Flexible full-printing strain sensing array for structural health detection

Abstract

The invention relates to the field of flexible sensors and structural health detection, in particular to a flexible full-printing strain sensing array for structural health detection, which comprises a sensing unit which adopts a full-bridge structure based on a Wheatstone bridge, four resistors of the sensing unit are arranged in a petal shape, a resistor on a diagonal line is mounted on the structure to sense strain, and a resistor on a diagonal line is mounted on the structure to sense the strain; the resistor on the other diagonal line is used for temperature compensation; the conductive electrode is prepared from printable and stretchable conductive silver paste through silk-screen printing; the excitation voltage source is used for providing current or potential difference for the four resistor branches of the Wheatstone bridge and driving the Wheatstone bridge to form a stable circuit structure; and the substrate layer is used as a carrier of the conductive electrode and the sensing unit and is made of a material with certain strain tensile property. The strain sensing array provided by the invention has the tensile capacity up to 40%, can realize multi-point detection at one position at the same time, and remarkably improves the resolution and precision of strain monitoring.

Description

Technical Field

[0001] The present invention relates to the field of flexible sensors and structural health detection, and in particular to a flexible fully printed strain sensor array for structural health detection. Background Art

[0002] With the continuous advancement of modern industry and technology, the complexity and safety requirements of engineering structures are increasing. In the fields of aerospace, construction engineering, automobile industry and energy equipment, structural health monitoring (SHM) has become an important means to ensure system stability and extend service life. Among the many monitoring technologies, strain detection is a key indicator for evaluating the health status of the structure, which can directly reflect the changes in the mechanical properties of the structure during loading, deformation and aging.

[0003] However, traditional rigid strain sensors face many challenges in dealing with efficient and accurate strain detection in complex environments, including: High hardness and limited adaptability: Traditional rigid strain sensors are usually difficult to fit perfectly with complex curved surfaces or flexible structures, which can easily lead to installation difficulties or measurement errors. Complex solution circuits and insufficient integration: Rigid sensor systems usually rely on complex solution circuits, with low overall integration, making it difficult to achieve multi-point distributed real-time and efficient monitoring. High cost and limitations: Traditional systems are usually difficult to maintain cost-effectiveness in large-area distributed monitoring, limiting their promotion in multi-sensing node applications. In response to the above problems, flexible strain sensors, with their unique flexibility and high sensitivity, have become an ideal choice for solving strain monitoring in complex environments. Flexible strain sensors use new materials and manufacturing processes, have excellent adaptability and customizability, can be easily attached to irregular surfaces, and have the characteristics of high sensitivity and low power consumption. However, in complex engineering systems, single-point flexible sensing alone is difficult to meet the monitoring needs of large-scale, real-time and multi-point strain data. Therefore, multi-point distributed strain monitoring technology based on flexible sensor arrays is a key research direction for realizing large-area strain detection in the future. The flexible strain sensor array can monitor the strain status of multiple locations simultaneously by modularizing the flexible strain sensors and forming an array structure, forming a distributed, multi-point coordinated monitoring network. This flexible strain sensor array can be fitted into scenes such as roads and bridges and aerospace vehicle skins. Summary of the invention

[0004] To solve the above problems, the present invention provides a flexible fully printed strain sensing array for structural health detection, which can realize multi-point detection at one position at the same time.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] A flexible all-printed strain sensing array for structural health monitoring, comprising:

[0007] The sensitive unit adopts a full-bridge structure based on the Wheatstone bridge, and its four resistors are arranged in a "petal-like" manner. The resistors on one diagonal (working piece) are installed on the structure to sense strain, and the resistors on the other diagonal (compensation piece) are used for temperature compensation;

[0008] Conductive electrodes are prepared by screen printing using a printable and stretchable conductive silver paste;

[0009] An excitation voltage source is used to provide current or potential difference to the four resistance branches of the Wheatstone bridge to drive the bridge to form a stable circuit structure;

[0010] The base layer, serving as a carrier of the conductive electrodes and sensitive units, is made of a material having certain strain and tensile properties.

[0011] In the present invention, the thickness of the sensitive unit is 20μm (or even thinner), which can fit tightly to complex surfaces and adapt to the monitoring needs of various forms of structures. The sensitive unit is made of independently prepared printable conductive ink, which is composed of carbon nanotubes, graphene, solvents, TPU particles and conductive liquids. The high conductivity of carbon nanotubes and graphene can significantly improve the electrical response capability of the sensing unit and achieve high sensitivity and stable strain detection. TPU elastomer provides excellent flexibility and mechanical durability to ensure that the sensor maintains structural integrity during repeated deformation. In order to improve conductivity, conductive substances such as ionic liquids are added thereto to enhance its conductivity. The three-dimensional conductive network formed by the combination of the three has both sensitivity and stability for long-term use. At the same time, TPU provides a uniformly dispersed matrix for the conductive material. The preparation process is as follows:

[0012] Weigh 0.5g of carbon nanotubes and 0.05g of graphene and place them in a beaker. Pour a certain amount of DMF (dimethylformamide) and stir. After the carbon nanotubes and graphene are completely dissolved, add 2g of TPU particles, then add 5mL of DMF, stir until the TPU particles are completely dissolved, then add 1-3mL of conductive liquid and stir evenly.

[0013] In the present invention, the conductive electrode is prepared by the following process:

[0014] Unsaturated polyester resin and ECA solvent (ethyl 2-hexanoate) were added into a three-necked flask at a mass ratio of 50%:50%, stirred at 65° C. for 24 hours, and filtered through a 300-mesh sieve to obtain a binder polymer dilution;

[0015] The obtained binder polymer dilution was mixed with silver powder, and then a dispersant (BYK-180), a coupling agent and a thermal initiator were added. After being ground 5 times with a three-powder machine for screen printing, it was ground with a ball mill at 10 rpm for 24 hours to obtain a silver paste; wherein the ratio of each raw material was: silver powder: binder polymer dilution: dispersant: thermal initiator: coupling agent = 80%: 18%: 1%: 0.5%: 0.5%.

[0016] The electrode pattern was printed on the silver paste by screen printing, and then the obtained electrode film was heated at 90° C. for 10 min in a box heater to solidify the electrode pattern.

[0017] In the present invention, the base layer is selected from silicon-based elastomer materials such as polydimethylsiloxane (PDMS) or Ecoflex, which have excellent elastic and mechanical properties, and the maximum strain can reach more than 1000%.

[0018] In the present invention, in order to enhance the adhesion with the conductive silver paste and the sensitive layer, the surface of the substrate layer is subjected to plasma treatment to form activated surface functional groups.

[0019] The present invention has the following beneficial effects:

[0020] The present invention provides a flexible fully printed strain sensor array for structural health detection. The sensor array is based on the structural design of the Wheatstone bridge, and the four resistance values ​​in the sensitive unit are equal, which effectively reduces the impact of temperature changes and achieves good temperature compensation performance. In addition, since the electrodes, sensitive units and substrates are all made of stretchable materials, the flexible strain sensor array has a stretchability of up to 40%. At the same time, within the design area, the sensor array can achieve strain detection of up to 16 points at a single location, significantly improving the resolution and accuracy of strain monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The figure is a schematic diagram of the structure of a flexible fully printed strain sensor array for structural health detection according to an embodiment of the present invention.

[0022] Figure 2 A diagram showing the structure and materials of a sensitive unit in an embodiment of the present invention;

[0023] In the figure: (a) is a schematic diagram of the structure of the sensitive unit; (b) is a diagram of material preparation.

[0024] Figure 3 Schematic diagram of the preparation of a conductive electrode in an embodiment of the present invention.

[0025] Figure 4 4 is a principle block diagram of a signal acquisition circuit in an embodiment of the present invention.

[0026] Figure 5 This is a specific application example of the present invention.

[0027] Figure 6 Testing the strain data of the sensitive unit of the present invention;

[0028] In the figure: 1-sensitive unit; 2-conductive electrode; 3-excitation voltage source; 4-base layer; 5-signal connection point; 6-sensitive unit arrangement point; 7-insulating ink area; 8-printed electrode; 9-left electrode; 10-right electrode; 11-middle electrode. DETAILED DESCRIPTION

[0029] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0030] Reference Figure 1 A flexible fully printed strain sensing array for structural health detection is a 4*4 distributed strain detection array, which is mainly composed of a sensitive unit 1, a conductive electrode 2, an excitation voltage source 3 and a substrate layer 4. The sensor is based on the structural design of a Wheatstone bridge. When a pair of bridge arms in the bridge is unbalanced due to external strain, the output voltage of the bridge will change accordingly, thereby realizing the detection of strain signals.

[0031] The sensitive unit 1 adopts a full-bridge structure based on the "Wheatstone bridge", and its four resistors are arranged in a "petal shape". The resistor on one diagonal (working sheet) is installed on the structure to sense strain, and the resistor on the other diagonal (compensation sheet) is used for temperature compensation, so that it has the advantage of being able to resist temperature compensation during the stretching process. When there is no strain and temperature signal, the resistance values ​​of the two pairs of diagonals are the same. At this time, the entire bridge is in a balanced state. The material of the sensitive unit 1 adopts independently prepared printable conductive ink, which is mainly composed of carbon nanotubes, graphene and elastomers, and has a strain elastic range of up to 50%, good resilience and repeatability. In the present invention, the thickness of the sensitive unit 1 is designed to be 20μm (or even thinner), which can effectively avoid wrinkling while ensuring conductive properties, thereby improving the stability and reliability of the sensor.

[0032] The material of the conductive electrode 2 is a printable and stretchable conductive silver paste prepared by screen printing. Its main components are silver powder and elastomer. The rheological properties are optimized by adding a dispersant to make it suitable for the screen printing process, ensuring the precise forming and stable conductivity of the electrode structure.

[0033] The excitation voltage source 3 is used to provide current or potential difference for the four resistance branches of the Wheatstone bridge, driving the bridge to form a stable circuit structure, thereby realizing signal detection and measurement, and its stability plays a key role in the accuracy of the sensor. When strain acts on the sensor, it will cause the strain-sensitive resistance value to change, and this change will break the balance of the bridge, thereby generating a voltage signal proportional to the strain at the output end. At the same time, the excitation voltage source 3 also enables the change of the temperature compensation resistor to be detected, allowing the system to distinguish and compensate for the resistance change caused by temperature change. Therefore, the excitation voltage source 3 not only ensures that the sensor can accurately measure strain, but also helps to achieve temperature compensation, improving the accuracy and reliability of the measurement.

[0034] The substrate layer 4 is used as a carrier of electrodes and sensitive units, and a material with certain strain tensile properties is selected, preferably a high-stretchability elastomer is mainly used to achieve large strain capacity. The substrate material selected in the present invention is a silicon-based elastomer material such as polydimethylsiloxane (PDMS) or Ecoflex, which has excellent elastic and mechanical properties, and the maximum strain can reach more than 1000%. In order to further improve the bonding force between the substrate surface and other functional layers, the surface of the substrate layer 4 is plasma treated to form activated surface functional groups, thereby significantly enhancing the adhesion with the conductive silver paste and the sensitive layer. On the treated substrate surface, the conductive silver paste and the sensitive layer are co-deposited using a printing process, which can achieve a firm bond between different materials and ensure the reliable performance of the sensor under large strain conditions.

[0035] Reference Figure 2 , Figure 2 The structure and material diagram of the sensitive unit in the present invention. In the present invention, the structure of the sensitive unit is similar to "four petals", and the resistances of the four petals are marked as R1, R2, R3, and R4 respectively. When the bridge is balanced, the following equation can be obtained according to Kirchhoff's law: R1 / R3=R2 / R4, at this time the detection voltage V out =0V. When the bridge loses balance, the output voltage is: Where E is the excitation voltage. Due to the consistency of the materials and structures of the four resistors during the printing process, the resistance values ​​are approximately the same and remain consistent under the influence of temperature. Here, resistors R1 and R3 are a pair, resistors R2 and R4 are another pair, and their temperature coefficients are the same. When the temperature changes, the change in each pair of resistors is the same, that is: R1′=R1(1+αΔT); R2′=R2(1+αΔT); R3′=R3(1+αΔT); R4′=R4(1+αΔT). Where α is the temperature coefficient of the resistor, ΔT is the temperature change, and V after the temperature change is out ' can be expressed as follows:

[0036]

[0037] Therefore, the structure of the sensitive unit of the present invention can resist the influence of temperature.

[0038] In the material preparation of sensitive units, such as Figure 2 As shown in Figure b, the raw materials are mainly composed of carbon nanotubes, graphene, solvent, TPU particles and conductive liquid. The preparation process is as follows: First, weigh 0.5g of carbon nanotubes and 0.05g of graphene, place them in a beaker, and then pour a certain amount of DMF (dimethylformamide) into it for stirring and dissolving. After observing that it is completely dissolved, add 2g of TPU particles, then add 5ml of DMF, stir until the particles are completely dissolved, and then add 1mL of conductive liquid, preferably ionic liquid, to further increase the conductivity of the ink, while maintaining a certain viscosity and stirring evenly.

[0039] Reference Figure 3 The conductive electrode 2 in the present invention is made of a printable and stretchable conductive silver paste. The preparation process of the conductive electrode 2 is as follows: First, unsaturated polyester resin and ECA solvent (ethyl 2-hexanoate) are added to a three-necked flask in a mass ratio of 50%:50%, stirred at 65°C for 24 hours, filtered with a 300-mesh sieve, and the obtained binder polymer diluent is mixed with silver powder, and then a dispersant (BYK-180), a coupling agent and a thermal initiator are added. After grinding 5 times in a three-powder machine for screen printing, a ball mill at 10rpm is used for grinding for 24 hours. Then, the obtained silver paste is used to print the electrode pattern by screen printing, and the film printed with the silver paste is heated at 90°C for 10 minutes in a box heater to solidify the electrode pattern. Among them, by mass percentage, silver powder: binder polymer diluent: dispersant: thermal initiator: coupling agent = 80%: 18%: 1%: 0.5%: 0.5%.

[0040] After preparing the conductive silver paste, Figure 4As shown, the pre-designed pattern is accurately printed by screen printing process. In the figure, 5 is the signal connection point; 6 is the sensitive unit arrangement point; 7 is the insulating ink area; 8 is the printed electrode; 9 is the left electrode; 10 is the right electrode; 11 is the middle electrode. It should be noted that in the entire printing process, in order to achieve electrical isolation and signal transmission between specific areas, it is also necessary to print insulating ink in the specified part. Specifically, when it is necessary to connect the left and right electrodes, if there is an electrode in the middle, insulating ink can be printed in this part, and then the electrode is printed on the insulating ink to make the left and right sides conductive. The insulating ink can be selected from elastomeric inks such as PDMS or TPU as a bridge for electrical isolation. After printing the insulating ink, conductive silver paste is printed on it to form an electrode structure. Through this process design, cross-conduction and signal acquisition between the left and right columns can be achieved. The present invention successfully leads to 32 wires required for 16 sensors through the arrangement of insulating ink, which significantly improves the integration and functionality of the overall circuit. Finally, the sensitive unit is printed. It should be noted that during the printing process, the self-prepared printable conductive ink needs to be printed on the substrate once and cannot be printed repeatedly. Then, an optical screen printing platform needs to be used for alignment so that the sensitive unit can be accurately printed at the connection of the four electrodes.

[0041] Figure 5 Typical application examples of the present invention are demonstrated, fully demonstrating its wide applicability and functionality. The present invention can be used for real-time detection of battery expansion force, providing key data support for battery safety assessment and optimization design. In addition, the present invention is also suitable for strain monitoring under bridge structures, and provides reliable technical support for structural health monitoring and safety assessment of bridges through high-precision, multi-point distributed strain sensing capabilities. At the same time, the present invention can perform refined strain observations on 16 parts within an area, support multi-channel synchronous data acquisition and analysis, and thus achieve a more comprehensive and accurate detection effect. This versatility and high-resolution characteristics give the present invention broad application prospects in the fields of energy, civil engineering, etc.

[0042] Figure 6 When a single sensor unit is stretched to different degrees (10% to 40%) by a stretching machine, the resistance value of the single sensor unit collected by the LCR test instrument is obtained. Figure 6 It can be seen that the strain of the sensor of the present invention can reach 40%, has a large strain degree, and has a high sensitivity, which can reach more than 5.

[0043] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A flexible fully printed strain sensing array for structural health monitoring, characterized in that: include: The sensitive unit adopts a full-bridge structure based on the Wheatstone bridge, and its four resistors are arranged in a "petal-like" manner. The resistors on one diagonal are installed on the structure to sense strain, and the resistors on the other diagonal are used for temperature compensation; Conductive electrodes are prepared by screen printing using a printable and stretchable conductive silver paste; An excitation voltage source is used to provide current or potential difference to the four resistance branches of the Wheatstone bridge to drive the bridge to form a stable circuit structure; The base layer, serving as a carrier of the conductive electrodes and sensitive units, is made of a material having certain strain and tensile properties.

2. A flexible fully printed strain sensing array for structural health detection as claimed in claim 1, characterized in that: The sensitive unit is made of independently prepared printable conductive ink, which is composed of carbon nanotubes, graphene, a solvent, TPU particles and a conductive liquid.

3. A flexible fully printed strain sensing array for structural health detection as claimed in claim 2, characterized in that: The preparation process of the self-prepared printable conductive ink is as follows: Weigh 0.5g of carbon nanotubes and 0.05g of graphene into a beaker, pour in a certain amount of dimethylformamide and stir. After the carbon nanotubes and graphene are completely dissolved, add 2g of TPU particles, then add 5mL of DMF, stir until the TPU particles are completely dissolved, then add 1-3mL of conductive liquid and stir evenly.

4. A flexible fully printed strain sensor array for structural health detection as claimed in claim 1, characterized in that: The thickness of the sensitive unit is 20 μm.

5. A flexible fully printed strain sensor array for structural health detection as claimed in claim 1, characterized in that: The conductive electrode is prepared by the following process: Unsaturated polyester resin and ECA solvent were added into a three-necked flask at a mass ratio of 50%:50%, stirred at 65°C for 24 hours, and filtered through a 300-mesh sieve to obtain a binder polymer dilution; The obtained binder polymer dilution was mixed with silver powder, and then a dispersant, a coupling agent and a thermal initiator were added, and after screen printing, the mixture was ground with a three-powder machine for 5 times, and then ground with a ball mill at 10 rpm for 24 hours to obtain a silver paste; The electrode pattern was printed on the silver paste by screen printing, and then the obtained electrode film was heated at 90° C. for 10 min in a box heater to solidify the electrode pattern.

6. A flexible fully printed strain sensing array for structural health detection as claimed in claim 5, characterized in that: By mass percentage, silver powder: binder polymer diluent: dispersant: thermal initiator: coupling agent = 80%: 18%: 1％：0.5％：0.5％。 7. A flexible fully printed strain sensor array for structural health detection as claimed in claim 1, characterized in that: The base layer is selected from polydimethylsiloxane or Ecoflex.

8. A flexible fully printed strain sensor array for structural health detection as claimed in claim 1, characterized in that: The surface of the substrate layer is plasma treated to form activated surface functional groups.

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