, and various shapes can be considered. For example, two points or lines drawn at distant positions may be used. Further, the arrangement direction of the reference length is not limited to the arrangement of the test pieces in the horizontal direction as viewed from the front as in the above-described example, but may be arranged in the vertical direction or in the oblique direction.
[0030]
The calculation for accurately determining the reference length from the image of the reference scale captured by the CCD image pickup device built in the video camera will be described. The CCD image sensor is composed of a large number of pixels arranged two-dimensionally at a predetermined pitch. Since the pitch of the arrangement of pixels is a finite value that is not infinitesimal, interpolation calculation between pixels is important in order to accurately determine the reference length. The case where the reference scale is two triangles as illustrated in FIG. 3B will be described. Accurately obtaining the reference length is equivalent to accurately obtaining the XY coordinates representing the vertices of the triangle on the CCD element, assuming that the XY coordinate axes are set based on the arrangement of the pixels of the CCD element.
[0031]
In order to obtain the vertices of the triangle as the reference scale, for example, the following image processing is performed. First, the contour is extracted from the photographed triangular image. The two sides of the outline are each approximated by a straight line, and the intersection of the two straight lines is defined as the vertex coordinates of the triangle. Assuming that two triangles constitute a reference scale, and when the coordinates of the respective vertices of each triangle are determined, a straight-line distance between the two points is calculated to be a reference length. By such processing, the reference length can be obtained with high accuracy even to a value finer than the pitch of the CCD elements.
[0032]
In order to obtain the reference length by the above-described image processing, it is desirable that the outer shape of the reference scale be a triangle or a quadrangle having two or more sides. At this time, it is preferable to arrange the two sides so that the length direction of the two sides and the arrangement direction of the pixels of the CCD element are not parallel to each other for accurate interpolation. The shapes illustrated in FIGS. 4A, 4B, and 4D are desirable shapes in view of this point. Also, to prevent the length direction of the reference scale from being parallel to the arrangement direction of the CCD pixels, instead of holding the video camera horizontally or at a right angle to it, shoot the entire video camera obliquely from the horizontal. It is also effective to do so.
[0033]
In the above-described embodiment, the description has been made on the assumption that the number of cameras of the video type extensometer is one. However, a configuration in which the upper and lower marking lines are imaged using two separate video cameras, and a total of three video cameras Of course, the present invention can be applied to an extensometer configured to photograph both the upper and lower marked lines with one video camera using a camera and separately photograph the upper and lower marked lines with the other two cameras. . Further, it goes without saying that the present invention can be applied not only to a tensile test but also to a compression test for measuring the amount of compression of a test piece.
[0034]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to the material testing machine of this invention, even if a test piece moves to a front-back direction with a simple structure, the distortion amount of a test piece can be measured accurately corresponding to it. Since a displacement meter for measuring the movement of the test piece in the front-rear direction as in the prior art is not required, the cost can be reduced and realized. If the reference scale is also used as a reference line, a more accurate strain gauge side can be obtained even though little effort is required as compared with the conventional test procedure.
[0035]
Further, according to the material testing machine of the present invention, it is possible to calibrate the distance between the upper and lower two marked lines using a reference scale whose exact reference length is known. It is possible to more accurately measure the properties of the material to be tested even if there is some error in the test.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing the whole of a tester main body and a video extensometer.
FIG. 2 is a diagram illustrating correction of a fluctuation due to a forward and backward movement of a test piece.
FIG. 3 is a diagram illustrating a method of attaching a reference scale.
FIG. 4 is a diagram illustrating an example of a reference scale.
FIG. 5 is a diagram illustrating an error of a measured value when a test piece moves in a front-back direction.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Test machine main body 2 ... Frame 3 ... Cross head 4 ... Lower chuck 5 ... Upper chuck 6 ... Load cell 7 ... Test piece 8 ... Lower marker 9 ... Upper marker 10 ... Reference scale 11 ... Video camera 12 ... Controller 13 ... field of view 14 ... video extensometer 21 ... mark 22 ... reference scale 23 ... reference scale and mark

Claims (3)

A material test having a video camera for photographing a marked line attached to a test piece to which a load is applied, and a video type strain measuring means for measuring a strain of the test piece from a movement amount of the marked line photographed by the video camera In the machine, a reference scale having a predetermined size is affixed to the surface of the test piece photographed by the video camera, and the measured value of the distortion of the test piece based on the size of the image photographed by the video camera. Material testing machine characterized by correcting The material testing machine according to claim 1, wherein the reference scale is also used by the mark line. The material testing machine according to claim 1, wherein the measured value of the movement amount of the reference line is corrected based on an absolute value of the reference scale.

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