An online monitoring flexible sensing element with large strain based on piezoresistive effect and its preparation method

Abstract

The present invention discloses a large-strain on-line monitoring flexible sensing element based on piezoresistive effect and a preparation method thereof. The sensing element includes a covering layer (1), a measuring 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 measuring unit and are connected to an external signal processing device through leads (3); the measuring unit is formed by vacuum hot pressing and sintering of a conductive filler-polymer condensate, and its thickness is (2 μm to 1000 μm), and it includes the following components in parts 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. The preparation process of the sensing element adopts a vacuum hot pressing and sintering process, which not only ensures the bonding property and uniformity between the high molecular polymer matrix and the conductive filler, but also greatly enhances the conductivity and sensitivity of the measuring unit, significantly improves the linear relationship between resistance and strain, and reduces the non-linear error.

Description

Technical Field

[0001] The present invention relates to a flexible sensing element, and particularly 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, a strain sensor visually presents the changes generated by structural stress and strain through the change of resistance, which is convenient for safety assessors to judge its health status, thereby preventing catastrophic damage. As the most widely used strain monitoring element at present, a resistance strain gauge is obtained by using photolithography technology to prepare a sensitive grid with a special pattern from a metal foil. It has a 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 that are 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 flexibility, light weight, and strong adaptability, but also have a wide range of application prospects 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 sensing material sources for flexible electronic components. Combining graphene or carbon nanotubes with a polymer having excellent flexibility to prepare a flexible sensitive material with a stable conductive path 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 a large-strain monitoring sensing element.

[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 above the graphene film to form a graphene film. In the forming process, the natural leveling method is adopted, but the treatment effect on the stress concentration and bubbles that are likely to occur in the film is relatively limited, resulting in insufficiently tight bonding of material particles or molecules, a relatively high porosity and poor elasticity of the formed graphene film. In addition, it is difficult to ensure the film thickness and repeatability by the method of three-layer sequential casting. Moreover, there is only one conductive path, i.e., the sensitive grid, in this flexible sensor, with poor response ability to small deformations, low resistance to external noise or interference, and a large influence 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 high molecular 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 of 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 property and uniformity between the high molecular 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 component mass fraction groups: 10 parts to 100 parts of a high molecular polymer and 1 part to 3 parts of a conductive filler; the covering layer is composed of a thermoplastic high molecular polymer.

[0008] For the sensing measurement unit provided by the present invention, it is necessary to strictly control its thickness and the ratio between the polymer and the conductive filler in its composition; 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, resulting in 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 the stability, while too little addition will lead to too slow response rate of the measurement unit and it cannot maintain timely response.

[0009] As a preferred solution, the composite material base layer includes a polymer substrate and a fiber reinforcement 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 reinforcement 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 preparation process and meets the large tensile range required for flexible sensor materials. In addition, after the above polymer is uniformly mixed with the conductive material, it can also have certain conductivity to ensure the conversion of pressure signal 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 large-strain online monitoring flexible sensing element based on the piezoresistive effect, including: fully mixing a polymer substrate and a fiber-reinforced phase material and then coating it 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 it with the covering layer through vacuum thermoforming above, and connecting it with the measuring unit through vacuum hot pressing below, 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 the 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, and kept warm 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 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:

[0027] 1) Compared with the existing single-gate sensor, the sensing element provided by the present invention also has the advantage of multiple gates. Under the action of strain, it can generate more resistance changes, effectively improving its sensitivity and measurement accuracy. Moreover, the multi-gate 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 gates also help 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.

[0028] 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] 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;

[0030] Among them, 1 - covering layer, 2 - electrode, 3 - lead wire, 4 - measuring unit, 5 - base layer, 6 - flexible sensing element;

[0031] 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;

[0032] 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;

[0033] 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;

[0034] 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

[0035] 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 specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0036] Embodiment 1

[0037] 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 the base layer;

[0038] 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 - press it in a vacuum environment to obtain the covering layer;

[0039] 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;

[0040] 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;

[0041] 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 the 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.

[0042] 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.

[0043] 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. Subsequently, 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.

[0044] As Figure 2 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 10.8, which is significantly higher than that of a metal foil strain gauge (K≈2). By comparing the test data with 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 repeated tests in the same group is less than 10%.

[0045] Example 2

[0046] 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.

[0047] 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.

[0048] 3) Weigh 0.2 g of graphene and pour it into a test tube with 60 ml of tetrahydrofuran solution. First, stir mechanically for 30 min, and then disperse it ultrasonically for 30 min to obtain a stable graphene dispersion.

[0049] 4) Weigh 4 g of thermoplastic polyurethane into the graphene dispersion. First, stir mechanically for 30 min, and then sonicate for 30 min to obtain a stable graphene-polymer dispersion.

[0050] 5) Add 120 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 the 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;

[0051] 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;

[0052] 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. Subsequently, 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.

[0053] As Figure 3 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 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 in the same group is less than 10%.

[0054] Example 3

[0055] 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;

[0056] 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;

[0057] 3) Weigh 0.2 g of graphene and pour 90 ml of tetrahydrofuran solution into a test tube, first mechanically stir for 30 min, and then ultrasonically disperse for 30 min to obtain a stable graphene dispersion;

[0058] 4) Weigh 6 g of thermoplastic polyurethane and put it into the graphene dispersion liquid. First, stir mechanically for 30 min, and then sonicate for 30 min to obtain a stable graphene-polymer dispersion liquid;

[0059] 5) Add 180 ml of methanol solution to the graphene-polymer dispersion liquid and stir. Separate the graphene-polymer from tetrahydrofuran and methanol, pour the tetrahydrofuran and methanol solution into the 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;

[0060] 6) Place the graphene-polymer condensate in the briquette of a vacuum hot pressing sintering furnace. Set a copper sheet of 150 mm × 150 mm × 0.4 mm in the middle of the briquette, and place two cushion strips of 50 mm × 10 mm × 0.04 mm thick 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, obtain the measuring unit;

[0061] 7) Connect two pieces of metal copper foil as electrodes at 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, obtain a flexible sensing element for large strain monitoring based on the piezoresistive effect.

[0062] As Figure 4 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 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 test data with linear fitting, the goodness of fit R 2 = 0.995, indicating that the linearity of the test results is good and the error range of the repeated tests in the same group is less than 10%.

[0063] Example 4

[0064] 1) Mix 0.5 g of polyimide and 0.5 g of surface-treated aramid fiber, coat it on the mold and dry for 3 h to obtain the base layer;

[0065] 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;

[0066] 3) Weigh 0.2 g of graphene and pour it into a test tube together with 30 ml of tetrahydrofuran solution. First, stir mechanically for 30 min, and then disperse ultrasonically for 30 min to obtain a stable graphene dispersion;

[0067] 4) Weigh 2 g of thermoplastic polyurethane into the graphene dispersion. First, stir mechanically for 30 min, and then sonicate for 30 min to obtain a stable graphene-polymer dispersion;

[0068] 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 the 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;

[0069] 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 placed 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;

[0070] 7) Connect two pieces of metal copper foil as electrodes to the end of the measurement unit, weld the leads to 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.

[0071] As Figure 5 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 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 due to the fact that when the thickness decreases, the sensitivity of the flexible sensing element to the stress distribution on the material surface and interface will be enhanced, thus affecting the change in resistance and improving the sensitivity; by comparing the test data with 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 in the same group is less than 10%.

Claims

1. An online monitoring flexible sensing element based on piezoresistive effect for large strain, characterized in that: It includes a cover 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 a conductive filler-polymer condensate, with a thickness of 5 to 1000 μm, and includes the following components in parts by mass: 10 to 100 parts of a high molecular polymer and 1 to 3 parts of a conductive filler; The cover layer is composed of a thermoplastic high molecular polymer; The preparation method of the on-line monitoring flexible sensing element includes: fully mixing a polymer substrate and a fiber reinforced phase material and then coating it on a mold, and drying to obtain a base layer; Placing the thermoplastic high molecular polymer on a briquette and obtaining a cover layer through vacuum hot pressing; fully mixing a conductive filler with a type A organic solvent, adding a high molecular polymer to obtain a conductive filler-polymer dispersion liquid, then adding a type B organic solvent and fully mixing, and successively performing separation, drying, and vacuum hot pressing and sintering to obtain a measurement unit; after attaching electrodes and leads to both ends of the measurement unit, fitting the upper part with the cover layer through vacuum thermoforming, and connecting the lower part with the measurement unit through vacuum hot pressing, and cooling to obtain.

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

3. The large-strain on-line monitoring flexible sensing element based on piezoresistive effect according to claim 1, characterized in that: 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 nanotube, copper powder, silver powder, gold powder, aluminum powder, and nickel powder.

4. The large-strain on-line monitoring flexible sensing element based on piezoresistive effect according to claim 3, characterized in that: The number of graphene layers is 3 to 200 layers, 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 nanotube is a multi-walled carbon nanotube, with an outer diameter of 1 nm to 100 nm, a length of 1 μm to 50 μm, and a hydroxyl content of ≤5 wt%.

5. A large-strain on-line monitoring flexible sensing element based on piezoresistive effect according to claim 1, characterized in that: The thickness of the cover layer and the base layer is 5 μm to 500 μm; the porosity of the measurement unit is 0.5% to 30%.

6. The large-strain on-line monitoring flexible sensing element based on piezoresistive effect according to claim 1, characterized in that: The process of the 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; the concentration of the conductive filler dispersion liquid is 0.1 mg / ml to 50 mg / ml.

7. A large-strain online monitoring flexible sensing element based on the piezoresistive effect according to claim 1, characterized in that: 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; 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.

8. A large-strain on-line monitoring flexible sensing element based on piezoresistive effect according to claim 1, characterized in that: The drying method is vacuum drying, and the conditions are: the temperature is 50°C to 100°C, and the time is 3h to 12h; the process of vacuum hot pressing and sintering is: under the condition of a pressure of 0.5MPa to 5MPa, heating from room temperature to 100°C to 500°C at a rate of 5°C / min to 40°C / min, holding for 0.5h to 6h, and the vacuum degree is less than 100Pa.

9. The large-strain online monitoring flexible sensing element based on piezoresistive effect according to claim 1, characterized in that: 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 in the pressure and vacuum degree environment of vacuum hot pressing and sintering.

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