Piezoresistive effect-based large strain on-line monitoring flexible sensing element and preparation method thereof

Abstract

The invention discloses a large-strain on-line monitoring flexible sensing element based on a piezoresistive effect and a preparation method of the large-strain on-line monitoring flexible sensing element. The sensing element comprises a covering layer (1), a measuring unit (4) and a composite material substrate layer (5) which are sequentially stacked from top to bottom, electrodes (2) are attached to the two ends of the measuring unit, and the measuring unit is connected with external signal processing equipment through leads (3); the measuring unit is formed by performing vacuum hot pressing sintering on a conductive filler-polymer condensation body, the thickness of the measuring unit is 2-1000 [mu] m, and the measuring unit comprises the following components in parts by mass: 10-100 parts of a high-molecular polymer and 1-3 parts of a conductive filler; and the covering layer consists of a thermoplastic high-molecular polymer. The preparation process of the sensing element adopts a vacuum hot pressed sintering process, the conductivity and sensitivity of a measuring unit are greatly enhanced while the combinability and uniformity between the high-molecular polymer matrix and the conductive filler are ensured, the linear relation between resistance and strain is remarkably improved, and nonlinear errors are reduced.

Description

Technical Field

[0001] The present invention relates to a flexible sensing element, in particular to a large-strain on-line monitoring flexible sensing element based on piezoresistive effect and a preparation method thereof, belonging to the technical field of flexible strain sensors. Background Art

[0002] As one of the most important technical devices for structural health monitoring of engineering structures, the strain sensor intuitively presents the changes generated by structural stress and strain through the change of resistance, which is convenient for safety assessment personnel to judge its health status, thereby preventing catastrophic damage. As the most widely used strain monitoring element at present, the resistance strain gauge is obtained by using lithography technology to prepare a sensitive grid with a special pattern from metal foil. It has high linearity and good durability. Due to the limitation of the elongation rate of metal materials, its range is small and the sensitivity coefficient is low, which has limitations for the structural health monitoring of large-span bridges, dams, tunnels and other structures prone to large strains. Therefore, the research and preparation of a large-strain monitoring sensing element for structures is particularly important.

[0003] Flexible electronic components have significant advantages not only in innovative products such as wearable devices and robotics technology due to their bendability, light weight, strong adaptability, etc., but also have a wide application prospect in the field of civil engineering health monitoring. As key materials for future multiple technological revolutions, graphene and carbon nanotubes exhibit high electron mobility and chemical stability due to their unique structures and highly ordered carbon atom arrangements, becoming the main sources of sensing materials for flexible electronic components. Combining graphene or carbon nanotubes with polymer polymers with excellent flexibility to prepare flexible sensitive materials with stable conductive paths and good resilience not only conforms to the piezoresistive effect of converting the changes of structural stress and strain into resistance changes, but also can overcome the disadvantages of existing strain sensors such as small range and low sensitivity coefficient, making it possible to prepare large-strain monitoring sensing elements.

[0004] Chinese Patent (CN109990695A) discloses a flexible graphene-based piezoresistive sensor and its preparation method. In this invention, a graphene dispersion is coated on a flexible polymer substrate by the sandwich method, and then a polymer liquid is evenly coated on top of the graphene film to form a graphene film. During the forming process, the natural leveling method is adopted, but the treatment effect on the stress concentration and bubbles that are likely to appear in the film is relatively limited, resulting in insufficiently tight binding of material particles or molecules, a relatively high porosity and poor elasticity of the formed graphene film. In addition, the method of successively casting and forming in three layers in this invention is difficult to ensure the film thickness and repeatability. Moreover, there is only one conductive path, i.e., the sensing grid, in this flexible sensor, resulting in poor response ability to small deformations, low resistance to external noise or interference, and a large impact of local material defects or uneven stress on the measurement results, which is not conducive to improving the linear relationship between resistance and strain and increases the non-linear error. Especially, the error is relatively large in high-precision measurements, thus reducing the stability and reliability for long-term use. Summary of the Invention

[0005] Aiming at the problems existing in the prior art, the first object of the present invention is to provide a large-strain on-line monitoring flexible sensing element based on the piezoresistive effect. This sensing element is based on the synergistic effect between a polymer and a conductive filler. By controlling the ratio of the two and the thickness of the sensing element, quantitative regulation of the measurement accuracy and sensitivity is achieved, the non-linear error between resistance and strain is greatly reduced, and the stability and reliability of the element are significantly improved.

[0006] The second object of the present invention is to provide a preparation method for a large-strain on-line monitoring flexible sensing element based on the piezoresistive effect. This method adopts a vacuum hot pressing and sintering process, which not only ensures the bonding and uniformity between the polymer matrix and the conductive filler but also greatly enhances the conductivity and sensitivity of the measurement unit. Further, this method greatly simplifies the forming process of the element, maintains the yield and stability of the element, and endows the element with excellent structural stability and anti-interference characteristics.

[0007] To achieve the above technical objects, the present invention provides a large-strain on-line monitoring flexible sensing element based on the piezoresistive effect, which includes a covering layer (1), a measurement unit (4), and a composite material base layer (5) stacked in sequence from top to bottom; electrodes (2) are attached to both ends of the measurement unit and are connected to an external signal processing device through leads (3); the measurement unit is formed by vacuum hot pressing and sintering of a conductive filler-polymer condensate, with a thickness of 5 μm to 1000 μm, and includes the following components in parts by mass: 10 parts to 100 parts of a polymer and 1 part to 3 parts of a conductive filler; the covering layer is composed of a thermoplastic polymer.

[0008] The thickness of the sensing measurement unit provided by the present invention and the ratio between the polymer and the conductive filler in its components should be strictly controlled; there is a positive correlation between the thickness of the measurement unit and the sensitivity. If the thickness of the measurement unit is too large, it will cause more loss during strain transfer of the structure and inaccurate measurement results; while the addition amount of the conductive filler will affect the conductivity of the measurement unit. Excessive addition will increase the non-linear error of the sensing element and reduce stability, while too little addition will result in too slow response rate of the measurement unit and inability to maintain timely response.

[0009] As a preferred solution, the composite material base layer includes a polymer substrate and a fiber reinforcing phase, and the mass ratio of the two is 1:0.02 - 5.

[0010] As a preferred solution, the polymer substrate is polyimide and / or polyester.

[0011] As a preferred solution, the fiber reinforcing phase is at least one of aramid fiber, carbon fiber, carbon nanotube and graphene.

[0012] As a preferred solution, the polymer is at least one of thermoplastic polyurethane, thermoplastic polyethylene terephthalate, styrene-butadiene-styrene, thermoplastic polyamide, thermoplastic polyolefin, thermoplastic rubber, thermoplastic polyether, styrene-isoprene-styrene, polypropylene, polyvinyl chloride, polycarbonate and polyethylene. The polymer adopted in the present invention has high resilience and a fixed glass transition temperature, is suitable for hot pressing molding preparation process and meets the large tensile range required by flexible sensor materials. In addition, the above polymer can also have certain conductivity after being uniformly mixed with the conductive material, ensuring that the pressure signal is converted into an electrical signal that can be captured.

[0013] As a preferred solution, the number of graphene layers is 3 - 50 layers, the sheet diameter distribution is 0.5μm - 500μm, and the proportion of sheet diameter of 0.5μm - 10μm is ≥80%.

[0014] As a preferred solution, the carbon nanotube is a multi-walled carbon nanotube, with an outer diameter of 1nm - 100nm, a length of 1μm - 50μm, and a hydroxyl content of ≤5wt%.

[0015] As a preferred solution, the thickness of the covering layer and the base layer is 5 μm to 500 μm; the porosity of the measuring unit is 0.5% to 30%. In the present invention, the porosity of the measuring unit must be strictly implemented according to the above requirements. The porosity is a measure of the density of the measuring unit. If the porosity is too low, the density inside the unit is too high, resulting in a decrease in the deformation ability of the measuring unit, thereby reducing its sensitivity to pressure changes and affecting the linearity and accuracy of the output signal; if the porosity is too high, it will cause a sudden change when the flexible sensing element is deformed by force, thereby affecting its sensitivity.

[0016] The present invention also provides a preparation method of a flexible sensing element for large-strain on-line monitoring based on the piezoresistive effect, including: fully mixing a polymer base material and a fiber-reinforced phase material and then coating them on a mold, and drying to obtain a base layer; placing a thermoplastic polymer on a pressing block and obtaining a covering layer through vacuum hot pressing; fully mixing a conductive filler and a type A organic solvent, adding a polymer, obtaining a conductive filler-polymer dispersion liquid, then adding a type B organic solvent and fully mixing, and sequentially performing separation, drying and vacuum hot pressing and sintering to obtain a measuring unit; after attaching electrodes and leads to both ends of the measuring unit, fitting the upper part with the covering layer through vacuum thermoforming, and connecting the lower part with the measuring unit through vacuum hot pressing, and cooling to obtain.

[0017] Aiming at the problem that the viscosity coefficient of the polymer is relatively large and it is difficult to uniformly mix with the conductive filler, based on the aggregation principle of the polymer, the polymer and the conductive filler are fully dispersed in an organic solvent, and then stirred to fully mix the two, and then the organic solvent is volatilized to obtain a uniformly mixed conductive filler-polymer aggregate.

[0018] As a preferred solution, the process of full mixing is: mechanically stirring at a rotation speed of 500 r / min to 6000 r / min for 0.1 h to 6 h, and then ultrasonically dispersing for 0.1 h to 6 h.

[0019] As a preferred solution, the concentration of the conductive filler dispersion liquid is 0.1 mg / ml to 50 mg / ml.

[0020] As a preferred solution, the type A organic solvent is at least one of tetrahydrofuran, N-methylpyrrolidone, N,N-dimethylformamide, ethyl acetate, butyl acetate, acetone, isopropanol, benzene, toluene, xylene and dichloromethane.

[0021] As a preferred solution, the type B organic solvent can be at least one of methanol, ethanol, isopropanol, n-hexane, cyclohexane, n-heptane, ethylene, propylene, chloroform, vinyl chloride and styrene.

[0022] In the present invention, the selection criterion for Class A organic solvents is that they can dissolve the high molecular polymer, disperse it evenly in the organic solvent, and reduce the viscosity of the high molecular polymer; the selection criterion for Class B organic solvents is that they have a lower solubility of the high molecular polymer than Class A organic solvents, and they are co-precipitated with the conductive filler to form a conductive filler-polymer condensate.

[0023] As a preferred embodiment, the drying method is vacuum drying, and the conditions are: the temperature is 50~100°C, and the time is 3~12h.

[0024] As a preferred embodiment, the process of vacuum hot pressing and sintering is as follows: under the condition of a pressure of 0.5~5 Mpa, the temperature is raised from room temperature to 100~500°C at a rate of 5~40°C / min, held for 0.5~6h, and the vacuum degree is less than 100 Pa.

[0025] As a preferred embodiment, the cooling method is at least one of natural cooling, water cooling, oil cooling, air cooling, forced cooling and intermittent cooling methods, and the whole cooling process is carried out in the pressure and vacuum degree environment of vacuum hot pressing and sintering.

[0026] Compared with the prior art, the beneficial technical effects of the technical solution provided by the present invention are as follows: 1) The sensing element provided by the present invention has the advantage of multiple grids compared with the existing single-grid sensor. Under the action of strain, it can generate more resistance changes, effectively improving its sensitivity and measurement accuracy. Moreover, the multi-grid structure can effectively distribute the strain signal, reduce the influence of local material defects or uneven stress on the measurement result, thereby improving the stability and reliability of long-term use. In addition, the multi-sensitive grid helps to improve the linear relationship between resistance and strain, thereby providing a more linear output, reducing the non-linear error, and effectively solving the problems of large non-linear error and low stability of the sensing element in the prior art.

[0027] 2) The preparation method provided by the present invention adopts the vacuum hot pressing and forming process, which promotes the close combination between the high molecular polymer and the filler under high temperature and high pressure, greatly improving the uniformity and stability of the two. Moreover, the vacuum hot pressing process can also improve the mechanical flexibility of the sensing element, optimize the internal conductive channels of the conductive filler and the polymer, and improve the response speed and sensitivity of the measurement unit; compared with the traditional process, this method has the advantages of simple operation, easy operation and high forming yield, and can effectively ensure the structural stability and anti-interference ability of the sensing element. Description of the Drawings

[0028] Figure 1 It is a structural diagram of a flexible sensing element for large strain monitoring based on the piezoresistive effect provided by the present invention; Among them, 1 - covering layer, 2 - electrode, 3 - lead wire, 4 - measurement unit, 5 - base layer, 6 - flexible sensing element; Figure 2 This is the relationship diagram between the resistance change rate and strain of the flexible sensing element for large strain monitoring of the structure based on the piezoresistive effect in Embodiment 1 of the present invention; Figure 3 This is the relationship diagram between the resistance change rate and strain of the flexible sensing element for large strain monitoring of the structure based on the piezoresistive effect in Embodiment 2 of the present invention; Figure 4 This is the relationship diagram between the resistance change rate and strain of the flexible sensing element for large strain monitoring of the structure based on the piezoresistive effect in Embodiment 3 of the present invention; Figure 5 This is the relationship diagram between the resistance change rate and strain of the flexible sensing element for large strain monitoring of the structure based on the piezoresistive effect in Embodiment 4 of the present invention. Detailed implementation manners

[0029] To facilitate the understanding of the present invention, the present invention will be described more comprehensively below with reference to specific implementation cases. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, these embodiments are provided to make the understanding of the disclosure content of the present invention more thorough and comprehensive. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the description of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0030] Embodiment 1

[0031] 1) Mix 0.5 g of polyimide with 0.5 g of surface-treated aramid fiber, coat it on a mold and dry for 3 h to obtain a base layer; 2) Weigh 1 g of thermoplastic polyurethane and place it on a pressing block. Place spacer bars on both sides of the pressing block to control the thickness of the covering layer, and heat and press it in a vacuum environment to obtain a covering layer; 3) Weigh 0.2 g of graphene and pour it into a test tube with 30 ml of tetrahydrofuran solution. First, mechanically stir for 30 min, and then ultrasonically disperse for 30 min to obtain a stable graphene dispersion; 4) Weigh 2 g of thermoplastic polyurethane into the graphene dispersion. First, mechanically stir for 30 min, and then ultrasonically stir for 30 min to obtain a stable graphene-polymer dispersion; 5) Add 60 ml of methanol solution to the graphene-polymer dispersion and stir. Separate the graphene-polymer from tetrahydrofuran and methanol, pour the tetrahydrofuran and methanol solution into a waste liquid collection bottle to obtain a graphene-polymer condensate. Place the graphene-polymer condensate in a vacuum drying oven, set the temperature to 80 °C and the time to 6 h; 6) Place the graphene-polymer condensate in the briquette of a vacuum hot-pressing sintering furnace. A copper sheet with dimensions of 150 mm × 150 mm × 0.4 mm is arranged in the middle of the briquette, and two cushion strips with dimensions of 50 mm × 10 mm × 0.04 mm thick are placed on both sides of the copper sheet. Set the pressure to 2.2 MPa, the temperature to 240 °C, the heating rate to 15 °C / min, the heating holding time to 30 min, and evacuate to a vacuum degree within 10 Pa. After cooling and demolding, a measuring unit is obtained; 7) Connect two pieces of metal copper foil as electrodes to the end of the measuring unit, weld the leads on the electrodes, place the covering layer above the measuring unit and bond it by vacuum thermoforming, and then place the base layer below the measuring unit and connect it by vacuum hot pressing. After cooling, a flexible sensing element for large strain monitoring based on the piezoresistive effect is obtained.

[0032] As Figure 2 shown, the resistance change rate of this embodiment has a good linear relationship with the strain. After calculation, the sensitivity coefficient K of this flexible sensing element is 10.8, which is significantly higher than that of the metal foil strain gauge (K≈2). By comparing the test data with the linear fitting, the goodness of fit R 2 = 0.999, indicating that the linearity of the test results is good and the error range of the repeated tests of the same group is less than 10%.

[0033] Example 2 1) Mix 0.5 g of polyimide with 0.5 g of surface-treated aramid fiber, coat it on the mold and dry for 3 h to obtain the base layer; 2) Weigh 1 g of thermoplastic polyurethane and place it on the briquette. Place cushion strips on both sides of the briquette to control the thickness of the covering layer, and heat and press it in a vacuum environment to obtain the covering layer; 3) Weigh 0.2 g of graphene and pour it into a test tube with 60 ml of tetrahydrofuran solution. First, mechanically stir for 30 min, and then ultrasonically disperse for 30 min to obtain a stable graphene dispersion; 4) Weigh 4 g of thermoplastic polyurethane into the graphene dispersion. First, mechanically stir for 30 min, and then ultrasonically stir for 30 min to obtain a stable graphene-polymer dispersion; 5) Add 120 ml of methanol solution to the graphene-polymer dispersion and stir to separate the graphene-polymer from tetrahydrofuran and methanol. Pour the tetrahydrofuran and methanol solution into the waste liquid collection bottle to obtain the graphene-polymer condensate. Place the graphene-polymer condensate in a vacuum drying oven, set the temperature to 80 °C and the time to 6 h; 6) Place the graphene-polymer condensate in the briquette of a vacuum hot-pressing sintering furnace. A copper sheet with dimensions of 150 mm × 150 mm × 0.4 mm is arranged in the middle of the briquette, and two cushion strips with dimensions of 50 mm × 10 mm × 0.04 mm thick are placed on both sides of the copper sheet. Set the pressure to 2.2 MPa, the temperature to 240 °C, the heating rate to 15 °C / min, the heating holding time to 30 min, and evacuate to a vacuum degree within 10 Pa. After cooling and demolding, a measurement unit is obtained; 7) Connect two pieces of metal copper foil as electrodes to the end of the measurement unit, weld the leads on the electrodes, place the cover layer above the measurement unit and bond it by vacuum thermoforming, and then place the base layer below the measurement unit and connect it by vacuum hot-pressing. After cooling, a flexible sensing element for large-strain monitoring based on the piezoresistive effect is obtained.

[0034] As Figure 3 shown, the resistance change rate of this embodiment has a good linear relationship with the strain. After calculation, the sensitivity coefficient K of this flexible sensing element is 14.4, which is significantly higher than that of the metal foil strain gauge (K≈2). Compared with Example 1, the increase in the sensitivity coefficient is mainly due to the relatively low graphene content, the poor conductivity of the sensing element, and the high resistance change rate. By comparing the test data with linear fitting, the goodness of fit R 2 = 0.996, indicating that the linearity of the test results is good and the error range of the repeated tests within the same group is less than 10%.

[0035] Example 3

[0036] 1) Mix 0.5 g of polyimide with 0.5 g of surface-treated aramid fiber, coat it on a mold and dry for 3 h to obtain a base layer; 2) Weigh 1 g of thermoplastic polyurethane and place it on the briquette. Place cushion strips on both sides of the briquette to control the thickness of the cover layer, and heat and press it in a vacuum environment to obtain a cover layer; 3) Weigh 0.2 g of graphene and pour it into a test tube with 90 ml of tetrahydrofuran solution. First, mechanically stir for 30 min, and then ultrasonically disperse for 30 min to obtain a stable graphene dispersion; 4) Weigh 6 g of thermoplastic polyurethane into the graphene dispersion. First, mechanically stir for 30 min, and then ultrasonically disperse for 30 min to obtain a stable graphene-polymer dispersion; 5) Add 180 ml of methanol solution to the graphene-polymer dispersion and stir to separate the graphene-polymer from tetrahydrofuran and methanol. Pour the tetrahydrofuran and methanol solution into a waste liquid collection bottle to obtain a graphene-polymer condensate. Place the graphene-polymer condensate in a vacuum drying oven, set the temperature to 80 °C, and the time to 6 h; 6) Place the graphene-polymer condensate in the briquette of a vacuum hot-pressing sintering furnace. A copper sheet with dimensions of 150 mm × 150 mm × 0.4 mm is arranged in the middle of the briquette, and two cushion strips with dimensions of 50 mm × 10 mm × 0.04 mm thick are placed on both sides of the copper sheet. Set the pressure to 2.2 MPa, the temperature to 240 °C, the heating rate to 15 °C / min, the heating holding time to 30 min, and evacuate the vacuum to within 10 Pa. After cooling and demolding, a measuring unit is obtained; 7) Connect two metal copper foils as electrodes to the end of the measuring unit, weld the leads to the electrodes, place the cover layer above the measuring unit and bond it by vacuum thermoforming, and then place the base layer below the measuring unit and connect it by vacuum hot-pressing. After cooling, a flexible sensing element for large-strain monitoring based on the piezoresistive effect is obtained.

[0037] As Figure 4 shown, the resistance change rate of this embodiment has a good linear relationship with strain. After calculation, the sensitivity coefficient K of this flexible sensing element is 49.5, which is significantly higher than that of the metal foil strain gauge (K≈2). Compared with Example 2, the increase in the sensitivity coefficient is mainly due to the relatively low graphene content, the poor conductivity of the sensing element, and the high resistance change rate. By comparing the experimental data with linear fitting, the goodness of fit R 2 = 0.995, indicating that the linearity of the experimental results is good and the error range of the repeated tests in the same group is less than 10%.

[0038] Example 4

[0039] 1) Mix 0.5 g of polyimide with 0.5 g of surface-treated aramid fiber, coat it on a mold and dry for 3 h to obtain a base layer; 2) Weigh 1 g of thermoplastic polyurethane and place it on the briquette. Place cushion strips on both sides of the briquette to control the thickness of the cover layer, and heat and press it in a vacuum environment to obtain a cover layer; 3) Weigh 0.2 g of graphene and pour it into a test tube with 30 ml of tetrahydrofuran solution. First, mechanically stir for 30 min, and then ultrasonically disperse for 30 min to obtain a stable graphene dispersion; 4) Weigh 2 g of thermoplastic polyurethane into the graphene dispersion. First, mechanically stir for 30 min, and then ultrasonically disperse for 30 min to obtain a stable graphene-polymer dispersion; 5) Add 60 ml of methanol solution to the graphene-polymer dispersion and stir to separate the graphene-polymer from tetrahydrofuran and methanol. Pour the tetrahydrofuran and methanol solution into a waste liquid collection bottle to obtain a graphene-polymer condensate. Place the graphene-polymer condensate in a vacuum drying oven, set the temperature to 80 °C and the time to 6 h; 6) Place the graphene-polymer condensate in the briquette of a vacuum hot pressing sintering furnace. A copper sheet with dimensions of 150 mm × 150 mm × 0.4 mm is arranged in the middle of the briquette, and two cushion strips with dimensions of 50 mm × 10 mm × 0.02 mm thick are placed on both sides of the copper sheet. Set the pressure to 2.2 MPa, the temperature to 240 °C, the heating rate to 15 °C / min, the heating holding time to 30 min, and evacuate to a vacuum degree within 10 Pa. After cooling and demolding, a measurement unit is obtained; 7) Connect two pieces of metal copper foil as electrodes at the end of the measurement unit, weld the leads on the electrodes, place the covering layer above the measurement unit and bond it by vacuum thermoforming, and then place the base layer below the measurement unit and connect it by vacuum hot pressing. After cooling, a flexible sensing element for large strain monitoring based on the piezoresistive effect is obtained.

[0040] As Figure 5 shown, the resistance change rate of this embodiment has a good linear relationship with the strain. After calculation, the sensitivity coefficient K of this flexible sensing element is 24.6, which is significantly higher than that of the metal foil strain gauge (K≈2). Compared with Example 2, the increase in the sensitivity coefficient is mainly because when the thickness is reduced, the sensitivity of the flexible sensing element to the stress distribution on the material surface and interface will be improved, thus affecting the change in resistance and enhancing the sensitivity; by comparing the test data with the linear fitting, the goodness of fit R 2 = 0.992, indicating that the linearity of the test results is good and the error range of the repeated tests within the same group is less than 10%.

Claims

1. A large strain online monitoring flexible sensor element based on piezoresistive effect, characterized by: The invention comprises a covering layer (1), a measuring unit (4) and a composite material base layer (5) which are stacked in sequence from top to bottom; electrodes (2) are attached to both ends of the measuring unit and are connected to an external signal processing device via leads (3); the measuring unit is formed by vacuum hot pressing and sintering a conductive filler-polymer aggregate, has a thickness of 5 to 1000 μm, and comprises the following components by mass: 10 to 100 parts of a high molecular polymer and 1 to 3 parts of a conductive filler; the covering layer is composed of a thermoplastic high molecular polymer.

2. According to claim 1, a large strain online monitoring flexible sensor element based on piezoresistive effect is characterized by: The composite material base layer includes a polymer matrix and a fiber reinforcement phase, and the mass ratio of the two is 1:0.02-5; the polymer matrix is ​​polyimide and / or polyester; the fiber reinforcement phase is at least one of aramid fiber, carbon fiber, carbon nanotube and graphene.

3. The large strain online monitoring flexible sensor element based on piezoresistive effect according to claim 1 is characterized by: The high molecular polymer is at least one of thermoplastic polyurethane, thermoplastic polyethylene terephthalate, styrene-butadiene-styrene, thermoplastic polyamide, thermoplastic polyolefin, thermoplastic rubber, thermoplastic polyether, styrene-isoprene-styrene, polypropylene, polyvinyl chloride, polycarbonate and polyethylene; the conductive filler is at least one of graphite, graphene, carbon black, carbon nanotubes, copper powder, silver powder, gold powder, aluminum powder and nickel powder.

4. The large strain online monitoring flexible sensor element based on piezoresistive effect according to claim 3 is characterized by: The number of graphene layers is 3 to 200, the sheet diameter distribution is 0.5 μm to 500 μm, and the proportion of sheet diameters of 0.5 μm to 50 μm is ≥80%; the carbon nanotubes are multi-walled carbon nanotubes with an outer diameter of 1 nm to 100 nm, a length of 1 μm to 50 μm, and a hydroxyl content of ≤5wt%.

5. The large strain online monitoring flexible sensor element based on piezoresistive effect according to claim 1 is characterized by: The thickness of the covering layer and the base layer is 5 μm to 500 μm; the porosity of the measuring unit is 0.5% to 30%.

6. The method for preparing a large strain online monitoring flexible sensor element based on piezoresistive effect according to any one of claims 1 to 5, characterized in that: The method comprises: fully mixing a polymer substrate and a fiber-reinforced phase material, coating the mixture on a mold, and drying the mixture to obtain a base layer; placing a thermoplastic high molecular polymer on a pressing block, and performing vacuum hot pressing to obtain a covering layer; fully mixing a conductive filler with a Class A organic solvent, adding the high molecular polymer to obtain a conductive filler-polymer dispersion, and then adding a Class B organic solvent to fully mix the mixture, and performing separation, drying, and vacuum hot pressing and sintering in sequence to obtain a measuring unit; attaching electrodes and leads to both ends of the measuring unit, laminating the upper portion with the covering layer by vacuum blistering, and connecting the lower portion with the measuring unit by vacuum hot pressing, and cooling the mixture to obtain the measuring unit.

7. The method for preparing a large strain online monitoring flexible sensor element based on piezoresistive effect according to claim 6, characterized in that: The fully mixing process is: mechanical stirring at a rotation speed of 500 r / min to 6000 r / min for 0.1 h to 6 h, and then ultrasonic dispersion for 0.1 h to 6 h; the concentration of the conductive filler dispersion is 0.1 mg / ml to 50 mg / ml.

8. The method for preparing a large strain online monitoring flexible sensor element based on piezoresistive effect according to claim 6, characterized in that: The Class A organic solvent is at least one of tetrahydrofuran, N-methylpyrrolidone, N,N-dimethylformamide, ethyl acetate, butyl acetate, acetone, isopropanol, benzene, toluene, xylene and dichloromethane; the Class B organic solvent can be at least one of methanol, ethanol, isopropanol, n-hexane, cyclohexane, n-heptane, ethylene, propylene, chloroform, vinyl chloride and styrene.

9. The method for preparing a large strain online monitoring flexible sensor element based on piezoresistive effect according to claim 6, characterized in that: The drying method is vacuum drying, and the conditions are: temperature of 50°C~100°C, time of 3h~12h; the process of vacuum hot pressing sintering is: under the condition of pressure of 0.5MPa~5MPa, heating from room temperature to 100°C~500°C at 5°C / min~40°C / min, keeping warm for 0.5h~6h, and vacuum degree is less than 100Pa.

10. The method for preparing a large strain online monitoring flexible sensor element based on piezoresistive effect according to claim 9, characterized in that: The cooling method is at least one of natural cooling, water cooling, oil cooling, air cooling, forced cooling and intermittent cooling, and the entire cooling process is in the pressure and vacuum environment of vacuum hot pressing sintering.

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