Two-dimensional large-strain flexible FBG sensor based on PDMS / SiO2 hybrid substrate

Abstract

The two-dimensional large-strain flexible FBG sensor based on PDMS / SiO2 mixed substrate relates to the field of optical fiber sensing technology, and is characterized by comprising a PDMS / SiO2 mixed flexible film, an optical fiber arranged on the surface of the PDMS / SiO2 mixed flexible film, and an FBG sensing unit inscribed on the optical fiber; the optical fiber is arranged in a continuous bending path on the surface of the PDMS / SiO2 mixed flexible film, and the FBG sensing unit is located in a semicircular characteristic region of the continuous bending path. The optical fiber is arranged in a continuous bending path on the surface of the PDMS / SiO2 mixed flexible film, and the external large deformation is geometrically modulated by the semicircular bending path, so that the axial strain directly borne by the FBG region can be reduced, and the large-strain adaptability of the sensor is improved. The PDMS / SiO2 mixed flexible film is adopted as the substrate, the large deformation capability of PDMS is retained, the mechanical stability of the substrate is improved through SiO2 particles, and the viscoelastic hysteresis and output drift in the cyclic stretching are reduced.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, specifically to a two-dimensional large-strain flexible FBG sensor based on a PDMS / SiO2 hybrid substrate and its fabrication and testing methods. Background Technology

[0002] Fiber Bragg grating (FBG) sensors are characterized by their small size, light weight, strong resistance to electromagnetic interference, corrosion resistance, and multi-point multiplexing capability. They have been widely used in structural health monitoring, flexible structure deformation measurement, aerospace, smart materials, and wearable sensing. FBG sensors can sense external strain or temperature changes by detecting changes in the Bragg center wavelength, exhibiting high measurement accuracy and stability.

[0003] However, traditional linearly bonded FBG sensors have significant limitations in large strain measurements. Because the axial tensile strain that the silica fiber itself can withstand is limited, when the flexible structure under test undergoes large deformation, if the fiber is arranged linearly along the tensile direction, a large proportion of the external strain will be transferred to the fiber axis, which can easily lead to fiber breakage, reflection spectrum distortion, or grating failure.

[0004] Existing flexible FBG sensors typically use flexible polymers such as PDMS as encapsulation or support substrates to improve the compatibility between the sensor and the flexible structure. However, pure PDMS materials suffer from significant viscoelasticity, large cyclic tensile hysteresis, and insufficient stability under long-term loading, which affects the repeatability and reliability of the sensor in large strain cyclic testing.

[0005] Furthermore, for two-dimensional in-plane strain measurement, FBGs arranged in a single direction have difficulty distinguishing strain inputs from different directions, especially on complex flexible structural surfaces where tension, lateral contraction, and geometric coupling may coexist. Therefore, there is a need for a flexible FBG sensor that is simple in structure, easy to fabricate, capable of achieving two-dimensional large strain measurement on flexible substrates, and can be further arrayed to achieve multi-point measurement. Summary of the Invention

[0006] The purpose of this invention is to provide a two-dimensional large-strain flexible FBG sensor based on a PDMS / SiO2 hybrid substrate, in order to solve the problems of limited large-strain measurement range of existing linear FBGs, large hysteresis of flexible substrates, and difficulty in effectively identifying two-dimensional in-plane strain.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a two-dimensional large-strain flexible FBG sensor based on a PDMS / SiO2 hybrid substrate, comprising a PDMS / SiO2 hybrid flexible film, an optical fiber disposed on the surface of the PDMS / SiO2 hybrid flexible film, and an FBG sensing unit etched on the optical fiber; the optical fiber is disposed on the surface of the PDMS / SiO2 hybrid flexible film in a continuous curved path, and the FBG sensing unit is located within a semi-circular feature region of the continuous curved path.

[0008] Preferably, the optical fiber is located entirely on the upper surface of the PDMS / SiO2 hybrid flexible film and is tightly bonded to the surface of the PDMS / SiO2 hybrid flexible film by an adhesive. The adhesive is 502 glue, and the thickness of the adhesive layer is ignored in the theoretical analysis, and the optical fiber and the surface of the flexible film are regarded as a bonding interface without relative slippage.

[0009] Preferably, the plane containing the PDMS / SiO2 hybrid flexible thin film is taken as the XY plane, with the length direction of the film as the x-direction and the width direction as the y-direction; the FBG sensing unit includes at least two FBGs corresponding to the x-direction and the y-direction, respectively. More preferably, the FBG sensing unit includes four FBGs (FBGx1, FBGy1, FBGy2, and FBGx2) located on the same continuous optical fiber path, corresponding to four semi-circular sensitive bending regions (X1, Y1, Y2, and X2), respectively. X1 and X2 are sensitive bending regions corresponding to in-plane strain response in the x-direction, and Y1 and Y2 are sensitive bending regions corresponding to in-plane strain response in the y-direction; the four sensitive bending regions are not four independent optical fibers, but different local regions on the same continuous optical fiber.

[0010] Preferably, the X1, Y1, Y2, and X2 sensitive bending regions are all arc-shaped regions, with a central angle preferably of 180°, i.e., a semi-circular bending segment. Without affecting strain transfer and optical signal stability, the central angle can also be appropriately adjusted according to the thin film size, the minimum bending radius of the optical fiber, and the number of sensing units. Each FBG gate area is preferably arranged at the top of the arc or at a point of continuous curvature in the corresponding arc-sensitive region to avoid the FBG being located at abrupt transitions between straight and curved sections.

[0011] Preferably, the PDMS / SiO2 hybrid flexible film is made by mixing PDMS base agent, PDMS curing agent, and SiO2 particles. In this embodiment, the PDMS used is Dow Corning SYLGARD™ 184 two-component silicone rubber elastomer, product name SYLGARD™ 184 Silicone Elastomer Kit, manufactured by Dow Corning Corporation, and purchased from Chang'an Kaihui Adhesives Store (a sole proprietorship) in Dongguan City. The product includes SYLGARD™ 184 Silicone Elastomer Base and SYLGARD™ 184 Silicone Elastomer Curing Agent. The mass ratio of the PDMS base agent, PDMS curing agent, and SiO2 particles is 10:1:1.22.

[0012] Preferably, the SiO2 particles have a particle size of 2μm, and the PDMS / SiO2 hybrid flexible film is prepared by solution casting and thermosetting, with a curing temperature of 80℃ and a curing time of 60min.

[0013] Preferably, the PDMS / SiO2 hybrid flexible film is formed using a PMMA mold, which is a square plate with a length of 100mm, a width of 100mm, and a thickness of 3mm, and a rectangular groove with a length of 88mm, a width of 82mm, and a depth of 0.9mm is opened in the center.

[0014] Preferably, the method for manufacturing the sensor includes the following steps:

[0015] (1) Add PDMS main agent, PDMS curing agent and SiO2 particles to a beaker at a mass ratio of 10:1:1.22 and stir in two stages using an electric stirrer. First stir at 200 rpm for 2 hours, then stir at 100 rpm for 2 hours to obtain PDMS / SiO2 mixture.

[0016] (2) The PDMS / SiO2 mixture is injected into the PMMA film mold and left to stand at room temperature for 4 hours to allow the bubbles in the mixture to escape completely; the room temperature refers to the laboratory ambient temperature without additional heating or cooling, preferably 15°C to 30°C, and the room temperature in this embodiment is about 17.6°C;

[0017] (3) Place the mold after standing in a high temperature oven and cure it at 80°C for 60 min. After cooling, demold to obtain a PDMS / SiO2 mixed flexible film.

[0018] An optical fiber routing path is traced on the surface of the PDMS / SiO2 hybrid flexible film. Optical fibers with FBG sensing units inscribed on them are placed along the path and fixed using 502 adhesive dots. The amount of 502 adhesive used should be sufficient to achieve fiber positioning, prevent significant accumulation after curing, and avoid covering or encapsulating the effective FBG grating area. Obvious adhesive droplets, clumps, or localized rigid accumulation in the effective FBG grating area should be avoided. However, the adhesive should be allowed to form a thin and continuous bonding layer between the FBG grating area and the film under capillary action to ensure strain can be transferred to the effective FBG grating area. In actual operation, adhesive can be added dropwise at several fixed points along the optical fiber path, allowing the adhesive to spread naturally along the contact gap between the optical fiber and the film under surface tension and capillary action.

[0019] The thickness of the adhesive layer is not limited by an absolute value, but it should meet the following requirements: after curing, the adhesive layer is a thin bonding layer that does not significantly change the designed bending radius of the optical fiber, does not form local rigid protrusions around the FBG gate region, does not significantly change the original strain distribution on the surface of the PDMS / SiO2 hybrid flexible film, and can ensure no relative slippage between the optical fiber and the film during stretching. In theoretical analysis, since the thickness of this adhesive layer is relatively small compared to the film thickness and the bending radius of the optical fiber, its thickness can be approximately ignored.

[0020] (5) The optical fiber is fixed to the surface of the PDMS / SiO2 hybrid flexible film by using 502 glue dots. After the adhesive is cured, a two-dimensional large strain flexible FBG sensor is obtained.

[0021] Preferably, in step (5), the "small amount of adhesive application" refers to minimizing the amount of adhesive applied in a single application while ensuring fiber positioning and fixation and preventing relative slippage between the fiber and the film. This ensures the adhesive only wets the contact area between the fiber and the film surface, without forming significant accumulation or blocky hardened areas around the FBG gate region. The adhesive layer formed after curing the 502 adhesive should be a thin bonding layer, with a thickness that does not significantly change the fiber bending radius, does not significantly increase local stiffness, and is negligible in theoretical analysis.

[0022] Preferably, the PDMS / SiO2 hybrid flexible thin film is subjected to scanning electron microscopy to observe the dispersion of SiO2 particles in the PDMS matrix; the optical fiber bending path is simulated using COMSOL wave optics to determine the influence of different bending radii on the mode field distribution and bending loss; and the sensor is simulated using Abaqus finite element method to analyze the geometric changes of the optical fiber path under tension in different directions. The minimum bending radius of the optical fiber path is preferably determined based on simulation results, the effective area of ​​the thin film, and the arrangement requirements of the sensing units. In this embodiment, the minimum bending radius is 4 mm.

[0023] Preferably, the sensor is connected to a fiber Bragg grating demodulator during testing. The fiber Bragg grating demodulator has a sampling frequency of 1 Hz and a wavelength resolution of 1 pm, and is used to collect the center wavelength of each FBG sensing unit in real time.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] (1) In this invention, optical fibers are laid on the surface of PDMS / SiO2 hybrid flexible film in a continuous bending path. The semi-circular bending path is used to geometrically modulate the large external deformation, which can reduce the axial strain directly borne by the FBG region and improve the large strain adaptability of the sensor.

[0026] (2) The present invention uses a PDMS / SiO2 hybrid flexible film as a substrate, which retains the large deformation capability of PDMS and improves the mechanical stability of the substrate through SiO2 particles, which helps to reduce viscoelastic hysteresis and output drift during cyclic stretching.

[0027] (3) The present invention adopts a surface-mount integration method, which avoids the damage to the integrity of the flexible thin film structure caused by slotting embedding. The preparation steps are simple and the optical fiber path can be flexibly designed according to the needs of two-dimensional strain measurement.

[0028] (4) The present invention realizes two-dimensional in-plane strain measurement through FBG sensing units in sensitive bending regions in the x and y directions, and the coupling response between different directions can be characterized by the sensitivity matrix, which is suitable for two-dimensional large strain monitoring of flexible structures; furthermore, the continuous bending optical fiber path and FBG sensing units can be arrayed along the surface of the flexible film to form a multi-point, multi-region strain monitoring structure.

[0029] (5) The sensor of the present invention can obtain a stable center wavelength response in temperature cycling, strain cycling and constant strain holding tests, providing a feasible solution for long-term monitoring of complex flexible structures.

[0030] (6) The single sensor unit of the present invention can be used independently for two-dimensional large strain measurement, or multiple sensor units can be arranged in an array on the same flexible film or multiple flexible films to realize multi-point strain detection and regional deformation monitoring of the flexible structure surface. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the sensor;

[0032] Figure 2 This is a schematic diagram of the equivalent transformation of fiber bending;

[0033] Figure 3 This is a straight fiber model;

[0034] Figure 4This is a map showing the distribution of the equivalent refractive index region.

[0035] Figure 5 The electric field distribution inside a bent optical fiber with different radii of curvature is shown.

[0036] Figure 6 This is a diagram of the fiber optic structure.

[0037] Figure 7 This is an internal view of the flexible substrate model;

[0038] Figure 8 Mesh partitioning diagram for the fiber optic model;

[0039] Figure 9 Mesh generation diagram for the flexible substrate model;

[0040] Figure 10 The diagram shows the change in fiber shape during the x-direction stretching simulation.

[0041] Figure 11 The diagram shows the change in fiber shape during the stretching simulation in the y-direction.

[0042] Figure 12 This is a diagram showing the shape change of the optical fiber during the xy bidirectional stretching simulation.

[0043] Figure 13 Diagram showing solution preparation;

[0044] Figure 14 This is a diagram showing the solution being stirred.

[0045] Figure 15 Diagram of the solution in the mold;

[0046] Figure 16 This is a diagram of a molded sensor.

[0047] Figure 17 SEM image of the PDMS / SiO2 hybrid flexible substrate surface;

[0048] Figure 18 SEM image of a cross-section of a PDMS / SiO2 hybrid flexible substrate;

[0049] Figure 19 Diagram of the displacement stage tensile test platform;

[0050] Figure 20 For the scatter plot and fitted curve of the temperature rise data;

[0051] Figure 21 The scatter plot and fitted curve of the temperature cycle are shown.

[0052] Figure 22 This is a diagram illustrating the tensile strain test process.

[0053] Figure 23The graph shows the relationship between the wavelength shift of FBGx1 and FBGy1 in the x-direction and strain.

[0054] Figure 24 The graph shows the relationship between the wavelength shift of FBGx1 and FBGy1 in the y-direction and strain.

[0055] Figure 25 The strain sensitivity and fitting curves of FBGx1 and FBGy1 in the principal strain direction are shown.

[0056] Figure 26 The strain stability diagram is shown in the x-direction.

[0057] Figure 27 This is a strain stability diagram in the y-direction. Detailed Implementation

[0058] 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 a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0059] In the description of this invention, it should be noted that the terms "upper," "lower," "left," "right," "inner," "outer," "front," "rear," "one end," and "the other end," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0060] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "setting," "connection," "adhesion," etc., should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an adhesive connection; it can be a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0061] In the description of this invention, "room temperature" refers to the laboratory ambient temperature without additional heating or cooling, preferably between 15°C and 30°C. In this embodiment, the room temperature during the preparation, settling, and initial temperature testing phases is approximately 17.6°C. This value is only used to illustrate the actual experimental conditions of this embodiment and does not constitute a limitation on the scope of protection of this invention.

[0062] Please see Figure 1 This invention provides a technical solution: a two-dimensional large-strain flexible FBG sensor based on a PDMS / SiO2 hybrid substrate, comprising a PDMS / SiO2 hybrid flexible film, a continuously bent optical fiber disposed on the upper surface of the PDMS / SiO2 hybrid flexible film, and FBG sensing units inscribed on the continuously bent optical fiber. The continuously bent optical fiber is arranged along a semi-circular characteristic path along the film surface, and the FBG sensing units are disposed in the semi-circular characteristic region; multiple sensor units can also be arrayed along the surface of the structure under test to achieve multi-point strain detection.

[0063] The optical fiber is entirely located on the surface of the PDMS / SiO2 hybrid flexible thin film and is bonded and fixed to the film surface with 502 adhesive. In actual use, the 502 adhesive only serves as a fixing and strain transfer medium. The thickness of the adhesive layer is relatively small compared to the film thickness and the bending radius of the optical fiber. Therefore, it can be approximately assumed that there is no relative slippage between the optical fiber and the film surface.

[0064] The FBG sensing unit preferably includes four FBGs, namely FBGx1, FBGy1, FBGy2, and FBGx2. To clarify the arrangement of each sensitive region, the plane containing the PDMS / SiO2 hybrid flexible film is taken as the XY plane, the length direction of the film is taken as the x-direction, and the width direction of the film is taken as the y-direction. Four semi-circular sensitive bending regions, X1, Y1, Y2, and X2, are formed sequentially along the same continuous optical fiber path. FBGx1 is set in region X1, FBGy1 is set in region Y1, FBGy2 is set in region Y2, and FBGx2 is set in region X2.

[0065] Wherein, X1 and X2 are the sensitive bending regions corresponding to the in-plane strain response in the x-direction, and Y1 and Y2 are the sensitive bending regions corresponding to the in-plane strain response in the y-direction. X1, Y1, Y2, and X2 are not four independent optical fibers, but rather four local arc regions at different locations within the same continuous optical fiber path. Preferably, each arc region uses the same or similar bending radius to ensure that each FBG has similar initial geometric conditions.

[0066] Furthermore, the central angle of the arc region where the FBG is located is preferably 180°, i.e., a semi-circular sensitive bending segment. The FBG gate area is preferably located at the top of the arc or in a region of continuous curvature within the corresponding arc region, avoiding placement at abrupt transitions between straight and curved sections. While meeting the requirements for fiber bending loss and thin film size, the central angle can also be appropriately adjusted according to the sensor size, array quantity, and strain measurement direction.

[0067] When the optical fiber is laid out in a semi-circular continuous curved path, and the PDMS / SiO2 hybrid flexible film is subjected to in-plane stretching, the fiber path deforms in tandem with the substrate. The large external strain is initially manifested as changes in the arc length, local rotation angle, and curvature of the curved path, which in turn leads to axial strain in the FBG region. The drift of the FBG center wavelength is mainly caused by the axial strain in the FBG region, and the curvature change is used to characterize the geometric modulation effect of the curved path on the large external deformation.

[0068] To facilitate the explanation of the working principle of the FBG sensing unit, the FBG center wavelength, axial strain response, and temperature coupling response can be expressed by the following key formulas:

[0069]

[0070]

[0071] in, Let be the Bragg center wavelength of the i-th FBG. The effective refractive index of the optical fiber core. For the grating period, Let be the average strain along the fiber axis of the i-th FBG. For ordinary quartz fiber, It is usually around 0.22.

[0072] Please see Figures 2 to 5 To determine the impact of fiber bending radius on transmission modes, this embodiment uses the COMSOL MultiphySiOcs wave optics module to establish an equivalent model of the bent fiber. Based on the conformal mapping concept, the circularly curved fiber is equivalent to a straight fiber model with a non-uniform refractive index distribution, and then the electric field distribution under different bending radii is solved.

[0073] In this embodiment, the refractive index of the external environment is set to 1.33. It should be noted that this refractive index of 1.33 is used to characterize the bending mode variation trend under low refractive index external medium conditions. When the optical fiber is embedded in a PDMS or PDMS / SiO2 composite substrate, the external equivalent refractive index can be further modified according to the actual substrate material. The simulation results are mainly used to determine the variation trend of mode leakage enhancement as the bending radius decreases, rather than to make an absolute prediction of the actual loss value. Numerical ports are set at both ends of the model, and perfect matching layers and scattering boundary conditions are set at the upper and lower boundaries. Bending radii R are set to 7mm, 9mm, 11mm, 13mm, 15mm, and 17mm respectively, and parameter scans are performed under the same boundary conditions.

[0074] Simulation results show that when the bending radius is large, the optical field is mainly concentrated near the fiber core; as the bending radius decreases, the fundamental mode field shifts outward from the bend, and the electric field component in the cladding region increases. When the bending radius is less than approximately 15 mm, the bend causes a strong disturbance to the propagation of the guided mode, exhibiting obvious whispering-gallery mode characteristics. Therefore, in the design of the sensor structure, both structural compactness and optical transmission stability should be considered comprehensively, and excessively small bending radii should be avoided in the FBG grating region.

[0075] In this embodiment, the minimum bending radius of the fiber path is 4 mm. This structure was not determined solely by simulation results, but rather by a trade-off between simulation conclusions and practical application constraints: on the one hand, simulations show that reducing the bending radius increases the risk of bending loss, therefore the sensitive FBG region should be located in a position with continuous curvature and smooth bending as much as possible; on the other hand, in order to form four sensitive bending regions X1, Y1, Y2 and X2 on a finite area PDMS / SiO2 hybrid flexible film and to achieve two-dimensional large strain measurement, the fiber path needs to have a sufficiently compact spatial arrangement.

[0076] Therefore, the minimum bending radius of 4mm can be considered as the lower limit design value selected in this embodiment among the thin film area, the number of sensing units, the two-dimensional strain sensing capability, and the stability of optical signal transmission. For subsequent structural optimization, if the thin film area allows, the bending radius of the arc region where the FBG is located can be further increased, or bending loss can be reduced by expanding the effective area of ​​the thin film, optimizing the turning connection section, and the arraying deployment method. The simulation results can explain the reasons for the attenuation of some FBG reflected signals in actual tests and provide guidance for the selection of fiber path radius and FBG deployment location.

[0077] Please see Figures 6 to 12To analyze the deformation characteristics of the sensor under tensile conditions in different directions, this embodiment uses Abaqus software to establish a finite element model. First, the bending path of the optical fiber is drawn in the XY plane, and a three-dimensional optical fiber structure is generated using the Sweep method. Then, a flexible substrate model with a thickness of 0.8 mm, a length of 88 mm, and a width of 52 mm is established, and the optical fiber is assembled with the flexible substrate.

[0078] The fiber optic model uses C3D8 eight-node linear hexahedral elements with a global mesh size of 0.05 mm; the flexible substrate model uses C3D10 ten-node quadratic tetrahedral elements with a global mesh size of 0.45 mm. The fiber-substrate interface is constrained using Tie Constraint to simulate the cooperative deformation relationship between the fiber and the flexible substrate.

[0079] In the finite element simulation, three working conditions were set up: uniaxial tension in the x-direction, uniaxial tension in the y-direction, and biaxial tension in the xy direction. The results show that during tension in the x-direction, the bending region where the transverse FBG is located gradually expands and the curvature continuously decreases; during tension in the y-direction, the bending region where the longitudinal FBG is located gradually expands and the curvature continuously decreases; during biaxial tension in the xy direction, the regions where both the transverse and longitudinal FBGs are located undergo geometric reconstruction.

[0080] The PDMS used in this embodiment is a commercially available two-component silicone elastomer, product name Dow Corning SYLGARD™ 184 Silicone Elastomer Kit, manufactured by Dow Corning Corporation, and purchased from Chang'an Kaihui Adhesives Store (a sole proprietorship) in Dongguan City. The product consists of SYLGARD™ 184 Silicone Elastomer Base and SYLGARD™ 184 Silicone Elastomer Curing Agent. The PDMS base is Sylgard 184 Silicone Elastomer Base, whose main component is dimethylvinylsiloxy-terminated polydimethylsiloxane, and it contains surface-treated silica reinforcing filler. The PDMS curing agent is Sylgard 184 Silicone Elastomer Curing Agent, whose main component is a trimethylsiloxy-terminated poly(methylhydrosiloxane-co-dimethylsiloxane) copolymer, and contains a platinum catalyst. The platinum catalyst is preferably a platinum-divinyltetramethyldisiloxane complex, i.e., the Karstedt platinum catalyst.

[0081] The Sylgard 184 Silicone Elastomer Base, Sylgard 184 Silicone Elastomer Curing Agent, and SiO2 particles were added to a beaker in a mass ratio of 10:1:1.22, wherein the SiO2 particles had a particle size of 2 μm. The mixture was stirred directly in the beaker using an electric stirrer, first at 200 rpm for 2 hours, and then at 100 rpm for 2 hours. This two-stage stirring ensured that the SiO2 particles were more fully dispersed in the PDMS system and reduced air bubbles introduced by high-speed stirring.

[0082] The stirred PDMS / SiO2 mixture was injected into a PMMA film mold. The PMMA mold was a square plate 100mm long, 100mm wide, and 3mm thick, with a rectangular groove in the center measuring 88mm long, 82mm wide, and 0.9mm deep. After injection, the mold was left to stand at room temperature for 4 hours to allow air bubbles in the mixture to escape completely. Room temperature refers to a laboratory environment without additional heating or cooling, preferably between 15°C and 30°C; in this embodiment, it is approximately 17.6°C.

[0083] After settling, the mold is placed in a high-temperature oven and cured at 80°C for 60 minutes. After curing and cooling to room temperature, the mold is demolded along the edge using a blade to obtain a PDMS / SiO2 mixed flexible film.

[0084] Table 1 Key parameters of PDMS / SiO2 hybrid flexible thin films

[0085] An optical fiber routing path was drawn on the surface of the PDMS / SiO2 hybrid flexible thin film. An optical fiber with an FBG sensing unit inscribed on it was placed along the path and fixed using 502 adhesive dots. This dot-bonding method utilizes the surface tension and capillary action of the adhesive to form a thin adhesive layer between the optical fiber and the thin film, thereby ensuring a secure bond while reducing the disturbance of the adhesive layer to the strain field of the flexible thin film.

[0086] To characterize the microstructure of the PDMS / SiO2 hybrid flexible film, scanning electron microscopy was used to observe the film surface and cross-section in this embodiment. The results showed that the film surface was generally continuous, without obvious through cracks or large-sized pores; the SiO2 particles were dispersed in the PDMS matrix, and no severe large-scale aggregation was observed.

[0087] Please see Figure 19 This embodiment constructs a thin film tensile testing platform based on a two-dimensional displacement stage. The platform includes a two-dimensional manual displacement stage, a thin film fixture, an extended connection structure, and a fixed base. The displacement stage is installed in reverse, allowing the loading process to be directly driven mechanically by a micrometer rotation mechanism.

[0088] Since the optical fiber is located on the upper surface of the flexible film, a rubber pad is placed on the upper surface of the clamping nozzle to prevent localized micro-bending loss or fiber breakage caused by pressure on the fiber during clamping. During the test, the two ends of the sensor sample are fixed in the clamp, and tensile displacement in the x or y direction is applied by rotating a micrometer.

[0089] The optical response of the sensor is acquired by a fiber Bragg grating demodulator. The demodulator has a sampling frequency of 1 Hz and a wavelength resolution of 1 pm. The sensor input is connected to the demodulator. The broadband light emitted by the demodulator is transmitted to the FBG array and reflected at the corresponding Bragg wavelength. The reflected signal returns to the demodulator and is demodulated to the center wavelength.

[0090] Please see Figure 20 and Figure 21 In this embodiment, a constant temperature chamber was used for temperature cycling tests. The initial ambient temperature was 17.6℃, which was the room temperature at the start of the temperature experiment in this embodiment. The temperature was first raised to 20℃, which was then used as the starting temperature for the temperature cycling test. Subsequently, the temperature was increased to 80℃ in 20℃ increments, and then allowed to cool naturally back to 20℃. The test results showed that the center wavelengths of FBGx1, FBGy1, FBGy2, and FBGx2 all exhibited a redshift with increasing temperature, and showed good linear temperature response within the range of 20℃ to 80℃.

[0091] Please see Figures 22 to 25 In this embodiment, tensile cyclic tests were performed in the x and y directions respectively. During the test, the strain was gradually increased from the original length to 10% in 1% strain increments, and then relaxed back to the original length in the same 1% strain increments. The complete loading-unloading process was repeated three times. Due to bending losses in some bending areas, the two effective reflection signals FBGx1 and FBGy1 were mainly used in the experimental analysis.

[0092] Test results show that FBGx1 can produce a significant wavelength shift when stretched in the x-direction, and FBGy1 can produce a significant wavelength shift when stretched in the y-direction. At the same time, when stretched in the y-direction, FBGx1 also exhibits a relatively obvious and linear wavelength response. This phenomenon is related to the geometric coupling of the continuously bent optical fiber path and the in-plane deformation coupling of the PDMS / SiO2 flexible substrate, indicating that the present invention can sense the coupled two-dimensional strain field.

[0093] When using two effective reflected signals, FBGx1 and FBGy1, for two-dimensional strain measurement, the responses of the two FBGs to the x and y directions can be represented as a two-dimensional sensitivity matrix. This matrix includes both the sensitivity in the main sensing direction and the sensitivity in the cross direction, and is used to characterize the two-dimensional strain coupling relationship in a continuously bent fiber path.

[0094]

[0095] In the formula, This represents the in-plane normal strain of the flexible thin film containing the sensor along the (x) direction. This represents the in-plane normal strain along the (y) direction of the flexible thin film containing the sensor; and These represent the Bragg center wavelength shifts of FBGx1 and FBGx2 under stress relative to their initial states, respectively. and These represent the Bragg center wavelength shifts of FBGy1 and FBGy2 under stress relative to their initial states, respectively. and Let represent the sensitivity coefficients of FBGx1 and FBGx2 to strain in the (x) direction, respectively. and These represent the sensitivity coefficients of FBGy1 and FBGy2 to strain in the (y) direction, respectively.

[0096] in, This represents the initial Bragg center wavelength of the (j)th (x)-direction sensitive FBG. Let represent the initial Bragg center wavelength (j=1,2) of the (j)th (y)-th sensitive FBG, which corresponds to two sensitive FBGs in the same direction respectively; The effective elastic-optical coefficient of an optical fiber reflects the effect of the change in effective refractive index caused by axial strain on the Bragg wavelength shift. For ordinary single-mode quartz fiber, It is usually around 0.22.

[0097] It should be noted that the average value of the responses of two FBGs in the same direction is used for strain calculation in equation (4) to reduce the influence of local non-uniform bonding, local differences in the fiber bending path, and fluctuations in the measurement of a single FBG on the two-dimensional strain calculation results. Equations (5) and (6) give the linear sensitivity relationship between FBG wavelength drift and axial strain under constant temperature or temperature compensation conditions. In practical applications, the aforementioned and It can also be obtained through tensile calibration experiments to further improve the accuracy of two-dimensional strain measurement.

[0098] Please see Figure 26 and Figure 27 In this embodiment, a constant strain stability test was further conducted. Fixed strains of 3%, 6%, and 9% were applied in the x and y directions, respectively, and each strain level was maintained for 1 hour, with the center wavelength recorded every 5 minutes. The results show that at each fixed strain level, the FBG center wavelength only fluctuated slightly and did not exhibit significant and continuous drift, indicating that the sensor has good output stability under constant strain conditions.

[0099] This invention incorporates SiO2 particles into a PDMS flexible substrate, arranges FBG optical fibers in a semi-circular continuous bending path, and achieves a tight bond between the fiber and the substrate using surface spot bonding. This enables the sensor to possess flexibility, stability, and ease of fabrication in two-dimensional large strain measurement. The structure can be used for in-plane strain monitoring of flexible structures, soft materials, complex curved surfaces, and components with large deformation. Furthermore, the continuous bending fiber path can be expanded according to the size of the measured area and the number of monitoring points. Multiple sensor units can be arrayed on the same flexible film or networked across multiple flexible films, thereby achieving multi-point, multi-region two-dimensional large strain monitoring.

[0100] It should be noted that, in this document, terms such as "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0101] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A two-dimensional large-strain flexible FBG sensor based on a PDMS / SiO2 hybrid substrate, characterized in that, The device includes a PDMS / SiO2 hybrid flexible film, an optical fiber disposed on the surface of the PDMS / SiO2 hybrid flexible film, and an FBG sensing unit inscribed on the optical fiber; the optical fiber is disposed on the surface of the PDMS / SiO2 hybrid flexible film in a continuous curved path, and the FBG sensing unit is located in a semi-circular feature area of ​​the continuous curved path.

2. The sensor according to claim 1, characterized in that, The optical fiber is located entirely on the upper surface of the PDMS / SiO2 hybrid flexible film and is tightly bonded to the surface of the PDMS / SiO2 hybrid flexible film by an adhesive.

3. The sensor according to claim 1, characterized in that, Taking the plane containing the PDMS / SiO2 hybrid flexible thin film as the XY plane, the length direction of the film is taken as the x-direction, and the width direction as the y-direction; the FBG sensing unit includes at least two FBGs corresponding to the x-direction and the y-direction respectively; the FBG sensing unit includes four FBGs located on the same continuous optical fiber path: FBGx1, FBGy1, FBGy2, and FBGx2, corresponding to four semi-circular sensitive bending regions X1, Y1, Y2, and X2 respectively; wherein, X1 and X2 are sensitive bending regions corresponding to the in-plane strain response in the x-direction, and Y1 and Y2 are sensitive bending regions corresponding to the in-plane strain response in the y-direction; the four sensitive bending regions are not four independent optical fibers, but different local regions on the same continuous optical fiber.

4. The sensor according to claim 3, characterized in that, The sensitive bending regions X1, Y1, Y2, and X2 are all arc-shaped regions with a central angle of 180°, i.e., semi-circular bending segments; each FBG grid area is arranged at the top of the arc or at the point of curvature continuity of the corresponding arc-sensitive region.

5. The sensor according to claim 1, characterized in that, The PDMS / SiO2 hybrid flexible film is made by mixing PDMS main agent, PDMS curing agent and SiO2 particles; the PDMS main agent is component A in the two-component PDMS elastomer system, that is, the base adhesive with polydimethylsiloxane prepolymer as the main body; the PDMS curing agent is component B that is matched with the main agent, that is, the curing agent with hydrogen-containing siloxane crosslinking agent and catalyst; the mass ratio of PDMS main agent, PDMS curing agent and SiO2 particles is 10:1:1.22; The PDMS / SiO2 hybrid flexible film was prepared by solution casting and thermosetting, with a curing temperature of 80℃ and a curing time of 60 min. The method for preparing the sensor includes the following steps: (1) Add PDMS main agent, PDMS curing agent and SiO2 particles to a beaker at a mass ratio of 10:1:1.22 and stir in two stages using an electric stirrer. First stir at 200 rpm for 2 hours, then stir at 100 rpm for 2 hours to obtain PDMS / SiO2 mixture. (2) The PDMS / SiO2 mixture is injected into the PMMA film mold and left to stand for 4 hours to allow the bubbles in the mixture to escape completely; (3) Place the mold after standing in a high temperature oven and cure it at 80°C for 60 min. After cooling, demold to obtain a PDMS / SiO2 mixed flexible film. (4) Draw the optical fiber routing path on the surface of the PDMS / SiO2 hybrid flexible film, place the optical fiber with the FBG sensing unit written on it along the path, and fix it with 502 glue dots. (5) The optical fiber is fixed to the surface of the PDMS / SiO2 hybrid flexible film by adhesive dots. After the adhesive is cured, a two-dimensional large strain flexible FBG sensor is obtained.

6. The sensor according to claim 1, characterized in that, After the optical fiber is laid out in a semi-circular continuous curved path, when the PDMS / SiO2 hybrid flexible film is subjected to in-plane stretching, the optical fiber path deforms in tandem with the substrate. The large external strain is first manifested as changes in the arc length, local rotation angle, and curvature of the curved path, and then axial strain is formed in the FBG region. The drift of the FBG center wavelength is caused by the axial strain in the FBG region, and the curvature change is used to characterize the geometric modulation effect of the curved path on the large external deformation.

7. The sensor according to claim 1, characterized in that, The FBG center wavelength, axial strain response, and temperature coupling response can be expressed by the following key formulas: ； ； ； in, Let be the Bragg center wavelength of the i-th FBG. The effective refractive index of the optical fiber core. For the grating period, Let be the average strain of the i-th FBG along the fiber axis. For a quartz fiber, It is 0.

22.

8. The sensor according to claim 1, characterized in that, The minimum bending radius of the optical fiber path is 4mm.

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