Liquid forming vari process resin flow optical fiber monitoring method

Abstract

The application discloses a liquid forming VARI process resin flow optical fiber monitoring method, which comprises the following steps: step 1, calculating the strain sensitivity coefficient of each grating in the optical fiber; step 2, in the process of laminating and pasting to form the fiber preform, selecting two adjacent fibers in each layer of fiber woven cloth to be monitored, and inserting an optical fiber between the two selected adjacent fibers; step 3, performing the liquid forming VARI process, under the action of vacuum negative pressure, the liquid resin and the curing agent mixture infiltrate the fiber preform, the real-time strain of the position of each grating is obtained according to the position information of each grating and the strain sensitivity coefficient of each grating, and the real-time three-dimensional resin flow front inside the fiber preform is determined. The application realizes real-time monitoring of internal flow field information under the condition that the influence on the fiber preform is minimum, so as to perform feedback control on process parameters such as injection pressure and injection port position, ensure the product quality, and further realize intelligent manufacturing of the composite material.

Description

Technical Field

[0001] This invention relates to the field of fiber optic grating sensing technology, specifically to a method for monitoring the flow of resin in an optical fiber during liquid molding VARI process. Background Technology

[0002] Due to their advantages such as high specific strength and specific stiffness, designable material mechanical properties, and ease of integral molding, advanced composite material structures are increasingly widely used in large-scale engineering structures in aerospace, transportation, and other fields, becoming an effective way to reduce weight, improve efficiency, and lower operating costs in large-scale engineering structures. Among these, Vacuum Assisted Resin Infusion (VARI) applies vacuum bagging technology to fiber preforms. Because it eliminates the need for autoclaves and uses single-sided rigid molds, it is suitable for manufacturing large-scale, complex three-dimensional structures, possessing the potential for low-cost, mass production, and has attracted considerable attention from manufacturers. The resin infusion process involves many factors, including resin viscosity, temperature, flow front, and the porosity and permeability of the fiber preform. If the process parameters are not properly selected or combined, defects such as dry spots or resin-rich areas can occur in the finished composite material, severely weakening its performance. Furthermore, due to the high variability of composite material manufacturing processes, using repeated experiments and trial-and-error methods to determine suitable process parameters is very inefficient and expensive, making it difficult to correct process parameter issues in a timely manner. Therefore, it is necessary to use real-time monitoring technology to monitor resin flow, so as to adjust the process parameters accordingly and reduce the occurrence of defects in a timely manner.

[0003] Non-invasive measurements such as those using cameras and ultrasound rely on assumptions to indirectly acquire real-time flow front data. Changes in pressure, heat, electrical impedance, and even optical signals occur before and after the resin reaches the dry fibers, potentially leading to errors or even erroneous monitoring. Therefore, lightweight, compact, low-interference embedded fiber optic grating sensors compatible with conductive fibers (carbon fibers) are needed as ideal invasive sensors for real-time online monitoring of the flow front. Summary of the Invention

[0004] The purpose of this invention is to provide a method for monitoring the resin flow in optical fibers during the liquid molding VARI process. This invention achieves real-time monitoring of internal flow field information with minimal impact on the fiber preform, enabling feedback control of process parameters such as injection pressure and injection port position, reducing defect rate, ensuring finished product quality, and ultimately realizing intelligent manufacturing of composite materials.

[0005] To achieve this objective, the present invention provides a method for monitoring resin flow in a liquid molding VARI process using optical fibers, characterized by comprising the following steps:

[0006] Step 1: Calculate the strain sensitivity coefficient of each grating in the optical fiber;

[0007] Step 2: Lay out multiple layers of fiber braided fabric to form a fiber preform. During the process of laying out the fiber preform, select two adjacent bundles of fibers in each layer of fiber braided fabric that needs to be monitored, insert optical fibers between the selected two adjacent bundles of fibers, and use liquid resin to pre-position the optical fibers. Then continue to lay out the fiber braided fabric to cover the optical fibers until all layers of fiber braided fabric are laid out to form a fiber preform. The optical fibers laid in each layer of fiber braided fabric that needs to be monitored have optical fiber interfaces reserved on the outside of the fiber preform.

[0008] Step 3: Vacuum-assisted resin infusion is performed on the fiber preform. During the vacuum-assisted resin infusion process, the spectral signal of each grating in the fiber preform is continuously collected through the reserved optical fiber interface at preset time intervals. Based on the position information of each grating and the strain sensitivity coefficient of each grating, the real-time strain at the position of each grating is obtained. The time when the flow front reaches the gratings at different positions in different layers of the fiber preform is determined, thereby determining the real-time three-dimensional resin flow front inside the fiber preform.

[0009] The beneficial effects of this invention are:

[0010] This invention achieves multi-point measurement of a single fiber using a multi-wavelength grating array, enabling real-time monitoring of the three-dimensional resin flow front and solving the "black box" problem of the invisible flow field inside the fiber preform. This invention can be applied to real-time monitoring of the flow front during composite material molding, allowing for feedback control of process parameters such as injection pressure and injection port location, reducing defect rates, ensuring finished product quality, and ultimately realizing intelligent manufacturing of composite materials. Furthermore, the integrated molding method for fiber optic intelligent composite materials shows great promise for monitoring the curing process and structural health during service. Attached Figure Description

[0011] Figure 1 A schematic diagram of fiber optic grating strain sensitivity coefficient calibration;

[0012] Figure 2 This is a partial schematic diagram of fiber Bragg gratings embedded in composite materials;

[0013] Figure 3 A schematic diagram of the fiber optic grating embedded in composite material;

[0014] Figure 4 Schematic diagram of the method for establishing the coordinate system of fiber preforms and positioning fiber optic gratings;

[0015] Figure 5 An internal fiber optic in-situ demodulation system is embedded in the composite material manufacturing process;

[0016] Figure 6 This is a schematic diagram of fiber optic monitoring of the resin flow front in the VARI process.

[0017] Among them, 1—optical fiber, 2—displacement sensor, 3—platform, 4—fiber braided cloth, 5—liquid resin, 6—printed grid coordinates, 7—grating, 8—coordinate origin, 9—reserved optical fiber interface, 10—demodulator, 11—liquid resin and curing agent mixture, 12—sealing strip, 13—coated release agent, 14—rigid mold, 15—guide tube, 16—injection port, 17—vacuum bag, 18—release cloth, 19—air extraction port, 20—resin collector, 21—vacuum pump. Detailed Implementation

[0018] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0019] like Figures 1-6 The optical fiber monitoring method for resin flow in the liquid molding VARI process is characterized by comprising the following steps:

[0020] Step 1: Calculate the strain sensitivity coefficient of each grating 7 in fiber 1;

[0021] Step 2: Multiple layers of fiber woven fabric 4 are stacked and laid to form a fiber preform. During the stacking and laying process, two adjacent fiber bundles are selected from each layer of fiber woven fabric 4 that needs to be monitored, and an optical fiber 1 is inserted between the selected two adjacent fiber bundles, such as... Figure 2 Liquid resin 5 is used to pre-position the optical fiber 1 to avoid relative movement between the optical fiber sensor 1 and the dry fiber 4 during the stacking and laying of multiple layers of dry fiber 4. The liquid resin 5 is compatible with the mixture of liquid resin and curing agent 11. Then, fiber braided fabric 4 is laid to cover the optical fiber 1 until all layers of fiber braided fabric 4 are laid to form a fiber preform. Figure 3 The optical fibers 1 laid in each layer of fiber woven fabric 4 that need to be monitored have optical fiber interfaces 9 reserved on the fiber prefabrication body respectively.

[0022] Step 3: Coat the surface of the rigid mold 14 with release agent 13, place the fiber preform on the release agent 13, connect the reserved fiber interface 9 of each fiber 1 to the demodulator 10, lay the release cloth 18 on the top layer of the fiber preform, set the injection port 16 on one side of the fiber preform, and set the air extraction port 19 on the other side of the fiber preform, use a vacuum bag 17 to cover the entire release cloth 18, fiber preform, injection port 16 and air extraction port 19, and connect the vacuum bag 17 to the rigid mold 14 through the sealing strip 12. 4. Bonding is performed to seal the entire release fabric 18, fiber preform, injection port 16, and air extraction port 19. The injection port 16 is connected to the liquid resin and curing agent mixture 11 through the guide pipe 15. The vacuum pump 21 is connected to the air extraction port 19 through the guide pipe 15, thereby extracting air from the vacuum bag 17, so that the liquid resin and curing agent mixture 11 is drawn into the vacuum bag 17. Under the action of vacuum negative pressure, the liquid resin and curing agent mixture 11 impregnates the fiber preform, and at a preset time interval (1 second)... -1 The spectral signal of each grating 7 is continuously acquired. Based on the position information and strain sensitivity coefficient of each grating, the real-time strain at the position of each grating 7 is obtained (liquid molding involves resin injection at a constant temperature, so temperature changes are negligible). The time it takes for the flow front to reach the grating 7 at different positions in different layers of the fiber preform is determined, thereby determining the real-time three-dimensional resin flow front inside the fiber preform. Figure 5 and Figure 6 The design is based firstly on fiber optic sensors that are highly compatible with fiber fabric, minimizing their impact on the mechanical properties of the finished composite material. Secondly, it enables real-time monitoring of the internal flow field of the fiber preform during manufacturing, which can effectively prevent defect formation and reduce scrap rate and manufacturing costs.

[0023] In step 1 of the above technical solution, the optical fiber 1 containing the grating 7 is respectively bonded and fixed to the two displacement sensors 2, such as... Figure 1 The optical fiber 1 between the two displacement sensors 2 is in a taut state. One displacement sensor 2 is fixed on the platform 3. Then, the other displacement sensor 2 is moved stepwise along the axial direction of the optical fiber 1 to lengthen the optical fiber 1, and the strain sensitivity coefficient of each grating 7 in the optical fiber 1 is calculated. This design can achieve precise control of the optical fiber displacement, thereby realizing a linear increase in strain in the grating region, which facilitates the determination of the strain sensitivity coefficient.

[0024] In the above technical solution, each optical fiber 1 has 5 to 10 gratings 7, and the length of each grating is 4 to 5 mm.

[0025] In step 1 of the above technical solution, the process of moving another displacement sensor 2 in steps of 0.01mm is to move it 2mm.

[0026] In step 1 of the above technical solution, the method for calculating the strain sensitivity coefficient of the grating in optical fiber 1 is as follows:

[0027] The demodulator 10 sends a spectral signal to the optical fiber 1. The gratings 7 in the optical fiber 1 reflect the spectral signal. The demodulator 10 obtains the center wavelength of the spectral signal corresponding to each grating 7 from the reflected signal.

[0028] The strain of fiber 1 is equal to the distance moved by another displacement sensor 2 divided by the distance between the two displacement sensors 2. The strain of fiber 1 and the center wavelength of the spectral signal corresponding to each grating 7 are recorded after each step movement. The least squares method is used to fit the linear function of the center wavelength of the spectral signal corresponding to each grating 7 with respect to the strain of fiber 1 to obtain the strain sensitivity coefficient of each grating in fiber 1. The slope of the curve is the strain sensitivity coefficient of each grating. This design can realize the precise control of fiber displacement, thereby realizing the linear increase of strain in the grating region, and thus facilitating the determination of the strain sensitivity coefficient.

[0029] In the fiber preform of step 2 of the above technical solution, the axial direction of the optical fiber 1 in the fiber braided fabric 4 to be monitored is the same as the fiber direction of the upper and lower layers of fiber braided fabric 4.

[0030] In step 3 of the above technical solution, the vacuum pump 21 is connected to the guide pipe 15 via the resin collector 20, and the guide pipe 15 is connected to the air extraction port 19. The resin collector 20 acts as a transition device; the resin sucked into the fiber preform under vacuum negative pressure will be drawn to the air outlet, but to prevent it from being sucked into the vacuum pump and damaging the pump body, it first passes through the resin collector. The principle is that one end of the resin collector is connected to the resin guide pipe at its inlet, and the other end is led out to only suck in air, so that the liquid resin will be stored in the resin collector.

[0031] In step 3 of the above technical solution, the change in the center wavelength of the spectral signal of each grating 7 is divided by the strain sensitivity coefficient of the grating 7 to obtain the real-time strain at the location of the grating 7.

[0032] The specific method for determining the arrival time of the flow front at different positions of the grating 7 in different layers of the fiber preform is as follows: After the flow front arrives, the injection pressure will be released, causing the strain to rise suddenly. The center wavelength of the spectral signal of the grating 7 jump is used to determine the arrival time.

[0033] The specific method for determining the real-time three-dimensional resin flow front inside the fiber preform is as follows: determine whether the flow front has been reached by the change of the center wavelength of the spectral signal of each grating 7, and draw the resin flow front accordingly.

[0034] In step 2 of the above technical solution, each grating 7 in each layer of fiber woven fabric 4 is positioned so that the position information of each grating can be obtained in step 3.

[0035] The specific method for positioning each grating 7 in each layer of fiber woven fabric 4 is as follows: A plane coordinate system and origin 8 are established using printed grid coordinates 6 within the plane of the fiber woven fabric. The coordinates along the thickness direction of the fiber woven fabric represent the number of fiber woven fabric layers 4. Based on this, a three-dimensional Cartesian coordinate system is established inside the fiber preform, such as... Figure 4 The coordinates of the grating center point 7 are recorded, and the grating 7 is positioned according to the corresponding coordinates after the fiber preform is formed. This design enables precise control of fiber displacement, thereby achieving a linear increase in strain in the grating region, which facilitates the determination of the strain sensitivity coefficient.

[0036] In the above technical solution, the least squares method is used to fit the linear function of the center wavelength of the spectral signal corresponding to each grating 7 with respect to the strain of the fiber 1, and the specific formula for the strain sensitivity coefficient of each grating 7 in the fiber 1 is as follows:

[0037]

[0038] Where, x i This refers to the center wavelength data of the i-th group of spectral signals, where m is the total number of spectral signal datasets, and y... i The strain value ω at the grating is calculated by displacement sensor 2. i It is the intercept of a linear function, f(x) i, ω i ) is the fitted linear function, L i f(x) is the standard deviation of the strain value at the grating obtained by displacement sensor 2 and the fitting function, and f(x) is the objective function, whose slope is the strain sensitivity coefficient.

[0039] In the above technical solution, the resin flow front refers to the flow front formed when the three-dimensional resin flow front inside the dried fiber preform arrives during the liquid molding process of the composite material. Monitoring the resin flow front specifically refers to real-time detection during the resin infusion process of the composite material molding.

[0040] In the above technical solution, the strain sensitivity coefficient calibration method refers to obtaining different strain results by directly applying displacement on the optical fiber 1 through the platform 3, and then establishing a linear relationship with the wavelength change to fit the strain sensitivity coefficient.

[0041] The optical fiber 1 has a cladding diameter of less than 125 μm, a coating thickness of less than 25 μm, a grating length of less than 10 mm, and is coated with polyimide throughout, making it resistant to temperatures above 200°C.

[0042] The monitoring and recording of the grating spectrum can be achieved using a spectrum analyzer or other fiber optic grating demodulator.

[0043] The establishment of the three-dimensional Cartesian coordinate system of the fiber preform can be achieved by covering the surface of a single layer of fiber cloth 4 with a transparent plastic sheet with printed grid coordinates 6 when positioning is required.

[0044] The contents not described in detail in this specification are existing technologies known to those skilled in the art.

Claims

1. A method for monitoring resin flow in a liquid molding VARI process using optical fibers, characterized in that, It includes the following steps: Step 1: Calculate the strain sensitivity coefficient of each grating (7) in the optical fiber (1); Step 2: Stack and lay multiple layers of fiber braided fabric (4) to form a fiber preform. During the stacking and laying process, select two adjacent bundles of fibers in each layer of fiber braided fabric (4) that need to be monitored, insert an optical fiber (1) between the selected two adjacent bundles of fibers, and use liquid resin (5) to pre-position the optical fiber (1). Then continue to lay fiber braided fabric (4) to cover the optical fiber (1) until all layers of fiber braided fabric (4) are laid to form a fiber preform. The optical fiber (1) laid in each layer of fiber braided fabric (4) that needs to be monitored has a reserved optical fiber interface (9) on the outside of the fiber preform. Step 3: Vacuum-assisted resin infusion treatment is performed on the fiber preform. During the vacuum-assisted resin infusion treatment, the spectral signal of each grating (7) in the fiber preform is continuously collected through the reserved optical fiber interface (9) at preset time intervals. Based on the position information of each grating and the strain sensitivity coefficient of each grating, the real-time strain of each grating (7) is obtained, and the time for the flow front to reach the grating (7) at different positions in different layers of the fiber preform is determined, thereby determining the real-time three-dimensional resin flow front inside the fiber preform.

2. The optical fiber monitoring method for resin flow in the liquid molding VARI process according to claim 1, characterized in that: In step 1, the optical fiber (1) containing the grating (7) is bonded and fixed to two displacement sensors (2) respectively. The optical fiber (1) between the two displacement sensors (2) is in a taut state. One of the displacement sensors (2) is fixed on the platform (3). Then, the other displacement sensor (2) is moved step by step along the axial direction of the optical fiber (1) to lengthen the optical fiber (1) and the strain sensitivity coefficient of each grating (7) in the optical fiber (1) is calculated.

3. The optical fiber monitoring method for resin flow in the liquid molding VARI process according to claim 1, characterized in that: The specific process of vacuum-assisted resin infusion treatment of the fiber preform is as follows: a release agent (13) is coated on the surface of the rigid mold (14), the fiber preform is placed on the release agent (13), the reserved optical fiber interface (9) is connected to the demodulator (10), a release cloth (18) is laid on the top layer of the fiber preform, an injection port (16) is set on one side of the fiber preform, and an air extraction port (19) is set on the other side of the fiber preform. A vacuum bag (17) is used to cover the entire release cloth (18), the fiber preform, the injection port (16) and the air extraction port (19), and the vacuum bag (17) is then passed through a dense... The sealing strip (12) is bonded to the rigid mold (14) to seal the entire release cloth (18), fiber preform, injection port (16) and air extraction port (19). The injection port (16) is connected to the liquid resin and curing agent mixture (11) through the guide tube (15). The vacuum pump (21) is connected to the air extraction port (19) through the guide tube (15) to extract the air from the vacuum bag (17), so that the liquid resin and curing agent mixture (11) is sucked into the vacuum bag (17). The liquid resin and curing agent mixture (11) impregnates the fiber preform under the action of vacuum negative pressure.

4. The optical fiber monitoring method for resin flow in the liquid molding VARI process according to claim 2, characterized in that: In step 1, the process of moving another displacement sensor (2) in steps of 0.01 mm is to move it 2 mm.

5. The optical fiber monitoring method for resin flow in the liquid molding VARI process according to claim 2, characterized in that: In step 1, the method for calculating the strain sensitivity coefficient of the grating in the optical fiber (1) is as follows: The demodulator (10) sends a spectral signal to the optical fiber (1). The grating (7) in the optical fiber (1) reflects the spectral signal. The demodulator (10) obtains the center wavelength of the spectral signal corresponding to each grating (7) from the reflected signal. The strain of the optical fiber (1) is equal to the distance moved by another displacement sensor (2) divided by the distance between the two displacement sensors (2). The strain of the optical fiber (1) and the center wavelength of the spectral signal corresponding to each grating (7) are recorded after each step movement. The least squares method is used to fit the linear function of the center wavelength of the spectral signal corresponding to each grating (7) with respect to the strain of the optical fiber (1) to obtain the strain sensitivity coefficient of each grating in the optical fiber (1).

6. The optical fiber monitoring method for resin flow in the liquid molding VARI process according to claim 1, characterized in that: In the fiber preform of step 2, the axial direction of the optical fiber (1) in the fiber braided fabric (4) to be monitored is the same as the fiber direction of the upper and lower layers of fiber braided fabric (4).

7. The optical fiber monitoring method for resin flow in the liquid molding VARI process according to claim 1, characterized in that: In step 3, the vacuum pump (21) is connected to the guide pipe (15) through the resin collector (20), and the guide pipe (15) is connected to the air extraction port (19).

8. The optical fiber monitoring method for resin flow in the liquid molding VARI process according to claim 1, characterized in that: In step 3, the change in the center wavelength of the spectral signal of each grating (7) is divided by the sensitivity coefficient of the grating (7) to obtain the real-time strain at the position of the grating (7); The specific method for determining the time when the flow front reaches the grating (7) at different positions in different layers of the fiber preform is as follows: After the flow front arrives, the injection pressure will be released, causing the strain to rise suddenly. The center wavelength of the spectral signal of the grating (7) jump is used to determine the arrival time. The specific method for determining the real-time three-dimensional resin flow front inside the fiber preform is as follows: determine whether the flow front has been reached by the change of the center wavelength of the spectral signal of each grating (7), and reconstruct the resin flow front accordingly.

9. The optical fiber monitoring method for resin flow in the liquid molding VARI process according to claim 1, characterized in that: In step 2, each grating (7) in each layer of fiber woven fabric (4) is positioned so that the position information of each grating can be obtained in step 3. The specific method for positioning each grating (7) in each layer of fiber woven fabric (4) is as follows: a coordinate system and coordinate origin (8) are established in the plane of the fiber woven fabric using printed grid coordinates (6). The coordinates in the thickness direction of the fiber woven fabric are the number of layers of fiber woven fabric (4). Based on this, a three-dimensional Cartesian coordinate system is established inside the fiber preform to record the coordinate position of the center point of the grating. After the fiber preform is formed, the grating (7) is positioned according to the corresponding coordinate position.

10. The optical fiber monitoring method for resin flow in the liquid molding VARI process according to claim 2, characterized in that: The least squares method is used to fit the linear function of the center wavelength of the spectral signal corresponding to each grating (7) with respect to the strain of the optical fiber (1), and the specific formula for the strain sensitivity coefficient of each grating in the optical fiber (1) is obtained as follows: Where, x i This refers to the center wavelength data of the i-th group of spectral signals, where m is the total number of spectral signal datasets, and y... i The strain value at the grating is calculated by the displacement sensor (2), ω i It is the intercept of a linear function, f(x) i, ω i ) is the fitted linear function, L i f(x) is the standard deviation of the strain value at the grating obtained by the displacement sensor (2) and the fitting function, and f(x) is the objective function, whose slope is the strain sensitivity coefficient.

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