Method for measuring elastic modulus

Abstract

This invention provides a method for measuring the elastic modulus of a material that can accurately measure its elastic modulus by imparting appropriate tensile strength with a simple configuration. [Solution] The elastic modulus measurement method includes: a change degree measurement step in which a tensile test is performed on an evaluation test piece of a predetermined composition to which strain gauges 40a and 40b are attached, and a first change degree, which is the degree of change in the strain of the evaluation test piece recognized from the displacement of the grips 20a and 20b, and a second change degree, which is the degree of change in the strain of the strain gauges 40a and 40b are measured; and a correction element setting step in which a correction element is set to correct the strain of the target test piece recognized from the displacement of the grips 20 and 20b when a tensile test is performed on a target test piece of a predetermined composition, based on the difference between the first change degree and the second change degree.

Description

Technical Field

[0001] The present invention relates to a method for measuring elastic modulus.

Background Art

[0002] In a high-speed tensile test, a hydraulic control type impact tester that can continuously set the test speed and continuously apply a constant speed to a test piece is generally used (for example, see Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In order to measure the elastic modulus of a material by a high-speed tensile test, it is necessary to perform accurate displacement measurement. However, a contact extensometer used in a static tensile test may be damaged, so it cannot be attached to a hydraulic control type impact tester for use. Therefore, in a high-speed tensile test, it is conceivable to measure the displacement of a material by attaching a strain gauge to the material and measuring the displacement of the material by the strain gauge, or by analyzing a photographed image of the material by a video camera. However, when strain gauges are attached to a material, the material may fracture at the point where the strain gauge is attached during a high-speed tensile test, before reaching its intended tensile strength. Therefore, while using strain gauges allows for accurate measurement of the material's elastic modulus, it presents the disadvantage of difficulty in setting an appropriate tensile strength. Furthermore, when analyzing images captured by a video camera, the accuracy of elastic modulus measurement depends on the shooting interval and resolution. Depending on the conditions of the high-speed tensile test, the accuracy of elastic modulus measurement may be insufficient, and the video camera setup during shooting must be precise, which is another disadvantage.

[0005] This invention was made in view of the above background, and aims to provide a method for measuring the elastic modulus of a material that can accurately measure the elastic modulus of a material by imparting appropriate tensile strength with a simple configuration. [Means for solving the problem]

[0006] This disclosure relates to a computer-based method for measuring the elastic modulus of a test specimen, using an impact testing machine having a pair of grips for gripping both ends of a test specimen, a loading mechanism for displacing the grips to apply tensile force to the test specimen, and a grip displacement detection unit for detecting the amount of displacement of the grips, wherein a tensile test is performed on an evaluation test specimen of a predetermined composition to which strain gauges are attached, by gripping the evaluation test specimen with the grips and applying a predetermined tensile force with the loading mechanism, and a first degree of change, which is the degree of change in the strain of the evaluation test specimen recognized from the amount of displacement of the grips detected by the grip displacement detection unit, and the change in the strain of the strain gauges... The present invention relates to an elastic modulus measurement method comprising: a change degree measurement step of measuring a second change degree, which is the degree of chemical change; a correction element setting step of setting a correction element for correcting the strain of the target test piece recognized from the amount of displacement of the grip detected by the grip detection unit when the tensile test is performed on the target test piece of a predetermined composition, based on the difference between the first change degree and the second change degree; and an elastic modulus calculation step of calculating the elastic modulus of the target test piece based on the corrected strain obtained by correcting the strain of the target test piece recognized from the amount of displacement of the grip detected by the grip detection unit when the tensile test is performed on the target test piece using the correction element. [Effects of the Invention]

[0007] According to the elastic modulus measurement method of this disclosure, the elastic modulus of a target test specimen can be measured without using strain gauges, by correcting the strain of the target test specimen, which is recognized from the displacement of the grip, using a correction element. Therefore, an appropriate tensile strength can be applied with a simple configuration, and the elastic modulus of the material can be measured with high accuracy. [Brief explanation of the drawing]

[0008] [Figure 1] This diagram schematically shows the configuration of a high-speed tensile testing machine according to the embodiment. [Figure 2] This figure shows the functional configuration of the control unit and the arithmetic unit. [Figure 3] This flowchart shows the procedure for setting the correction factor and measuring the elastic modulus. [Figure 4] This stress-strain diagram compares strain measurement data from strain gauges with strain measurement data from chuck displacement. [Figure 5] This stress-strain diagram compares strain measurement data from strain gauges with strain measurement data from chuck displacement after correction using a correction coefficient. [Figure 6] This is measurement data for PMMA samples. [Figure 7] This is measurement data for PA samples. [Modes for carrying out the invention]

[0009] Embodiments of the present invention will be described below with reference to the drawings. In this embodiment, a high-speed tensile testing machine will be described as an impact testing machine according to the present invention.

[0010] [1. Configuration of a high-speed tensile testing machine] Figure 1 is a schematic diagram showing the configuration of the high-speed tensile testing machine 1 according to this embodiment. The high-speed tensile testing machine 1 performs a high-speed tensile test by applying a tensile force (load) at a predetermined impact speed to a test specimen TP of the material to be tested. The high-speed tensile testing machine 1 comprises a testing machine body 2, a control device 5, and a calculation device 6. The impact speed is set to, for example, 1 m / second. The materials to be tested are various materials, industrial products, or parts or components of industrial products, and the test specimen TP is prepared according to a predetermined standard for material testing. In this embodiment, the test specimen TP is a resin material of a predetermined composition such as PMMA (Poly Methyl Methacrylate) or PP (Polypropylene).

[0011] The testing machine body 2 comprises a table 10, a pair of support columns 11a and 11b erected on the table 10, a cross yoke 12 spanning the pair of support columns 11a and 11b, and a hydraulic cylinder 30 fixed to the cross yoke 12. The hydraulic cylinder 30 constitutes a load mechanism that applies tensile force to the test piece TP in the testing machine body 2 and is operated by hydraulic fluid supplied via a servo valve 33 from a hydraulic source (not shown) located inside the table 10.

[0012] An upper chuck 20a is connected to the piston rod 31 of the hydraulic cylinder 30 via a run-up jig 25 and a joint 26. A lower chuck 20b is provided on the table 10 via a load cell 27, which is a force sensor. The upper chuck 20a and the lower chuck 20b correspond to the pair of gripping devices of this disclosure. With this configuration, the testing machine body 2 pulls up the piston rod 31 at a predetermined impact speed. As a result, a force acts on the upper chuck 20a and the lower chuck 20b that rapidly separates them, and this force acts as a tensile force on the test piece TP gripped by the upper chuck 20a and the lower chuck 20b.

[0013] The testing machine body 2 is equipped with a stroke sensor 32 that outputs a stroke detection signal D1 to the control device 5 corresponding to the amount of movement of the piston rod 31 which corresponds to the displacement of the upper chuck 20a, and a load cell 27 which is a force detector that outputs a force detection signal D2 indicating tensile force to the control device 5. The displacement of the upper chuck 20a (amount of movement of the piston rod 31) corresponds to the deformation of the test specimen TP, and the strain of the test specimen TP is measured based on the stroke detection signal D1.

[0014] The control device 5 performs a high-speed tensile test by controlling the operation of the testing machine body 2, and acquires time-series data Q1 of the stroke detection signal D1, which indicates the amount of deformation of the test specimen TP, and time-series data Q2 of the force detection signal D2, which indicates the tensile force, during the execution of the high-speed tensile test. A computing device 6, which is composed of a personal computer or the like, is connected to the control device 5 by wire or wireless.

[0015] Each time-series data Q1, Q2 is sent from the control device 5 to the arithmetic device 6 at an appropriate timing, and the arithmetic device 6 measures the elastic modulus of the test piece TP based on the time-series data Q1, Q2. The arithmetic device 6 also functions as an input interface for setting various setting parameters (such as impact speed, etc.) related to the high-speed tensile test to the control device 5.

[0016] FIG. 2 is a diagram showing the functional configuration of the control device 5 and the arithmetic device 6. The control device 5 includes a communication unit 50, a servo drive unit 51, and a detection signal capture unit 52. The control device 5 is a computer unit including a processor such as a CPU or MPU, a memory such as a ROM or RAM, a storage device such as an HDD or SSD, and an interface circuit for connecting sensors and peripheral devices. The processor of the control device 5 realizes the functions shown in FIG. 2 by executing a control program of the control device 5 stored in the memory or the storage device.

[0017] The servo drive unit 51 outputs a control signal to the servo valve 33 to operate the hydraulic cylinder 30 so that the piston rod 31 is pulled up at a predetermined speed. The detection signal capture unit 52 includes A / D conversion circuits 53a, 53b that sequentially capture the stroke detection signal D1 and the force detection signal D2 respectively, and perform A / D conversion on them and output the results. Further, the detection signal capture unit 52 includes A / D conversion circuits 53c, 53d that perform A / D conversion on and output the strain detection signals D3a, D3b output from the strain gauges 40a, 40b attached to the evaluation test piece TPe used when calculating the correction coefficient described later.

[0018] The output of the A / D conversion circuit 53a becomes the time-series data Q1 in which the stroke detection signal D1 is digitized, and the output of the A / D conversion circuit 53b becomes the time-series data in which the force detection signal D2 is digitized. Also, the outputs of the A / D conversion circuits 53c, 53d become the time-series data in which the strain detection signals D3a, D3b are digitized. Hereinafter, the strain detection signals D3a, D3b are also collectively referred to as the strain detection signal D3.

[0019] [[ID=·15]] The A / D conversion circuits 53a to 53d operate in synchronization with a common clock signal, and synchronously capture the stroke detection signal D1, the force detection signal D2, and the strain detection signal D3 at a predetermined measurement sampling period Ts, and output the time-series data Q1, Q2, and Q3 thereof. As the source of the clock signal, an appropriate clock circuit may be used, for example, a clock circuit provided in a processor may be used.

[0020] The measurement sampling period Ts is set to a frequency (on the order of several MHz or higher) higher than the frequency (generally several kHz) used in a material testing machine that applies a static load to the test piece TP. As the set value of the impact speed increases, a measurement sampling period Ts with a higher frequency is set. Thereby, even when a larger impact speed is set and the change in the strain of the test piece TP becomes more rapid, the stroke detection signal D1, the force detection signal D2, and the strain detection signal D3 can always be captured with sufficient resolution for the rapid change.

[0021] The arithmetic unit 6 is a computer unit having a communication unit 60, a processor 61, a memory 65, and the like. In the memory 65, a program 66 for controlling the arithmetic unit 6 and data 67 of a correction coefficient described later are stored. The processor 61 functions as a correction coefficient setting unit 63 and an elastic modulus measurement unit 64 by reading and executing the program 66. The correction coefficient setting unit 63 executes the processing according to the degree-of-change measurement step and the correction element setting step in the elastic modulus measurement method of the present disclosure. The elastic modulus measurement unit 64 executes the processing according to the elastic modulus calculation step in the elastic modulus measurement method of the present disclosure.

[0022] [2. Calculation Process of Correction Coefficient] Steps S1 to S3 in Figure 4 represent the setting process of the correction coefficient K, which is performed by the correction coefficient setting unit 63. In step S1, the correction coefficient setting unit 63 receives and acquires from the control device 5 the time-series data Q1 of the stroke detection signal D1, the time-series data Q2 of the force detection signal D2, and the time-series data Q3 of the strain detection signal D3 when an evaluation test piece TPe, with strain gauges 40a and 40b attached to both sides, is gripped by the upper chuck 20a and the lower chuck 20b and subjected to a high-speed tensile test, as shown in Figure 2.

[0023] Here, Figure 4 is a stress-strain diagram in which the vertical axis is set to the stress (MPa) applied to the evaluation specimen TPe and the horizontal axis is set to the strain (%) of the evaluation specimen TPe. M11 in Figure 4 shows the change in strain of the evaluation specimen TPe recognized from the time-series data Q3 of the strain detection signal D3, and M12 shows the change in strain of the evaluation specimen TPe recognized from the displacement amount of the upper chuck 20a indicated by the time-series data Q1 of the stroke detection signal D1. Stress is recognized from the time-series data Q2 of the force detection signal D2.

[0024] Here, the inclinations of M11 and M12 indicate the elastic modulus of the evaluation specimen TPe. However, since the strain gauges 40a and 40b directly detect the strain of the evaluation specimen TPe, the strain can be accurately detected and the elastic modulus measured by using the strain detection signal D3. In contrast, the stroke sensor 32 detects the displacement of the upper chuck 20a based on the amount of movement of the piston rod 31, so the detected value is superimposed with the displacement amounts of intervening mechanical components such as the joint 26 and the run-up jig 25.

[0025] Therefore, when using the stroke detection signal D1, the measurement accuracy of the strain of the evaluation specimen TPe is lower than when using the strain detection signal D3, and accordingly, the measurement accuracy of the elastic modulus of the evaluation specimen TPe is also lower. To address this decrease in measurement accuracy when using the stroke detection signal D1, the correction coefficient setting unit 63 calculates a correction coefficient K using the following equation (1) to bring the strain recognized from the measured displacement amount by the stroke detection signal D1 closer to the measured strain by the strain detection signals D3a and D3b.

number

[0026] Here, the slope of M11 corresponds to the first degree of change in this disclosure, and the slope of M12 corresponds to the second degree of change in this disclosure. The correction coefficient K reduces the difference between the measured strain of the test piece recognized from the stroke detection signal D1 and the measured strain from the strain detection signal D3. The correction coefficient setting unit 63 stores the data of the correction coefficient K calculated by the above formula (1) in the memory 65.

[0027] Figure 5 is a stress-strain diagram, similar to Figure 4, in which the vertical axis is set to stress (MPa) and the horizontal axis to strain (%), showing the progression of strain in specimen TP measured by strain detection signal D3 at M21, and the progression of corrected strain, which is obtained by multiplying the strain of specimen TP recognized from the measured displacement amount of specimen TP by the stroke detection signal D1 by a correction factor K, when a high-speed tensile test is performed on specimen TP. By applying the correction factor K, the difference between M21 and M22 in Figure 5 after correction is reduced compared to the difference between M11 and M12 in Figure 4 before correction. This indicates that the measurement error when measuring the strain of specimen TP using the stroke detection signal D1 can be reduced by applying the correction factor K.

[0028] [3. Measurement process for elastic modulus] Step S4 in Figure 3 is a process performed by the elastic modulus measuring unit 64. The elastic modulus measuring unit 64 measures the elastic modulus of a target test specimen TP with the same composition as the evaluation test specimen TPe using a correction coefficient K stored in memory 65. The target test specimen TP is a normal test specimen without strain gauges attached.

[0029] In step S4-1, the elastic modulus measuring unit 64 performs a high-speed tensile test on the target test piece TP using the high-speed tensile testing machine 1 and acquires time-series data Q1 of the stroke detection signal D1 output from the stroke sensor 32 and time-series data Q2 of the force detection signal D2 output from the load cell 27. The elastic modulus measuring unit 64 operates the hydraulic cylinder 30 to perform the high-speed tensile test by transmitting control data including impact velocity setting information to the control device 5.

[0030] In the following step S4-2, the corrected strain DS_c is calculated as the measured value of the target test specimen TP by multiplying the strain Ds of the target test specimen TP measured using the stroke detection signal D1 by the correction coefficient K, as shown in equation (2) below.

number

[0031] This allows for improved measurement accuracy in measuring the strain of the target test specimen TP without using strain gauges. The elastic modulus measuring unit 64 then calculates the slope of the stress-strain diagram (corresponding to W22 in Figure 5) for the corrected strain Ds_c as the elastic modulus of the target test specimen TP. This enables accurate measurement of the elastic modulus of the target test specimen TP through a simple measurement process using the stroke detection signal D1 from the stroke sensor 32.

[0032] [4. Verification Results] Referring to Figures 6 and 7, we will now explain the verification results of the effect of improving the measurement accuracy of the elastic modulus by correcting the strain of the target test piece TP, measured using the stroke detection signal D1 of the stroke sensor 32, with the correction coefficient K, as described above.

[0033] Figure 6 shows the elastic modulus calculated using the strain measured with strain gauges 40a and 40b (using strain detection signal D3) and the correction factor K, as well as the elastic modulus calculated using the corrected strain (corrected by the correction factor K) obtained by correcting the strain measured with stroke detection signal D1 (using the displacement of the upper chuck 20a), when high-speed tensile tests were performed on PMMA specimen TP at four impact speeds: 0.01 m / sec, 0.1 m / sec, 1 m / sec, and 10 m / sec. Figure 6 also shows the measurement results averaged over multiple high-speed tensile tests.

[0034] Figure 6 shows that for the PMMA specimen TP, the difference between the elastic modulus calculated using the corrected strain (calculated by correcting the strain measurement value based on the stroke detection signal D1 with a correction factor K) and the elastic modulus calculated using the strain measurement value based on the strain detection signal D3 is within 2%. This confirms that applying the correction factor K improves the measurement accuracy when measuring the elastic modulus using the stroke detection signal D1.

[0035] Figure 7 shows the elastic modulus calculated using the strain measured using strain gauges 40a and 40b with the impact velocity set to four levels: 0.01 m / sec, 0.1 m / sec, 1 m / sec, and 10 m / sec, for a PP test specimen TP, similar to Figure 6. It also shows the elastic modulus calculated using the corrected strain obtained by correcting the strain measured using the stroke detection signal D1 (using the displacement of the upper chuck 20a) with the correction factor K, and the difference between the two. Figure 7 shows the measurement results averaged after performing multiple high-speed tensile tests.

[0036] Figure 7 shows that for the PP test specimen TP, the difference between the elastic modulus calculated using the corrected strain (calculated by correcting the strain measurement value based on the stroke detection signal D1 with a correction factor K) and the elastic modulus calculated using the strain measurement value based on the strain detection signal D3 is within 4%. This confirms that applying the correction factor K improves the measurement accuracy when measuring the elastic modulus using the stroke detection signal D1.

[0037] [5. Other Embodiments] In the above embodiment, the correction coefficient K according to equation (1) above was used as the correction element of this disclosure. In another embodiment, for example, a correlation map showing the correlation between the strain of the test piece measured using the stroke detection signal D1 (input strain) and the strain of the test piece measured using the strain detection signal D3 (output strain) may be created by performing the same processing as in step S1 of Figure 3, and this correlation map may be set as the correction element. In this case, the strain of the test piece measured using the stroke detection signal D1 (input strain) is applied to the correlation map, and the corresponding output strain is set as the correction strain.

[0038] In the above embodiment, as shown in Figure 2, when calculating the correction coefficient K, the measurement accuracy was improved by using an evaluation test piece TPe with two strain gauges 40a and 40b attached, and averaging the detected values ​​of both. In other embodiments, an evaluation test piece with three or more strain gauges attached may be used, or an evaluation test piece with one strain gauge attached may be used.

[0039] [6. Modes] Those skilled in the art will understand that the above-described exemplary embodiments are specific examples of the following embodiments.

[0040] (1) A method for measuring the elastic modulus of a test specimen, performed by computer, using an impact testing machine having a pair of grips for gripping both ends of a test specimen, a loading mechanism for displacing the grips and applying a tensile force to the test specimen, and a grip displacement detection unit for detecting the amount of displacement of the grips, wherein a tensile test is performed on an evaluation test specimen of a predetermined composition to which a strain gauge is attached, by gripping the evaluation test specimen with the grips and applying a predetermined tensile force with the loading mechanism, and a first degree of change, which is the degree of change in the strain of the evaluation test specimen recognized from the amount of displacement of the grips detected by the grip displacement detection unit, and the strain of the strain gauge A method for measuring elastic modulus, comprising: a change degree measurement step of measuring a second change degree, which is the degree of change of; a correction element setting step of setting a correction element for correcting the strain of the target test piece recognized from the amount of displacement of the grip detected by the grip detection unit when the tensile test is performed on the target test piece of a predetermined composition, based on the difference between the first change degree and the second change degree; and an elastic modulus calculation step of calculating the elastic modulus of the target test piece based on the corrected strain obtained by correcting the strain of the target test piece recognized from the amount of displacement of the grip detected by the grip detection unit when the tensile test is performed on the target test piece using the correction element. According to the elastic modulus measurement method described in paragraph 1, a correction coefficient is set to reduce the error in the elastic modulus of a test piece of a predetermined composition measured based on the displacement of the gripper, based on the difference between the first evaluation elastic modulus based on the displacement of the gripper detected by the gripper displacement detection unit and the second evaluation elastic modulus based on the strain of the strain gauge. Furthermore, since the elastic modulus of the target test piece can be measured by correcting the elastic modulus based on the displacement of the gripper with the correction coefficient without using a strain gauge, the elastic modulus of the material can be measured with an appropriate tensile strength using a simple configuration.

[0041] (Clause 2) The method for measuring the elastic modulus according to Clause 1, wherein in the correction element setting step, a correction coefficient, which is the ratio of the second degree of change to the first degree of change, is set as the correction element, and in the elastic modulus calculation step, the correction is performed by multiplying the strain of the target test piece recognized from the amount of displacement of the grip detected by the grip detection unit by the correction coefficient. According to the elastic modulus measurement method described in paragraph 2, the measurement accuracy can be improved by applying a correction coefficient to the strain of the target test piece, which is measured based on the displacement of the gripping device.

[0042] (Clause 3) The method for measuring the elastic modulus according to Clause 1 or 2, wherein in the correction element setting step, the tensile test is performed on the evaluation test piece to which the plurality of strain gauges are attached, and the average value of the degree of change of the strain of the plurality of gauges is set as the second degree of change. According to the elastic modulus measurement method in paragraph 2, the influence of variations in strain detection values ​​due to the placement of strain gauges on the evaluation test specimen can be reduced, and the second degree of change can be measured with high accuracy. [Explanation of symbols]

[0043] 1. High-speed tensile testing machine 2. Test machine body 5 Control device 6 Arithmetic unit 20a Upper chuck (gripping tool) 20b Lower check (gripping tool) 30. Hydraulic cylinder (load mechanism) 32 Stroke sensor (gripping tool displacement detection unit) 40a, 40b strain gauges 61 processors 63 Correction coefficient setting section 64 Elastic modulus measurement section 65 memory 66 Programs 67 Correction coefficient data TPe evaluation test specimens TP target test specimen

Claims

1. A method for measuring the elastic modulus of a test specimen, performed by a computer, using an impact testing machine having a pair of grips for gripping both ends of a test specimen, a loading mechanism for displacing the grips to apply tensile force to the test specimen, and a grip displacement detection unit for detecting the amount of displacement of the grips, wherein the elastic modulus of a test specimen is measured, A tensile test is performed on an evaluation test piece of a predetermined composition to which a strain gauge is attached, by gripping the evaluation test piece with the gripper and applying a predetermined tensile force with the load mechanism, and a first degree of change, which is the degree of change in the strain of the evaluation test piece recognized from the displacement of the gripper detected by the gripper displacement detection unit, and a second degree of change, which is the degree of change in the strain of the strain gauge, are measured in a degree of change measurement step. Based on the difference between the first degree of change and the second degree of change, a correction element setting step is performed to set a correction element for correcting the strain of the target test piece recognized from the amount of displacement of the grip detected by the grip detection unit when the tensile test is performed on the target test piece of the predetermined composition, An elastic modulus calculation step is performed on the target test specimen, and the elastic modulus of the target test specimen is calculated based on the corrected strain obtained by correcting the strain of the target test specimen, which is recognized from the amount of displacement of the grip detected by the grip detection unit, using the correction element. A method for measuring elastic modulus, including the method itself.

2. In the correction element setting step, a correction coefficient, which is the ratio of the second degree of change to the first degree of change, is set as the correction element. In the elastic modulus calculation step, the correction is performed by multiplying the strain of the target test piece, recognized from the displacement of the grip detected by the grip detection unit, by the correction coefficient. The method for measuring the elastic modulus according to claim 1.

3. In the correction element setting step, the tensile test is performed on the evaluation test piece to which the multiple strain gauges are attached, and the average value of the degree of change in strain of the multiple gauges is set as the second degree of change. The method for measuring the elastic modulus according to claim 1 or claim 2.

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