Adhesive selection method

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

Technical Field

[0001] The present invention relates to 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.

Background Art

[0002] Conventionally, a technique for measuring the strain distribution and temperature distribution of an object over the entire length of an optical fiber cable has been known. In this technique, an optical fiber cable is installed on the object, and scattered light observed by irradiating the optical fiber cable with pulsed light from a laser is analyzed. Such measurement is called distributed measurement, and examples of objects to be measured include, for example, concrete and steel materials (reinforcing bars, steel frames, steel sheet piles, etc.). For example, in the technique of Patent Document 1 below, an optical fiber cable is adhered to a steel sheet pile, and the strain distribution of the steel sheet pile is measured.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In this type of strain measurement, the optical fiber cable is securely adhered to the object over the entire length of the measurement target section so that the optical fiber cable expands and contracts following the expansion and contraction of the object. However, in a large strain region such as when the object is irreversibly damaged (for example, after the yield of steel materials), the shear force acting between the object and the optical fiber cable becomes extremely large. Therefore, in order to measure such a large strain, an adhesive is required that prevents the optical fiber cable and the adhesive layer from peeling off from the object due to the above shear force. However, the characteristics of adhesives capable of coping with such large strain measurements are not clarified even in Patent Document 1.

[0005] The present invention aims to provide an adhesive selection method that enables the selection of an adhesive capable of handling large strain measurements in an adhesive structure for bonding an optical fiber cable for strain measurement to an object. [Means for solving the problem]

[0006] The gist of this invention is as follows:

[0007] [1] An adhesive selection method for selecting an adhesive in an adhesive structure for bonding a fiber optic cable for strain measurement to a predetermined object, wherein the adhesive is selected such that the adhesive strength of the adhesive to the object is greater than the restoring force generated in the adhesive after curing in response to the strain when the maximum strain that should be measurable by the fiber optic cable occurs in the object.

[0008] [2] When the shear bonding 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, and a predetermined length related to the length of the region where the strain ε1 occurs is L, Eg / τg <L / (φ·ε1) …(1) The adhesive selection method described in [1], which involves selecting an adhesive such that the above condition is met.

[0009] [3] In the adhesive structure, the optical fiber cable is bonded to the measurement target area on the object with the adhesive, and a protective tube that protects the portion of the optical fiber cable outside the measurement target area by inserting it into the hollow part is bonded to the object with a second adhesive at a portion connected to the bonding portion of the adhesive, and when the shear bonding strength of the second adhesive to the object is τg2, the elastic modulus of the second adhesive after curing is Eg2, and the outer diameter of the protective tube is D, Eg2 / τg2 <L / (D·ε1) …(2) The adhesive selection method described in [2], which involves selecting the second adhesive such that the condition is met.

[0010] 〔4〕In the bonding structure, the optical fiber cable is bonded to the measurement target area on the object with the adhesive, and a protective tube that protects by inserting a portion of the optical fiber cable outside the measurement target area into a hollow portion is bonded to the object with a second adhesive, and there is a non-bonded portion between the bonded portion of the adhesive and the bonded portion of the second adhesive. The adhesive selection method according to 〔2〕 or 〔3〕.

[0011] 〔5〕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 〔2〕 to 〔4〕.

[0012] 〔6〕The object is a steel material, and the value of ε1 is greater than 1000 μ. The adhesive selection method according to any one of 〔2〕 to 〔5〕.

Advantages of the Invention

[0013] According to the present invention, in an adhesive structure for bonding an optical fiber cable for strain measurement to an object, an adhesive selection method capable of selecting an adhesive capable of corresponding to measurement of large strain can be provided.

Brief Description of the Drawings

[0014] [Figure 1] (a) is a cross-sectional view showing a strain measurement unit and a reinforcing bar including the adhesive structure of the embodiment, and (b) is a cross-sectional view taken along line Ib-Ib in (a). [Figure 2] (a) is a cross-sectional view showing a model of the strain measurement unit in a cross-section orthogonal to the axis of the reinforcing bar, and (b) is a perspective view thereof. (c) is a cross-sectional view showing a model of the bonded portion of the protective tube of the strain measurement unit. [Figure 3] A cross-sectional view showing a strain measurement unit according to a modification.

Modes for Carrying Out the Invention

[0015] Hereinafter, an embodiment of the adhesive selection method according to the present invention will be described in detail with reference to the drawings.

[0016] Figure 1(a) is a cross-sectional view showing the strain measurement unit 2 and reinforcing bar 3, including the adhesive structure 1 of this embodiment, and Figure 1(b) is a cross-sectional view taken along the 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 strain measurement. The reinforcing bar 3 is embedded in concrete in a reinforced concrete structure (not shown). The strain measurement unit 2 is constructed by bonding the optical fiber cable 5 to the surface of the reinforcing bar 3 using the adhesive structure 1.

[0017] A fiber installation surface 7, which forms a flat surface for installing an optical fiber cable 5, is provided on the side of the reinforcing bar 3. The reinforcing bar 3 has a circular cross-section, and a flat portion is part of it, which is the fiber installation surface 7. Commercially available products may be used as such reinforcing bars 3. The fiber installation surface 7 extends along 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 along the entire length of the measurement target section 9 (measurement target area) for strain measurement of the reinforcing bar 3. The optical fiber cable 5 is, for example, an optical fiber core with a diameter of 0.9 mm, in which optical fiber strands are covered with a non-halogen resin or the like. Note that optical fiber strands are optical fibers mainly made of quartz glass covered with a coating such as an ultraviolet-curing resin. In this way, a strain measurement section 2 is formed on the surface of the reinforcing bar 3 by bonding the optical fiber cable 5 with adhesive. The strain measurement section 2 includes an adhesive layer 15 made of hardened adhesive.

[0018] When the optical fiber cable 5 is adhered to the reinforcing bar 3, the surface of the reinforcing bar 3 is polished to remove rust, plating, mill scale, etc. Then, degreasing and cleaning of the bonding position (fiber installation surface 7) are performed using an industrial tissue paper with a solvent such as ethanol. Also, the surface of the fiber installation surface 7 is polished to remove rust, plating, mill scale, etc. Then, the surface of the fiber installation surface 7 may be polished in multiple directions with sandpaper so as not to have a directionality and finished. Then, after the optical fiber cable 5 is temporarily fixed on the fiber installation surface 7 at an interval of about 1 m with an instant adhesive or tape, etc., an adhesive is applied on the fiber installation surface 7 over the entire length of the measurement target section 9, and the optical fiber cable 5 is fixed on the fiber installation surface 7 by curing the adhesive. The application thickness of the adhesive is preferably about the diameter or less of the optical fiber cable 5.

[0019] As the above-mentioned adhesive, an acrylic resin-based adhesive, an epoxy resin-based adhesive, etc. can be used. Also, the adhesive is classified into a dry-curing type, a chemical reaction type, a hot-melt type, a pressure-sensitive type, etc. according to the method of solidification, and any of them can be used. Considering durability, it is preferable that an epoxy resin-based adhesive is adopted as the above-mentioned adhesive. Also, considering workability, it is more preferable that a two-component mixing room temperature curing type epoxy resin-based adhesive is adopted rather than a heat-curing type epoxy resin-based adhesive.

[0020] At one end of the strain measurement unit 2, 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 stainless steel pipe, and its outer diameter 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 portion is inserted into the hollow part of the protective tube 11. The optical fiber cable 5 then 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 of the optical fiber cable 5 bending during installation and wiring, and suppresses the breakage of the optical fiber cable 5. Furthermore, near the end opening of the protective tube 11, a heat-shrinkable rubber tube 13 is covered around the optical fiber cable 5 so that it does not directly contact the edge of the end opening, thereby suppressing the breakage of the optical fiber cable 5. The adhesive used to bond the protective tube 11 to the reinforcing bar 3 will be called the "second adhesive". The second adhesive layer 17, which consists of the hardened second adhesive, is connected to the adhesive layer 15.

[0021] The end of the optical fiber cable 5 drawn out to the outside of the reinforced concrete structure is connected to a measuring instrument (not shown) that performs strain measurement. Further, an analysis device such as a computer is connected to this measuring instrument. The measuring instrument irradiates the optical fiber cable 5 with pulsed light, receives various scattered lights returning from each position in the longitudinal direction of the optical fiber cable 5, and transmits information regarding the intensity, wavelength, etc. of the received scattered light to the analysis device. Examples of the above-mentioned scattered light include Rayleigh scattered light, Brillouin scattered light, etc. As an example of the measuring instrument, for example, an OTDR (Optical Time Domain Reflectometer) that uses Rayleigh scattered light, a BOTDR (Brillouin Optical Time Domain Reflectometer) that uses Brillouin scattered light, etc. can be used. In the analysis device, based on the principle that the intensity and wavelength of the above-mentioned scattered light depend on the strain applied to the optical fiber cable 5, the intensity and wavelength of the scattered light at each position in the longitudinal direction of the optical fiber cable 5 are analyzed. By this analysis, in the analysis device, the strain generated at each position in the longitudinal direction of the optical fiber cable 5 is acquired, for example, at a pitch of several centimeters. Such a strain measurement technique is called distributed measurement.

[0022] The strain measurement unit 2 is capable of measuring large strains of the reinforcing bar 3 such as 1000 μ or more. For example, when the reinforcing bar 3 is SD345 (yield strain 1725 μ), the strain measurement unit 2 can measure the strain after the yield of the reinforcing bar 3 (for example, tens of thousands of μ). Since such large strains can be measured, the strain measurement unit 2 can be used for purposes such as the soundness evaluation of the reinforced concrete structure after an earthquake.

[0023] When large strains occur, such as those after the yielding of reinforcing bar 3, the shear force acting between reinforcing bar 3 and optical fiber cable 5 becomes extremely large. Therefore, in order to measure such large strains, an adhesive is required that prevents the optical fiber cable 5 and adhesive layer 15 from delaminating from reinforcing bar 3 due to the above-mentioned shear force. Below, we will explain how to select an adhesive that will enable the strain measurement unit 2 to handle the measurement of such large strains.

[0024] [Method for selecting adhesives] Generally, after yielding of steel, large localized strains of tens of thousands of micrometers can occur in a narrow region of about 1 to 20 mm. In other words, in the case of reinforcing bar 3, large localized strains can occur in a narrow region of about 1 to 20 mm in the longitudinal direction of the reinforcing bar 3 after yielding. When 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 such a narrow region where large localized strains occur.

[0025] Figure 2(a) is a cross-sectional view showing a model of the strain measurement section 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 a predetermined width bg that is the same height as the diameter of the optical fiber cable 5 and wider than the diameter of the optical fiber cable 5, and is formed on the fiber installation surface 7 in a rectangular cross section that encompasses the cross section of the optical fiber cable 5.

[0026] Here, let τg be the shear bonding strength of the adhesive to the reinforcing bar 3, Eg be the elastic modulus of the hardened adhesive (adhesive layer 15), and φ be the diameter of the optical fiber cable 5. Also, let L be the length of the region in the reinforcing bar 3 where localized large strain occurs, and let ε1 be the maximum strain that should be measurable by the optical fiber cable 5. This strain ε1 is set based on the performance required of the strain measurement unit 2 (measurable strain range). That is, the strain measurement unit 2 is required to be able to measure the strain of the reinforcing bar 3 up to a maximum of ε1. Furthermore, as shown in Figure 2(b), consider part A on the adhesive layer 15 side that is bonded to the region in the reinforcing bar 3 where localized large strain occurs. Part A is a rectangular parallelepiped shape with length L, width bg, and height φ, composed of the optical fiber cable 5 and the adhesive layer 15 that embeds the optical fiber cable 5.

[0027] When a local strain ε1 occurs in a region of length L of the reinforcing bar 3, the strain ε1 is also transmitted to the part A to which it is bonded. Let F1 be the restoring force in the longitudinal direction of the reinforcing bar 3 that occurs in part A in response to this strain ε1. That is, F1 is the force acting on part A in the longitudinal direction of the reinforcing bar 3 when the above strain ε1 occurs, and the force F1 is expressed by the following equation (a). F1 = φ·bg·Eg·ε1 …(a) Furthermore, the shear adhesive force F2 of part A to reinforcing bar 3 is expressed by the following equation (b). F2 = τg·bg·L …(b)

[0028] In order for part A not to separate from rebar 3, F1 <F2 …(c) Therefore, from equations (a) to (c), φ·bg·Eg·ε1<τg·bg·L …(d) It would be good if that were satisfied. To summarize, Eg / τg <L / (φ·ε1) …(1) It is sufficient if it is satisfied. Therefore, if the adhesive of the adhesive structure 1 of the strain measuring unit 2 is selected so as to satisfy the above formula (1), when a strain ε1 occurs in the reinforcing bar 3, the optical fiber cable 5 and the adhesive layer 15 will not peel off from the reinforcing bar 3. That is, if the above formula (1) is satisfied, the strain ε1 can be measured by the strain measuring unit 2. Here, since Eg and τg on the left side of formula (1) are characteristic values of the adhesive, an adhesive having characteristics such that the ratio of these characteristic values τg and Eg is less than L / (φ·ε1) may be selected.

[0029] Incidentally, L in formula (1) is the length of the region where local large strains occur in the reinforcing bar 3. As described above, generally after the yield of the steel material, local large strains can occur in a narrow region of about 1 to 20 mm. Therefore, as the value of L, a value within the range of 1 mm < L < 20 mm may be adopted. Furthermore, in order to obtain a proper adhesive selection result, it is preferable to adopt a value within the range of 1 mm < L < 5 mm. For example, here, it is assumed that L = 2 mm is adopted.

[0030] For example, if the length L of the region where local large strains occur 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 to be measurable by the optical fiber cable 5 is 30000 μ, then from formula (1), an adhesive for which (Eg / τg) is less than 74 may be selected. Also, under the same conditions, when the above maximum strain ε1 is, for example, 20000 μ, an adhesive for which (Eg / τg) is less than 111 may be selected, and when the above maximum strain ε1 is, for example, 15000 μ, an adhesive for which (Eg / τg) is less than 148 may be selected.

[0031] 〔Adhesive for bonding the protective tube 11〕[[ID=...]] Furthermore, in order to enable the strain measurement unit 2 to handle the large strain of the reinforcing bar 3 as described above, the second adhesive must also be considered. That is, if the second adhesive layer 17 is connected to the adhesive layer 15, and the protective tube 11 delaminates from the reinforcing bar 3 due to the large strain of the reinforcing bar 3, the delamination will extend from this point of delamination to the measurement target section 9, and there is a risk that the optical fiber cable 5 in the measurement target section 9 will delaminate 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 delaminating from the reinforcing bar 3 even if a localized large strain ε1 of the reinforcing bar 3 occurs in the section where the protective tube 11 is installed.

[0032] As shown in Figure 2(c), the second adhesive layer 17 is formed on the fiber installation surface 7 with a rectangular cross-section encompassing the cross-section of the protective pipe 11, having a predetermined width bg2 that is the same height as the outer diameter of the protective pipe 11 and wider than the outer diameter of the protective pipe 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 pipe 11 is D, following the above equation (1), Eg2 / τg2 <L / (D·ε1) …(2) If this condition is satisfied, even if a local strain ε1 of the reinforcing bar 3 occurs in the section where the protective pipe 11 is installed, 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 measured more favorably by the strain measuring unit 2.

[0033] For example, if the length L of the region 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 μm, then from equation (2), a second adhesive with (Eg / τg) less than 33 should be selected. Note that this second adhesive and the adhesive constituting the adhesive layer 15 may be the same adhesive.

[0034] Generally, the properties of an adhesive (shear bonding strength, elastic modulus after curing, etc.) depend on the material of the object to be bonded, curing conditions, and operating temperature, making it difficult to specify them uniquely. In the adhesive selection method of this embodiment, the characteristic values ​​Eg, τg, Eg2, and τg2 in equations (1) and (2) may be, for example, values ​​that assume a standard operating environment as described in an adhesive catalog. Alternatively, characteristic values ​​corresponding to the operating environment of the strain measurement unit 2, as described in a catalog, may be used.

[0035] The present invention can be implemented in various forms, including the embodiments described above, by making various changes and improvements based on the knowledge of those skilled in the art. Furthermore, it is possible to construct modified versions by utilizing the technical matters described in the embodiments described above. The configurations of each embodiment may be used in appropriate combinations.

[0036] As shown in Figure 3, a non-adhesive section 21 (non-adhesive portion) may be provided on the fiber installation surface 7. The non-adhesive section 21 is the section between the measurement target section 9 and the adhesive section 23 (adhesive portion) between the reinforcing bar 3 and the protective tube 11. The measurement target section 9 is the adhesive section (adhesive portion) where the reinforcing bar 3 and the optical fiber cable 5 are bonded together. In the non-adhesive section 21, there is no adhesive, and the optical fiber cable 5 and the protective tube 11 are not bonded 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 existence of the non-adhesive section 21 reduces the possibility that the optical fiber cable 5 in the measurement target section 9 will detach from the reinforcing bar 3 starting from the point of detachment, even if the protective tube 11 detaches from the reinforcing bar 3. Therefore, if a non-adhesive section 21 exists, the selection of the second adhesive using the aforementioned formula (2) may be omitted.

[0037] Furthermore, it is not essential to adhere the protective tube 11 to 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 binding wire or cable ties to prevent the optical fiber cable 5 from bending during installation and wiring. Also, it is not essential to cover the optical fiber cable 5 with the protective tube 11 outside the measurement target section 9, and a larger diameter communication optical fiber cable may be connected to the optical fiber cable 5 outside the measurement target section 9. 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 use a fixing method with lower rigidity than the second adhesive for the protective tube 11. For example, a low-rigidity acrylic resin-based adhesive may be used.

[0038] Furthermore, while the embodiment describes an example where reinforcing bars 3 are used as the object of strain measurement, the method is not limited to this, and any steel material in general, such as steel frames or steel sheet piles, can be used as the object. For example, even when SS400 steel (yield strain 1225μ) is used as the object, it is possible to measure the strain after yield, similar to the reinforcing bars 3. Also, the object is not limited to steel materials; for example, it may be a concrete member. Furthermore, while the embodiment describes an example where the optical fiber cable 5 is an optical fiber core with a diameter of 0.9 mm, the optical fiber cable 5 may be a single strand with a diameter of 0.25 mm, or it may be a ribbon core, etc.

[0039] Furthermore, in this embodiment, the cross-sectional shape of the adhesive layer 15 (Figure 2(a)) is modeled as a rectangle with height φ and width bg, but it is not limited to this. The cross-sectional shape of the adhesive layer 15 may be modeled as other shapes and dimensions, such as a trapezoid, to match the actual adhesive application conditions. Based on the modeled cross-sectional shape and dimensions of the adhesive layer 15, an adhesive may be selected according to formula (1) such that part A does not peel off from the reinforcing bar 3. The same applies to the cross-sectional shape of the second adhesive layer 17 (Figure 2(b)) and the method for selecting the second adhesive. [Explanation of Symbols]

[0040] 1...Adhesive structure, 3...Reinforcement bar (object), 5...Optical fiber cable, 9...Measurement target section (measurement target area, adhesive part), 17...Second adhesive layer, 21...Non-adhesive section (non-adhesive part), 23...Adhesive section (adhesive part).

Claims

1. A method for selecting an adhesive for an adhesive structure used to bond a fiber optic cable for strain measurement to a predetermined object, An adhesive selection method is provided, which involves selecting an adhesive such that the adhesive strength of the adhesive to the object is greater than the restoring force generated in the cured adhesive to counteract the strain when the maximum strain that should be measurable by the optical fiber cable occurs in the object, The aforementioned object is a steel material, Let τg be the shear bonding strength of the adhesive to the object. Let Eg be the elastic modulus of the adhesive after curing. Let the diameter of the optical fiber cable be φ. Let the maximum strain be ε1. When the length of the region where the strain ε1 occurs is L, The strain ε1 is the strain of the object after yielding, Eg / τg<L / (φ・ε1) …(1) A method for selecting an adhesive that satisfies the condition described above.

2. In the aforementioned adhesive structure, The optical fiber cable is bonded to the measurement target area on the object with the adhesive. A protective tube, which protects the portion of the optical fiber cable outside the measurement target area by inserting it into the hollow section, is bonded with a second adhesive at a portion connected to the bonding portion of the adhesive on the object. Let τg2 be the shear bonding strength of the second adhesive to the object. Let Eg2 be the elastic modulus of the second adhesive after curing. When the outer diameter of the protective tube is D, Eg2 / τg2<L / (D・ε1)…(2) The adhesive selection method according to claim 1, which involves selecting the second adhesive such that the following condition is met.

3. In the aforementioned adhesive structure, The optical fiber cable is bonded to the measurement target area on the object with the adhesive. A protective tube, which protects the portion of the optical fiber cable outside the measurement target area by inserting it into the hollow section, is bonded to the object with a second adhesive. The adhesive selection method according to claim 1, wherein a non-adherent portion exists between the adhesive portion of the first adhesive and the adhesive portion of the second adhesive.

4. The adhesive selection method according to any one of claims 1 to 3, wherein the object is a steel material, and the value of L is set within the range of 1 mm < L < 20 mm.

5. The adhesive selection method according to any one of claims 1 to 3, wherein the object is a steel material and the value of ε1 is greater than 1000 μm.

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