JPH085306A - Method of measuring creep strain - Google Patents

Abstract

(57) [Summary] [Purpose] A method for accurately, stably and easily measuring the creep strain of various members including composite materials such as CFRP using a strain gauge. [Structure] After applying a creep load to a strand test piece made of CFRP, measuring gauges R1 and R2 are attached to the front and back of the test piece.
And the dummy gauges r1 and r2 are attached to the front and back sides of the control strand which is placed in the vicinity thereof and is not loaded with a load. With these gauges R1, R2 and gauges r1, r2,
A 4-resistor bridge circuit was arranged in which the gauges R1 and R2 were located on opposite sides. And gauge R1 and gauge r
The battery E is connected between the terminal A at the connection point of 1 and the terminal B at the connection point of the gauge R2 and the gauge r2, and the terminal C at the connection point of the gauge R2 and the gauge r1 and the gauge R1 and the gauge r2.
A digital microvoltmeter V1 is connected between the terminal D of the connection point of and and the potential difference Ei between the terminals C and D is measured,
The creep strain amount of the test piece 1 was determined.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring creep strain, and more particularly to a method suitable for measuring creep strain of a composite material such as carbon fiber reinforced plastic.

[0002]

2. Description of the Related Art In recent years, carbon fiber reinforced plastics (CF
Composite materials such as RP) are becoming very frequently used for structural materials and the like. Measuring the creep behavior of such composites is very important when using them, for example when building structures are subjected to long-term loads.

However, a load is applied to a thin member such as a composite material, and the load causes a load of several thousand to several 10,000 μs.
At present, there is no good method for measuring the creep strain of about several tens of μst for a long period of time for a member that has undergone a large deformation of about t (μ strain).

[0004]

Conventionally, as a method for measuring creep strain, a method using an extension meter, a method using a strain gauge, etc. are known.

Of these, the extension meter method uses an electric amplifier, and is not suitable for measuring the increase / decrease of minute distortion for a long period of time because of its amplification degree, stability of zero point, and the like. It also has the drawback of being heavy and expensive.
On the other hand, the strain gauge method is relatively easy and highly accurate, and is a widely used method.

This is to detect a strain by attaching a strain gauge to a member and converting a resistance change of the gauge due to the deformation of the member caused by the deformation of the member into a voltage change.

However, (1) a large strain is applied to the strain gauge when a creep load is applied to the member, and the resistance wire itself constituting the strain gauge undergoes large creep deformation. (2) The creep deformation measurement requires a long period of 1 to several tens of months, but a strain amplifier is used to measure the resistance change of the strain gauge. There is an effect due to fluctuations in amplification factor, fluctuations in zero point, etc.

For this reason, even with the strain gauge method, it is not possible to accurately measure the minute creep strain exhibited by the member which is largely deformed.

The main object of the present invention is to eliminate the creep strain of a composite material such as CFRP, which is important but difficult to measure.
Although there is a method that enables accurate, stable, and simple measurement using a strain gauge, it is of course possible to measure the creep strain of a member other than the composite material by the measuring method of the present invention.

[0010]

The above object can be achieved by the creep measuring method according to the present invention. In summary, the present invention applies a creep test load to a test piece, and while the load is applied, the strain gauges for measurement are attached to the front and back of the test piece respectively, while the same material as the test piece is not loaded. It is placed near a test piece as a control piece, and dummy strain gauges are attached to the front and back sides of the control piece, respectively. A four-resistance bridge circuit arranged so that the measurement strain gauge and the dummy strain gauge are connected to each other, and a DC power supply is connected between two opposing connection points. A creep strain measuring method characterized in that the amount of creep strain of the test piece is known by connecting a voltmeter between the other two opposing connection points and measuring the potential difference therebetween. It is. According to one aspect of the present invention, the test piece is made of fiber-reinforced plastic obtained by impregnating unidirectionally arranged reinforcing fibers with a matrix resin and curing the resin.

[0011]

The present invention will be described below. One of the conventional problems in measuring the creep strain of a member such as a CRFP composite material using a strain gauge is that a large strain is applied to the strain gauge when a creep load is applied to the member,
This means that the resistance wire that constitutes the strain gauge itself undergoes large creep deformation. Therefore, in the present invention, in order to avoid this, the member is set in the creep tester,
After applying the creep load, the strain gauge was attached to the member.

Another problem with the conventional measuring method using a strain gauge is that the measurement of creep deformation requires a long period of 1 to several tens of months. This is due to the fluctuation of points. In the present invention, in order to eliminate this problem, a simple measurement system including a 4-resistor bridge circuit using a strain gauge, its DC power supply, and a microvolt meter is adopted.

The present invention will be described below with reference to examples.
In this example, a minute change in creep strain was measured for a strand of carbon fiber reinforced plastic over 1000 hours.

FIG. 1 is a partial sectional view showing a test piece having a tab attached thereto in an embodiment of the measuring method of the present invention. As shown in FIG. 1, tabs 2 are attached to both ends of the strand test piece 1 so that the strand test piece 1 can be directly pinch-chucked to the creep tester. The tab 2 is a cylindrical body having a projecting piece 2a provided with a pin hole 2b.

The strand test piece 1 is made of carbon fiber reinforced plastic (CFRP) prepared by a resin impregnation method. Carbon fiber reinforced plastic strands are formed by using high elasticity and high strength carbon fibers as reinforcements and room temperature curing epoxy resin as matrix resin, impregnating carbon fibers arranged in one direction with epoxy resin and curing. Test piece 1 was prepared. Cure at room temperature (about 25
(° C) The sample was left standing for 1 week or longer.
The shape of the test piece was a round bar, and the radius was 0.3 mm and the length was 80 mm. The number of carbon fibers was 6000 and the amount of epoxy resin was 10 g / 1 m.

The tab 2 was attached to the test piece 1 by inserting the test piece 1 into the hole 2c formed in the tab 2 and fixing it with an adhesive. The tab 2 was attached such that the distance L1 between the pin holes 2b of the two tabs 2 was 221 mm and the exposed length L2 of the test piece 1 between the two tabs 2 was 80 mm. A room temperature-curable epoxy adhesive was used as the mounting adhesive.

A dummy gauge strand (control strand) 3 is further attached to one of the tabs 2 described above. This control strand 3 is made of the same carbon fiber plastic as the strand test piece 1 and has a length of 60 mm.
It is short, and the only difference is that no load is applied. Similarly, the tab 2 was provided with a hole into which the control strand 3 was inserted and fixed with an adhesive. The exposed length of the control strand 3 was 40 mm.

The test piece 1 to which the tab 2 was attached was pin-chucked and attached to the creep tester through the pin hole 2b. Then, as shown in FIG. 2, a creep load P = 100 kg was constantly applied by a tester, and 1000% load was applied under the load.
After standing for a time, the creep deformation of the test piece 1 was measured.

Assuming that the CFRP composite material is used as a structural material, the strand test piece 1 has 10,000 μ
In the large deformation region where the deformation of st is generated, the test piece 1
It is necessary to measure the creep of. However, in this large deformation area, the strain gauge itself will be several 100st in a few days.
Will cause creep. In order to avoid this, in the present invention, the strain gauge is attached after the creep load P is applied. This can minimize strain gauge creep.

In the above, st (strain) is a unit of strain, and when the test piece is doubled, that is, the strain amount of 100% is defined as 1st. Therefore, the distortion amount of 1% is 1st / 1
00 = 10000 μst.

In the present invention, two strain gauges for measurement (measurement gauge) and one for temperature compensation are used in order to avoid the influence of the resistance change of the gauge due to the temperature change, which is caused by the long time required for the measurement of creep strain. A four-resistor bridge circuit was constructed using the two dummy strain gauges (dummy gauges). These gauges were attached as shown in FIG. That is, the measuring gauge R is attached to the front and back of the strand test piece 1.
1 and R2 were attached, and dummy gauges r1 and r2 were attached to the front and back of the control strand 3. Dummy gauge r1, r
In order to eliminate the temperature difference between 2 and the measuring gauges R1 and R2, the dummy gauges r1 and r2 were positioned in the immediate vicinity of the measuring gauges R1 and R2. Of course, it is desirable that the temperature atmosphere of the test be constant during the long-term test.

The structure of the 4-resistor bridge circuit according to the present invention is shown in FIG. As shown in FIG. 3, measuring gauges R1, R
2. Replace the dummy gauges r1 and r2 with the measurement gauges R1 and R2.
Are placed so that they are on opposite sides, and are connected so as to form a quadrangle. The battery E was connected between the terminal A at the connection point between the measurement gauge R1 and the dummy gauge r1 and the terminal B at the connection point between the measurement gauge R2 and the dummy gauge r2. Then, a digital microvoltmeter V1 is connected between the terminal C at the connection point between the measurement gauge R2 and the dummy gauge r1 and the terminal D at the connection point between the measurement gauge R1 and the dummy gauge r2, and the potential difference between the terminals C and D is established. Measure Ei. A digital millivolt meter V2 is connected to the terminals A and B, and the potential difference between the terminals A and B, that is, the electromotive force E _{0 of the} battery E is also measured, and it is confirmed that there is no change in the electromotive force E ₀ during the test.

In the above, the gauges R1, R2, r1,
The lengths of the lead wires of r2 were all the same because they were affected by the change in resistance with temperature. Measuring gauge R1, R2
Was placed so as to be on the opposite side of the bridge because the bending load expected to be applied to the test piece 1 during measurement (this bending load is almost 0, but may change in the long term, This may cancel the minute amount of creep strain). The potential difference between the terminals C and D of this circuit, that is, the voltage output is almost stable up to the order of 1 μV (about 0.6 μst).

In the bridge circuit, gauges R1 and R
The resistance values of 2, r1 and r2 are R1 = R2 = r1 = r2 = R
Then, when the resistance of the measurement gauges R1 and R2 increases by ΔR due to creep strain, the voltage change Δe (change in potential difference Ei) and the strain change Δε (that is, creep strain) that occur between the terminals C and D are as follows. Become.

Δe = 1/2 ・ ΔR / R ・ E _O = 1/2 ・
K _S · E _O · Δε (however, K _S : Gauge ratio) ∴ Δε = 2 Δe / (K _S · E _O ) Therefore, if the potential difference Ei between the terminals C and D is measured, the creep of the test piece 1 is observed from the change. You can know the amount of distortion.

The measurement results of creep strain by the method of the present invention are shown in FIG. Two test pieces 1 were prepared and tested. One of the test pieces is indicated by a circle in the figure and the other is indicated by a square. As shown in FIG. 4, the minute creep strain of the CFRP composite material, which was difficult to measure in the past, can be clearly detected.

As described above, in the present invention, the measuring gauge R
1 and R2 are attached to the test piece 1, and dummy gauges r1 and r
2 is attached to the control piece 3 to which no creep load is applied, and a 4-resistor bridge circuit is configured by these to measure the creep strain of the test piece 1. There is no influence of amplification rate fluctuations, zero point fluctuations, etc. due to the measurement of the period. After the creep load is applied to the test piece 1, the measuring gauges R1 and R2 are attached to the test piece 1.
Since it is attached, the measuring gauge R under load
No creep deformation due to large strain of 1 and R2. As a result, the creep strain of the test piece 1 could be measured accurately and stably.

[0028]

As described above, according to the method of the present invention, it is possible to accurately, stably and simply measure the creep strain of various members including a composite material such as CFRP using a strain gauge. You can

[Brief description of drawings]

FIG. 1 is a partial sectional view showing a tab-attached test piece in an embodiment of a measuring method of the present invention.

FIG. 2 is an explanatory diagram showing that strain gauges are attached to the front and back of each of the test piece and the control piece of FIG.

FIG. 3 is a diagram showing a 4-resistor bridge circuit configured by the strain gauge of FIG.

FIG. 4 is a graph showing the results of measurement of creep strain according to the method of the present invention.

[Explanation of symbols]

 1 Strand test piece 2 Tab 2b Pin hole 3 Control piece E Battery r1, r2 Dummy gauge R1, R2 Measuring gauge V1 Digital microvolt meter V2 Digital millivolt meter

Claims (1)

[Claims] 1. A control piece in which a creep test load is applied to a test piece, and strain gauges for measurement are attached to the front and back sides of the test piece while the load is applied, while the same material as the test piece is not applied to the load. Place it near the test piece as, and attach dummy strain gauges to the front and back of the control piece,
The two strain gauges for measurement and the two strain gauges for dummy constitute a 4-resistor bridge circuit in which the strain gauges for measurement are arranged to face each other, and the strain gauge for measurement and the strain gauge for dummy are arranged. By connecting a DC power supply between the two opposing connection points of one of the four connected connection points and connecting a voltmeter between the two opposing connection points of the other, and measuring the potential difference therebetween, A method for measuring creep strain, characterized in that the amount of creep strain of the test piece is known.