Graphene-carbon nanotube composite film strain sensor and preparation method thereof

Abstract

A graphene-carbon nanotube composite film strain sensor provided by the present invention comprises a substrate layer, a smart layer and a protection layer, the substrate layer is located at the bottom of the smart layer, the protection layer is located at the top of the smart layer, and two ends of the smart layer are provided with a plurality of conductive wires. The PDMS substrate layer is covered with a graphene smart layer, and the two ends of the graphene smart layer are connected with wires; preparing a PDMS mixed solution on the graphene smart layer, removing bubbles, and curing to form a PDMS protective layer; and demolding and cutting into a proper size to prepare the graphene-carbon nanotube composite film strain sensor. The sensor has a large measuring range of 50000 mu epsilon and excellent sensitivity coefficient and weather resistance, and can give a response and continuously monitor the expansion process of a crack when the crack of the concrete member is generated. The sensor is simple in preparation method and low in cost, and has great application potential in structural monitoring of civil engineering.

Description

Technical Field

[0001] The present invention relates to the field of flexible sensors, and in particular to a graphene-carbon nanotube composite film strain sensor and a preparation method thereof. Background Art

[0002] Strain sensors are functional devices that measure the deformation of objects under force and are widely used in structural damage detection and health monitoring.

[0003] Traditional strain sensors include resistive strain sensors, vibrating wire strain sensors, and grating strain sensors. Resistive strain sensors are primarily fabricated using the piezoresistive effect of metal or semiconductor materials. They can only monitor strain before a structure or component cracks, and their performance is unstable at high temperatures, making them unsuitable for long-term monitoring. Grating strain sensors have yet to gain widespread popularity due to high equipment costs, limited monitoring areas, insensitivity to specific cracks, and the tendency for embedded monitoring to introduce defects. While vibrating wire strain sensors are widely used in practical engineering, they still face challenges such as short service life, high cost, and limited measurement range.

[0004] Existing research on graphene-based strain sensors typically involves incorporating graphene nanomaterials as conductive fillers into polymers. This results in a high percolation threshold for the polymer material, requiring a high content of conductive filler to produce a significant electrical signal change during deformation testing. Furthermore, laboratory tests of existing graphene-based strain sensors have shown that their initial resistance is too high, making it difficult to achieve the same stable readings as metal strain gauges. To address this issue, we propose a graphene-carbon nanotube composite film strain sensor and its preparation method to address these issues. Summary of the Invention

[0005] The present invention provides a graphene-carbon nanotube composite film strain sensor and a preparation method thereof, which solves the problems of unstable performance of resistive strain sensors at high temperatures and unsuitability for long-term monitoring; high cost and insensitivity to specific cracks of grating strain sensors; short life of vibrating wire strain sensors; and large percolation threshold of graphene-based strain sensor materials, excessively high initial resistance, and difficulty in achieving stable readings like metal strain gauges.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a graphene-carbon nanotube composite film strain sensor and its preparation method, including a base layer, a sensitive layer and a protective layer, the base layer is located at the bottom of the sensitive layer, the protective layer is located at the top of the sensitive layer, and multiple conductive wires are provided at both ends of the sensitive layer.

[0007] In a preferred embodiment, the materials of the base layer and the protective layer are polydimethylsiloxane (PDMS). The sensitive layer includes a double-layer graphene-carbon nanotube composite polyethylene (PE) film.

[0008] In a preferred embodiment, the thickness of the base layer and the protective layer are both 0.8-1.2 mm, and the double-layer graphene-carbon nanotube composite polyethylene (PE) film is bonded by an inorganic adhesive.

[0009] In a preferred embodiment, the conductive wire is connected to the sensitive layer via conductive silver glue.

[0010] A graphene-carbon nanotube composite film strain sensor and a preparation method thereof, characterized by: S1, preparing a base layer (1): preparing a PDMS mixed liquid and transferring it into a mold, removing bubbles, and curing to form a dimethylsiloxane (PDMS) base layer; S2. Fabricating a graphene sensitive layer: Covering the surface of a dimethylsiloxane (PDMS) substrate with a graphene sensitive layer, and connecting conductive wires at both ends of the graphene sensitive layer; S3. Fabricating a protective layer: preparing a PDMS mixture on the graphene sensitive layer, removing bubbles, and curing to form a polydimethylsiloxane (PDMS) protective layer; S4. Demolding and cutting the entire structure to form a graphene-carbon nanotube composite film strain sensor.

[0011] In a preferred solution, in S1, the material of the mold is polytetrafluoroethylene, and the size of the internal groove of the mold is 100 mm×50 mm×5 mm.

[0012] In a preferred embodiment, the steps for preparing the base layer in S1 are as follows: A1, mixing the PDMS main agent and the curing agent in a mass ratio of 10:1, pouring the mixture into a mold, and vibrating the mixture several times; A2. Move the mold to a vacuum drying oven and dry it in a vacuum oven for 10 minutes to remove bubbles. A3. After curing under constant temperature and humidity conditions, a PDMS base layer is formed.

[0013] In a preferred embodiment, in S2, the steps of preparing the graphene sensitive layer are: B1, bonding two graphene-carbon nanotube composite PE films by an inorganic adhesive; B2. Conductive silver glue is used at intervals of 10 mm at both ends of the sensitive layer composite film to connect the conductive wires to the double-layer graphene-carbon nanotube composite PE film; B3. Transferring the double-layer graphene-carbon nanotube composite PE film onto the surface of the PDMS substrate layer to form a graphene sensitive layer, with the conductive wires located on the upper surface of the sensitive layer.

[0014] In a preferred embodiment, in S3, the steps for preparing the protective layer of dimethylsiloxane (PDMS) are the same as the steps for preparing the base layer.

[0015] The present invention has the following beneficial effects: The present invention comprises a layered structure comprising a PDMS base layer, a graphene material sensitive layer, and a PDMS protective layer. The strain sensor, which utilizes a double-layer graphene-carbon nanotube composite film as the sensitive layer, has a range of 50,000 με, a significant improvement over conventional resistance strain gauges. Furthermore, the strain sensor's sensitivity coefficient, which is comprised of the double-layer graphene-carbon nanotube composite film as the sensitive layer, ranges from 15 to 30, far exceeding that of metal strain gauges.

[0016] Graphene-carbon nanotube composite PE film has excellent electrical conductivity and tensile properties. The graphene-carbon nanotube composite PE film is cut into a single-layer film, and an inorganic adhesive is used to form a double-layer film with a sensitive layer. This double-layer film has stable and low resistance. Tensile tests on the double-layer graphene-carbon nanotube composite film strain sensor showed an average tensile strain of 32% and failure at the end.

[0017] The strain sensor in this application demonstrates an increase in resistance with increasing temperature under constant humidity conditions. At constant temperature, the resistance is virtually unaffected by humidity. After ultraviolet irradiation, the strain sensor's initial resistance remains relatively stable, and its tensile strength significantly increases. Compared to traditional resistance strain gauges, the graphene-based strain sensor in this application offers superior basic performance, sensitivity, weather resistance, and cost-effectiveness.

[0018] Conventional resistance strain gauges experience a sudden increase in strain and subsequent signal loss after concrete cracking. The graphene-carbon nanotube composite film strain sensor described in this application still exhibits a change in resistance after concrete cracking, but this change no longer follows a linear pattern. The graphene-based strain sensor not only responds to concrete cracks as they occur but also continuously monitors their expansion, offering greater reliability and practicality, and thus significant potential for widespread adoption. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be further described below with reference to the accompanying drawings and examples; Figure 1 It is a structural diagram of the overall structure of the present invention; Figure 2 This is a graph showing the relationship between the resistance change rate and strain of the sensor of the present invention; Figure 3 This is a test chart of the sensitivity coefficient of sensors of different batches of the present invention; Figure 4 This is a graph showing how the resistance of sensors from different batches of the present invention changes with humidity at a constant temperature of 20°C; Figure 5 This is a graph showing the resistance variation of the variable sensors of different batches under the condition of 50% relative humidity; Figure 6This is a graph showing the effect of temperature and humidity cycles on the initial resistance of the sensor of the present invention; Figure 7 This is a graph showing the effect of temperature and humidity cycles on the tensile strength of the sensor of the present invention; Figure 8 This is a diagram showing the effect of temperature and humidity cycles on the tensile strength of the sensor of the present invention; Figure 9 This is a graph showing the effect of ultraviolet irradiation on the tensile strength of the sensor of the present invention; Figure 10 The strain of the resistance strain gauge, the resistance change rate of the sensor, and the measured crack width change curve during the specimen loading process of the present invention; In the figure: base layer 1; sensitive layer 2; protective layer 3; conductive wire 4. DETAILED DESCRIPTION

[0020] Example 1: like Figure 1-10 In the invention, a graphene-carbon nanotube composite film strain sensor and its preparation method are provided, comprising a base layer 1, a sensitive layer 2 and a protective layer 3, wherein the base layer 1 is located at the bottom of the sensitive layer 2, the protective layer 3 is located at the top of the sensitive layer 2, and a plurality of conductive wires 4 are provided at both ends of the sensitive layer 2. Based on this structure, the present application is a layered structure of a base layer 1 of PDMS, a sensitive layer 2 of graphene material and a protective layer 3 of PDMS. The strain sensor with a double-layer graphene-carbon nanotube composite film as the sensitive layer 2 has a measuring range of 50,000 με, which is significantly improved compared to the traditional resistance strain gauge which is generally 20,000 με. Secondly, the strain sensor of the sensitive layer 2 of the double-layer graphene-carbon nanotube composite film has a sensitivity coefficient of 15 to 30, which is much larger than the sensitivity coefficient of the metal strain gauge which is generally 0.8.

[0021] Polydimethylsiloxane (PDMS) material can be directly bonded to the graphene-carbon nanotube composite film.

[0022] Graphene-carbon nanotube composite PE film has excellent electrical conductivity and tensile properties. The graphene-carbon nanotube composite PE film is cut into a single-layer film, and an inorganic adhesive is used to form a double-layer film with a sensitive layer 2. This double-layer film has stable and low resistance. Tensile tests on the double-layer graphene-carbon nanotube composite film strain sensor showed an average tensile strain of 32% and failure at the end.

[0023] The strain sensor in this application demonstrates an increase in resistance with increasing temperature under constant humidity conditions. At constant temperature, the resistance is virtually unaffected by humidity. After ultraviolet irradiation, the strain sensor's initial resistance remains relatively stable, and its tensile strength significantly increases. Compared to traditional resistance strain gauges, the graphene-based strain sensor in this application offers superior basic performance, sensitivity, weather resistance, and cost-effectiveness.

[0024] Conventional resistance strain gauges experience a sudden increase in strain and subsequent signal loss after concrete cracking. The graphene-carbon nanotube composite film strain sensor described in this application still exhibits a change in resistance after concrete cracking, but this change no longer follows a linear pattern. The graphene-based strain sensor not only responds to concrete cracks as they occur but also continuously monitors their growth, offering greater reliability and practicality.

[0025] In a preferred embodiment, the materials of the base layer 1 and the protective layer 3 are polydimethylsiloxane (PDMS). The sensitive layer 2 comprises a double-layer graphene-carbon nanotube composite polyethylene PE film. In a preferred embodiment, the thickness of both the base layer 1 and the protective layer 3 is 0.8-1.2 mm. The double-layer graphene-carbon nanotube composite polyethylene (PE) film is bonded together with an inorganic adhesive. This structure allows for a 1 mm thick PDMS base layer, making it easy to demold, less susceptible to damage, and exhibiting minimal attenuation, resulting in a smaller overall sensor thickness. After curing for 24 hours, the PDMS base layer 1 fully solidifies, exhibiting strong adhesion and elasticity, allowing for excellent bonding to the graphene sensitive layer 2.

[0026] In a preferred embodiment, the conductive wire 4 is connected to the sensitive layer 2 via conductive silver glue. In this structure, the conductive wire 4 is used to measure the strain of the component. The conductive wire 4 is connected to the wire by soldering and is connected to the instrument for collecting strain.

[0027] Carbon nanotubes (CNTs) possess excellent mechanical properties, low density, and self-lubrication. Their Vickers hardness is comparable to that of diamond, their tensile strength reaches 50-200 GPa, and they share electrical and mechanical properties with graphene. Carbon nanotubes and graphene are excellent one-dimensional and two-dimensional carbon structures, respectively, and their combined advantages can be used in composite materials. The sensor described in this application offers a wide range, high sensitivity, stable readings, excellent durability, and low cost.

[0028] Example 2: Further described in conjunction with Example 1: A graphene-carbon nanotube composite film strain sensor and a preparation method thereof are characterized by: S1, preparing a base layer 1: preparing a PDMS mixed liquid and transferring it into a mold, removing bubbles, and curing it to form a dimethylsiloxane PDMS base layer; S2, preparing the graphene sensitive layer 2: covering the surface of the dimethylsiloxane (PDMS) base layer 1 with the graphene sensitive layer 2, and connecting conductive wires at both ends of the graphene sensitive layer 2; S3, preparing protective layer 3: preparing a PDMS mixed solution on the graphene sensitive layer 2, removing bubbles, and curing to form a polydimethylsiloxane (PDMS) protective layer; S4. Demolding and cutting the entire structure to form a graphene-carbon nanotube composite film strain sensor.

[0029] In a preferred solution, in S1, the material of the mold is polytetrafluoroethylene, and the size of the internal groove of the mold is 100 mm×50 mm×5 mm.

[0030] Example 3: Further explanation based on Examples 1 and 2: In S1, the preparation steps for base layer 1 are as follows: PDMS base agent and curing agent are mixed in a mass ratio of 10:1, the total mass of the PDMS base agent and curing agent is 4g, and the mixture is poured into a mold and shaken multiple times; the mold is placed in a vacuum drying oven and vacuum dried for 10 minutes to remove air bubbles; and finally, the mold is cured at 25°C and 50% humidity for 24 hours to form the 1mm thick PDMS base layer 1. The mold is made of polytetrafluoroethylene (PTFE) with external dimensions of 120mm × 70mm × 8mm and an internal groove of 100mm × 50mm × 5mm. This ensures that the PDMS base layer 1 can be smoothly demolded after curing and does not stick to the mold. PTFE molds are reusable, effectively reducing costs.

[0031] In a preferred embodiment, in S2, the steps for preparing the graphene sensitive layer 2 are as follows: after forming the base layer 1, two sheets of graphene-carbon nanotube composite PE films measuring 100 mm x 50 mm are bonded together using an inorganic adhesive; conductive silver adhesive is used to connect the conductive wires 4 to the double-layer graphene-carbon nanotube composite PE film at 10 mm intervals on both ends of the composite film of the sensitive layer 2, for a total of eight conductive wires. The double-layer graphene-carbon nanotube composite PE film is transferred to the surface of the PDMS base layer 1 to form the graphene sensitive layer, with the conductive wires 4 located on the upper surface of the sensitive layer.

[0032] The graphene-carbon nanotube composite film strain sensor has a large measuring range of 50,000 με; a sensitivity coefficient between 15 and 30; a linearity of about 7% and a repeatability of 2.47% to 5.68%; and a resolution of less than 1.04×10-5 strain.

[0033] In a preferred embodiment, in S3 , the steps for preparing the protective layer 3 of dimethylsiloxane (PDMS) are the same as those for preparing the base layer 1 .

[0034] Example 4: Further explanation is given in conjunction with Examples 1 to 3: The prepared sensor is subjected to resistance change rate-strain relationship test, as shown in FIG. Figure 2As shown in the figure, different batches of graphene-carbon nanotube composite film strain sensors show a good linear relationship between the resistance change rate and the strain within a certain deformation range of less than 8%. However, when the strain exceeds this range, the relationship between the resistance change rate and strain becomes complex. Considering a certain margin for the measuring range, the sensor's measuring range is determined to be 5%, or 50,000 με.

[0035] Initial resistance measurements were performed on sensors from different batches under constant temperature and humidity conditions. Four sensors were randomly selected from each batch. The resistance test was performed to minimize disturbance to the strain gauges, as shown in Table 1. Although the initial resistance of the sensors is not as stable as that of metal strain gauges and exhibits some deviation, the error is generally around 10%, which does not affect normal measurements.

[0036]

[0037] Table 1: Initial resistance measurements of sensors from different batches under constant temperature and humidity conditions The sensitivity coefficient of the sensor prepared in this embodiment was tested under constant temperature and humidity conditions, as shown in Table 2. The sensors that have been tested for sensitivity coefficient have all been confirmed to be able to be stretched cyclically for more than 5 times to avoid the inaccuracy of the sensitivity coefficient tested due to the residual resistance of the strain sensor. It can be seen that there is a certain gap in the sensitivity coefficient between different batches of sensors, but the sensitivity coefficient fluctuation of the same batch of sensors is relatively stable. The sensitivity test of different batches of sensors was carried out within 5% of the test range. Figure 3 shown.

[0038]

[0039] Table 2: Sensitivity test of different batches of sensors The linearity of the sensor is the maximum deviation between the measured resistance change rate-strain curve and the fitted resistance change rate-strain line, expressed as a percentage of the resistance change rate at 5% strain within the test range, as shown in Table 3. While errors may exist between sensor batches due to factors such as manufacturing process and temperature, the linearity remains relatively stable, generally around 7%.

[0040]

[0041] Table 3: Linearity of graphene-carbon nanotube composite film strain sensors from different batches Sensor repeatability refers to the inconsistency of the characteristic curve obtained when the input variable is continuously varied in the same direction over the full measurement range under the same operating conditions, as shown in Table 4. As can be seen, the repeatability of sensors from different batches ranges from 2.47% to 5.68%. This variation in repeatability is related to the preparation and packaging of the sensitive layer. However, the repeatability is generally around 5%, which meets the requirements of practical engineering applications.

[0042]

[0043] Table 4: Repeatability of different batches of graphene-carbon nanotube composite film strain sensors Example 5: Further explanation is given in conjunction with Examples 1 to 4: At room temperature of 20°C, the sensor is placed in an environment with a relative humidity change of 10% to 90%, and the initial resistance of the sensor is measured. The initial resistance of the strain sensor in an environment with a relative humidity of 50% is used as a reference point to calculate the resistance change rate of the sensor, as shown in FIG. Figure 4 As shown in the figure, the sensor's resistance changes almost negligibly with increasing humidity. Because the graphene-carbon nanotube composite film's sensitive layer 2 is encapsulated between two layers of PDMS, and the PDMS structure is dense, hydrophobic, and has excellent water-repellent properties, the sensor's initial resistance is largely unaffected by humidity.

[0044] For the sensor prepared in this embodiment, the initial resistance of the sensor at room temperature of 20°C was used as the reference point under constant humidity conditions, and the initial resistance of the sensor was measured in the temperature range of -20°C to 60°C. Figure 5 As shown in Figure 2, it can be seen that as the temperature increases, the initial resistance of the sensor gradually increases.

[0045] For the sensor prepared in this embodiment, four working conditions were set to conduct temperature and humidity cycle tests. Working condition 1: Under the condition of 70% relative humidity, the sensor was first placed at -20% for 2 hours, then the temperature was increased to 60% at a rate of 2°C / min, left for 2 hours, then the temperature was reduced to -20°C at a rate of 2% / min, left for 2 hours, and measurement was performed; working condition 2: Under the condition of 90% relative humidity, the sensor was first placed at a low temperature of -20°C for 2 hours, then the temperature was increased to 60°C at a rate of 2°C / min, left for 2 hours, then the temperature was reduced to -20°C at a rate of 2°C / min, left for 2 hours, and measurement was performed; Working condition 3: Under the condition of relative humidity of 70%, the sensor is first placed at a low temperature of -20℃ for 2 hours, then the temperature is increased to 40℃ at a rate of 2℃ / min, left for 2 hours, then the temperature is decreased to -20℃ at a rate of 2℃ / min, left for 2 hours, and then the measurement is carried out; working condition 4: Under the condition of relative humidity of 90%, the sensor is first placed at a low temperature of -20℃ for 2 hours, then the temperature is increased to 40℃ at a rate of 2℃ / min, left for 2 hours, then the temperature is decreased to -20℃ at a rate of 2℃ / min, left for 2 hours, and then the measurement is carried out. Figure 6 It can be seen that the cyclic change of temperature and humidity will cause the initial resistance of the sensor to decrease, and the reduction is very small and can be almost ignored. The initial resistance of the sensor prepared in this embodiment is relatively stable.

[0046] For the sensor prepared in this embodiment, the tensile strength test was carried out after the temperature and humidity cycle changed under four working conditions, as shown in FIG. Figure 7 As shown in the figure, it can be seen that with the increase of the cycle, the change rate of the tensile strength of the sensor has no fixed rule, but the tensile strength of most sensors is improved.

[0047] The sensor prepared in this embodiment was irradiated with ultraviolet light of 245 nm and cycled for 100 times, while the resistance of the sensor was measured. Figure 8 As shown in the figure, it can be seen that with the increase in the number of cycles, the initial resistance of the PP2-4 and PP2-5 sensors is relatively stable, while the initial resistance of the PP2-6 sensor fluctuates relatively greatly in the first 50 measurements and gradually stabilizes after more than 50 times.

[0048] The sensor prepared in this embodiment was irradiated with ultraviolet light of 245 nm wavelength to measure the tensile strength of the sensor. Figure 9 It can be seen that with the increase of UV cycle irradiation period, the tensile strength of the sensor is significantly increased.

[0049] The resistance strain gauge and the sensor prepared in this embodiment are mounted on the surface of the specimen to carry out a loading test. Figure 10As shown in the figure, when the external load reaches the cracking load, a conventional resistance strain gauge experiences a sudden increase in strain and subsequent signal loss. However, the graphene-carbon nanotube composite film strain sensor still exhibits a change in resistance rate after concrete cracking, although this change no longer follows a linear pattern. The graphene-carbon nanotube composite film strain sensor prepared in this example not only responds to the onset of concrete cracks but also continuously monitors the crack's propagation, demonstrating enhanced reliability and practicality.

[0050] The above embodiments are merely preferred technical solutions of the present invention and should not be construed as limiting the present invention. The scope of protection of the present invention shall be the technical solutions set forth in the claims, including equivalent alternatives to the technical features of the technical solutions set forth in the claims. In other words, equivalent alternatives and improvements within this scope are also within the scope of protection of the present invention.

Claims

1. A graphene-carbon nanotube composite film strain sensor, characterized by: The invention comprises a base layer (1), a sensitive layer (2) and a protective layer (3), wherein the base layer (1) is located at the bottom of the sensitive layer (2), the protective layer (3) is located at the top of the sensitive layer (2), and a plurality of conductive wires (4) are provided at both ends of the sensitive layer (2).

2. The graphene-carbon nanotube composite film strain sensor according to claim 1, wherein: The materials of the base layer (1) and the protective layer (3) are polydimethylsiloxane (PDMS). The sensitive layer (2) comprises a double-layer graphene-carbon nanotube composite polyethylene (PE) film.

3. The graphene-carbon nanotube composite film strain sensor according to claim 1, wherein: The thickness of the base layer (1) and the protective layer (3) are both 0.8-1.2 mm, and the double-layer graphene-carbon nanotube composite polyethylene (PE) film is bonded by an inorganic adhesive.

4. The graphene-carbon nanotube composite film strain sensor according to claim 1, wherein: The conductive wire (4) is connected to the sensitive layer (2) via conductive silver glue.

5. A graphene-carbon nanotube composite film strain sensor and a method for preparing the same according to any one of claims 1 to 4, characterized in that: S1. Preparation of base layer (1): Prepare PDMS mixture and transfer it into the mold, remove bubbles, and solidify to form a polydimethylsiloxane (PDMS) base layer; S2. Fabricating a graphene sensitive layer (2): covering the surface of a polydimethylsiloxane (PDMS) substrate layer (1) with a graphene sensitive layer (2), and connecting conductive wires at both ends of the graphene sensitive layer (2); S3, preparing a protective layer (3): preparing a PDMS mixed solution on the graphene sensitive layer (2), removing bubbles, and curing to form a polydimethylsiloxane (PDMS) protective layer; S4. Demolding and cutting the entire structure to form a graphene-carbon nanotube composite film strain sensor.

6. The graphene-carbon nanotube composite film strain sensor according to claim 5, characterized in that: In S1, the material of the mold is polytetrafluoroethylene, and the internal groove size of the mold is 100 mm × 50 mm × 5 mm.

7. The graphene-carbon nanotube composite film strain sensor according to claim 5, characterized in that: The preparation steps of the base layer (1) in S1 are as follows: A1, the main agent and curing agent of PDMS are mixed in a mass ratio of 10:1, poured into a mold, and vibrated several times; A2. Move the mold to a vacuum drying oven and dry it in a vacuum oven for 10 minutes to remove bubbles. A3. After curing under constant temperature and humidity conditions, a PDMS base layer (1) is formed.

8. The graphene-carbon nanotube composite film strain sensor according to claim 5, The method is characterized in that: in S2, the preparation steps of the graphene sensitive layer (2) are as follows: B1, bonding two graphene-carbon nanotube composite material PE films by an inorganic glue; B2. Using conductive silver glue at intervals of 10 mm at both ends of the composite film of the sensitive layer (2) to connect the conductive wire (4) and the double-layer graphene-carbon nanotube composite PE film; B3. The double-layer graphene-carbon nanotube composite PE film is transferred to the surface of the PDMS substrate layer to form a graphene sensitive layer, and the conductive wire (4) is located on the upper surface of the sensitive layer.

9. The graphene-carbon nanotube composite film strain sensor according to claim 5, characterized in that: in S3, the steps for making the polydimethylsiloxane (PDMS) protective layer (3) are the same as the steps for making the base layer (1).

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