JP2024062036A - Method for selecting adhesive - Google Patents

Abstract

To provide a method for selecting an adhesive which allows selection of an adhesive usable for measuring a large strain in an adhesion structure for attaching an optical fiber cable for measuring a strain to a target object.SOLUTION: The present invention relates to a method for selecting an adhesive in an adhesion structure 1 for adhering an optical fiber cable 5 for measuring a strain to a reinforcement 3. An adhesive is selected which has a larger adhesive force on the reinforcement 3 than a restoration force generated in an adhesive layer 15 made of a cured adhesive against the largest possible strain that can be measured by the optical fiber cable 5 when the largest possible strain is generated in the reinforcement 3.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for selecting an adhesive for a bonding structure in which an optical fiber cable for measuring strain is bonded to a specified object.

Conventionally, a technique is known in which an optical fiber cable is installed on an object, and the scattered light observed by irradiating the optical fiber cable with pulsed laser light is analyzed to measure the strain distribution and temperature distribution of the object over the entire length of the optical fiber cable. This type of measurement is called distributed measurement, and examples of objects to be measured include concrete and steel materials (reinforcing bars, steel frames, steel sheet piles, etc.). For example, in the technique described in Patent Document 1 below, an optical fiber cable is attached to a steel sheet pile and the strain distribution of the steel sheet pile is measured.

Patent Publication No. 2022-18656

In this type of strain measurement, the optical fiber cable is reliably bonded to the object over the entire length of the measurement target section so that the optical fiber cable expands and contracts in response to the expansion and contraction of the object. However, in large strain regions, such as when the object is irreversibly damaged (for example, after steel material yields), the shear force acting between the object and the optical fiber cable becomes extremely large. Therefore, in order to measure such large strains, an adhesive is required that will not cause the optical fiber cable and adhesive layer to peel off from the object due to the shear force. However, even Patent Document 1 does not disclose the properties of an adhesive that can be used to measure such large strains.

The present invention aims to provide a method for selecting an adhesive that enables the selection of an adhesive that can be used to measure large strains in a bonding structure in which an optical fiber cable for measuring strain is bonded to an object.

The gist of the present invention is as follows:

[1] An adhesive selection method for selecting an adhesive for an adhesive structure in which an optical fiber cable for measuring strain is bonded to a specified object, the adhesive selection method selecting an adhesive that has a greater adhesive strength for the object than the restoring force that occurs in the adhesive after hardening in response to the maximum strain that should be measurable by the optical fiber cable when the maximum strain occurs in the object.

[2] When the shear adhesive strength of the adhesive to the object is τg, the elastic modulus of the adhesive after hardening is Eg, the diameter of the optical fiber cable is φ, the maximum strain is ε1, and a predetermined length related to the length of the region in which the strain ε1 occurs is L,
Eg/τg<L/(φ·ε1) ... (1)
The adhesive selection method according to [1], further comprising selecting the adhesive such that the following holds true.

[3] In the adhesive structure, the optical fiber cable is adhered to a measurement target area on the object with the adhesive, and a protective tube that protects a portion of the optical fiber cable outside the measurement target area by inserting it through a hollow portion is adhered to the object with a second adhesive at an adhesive portion that is continuous with the adhesive portion, and when the shear adhesive strength of the second adhesive to the object is τg2, the elastic modulus of the second adhesive after hardening is Eg2, and the outer diameter of the protective tube is D,
Eg2/τg2<L/(D·ε1) ... (2)
The adhesive selection method according to [2], further comprising selecting the second adhesive such that the following holds true.

[4] The adhesive selection method described in [2] or [3], in which the adhesive structure is such that the optical fiber cable is adhered to the measurement target area on the object with the adhesive, a protective tube that protects the portion of the optical fiber cable outside the measurement target area by inserting it into a hollow portion is adhered to the object with a second adhesive, and a non-adhesive portion exists between the adhesive portion and the adhesive portion of the second adhesive.

[5] An adhesive selection method according to any one of [2] to [4], in which the object is a steel material and the value of L is set within the range of 1 mm < L < 20 mm.

[6] An adhesive selection method according to any one of [2] to [5], in which the object is a steel material and the value of ε1 is greater than 1000μ.

The present invention provides a method for selecting an adhesive that allows the selection of an adhesive that can be used to measure large strains in a bonding structure in which an optical fiber cable for measuring strain is bonded to an object.

1A is a cross-sectional view showing a strain measurement unit and a reinforcing bar including an adhesive structure according to an embodiment, and FIG. 1B is a cross-sectional view taken along the line Ib-Ib in FIG. 1A is a cross-sectional view showing a model of the strain gauge unit in a cross section perpendicular to the axis of the rebar, and FIG. 1B is a perspective view of the model, and FIG. 1C is a cross-sectional view showing a model of the adhesive part of the protective tube of the strain gauge unit. FIG. 11 is a cross-sectional view showing a strain measuring unit according to a modified example.

Below, an embodiment of the adhesive selection method according to the present invention will be described in detail with reference to the drawings.

Figure 1(a) is a cross-sectional view showing a strain measurement unit 2 and a reinforcing bar 3 including an adhesive structure 1 of this embodiment, and Figure 1(b) is a cross-sectional view taken along line Ib-Ib in Figure 1(a). The strain measurement unit 2 measures the strain of the reinforcing bar 3 using an optical fiber cable 5 for measuring strain. The reinforcing bar 3 is embedded in concrete in a reinforced concrete structure (not shown). The strain measurement unit 2 is constructed by adhering the optical fiber cable 5 to the surface of the reinforcing bar 3 with an adhesive structure 1.

The side of the reinforcing bar 3 is provided with a fiber installation surface 7 that forms a flat surface for installing the optical fiber cable 5. The reinforcing bar 3 has a flat portion in a part of its circular cross section, and this flat portion is the fiber installation surface 7. A commercially available product may be used as such a reinforcing bar 3. The fiber installation surface 7 extends over the entire length of the reinforcing bar 3. The optical fiber cable 5 extends parallel to the reinforcing bar 3 and is bonded to the fiber installation surface 7 over the entire length of the measurement target section 9 (measurement target area) of the reinforcing bar 3 for strain measurement. The optical fiber cable 5 is, for example, an optical fiber core wire with a diameter of 0.9 mm in which an optical fiber strand is coated with a non-halogen resin or the like. Note that the optical fiber strand is an optical fiber mainly made of quartz glass covered with a coating of an ultraviolet-curing resin or the like. In this way, the strain measurement unit 2 is formed by bonding the optical fiber cable 5 to the surface of the reinforcing bar 3 with an adhesive. The strain measurement unit 2 includes an adhesive layer 15 made of a cured adhesive.

When the optical fiber cable 5 is attached to the reinforcing bar 3, the surface of the reinforcing bar 3 is polished to remove rust, plating, black skin, etc. Then, the attachment position (fiber installation surface 7) is degreased and cleaned using industrial tissue paper with a solvent such as ethanol. The surface of the fiber installation surface 7 is also polished to remove rust, plating, black skin, etc. Then, the surface of the fiber installation surface 7 may be polished in multiple directions with sandpaper so that it has no directionality. Then, the optical fiber cable 5 is temporarily fixed on the fiber installation surface 7 at intervals of about 1 m using instant adhesive or tape, etc., and then adhesive is applied to the fiber installation surface 7 over the entire length of the measurement target section 9, and the adhesive hardens to fix the optical fiber cable 5 on the fiber installation surface 7. The thickness of the applied adhesive is preferably about the diameter of the optical fiber cable 5 or less.

Acrylic resin adhesives, epoxy resin adhesives, etc. can be used as the adhesives mentioned above. Adhesives are also classified into dry-setting, chemical reaction, hot melt, pressure-sensitive, etc., depending on how they solidify, and any of these can be used. Taking durability into consideration, it is preferable to use epoxy resin adhesives as the adhesives mentioned above. Also, taking workability into consideration, it is preferable to use two-part mixed room temperature curing epoxy resin adhesives rather than heat-curing epoxy resin adhesives.

At one end of the strain measuring unit 2, at a position outside the measurement target section 9, a protective tube 11 is bonded to the fiber installation surface 7. The protective tube 11 is made of, for example, a SUS tube, and the outer diameter of the protective tube 11 is, for example, about 2 mm. At the end of the measurement target section 9, the optical fiber cable 5 is pulled out from the fiber installation surface 7, and the pulled out part is inserted into the hollow part of the protective tube 11. Then, the optical fiber cable 5 passes through the hollow part of the protective tube 11 and is pulled out to the outside of the reinforced concrete structure. This structure reduces the possibility that the optical fiber cable 5 will be bent when the optical fiber cable 5 is mounted and wired, and the breakage of the optical fiber cable 5 is suppressed. In addition, in the vicinity of the end opening of the protective tube 11, the optical fiber cable 5 is covered with a heat-shrinkable rubber tube 13 around the optical fiber cable 5 so that the optical fiber cable 5 does not directly contact the edge of the end opening, thereby suppressing the breakage of the optical fiber cable 5. The adhesive that bonds the protective tube 11 to the reinforcing bar 3 is called the "second adhesive". The second adhesive layer 17, which is made of the cured second adhesive, is connected to the adhesive layer 15.

The end of the optical fiber cable 5 drawn out from the reinforced concrete structure is connected to a measuring instrument (not shown) that performs strain measurement. Furthermore, an analysis device (not shown) such as a computer is connected to this measuring instrument. The measuring instrument inputs pulsed light into the optical fiber cable 5, receives various scattered lights returning from each position in the longitudinal direction of the optical fiber cable 5, and transmits information on the intensity, wavelength, etc. of the received scattered light to the analysis device. The scattered light includes Rayleigh scattered light and Brillouin scattered light. Examples of measuring instruments that can be used include an OTDR (Optical Time Domain Reflectometer) that uses Rayleigh scattered light and a Brillouin Optical Time Domain Reflectometer (BOTDR) that uses Brillouin scattered light. The analysis device analyzes the intensity and wavelength of the scattered light at each position in the longitudinal direction of the optical fiber cable 5 based on the principle that the intensity and wavelength of the scattered light depend on the strain applied to the optical fiber cable 5. Through this analysis, the analysis device obtains the strain occurring at each position in the longitudinal direction of the optical fiber cable 5 at a pitch of, for example, several centimeters. This type of strain measurement technology is called distributed measurement.

The strain measurement unit 2 is capable of measuring large strains in the reinforcing bars 3, such as 1000μ or more. For example, when the reinforcing bars 3 are SD345 (yield strain 1725μ), the strain measurement unit 2 can measure the strain after the yield of the reinforcing bars 3 (e.g., tens of thousands ofμ). Because it is capable of measuring such large strains, the strain measurement unit 2 can be used for purposes such as evaluating the soundness of reinforced concrete structures after an earthquake.

When large strains occur, such as those occurring after the yielding of the reinforcing bar 3, the shear force acting between the reinforcing bar 3 and the optical fiber cable 5 becomes extremely large. Therefore, in order to measure such large strains, an adhesive is required that will not cause the optical fiber cable 5 and adhesive layer 15 to peel off from the reinforcing bar 3 due to the shear force. Below, we will explain how to select an adhesive that will enable the strain measuring unit 2 to measure such large strains.

[How to select adhesive]
Generally, after the yielding of steel, large strains of tens of thousands of μ may occur locally in a narrow region of about 1 to 20 mm. That is, after the yielding of the reinforcing bar 3, large strains may occur locally in a narrow region of about 1 to 20 mm in the longitudinal direction of the reinforcing bar 3. In selecting an adhesive, it is sufficient to satisfy the condition that the optical fiber cable 5 and the adhesive layer 15 do not peel off from the reinforcing bar 3 in the narrow region where such localized large strain occurs.

Figure 2(a) is a cross-sectional view showing a model of the strain measurement unit 2 in a cross section perpendicular to the axis of the reinforcing bar 3. As shown in the figure, in this model, the adhesive layer 15 has the same height as the diameter of the optical fiber cable 5, a predetermined width bg wider than the diameter of the optical fiber cable 5, and is formed on the fiber installation surface 7 with a rectangular cross section that includes the cross section of the optical fiber cable 5.

Here, the shear adhesive strength of the adhesive to the reinforcing bar 3 is τg, the elastic modulus of the adhesive (adhesive layer 15) after hardening is Eg, and the diameter of the optical fiber cable 5 is φ. The length of the region in which the localized large strain of the reinforcing bar 3 occurs is L, and the maximum strain that should be measurable by the optical fiber cable 5 is ε1. This strain ε1 is set based on the performance (measurable strain range) required of the strain measurement unit 2. In other words, the strain measurement unit 2 is required to be able to measure the strain of the reinforcing bar 3 up to ε1. Also, here, as shown in FIG. 2(b), consider a portion A on the adhesive layer 15 side that is adhered to the region in which the localized large strain of the reinforcing bar 3 occurs. The portion A is a rectangular parallelepiped portion with a length L, a width bg, and a height φ, composed of the optical fiber cable 5 and the adhesive layer 15 in which the optical fiber cable 5 is embedded.

When strain ε1 occurs locally in an area of length L of reinforcing bar 3, strain ε1 is also transmitted to part A attached to this area. The restoring force in the longitudinal direction of reinforcing bar 3 that occurs in part A against strain ε1 is defined as F1. In other words, the force in the longitudinal direction of reinforcing bar 3 that acts on part A when strain ε1 occurs is F1, and force F1 is expressed by the following formula (a).
F1 = φ bg Eg ε1 ... (a)
Further, the adhesive strength F2 in the shear direction of the portion A relative to the reinforcing bar 3 is expressed by the following formula (b).
F2 = τg bg L ... (b)

In order for part A to not peel off from the reinforcing bar 3,
F1<F2 ... (c)
Therefore, from the formulas (a) to (c),
φ bg Eg ε1 < τg bg L ... (d)
It is enough if the above is satisfied.
Eg/τg<L/(φ·ε1) ... (1)
is satisfied. Therefore, if the adhesive of the adhesive structure 1 of the strain measuring unit 2 is selected to satisfy the above formula (1), the optical fiber cable 5 and the adhesive layer 15 will not peel off from the reinforcing bar 3 when strain ε1 occurs in the reinforcing bar 3. In other words, if the above formula (1) is satisfied, the strain ε1 can be measured by the strain measuring unit 2. Here, Eg and τg on the left side of formula (1) are characteristic values of the adhesive, so it is only necessary to select an adhesive having characteristics such that the ratio of these characteristic values τg and Eg is less than L/(φ·ε1).

In addition, L in formula (1) is the length of the area in which localized large strain occurs in the reinforcing bar 3. As mentioned above, after steel material generally yields, localized large strain can occur in a narrow area of about 1 to 20 mm, so a value in the range of 1 mm < L < 20 mm should be adopted for L. Furthermore, to obtain appropriate adhesive selection results, it is preferable to adopt a value in the range of 1 mm < L < 5 mm. For example, let us assume that L = 2 mm is adopted here.

For example, if the length L of the area where localized large strain occurs in the reinforcing bar 3 is 2 mm, the diameter φ of the optical fiber cable 5 is 0.9 mm, and the maximum strain ε1 that should be measurable by the optical fiber cable 5 is 30,000μ, then an adhesive with (Eg/τg) of less than 74 should be selected from formula (1). Also, under the same conditions, if the maximum strain ε1 is, for example, 20,000μ, an adhesive with (Eg/τg) of less than 111 should be selected, and if the maximum strain ε1 is, for example, 15,000μ, an adhesive with (Eg/τg) of less than 148 should be selected.

[Adhesive for Adhering Protective Tube 11]
Also, in order to enable the strain measuring unit 2 to measure the large strain of the reinforcing bar 3 as described above, it is necessary to take the second adhesive into consideration. That is, in the case where the second adhesive layer 17 is connected to the adhesive layer 15, if the protective tube 11 peels off from the reinforcing bar 3 due to the large strain of the reinforcing bar 3, the peeled portion will extend from this peeled point to the measurement target section 9, and there is a risk that the optical fiber cable 5 in the measurement target section 9 will peel off from the reinforcing bar 3. Therefore, in a structure in which the second adhesive layer 17 is connected to the adhesive layer 15, it is necessary to prevent the protective tube 11 from peeling off from the reinforcing bar 3 even if a local large strain ε1 of the reinforcing bar 3 occurs in the installation section of the protective tube 11.

2(c), the second adhesive layer 17 has the same height as the outer diameter of the protective tube 11, a predetermined width bg2 wider than the outer diameter of the protective tube 11, and is formed on the fiber installation surface 7 with a rectangular cross section that includes the cross section of the protective tube 11. Then, when the shear adhesive strength of the second adhesive to the reinforcing bar 3 is τg2, the elastic modulus of the second adhesive layer 17 is Eg2, and the outer diameter of the protective tube 11 is D, following the above formula (1),
Eg2/τg2<L/(D·ε1) ... (2)
is satisfied, even if a local strain ε1 of the reinforcing bar 3 occurs in the installation section of the protective pipe 11, the protective pipe 11 will not peel off from the reinforcing bar 3. Therefore, if the ratio of the characteristic values τg2 and Eg2 of the second adhesive is selected to be less than L/(D·ε1), the protective pipe 11 and the second adhesive layer 17 will not peel off from the reinforcing bar 3, and the strain ε1 can be more suitably measured by the strain measuring unit 2.

For example, if the length L of the area where localized large strain occurs in the reinforcing bar 3 is 2 mm, the outer diameter D of the protective tube 11 is 2 mm, and the maximum strain ε1 that should be measurable by the optical fiber cable 5 is 30,000 μ, then a second adhesive having (Eg/τg) of less than 33 can be selected from formula (2). Note that this second adhesive and the adhesive constituting the adhesive layer 15 may be the same adhesive.

In general, adhesive properties (shear adhesive strength, elastic modulus after hardening, etc.) depend on the material of the object to be bonded, curing conditions, use temperature, etc., and are therefore difficult to uniquely indicate. In the adhesive selection method of this embodiment, the characteristic values Eg, τg, Eg2, and τg2 in formulas (1) and (2) may be values assuming a standard use environment listed in an adhesive catalog, for example. In addition, characteristic values corresponding to the use environment of the strain measurement unit 2 listed in a catalog, etc. may be used.

The present invention can be implemented in various forms, including the above-described embodiment, with various modifications and improvements based on the knowledge of those skilled in the art. It is also possible to construct modified versions by utilizing the technical matters described in the above-described embodiment. The configurations of each embodiment may be used in appropriate combination.

As shown in FIG. 3, a non-adhesive section 21 (non-adhesive portion) may be provided on the fiber installation surface 7. The non-adhesive section 21 is a section between the measurement target section 9 and an adhesive section 23 (adhesive portion) between the reinforcing bar 3 and the protective tube 11. The measurement target section 9 is an adhesive section (adhesive portion) where the reinforcing bar 3 and the optical fiber cable 5 are adhered. In the non-adhesive section 21, no adhesive is present, and the optical fiber cable 5 and the protective tube 11 are not adhered to the reinforcing bar. If such a non-adhesive section 21 exists, the shear force generated in the protective tube 11 occurs independently of the shear force generated in the measurement target section 9, and has almost no effect on the measurement target section 9. In other words, the presence of the non-adhesive section 21 reduces the possibility that even if the protective tube 11 peels off from the reinforcing bar 3, the optical fiber cable 5 in the measurement target section 9 will peel off from the reinforcing bar 3 starting from the peeling point. Therefore, if the non-adhesive section 21 exists, the selection of the second adhesive using the above-mentioned formula (2) may be omitted.

In addition, it is not necessary to glue the protective tube 11 onto the fiber installation surface 7 in order to fix the protective tube 11 to the reinforcing bar 3. For example, the protective tube 11 may be tied to the reinforcing bar 3 using a cable tie or an insulok to secure the optical fiber cable 5 to a degree that does not cause bending when the optical fiber cable 5 is mounted and wired. In addition, the structure in which the optical fiber cable 5 is covered with the protective tube 11 outside the measurement target section 9 is not necessary, and outside the measurement target section 9, a large-diameter communication optical fiber cable may be connected to the optical fiber cable 5. When fixing the communication optical fiber to a position adjacent to the measurement target section 9 of the reinforcing bar 3, it is preferable to fix it with a second adhesive for the protective tube 11 that is less rigid than the second adhesive. For example, an acrylic resin-based adhesive with low rigidity may be used.

In the embodiment, an example is described in which the reinforcing bar 3 is used as the object of strain measurement, but the present invention is not limited to this, and the object can be any steel material such as a steel frame or a steel sheet pile. For example, when the object is a steel material such as SS400 (yield strain 1225μ), it is possible to measure the strain after yielding, as with the reinforcing bar 3. The object is also not limited to steel, and may be, for example, a concrete member. In the embodiment, an example is described in which the optical fiber cable 5 is an optical fiber core wire with a diameter of 0.9 mm, but the optical fiber cable 5 may be a 0.25 mm diameter wire, a ribbon core wire, or the like.

In the embodiment, the cross-sectional shape of the adhesive layer 15 (FIG. 2(a)) is modeled as a rectangle with height φ and width bg, but is not limited to this. The cross-sectional shape of the adhesive layer 15 may be modeled with other shapes and dimensions, such as a trapezoid, in accordance with the actual adhesive application conditions, etc. Then, based on the cross-sectional shape and dimensions of the modeled adhesive layer 15, an adhesive may be selected under the condition that portion A does not peel off from the reinforcing bar 3, following formula (1). The same applies to the cross-sectional shape of the second adhesive layer 17 (FIG. 2(b)) and the method of selecting the second adhesive.

1...bonded structure, 3...reinforcing bar (object), 5...optical fiber cable, 9...measurement target section (measurement target area, bonded portion), 17...second adhesive layer, 21...non-bonded section (non-bonded portion), 23...bonded section (bonded portion).

Claims (6)

1. An adhesive selection method for selecting an adhesive in an adhesive structure for adhering an optical fiber cable for strain measurement to a predetermined object, comprising:
An adhesive selection method for selecting an adhesive that has a greater adhesive strength with respect to the object than the restoring force generated in the adhesive after hardening in response to the maximum strain that should be measurable by the optical fiber cable when the maximum strain occurs in the object. The shear adhesive strength of the adhesive to the object is τg,
The elastic modulus of the adhesive after curing is Eg,
The diameter of the optical fiber cable is φ,
The maximum strain is ε1,
When a predetermined length related to the length of the region in which the strain ε1 occurs is L,
Eg/τg<L/(φ·ε1) ... (1)
The adhesive selection method according to claim 1 , further comprising the step of selecting the adhesive such that the following expression is satisfied: In the adhesive structure,
the optical fiber cable is adhered to a measurement target area on the object with the adhesive;
a protective tube for protecting a portion of the optical fiber cable outside the measurement target area by inserting the protective tube into a hollow portion is bonded with a second adhesive at an adhesive portion connected to the adhesive portion on the object,
The shear adhesive strength of the second adhesive to the object is τg2,
The elastic modulus of the second adhesive after curing is Eg2,
When the outer diameter of the protective tube is D,
Eg2/τg2<L/(D·ε1) ... (2)
The adhesive selection method according to claim 2 , further comprising the step of selecting the second adhesive such that the following expression is satisfied: In the adhesive structure,
the optical fiber cable is adhered to a measurement target area on the object with the adhesive;
a protective tube for protecting a portion of the optical fiber cable outside the measurement target area by inserting the protective tube into a hollow portion is bonded onto the target object with a second adhesive;
The adhesive selection method according to claim 2 , wherein a non-bonded portion exists between the bonded portion of the adhesive and the bonded portion of the second adhesive. The adhesive selection method according to any one of claims 2 to 4, wherein the object is a steel material, and the value of L is set within the range of 1 mm < L < 20 mm. The adhesive selection method according to any one of claims 2 to 4, wherein the object is a steel material and the value of ε1 is greater than 1000μ.

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