Material Testing Machine

Abstract

To determine a proper control parameter when characteristics of a specimen include nonlinearity.SOLUTION: A fatigue testing machine 1 has a hydraulic actuator 15. The fatigue testing machine 1 applies test power FP to a specimen SP by the hydraulic actuator 15, and deforms the specimen SP to perform a fatigue test. The fatigue testing machine comprises a rigidity calculation unit 212 that obtains the maximum value MR of a rigidity value RG of the specimen SP when the test power FP changes nonlinearly with respect to a displacement X of the hydraulic actuator 15, and a control determination unit 214 that determines values of control parameters (PG, IG, DG) that define the operation of the hydraulic actuator 15 based on the maximum value MR of the rigidity value RG.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a materials testing machine. [Background technology]

[0002] Various techniques are known for reducing the workload of operators in fatigue testing machines. For example, the fatigue testing machine described in Patent Document 1 is provided with a means for setting the amplitude and frequency range of the repeating waveform, and while the test piece is attached, automatically changes the frequency sequentially within the set frequency range, drives and controls the loading mechanism (hydraulic cylinder) so that a test force is applied to the test piece at the set amplitude, takes in the output of the force detection means for each frequency, creates a graph showing the frequency response characteristics of the system including the test piece, and outputs it for display or the like. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-292400 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the material testing machine such as the fatigue testing machine described in Patent Document 1, the control parameters are adjusted without taking into consideration the nonlinearity of the characteristics of the test specimen. For a test specimen with nonlinear characteristics, if the control parameters are adjusted assuming that the test force changes linearly with the actuator displacement, the control may become unstable and the test specimen may be overloaded. Furthermore, for example, when an operator sets safe control parameters, the operator has no choice but to perform the test in a state of poor responsiveness.

[0005] The present invention has been made in consideration of the above circumstances, and aims to provide a materials testing machine that can determine appropriate control parameters when the characteristics of a test specimen are nonlinear. [Means for solving the problem]

[0006] The material testing machine of the present invention is a material testing machine that has an actuator, applies a test force to a specimen by the actuator, and performs a material test by deforming the specimen, and is equipped with a stiffness calculation unit that calculates the maximum stiffness value of the specimen when the test force changes nonlinearly with the displacement of the actuator, and a control determination unit that determines the value of a control parameter that specifies the operation of the actuator based on the maximum stiffness value. [Effects of the Invention]

[0007] The material testing machine according to the present invention determines the value of the control parameter that defines the operation of the actuator based on the maximum stiffness value of the test piece, and therefore can determine the appropriate control parameter. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram showing an example of the configuration of a fatigue testing machine according to the present embodiment. [Figure 2] 1 is a graph showing an example of the relationship between the displacement of a test specimen and the test force. [Figure 3] FIG. 2 is a diagram illustrating an example of the configuration of a control device. [Figure 4] 10 is a graph showing an example of a change in stiffness of a test specimen. [Figure 5] 1 is a graph showing an example of the relationship between the displacement of a test specimen and the test force. [Figure 6] 10 is a table showing an example of control parameters. [Figure 7] 5 is a flowchart illustrating an example of processing of the control device according to the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, this embodiment will be described with reference to the drawings.

[0010] FIG. 1 is a diagram showing an example of the configuration of a fatigue testing machine 1 according to this embodiment. The fatigue testing machine 1 performs a fatigue test on a specimen SP. The fatigue testing machine 1 repeatedly applies a test force FP to the specimen SP, for example. The test force FP and the number of repetitions are set in advance. The number of repetitions is, for example, 10 2 times ~ 10 8 times. As shown in FIG. 1, the fatigue testing machine 1 includes a testing machine main body 10 and a control unit 20. The testing machine main body 10 carries out a fatigue test on the specimen SP in accordance with instructions from the control unit 20. The control unit 20 controls the operation of the testing machine main body 10. The fatigue testing machine 1 corresponds to an example of a "material testing machine."

[0011] As shown in FIG. 1, a testing machine main body 10 is configured by forming a load frame on a base 11 with a pair of support columns 12a, 12b and a yoke 13, and by fixing a crosshead 14 to the support columns 12a, 12b.

[0012] A hydraulic actuator 15 is disposed on the base 11, and a lower jig 16a for fixing the lower end of the specimen SP is attached to a piston rod 15a of the hydraulic actuator 15. An upper jig 16b for fixing the upper end of the specimen SP is attached to the crosshead 14 via a load cell 17. The load cell 17 detects the test force FP acting on the specimen SP.

[0013] The hydraulic actuator 15 extends and retracts its piston rod 15a by controlling the direction and amount of pressure oil by a servo valve 18. As a result, a test force FP is applied to a specimen SP fixed between an upper jig 16b and a lower jig 16a. The stroke of the hydraulic actuator 15, i.e., the displacement X of the specimen SP, is detected by a differential transformer 19 attached to the hydraulic actuator 15. The hydraulic actuator 15 corresponds to an example of an "actuator."

[0014] The control unit 20 includes a signal input / output device 21A, a control device 21B, and an input / output device 22. The signal input / output device 21A is disposed between the testing machine main body 10 and the control device 21B, and processes various signals input to and output from the control device 21B. The signal input / output device 21A includes a first amplifier 23, a first A / D converter 24, a second amplifier 27, a second A / D converter 28, a third amplifier 25, and a D / A converter 26.

[0015] The first amplifier 23 amplifies the test force signal SG1 output from the load cell 17 and outputs the amplified signal to the first A / D converter 24. The first A / D converter 24 A / D converts the test force signal amplified by the first amplifier 23 to generate test force information FD, and outputs the generated test force information FD to the control device 21 B. The test force information FD indicates the test force FP applied to the specimen SP. The second amplifier 27 amplifies the displacement signal SG2 output from the differential transformer 19 and outputs the amplified signal to the second A / D converter . The second A / D converter 28 A / D converts the displacement signal amplified by the second amplifier 27 to generate displacement information XD, and outputs the generated displacement information XD to the control device 21B. The displacement information XD indicates the displacement X of the hydraulic actuator 15.

[0016] The control device 21B generates command information dX based on the test force information FD input from the first A / D converter 24 and the displacement information XD input from the second A / D converter 28, and outputs the command information dX to the D / A converter 26. The D / A converter 26 performs D / A conversion on the command information dX input from the control device 21B to generate a command signal, and outputs the generated command signal to the third amplifier 25. The third amplifier 25 amplifies the control signal output from the D / A converter 26 and outputs the amplified control signal SG3 to the servo valve 18. The servo valve 18 controls the direction and amount of pressure oil supplied to the hydraulic actuator 15 in accordance with a control signal SG3 output from the third amplifier 25.

[0017] The input / output device 22 includes, for example, an input unit that inputs the minimum value XN of the displacement X of the hydraulic actuator 15, the maximum value XM of the displacement X, the frequency of the displacement X, etc. to the control device 21B, and an output unit that outputs the load-displacement characteristics of the specimen SP, etc. as test results.

[0018] The fatigue test performed by the fatigue testing machine 1 is continuously performed until the number of repetitions reaches, for example, 10. 2 times ~ 10 8 One repetition corresponds to one period W of the change in displacement X. In other words, the fatigue testing machine 1 continuously executes the fatigue test for a time period equal to the number of repetitions of one cycle W of the change in displacement X. However, if it is determined that the specimen SP has been destroyed or broken, the fatigue testing machine 1 terminates the fatigue test.

[0019] 2 is a graph showing an example of the relationship between the displacement X of the specimen SP and the test force FP. The specimen SP is made of, for example, plastic, and more specifically, epoxy resin. 2 represents the displacement X, and the vertical axis of Fig. 2 represents the test force FP. Graph G1 shows an example of the relationship between the displacement X and the test force FP. The displacement X is the displacement X of the hydraulic actuator 15.

[0020] Graph G1 includes graph G11 and graph G12. Graph G11 shows an example of the relationship between the displacement X and the test force FP when the displacement X increases. Graph G12 shows an example of the relationship between the displacement X and the test force FP when the displacement X decreases. The slope of the graph G1 indicates the stiffness value RG of the specimen SP. The stiffness value RG is calculated using the following formula (1). RG=ΔFP / ΔX (1) That is, the stiffness value RG is determined from the amount of change ΔFP in the test force FP when the displacement X increases by a small displacement ΔX. The small displacement ΔX is, for example, 0.01 mm.

[0021] As shown in graph G1, the stiffness value RG of the specimen SP increases as the displacement X increases. Also, as shown in Fig. 2, since graph G1 is a downward convex curve, the stiffness value RG of the specimen SP is nonlinear. In other words, in Fig. 2, the test force FP changes nonlinearly with respect to the displacement X of the hydraulic actuator 15.

[0022] FIG. 3 is a diagram showing an example of the configuration of the control device 21B. The control device 21B causes the testing machine main body 10 to carry out a fatigue test. The control device 21B includes a computer having a storage device such as an HDD (Hard Disk Drive) or an SSD (Solid State Drive), interface circuits with the signal input / output device 21A and the input / output device 22, and various electronic circuits.

[0023] The control device 21B is configured by, for example, a personal computer, and controls the operation of the test machine main body 10. The control device 21B includes a processor 21C and a memory 21D. The processor 21C is configured with a CPU (Central Processing Unit), an MPU (Micro-Processing Unit), and the like. The memory 21D is configured with a ROM (Read Only Memory), a RAM (Random Access Memory), etc. The memory 21D stores the control program PGM.

[0024] The control device 21B is not limited to a personal computer, and may be configured by one or more appropriate circuits such as an integrated circuit such as an IC chip or an LSI. The control device 21B may also be configured by, for example, a tablet terminal or a smartphone. The control device 21B may also include programmed hardware such as a DSP (Digital Signal Processor) or an FPGA (Field Programmable Gate Array). The control device 21B may also include a SoC (System-on-a-Chip)-FPGA.

[0025] As shown in FIG. 3, the control device 21B includes an acquisition unit 211, a stiffness calculation unit 212, a determination unit 213, a control decision unit 214, a test execution unit 215, a measurement value storage unit 216, and a parameter storage unit 217. Specifically, processor 21C of control device 21B executes control program PGM to function as an acquisition unit 211, a stiffness calculation unit 212, a determination unit 213, a control decision unit 214, and a test execution unit 215. Furthermore, processor 21C of control device 21B executes control program PGM to cause memory 21D to function as a measurement value storage unit 216 and a parameter storage unit 217.

[0026] The measurement value storage unit 216 stores the test force information FD, displacement information XD, and time T acquired by the acquisition unit 211 in association with each other. The time T is the time elapsed since the start of the fatigue test. The fatigue test is instructed to the testing machine main body 10 by the test execution unit 215. The testing machine main body 10 executes the fatigue test in accordance with the instruction from the test execution unit 215. As will be described with reference to FIG. 4, the acquisition unit 211 acquires the test force information FD and displacement information XD, for example, every 1 msec.

[0027] The parameter storage unit 217 stores the control parameters determined by the control determination unit 214. The control parameters are read by the test execution unit 215. In this embodiment, the test execution unit 215 controls the command information dX by PID control, where the control parameters include a proportional gain PG, an integral gain IG, and a derivative gain DG.

[0028] The acquiring unit 211 acquires the displacement X and the test force FP every predetermined time ΔT. Specifically, the acquiring unit 211 acquires the displacement information XD and the test force information FD from the signal input / output device 21A every predetermined time ΔT. The predetermined time ΔT is, for example, 1 msec. In other words, the predetermined time ΔT indicates the time interval between two adjacent measurement points. The measurement points indicate the timing at which the acquiring unit 211 acquires the displacement X and the test force FP. The acquiring unit 211 also acquires, for example, a time T that is the elapsed time from the start of the fatigue test from the test executing unit 215. The acquiring unit 211 may also acquire, for example, the time T that is the elapsed time from the first acquisition of the displacement information XD and the test force information FD from the signal input / output device 21A. Furthermore, the acquiring unit 211 stores the acquired test force information FD, displacement information XD, and time T in the measurement value storage unit 216 in association with each other.

[0029] In this embodiment, the predetermined time ΔT is 1 msec, but is not limited to this. The longer the predetermined time ΔT, the simpler the processing of the control device 21B. The shorter the predetermined time ΔT, the more accurately the stiffness value RG can be obtained.

[0030] The stiffness calculation unit 212 divides into a plurality of sections the range between the minimum value of the displacement X in the fatigue test and the maximum value of the displacement X. Then, the stiffness calculation unit 212 calculates the stiffness value RG in each of the plurality of sections, and obtains the maximum value MR of the stiffness value RG. In this embodiment, when the acquisition unit 211 acquires, for example, displacement information XD and test force information FD at time TA, the stiffness calculation unit 212 calculates the stiffness value RG corresponding to time TA based on 100 measurement points before and after time TA. The 100 measurement points before and after time TA are composed of, for example, 50 measurement points before time TA, 49 measurement points after time TA, and a measurement point at time TA. One measurement point corresponds to the timing at which the acquisition unit 211 acquires the displacement X and the test force FP. For example, the stiffness calculation unit 212 calculates an approximate straight line by the least squares method for 100 measurement points before and after the time TA, with the horizontal axis representing the displacement X and the vertical axis representing the test force FP. Then, the stiffness calculation unit 212 obtains the slope of the approximate straight line as the stiffness value RG. The "100" in the 100 measurement points corresponds to an example of a "predetermined number."

[0031] In this embodiment, a case where the stiffness value RG is calculated using 100 measurement points before and after the time TA will be described, but the present invention is not limited to this. The fewer the number of measurement points, the simpler the processing of the control device 21B. The more the number of measurement points, the more accurately the stiffness value RG can be calculated.

[0032] The 100 measurement points before and after time TA correspond to an example of a “section.” Note that since the predetermined time ΔT is, for example, 1 msec, the measurement time of the 100 measurement points before and after time TA, i.e., the length of the section, is, for example, 0.1 sec (= 1 msec × 100). 100 points corresponds to an example of a "predetermined number." In this embodiment, the stiffness calculation unit 212 increases the time TA by a predetermined time ΔT, and calculates the stiffness value RG corresponding to the time TA every time the time TA is increased by the predetermined time ΔT. That is, the stiffness calculation unit 212 shifts the "section" by the predetermined time ΔT to calculate the stiffness value RG. In other words, in this embodiment, the stiffness calculation unit 212 calculates the stiffness value RG every 1 msec. The stiffness value RG will be further explained with reference to FIG.

[0033] In this embodiment, the stiffness value RG is calculated every predetermined time ΔT, but this is not limiting. The longer the time interval for calculating the stiffness value RG, the simpler the processing of the control device 21B. The shorter the time interval for calculating the stiffness value RG, the more accurately the change in the stiffness value RG can be calculated.

[0034] The determination unit 213 determines whether the test force FP of the specimen SP changes nonlinearly with respect to the displacement X. The determining unit 213 determines whether or not the test force FP of the specimen SP changes nonlinearly with respect to the displacement X, based on, for example, the first stiffness value RG1 and the second stiffness value RG2. The first stiffness value RG1 is the stiffness value RG in the section including the maximum value XM of the displacement X. The first stiffness value RG1 coincides with the maximum value MR of the stiffness value RG. The second stiffness value RG2 is the stiffness value RG in the section including the minimum value XN of the displacement X. Specifically, when the ratio of the first stiffness value RG1 to the second stiffness value RG2 is equal to or greater than a predetermined threshold, the determination unit 213 determines that the test force FP of the specimen SP changes nonlinearly with respect to the displacement X. The predetermined threshold is, for example, 1.1. The first stiffness value RG1 and the second stiffness value RG2 will be further described with reference to FIG.

[0035] Based on the maximum value MR of the stiffness value RG, the control determination unit 214 determines the value of the control parameter that defines the operation of the hydraulic actuator 15. The maximum value MR of the stiffness value RG coincides with the first stiffness value RG1. In this embodiment, the test execution unit 215 controls the command information dX by PID control, where the control parameters include a proportional gain PG, an integral gain IG, and a derivative gain DG.

[0036] The test execution unit 215 executes a fatigue test on the specimen SP. The test execution unit 215 controls the command information dX by PID control. The test execution unit 215 executes a preliminary test for determining the values ​​of the control parameters and a fatigue test on the specimen SP. In the preliminary test, for example, a simulated fatigue test is performed using a mock specimen having characteristics equivalent to those of the specimen SP, with the same amplitude and frequency as those of the fatigue test. For convenience, the case where the mock specimen is the same as the specimen SP will be described here. However, in fatigue tests, the number of repetitions is, e.g., 10 2 times ~ 10 8In the preliminary test, the control parameters are set to safe values ​​that will not cause unstable control, whereas in the simulated fatigue test, the number of repetitions is, for example, 5.

[0037] 3 are executed during the preliminary test. Note that at least one of the processes of the stiffness calculation unit 212, the determination unit 213, and the control determination unit 214 may be executed after the preliminary test is completed.

[0038] 4 is a graph showing an example of changes in the stiffness value RG of the specimen SP, where the graph shown in FIG. 4 is generated based on measurement points in a preliminary test executed by the test execution unit 215. The horizontal axis of FIG. 4 represents time T, the right vertical axis of FIG. 4 represents test force FP, and the left vertical axis of FIG. 4 represents stiffness value RG. Graph G21 shows the change in test force FP. Graph G22 shows the change in stiffness value RG. Graph G23 shows the average value of stiffness value RG.

[0039] The period W is the period of the simulated fatigue test in the preliminary test. The period W is, for example, 2 seconds. In other words, the frequency of the simulated fatigue test is 0.5 Hz.

[0040] The stiffness value RG shown in graph G22 is calculated by the stiffness calculation unit 212. The stiffness value RG increases with an increase in the test force FP shown in graph G21, and decreases with a decrease in the test force FP shown in graph G21. However, because the specimen SP in FIG. 4 has nonlinear characteristics, the change in the stiffness value RG shown in graph G22 is not a sine wave but rather exhibits a complex waveform. However, the waveforms showing the change in stiffness value RG for each of the four periods shown in Fig. 4 are generally consistent. In other words, as shown in Fig. 5, the relationship between displacement X and test force FP varies substantially along a single curve, as shown in graph G31. Note that the curve shown in graph G31 in Fig. 5 is a downwardly convex curve, similar to the curve shown in Fig. 2. Moreover, as shown in graph G22, the maximum value MR of the stiffness value RG is approximately 85 kN.

[0041] The average value of the stiffness values ​​RG shown in graph G23 indicates the average value up to that point of time of the stiffness values ​​RG calculated by the stiffness calculation unit 212. For example, the average value of the stiffness values ​​RG when time T is 2 seconds indicates the average value of 2000 stiffness values ​​RG calculated by the stiffness calculation unit 212 from time T 0 seconds to 2 seconds. As shown in graph G23, the average value of the stiffness values ​​RG converges to approximately 70 kN / mm.

[0042] 5 is a graph showing an example of the relationship between the displacement X of the specimen SP and the test force FP. The graph shown in FIG. 5 is generated based on measurement points in a preliminary test executed by the test execution unit 215. The horizontal axis of FIG. 5 represents the displacement X, and the vertical axis of FIG. 5 represents the test force FP. Graph G31 shows the relationship between the displacement X and the test force FP. The curve shown in graph G31 is a downward convex curve, similar to the curve shown in FIG. As shown in graph G31, the minimum value XN of the displacement X is 0.10 mm, and the maximum value XM of the displacement X is 0.20 mm. Also, as shown in graph G31, the minimum value of the test force FP is 1500 N, and the maximum value of the test force FP is 8500 N.

[0043] Graph G32 represents the tangent to graph G31 at point PM where displacement X is maximum. The slope of graph G32 represents the first stiffness value RG1 at point PM. Graph G33 represents the tangent to graph G31 at point PN where displacement X is minimum. The slope of graph G33 represents the second stiffness value RG2 at point PM. In FIG. 5, the ratio of the first stiffness value RG1 to the second stiffness value RG2 is approximately 1.4, which is greater than or equal to the predetermined threshold value (=1.1), so the judgment unit 213 judges that the test force FP of the specimen SP changes nonlinearly with respect to the displacement X.

[0044] Graph G31 shown in Fig. 5 may be displayed on a display such as an LCD (Liquid Crystal Display) or printed out by a printer. In this case, the user can easily confirm that the test force FP of the specimen SP changes nonlinearly with respect to the displacement X. Furthermore, if the user knows in advance whether the specimen SP is nonlinear, the user can infer a jig defect or an installation problem by displaying or printing graph G31 shown in Fig. 5. For example, when the specimen SP does not have nonlinearity, if a result is obtained indicating that the specimen SP has nonlinearity, as in graph G31 shown in Fig. 5, the user can infer a jig defect or an installation problem.

[0045] FIG. 6 is a table showing an example of control parameters. As described with reference to Fig. 4, the average value of the stiffness value RG is approximately 70 kN / mm. When the stiffness value RG is 70 kN / mm, the control parameters are, for example, as shown in Fig. 6, with the proportional gain PG set to "255", the integral gain IG set to "5", the derivative gain DG set to "0.1", and the filter coefficient N set to "100".

[0046] The proportional gain PG, integral gain IG, differential gain DG, and filter coefficient N that constitute the control parameters are expressed by the following equation (2).

number

[0047] Furthermore, as shown in graph G22 of FIG. 4, when the maximum value of the stiffness value RG is 85 kN / mm, the control parameters are, for example, as shown in FIG. 6, with the proportional gain PG set to "215", the integral gain IG set to "5", the differential gain DG set to "0.1", and the filter coefficient N set to "100".

[0048] That is, for example, when the control parameters are set based on the average stiffness value RG, the proportional gain PG is set to an excessively large value compared to when the control parameters are set based on the maximum stiffness value MR of RG, which can result in unstable control and an overload on the specimen SP.

[0049] By setting the control parameters based on the maximum value MR of the stiffness value RG, it is possible to determine appropriate control parameters, which can prevent the control from becoming unstable and the specimen SP from being overloaded.

[0050] Furthermore, when an operator sets safe control parameters, they are forced to conduct tests in a state of poor responsiveness, but by setting the control parameters based on the maximum value MR of the stiffness value RG, appropriate control parameters can be determined, allowing tests to be conducted in a state of good responsiveness.

[0051] FIG. 7 is a flowchart showing an example of processing by the control device 21B according to this embodiment. First, in step S101, the acquisition unit 211 acquires the displacement X and the test force FP every predetermined time ΔT, which is, for example, 1 msec. Next, in step S103, the stiffness calculation unit 212 calculates the stiffness value RG. When the acquisition unit 211 acquires, for example, displacement information XD and test force information FD at time TA, the stiffness calculation unit 212 calculates the stiffness value RG corresponding to time TA based on 100 measurement points before and after time TA. Specifically, the stiffness calculation unit 212 calculates an approximate straight line by the least squares method for the 100 measurement points before and after time TA, with displacement X as the horizontal axis and test force FP as the vertical axis. Then, the stiffness calculation unit 212 determines the slope of the approximate straight line as the stiffness value RG.

[0052] Next, in step S105, the determination unit 213 calculates a first stiffness value RG1. The first stiffness value RG1 is the stiffness value RG in a section that includes the maximum value XM of the displacement X. In this embodiment, the section corresponds to 100 measurement points. Next, in step S107, the determination unit 213 calculates a second stiffness value RG2. The second stiffness value RG2 is the stiffness value RG in the section including the minimum value XN of the displacement X. Next, in step S109, the determination unit 213 determines whether the ratio of the first stiffness value RG1 to the second stiffness value RG2 (=RG1 / RG2) is equal to or greater than a predetermined threshold value, for example, 1.1.

[0053] If the determination unit 213 determines that the ratio of the first stiffness value RG1 to the second stiffness value RG2 is not equal to or greater than the predetermined threshold (step S109; NO), the determination unit 213 determines that the test force FP of the specimen SP does not change nonlinearly with respect to the displacement X, and then the processing ends. If the determination unit 213 determines that the ratio of the first stiffness value RG1 to the second stiffness value RG2 is equal to or greater than the predetermined threshold (step S109; YES), the processing proceeds to step S111. Then, in step S111, the determining unit 213 determines that the test force FP of the specimen SP changes nonlinearly with respect to the displacement X.

[0054] Next, in step S113, the control determination unit 214 acquires the maximum value MR of the stiffness value RG. In this embodiment, the maximum value MR of the stiffness value RG matches the first stiffness value RG1. Next, in step S115, the control determination unit 214 determines the values ​​of the control parameters that define the operation of the hydraulic actuator 15 based on the maximum value MR of the stiffness value RG. Then, the processing ends. In this embodiment, the test execution unit 215 controls the operation of the hydraulic actuator 15 by PID control. Therefore, the control parameters include a proportional gain PG, an integral gain IG, and a derivative gain DG. The proportional gain PG, the integral gain IG, and the derivative gain DG are defined by the above formula (2).

[0055] As described with reference to FIGS. 3 to 7, the stiffness calculation unit 212 divides the range between the minimum value of the displacement X and the maximum value of the displacement X in the fatigue test into a plurality of intervals. Then, the stiffness calculation unit 212 calculates the stiffness value RG in each of the plurality of intervals, and obtains the maximum value MR of the stiffness value RG. Therefore, by appropriately setting the intervals, the maximum value MR of the stiffness value RG can be appropriately calculated. In this embodiment, when the acquisition unit 211 acquires, for example, displacement information XD and test force information FD at time TA, the stiffness calculation unit 212 calculates the stiffness value RG corresponding to time TA based on 100 measurement points before and after time TA. The 100 measurement points before and after time TA correspond to an example of an "interval." By setting the interval as in this embodiment, the maximum value MR of the stiffness value RG can be calculated appropriately.

[0056] Furthermore, the stiffness calculation unit 212 calculates the stiffness value RG from the displacement X and test force FP corresponding to each of a predetermined number (100) of measurement points, and therefore can calculate the stiffness value RG appropriately. In this embodiment, the stiffness calculation unit 212 calculates an approximate straight line by the least squares method for 100 measurement points before and after the time TA, for example, with the displacement X as the horizontal axis and the test force FP as the vertical axis. Then, the stiffness calculation unit 212 determines the slope of the approximate straight line as the stiffness value RG. Therefore, the stiffness value RG can be calculated appropriately.

[0057] Furthermore, the determination unit 213 determines whether the test force FP of the specimen SP changes nonlinearly with respect to the displacement X, based on the first stiffness value RG1 and the second stiffness value RG2. The first stiffness value RG1 is the stiffness value RG in the section including the maximum value XM of the displacement X. The second stiffness value RG2 is the stiffness value RG in the section including the minimum value XN of the displacement X. Therefore, it can be properly determined whether the test force FP of the specimen SP changes nonlinearly with respect to the displacement X.

[0058] Furthermore, when the ratio of the first stiffness value RG1 to the second stiffness value RG2 (=RG1 / RG2) is equal to or greater than a predetermined threshold, the determination unit 213 determines that the test force FP of the specimen SP changes nonlinearly with respect to the displacement X. Therefore, by setting the predetermined threshold to an appropriate value, it is possible to properly determine whether the test force FP of the specimen SP changes nonlinearly with respect to the displacement X.

[0059] It will be understood by those skilled in the art that the above-described embodiment is a specific example of the following aspects.

[0060] (Section 1) The material testing machine according to the first aspect is a material testing machine that includes an actuator, applies a test force to a specimen by the actuator, and performs a material test by deforming the specimen. The material testing machine includes a stiffness calculation unit that calculates the maximum stiffness value of the specimen when the test force changes nonlinearly with respect to the displacement of the actuator, and a control determination unit that determines the value of a control parameter that specifies the operation of the actuator based on the maximum stiffness value.

[0061] According to the material testing machine described in paragraph 1, when the test force changes nonlinearly with respect to the displacement of the actuator, the value of the control parameter that defines the operation of the actuator is determined based on the maximum stiffness value. Therefore, when the test force changes nonlinearly with respect to the displacement of the actuator, the value of the control parameter can be appropriately determined, thereby preventing the control from becoming unstable and preventing an overload from being applied to the test specimen, and enabling the test to be performed with good responsiveness.

[0062] (Section 2) In the material testing machine described in paragraph 1, the stiffness calculation unit divides the range between the minimum value of the displacement and the maximum value of the displacement in the material test into a plurality of intervals, calculates the stiffness value in each of the plurality of intervals, and finds the maximum stiffness value.

[0063] According to the material testing machine described in paragraph 2, the range between the minimum value of the displacement and the maximum value of the displacement in the material test is divided into a plurality of intervals, the stiffness value in each of the plurality of intervals is calculated, and the maximum stiffness value is found. Therefore, by setting the interval appropriately, the maximum stiffness value can be calculated appropriately. In this embodiment, when the acquisition unit 211 acquires, for example, displacement information XD and test force information FD at time TA, the stiffness calculation unit 212 calculates the stiffness value RG corresponding to time TA based on 100 measurement points before and after time TA. The 100 measurement points before and after time TA correspond to an example of an "interval." By setting the interval as in this embodiment, the maximum value MR of the stiffness value RG can be calculated appropriately.

[0064] (Section 3) The material testing machine described in paragraph 2 is provided with an acquisition unit that acquires the displacements and the test forces at predetermined time intervals, each of the multiple sections corresponds to a period during which the acquisition unit acquires a predetermined number of the displacements and the test forces, and the stiffness calculation unit calculates the stiffness value from the predetermined number of the displacements and the test forces.

[0065] According to the material testing machine described in paragraph 3, the stiffness value is calculated from the predetermined number of displacements and test forces acquired by the acquisition unit. Therefore, the stiffness value can be calculated appropriately. In this embodiment, the stiffness calculation unit 212 calculates an approximate straight line by the least squares method for 100 measurement points before and after the time TA, for example, with the displacement X as the horizontal axis and the test force FP as the vertical axis. Then, the stiffness calculation unit 212 determines the slope of the approximate straight line as the stiffness value RG. Therefore, the stiffness value RG can be calculated appropriately.

[0066] (Section 4) The material testing machine described in paragraph 2 or 3 is provided with a judgment unit that judges whether the test force of the specimen changes nonlinearly with respect to the displacement based on a first stiffness value, which is the stiffness value in the section including the maximum value of the displacement, and a second stiffness value, which is the stiffness value in the section including the minimum value of the displacement.

[0067] According to the material testing machine described in paragraph 4, it is determined whether the test force of the specimen changes nonlinearly with respect to the displacement based on the first stiffness value, which is the stiffness value in the section including the maximum value of the displacement, and the second stiffness value, which is the stiffness value in the section including the minimum value of the displacement. Therefore, with a simple process, it is possible to properly determine whether or not the test force of the specimen changes nonlinearly with respect to the displacement. In this embodiment, as described with reference to Fig. 4, the first stiffness value RG1 corresponding to the slope of graph G32 is larger than the second stiffness value RG2 corresponding to the slope of graph G33. Therefore, as shown in graph G31, the curve showing the relationship between the displacement X and the test force FP is a downward convex curve. Therefore, it is possible to properly determine whether the test force of the test specimen changes nonlinearly with respect to the displacement.

[0068] (Section 5) In the material testing machine described in paragraph 4, the judgment unit judges that the test force of the specimen changes nonlinearly with respect to the displacement when the ratio of the first stiffness value to the second stiffness value is greater than or equal to a predetermined threshold value.

[0069] According to the material testing machine described in paragraph 5, when the ratio of the first stiffness value to the second stiffness value is equal to or greater than a predetermined threshold value, it is determined that the test force of the specimen changes nonlinearly with respect to the displacement. Therefore, with a simple process, it is possible to properly determine whether the test force of the specimen changes nonlinearly with respect to the displacement. In this embodiment, as described with reference to FIG. 4, the ratio of the first stiffness value RG1 to the second stiffness value RG2 (=RG1 / RG2) is approximately 1.4, which is greater than or equal to the predetermined threshold value (=1.1), so the judgment unit 213 judges that the test force FP of the specimen SP changes nonlinearly with respect to the displacement X.

[0070] The fatigue testing machine 1 according to this embodiment is merely an example of the form of the "material testing machine" according to the present invention, and can be modified and applied as desired within the scope of the gist of the present invention. For example, in this embodiment, the "material testing machine" will be described as a fatigue testing machine 1, but is not limited to this. The "material testing machine" may be equipped with an actuator, and the actuator may apply a test force to a specimen to deform the specimen and perform a material test. The "material testing machine" may be, for example, a tensile testing machine, a compression testing machine, a bending testing machine, a torsion testing machine, a creep testing machine, an impact testing machine, or the like.

[0071] In this embodiment, the "actuator" is a hydraulic actuator 15, but is not limited to this. The "actuator" may be, for example, an electric actuator, that is, a motor.

[0072] In the present embodiment, the case where the test execution unit 215 controls the hydraulic actuator 15 by PID control has been described, but the present invention is not limited to this. The test execution unit 215 may also control the hydraulic actuator 15 by, for example, PI control or PD control.

[0073] 1 and 3 show functional configurations, and the specific implementation form is not particularly limited. In other words, it is not necessary to implement hardware corresponding to each functional unit individually, and it is of course possible to implement a configuration in which a single processor executes a program to realize the functions of multiple functional units. Furthermore, some of the functions realized by software in the above embodiments may be realized by hardware, or some of the functions realized by hardware may be realized by software.

[0074] 7 are divided according to the main processing content to make it easier to understand the processing of the control device 21B. There is no limitation to the division method or names of the processing units shown in the flowchart of FIG. 7, and the processing can be divided into more processing units according to the processing content, or one processing unit can be divided so as to include more processes. Furthermore, the processing order of the above flowchart is not limited to the example shown in the figure.

[0075] The control device 21B of the fatigue testing machine 1 causes the processor 21C to execute a control program PGM. This control program PGM can also be recorded on a computer-readable recording medium. The recording medium can be a magnetic or optical recording medium or a semiconductor memory device. Specifically, examples include portable or fixed recording media such as flexible disks, HDDs, CD-ROMs (Compact Disk Read Only Memory), DVDs, Blu-ray (registered trademark) discs, magneto-optical disks, flash memories, and card-type recording media. The recording medium may be a non-volatile storage device such as a RAM, a ROM, or a HDD, which is an internal storage device provided in the control device 21B. The control program PGM may be stored in a server device or the like, and the control program PGM may be downloaded from the server device to the control device 21B. [Explanation of symbols]

[0076] 1. Fatigue testing machine (material testing machine) 10 Testing machine body 14 Crosshead 15 Hydraulic actuator (actuator) 15a Piston rod 16a Lower jig 16b Upper jig 17 Load Cell 18 Servo valve 19 Differential transformer 20 Control Unit 21A signal input / output device 21B Control device 21C processor 21D Memory 211 Acquisition Department 212 Rigidity calculation section 213 Judgment section 214 Control Decision Unit 215 Testing Department 216 Measurement value memory unit 217 Parameter storage section PGM control program PG Proportional Gain IG Integral Gain DG Differential Gain N filter coefficients FP test force FD test force information RG stiffness value RG1 First stiffness value RG2 Second stiffness value MR maximum value SG1 Test force signal SG2 Displacement signal SG3 control signal SP specimen T, TA time W period X Displacement XD Displacement Information XM Maximum XN Minimum dX command information ΔFP change ΔT Predetermined time ΔX small displacement

Claims

1. A material testing machine comprising an actuator, the actuator applying a test force to a specimen to deform the specimen and perform a material test, a stiffness calculation unit that calculates a maximum stiffness value of the specimen when the test force changes nonlinearly with respect to the displacement of the actuator; a control determination unit that determines a value of a control parameter that defines an operation of the actuator based on the maximum stiffness value; Equipped with The stiffness calculation unit Dividing a range between the minimum value of the displacement and the maximum value of the displacement in the material test into a plurality of sections; Calculating the stiffness value in each of the plurality of sections; The maximum value of the stiffness value is determined, a determination unit that determines whether the test force of the specimen changes nonlinearly with respect to the displacement based on a first stiffness value that is the stiffness value in the section including the maximum value of the displacement and a second stiffness value that is the stiffness value in the section including the minimum value of the displacement, Material testing machine.

2. The determination unit determines that the test force of the specimen changes nonlinearly with respect to the displacement when a ratio of the first stiffness value to the second stiffness value is equal to or greater than a predetermined threshold.

2. The material testing machine according to claim 1.

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