JPH0361900B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to a material testing machine, and particularly to a material testing machine suitable for conducting a J _1c test according to the unloading compliance method.

(b) Prior art In general, in the J _1c test based on the unloading compliance method, a crack is formed in a test piece in advance as shown in Figure 2, and a tensile load P is applied to this test piece. As shown in Figure 2, approximately 10% of the load is partially unloaded at predetermined intervals, and the load P and load point displacement V at each unloading time are detected, and the stable crack length is determined from the compliance determined by these. Δa is calculated, and the J-integral value at the time of unloading determined by the following formula (1) and its Δa are used to calculate J- as shown in Fig. 4.
A Δa curve (R curve) is formed and the J _1c value is determined.

J=A/B(W-a)・f(a ₀ /W)...(1) Here, A; Area under the load-displacement curve B; Thickness of the test piece W; Width of the test piece a; Crack length (Δa = a-a ₀ ) f (a ₀ /W); axial force correction term When forming the J-Δa curve, it is most important to accurately measure the crack length a determined from compliance. It is.

In a load test as shown in FIG. 3, the test is usually conducted under displacement control in order to continue the test beyond the maximum load point. However, since unloading when determining compliance is about 10%, the displacement at that time is usually less than 2%, and in most cases is about 1%, so unloading by displacement control is problematic.

In other words, when unloading is performed manually or semi-automatically, it is difficult to maintain a constant unloading speed, and the unloading range is less than 10% and 2% of the measurement amplifier range for load and displacement, respectively. However, since the absolute values of load and displacement reach 50% or more, it is impossible to increase the amplitude by 10 times, 50 times, etc. on the recorder, and it is impossible to increase the accuracy of compliance measurement. There is. Therefore, the shape of the obtained J-Δa curve often deviates from the ideal shape, and as a result, the J _1c value often varies.

In order to solve the above-mentioned problems, many conventional test apparatuses are configured to perform data processing and load control using a computer.
In other words, the accuracy of compliance measurement can be improved by increasing the load by 10 times and increasing the displacement by using a partial enlargement device for the unloading section that is automatically operated from a computer.
This is done by measuring at 50x magnification. Here, a 12-bit D-A converter is normally used for setting a target value signal for load control of a test machine by a computer. 12bit D-A
The converter is appropriate for material testing equipment because its resolution is ±0.05% of full scale. However, in order to control the load as shown in Figure 3, the maximum estimate of unloading using displacement control is 2%, so 2%/0.05% = 40, and unloading is performed in 40 steps. You will be killed. 40
When unloading is performed in steps, it is meaningless to collect more than 40 points as data for compliance measurement, and it is difficult to perform highly accurate measurements.
Furthermore, bad data is often mixed near the start point of unloading, so it is common practice to conduct tests with a large amount of unloading. Hysteresis appears, impairing the quality of the data used to determine the crack length.

(c) Purpose The present invention was made in view of the above, and is capable of unloading approximately 5 times more smoothly than conventional devices, and also allows data for compliance measurement of the unloading section to be It is possible to collect approximately 5 times the amount of J _1c , form an ideal J-Δa curve, and obtain J _1c values with good reproducibility.
The purpose is to provide material testing machines for testing.

(d) Structure The present invention is characterized by a means for generating a target value signal for a loading mechanism that applies a load to a test piece, and a means for generating a target value signal to which both a detected load value and a detected load point displacement value are provided. The present invention has a negative feedback loop for feeding back, and is configured such that a control operation signal for the load mechanism is obtained from a difference signal between a target value signal and both detected value signals.

(e) Examples Examples of the present invention will be described below based on the drawings.

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention.

The loading mechanism 1 is composed of an electro-hydraulic test device,
A servo valve converts an electrical input signal into a hydraulic signal to drive a hydraulic actuator. The load P acting on the test piece due to the drive is detected by the load cell 2 and the load amplifier 3, and the load point displacement V of the test piece is detected by the clip gauge 4 and the displacement amplifier 5, respectively. After unnecessary frequency components are removed from each detection signal by filters 6 and 7, the signals are digitized by A-D converter 10 via partial enlargers 8 and 9, and input into computer 11. The partial magnifying devices 8 and 9 are for magnifying the load in the unloading section and the load point displacement detection signal by 10 times and 50 times, respectively, and are automatically operated by a command from the computer 11.

The load detection value P and the load point displacement detection value V are also directly input into the computer 11 via the A-D converter 10, and the load detection signal P and the load point displacement detection signal V are monitored. The target value signal to be given to the load mechanism 1 is determined by the third
The load is switched in the direction of loading or unloading in the manner shown in the figure.

The target value signal output from the computer 11 is converted into an analog signal by the DA converter 12,
It is supplied to the load mechanism 1. Note that this DA converter 12 uses a 12-bit converter similar to the conventional one. A signal R obtained by mixing a load detection signal P and a load point displacement signal V in a one-to-one ratio is fed back to the analog target value signal S. That is, the feedback signal R is formed by adding the load detection signal P and the load point displacement signal V on a one-to-one basis, and multiplying the result by 1/2 in the arithmetic unit 13.

FIG. 5 is a flowchart showing a program for load control and data processing in the computer 11. Based on the load control curves of FIG. 5 and FIG. 3, the operation of the embodiment of the present invention will be described below.

First, prior to the test, initial conditions such as test conditions are input. Based on this test condition, the target value signal S is increased until the detected load value reaches the first unloading start point at point A in FIG. This results in
Both the load detection signal P and the load point displacement signal V increase, and the load mechanism 1 follows them so that the average value R is fed back so that S=R. In this initial state, the rate of change in load appears to be greater than the rate of change in displacement due to the characteristics of the material, and therefore, in this state, the load is mainly applied under load control. Then, after exceeding point B in FIG. 3 where the gradient of load and displacement becomes 1:1, the load is mainly applied by displacement control. Also,
In each unloading section, the rate of change in load is greater than the rate of change in displacement, as in the initial state, and the load is unloaded mainly by load control. Load detection signal P
Switching of the target value signal from the computer 11 to the loading direction or the unloading direction by monitoring the load point displacement detection signal V is performed by monitoring the load point displacement detection signal V up to point B in Figure 3 and the end point of each unloading section. With the supervision of P.
Others are executed by monitoring the load point displacement detection signal V. From the above, after passing point B, the displacement is controlled until reaching the unloading start point C at each unloading section.
After reaching the unloading end point D, control is performed by load control.

Note that sufficient relaxation is performed before each unloading so that the unloading compliance is not affected by the relaxation. In addition, data for compliance measurement is collected via partial magnification devices 8 and 9 during each unloading, and energy is determined by calculating the area under the load-displacement curve during loading and unloading. From these, a J-Δa curve as shown in FIG. 4 is formed, and the J _1c value is automatically determined.

As described above, since the control of the load mechanism at each time of unloading can be regarded as load control, the target value signal can be changed approximately five times or more compared to conventional displacement control, and D -A converter 12 is conventionally 40
If it were a step change, it would represent a change of more than 200 steps.

(F) Effects As explained above, according to the present invention, the control operation signal of the load mechanism that applies a load to the test piece is fed back by mixing the load detection signal and the load point displacement detection signal with the target value signal. Therefore, depending on the characteristics of the material, load control is mainly used in places where the rate of change in load is large, and displacement control is mainly used in places where the rate of change in displacement is large. In other words, the load up to the next unloading point is by displacement control, and the load mechanism is driven by load control for approximately 10% unloading, but this conversion is automatically done depending on the characteristics of the material itself, and J _1c Very convenient for exams. In other words, compared to the control method using displacement in the conventional device, it is possible to provide approximately 5 times the amount of change in the target value in the unloading section;
Unloading control is now twice as smooth, and
Approximately five times as much data for measuring unloading compliance can be collected, a more ideal J-Δa curve can be formed, and a J _1c value with good reproducibility can be obtained.

Note that if the load mechanism is controlled only by load control, it will not be possible to obtain an appropriate load speed unless the rate of change of the target value signal is adjusted in the gentle portion of the load that exceeds point B on the load-displacement curve in Figure 3. Furthermore, if the maximum load point is exceeded, the load decreases even if the displacement is changed, making it uncontrollable. Furthermore, even if you try to artificially switch between this kind of load control and the conventional displacement control described above, it is difficult to perform error-free control that completely eliminates disturbances when switching the feedback signal. This is impossible if we take into account the changes in the pieces over time. According to the present invention, such disturbances do not occur because load control and displacement control are inevitably switched depending on the material properties.

The above effects can be obtained by using the 12-bit D-A converter that is standard equipment in the material testing machine as is, which has great merits.

[Brief explanation of drawings]

Figure 1 is a block diagram showing the configuration of an embodiment of the present invention, Figure 2 is a front view of a test piece used in the J _1c test, Figure 3 is a load-displacement curve diagram of the J 1c test, and Figure 4 is a diagram showing the load-displacement curve of the J _1c test.
The figure is a graph showing the J-Δa curve, and FIG. 5 is a flowchart showing the load control and data acquisition program of the computer 11 in FIG. DESCRIPTION OF SYMBOLS 1... Load mechanism, 2... Load cell, 4... Clip gauge, 8, 9... Partial enlargement device, 10...
...A-D converter, 11...computer, 12...
...D-A converter.

Claims (1)

[Claims] 1 A tensile load is applied to a test piece with a crack formed in advance by a loading mechanism, and the load is repeatedly unloaded at a predetermined rate at a predetermined interval, and the load acting on the test piece and the displacement at the load point are calculated. In a testing device that can detect, calculate the stable crack length of a test piece from the compliance at each unloading time, and determine the J _1c value of the test piece from the calculated value and the J-integral value, the target of the loading mechanism is It has means for generating a value signal, and a negative feedback circuit that feeds back both the load detection value and the load point displacement detection value to the target value signal, and the control operation signal for the load mechanism is based on both of the detection value signals. A material testing machine for _J1c testing, characterized in that it is configured to obtain a difference signal between the target value signal and the target value signal.