Fiber bragg grating strain sensor and preparation method thereof

Abstract

The invention relates to a fiber bragg grating strain sensor and a preparation method thereof. The method comprises the following steps: providing a metal substrate of which the surface is provided with a strain groove for accommodating a fiber bragg grating; placing a fiber bragg grating in the strain groove, and applying pre-strain to the fiber bragg grating; covering a packaging material on the strain groove area provided with the fiber bragg grating, wherein the packaging material comprises a mixture of metal powder and glass powder; and sintering the substrate covered with the packaging material to melt the glass powder and bond and solidify the metal powder to form a composite packaging body for packaging and fixing the fiber grating in the strain groove. According to the invention, the interface thermal stress caused by the difference of thermal expansion coefficients of materials is reduced, the temperature cross sensitivity of the sensor is effectively inhibited, and the strain measurement precision and long-term stability of the sensor in a wide temperature range are improved.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and in particular to a fiber optic strain sensor and its fabrication method. Background Technology

[0002] Fiber Bragg grating strain sensors are widely used in structural health monitoring due to their high sensitivity and resistance to electromagnetic interference. Traditional fiber Bragg grating strain sensors typically consist of a substrate for transmitting strain, a fiber Bragg grating fixed to the substrate, and a protective encapsulation layer covering the grating. A common implementation uses a metal material (such as stainless steel or titanium alloy) as the substrate, fixing the fiber Bragg grating via mechanical clamping or bonding, and encapsulating it with organic polymer materials such as epoxy resin. However, this type of structure has significant technical drawbacks in practical applications. Because of the inherent differences in the coefficients of thermal expansion between the metal substrate, encapsulation material, and the silica fiber itself, the varying degrees of expansion or contraction between these materials when the ambient temperature changes generate additional thermal stress on the fiber Bragg grating. This causes a non-strain-induced drift in its center wavelength, known as the temperature cross-sensitivity effect. This effect introduces significant measurement errors, severely reducing the sensor's measurement accuracy and reliability over a wide temperature range or in varying temperature environments. Summary of the Invention

[0003] Therefore, it is necessary to provide a fiber optic strain sensor and its fabrication method to address the technical problem of large temperature drift caused by the mismatch of the thermal expansion coefficients of traditional fiber optic strain sensors.

[0004] This invention provides a method for fabricating a fiber Bragg grating strain sensor, the method comprising: A metal substrate with strain grooves on its surface for accommodating fiber Bragg gratings is provided; The fiber grating is placed in the strain groove, and a pre-strain is applied to the fiber grating; A sealing material comprising a mixture of metal powder and glass powder is used to cover the strain groove region where a fiber Bragg grating is provided. The substrate covered with encapsulation material is sintered to melt the glass powder and bond and solidify the metal powder, forming a composite encapsulation that encapsulates and fixes the fiber grating within the strain groove.

[0005] In one embodiment, the metal substrate is an aluminum substrate. In one embodiment, the strain groove is a V-groove.

[0006] In one embodiment, the provision of a metal substrate with strain grooves on its surface for accommodating fiber Bragg gratings further includes: The metal substrate is subjected to a surface treatment to obtain a specific surface roughness, the surface treatment including chemical etching using a hydrofluoric acid solution.

[0007] In one embodiment, the metal powder is aluminum powder.

[0008] In one embodiment, the mass ratio of the metal powder to the glass powder is 6:4 to 7.5:2.5.

[0009] In one embodiment, the particle size of the metal powder ranges from 5 micrometers to 15 micrometers. In one embodiment, the sintering process is a gradient sintering process performed under a protective atmosphere.

[0010] In one embodiment, the gradient sintering process includes: The first stage involves raising the temperature to 240°C to 260°C and holding it there to remove organic matter. In the second stage, the temperature is raised to 370°C to 390°C and held to allow the glass powder to fully melt.

[0011] In one embodiment, the heat preservation time for the first stage is 25 to 35 minutes, and the heat preservation time for the second stage is 50 to 70 minutes.

[0012] In one embodiment, after the sintering process forms the composite package, the process further includes armoring the fiber optic cable leading out of the fiber Bragg grating, specifically including: The lead-out optical fiber is inserted into a flexible protective tube and then sealed and fixed at the end.

[0013] In one embodiment, the process of covering the encapsulation material also includes a process of vibrating and pressurizing the encapsulation material to control the density of the encapsulation layer.

[0014] This invention also provides a fiber Bragg grating strain sensor, fabricated by the above-described method for preparing a fiber Bragg grating strain sensor, comprising: A metal substrate with strain grooves on its surface for accommodating fiber Bragg gratings; A fiber grating is disposed within the strain groove and is in a pre-stretched state; A composite package covers the strain groove area and encapsulates and fixes the fiber grating inside the strain groove; The composite encapsulation body is made of a mixture of sintered metal powder and glass powder.

[0015] The aforementioned fiber Bragg grating strain sensor and its fabrication method utilize a metal substrate and a packaging material comprising a mixture of metal powder and glass powder. A sintering process melts the glass powder and bonds and solidifies the metal powder to form a composite package. The resulting fiber Bragg grating strain sensor exhibits a more thermally coordinated integrated structure between the metal substrate, the composite package, and the internal fiber Bragg grating through inorganic sintering. The metal powder component in this composite package effectively enhances the thermal conductivity of the packaging layer, while its thermal expansion characteristics are compatible with the metal substrate. The molten and solidified glass phase not only provides robust packaging protection but also acts as a stress buffer and transition between the metal powder and the fiber Bragg grating. This ensures that the optical path from the substrate through the package to the fiber undergoes relatively uniform thermal deformation during temperature changes, thereby reducing interfacial thermal stress caused by differences in material thermal expansion coefficients. This effectively suppresses temperature cross-sensitivity of the sensor and improves its strain measurement accuracy and long-term stability over a wide temperature range. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0017] Figure 1 This is a flowchart illustrating a method for fabricating a fiber Bragg grating strain sensor according to one embodiment. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] It should be noted that when a component is referred to as being "fixed to" or "set on" another component, it can be directly on the other component or there may be an intermediate component. When a component is considered to be "connected to" another component, it can be directly connected to the other component or there may be an intermediate component present. The terms "vertical," "horizontal," "upper," "lower," "left," "right," and similar expressions used in this specification are for illustrative purposes only and do not represent the only possible implementation.

[0020] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0021] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature and the second feature are in indirect contact through an intermediate medium. Furthermore, "above," "over," and "on top" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0022] Unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used in this specification includes any and all combinations of one or more of the associated listed items.

[0023] The following is combined Figure 1 The fiber optic strain sensor and its fabrication method of the present invention are described.

[0024] like Figure 1 As shown, in one embodiment, a method for fabricating a fiber optic strain sensor includes the following steps: Step S110: A metal substrate with a strain groove on its surface for accommodating a fiber Bragg grating is provided.

[0025] The metal substrate is an aluminum substrate, using 6061 aluminum alloy sheet. The aluminum substrate and the aluminum powder in the subsequent packaging material form a systematic match in terms of thermal expansion coefficient, which effectively reduces the temperature cross-sensitivity caused by material thermal mismatch, so that the temperature drift coefficient of the sensor is ≤0.81με / °C in the entire temperature range (-40℃ to 80℃), see Table 6.

[0026] Before providing the metal substrate, the substrate undergoes surface treatment to achieve a specific surface roughness. This treatment includes chemical etching using a hydrofluoric acid solution. Increasing the substrate surface roughness enhances the mechanical interlocking and bonding force between the encapsulation material and the substrate, improving the shear strength of the encapsulation interface. See Table 1 for encapsulation results; test results show a shear strength of 35.2 MPa, ensuring long-term stability of strain transfer. Alternatively, the aluminum substrate is immersed in acetone and ultrasonically treated for 10 minutes using an ultrasonic cleaner (40 kHz) to remove surface oil. It is then chemically etched by soaking in a 10% hydrofluoric acid solution for 3 minutes, rinsed with deionized water until neutral, and then dried. After nitrogen drying, it is placed in an 80°C oven for 30 minutes. The treated metal substrate, verified by a surface roughness tester, has Ra ≤ 0.1 μm.

[0027] Step S120: Place the fiber grating in the strain groove and apply pre-strain to the fiber grating.

[0028] The strain gauge is a V-groove. The fiber grating is placed inside the V-groove, and both ends are temporarily fixed with high-temperature resistant adhesive (model: HT-400). A pre-strain of 0.3% (corresponding to a displacement ΔL = 0.15 mm) is applied using a micro-displacement stage (accuracy 0.1 μm), maintaining the pre-strain error <0.02%. Tension fixing is then applied, followed by secondary fixing with UV-curable adhesive (wavelength 365 nm, strength 50 mW / cm²). The V-groove provides precise positioning and a good strain transfer interface for the fiber grating, improving the strain transfer efficiency and measurement linearity of the sensor. See Table 3. Experimental data show that its sensitivity remains stable at 1.25 pm / με throughout the entire measurement range, and the linearity R² = 0.998.

[0029] Step S130: Cover the strain groove area where the fiber optic grating is provided with an encapsulation material, the encapsulation material comprising a mixture of metal powder and glass powder.

[0030] The metal powder is aluminum powder, which forms a material system with the same coefficient of thermal expansion as the aluminum substrate. As a high thermal conductivity filler, the encapsulation layer formed after being combined with glass powder has excellent thermal conductivity, reducing the dynamic response time. At the same time, the thermal matching of the material system ensures the stability of the sensor in thermal cycling. See Table 1. After 100 thermal cycles, the maximum wavelength drift is only 8.5 pm.

[0031] Table 1. Packaging effect The mass ratio of metal powder to glass powder is 6:4 to 7.5:2.5. This ensures that the glass phase can fully wet and bind the aluminum powder particles to form a dense structure, while maximizing the high thermal conductivity of the aluminum powder. This achieves high density and low porosity in the encapsulation layer, which is fundamental to achieving high thermal conductivity and excellent mechanical properties. Vibration filling and pressurization are performed during the encapsulation material coating process to control the density of the encapsulation layer. External force promotes the tight and uniform filling of the mixed powder in the strain groove, facilitating subsequent sintering to obtain a highly dense encapsulation layer. This is key to achieving low porosity, high thermal conductivity, and strong interfacial bonding. The particle size of the metal powder ranges from 5 micrometers to 15 micrometers. Optimizing the powder packing density and sintering activity allows for tighter packing during vibration filling and pressurization, ensuring the density and uniformity of the final encapsulation, providing a foundation for achieving low porosity and high shear strength. As shown in Table 1, test data indicate that the encapsulation layer porosity can be as low as 0.3%, and the shear strength can reach 35.2 MPa. Alternatively, aluminum powder and low-temperature glass powder (softening point 320℃) are dry-mixed at a mass ratio of 7:3, and the mixture is filled using a three-dimensional vibration table (amplitude 0.5mm) in three batches. After each filling, a pressure of 5MPa is applied to ensure that the filling uniformity error is <5% and the final encapsulation layer density is ≥2.8g / cm³ (Archimedes method test).

[0032] Step S140: The substrate covered with the encapsulation material is sintered to melt the glass powder and bond and solidify the metal powder to form a composite encapsulation that fixes the fiber grating in the strain groove.

[0033] The sintering process is a gradient sintering process carried out under a protective atmosphere. The gradient sintering process includes: a first stage, heating to 240°C to 260°C and holding at that temperature to remove organic matter; and a second stage, heating to 370°C to 390°C and holding at that temperature to fully melt the glass powder. The holding time for the first stage is 25 to 35 minutes, and the holding time for the second stage is 50 to 70 minutes.

[0034] The sintering process is a gradient sintering process carried out under a protective atmosphere to prevent the metal powder from oxidizing at high temperatures and to control the decomposition of organic matter and the melting process of glass powder. This avoids the generation of oxidation impurities and defects inside the encapsulation layer, ensuring the chemical stability and physical integrity of the encapsulation body, and directly contributing to the sensor's excellent corrosion resistance. After immersion in 3.5% NaCl solution for 90 days, the wavelength drift is less than 15 pm (see Table 7). The gradient sintering process includes: a first stage of heating to 240℃ to 260℃ and holding to remove organic matter; and a second stage of heating to 370℃ to 390℃ and holding to fully melt the glass powder. The process of removing the binder and melting the glass matrix is ​​precisely controlled in stages to achieve a uniform transformation of the encapsulation material from powder to a dense sintered body, avoiding the generation of bubbles and cracks. This results in a composite encapsulation body with a uniform structure and strong bonding. Its performance, as tested (see Table 2), shows that the strain sensitivity is improved by 12% compared to the traditional structure. The first stage of heat preservation lasts for 25 to 35 minutes, and the second stage lasts for 50 to 70 minutes. This ensures that the physicochemical reactions in each stage are fully carried out, that is, that volatile organic compounds are fully discharged and the glass powder is completely melted to wet all aluminum powder particles. This ensures that the encapsulation layer is fully densified, meeting the design density and porosity requirements (see Table 1), and giving the encapsulation excellent fatigue resistance (see Table 5). Fatigue life tests show that the sensitivity decay is only 4% after 5 million cycles. Optionally, a programmable temperature-controlled sintering furnace is used, with an oxygen content of <100ppm (nitrogen protection). The first stage (degreasing): heating to 250℃ at 10℃ / min and holding for 30 minutes to discharge volatile organic compounds; the second stage (melting): heating to 380℃ at 5℃ / min and holding for 60 minutes to allow the glass powder to flow fully; finally (cooling): cooling to 80℃ at a rate of ≤2℃ / min to avoid thermal stress cracking.

[0035] Table 2. Test Results The fabrication method of the fiber Bragg grating strain sensor in this embodiment involves using a metal substrate and preparing a mixture of metal powder and glass powder as the encapsulation material. The glass powder is then melted and the metal powder is bonded and solidified through sintering to form a composite encapsulation. The resulting fiber Bragg grating strain sensor exhibits a more thermally coordinated integrated structure between the metal substrate, the composite encapsulation, and the internal fiber Bragg grating through inorganic sintering. The metal powder component in the composite encapsulation effectively improves the thermal conductivity of the encapsulation layer, while its thermal expansion characteristics are compatible with the metal substrate. The solidified glass phase not only provides robust encapsulation protection but also acts as a stress buffer and transition between the metal powder and the fiber Bragg grating. This ensures that the optical path structure from the substrate through the encapsulation to the optical fiber undergoes relatively uniform thermal deformation during temperature changes, thereby reducing interfacial thermal stress caused by differences in the thermal expansion coefficients of the materials. This effectively suppresses temperature cross-sensitivity of the sensor and improves its strain measurement accuracy and long-term stability over a wide temperature range.

[0036] After the composite package is formed by sintering, the process of armoring the fiber optic cable leading fiber also includes: inserting the leading fiber into a flexible protective tube and sealing and fixing it at the end, providing mechanical protection and stress isolation for the fragile fiber leading part, which improves the durability and anti-lateral interference capability of the sensor in complex installation and use environments. Lateral interference suppression test (see Table 4) shows that the strain error is only 2.22με under 500N lateral force, and the suppression rate is greater than 32%.

[0037] Furthermore, the present invention also provides a fiber Bragg grating strain sensor. This fiber Bragg grating strain sensor is fabricated using the aforementioned method for fabricating fiber Bragg grating strain sensors, and includes a metal substrate, a fiber Bragg grating, and a composite encapsulation body. A strain groove for accommodating the fiber Bragg grating is provided on the surface of the metal substrate. The fiber Bragg grating is disposed within the strain groove and is in a pre-stretched state. The composite encapsulation body covers the strain groove area and encapsulates and fixes the fiber Bragg grating within the strain groove. The composite encapsulation body is made of a mixture of sintered metal powder and glass powder.

[0038] The sensor itself possesses characteristics of low temperature drift (see Table 6), high strain sensitivity (see Table 3), excellent anti-interference performance (see Table 4), and long-term stability (see Tables 5 and 7). The metal substrate is an aluminum substrate, using 6061 aluminum alloy. The strain groove is a V-groove with a depth of 0.3 mm and a bottom curvature radius of 0.5 mm. The surface of the aluminum substrate is chemically etched, with a surface roughness Ra ≤ 0.1 μm. The pre-stretch strain of the fiber Bragg grating is 0.2%~0.4%. The metal powder is aluminum powder, the density of the composite package is ≥ 2.8 g / cm³, and the porosity of the composite package is ≤ 0.5%.

[0039] Experimental verification shows that the fiber optic strain sensor of this invention significantly outperforms traditional structures in terms of sensitivity, anti-interference ability, fatigue life, temperature stability, and corrosion resistance, meeting the needs of long-term monitoring in extreme environments. Specific data and comparison results are shown in Tables 3-7.

[0040] Table 3. Strain sensitivity test (-5000με~5000με) Conclusion: The full-range sensitivity is stable at 1.25 pm / με, and the linearity error is <0.2%.

[0041] Table 4. Lateral Interference Suppression Test (50-500N Lateral Force) Conclusion: The error is only 2.22με under a 500N lateral force, with a suppression rate of >32%.

[0042] Table 5. Fatigue life test (±5000με cyclic loading) Conclusion: Sensitivity decays by only 4% after 5 million cycles, and lifespan is 3 times that of traditional structures.

[0043] Table 6. Temperature Drift Test (-40°C to 80°C) Conclusion: The temperature drift coefficient across the entire temperature range is ≤0.81με / °C, meeting the requirements for extreme environments.

[0044] Table 7. Corrosion resistance test (3.5% NaCl solution) Conclusion: After 90 days of immersion, the wavelength drift was <15pm and the corrosion depth was <0.2mm.

[0045] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0046] The above-described embodiments are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the appended claims.

Claims

1. A method for fabricating a fiber optic strain sensor, characterized in that, The method includes: A metal substrate with strain grooves on its surface for accommodating fiber Bragg gratings is provided; The fiber grating is placed in the strain groove, and a pre-strain is applied to the fiber grating; A sealing material comprising a mixture of metal powder and glass powder is used to cover the strain groove region where a fiber Bragg grating is provided. The substrate covered with encapsulation material is sintered to melt the glass powder and bond and solidify the metal powder, forming a composite encapsulation that encapsulates and fixes the fiber grating within the strain groove.

2. The method for fabricating a fiber optic strain sensor according to claim 1, characterized in that, The metal substrate is an aluminum substrate, and the metal powder is aluminum powder.

3. The method for fabricating a fiber optic strain sensor according to claim 1, characterized in that, The strain groove is a V-shaped groove.

4. The method for fabricating a fiber optic strain sensor according to claim 1, characterized in that, The metal substrate with strain grooves on its surface for accommodating fiber Bragg gratings is further comprising: The metal substrate is subjected to a surface treatment to obtain a specific surface roughness, the surface treatment including chemical etching using a hydrofluoric acid solution.

5. The method for fabricating a fiber optic strain sensor according to claim 1, characterized in that, The mass ratio of the metal powder to the glass powder is 6:4 to 7.5:2.5, and the particle size of the metal powder ranges from 5 micrometers to 15 micrometers.

6. The method for fabricating a fiber optic strain sensor according to claim 1, characterized in that, The sintering process is a gradient sintering process performed under a protective atmosphere.

7. The method for fabricating a fiber optic strain sensor according to claim 6, characterized in that, The gradient sintering process includes: The first stage involves raising the temperature to 240°C to 260°C and holding it there to remove organic matter. In the second stage, the temperature is raised to 370°C to 390°C and held to allow the glass powder to fully melt.

8. The method for fabricating a fiber optic strain sensor according to claim 7, characterized in that, The heat preservation time for the first stage is 25 to 35 minutes, and the heat preservation time for the second stage is 50 to 70 minutes.

9. The method for fabricating a fiber optic strain sensor according to claim 1, characterized in that, The process of covering the encapsulation material also includes a process of vibrating and pressurizing the encapsulation material to control the density of the encapsulation layer.

10. A fiber optic strain sensor, characterized in that, The fiber optic strain sensor is prepared by the method of any one of claims 1 to 9, comprising: A metal substrate with strain grooves on its surface for accommodating fiber Bragg gratings; A fiber grating is disposed within the strain groove and is in a pre-stretched state; A composite package covers the strain groove area and encapsulates and fixes the fiber grating inside the strain groove; The composite encapsulation body is made of a mixture of sintered metal powder and glass powder.

Images