Flexible strain sensor, method of manufacture and deep sea equipment structural health monitoring system

Abstract

The application discloses a flexible strain sensor, a preparation method and a deep-sea equipment structure health monitoring system. The flexible strain sensor comprises a flexible packaging layer, a composite conductive network structure packaged in the flexible packaging layer, at least one first conductive layer and at least one second conductive layer, wherein the first conductive layer is a conductive layer with a compact connection structure after mechanical pressure treatment, the second conductive layer is a conductive layer with a loose stacking structure without mechanical pressure treatment, and a conductive part electrically connected with the composite conductive network structure and led out. The application realizes the unity of ultrahigh strain sensitivity and high signal stability through the synergistic mechanism of macroscopic cracks and network microcracks generated by the two types of conductive layers under strain, and simultaneously endows the sensor with excellent flexibility and conformal attachment capability, so that the sensor can accurately collect the small strain signals on the surface of the deep-sea equipment structure and is suitable for the monitoring of the health of the deep-sea equipment structure.

Description

Technical Field

[0001] This application relates to the field of physical measurement and monitoring technology, and in particular to a flexible strain sensor, its preparation method, and a deep-sea equipment structural health monitoring system. Background Technology

[0002] The ocean covers more than 70% of the Earth's surface, with the deep sea (below 200 meters) accounting for 90% of the marine environment. It is the largest habitat for biological communities on Earth, containing abundant biological, energy, and mineral resources. Deep-sea scientific research is of irreplaceable value for understanding biological evolution, assessing climate change, and exploring marine resources. Currently, remotely operated vehicles (ROVs), manned submersibles (HOVs), and autonomous underwater vehicles (AUVs) are the main platforms for conducting such research. These vehicles operate for extended periods in extreme environments of high pressure (up to hundreds of megapascals), low temperatures, and high salt corrosion. Their pressure-bearing structures undergo continuous and minute deformations (typically less than 1% strain) under immense hydrostatic pressure. If these minute deformations are not detected and assessed in time, they may gradually evolve into microcracks and propagate, ultimately threatening the structural integrity of the equipment and posing significant safety risks to personnel and expensive scientific instruments.

[0003] Therefore, real-time, online monitoring of the structural health of deep-sea equipment is crucial. However, existing structural strain monitoring technologies face significant challenges when applied to the deep-sea environment: The first category is traditional resistive sensors, represented by resistive strain gauges. Although these sensors are small and resistant to electromagnetic interference, their sensitivity (measured by the strain factor GF) is generally low. For the minute strains, typically less than 1%, in deep-sea structures, the resulting resistance change signals are extremely weak, resulting in a low signal-to-noise ratio and making accurate and reliable detection difficult. The second category is fiber-optic strain sensors. Some fiber-optic sensors can achieve high strain sensitivity through complex optical structure designs (such as Fabry-Perot interferometers and long-period gratings). However, optical fibers themselves are brittle and rigid, with limited bending radii, making it difficult to achieve a tight, conformal fit with the complex curved surfaces of deep-sea equipment (such as the cylindrical shell of a pressure vessel and the frustum structure of an observation window). Insufficient fit leads to low strain transfer efficiency and distorted measurement signals. Furthermore, fiber-optic sensing systems are typically complex in structure, difficult to deploy and maintain, limiting their application in deep-sea equipment in space-constrained and harsh environments. The third category is detection technology based on acoustic emission. This method locates and assesses defects by capturing the stress waves released when materials are damaged. However, deep-sea equipment typically integrates thrusters, robotic arms, pumps, valves, and various scientific instruments, which generate complex broadband acoustic noise during operation. This noise can easily drown out weak acoustic emission signals from damage, resulting in a high false alarm rate and poor reliability of this method in practical applications.

[0004] In general, flexible strain sensors have low sensitivity under small strains; traditional fiber optic sensors have complex structures and cannot conformally fit onto complex deep-sea equipment surfaces to accurately collect strain signals due to the bending characteristics of the fiber itself; acoustic emission sensors are easily affected by external signals. Summary of the Invention

[0005] In view of this, this application provides a flexible strain sensor, a preparation method, and a deep-sea equipment structural health monitoring system to solve the problem of insufficient sensitivity of resistive sensors to small strains in the prior art.

[0006] To solve the above-mentioned technical problems, one technical solution adopted in this application is: to provide a flexible strain sensor, comprising: Flexible encapsulation layer; A composite conductive network structure encapsulated within a flexible encapsulation layer, the composite conductive network structure comprising at least one first-type conductive layer and at least one second-type conductive layer; wherein... The first type of conductive layer is a conductive layer with a dense connection structure that has been mechanically pressurized; The second type of conductive layer is a conductive layer with a loosely stacked structure that has not undergone mechanical pressure treatment; and, Conductive parts that are electrically connected to and led out from the composite conductive network structure.

[0007] As a further improvement of this application, the ratio of the number of first-type conductive layers to the number of second-type conductive layers is 9:3.

[0008] As a further improvement of this application, the first conductive layer and the second conductive layer comprise nanoscale conductive materials, which are selected from at least one of metal nanowires, carbon nanotubes, and graphene.

[0009] As a further improvement to this application, the nanoscale conductive material includes silver nanowires.

[0010] As a further improvement to this application, the flexible encapsulation layer is made of an insulating, flexible and stretchable elastomer material.

[0011] As a further improvement to this application, the material of the flexible encapsulation layer is polydimethylsiloxane.

[0012] As a further improvement to this application, the mass ratio of curing agent to substrate in polydimethylsiloxane is 1:15.

[0013] To solve the above-mentioned technical problems, another technical solution adopted in this application is: providing a method for fabricating a flexible strain sensor as described in any of the above claims, comprising: An initial conductive layer is formed on a substrate by at least one coating of a conductive material solution and drying. The initial conductive layer is subjected to mechanical pressure treatment to obtain the first type of conductive layer; A second type of conductive layer is formed on the first type of conductive layer by at least one coating of a conductive material solution and drying process; A composite conductive network structure is constructed using a first-type conductive layer and a second-type conductive layer; Electrically connect the two ends of the composite conductive network structure to the conductive part; The composite conductive network structure and the electrical connection points between the composite conductive network structure and the conductive parts are encapsulated using flexible encapsulation materials.

[0014] As a further improvement to this application, before electrically connecting the two ends of the composite conductive network structure to the conductive part, it further includes: The composite conductive network structure is subjected to thermal annealing.

[0015] To solve the above-mentioned technical problems, another technical solution adopted in this application is: to provide a deep-sea equipment structure health monitoring system, including at least one flexible strain sensor as described above, wherein the flexible strain sensor is conformally attached to the surface of the deep-sea equipment structure.

[0016] The beneficial effects of this application are: The flexible strain sensor provided in this application comprises a composite conductive network structure consisting of at least one first type of conductive layer with a dense interconnection structure obtained through mechanical compression and at least one second type of conductive layer with a loosely stacked structure that has not undergone mechanical compression. Under tensile strain, the dense first type of conductive layer generates a small number of macroscopic through cracks with large sizes due to internal stress concentration, resulting in a sharp, nonlinear step increase in resistance. At the same time, the loosely stacked second type of conductive layer forms a large number of dense network microcracks, whose resistance changes relatively slowly, but its key role is to maintain the continuity of the entire conductive network and prevent the sensor from failing under small strain. The synergy of these two crack modes enables the sensor to generate an electrical signal response that is much stronger than that of traditional resistive sensors in an extremely low strain range. Furthermore, the flexible encapsulation layer endows the sensor with excellent flexibility and elasticity, enabling it to withstand complex deformations such as stretching, bending, and torsion. It perfectly conforms to the complex curved surfaces of cylindrical and spherical structures commonly found in deep-sea equipment, achieving stress-free and gapless conformal attachment. This ensures the accuracy and reliability of strain signal acquisition, overcoming the inherent limitation of fiber optic sensors in adapting to complex surfaces. In addition, based on the principle of resistance change, this sensor exhibits strong resistance to electromagnetic and acoustic interference, avoiding the drawbacks of acoustic emission sensors being susceptible to environmental noise. It can output stable and pure strain signals, providing a high-quality data foundation for subsequent structural health assessments and safety early warnings. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of one embodiment of the flexible strain sensor of the present invention; Figure 2 These are schematic diagrams of the first type of conductive layer and crack in one embodiment of the flexible strain sensor of the present invention; Figure 3 This is a schematic diagram of the second type of conductive layer and a crack diagram of an embodiment of the flexible strain sensor of the present invention; Figure 4 This is a schematic diagram of the resistance change of a flexible strain sensor with different conductive layer ratios under the same strain, according to one embodiment of the present invention. Figure 5 This is a schematic diagram showing the sensitivity of different PDMS ratios and the maximum stress change at 3% tension in one embodiment of the flexible strain sensor of the present invention. Figure 6 This is a schematic flowchart of an embodiment of the fabrication method of the flexible strain sensor of the present invention; Figure 7 This is a schematic diagram of a structure of an embodiment of the deep-sea equipment structural health monitoring system of the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0019] The terms "first," "second," and "third" in this application are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first," "second," or "third" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified. All directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of this application are only used to explain the relative spatial positions and movements of components in a specific orientation (as shown in the figures). If the specific orientation changes, the directional indications also change accordingly. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or devices.

[0020] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0021] Figure 1 This is a schematic diagram of the structure of a flexible strain sensor according to an embodiment of the present invention. Figure 1 As shown, the flexible strain sensor includes: a flexible encapsulation layer 1; a composite conductive network structure 2 encapsulated within the flexible encapsulation layer 1, the composite conductive network structure 2 including at least one first type conductive layer 21 and at least one second type conductive layer 22; the first type conductive layer 21 is a conductive layer with a dense connection structure obtained through mechanical pressure treatment; the second type conductive layer 22 is a conductive layer with a loose stacking structure obtained without mechanical pressure treatment; and a conductive portion 3 electrically connected to and led out from the composite conductive network structure 2.

[0022] The flexible encapsulation layer 1 forms the sensor's body, providing physical protection and environmental isolation for the internal structure and giving the sensor overall flexibility. It completely encapsulates the internal components, ensuring the sensor can operate stably in underwater environments.

[0023] The composite conductive network structure 2 is the core sensing unit of the sensor, responsible for converting mechanical strain into resistance change. Its innovation lies in the fact that it is not a single homogeneous conductive layer, but is composed of at least one first-type conductive layer 21 and at least one second-type conductive layer 22.

[0024] The first type of conductive layer 21 is a conductive layer with a dense interconnect structure obtained through mechanical pressure treatment. Here, "mechanical pressure treatment" refers to the process of applying external mechanical pressure (such as through rolling or flat plate pressing) after the conductive material has been formed into a film. This treatment causes the contact points between the conductive nanomaterials (such as silver nanowires) within the layer to be strongly compressed, deformed, and even undergo localized "cold welding," thereby forming a denser interconnect structure with lower contact resistance at the microscopic level. When not stretched, the nanowire intersections within this layer are firmly established, forming a low-resistance main conductive path.

[0025] The second type of conductive layer 22 is a conductive layer with a loosely stacked structure that has not undergone the aforementioned mechanical pressure treatment. This means that after its formation, the conductive layer did not undergo the additional pressure process described above. Therefore, the conductive nanomaterials within the layer mainly rely on weak interaction forces such as van der Waals forces and mechanical entanglement to form contacts, exhibiting a loosely stacked structure at the microscopic level. There are many contact points between the nanowires, but the connections are relatively fragile.

[0026] It should be noted that the first type of conductive layer 21 and the second type of conductive layer 22 can be stacked vertically in space. Preferably, in some embodiments, the first type of conductive layer 21 and the second type of conductive layer 22 can also be arranged horizontally. For example, a 40mm*20mm composite conductive network structure 2 can be composed of two 20mm*20mm first type conductive layers 21 and second type conductive layers 22 arranged horizontally. The conductive part 3 is electrically connected to both ends of the composite conductive network structure 2 to lead the network resistance signal to an external measurement circuit (such as a digital multimeter or data acquisition card). The conductive part 3 may include welded or bonded metal wires (such as copper wires), printed conductive electrodes, conductive tape, etc. Preferably, in this embodiment, the conductive part 3 includes two conductive silver pastes 31 and two copper wires 32, wherein one conductive silver paste 31 is electrically connected to one end of the composite conductive network structure 2, the other conductive silver paste 31 is electrically connected to the other end of the composite conductive network structure 2, and the two conductive silver pastes 31 are respectively electrically connected to the two copper wires 32.

[0027] Specifically, the functional differentiation between the first type of conductive layer 21 and the second type of conductive layer 22 is the basis for the coexistence of high sensitivity and high stability in this invention. For example... Figure 2 As shown, when subjected to tensile strain, the first type of conductive layer 21 (see [reference]) Figure 2 (As shown in Figure A) Due to the relatively "brittle and hard" structure, stress tends to concentrate at defects. These defects originate from "inherent microscopic defects and structural non-uniformity generated during the manufacturing process," leading to a few macroscopic through-cracks (see Figure A). Figure 2 The rapid expansion (as shown in Figure B) instantly and significantly increases the resistance, contributing the main to the sensitivity. For example... Figure 3 As shown, the second type of conductive layer 22 (see [reference]). Figure 3 As shown in Figure A, due to its loose structure, it can uniformly distribute stress, resulting in a large number of fine, network-like microcracks (see Figure A). Figure 3As shown in Figure B, although the resistance increases under tensile strain, it effectively maintains the continuity of the current path, preventing the sensor from being completely disconnected due to a single large crack, thus ensuring signal stability and device reliability. This synergistic mechanism, where macroscopic cracks are responsible for sensitive abrupt changes and microscopic cracks are responsible for ensuring the path, enables the sensor to generate an electrical signal response far stronger than that of traditional resistive sensors within an extremely low strain range. It should be noted that the second type of conductive layer 22 can effectively maintain the continuity of the current path in the following ways: First, irregular network cracks will exhibit an unstable process of "cracking-connecting-cracking" during the stretching process; second, the edges of the network cracks are not smooth and may form conductive islands (formed after the network cracks crack), for example, some longer silver nanowires may cross the cracks, achieving interconnection between conductive islands; third, some conductive islands just cross through large through cracks, which can also effectively maintain the continuity of the current path.

[0028] Furthermore, based on the above embodiments, in other embodiments, the ratio of the number of the first type of conductive layer 21 to the number of the second type of conductive layer 22 is 9:3.

[0029] Specifically, the number of layers for each type of conductive layer is precisely controlled by controlling the number of coating-drying cycles. For example, to fabricate a sensor with a layer ratio of 9:3, the substrate is first coated and dried repeatedly 9 times to form 9 initial conductive layers. These 9 layers are then subjected to a one-time mechanical pressure treatment, transforming them into a first-type conductive layer 21. Subsequently, the coating and drying process is repeated 3 times on this structure to form 3 new conductive layers. These 3 layers are not pressure-treated and directly serve as the second-type conductive layer 22. Ultimately, the composite conductive network structure 2 consists of a total of 12 layers, of which 9 are pressure-treated layers (first type) and 3 are unpressure-treated layers (second type).

[0030] This specific ratio (9:3) was obtained through systematic experimental optimization. In this embodiment, sensors with different layer ratios were pre-prepared and subjected to constant small tensile strain cycles of 0-3% at a strain rate of 1% per minute. Figure 4As shown, the sensor's sensitivity increases with the increase of the pressurized layer ratio. The Gauge Factor (GF) is a key indicator of strain sensor sensitivity, defined as GF = (ΔR / R0) / ε, where ΔR / R0 is the rate of change of resistance and ε is the strain. Specifically, the sensor with a layer ratio of 9:3 (green) achieves a GF value of 430 at 3% strain, an improvement of approximately 1400% compared to the maximum GF value of 30 for the sensor without a pressurized layer (i.e., a layer ratio of 0:12, blue). In contrast, the sensor composed entirely of pressurized layers (yellow) fractures at approximately 0.288% strain, with a sharp increase in resistance. This indicates that the sensor without any pressurized layers has the lowest sensitivity. Sensitivity increases significantly with the increase of the pressurized layer ratio, reaching its peak at a ratio of 9:3. However, when the sensor is entirely composed of pressurized layers, although the initial sensitivity increases, its brittleness increases, leading to catastrophic fracture at extremely low strain, with resistance approaching infinity and complete loss of stability. Therefore, the 9:3 ratio achieves the best balance between ultra-high sensitivity and good stability and fracture toughness.

[0031] It should be noted that the layer ratio can be adjusted according to different application requirements. For example, for applications requiring higher sensitivity but with acceptable brittleness, other ratios can be tried. For applications emphasizing durability and a large strain range, different balance ratios can be tried. This embodiment protects the proven optimal ratio of 9:3, but the scope of protection of this invention is not limited to this. Other ratios, as long as they are based on the core idea of ​​"composite pressurized and unpressurized layers," may fall within the scope of this invention.

[0032] Furthermore, based on the above embodiments, in other embodiments, the first conductive layer 21 and the second conductive layer 22 contain nanoscale conductive materials, which are selected from at least one of metal nanowires, carbon nanotubes, and graphene.

[0033] Specifically, nanoscale conductive materials refer to conductive materials with at least one dimension ranging from 1 to 100 nanometers. Their nanoscale effects give them advantages in constructing flexible conductive networks: the huge specific surface area facilitates the formation of abundant conductive pathways; good mechanical properties (such as the flexibility of nanowires) allow them to adapt to the deformation of flexible substrates; and they can be processed by solution methods, making the process simple.

[0034] 1. Metal nanowires: such as silver nanowires (AgNW), gold nanowires (AuNW), and copper nanowires (CuNW). Among them, silver nanowires (AgNW) are the preferred choice. They have the highest bulk conductivity among known metals, their synthesis process is mature, they have a high aspect ratio, are easy to disperse in solution, and can be formed into films through drop coating, spin coating, spray coating, etc., to form a highly conductive and highly transparent network.

[0035] 2. Carbon nanotubes (CNTs): Single-walled or multi-walled carbon nanotubes possess extremely high mechanical strength and excellent electrical conductivity. Their tubular structure easily overlaps to form a network. They can be dispersed in suitable solvents to form solutions for coating.

[0036] 3. Graphene: A two-dimensional sheet structure with excellent electrical conductivity. Graphene sheets or solutions prepared by redox methods can be used to construct conductive layers. Slippage between graphene sheets can also contribute to the change in electrical resistance under strain.

[0037] In practice, one of the above materials can be used alone, or two or more materials can be used in combination, such as AgNW and CNTs, in order to combine their respective advantages.

[0038] Preferably, the nanoscale conductive material is silver nanowire.

[0039] It should be noted that the morphology of conductive materials is not limited to "wires" and "tubes". Other one-dimensional, two-dimensional, or zero-dimensional nanoconductive materials, as long as they can form a similar strain-tunable conductive network, can be considered within the scope of equivalent substitution of the concept of this invention. The choice of materials will affect specific parameters such as the initial conductivity, strain response curve, and cyclic stability of the network, but the core concept of "creating two types of conductive layers with different structures / response characteristics by applying pressure and using them to adjust sensor performance" remains unchanged.

[0040] Furthermore, based on the above embodiments, in other embodiments, the flexible encapsulation layer 1 is made of an insulating, flexible, and stretchable elastomer material. Specifically, the insulating, flexible, and stretchable elastomer material ensures the basic requirements of normal sensor operation (insulation), adaptability to curved surface bonding (flexibility), and following substrate deformation without detachment or damage (stretchability).

[0041] Preferably, the flexible encapsulation layer 1 is made of polydimethylsiloxane (PDMS). PDMS is a widely used silicone-based organic polymer elastomer. Its main advantages include: excellent chemical stability and weather resistance, suitable for underwater and corrosive environments; a wide range of tunable Young's modulus, allowing control of flexibility by changing the degree of crosslinking; excellent tensile properties; good biocompatibility; easy casting and curing; and relatively low cost. PDMS is typically supplied in a two-component form: a prepolymer and a curing agent, which are mixed and then cured.

[0042] Preferably, the mass ratio of curing agent to substrate in polydimethylsiloxane is 1:15. For example... Figure 5As shown, this embodiment tested the performance of sensors encapsulated with PDMS at different ratios. It was found that as the curing agent ratio decreased, the resulting PDMS became softer, and the sensor sensitivity tended to increase. The highest sensitivity was achieved at a ratio of 1:20. However, excessively low crosslinking density resulted in insufficient mechanical strength of the PDMS, making it prone to irreversible deformation or debonding from the substrate during cyclic stretching, leading to unstable sensor response and poor repeatability. At a ratio of 1:15, the PDMS exhibited a moderate modulus and good elastic recovery, ensuring both high sensitivity and excellent stability and low hysteresis in long-term cyclic testing. Therefore, this embodiment preferably sets the mass ratio of curing agent to substrate in polydimethylsiloxane to 1:15.

[0043] Figure 6 A schematic flowchart illustrating one embodiment of the fabrication method of the flexible strain sensor of the present invention is shown. Figure 6 As shown, the fabrication method of this flexible strain sensor includes: Step S1: An initial conductive layer is formed on the substrate by coating a conductive material solution at least once and then drying it.

[0044] 1. Substrate preparation: Select a smooth, flat, and heat-resistant flexible film as a temporary substrate, such as a cleaned polyimide film. Fix it to a flat workbench with tape.

[0045] 2. Solution Preparation: This example uses a silver nanowire solution as an example. First, 25 g of ethylene glycol was placed in a 100 mL beaker and stirred using a magnetic stirrer. Then, 0.2 g of polyvinylpyrrolidone was added to the beaker, and the solution was heated to 110 °C while continuously stirring. After the polyvinylpyrrolidone was completely dissolved, 0.25 g of silver nitrate powder was slowly added, and the mixture was stirred for 5 minutes. Next, 3.4 g of ferric chloride solution was added to the stirring reaction system. After 45 seconds, the beaker was covered with plastic wrap and transferred to a preheated constant temperature furnace at 110 °C for 12 hours. After the reaction was completed, the solution was cooled to room temperature (25 °C), washed with acetone, and the product was collected by centrifugation. The precipitate was further washed with ethanol to remove residual acetone, finally obtaining a 3% (3 wt%) silver nanowire solution (ethanol as solvent) for subsequent experiments.

[0046] 3. Coating and Drying: A quantitative amount of solution is drawn using a pipette and dropped onto the substrate. The substrate is then placed in an oven at a preset temperature for a certain period to allow the solvent to evaporate, forming a continuous nanomaterial network film. This "drop-coating-drying" process is equivalent to "coating a conductive material solution and drying it in one step."

[0047] 4. Formation of the initial conductive layer: The above process can be performed only once to form a single initial conductive layer. However, to obtain a first-class conductive layer of a certain thickness, this process usually needs to be repeated multiple times. Each coating is applied to the surface of the previous layer after it has dried, thus stacking the layers one by one to form a stacked structure of "initial conductive layers" with the required number of layers. At this point, the layers are bonded together solely by physical stacking and van der Waals forces.

[0048] Step S2: Apply mechanical pressure to the initial conductive layer to obtain the first type of conductive layer.

[0049] Specifically, to prevent the conductive layer from sticking to the pressurizing tool during pressurization, a thin, smooth spacer layer, such as another clean film, can be placed over the initial conductive layer. In this embodiment, a suitable tool is used to apply uniform mechanical pressure to the area covered with the spacer layer. A custom-made roller press can be used to roll the material at constant pressure and speed, or a flat-head tool can be used to apply pressure manually. This process strongly compresses the conductive network, causing plastic deformation, tight contact, and even local welding at the intersections of the nanomaterials, thereby transforming the "initial conductive layer" into a first-type conductive layer with a dense interconnected structure. The spacer layer is removed after pressurization.

[0050] Step S3: A second conductive layer is formed on the first conductive layer by coating a conductive material solution at least once and then drying it.

[0051] Specifically, this step is similar to step S1, but the substrate being coated is the first type of conductive layer that has already undergone pressure treatment, along with the substrate on which it is located. Similarly, by repeatedly performing the "drop-coating-drying" cycle, a new conductive layer is directly constructed on top of the first type of conductive layer. The key difference is that these newly formed layers are no longer subjected to the mechanical pressure treatment of step S2. Therefore, these newly formed layers retain the original morphology of the loosely stacked nanomaterials, constituting the second type of conductive layer.

[0052] Step S4: Construct a composite conductive network structure using the first type of conductive layer and the second type of conductive layer.

[0053] Step S5: Electrically connect the two ends of the composite conductive network structure to the conductive part.

[0054] Specifically, to extract electrical signals, reliable electrodes need to be fabricated at both ends of the composite conductive network structure. A common method is to use conductive silver paste, applying it to predetermined areas at both ends of the network structure with a fine needle or scraper. Then, the ends of the wires are pressed into the uncured conductive silver paste. Finally, a heat treatment is performed to cure the conductive silver paste, thereby firmly fixing the wires and forming an ohmic contact with the conductive network, completing the connection of the "conductive part".

[0055] Step S6: Encapsulate the composite conductive network structure and the electrical connection points between the composite conductive network structure and the conductive parts using a flexible encapsulation material.

[0056] Specifically, the uniformly mixed and degassed liquid encapsulation material is poured or spin-coated onto the pre-connected composite conductive network structure, ensuring complete coverage of the network structure and connection points. It is then cured according to the material's properties to form a robust, flexible encapsulation layer that seals and protects the core sensing structure.

[0057] Furthermore, in order to enhance conductivity and strengthen interlayer bonding, in addition to the above embodiments, other embodiments further include: performing thermal annealing on the composite conductive network structure before electrically connecting the two ends of the composite conductive network structure to the conductive part.

[0058] Specifically, the substrate with the composite conductive network structure is placed entirely in a high-temperature oven or rapid annealing furnace. In an inert atmosphere or air, the temperature is raised to the target temperature at a certain rate and held for a period of time, then cooled in the furnace or naturally to room temperature. Heat treatment further removes residual organic matter from the conductive material, promotes closer contact between nanomaterials, reduces contact resistance, and thus lowers the initial resistance of the sensor, which is beneficial for improving the signal-to-noise ratio. For multilayer stacked structures, heat treatment can promote slight interdiffusion or sintering of nanomaterials between different layers, enhance the mechanical bonding between layers, and allow the composite network to deform more synergistically as a whole under strain, improving the consistency and stability of the response.

[0059] Figure 7 A schematic diagram of one embodiment of the deep-sea equipment structural health monitoring system of the present invention is shown. This deep-sea equipment structural health monitoring system includes at least one flexible strain sensor as described in any of the above embodiments, the flexible strain sensor being conformally attached to the surface of the deep-sea equipment structure.

[0060] Specifically, such as Figure 7 As shown, the deep-sea equipment structural health monitoring system comprises: 1. Sensing Unit 10: One or more flexible strain sensors 100. Depending on monitoring requirements, they are conformally attached to the surface of critical structural components of the deep-sea equipment. Attachment can be achieved using flexible adhesives compatible with underwater environments.

[0061] 2. Signal Acquisition and Conditioning Unit 11: Includes an excitation source circuit and a high-precision analog-to-digital converter, used to acquire the resistance change signal generated by the sensor due to strain in real time. A four-wire measurement method can be used to improve measurement accuracy.

[0062] 3. Data processing and communication unit 12: responsible for control signal acquisition, processing raw data, extracting feature parameters, and sending data to the control center via wired or wireless means.

[0063] 4. Early Warning Unit 13: A software system located in the control center. It receives data from various sensors and uses a preset algorithm model to determine whether the structural strain state is normal. When abnormal strain is detected, the system triggers an alarm and can execute safety contingency plans in conjunction with the system.

[0064] This embodiment employs a flexible strain sensor. This system can detect minute structural deformations that are difficult for traditional systems to detect, enabling early, online, and distributed monitoring of the structural health of deep-sea equipment, greatly improving the safety and reliability of deep-sea operations. The system can be expanded and integrated with other types of sensors for multi-parameter fusion monitoring. It can also network the sensors, using spatiotemporal correlation analysis of data from multiple sensors to locate and track crack propagation.

[0065] The above are merely embodiments of this application and do not limit the scope of this patent application. Any equivalent structural or procedural changes made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of this application.

Claims

1. A flexible strain sensor, characterized in that, include: Flexible encapsulation layer; A composite conductive network structure encapsulated within the flexible encapsulation layer, the composite conductive network structure comprising at least one first-type conductive layer and at least one second-type conductive layer; wherein... The first type of conductive layer is a conductive layer that has a dense connection structure at the microscopic level after being mechanically pressurized. The second type of conductive layer is a conductive layer that has a loosely stacked structure at the microscopic level without the aforementioned mechanical pressure treatment; and, The conductive parts are electrically connected to and led out from the composite conductive network structure; The ratio of the number of the first type of conductive layer to the number of the second type of conductive layer is adjusted according to different application requirements.

2. The flexible strain sensor according to claim 1, characterized in that, The ratio of the number of the first type of conductive layer to the number of the second type of conductive layer is 9:

3.

3. The flexible strain sensor according to claim 1, characterized in that, The first type of conductive layer and the second type of conductive layer contain nanoscale conductive materials, wherein the nanoscale conductive materials are selected from at least one of metal nanowires, carbon nanotubes, and graphene.

4. The flexible strain sensor according to claim 3, characterized in that, The nanoscale conductive material includes silver nanowires.

5. The flexible strain sensor according to claim 1, characterized in that, The flexible encapsulation layer is made of an insulating, flexible, and stretchable elastomer material.

6. The flexible strain sensor according to claim 5, characterized in that, The flexible encapsulation layer is made of polydimethylsiloxane.

7. The flexible strain sensor according to claim 6, characterized in that, The mass ratio of curing agent to substrate in the polydimethylsiloxane is 1:

15.

8. A method for fabricating a flexible strain sensor as described in any one of claims 1-7, characterized in that, include: An initial conductive layer is formed on a substrate by at least one coating of a conductive material solution and drying. The initial conductive layer is subjected to mechanical pressure treatment to obtain a first type of conductive layer; A second type of conductive layer is formed on the first type of conductive layer by at least one process of coating a conductive material solution onto it and then drying it. A composite conductive network structure is constructed using the first type of conductive layer and the second type of conductive layer; The two ends of the composite conductive network structure are electrically connected to the conductive part; The composite conductive network structure and the electrical connection points between the composite conductive network structure and the conductive part are encapsulated using a flexible encapsulation material.

9. The method for fabricating a flexible strain sensor according to claim 8, characterized in that, Before electrically connecting the two ends of the composite conductive network structure to the conductive part, the method further includes: The composite conductive network structure is subjected to thermal annealing.

10. A deep-sea equipment structural health monitoring system, characterized in that, It includes at least one flexible strain sensor as described in any one of claims 1-7, wherein the flexible strain sensor is conformally attached to the surface of the deep-sea equipment structure.

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