JP7199282B2 - Optical fiber cable for strain detection - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical fiber cable installed on an object to be measured and used for strain detection.

An optical fiber sensing system that confirms the modulation of light propagating in the optical fiber by embedding an optical fiber cable in the object to be measured, as one of the methods for detecting distortion that occurs in structures and the ground in the fields of civil engineering and construction. There is The optical fiber cable used for this purpose is generally improved in strength by coating the optical fiber bare wire with a fiber-reinforced resin or the like in order to prevent breakage during installation and ensure handleability.

Conventionally, there has been proposed a strain-sensing optical cable having an uneven structure formed on its outer surface in order to improve adhesion to an object to be measured such as a concrete structure (see Patent Documents 1 and 2). In addition, in order to improve the detection accuracy, an optical cable has been proposed in which a plurality of protrusions made of a synthetic resin or adhesive different from the outer covering are provided on the outer surface of the outer covering at predetermined intervals along the longitudinal direction (Patent Reference 3).

On the other hand, in order to prevent deterioration of measurement accuracy due to the influence of environmental humidity, a reinforced coating layer made of fiber-reinforced resin is not provided, and the tensile elastic modulus of the coating that protects the optical fiber is smaller than the tensile elastic modulus of the optical fiber. An optical fiber cable has also been proposed (Patent Document 4).

JP-A-2002-23030 JP-A-2006-64761 JP 2009-92467 A JP 2012-229992 A

However, the conventional techniques described above have the following problems. When the optical fiber cable for strain detection is embedded in the object to be measured, after placing the concrete, vibration is applied with a vibrator or the like to fill and densify the concrete. In the cable, the vibration propagates from the reinforcing coating layer to the optical fiber strand, causing breakage in the optical fiber. In addition, as in the manufacturing method described in Patent Document 1, when unevenness is formed on the surface using a roller, stress is applied to the fiber reinforced resin layer (reinforced coating layer) provided inside the extruded coating layer, The stress propagates to the optical fiber as it is, increasing the transmission loss.

On the other hand, the optical fiber cable described in Patent Document 4 can reduce the propagation of vibration and stress to the optical fiber strands, but since the reinforcing coating layer is not provided, the strength of the cable itself is reduced, and the installation is difficult. Occasionally, the optical fiber is prone to breakage or disconnection, which poses a problem in terms of handling. In addition, as in the optical fiber cable described in Patent Document 3, by forming projections on the surface of the cable jacket by dripping a thermosetting resin, it is possible to prevent an increase in transmission loss due to the application of stress during the formation of projections and depressions. Since there is an interface between the surface of the cable jacket and the projections, the projections are likely to fall off during laying, resulting in poor fixability of the cable to the object to be measured.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an optical fiber cable for strain detection that is easy to handle and has low transmission loss.

An optical fiber cable for strain detection according to the present invention is an optical fiber cable installed on an object to be measured and used for strain detection of the object to be measured. a resin coating layer provided around the wire, the resin coating layer being provided at least as an outermost layer and having a plurality of concave portions and/or convex portions formed thereon; a fixing layer; 1 or 2 or more layers are provided between the optical fiber and the reinforcing layer for securing the strength of the optical fiber, and 1 or 2 or more layers are provided between the fixing layer and the optical fiber. and a protective buffer layer formed of a resin material having a lower bending elastic modulus than the reinforcing layer , wherein the reinforcing layer is formed of a fiber-reinforced plastic having a bending elastic modulus of 10 to 700 GPa, and the protective buffer The layer has a flexural modulus of 3 to 65 GPa when laminated with the reinforcing layer.
Further, in the strain detecting optical fiber cable of the present invention, a release agent is attached to the outer surface of the optical fiber strand, and the diameter of the reinforcing fiber in the fiber reinforced plastic constituting the reinforcing layer is d μm in the cross section. Then, the difference between the diameter Di μm of the inscribed circle contacting the inner surface of the innermost layer of the resin coating layer provided around the optical fiber strand and the diameter Do μm of the circumscribed circle contacting the outer surface of the optical fiber strand is The range shown by the following formula 1 may be used.
In that case, the peripheral length S of the portion of the outer surface of the optical fiber that is not in direct contact with the innermost layer of the resin coating layer may be set to a range that satisfies Equation 2 below.
On the other hand, the protective buffer layer can be made of a resin having a flexural modulus of 0.3 to 5 GPa, for example.

When the optical fiber cable of the present invention has an irregular cross-sectional shape, each numerical value in the above formulas 1 and 2 is the inscribed circle having the maximum diameter in contact with the void portion of the cross section of the optical fiber cable or the optical fiber strand The inscribed circle of the maximum diameter having the same center point as is calculated as a pseudo circle, and the same applies to the following explanation.

According to the present invention, it is possible to obtain an optical fiber cable for strain detection that is less likely to break or break in the optical fiber strand during handling or construction work, and that has low transmission loss.

A and B are cross-sectional views showing the structure of the optical fiber cable according to the first embodiment of the present invention, where A is a vertical cross-sectional view and B is a cross-sectional view taken along line xx shown in A. FIG. FIG. 4 is a cross-sectional view of an optical fiber cable of a first modified example of the first embodiment of the present invention; FIG. 4 is a cross-sectional view of an optical fiber cable of a second modified example of the first embodiment of the present invention; A and B are cross-sectional views showing the structure of an optical fiber cable according to a second embodiment of the present invention, where A is a vertical cross-sectional view and B is a cross-sectional view taken along line yy shown in A. FIG. 3 is an enlarged cross-sectional view of a reinforcing layer 2; FIG.

EMBODIMENT OF THE INVENTION Hereinafter, the form for implementing this invention is demonstrated in detail with reference to attached drawing. In addition, this invention is not limited to embodiment described below.

(First embodiment)
First, an optical fiber cable according to a first embodiment of the present invention will be described. 1A and 1B are cross-sectional views showing the structure of the optical fiber cable of this embodiment, FIG. 1A being a vertical cross-sectional view, and FIG. 1B being a cross-sectional view taken along line xx shown in FIG. 1A. The optical fiber cable 10 of this embodiment is used for strain detection of an object to be measured, and as shown in FIGS. A resin coating layer 5 with an anchoring layer 4 is provided.

[Optical fiber strand 1]
The optical fiber strand 1 is formed by forming one or two layers of resin protective layers around an optical fiber made of a material having a high light transmittance such as quartz glass. As the resin for forming the protective layer, an ultraviolet curable resin such as an ultraviolet curable urethane acrylate is generally used. Although the thickness of the optical fiber wire 1 is not particularly limited, when the outer diameter of the optical fiber is 0.125 mm, the outer diameter of the optical fiber wire 1 is, for example, 0.25 mm.

[Reinforcing layer 2]
The reinforcing layer 2 is for securing the strength of the optical fiber strand 1 , and is provided between the fixing layer 4 and the optical fiber strand 1 in one or more layers. The reinforcing layer 2 has strength to protect the optical fiber strand 1 from being easily broken when laying or handling the optical fiber cable 10, and hardness to maintain the straightness of the cable when installed on the object to be measured. For example, it can be formed of a fiber-reinforced resin (fiber-reinforced plastic), which is a composite material of a reinforcing fiber such as glass and a resin.

If the flexural modulus of the reinforcing layer 2 is too low, the reinforcing effect may be insufficient. It is preferably made of fiber reinforced resin of 10 to 700 GPa. Thereby, the optical fiber strand 1 can be reinforced without lowering the strain detection sensitivity.

[Protective buffer layer 3]
The protective buffer layer 3 is intended to relieve the stress applied during manufacturing and to protect the optical fiber 1 from vibrations during installation, and is made of a resin material having a lower flexural modulus than the reinforcing layer 2. formed. One or more layers of the protective buffer layer 3 are provided between the fixing layer 4 and the optical fiber strand 1 . As a result, the physical stress applied during the manufacture of the optical fiber cable is relieved, thereby preventing an increase in transmission loss. Moreover, by providing the protective buffer layer 3, it is possible to prevent the occurrence of breakage or breakage due to direct transmission of vibration to the optical fiber.

From the viewpoint of stress relaxation, the protective buffer layer 3 is preferably made of a resin having a flexural modulus of 0.3 to 5 GPa. As a result, the effect of relieving the external force is enhanced, and the fiber wire 1 can be reliably protected. For example, when the reinforcing layer 2 is made of a fiber-reinforced resin obtained by impregnating glass rovings with a vinyl ester resin and curing, the protective buffer layer 3 can be made of polypropylene or the like.

Further, it is preferable to select the resin material and thickness of the protective buffer layer 3 so that the flexural modulus of the protective buffer layer 3 when laminated with the reinforcing layer 2 is 3 to 65 GPa. As a result, it is possible to improve the handleability, prevent the cable from breaking during installation, and prevent the disconnection of the optical fiber 1 and the increase in transmission loss.

[Fixing layer 4]
The fixing layer 4 is provided as the outermost layer, and is for ensuring adhesion between the cable and concrete, soil, or the like after being embedded in a measurement object such as a structure or the ground. /or a convex portion is formed. Although FIG. 1 shows an example in which recesses 4a are formed in the fixing layer 4 at predetermined intervals, the present invention is not limited to this, and the fixing layer 4 is provided with protrusions. Alternatively, an uneven shape may be formed. Further, the recesses and protrusions may be formed either regularly or irregularly (randomly).

By providing the fixing layer 4, it is possible to prevent only the cable from falling out of concrete, soil, etc., and the cable can easily follow the strain and stress applied to the measurement object after burying, and good detection performance can be obtained. The fixing layer 4 can be made of, for example, a resin material such as polyethylene.

[how to use]
The optical fiber cable 10 of this embodiment is embedded in, attached to, or wound around an object to be measured, and the strain of the object to be measured is detected by the optical cable strands 1 . Here, "distortion" means a change point of a physical quantity, sound, light, pressure, temperature, or the like. Examples of objects to be measured include, but are not limited to, concrete structures such as buildings, bridges, and tunnels, soil concrete structures such as embankments of rivers, slopes of mountains, and the like.

In the optical fiber cable of this embodiment, in addition to the reinforcing layer and the fixing layer, the protective buffer layer is provided around the optical fiber strands. An increase in transmission loss due to such stress can be prevented. As a result, the optical fiber cable of this embodiment is easy to handle and has low transmission loss.

Although the optical fiber cable of the present embodiment is effective for burial use, it can also be used by being attached or wound around an object to be measured, for example, without being burried. In that case, the fixing layer may not be provided on the optical fiber cable, and the cable may be fixed to the object to be measured with an adhesive or the like.

(First Modification of First Embodiment)
Next, an optical fiber cable according to a first modified example of the first embodiment of the invention will be described. FIG. 2 is a cross-sectional view of the optical fiber cable of this modification. 2, the same components as those of the optical fiber cable 10 of the first embodiment shown in FIG. 1B are denoted by the same reference numerals, and detailed description thereof will be omitted. In the optical fiber cable 10 shown in FIG. 1B, the layer in contact with the optical fiber strand 1 is the reinforcing layer 2, but the present invention is not limited to this. The layer in contact with the optical fiber strand 1 may be the protective buffer layer 3, and the reinforcing layer 2 may be provided on the outside thereof.

Like the optical fiber cable 20 of this modified example, even if the protective buffer layer 3 is provided inside the reinforcing layer 2, the optical fiber strands are protected during handling and installation, and transmission loss due to stress applied during manufacturing is reduced. Increase can be prevented. Note that the configuration and effect of this modified example other than those described above are the same as those of the above-described first embodiment.

(Second modification of the first embodiment)
Next, an optical fiber cable according to a second modification of the first embodiment of the invention will be described. FIG. 3 is a cross-sectional view of the optical fiber cable of this modification. 3, the same components as those of the optical fiber cable 10 of the first embodiment shown in FIG. 1B are denoted by the same reference numerals, and detailed description thereof will be omitted.

Although FIGS. 1A, B and 2 show an optical fiber cable having one optical fiber strand, the present invention is not limited to this, and may include two or more optical fiber strands. good too. In that case, for example, like the optical fiber cable 30 shown in FIG. and the fixing layer 4 is formed thereon.

As in the optical fiber cable of this modified example, even when there are two or more optical fiber strands, since a reinforcing layer, a protective buffer layer, and a fixing layer are provided around them, transmission loss is reduced while ensuring ease of handling. increase can be prevented. The configuration and effect of this modification other than the above are the same as those of the above-described first embodiment and its first modification.

(Second embodiment)
Next, an optical fiber cable according to a second embodiment of the invention will be described. 4A and 4B are cross-sectional views showing the structure of the optical fiber cable of this embodiment, FIG. 4A being a vertical cross-sectional view, and FIG. 4B being a cross-sectional view taken along line yy shown in FIG. 4A. 5 is an enlarged sectional view of the reinforcing layer 2. FIG. In FIGS. 4A and 4B, the same components as those of the optical fiber cable 10 of the first embodiment shown in FIGS. 1A and 1B are denoted by the same reference numerals, and detailed description thereof will be omitted.

As shown in FIGS. 4A and 4B, in the optical fiber cable 40 of this embodiment, the release agent 6 is adhered to part or all of the outer surface of the fiber strand 1 . That is, the optical fiber cable 40 has a portion where the release agent 6 exists between the fiber wire 1 and the resin coating layer 5, and the fiber wire 1 and the resin coating layer 5 are not bonded to each other in that portion. It is a non-adhesive part.

As in the optical fiber cable 40 of this embodiment, if the release agent 6 is applied to a part or all of the outer surface of the fiber strand 1, the fiber strand 1 will be deformed due to the difference in linear thermal expansion coefficient when the resin coating layer 5 is formed. , and micro-venting due to the unevenness of the surface of the reinforcing layer 2 are suppressed, thereby reducing the transmission loss.

[Release agent 6]
The release agent 6 may be any agent that does not adhere to either the outer surface of the fiber strand 1 or the inner surface of the innermost layer (the reinforcing layer 2 or the protective buffer layer 3) of the resin coating layer 5, and may be silicone oil or other known agents. Various waxes and powders can be used.

Waxes used for the release agent 6 include, for example, polyolefin waxes such as polyethylene wax and polypropylene wax, branched hydrocarbon waxes such as microcrystalline wax, long-chain hydrocarbon waxes such as paraffin wax and Sasol wax, Dialkyl ketone waxes such as distearyl ketone, carnauba wax, montan wax, behenic acid behenate, trimethylolpropane tribehenate, pentaerythritol tetrabehenate, pentaerythritol diacetate dibehenate, glycerin tribehenate, 1 , 18-octadecanediol distearate, tristearyl trimellitate and distearyl maleate, and amide waxes such as ethylenediaminebehenylamide and tristearylamide trimellitate.

Examples of powder include silicon nitride (Si ₃ N ₄ ) and boron nitride (BN). When boron nitride is used as the release agent 6, the thermal conductivity of the optical fiber cable 40 can be improved, so that the responsiveness of thermal measurement can be enhanced when thermal measurement is performed simultaneously with strain detection.

The amount of the release agent 6 attached is not particularly limited, but in the cross section of the optical fiber cable 40, the diameter Diμm of the inscribed circle contacting the inner surface of the innermost layer of the resin coating layer 5 and the optical fiber strand It is preferable that the difference from the diameter Do μm of the circumscribed circle contacting the outer surface of 1 is within the range shown by the following formula 3. Here, "d" in Equation 3 below is the diameter (μm) of the reinforcing fibers in the fiber-reinforced plastic constituting the reinforcing layer 2 (see FIG. 5).

The difference (Di-Do) between the diameter Diμm of the inscribed circle contacting the inner surface of the innermost layer of the resin coating layer 5 and the diameter Doμm of the circumscribed circle contacting the outer surface of the optical fiber strand 1 is the number of reinforcing fibers contained in the reinforcing layer 2. If the diameter is less than ⅓ of the diameter d μm, the area where the release agent 6 adheres becomes large, and the effect of reducing the transmission loss cannot be obtained sufficiently. Further, if Di-Do exceeds 30 μm, the internal shape may collapse and distortion may occur, resulting in variations in bending strength in the axial direction of the cylinder. On the other hand, when the release agent 6 is attached so that Di-Do falls within the range shown in the above formula 3, the decrease in transmission loss is prevented, and an optical fiber cable in which the axial angle dependence of bending strength is suppressed can be obtained. Obtainable.

In order to stably obtain the effect of reducing transmission loss, the peripheral length S (μm) preferably satisfies Equation 4 below. Here, Do (μm) in Equation 4 below is the diameter of the circumscribed circle of the optical fiber strand 1, and π is the circular constant.

The method of attaching the release agent 6 to a portion of the outer surface of the fiber 1 is not particularly limited, but when the release agent 6 is liquid, a method such as coating can be employed. Further, when the release agent 6 is powdery, for example, the optical fiber strand 1 is continuously passed through a tank in which the powdery release agent 6 such as boron nitride is stored in a previous step or the same step. Then, a liquid thermosetting resin is allowed to pass therethrough to form the resin coating layer 5 . The adhesion of the release agent 6 and the formation of the resin coating layer 5 may be performed continuously in the same process, or may be performed in separate processes.

In the optical fiber cable 40 of this embodiment, the releasing agent 6 is attached to the outer surface of the fiber wire 1, and the non-bonded portion is provided between the fiber wire 1 and the resin coating layer 5, so that the resin coating layer The transmission loss can be reduced by alleviating stress during the formation of 5 and irregularities on the surface of the reinforcing layer 2 . This effect is particularly remarkable in an optical fiber cable in which the layer in contact with the optical fiber strands 1 shown in FIGS. 4A and 4B is the reinforcing layer 2. 3, the transmission loss reduction effect can be obtained by attaching the release agent 6 to the outer surface of the fiber strand 1 .

EXAMPLES Hereinafter, the effects of the present invention will be specifically described with reference to examples and comparative examples. In this example, optical fiber cables of examples and comparative examples were produced by the following method, and their performance was evaluated.

<Example 1>
After impregnating three 280 tex glass rovings (Nitto Boseki Co., Ltd. RS28) with thermosetting vinyl ester resin (Showa Denko Co., Ltd. non-styrene type RF-313), an optical fiber with an outer diameter of 250 μm is placed in the center. The strands were placed and passed through a squeeze nozzle to form a reinforcing layer. The outer diameter after formation of the reinforcing layer was 0.9 mm, and the reinforcing fiber content in the reinforcing layer was 56.7% by volume.

Subsequently, linear low-density polyethylene (LLDPE; Nippon Polyethylene Co., Ltd. Novatec UF240, durometer hardness 49) was extruded around the reinforcing layer and immediately water-cooled to form a protective buffer layer. The outer diameter after forming the protective buffer layer was 4.2 mm. After that, heat treatment was performed to cure the thermosetting resin.

Next, LLDPE (Neo-Zex 2540R manufactured by Prime Polymer Co., Ltd., durometer hardness 57) was extruded around the protective buffer layer so as to have an outer diameter of 6.0 mm. Then, before cooling, the optical fiber cable of Example 1 was obtained by passing it between embossing rollers and embossing the surface to form a fixing layer.

<Example 2>
An ultraviolet curable resin (Dai-cure coat 8714A manufactured by Dainippon Ink and Chemicals Co., Ltd.) is coated around the optical fiber strand with an outer diameter of 250 μm so that the outer diameter is 350 μm, and is cured by irradiating ultraviolet rays to provide protective buffering. formed a layer. Next, 10 glass rovings of 280 tex (RS28 manufactured by Nitto Boseki Co., Ltd.) were impregnated with a thermosetting vinyl ester resin (non-styrene type RF-313 manufactured by Showa Denko Co., Ltd.), and then a protective buffer layer was placed in the center. was placed and passed through a squeeze nozzle to form a reinforced layer. The outer diameter after formation of the reinforcing layer was 1.6 mm, and the reinforcing fiber content in the reinforcing layer was 57.9% by volume.

Next, LLDPE (Neo-Zex 2540R manufactured by Prime Polymer Co., Ltd., durometer hardness 57) was extruded around the reinforcing layer so that the outer diameter was 3.4 mm, immediately cooled with water, and then heated in a curing tank to make it thermosetting. The resin was allowed to cure. After that, the surface was softened by heating to a temperature at which LLDPE melts, and in this state, the surface was embossed by passing it between embossing rollers to form a fixing layer, and an optical fiber cable of Example 2 was obtained. .

<Example 3>
LLDPE (Novatec UF240 manufactured by Nippon Polyethylene Co., Ltd., durometer hardness 49) was extruded around the optical fiber strand having an outer diameter of 250 μm so that the outer diameter became 1.0 mm, and immediately water-cooled to form a protective buffer layer. Next, 10 glass rovings of 280 tex (RS28 manufactured by Nitto Boseki Co., Ltd.) were impregnated with a thermosetting vinyl ester resin (non-styrene type RF-313 manufactured by Showa Denko Co., Ltd.), and then a protective buffer layer was placed in the center. was placed and passed through a squeeze nozzle to form a reinforced layer. The outer diameter after formation of the reinforcing layer was 2.0 mm, and the reinforcing fiber content in the reinforcing layer was 56.5% by volume.

Next, LLDPE (Neo-Zex 2540R manufactured by Prime Polymer Co., Ltd., durometer hardness 57) was extruded around the reinforcing layer so that the outer diameter was 3.8 mm, immediately cooled with water, and then heated in a curing bath to make it thermosetting. The resin was allowed to cure. After that, the surface was softened by heating to a temperature at which LLDPE melts, and in this state, the surface was embossed by passing it through embossing rollers to form a fixing layer, and the optical fiber cable of Example 3 was obtained. .

<Example 4>
After impregnating three 280 tex glass rovings (Nitto Boseki Co., Ltd. RS28) with thermosetting vinyl ester resin (Showa Denko Co., Ltd. non-styrene type RF-313), an optical fiber with an outer diameter of 250 μm is placed in the center. Placed the wires. Then, a waste cloth impregnated with silicone oil (KF-96-50CS manufactured by Shin-Etsu Silicone Co., Ltd.) was brought into contact with the surface of the optical fiber, and then passed through a squeeze nozzle to form a reinforcing layer. The outer diameter after formation of the reinforcing layer was 0.9 mm, and the reinforcing fiber content in the reinforcing layer was 56.7% by volume.

Subsequently, linear low-density polyethylene (LLDPE; Nippon Polyethylene Co., Ltd. Novatec UF240, durometer hardness 49) was extruded around the reinforcing layer and immediately water-cooled to form a protective buffer layer. The outer diameter after forming the protective buffer layer was 4.2 mm. After that, heat treatment was performed to cure the thermosetting resin.

Next, LLDPE (Neo-Zex 2540R manufactured by Prime Polymer Co., Ltd., durometer hardness 57) was extruded around the protective buffer layer so as to have an outer diameter of 6.0 mm. Then, before cooling, the optical fiber cable of Example 4 was obtained by passing it between embossing rollers and embossing the surface to form a fixing layer.

In the optical fiber cable of Example 4, in any cross section, the diameter d of the reinforcing fiber in the fiber-reinforced plastic constituting the reinforcing layer is 12.5 to 15.3 μm, and the diameter Do of the circumscribed circle of the optical fiber strand is was 248 to 256 μm, the diameter Di of the inscribed circle in contact with the inner surface of the reinforcing layer, which was the innermost layer of the resin coating layer, was 253 to 264 μm, and the Di-Do was 5 to 8 μm. In addition, in the optical fiber cable of Example 4, the peripheral length S of the portion where the innermost layer of the resin coating layer is not in direct contact is 475 μm (211°) in an arbitrary cross section, which satisfies Equation 4 above. there were.

<Comparative Example 1>
After impregnating three 280 tex glass rovings (Nitto Boseki Co., Ltd. RS28) with thermosetting vinyl ester resin (Showa Denko Co., Ltd. non-styrene type RF-313), an optical fiber with an outer diameter of 250 μm is placed in the center. The strands were placed and passed through a squeeze nozzle to form a reinforcing layer. The outer diameter after formation of the reinforcing layer was 0.9 mm, and the reinforcing fiber content in the reinforcing layer was 56.7% by volume.

Next, LLDPE (Neo-Zex 2540R manufactured by Prime Polymer Co., Ltd., durometer hardness 57) was extruded around the reinforcing layer so that the outer diameter was 6.0 mm, immediately cooled with water, and then heated in a curing bath to make it thermosetting. The resin was allowed to cure. After that, the surface was softened by heating to a temperature at which LLDPE melts, and in this state, the surface was embossed by passing it between embossing rollers to form a fixing layer, and an optical fiber cable of Comparative Example 1 was obtained. .

<Comparative Example 2>
An ultraviolet curable resin (Dai-cure coat 8714A manufactured by Dainippon Ink and Chemicals Co., Ltd.) is coated around the optical fiber strand with an outer diameter of 250 μm so that the outer diameter is 350 μm, and is cured by irradiating ultraviolet rays to provide protective buffering. formed a layer. Next, LLDPE (Neo-Zex 2540R manufactured by Prime Polymer Co., Ltd., durometer hardness 57) was extruded around the protective buffer layer so that the outer diameter was 3.4 mm, immediately cooled with water, and then heated in a curing bath to thermally cure. hardening resin. Thereafter, the cable was heated to a temperature at which LLDPE melted to soften the surface, and in this state, the cable was passed through embossing rollers to emboss the surface to form a fixing layer, and an optical fiber cable of Comparative Example 2 was obtained. .

(evaluation)
For each of the optical fiber cables of Examples and Comparative Examples produced by the above-described method, transmission loss, occurrence of breakage when embedded in concrete, and pull-out load were evaluated by the following methods.

<Transmission loss>
Transmission loss at a wavelength of 1.55 μm was measured using an OTDR (Optical Time Domain Reflectometer) AQ7250 manufactured by Ando Electric Co., Ltd. At that time, an optical fiber bare wire was taken out from one end of the optical fiber cable, cut, and connected to the OTDR by a mechanical splice through a dummy fiber.

<Concrete embedding test>
Using a forced twin-screw mixer with a capacity of 50 L, 350 kg/m 3 of cement, 870 kg/m ³ of fine aggregate, 901 kg/m ³ of coarse aggregate, and 175 kg/m ³ of water are mixed so that the total amount becomes 40 L. m ³ , high performance AE water reducing agent was blended at a ratio of 2.8 kg/m ³ and kneaded for 90 seconds. The materials used in this test are shown below.
・Cement: Ordinary portland cement manufactured by Taiheiyo Cement Co., Ltd. (specific gravity: 3.16)
・Fine aggregate: land sand (surface dry specific gravity 2.60, maximum particle size 5 mm)
・Coarse aggregate: crushed stone (surface dry specific gravity 2.67, maximum grain size 20mm)
・Water: city water ・High performance AE water reducing agent: SP8SV manufactured by BASF Japan Co., Ltd.

The optical fiber cable was embedded while pouring the obtained fresh concrete into the mold. At that time, the wooden frame was hit with a wooden mallet to vibrate it while filling so as not to cause poor filling of the concrete. Then, after curing at room temperature for 24 hours, the mold was released and further cured in water for 6 days. After that, the specimen was cured at room temperature in the atmosphere until the age of 28 days, and the continuity of the embedded optical fiber cable was confirmed with an optical fiber checker. did.

<Pullout load>
The transmission loss of an optical fiber cable tends to decrease as the optical fiber strands are in more firm contact with the resin coating layer. Therefore, in this example, the adhesive force between the optical fiber strand and the resin coating layer (innermost layer) was measured by a pull-out test, and the effect on the reduction in transmission loss was indirectly confirmed. The pull-out test was performed using a measurement sample cut out to an arbitrary length from each of the optical fiber cables of Examples 1 and 4. For each measurement sample, 10 cm from one end was left, and the resin coating layer and the like existing outside the optical fiber was removed from the other portions to expose the optical fiber. Then, a spring scale was attached to the exposed optical fiber strand and pulled out from the resin coating layer, and the maximum load obtained at that time was taken as the pull-out load.

The above results are summarized in Table 1 below.

As shown in Table 1 above, the optical fiber cable of Comparative Example 1 having the same structure as the conventional product without the protective buffer layer had a high transmission loss, and the optical fiber broke when embedded in concrete. In addition, the optical fiber cable of Comparative Example 2, in which no reinforcement layer was provided, had a low transmission loss, but the optical fiber was broken when embedded in concrete. On the other hand, the optical fiber cables of Examples 1 to 4 produced within the scope of the present invention had low transmission loss and did not break when buried in concrete.

The optical fiber cable of Example 4, in which silicone oil was applied to the surfaces of the fiber strands, had a particularly low transmission loss among the optical fiber cables of Examples. In the optical fiber cable of Example 4, the drawing load was reduced to 70% or less compared to the optical fiber cable of Example 1, and the degree of adhesion between the fiber strands and the reinforcing layer was low. From the above results, it was confirmed that an optical fiber cable for strain detection with excellent handleability and low transmission loss can be obtained according to the present invention.

REFERENCE SIGNS LIST 1 optical fiber strand 2 reinforcing layer 3, 33 protective buffer layer 4 fixing layer 4a concave portion 5 resin coating layer 6 release agent 10, 20, 30, 40 optical fiber cable

Claims (4)

An optical fiber cable installed on a measurement object and used for strain detection of the measurement object,
one or more optical fiber strands;
a resin coating layer provided around the optical fiber strand;
has
The resin coating layer has at least
a fixing layer provided as the outermost layer and having a plurality of concave portions and/or convex portions;
a reinforcing layer provided between the fixation layer and the optical fiber strand and having one or more layers to ensure the strength of the optical fiber strand;
one or more layers provided between the fixing layer and the optical fiber strand, and a protective buffer layer formed of a resin material having a lower flexural modulus than the reinforcing layer ,
The reinforcing layer is made of a fiber-reinforced plastic having a flexural modulus of 10 to 700 GPa,
The optical fiber cable for strain detection , wherein the protective buffer layer has a flexural modulus of 3 to 65 GPa when laminated with the reinforcing layer . A release agent is attached to the outer surface of the optical fiber,
In a cross section, when the diameter of the reinforcing fiber in the fiber-reinforced plastic constituting the reinforcing layer is dμm, the diameter of the inscribed circle in contact with the inner surface of the innermost layer of the resin coating layer provided around the optical fiber strand. 2. The optical fiber cable for strain detection according to claim 1 , wherein the difference between the diameter Di [mu]m and the diameter Do [mu]m of the circumscribed circle in contact with the outer surface of the optical fiber strand is within the range shown by the following formula (I).

3. The optical fiber cable for strain detection according to claim 2 , wherein a peripheral length S of a portion of the outer surface of the optical fiber that is not in direct contact with the innermost layer of the resin coating layer satisfies the following formula (II).

The optical fiber cable for strain detection according to any one of claims 1 to 3 , wherein the protective buffer layer is made of a resin having a flexural modulus of 0.3 to 5 GPa.

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