JP4033119B2 - Material testing method, material testing machine - Google Patents

Description

  The present invention relates to a test method for measuring material properties by applying a load to a test piece, and more particularly to a technique suitable for calculating material properties in a high-speed tensile test in which a tensile load is applied at a high speed.

  In general, a tensile test is a method in which a tensile load is applied to a test piece, and the elongation and load of the test piece are measured every moment to examine the mechanical properties of the material. A tensile testing machine for performing a tensile test usually holds both ends of a test piece with a jig and pulls the test piece by separating one jig from the other. The tensile load acting on the test piece is measured by a load cell attached to the jig, and the elongation of the test piece is measured by a displacement meter attached to the test piece.

  By the way, in recent years, a high-speed tensile test in which a tensile load is applied to a test piece at a high speed has been performed. This is because there is an increasing need to accurately evaluate characteristics and the like when a material is deformed at high speed in the event of a collision with an automobile or the like and to design with high accuracy. For example, in a hydraulic servo type high-speed tensile testing machine, an actuator is accelerated from a stationary state to a high-speed range using a running space, and then a test piece is pulled at a constant speed within the high-speed range. For example, it is often pulled at a speed of 1 to 20 m / s.

JP-A-10-318894

  However, in the high-speed tensile test, there is a problem that vibration is generated in the entire machine due to inertial force / impact of an actuator or a jig, and a vibration phenomenon appears in the output of the load cell, resulting in deterioration of load measurement accuracy.

  Therefore, a technique has been proposed in which a strain gauge is directly attached to a test piece and a load is measured by the strain gauge. However, since the strain gauge itself does not have a function to measure the load, a preliminary test is performed, the conversion coefficient between the strain gauge's strain output and the load output of the load cell is calculated, and then the load is loaded by the strain gauge in the actual tensile test. Was to measure. Therefore, about twice as much time is required for the measurement, there is a problem that the measurement efficiency is poor, and there is a problem that an error occurs if there is a difference in the way of attaching the strain gauge between the preliminary test and the main test.

(1) A first aspect of the present invention is a test method for measuring a material property by applying a load to a test piece, and a load measuring means for detecting a force acting on the one test piece while applying the load to the one test piece. The time change characteristics of load output due to the strain and the time change characteristics of strain output by the strain gauge attached to the test piece, and the time change characteristic of load output in the plastic region obtained when a load is applied to one test piece The conversion coefficient is calculated based on the relationship between the strain output and the time variation characteristic of the strain output, and the strain value of the time variation characteristic of the strain output obtained when a load is applied to one test piece using the conversion coefficient is the actual load. It is characterized in that the time change characteristic of the actual load acting on one test piece is calculated.
(2) According to the invention of claim 2, in the test method of claim 1, the moving average load F (t) at a specific time t in the plastic region of the time change characteristic of the load output is calculated and the time change characteristic of the strain output is calculated. The moving average strain ε (t) at a specific time t in the plastic region is calculated, and the spring constant k as a conversion coefficient is calculated from the moving average load F (t) and the moving average strain ε (t) as F (t) = k. It is calculated from the relationship of xε (t), and the strain value in the time change characteristic of the strain output is converted into the actual load using the spring constant k.
(3) According to the invention of claim 3, in the test method of claim 2, in place of the moving average load F (t) and the moving average strain ε (t), leveling is performed by performing Fourier transform to remove high frequency components. The average load F ′ (t) and the average strain ε ′ (t) are used.
(4) The invention of claim 4 is a material testing machine for measuring a material property by applying a load to a test piece, a load measuring means for measuring a load acting on the test piece, and a strain gauge attached to the test piece. The strain measurement means for measuring the strain of the test piece by the output, the time change characteristic of the load output by the measurement value of the load measurement means while applying a load to one test piece, and the time of the strain output by the measurement value of the strain measurement means Each change characteristic was acquired, and the conversion coefficient was calculated for one test piece based on the relationship between the time change characteristic of the load output and the time change characteristic of the strain output in the plastic region among the acquired time change characteristics. An actual load computing device that converts the strain value in the time change characteristic of the strain output into an actual load using the conversion coefficient, and calculates the time change characteristic of the actual load in one test piece.

  According to the present invention, it is possible to calculate the load acting on the test piece from the output value of the strain gauge without requiring a preliminary test, and it is possible to increase the measurement efficiency.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the best embodiment of the present invention will be described in detail with reference to the drawings.
FIG. 1 shows a hydraulic servo type high-speed tensile testing machine 10 according to the present embodiment. The high-speed tensile testing machine 10 includes an actuator 11, a flow control valve 12, a hydraulic pressure source 13, a load cell 14, a gripping device 15, a displacement detector 16, and the like. The gripping device 15 grips the grips on both ends of the test piece 1 so that the test piece 1 is fixed to the high-speed tensile testing machine 10.

  In particular, the gripping device 15 on the load cell 14 side has a substantially integrated structure with the load cell 14, and the natural frequency of the measurement system including the load cell 14 and the gripping device 15 is set high. As a result, the load acting on the test piece 1 can be measured with high accuracy.

  The strain gauge 20 is affixed in the vicinity of the end where the test piece 1 is gripped by the gripping device 15, in other words, other than the constricted part where the test piece 1 breaks. The strain gauge 20 can detect the strain of the test piece linearly. In other words, the central constricted portion of the test piece 1 is plastically deformed due to the tensile load and breaks, but the strain gauge 20 is attached to a portion of the end portion of the test piece that is not plastically deformed.

  Furthermore, the high-speed tensile testing machine 10 is provided with a calculation device 17 for calculating the load-strain characteristics by inputting detection signals from the strain gauge 20 and the load cell 14 and controlling the operation of the entire testing machine.

  The arithmetic unit 17 acquires the load signal from the load cell 14 at a predetermined sampling interval and stores the time change characteristic of the load. At the same time, a strain signal from the strain gauge 20 is acquired at a predetermined sampling interval, and the strain time change characteristic is stored. That is, the arithmetic unit 17 simultaneously acquires a load output-time change characteristic and a strain output-time change characteristic from output signals from the strain gauge 20 and the load cell 14 obtained in the process of applying a tensile load to the test piece 1. In addition, the meaning of simultaneous acquisition in this specification means acquiring in a single tensile test process. That is, it is not necessary to perform the preliminary test and the main test separately, and both characteristics are obtained by a single tensile test.

  The calculation device 17 performs calculation processing based on the relationship between the time change characteristics of these loads and the time change characteristics of the strain, converts the strain output in the time change characteristics of the strain into an actual load, and calculates the actual load of the test piece 1. Get time-varying characteristics.

  A monitor 18 is connected to the arithmetic unit 17, and the monitor 18 displays a graph of a load time change characteristic, a strain time change characteristic, or a load-strain characteristic.

Next, a tensile test using the high-speed tensile tester 10 will be described with reference to the flowchart of FIG.
First, in step S <b> 30, a tensile load is applied to the test piece 1 by the actuator 11. At the same time, output signals are obtained from the load cell 14 and the strain gauge 20 in step S32. In step 34, load output-time change characteristics (see FIG. 3A: also referred to as load time change characteristics) and strain output-time change characteristics (see FIG. 3B: also referred to as strain time change characteristics). Generate.

  As shown in FIG. 3A, in the high-speed tensile test, even if the load cell 14 having a high natural frequency is used, vibration is generated in the load output due to the impact of the test or the inertial force of the tensile testing machine 10. It is inevitable that it may occur. In particular, the influence of vibration is large in a region where the slope of the graph in the vicinity of the elastic limit greatly changes, such as in the portion A. On the other hand, as shown in FIG. 3B, the influence of vibration hardly occurs in the strain output.

  Thereafter, in step S36, based on the relationship between the load output-time characteristic and the strain output-time characteristic obtained simultaneously, the strain output in the strain output-time change characteristic is converted into an actual load, and the actual load in the test piece- Calculate time-varying characteristics. In particular, the strain output of the strain output-time change characteristic is converted into an actual load based on the relationship between the plastic regions.

Step S36 will be described more specifically with reference to the flowchart of FIG.
First, in step S361, a moving average load F (t1) at a specific time t1 and a moving average load F (t2) at a specific time t2 in the plastic region B of the load output-time characteristic of FIG. Next, in step S362, similarly, moving average strains ε (t1) and ε (t2) at specific times t1 and t2 in the plastic region C of the strain output-time characteristic of FIG. 3B are calculated.

  Note that these moving averages are obtained by subdividing the plastic zones B and C into minute sections and calculating the load output and strain output of each minute section, respectively, and at the specific times t1 and t2, the time before (or the time) It is calculated by obtaining an output average value for a predetermined section of (after). The plastic regions B and C here are preferably portions excluding the region A in the vicinity of the elastic limit.

  Next, in step S363, the spring constant k1 is calculated from the relationship of F (t1) = k1 × ε (t1) from the moving average F (t1) and the moving average strain ε (t1). The spring constant k2 is calculated from the relationship of F (t2) = k2 × ε (t2) from (t2) and the moving average strain ε (t2). Thereafter, in step S364, an average value k of the plurality of spring constants k1 and k2 is calculated. Although the spring constants k1 and k2 for two specific times are calculated here, three or more spring constants may be used, or one spring constant may be used. Further, a plurality of moving averages, for example, F (t1) F and (t2) may be further averaged to obtain one value F (t), and then the spring constant may be obtained.

  In step S365, the vertical axis of the strain output-time characteristic in FIG. 3B, that is, the strain output is converted into an actual load by multiplying the strain output by the spring constant k. This actual load can be assumed to be a load actually applied to the test piece after the influence of the impact vibration of the tensile test is removed.

  That is, in the tensile test apparatus according to this embodiment, based on the relationship between the time change characteristic of the load and the time change characteristic of the strain, the strain of the acquired time change characteristic is converted into an actual load and acts on the test piece. The time change characteristic of the actual load is calculated.

  FIG. 5 shows a load output-time characteristic (H) directly obtained from the load cell 14 in an actual high-speed tensile test, and an actual load-time characteristic (M) obtained by calculation by the tensile test apparatus 10. It can be seen that in the actual load-time characteristic (M), the influence of vibration is eliminated and a stable characteristic is obtained. This actual load-time characteristic is stored in the storage unit of the arithmetic unit 17 and displayed on the display unit 18.

  According to the present tensile test apparatus 10, since high-accuracy actual load-time characteristics can be obtained from data obtained by a single high-speed tensile test, it is not necessary to perform a preliminary test or the like, and the accuracy is simple and efficient. Measurement is possible. In addition, the data of the plastic area of load output and strain output are flattened (smoothed), and the comparison is made in the plastic area (special second-stage plastic area). This is because the strain output behavior is very close to that of the plastic region and the plastic region can be compared with a larger output value than the elastic deformation region, and the comparison accuracy is high. By combining these rationally, a highly accurate conversion result can be obtained synergistically.

  In this embodiment, the moving average value is used when obtaining the spring constant. Instead of this moving average, the moving constant value is obtained by performing a Fourier transform process on the load signal and the strain signal to remove high frequency components. The average load or the like may be used. Moreover, you may combine these. In short, it is sufficient if data (especially load output data) is smoothed to reduce the influence of vibration.

  Further, in the present embodiment, the case where the spring constant is determined using the plastic region data, particularly the plastic region data excluding large fluctuations in the vicinity of the elastic limit, as a preferred form is shown, but the present invention is not limited thereto, Other regions may be used as long as a predetermined relationship is obtained from the load output and strain output measured at the same time and can be used as a conversion coefficient.

  Furthermore, the high-speed tensile testing machine has been described, but if it is a testing machine that evaluates characteristics by loading a material at a speed at which the vibration component appears as noise in the load detection signal, a compression testing machine, a fatigue testing machine, The present invention can be applied to various material testing machines such as a bending testing machine.

Overall configuration diagram showing a tensile testing machine according to the present embodiment Flow chart showing the tensile test process using a tensile tester Conceptual diagram of data obtained by tensile test Flow chart showing detailed process in tensile test The figure which shows the actual load-time characteristic obtained by the tensile test

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Test piece 10 Tensile tester 11 Actuator 12 Flow control valve 13 Hydraulic source 14 Load cell 15 Grasp device 16 Displacement measuring device 17 Arithmetic device 18 Display device 20 Strain gauge

Claims (4)

In a test method for measuring material properties by applying a load to a test piece,
While applying the load to one of the specimen, the acquisition time change characteristic of the load output by the load measuring means for detecting a force acting, and time change characteristic of the distortion output by pasted strain gauges on the specimen to the specimen And
Calculating a conversion factor based on a relationship between time variation characteristic of the distortion output and time variation characteristics of the load output in the acquired plastic zone when given load to the one test piece,
Using the conversion coefficient, the time change of the actual load acting on the one test piece by converting the strain value of the time change characteristic of the strain output acquired when the load is applied to the one test piece into an actual load. A test method characterized by calculating characteristics. The test method of claim 1 ,
The moving average load F (t) at a specific time t in the plastic region of the time change characteristic of the load output is calculated, and the moving average strain ε (t) of the specific time t in the plastic region of the time change characteristic of the strain output. To calculate
From the moving average load F (t) and the moving average strain ε (t), the spring constant k as the conversion coefficient is calculated from the relationship of F (t) = k × ε (t),
A test method characterized by converting a strain value in the time change characteristic of the strain output into an actual load by using the spring constant k. The test method of claim 2 ,
Instead of the moving average load F (t) and moving average strain epsilon (t), the average load F'was leveled by removing high frequency components by performing a Fourier transform (t) and the average strain ε'a (t) A test method characterized by using. In a material testing machine that applies load to a specimen and measures material properties,
Load measuring means for measuring the load acting on the test piece;
And distortion measuring means for measuring the distortion of the test piece by the output of the strain gauge affixed to the test strip,
While applying the load to one of the specimen, the time change characteristic of the load output by the measurement value of the load measuring means, respectively to get the temporal change characteristics of the strain output by measurement of the strain measuring means, obtained both time variation characteristics of, and calculates the time change characteristic and group Hazuki transform coefficients for said one specimen in relation to the time change characteristic of the strain output of the load output in plastic zone, using the conversion coefficients the calculated, the strain output A material tester comprising: an actual load calculation device that converts a strain value in the time change characteristic of the actual load into an actual load and calculates a time change characteristic of the actual load in the one test piece.

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