JPH11248558A - Measuring method for strain distribution - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a measuring method in which the strain of a test member can be measured irrespective of the generation of a crack, by a method wherein an optical fiber is fixed to the surface of the test member via a shock absorbing member whose yield stress is higher than that of the test member and whose modulus of elasticity is smaller than that of the optical fiber. SOLUTION: Optical fibers 2 which are covered with nylon through a measuring apparatus B-OTDR 8 for a strain distribution are pasted in the center on the surface and in the center on the rear surface of a beam 1 via urethane resins, as shock absorbing members, whose yield stress is higher than that of concrete as the beam 1 and whose modulus of elasticity is smaller than that of the optical fibers 2. Accordingly, even when a crack is generated in the beam 1, the optical fibers 2 are not torn into pieces, and the strain distribution on the surface of the beam 1 can be measured easily. At this time, a strain measured value before the generation of the crack is precise, and the value of a strain in every position is increased rectlinearly due to an increase in a load after the generation of the crack. Consequently, when, after the generation of the crack, the strain measured value of the optical fibers 2 is proportionally calculated, the value of the strain which is generated actually can be found.

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 strain distribution, and more particularly to a method for measuring strain distribution of a cracked test member.

[0002]

2. Description of the Related Art Hitherto, it has been very important to accurately evaluate the strain accumulated in a structure and measure the stress distribution in order to diagnose the reliability of the structure. That is, in a strength test of a building material, how to accurately measure a stress distribution is an issue. For example, when constructing a bridge or building, the diameter of 1m
A cast-in-place concrete pile having a length of about 10 m is driven into the pile. In such a pile strength test, a strain gauge is interspersed and fixed on the surface of the pile, and the stress distribution is obtained based on the measured value. Was.

When measuring the strain of a structure, if the stress generated in the structure is equal to or lower than the yield stress, it is necessary to accurately measure the strain generated in the structure only by using a strain gauge. The stress distribution on the surface of the structure can be easily estimated from the measured strain value.

[0004] However, if the stress generated in the structure exceeds the yield stress and cracks occur in the structure, it becomes difficult to measure only by using a strain gauge. This is because some of the strain gauges on the surface of the structure are destroyed by cracks, and cannot be measured. In addition, even a measurable strain gauge remaining without being destroyed is affected by stress release due to the occurrence of cracks, so that it is impossible to obtain an accurate strain distribution over the entire structure surface.

[0005] Therefore, in order to solve such a problem, a reinforcing bar with a reinforcing bar is embedded in the structure in advance, and the strain on the surface of the structure is estimated based on the measured value of the strain inside the structure. In addition, a technique for evaluating the deformation state of a structure has been used.

Here, as an example, a simple beam strength test of a reinforced concrete beam performed by the present inventors will be described. FIG. 4 shows the dimensions of the beam 1, which is a test member used in the reinforced concrete beam strength test, the position of the strain gauge 5, the position of the reinforcing bar 6, the position of the reinforcing bar 6a, and the positions of the fulcrum 3 and the loading point 4.

[0007] As shown in FIG.
A reinforcing bar 6 having a reinforcing bar 6a attached thereto at regular intervals is embedded therein. Both ends of the beam 1 are supported by a fulcrum 3, and a load is applied to the central portion (loading point 4) of the beam 1 vertically downward from directly above, and a plurality of strain gauges 5 are attached to the surface of the beam 1. Have been. The dimensions of each part are as shown in the figure.
5 m, 0.4 m wide, the position of the strain gauge 5 is 2.5 m, 4 m, 5 m, 6 m, 7.5 m from the left end of the beam 1, and the rebar gauge 6
The position a is 1 m apart from the left end of the beam 1, and the position of the loading point 4 is 5 m from the left end of the beam 1.

FIG. 5 shows a cross section of the beam 1 orthogonal to the longitudinal direction. As shown in the figure, a total of ten rebars (rebars 6, 7) are embedded in the beam 1, and in particular, the rebar 6 is provided with the above-mentioned rebar gauge 6a. Then, strain gauges 5 are attached to the upper and lower surfaces of the beam 1. The dimensions of each part are as shown in the figure, and a total of six reinforcing bars 7 are provided at 0.05 m intervals from the lower end of the beam 1 and 0.05 m from the upper end.
At two positions. Similarly, the reinforcing bar 6 to which the reinforcing bar gauge 6a is attached is moved from the upper surface and the lower surface of the beam 1 by 0.0.
One of each was placed at a position of 5 m.

Here, the results of the simple beam strength test using the beam 1 are as follows. That is, the load shown in FIG. 6 was obtained by applying a load stepwise to the loading point 4 and measuring the strain gauge 5 at each step. FIG. 6 shows the values of the strain generated when the loaded load is 7t, 8t, and 11t, with the left end of the beam 1 in FIG. 4 as the origin of the horizontal axis. Each numerical value is based on the strain generated when the load is 5t.

[0010] As shown in FIG.
Since a crack has occurred in the lower surface of the beam 1, the strain on the lower surface at a position 5 m from the left end of the beam 1 has a smaller value when the loads are 8t and 11t than when the load is 7t. This is considered to be due to the fact that stress was released near the position at a distance of 5 m from the left end of the beam 1 due to the effect of the generated crack, and an accurate strain distribution was not obtained.

Further, at a load of 11 t, it was not possible to measure a strain value at a position 4 m away from the left end of the beam 1. This is considered that the strain gauge 5 at a position 4 m from the left end of the beam 1 was broken by a crack. That is, these results indicate that when a crack occurs in a structure, the deformation state of the structure cannot be evaluated only by using a strain gauge attached to the surface of the structure.

Therefore, on the cross section perpendicular to the neutral plane, it is assumed that the strain value is linearly distributed, and the beam 1 is obtained by using the strain value obtained from the reinforcing bar 6a installed in the structure.
The value of the strain generated on the upper surface and the lower surface of is determined.

FIG. 7 is a graph showing the measurement results of the strain gauge 5 and the rebar gauge 6a at a position 5 m from the left end of the beam 1, and the strain on the surface estimated based on the value obtained from the rebar gauge 6a. . In the drawing, the upper surface of the beam 1 is set as the origin of the horizontal axis, and the value of the strain in the cross section from the upper surface of the beam 1 to the vertically downward direction is shown.

When the distance from the upper surface of the beam 1 to the vertical downward direction is 5 cm and 45 cm, ● _* , black triangle _* , ■ _* are as follows:
The values measured by the rebar gauge 6a under the loads of 11t, 8t, and 7t are shown. ● at 0 cm and 50 cm, black triangle,
(2) shows the measured values of the strain gauge 5 on the surface of the beam 1 when the loads are 11t, 8t, and 7t, respectively. ○, △,
□ indicates the value of the strain on the upper surface and the lower surface of the beam 1 estimated from the value of the rebar gauge 6a when the load is 11t, 8t, and 7t, respectively.

As is apparent from the above results, when the load is 7 t before the occurrence of the crack, the measured value by the strain gauge 5 and the estimated value by the rebar gauge 6 a are both the same, but after the occurrence of the crack, It can be seen that the loads 8t and 11t do not match.

However, as a matter of course, it is difficult to install a reinforcing bar inside a structure already constructed, and in the conventional method, a plurality of strain gauges or reinforcing bars are discretely arranged. Therefore, the spatially continuous strain distribution could not be measured.

By the way, as a method for measuring the strain distribution of a structure, not only a method using the above-described strain gauge and a rebar gauge but also a method using a new optical fiber has been proposed. This is to attach an optical fiber to the surface of the structure, make a light pulse incident on this optical fiber,
By measuring the strain distribution of the Brillouin scattered light generated in the longitudinal direction of the optical fiber along the longitudinal direction of the optical fiber, the strain of the structure is indirectly measured.

That is, the characteristic of the light (that is, the backscattered light) of the scattered light generated at each position in the longitudinal direction of the optical fiber after the light pulse is incident from one end of the optical fiber and returning to the incident end side. Is analyzed to measure the tension applied to the optical fiber and the position where the tension is generated. As an apparatus for measuring the distribution of strain generated in the longitudinal direction of such an optical fiber, a strain-loss integrated OTDR (hereinafter, referred to as B-OTDR) (Japanese Patent Application No. Hei.
248169) is used.

[0019]

However, even if an attempt is made to measure the strain distribution using such an optical fiber, if the optical fiber is bonded using a strong adhesive as in the prior art, cracks generated in the test member may occur. Since the optical fiber is torn, it has been difficult to measure the strain distribution without worrying about the occurrence of cracks. The present invention is intended to solve such a problem, and it is possible to measure the strain of a test member regardless of the occurrence of cracks, and to easily obtain the strain distribution over the entire test member. It is intended to provide a measuring method.

[0020]

In order to achieve such an object, a method for measuring a strain distribution according to the present invention comprises the steps of: providing a surface of a test member having a higher yield stress than the test member and having a higher elasticity than an optical fiber; The optical fiber is fixed via a buffer member having a small ratio, and the strain distribution of the test member is obtained based on the strain distribution of the optical fiber. With such a configuration, according to the present invention, even if a crack occurs in the test member, the optical fiber is not cut by the crack, and the strain distribution on the surface of the test member can be measured. Further, since it is not necessary to embed a rebar gauge or the like in the test member, the strain distribution can be easily measured even in the already constructed test member, and the deformation state of the test member can be easily evaluated.

[0021]

Next, one embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view showing one embodiment of the present invention. 4, the same or equivalent members in FIG. 4 are denoted by the same reference numerals, and a beam 1 as a test member is a simple beam formed of concrete. An optical fiber 2 is attached to the beam 1 at the center between the upper surface and the lower surface thereof via a buffer member (not shown) made of a material having a higher yield stress and a lower elastic modulus than concrete.

For example, an adhesive made of urethane resin or the like may be used as the cushioning member. This is because if the optical fiber 2 is firmly fixed using a strong adhesive, the optical fiber may be broken due to cracks. Therefore, the optical fiber 2
The invention of the present application cannot be achieved only by affixing to the beam 1.

In this embodiment, the wavelength 1.
Nylon-coated optical fiber 2 for 55 μm;
A device (B-OTDR8) for measuring the distribution of strain occurring in the length direction of the optical fiber 2 for 55 μm and a yield stress higher than that of the beam 1 and more elastic than that of the optical fiber 2 for fixing the optical fiber 2 to the beam 1 And a urethane resin which is a buffer material having a low rate. Further, the surface of the beam 1 is coated with a urethane resin having a thickness of about 1 mm, and the optical fiber 2 is attached thereon.

Here, experimental results showing the effectiveness of the present embodiment will be described. FIG. 2 is a graph showing experimental results in the present embodiment shown in FIG. As shown in the figure, the relationship between the distance from the left end of the beam 1 and the strain is shown, and the left end of the beam 1 is taken as the origin of the horizontal axis and the load is 7
For t, 8t, and 11t, the value of the strain generated in the beam 1 is shown based on the case where the load is 5t.

The solid and dotted lines in the figure show the strain distribution obtained from the optical fiber. □, Δ, and ○ indicate the values of strain measured by a strain gauge (not shown) attached to the upper surface of the beam 1. ■, black triangle, ●
Indicates the value of strain measured by a strain gauge (not shown) attached to the lower surface of the beam 1. The installation position of the strain gauge is the same as that in FIG. 4, and a rebar gauge (not shown) is also attached.

Now, as is apparent from FIG. 2, on the upper surface of the beam 1, the measured value of the strain gauge and the measured value of the optical fiber 2 are almost the same. On the other hand, on the lower surface, stress is released because the beam 1 is cracked by the load of 8 t, and depending on the location, the stress is reduced by 8 t compared to the case of 7 t.
And 11t, the value of the strain is smaller. However, the measured value of the optical fiber 2 substantially coincides with the value of the strain gauge at 7t before the crack occurs. Further, it can be seen that the strain value at each position also increases with the increase in the load, similarly to the upper surface, after 8t at which the crack occurred.

Here, in order to compare the value of the strain on the lower surface due to the optical fiber with the value of the strain generated on the lower surface estimated from the value of the strain of the rebar meter, the value of the strain at the loading point was determined for each load. Each is shown in FIG.

In FIG. 3, ● represents the value of the strain on the lower surface estimated from the value of the rebar gauge, □ represents the value measured by the optical fiber, and the black triangle represents the value measured by the strain gauge.
The value of the strain on the lower surface estimated from the value of the rebar gauge and the value of the strain gauge installed on the lower surface show the same value up to 7 t, which indicates that the strain gauge accurately measures.

However, it can be seen that the value of the strain gauge hardly changes after 8 t due to the effect of the crack, and the value is not accurately measured by the strain gauge. The value of the strain on the lower surface estimated from the rebar meter and the value of the strain on the lower surface due to the optical fiber show the same value up to 7t.

Further, it can be seen that the inclination is different after 8t when the crack occurs, but both change linearly with the load. From this, it can be seen that the value of the strain due to the optical fiber is almost accurate until the crack occurs. Then, after the crack has occurred, the value of the strain actually occurring can be obtained from the strain value of the optical fiber by a proportional calculation.

That is, by using the optical fiber, the strain distribution on the beam surface could be continuously obtained spatially. Here, since the strain gauge was installed on the concrete beam, it was possible to discover whether cracks occurred. In actual measurement, it is considered difficult to determine whether or not a crack has occurred from only the value of the optical fiber.

However, as can be seen from the results of this embodiment, the value of the strain after the occurrence of the crack is larger than that before the occurrence of the crack, and the value of the strain at the time of the yield stress depends on the material of the structure. Since is known almost exactly, it is considered that it is possible to estimate the presence or absence of crack occurrence from the value and the measured value of the optical fiber. As described above, by using the present embodiment, the true strain distribution could be continuously obtained spatially regardless of the presence or absence of cracks.

In this embodiment, urethane resin is used as the cushioning member. However, any other material having a higher yield stress than the structure and a smaller elastic modulus than the optical fiber may be used. I don't care. For example, similar results could be obtained using epoxy resin or silicone rubber. Similar results could be obtained when the optical fiber was coated with a photosensitive resin such as a UV (ultraviolet) curable resin having a higher yield stress than concrete. Further, the surface of the beam 1 may be coated with the above-described cushioning member, or a groove may be formed on the surface of the beam 1 and the optical fiber 2 may be fixed therein. In the above, an example was given of a concrete structure, but the present invention is not limited to this, and the present invention can be applied to structures such as bridges and ships, and the size is not limited to the above.

[0034]

As described above, according to the present invention, even if a crack occurs in the test member, the strain distribution on the surface of the test member can be easily measured without breaking the optical fiber. Further, since it is not necessary to embed a reinforcing bar or the like in the test member, it is possible to easily measure the strain distribution even in the test member already constructed. Furthermore, the present invention is also effective when applied to structures such as bridges, ships, cast-in-place concrete piles, and the like.

[Brief description of the drawings]

FIG. 1 is a perspective view showing one embodiment of the present invention.

FIG. 2 is a graph showing experimental results in the present embodiment shown in FIG.

FIG. 3 shows the value of strain on the lower surface of the optical fiber 1 and
It is a graph for comparing with the value of the strain generated in the lower surface estimated from the value of the strain of the rebar gauge 6a.

FIG. 4 is a sectional view showing a conventional example.

5 is a sectional view showing the beam 1 according to FIG.

6 is a graph showing a result of a simple beam strength test using the beam 1 shown in FIG.

FIG. 7 is a graph showing measurement results of a strain gauge and a rebar gauge, and strain on a surface estimated based on numerical values obtained from the rebar gauge.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Beam, 2 ... Optical fiber, 3 ... Support point, 4 ... Loading point, 5 ...
Strain gauge, 6,7 ... Rebar, 6a ... Rebar gauge, 8 ... B-
OTDR.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Naka Naruse 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (9)

[Claims] 1. An optical fiber is fixed to a surface of a test member via a buffer member having a higher yield stress than the test member and a smaller elastic modulus than the optical fiber, and based on a strain distribution of the optical fiber. A strain distribution measuring method, wherein a strain distribution of the test member is obtained. 2. The strain distribution measuring method according to claim 1, wherein a groove is formed in the test member, and the optical fiber is installed in the groove. 3. The strain distribution measuring method according to claim 1, wherein the buffer member is formed of urethane resin. 4. The strain distribution measuring method according to claim 1, wherein the buffer member is formed of an epoxy resin. 5. The optical fiber according to claim 1, wherein the optical fiber is covered with a covering member having a higher yield stress than the test member and a smaller elastic modulus than the optical fiber, and then fixed to a surface of the test member. A strain distribution measuring method. 6. The strain distribution measuring method according to claim 5, wherein the covering member is formed of nylon. 7. The strain distribution measuring method according to claim 5, wherein the covering member is formed of an ultraviolet curable resin. 8. The optical fiber according to claim 1, wherein the test member is covered with a coating member having a higher yield stress than the test member and a smaller elastic modulus than the optical fiber, and then the surface of the test member is covered with the optical fiber. A strain distribution measuring method, characterized by fixing 9. The optical system according to claim 1, wherein an optical pulse is incident on the optical fiber, and a distribution of the Brillouin scattered light generated in the longitudinal direction of the optical fiber in the longitudinal direction is measured. Measuring the strain distribution of the optical fiber along the line, and obtaining the strain distribution of the test member from the strain distribution of the optical fiber.