A wide-range flexible strain sensing element and its array packaging method

Abstract

The application discloses a wide-range flexible strain sensing element and an array packaging method thereof. The element comprises a packaging film layer, a flexible strain sensing layer, a packaging gel layer and a vise base at both ends of the sensing element which are stacked in sequence; the vise base is fixed at both ends of the sensing element by mechanical pressure bonding, and the bonding part is provided with bonding sawteeth and anti-skid textures. The packaging process of the element is as follows: the packaging film is cut into a rectangular substrate, a mesh array template is fixed on the rectangular substrate, the packaging film solution is brushed, the sensing layer is placed in the mesh of the array template, the precursor is obtained after compaction and solidification, and then the self-compacting packaging gel is coated, and finally the mechanical fastening of the vise base at both ends is achieved. The application adopts array positioning design, realizes accurate alignment between the layers of the element, and eliminates the problem of sensor signal drift by combining with the mechanical bonding type anchor base, thereby effectively improving the yield, consistency and monitoring accuracy of the element.

Description

Technical Field

[0001] This invention relates to a flexible strain sensing element, specifically a large-range flexible strain sensing element and its array packaging method, belonging to the field of civil engineering structural crack monitoring. Background Technology

[0002] With the explosive growth of infrastructure construction, the civil engineering industry is shifting from a construction phase to one that emphasizes both construction and maintenance. Affected by concentrated construction cycles, future engineering structures will face the challenge of collective aging, making structural health monitoring technology a core means of ensuring the safe operation of facilities. Traditional resistance, vibrating wire, and fiber optic strain sensing technologies are limited by shortcomings such as short range, low sensitivity, and short fatigue life, making it difficult to meet the long-term monitoring needs of structures with large deformations. Flexible strain sensing elements, using carbon-based nanomaterials such as carbon nanotubes and graphene as conductive layers and elastic polymers such as polyurethane and polydimethylsiloxane as substrates, have become the preferred solution for next-generation strain monitoring technology due to their large strain range, high sensitivity, and strong conformal capability.

[0003] However, existing packaging processes for flexible strain sensors generally suffer from two major problems: First, the manufacturing process of the sensors is rudimentary, and a large-scale, standardized packaging process has yet to emerge. The manual process of sequentially coating the sensors with encapsulating adhesive material is inefficient, and the thickness variation of the encapsulation layer between different batches is significant, sometimes exceeding ±30%. Second, most current research uses traditional adhesive techniques to fix the sensors to the surface of the structure under test. Changes in temperature and humidity in the external environment accelerate interface peeling, making this installation method unreliable and unsuitable for the long-term monitoring needs of structural monitoring. Therefore, given the urgent need for highly reliable and long-life sensors for monitoring cracks in civil engineering structures, developing a standardized and environmentally robust packaging method for novel flexible strain sensors is urgently needed. Summary of the Invention

[0004] To address the problems existing in the prior art, the first objective of this invention is to provide a large-range flexible strain sensing element. This sensing element, through a mechanically fastened jaw clamp base, a sawtooth mechanical engagement, and a bolt anchoring installation method, eliminates the signal drift phenomenon caused by traditional adhesive installation methods, ensuring that the sensor and the structure under test deform together over a long period under complex working conditions, thereby improving the monitoring accuracy and reliability of the sensing element.

[0005] The second objective of this invention is to provide an array-type packaging method for a large-range flexible strain sensing element. This method first uses a high-precision mesh array template for positioning and packaging, ensuring that the sensing layer of the sensing element is located at the geometric center of the packaging structure. Combined with a standardized cutting process, this significantly reduces sensitivity deviation and yield between different batches of sensing elements. Furthermore, the mesh array template can be flexibly adjusted in size and hole arrangement according to requirements, and the number of elements processed in a single batch is controllable, making it suitable for large-scale industrial production applications.

[0006] To achieve the above-mentioned technical objectives, the present invention provides a large-range flexible strain sensing element, comprising a sequentially stacked encapsulation film layer, a flexible strain sensing layer, an encapsulation colloid layer, and a jaw clamp base located at both ends of the sensing element; the jaw clamp base is fixed to both ends of the sensing element by mechanical pressing and interlocking, and its interlocking part is provided with interlocking serrations and anti-slip texture.

[0007] The flexible strain sensing element provided by this invention replaces the traditional adhesive installation with a mechanical interlocking anchor base, effectively eliminating stress concentration and signal drift caused by the large material difference between the sensing element and the structure under test, thereby ensuring that the sensing element and the structure under test deform together in a long-term manner under complex working conditions.

[0008] As a preferred embodiment, the base of the clamp containing the wire at both ends of the sensing element is also provided with a through hole with a diameter of 1.5~3mm, an inner wall roughness Ra≤0.8μm and rounded edges.

[0009] As a preferred embodiment, the end of the mortise clamp base is provided with an anchor bolt hole with a diameter of 6~10mm and an anti-slip thread embedded in the hole wall.

[0010] As a preferred option, the anchor bolt holes can be fastened with standard bolts of specifications M6-M10.

[0011] As a preferred embodiment, the bolt has a pitch of 1.0~1.5mm, and the bolt head is provided with a self-locking washer or nylon insert to prevent loosening.

[0012] As a preferred embodiment, the material of the mortise clamp base is at least one of austenitic stainless steel, tool steel, and mold steel.

[0013] As a preferred embodiment, the surface of the mouth clip is coated with an anti-wear coating, the coating material being polytetrafluoroethylene or silicon carbide, with a thickness of 5~20μm.

[0014] This invention also provides an array-type packaging method for a large-range flexible strain sensing element, characterized in that:

[0015] Step S1: Disperse raw materials, including graphene powder and dispersant, in a flexible substrate solution to obtain a mixture. After curing in a mold, demolding, and cutting with a die, connect external wires to obtain a U-shaped sensing layer.

[0016] Step S2: After preparing the encapsulation film using spin coating, cut it into rectangular substrates of uniform size;

[0017] Step S3: Fix the mesh array template on a horizontal platform using a rectangular substrate, apply an encapsulation film solution, transfer the U-shaped sensing layer into the mesh of the array template, and remove the mold after compaction and curing to obtain the precursor of the flexible strain sensing element.

[0018] Step S4: After the precursor is coated with self-sealing colloid, both ends are mechanically fastened with mortise clamps to obtain the product.

[0019] The concentration of graphene powder in the mixture is 5-10 g / L, and the concentration of dispersant is 0.5-1 g / L. More preferably, the dispersant is PVP powder and / or SDS powder.

[0020] To address the problems of poor encapsulation consistency due to the rudimentary manual coating process in existing encapsulation technologies, and insufficient long-term monitoring stability caused by the susceptibility of adhesive mounting interfaces to environmental temperature, humidity, and aging, the encapsulation method provided by this invention first achieves precise alignment and gapless filling of the sensing layer, encapsulation layer, and thin film substrate through the synergistic effect of the arrayed positioning design of a high-precision perforated mold and the self-sealing encapsulating adhesive. Then, combined with a mechanical interlocking anchoring base, the problem of sensor signal drift is eliminated. Thus, while significantly improving the pass rate of mass production and the performance consistency of sensing elements, it also effectively improves the monitoring accuracy of sensing elements.

[0021] As a preferred embodiment, the cutting trajectory of the die-cutting coincides with the edge contour of the U-shaped sensing layer, and the cutting pressure is 10~50MPa, with a blade inclination angle of 15~30°.

[0022] As a preferred embodiment, the preparation process of the U-shaped sensing layer is as follows: graphene powder and dispersant are dispersed in deionized water under stirring, and then added to a flexible substrate solution. After ultrasonic dispersion, a mixture is obtained. The mixture is transferred to a polytetrafluoroethylene mold, cured at room temperature, dried, demolded, and cut with a die to obtain the final product.

[0023] As a preferred embodiment, the mesh array template is made of one of austenitic stainless steel, tool steel, and mold steel, and its thickness is 3-5 mm.

[0024] As a preferred embodiment, the mesh of the array template can completely fit the U-shaped sensing layer, the roughness of the inner wall of the mesh Ra≤1.6μm, and its positioning accuracy error≤0.1mm.

[0025] The parameters of the array template used in this invention must be strictly implemented in accordance with the above requirements. Through high-precision array positioning, it achieves precise alignment between the sensing layer and the encapsulation film, ensuring that the sensing layer is located at the geometric center of the encapsulation structure. Furthermore, the array design can adjust the mesh size and number according to actual production needs, which not only enables large-scale production in a single batch, but also effectively eliminates the sensitivity differences between different batches of the same size, meeting the requirements of industrial production.

[0026] As a preferred embodiment, the difference between the elastic modulus of the encapsulation film material and the elastic modulus of the strain sensing layer is within ±10%, and the thickness is 50~200μm.

[0027] As a preferred embodiment, the material of the encapsulating film is selected from aqueous polyurethane and / or polydimethylsiloxane.

[0028] As a preferred embodiment, the difference between the elastic modulus of the self-sealing encapsulant and the elastic modulus of the strain sensing layer is within ±10%, and its viscosity at 25°C is 500~5000 cP. The viscosity of the self-sealing encapsulant must strictly adhere to the above requirements, ensuring that it can autonomously fill the gap between the sensing layer and the encapsulation film during the encapsulation process, forming a fully enclosed protective structure without interface defects, thereby avoiding testing errors of the sensing element caused by air bubbles.

[0029] As a preferred embodiment, the coating thickness of the self-sealing encapsulant is 250~1000μm.

[0030] Compared with the prior art, the beneficial technical effects of the technical solution provided by the present invention are as follows:

[0031] 1) The sensing element provided by the present invention uses a mechanically fastened jaw clamp base, a sawtooth mechanical engagement and bolt anchoring installation method, which eliminates the signal drift phenomenon caused by the traditional adhesive installation method, ensures that the sensor and the structure under test deform together for a long time under complex working conditions, and improves the monitoring accuracy and reliability of the sensing element.

[0032] 2) In the preparation method provided by the present invention, the sensing layer of the sensing element is located at the geometric center of the packaging structure by positioning and packaging with a high-precision mesh array template. Then, combined with a standardized cutting process, the sensitivity deviation and yield of sensing elements in different batches are greatly reduced. In addition, the mesh array template can also flexibly adjust the size and hole arrangement according to the requirements, and the number of single processing is controllable, which is suitable for large-scale industrial production applications.

[0033] 3) In the technical solution provided by the present invention, a co-encapsulation scheme of an encapsulation film with matching elastic modulus and a self-sealing colloid is adopted. The colloid material fills the surface and edge gaps of the sensing layer through autonomous flow, forming a fully enclosed protective structure without interface defects. This effectively isolates the external environment such as water vapor penetration, chemical corrosion and ultraviolet aging from the disturbance to the sensing element, and extends the monitoring life of the sensing element. Attached Figure Description

[0034] Figure 1 This is a three-dimensional layered schematic diagram of the sensing element packaging method in Embodiment 1 of the present invention;

[0035] Figure 2 This is a schematic diagram of the sensor element packaging process in Embodiment 1 of the present invention;

[0036] in, Figure 2 (a) is a diagram of the array template fixing process. Figure 2 (b) is a diagram showing the process of fixing the sensing layer to the template. Figure 2 (c) is a process diagram showing the process of obtaining the component precursor after removing the mold. Figure 2 (d) is a diagram illustrating the process of obtaining the sensing element through curing and cutting;

[0037] Figure 3 This is a schematic diagram of the overall structural arrangement of the sensing element in Embodiment 1 of the present invention on a top view.

[0038] Figure 4 This is a schematic cross-sectional view of the sensing element in Embodiment 1 of the present invention;

[0039] Figure 5 This is a flowchart illustrating the fabrication process of the sensor element packaging technology in Embodiment 1 of the present invention. Detailed Implementation

[0040] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. Through these descriptions, the features and advantages of the present invention will become clearer and more apparent. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them.

[0041] Example 1

[0042] This embodiment provides a large-range flexible strain sensing element, including a sequentially stacked encapsulation film layer, a flexible strain sensing layer, an encapsulation colloid layer, and a jaw clamp base located at both ends of the sensing element; the jaw clamp base is fixed to both ends of the sensing element by mechanical pressing and interlocking, and its interlocking part is provided with interlocking serrations and anti-slip texture.

[0043] The flexible strain sensing element is 102 mm long, 14 mm wide, and 800 μm thick. Both the encapsulating colloid and the encapsulating film are made of water-based polyurethane.

[0044] Specifically, the fabrication process of this flexible strain sensing element is as follows:

[0045] Step S1: Pre-fabrication of the flexible strain sensing layer: Pour 5g of deionized water into a beaker, add the weighed graphene powder and PVP / SDS powder, and magnetically stir at 1000r / min for 10min to obtain a graphene dispersion. The concentration of graphene powder is 7g / L, and the concentration of dispersant is 0.7g / L. Add the dispersed graphene dispersion to waterborne polyurethane, initially stir magnetically at 1000r / min for 10min, and then ultrasonically disperse at 200W in an ultrasonic cell disruptor for 60min to obtain a graphene-waterborne polyurethane mixed dispersion. Transfer the mixed slurry to a polytetrafluoroethylene mold using a pipette, cure at room temperature for 48h, and then dry in a 50℃ vacuum drying oven for 6h. After demolding, cut the U-shaped strain sensing layer using a die. Coat the end with a 4mm×4mm silver paste electrode, and connect it with a 1mm outer diameter and 150mm length semi-stripped wire by soldering.

[0046] Step S2, Preparation and cutting of the encapsulation film: A waterborne polyurethane film with a thickness of 100 μm was prepared by spin coating. The spin coating acceleration was set to 500 rpm, and the rotation speed was stabilized at 300 rpm and held for 60 s. After the film was formed, it was cut into a rectangular substrate with a length of 210 mm and a width of 140 mm using a cutter.

[0047] Step S3, Precursor Preparation: Using a 20-hole mold arranged in a 10×2 pattern, the prepared waterborne polyurethane film is laid flat on a horizontal operating platform. The mold is fixed, and a thin layer of waterborne polyurethane colloid is applied to the hole area using a flat-head brush. The pre-made sensing layer is then embedded into the holes, and the wires of the sensing layer are passed through the holes reserved on the side of the mold. The sensing layer is rolled and compacted sequentially using a rubber roller to stabilize the colloid thickness within the range of 50±5μm. After being placed at room temperature for 4 hours, it is transferred to a 50℃ vacuum drying oven for 2 hours to dry completely. After complete curing, the mold is removed, and then the film is placed on the coating platform of a heated flatbed coating machine. The doctor blade angle is set to 45 degrees, the height to 800μm, the coating speed to 10mm / s, and the coating material to be waterborne polyurethane colloid. After coating, a stepped temperature curing process is used: pre-curing at 40℃ for 2 hours, followed by curing at 60℃ for 4 hours. The film is then cut using a die to obtain the final product.

[0048] Step S4: Install the anchoring base: Based on the characteristics of the strain sensing element in this example, stainless steel mortise clamps are used, with a width matching the sensing element of 14mm. The diameter of the reserved wire hole is 1.5mm, and the anchoring bolts are M6×1 threaded. During installation, first pass the wire through the reserved wire hole, with a bending radius of not less than 3mm. Use mechanical interlocking to fix the two ends of the specially made mortise clamps to the two ends of the sensing element.

[0049] This embodiment also tests and calibrates the obtained sensing element, and tests and calibrates the initial resistance, sensitivity, monitoring range, monitoring accuracy and fatigue life of flexible strain sensing elements prepared in the same batch.

[0050] In this embodiment, the encapsulated graphene-waterborne polyurethane strain sensing element has a length of 102 mm, a width of 14 mm, a thickness of 800 μm, and a sensitive test section length of 80 mm. Statistical analysis shows that the thickness error of the sensing element from different batches does not exceed ±8%, and the fluctuations in initial resistance and sensitivity coefficient are less than ±5%. Furthermore, by mounting the sensing element on a rubber substrate and conducting simulated tensile tests, the results show that in cyclic testing within a 10% strain amplitude, the strain transfer efficiency of the sensing element anchored using a specially designed clamp reaches 95%, while the traditional adhesive method only achieves 82%, and the latter exhibits interface peeling failure in the later stages of the cyclic test.

[0051] Example 2

[0052] This embodiment is exactly the same as Embodiment 1, except that: 1. The flexible strain sensing element is 52mm long, 14mm wide, and 1mm thick; 2. The encapsulation film and encapsulation colloid are both made of polydimethylsiloxane, and the cut dimensions are rectangular substrates with a length of 140mm and a width of 110mm.

[0053] In this embodiment, the encapsulated graphene-carbon nanotube-polydimethylsiloxane strain sensing element has a length of 52 mm, a width of 14 mm, a thickness of 1000 μm, and a sensitive test section length of 30 mm. Statistical analysis shows that the thickness error of the encapsulated sensing elements from different batches does not exceed ±8%, and the fluctuations in initial resistance and sensitivity coefficient are less than ±5%. Furthermore, by mounting the sensing element on a steel plate and conducting simulated tensile tests, the results show that in cyclic testing within a 15% strain amplitude, the strain transfer efficiency of the sensing element anchored with a specially designed clamp is greater than that of the adhesive-mounted method, and the latter exhibits interface peeling failure in the later stages of the cyclic test.

[0054] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A large range flexible strain sensing element, characterized by: The array type packaging process of the sensing element comprises: Step S1, dispersing raw materials including graphene powder and dispersant in a flexible substrate solution to obtain a mixed solution, and then performing mold curing, demolding and knife mold cutting on the mixed solution to obtain a U-shaped sensing layer with external lead wires; Step S2, preparing a packaging film by using a spin coating method, and then cutting the packaging film into rectangular substrates with the same size; Step S3, fixing a mesh array template on a horizontal platform through the rectangular substrate, brushing the packaging film solution, and then transferring the U-shaped sensing layer into the mesh of the array template, and performing compaction and curing in sequence, and then removing the mold to obtain a flexible strain sensing element precursor; Step S4, after the precursor is coated with a self-compacting packaging gel, the two ends are mechanically fastened through the vise base to obtain the flexible strain sensing element. The concentration of the graphene powder in the mixed solution is 5-10 g / L, and the concentration of the dispersant is 0.5-1 g / L. Among the two ends of the sensing element, the vise base containing one end of the lead wire also reserves a through hole with a pore size of 1.5-3 mm, the inner wall roughness of the hole is Ra≤0.8 μm, and the edge is rounded.

2. A wide range flexible strain sensing element according to claim 1, characterised in that: The end of the vise base is provided with an anchor bolt hole with a pore size of 6-10 mm, and the hole wall is embedded with anti-skid threads; the anchor bolt hole can be fastened by a standard bolt with a specification of M6-M10; the pitch of the bolt is 1.0-1.5 mm, and the head of the bolt is provided with a self-locking washer or a nylon insert to prevent loosening.

3. A multi-range flexible strain sensing element according to claim 1 or 2, characterised in that: The material of the vise base is at least one of austenitic stainless steel, tool steel and die steel; the surface of the vise is coated with an anti-wear coating, and the coating material is polytetrafluoroethylene or silicon carbide with a thickness of 5-20 μm.

4. A wide range flexible strain sensing element according to claim 1, wherein: The cutting trajectory of the knife mold cutting coincides with the edge profile of the U-shaped sensing layer, the cutting pressure is 10-50 MPa, and the knife edge inclination angle is 15-30°.

5. A wide range flexible strain sensing element according to claim 1, wherein: The preparation process of the U-shaped sensing layer is as follows: the graphene powder and the dispersant are dispersed in deionized water under stirring, and then the flexible substrate solution is added, and ultrasonic dispersion is performed to obtain a mixed solution; the mixed solution is transferred to a polytetrafluoroethylene mold, and normal temperature curing is performed, and then drying, demolding and knife mold cutting are performed.

6. A wide range flexible strain sensing element according to claim 1, wherein: The material of the mesh array template is one of austenitic stainless steel, tool steel and die steel, and the thickness is 3-5 mm; the mesh of the array template can completely fit the U-shaped sensing layer, the mesh inner wall roughness is Ra≤1.6 μm, and the positioning accuracy error is ≤0.1 mm.

7. A wide range flexible strain sensing element according to claim 1, wherein: The difference between the elastic modulus of the packaging film material and the elastic modulus of the strain sensing layer is within ±10%, and the thickness is 50-200 μm; the material of the packaging film is selected from water-based polyurethane and / or polydimethylsiloxane.

8. A wide range flexible strain sensing element according to claim 1, wherein: The difference between the elastic modulus of the self-compacting packaging gel and the elastic modulus of the strain sensing layer is within ±10%, and the viscosity at 25°C is 500-5000 cP; the coating thickness of the self-compacting packaging gel is 250-1000 μm.

9. A wide range flexible strain sensing element according to claim 1, wherein: ​

Images