Structural integrated sensor based on screen printing and invisible crack monitoring method

Abstract

The invention discloses a structure-integrated sensor based on screen printing and an invisible crack monitoring method, and belongs to the technical field of aviation structure health monitoring, the structure-integrated sensor comprises sensing circuits, an adjusting resistor and electrodes, multiple sensing circuits are connected in parallel between the two electrodes, an interval exists between every two adjacent sensing circuits, and the adjusting resistor is connected with the electrodes. The branch of the sensing circuit is connected in series with the single adjusting resistor, and the adjusting resistors are arranged in parallel. According to the invention, a silk-screen printing technology is adopted, the sensing circuit and the adjusting resistor are deposited on an aircraft structure for expression, and the change condition of the crack is judged through a step change electric signal generated when the branch is cut off through crack propagation. Small-space and small-size integration of the sensor is achieved, and the problems that a traditional sensor is low in integration efficiency, large in additional weight and the like are solved. Meanwhile, the sensor provided by the invention has relatively high sensitivity, so that the monitoring performance is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of aerospace structural health monitoring, and particularly to a structure-integrated sensor based on screen printing and a method for monitoring invisible cracks. Background Art

[0002] In the aerospace field, regions connected by riveting, bolts, etc. are prone to severe stress concentration phenomena, which in turn lead to the generation of cracks at the hole edges, negatively affecting the structural reliability of the aircraft. However, due to the occlusion of the lap joint structure or bolts, such cracks are difficult to be directly observed by the naked eye in the early initiation stage. Therefore, it is necessary to monitor such invisible cracks in a timely and effective manner.

[0003] Currently, commonly used structural crack monitoring methods include those based on guided waves, fiber Bragg gratings, eddy currents, etc. The sensors used in these monitoring methods are generally relatively thick and require secondary integration, which leads to problems such as small available integration areas at hole edges and structural lap joints, limited sensor installation space, and low integration efficiency. In addition, existing structural crack detection methods also have problems such as low monitoring sensitivity and high monitoring operation difficulty. Therefore, this application uses screen printing technology to prepare sensors with small thickness dimensions and high sensitivity, providing an effective means for solving the monitoring of invisible cracks in the above-mentioned aircraft structures. Summary of the Invention

[0004] The present invention aims to overcome problems in the prior art such as large sensor integration volume, low integration efficiency, and low monitoring accuracy, and provides a structure-integrated sensor based on screen printing and a method for monitoring invisible cracks. To achieve the above object, the technical solution of the present invention is: a structure-integrated sensor based on screen printing is provided, including:

[0005] A substrate;

[0006] An insulating layer covering the substrate;

[0007] A conductive structure integrated on the insulating layer; the conductive structure includes a plurality of sensing circuits, a plurality of adjusting resistors, and electrodes. The sensing circuits are arranged in parallel, and a single adjusting resistor is connected in series on the branch of the sensing circuit. There is a gap between any two adjacent branches of the sensing circuit;

[0008] A protective layer covering the conductive structure.

[0009] In one embodiment, no adjusting resistor is provided in one branch of the sensing circuit.

[0010] In one embodiment, the resistance values of the adjusting resistors are set in a gradient manner.

[0011] In one embodiment, the adjusting resistors are arranged in parallel.

[0012] In one embodiment, the conductive structure further includes electrodes, and multiple sensing circuits are connected in parallel between the two electrodes.

[0013] In one embodiment, the electrodes and the sensor circuits are connected by conductive silver paste and fine copper wires.

[0014] In one embodiment, both the adjusting resistors and the sensing circuits are made of one or several of metal-based conductive inks, carbon-based conductive inks, and polymer conductive inks, and the protective layer includes one or several of ceramic reinforcing phases and fiber reinforcing phases.

[0015] In one embodiment, the protective layer includes a high-temperature tape that covers the electrodes.

[0016] The present invention also provides a method for monitoring invisible cracks, including:

[0017] Integration of the above-mentioned sensor: Select the structure at the crack as the substrate, apply the insulating layer to the substrate, and perform surface modification; According to the set screen printing parameters, screen print and cure the sensing circuit and the adjusting resistors on the insulating layer; Install the electrodes, perform lead wire treatment on the electrodes and the sensing circuits to achieve the integration of the conductive structure, and cover and fix the protective layer on the integrated conductive structure;

[0018] Monitoring of the crack: Connect the sensor to an external acquisition circuit. Based on the damage of the crack to the sensing circuit, the resistance or potential of the sensor changes, and a step-change electrical signal generated is obtained at the external acquisition circuit to judge the generation and expansion of the crack.

[0019] In one embodiment, in the integration step of the sensor, the crack passes through at least one branch of the sensing circuit.

[0020] In summary, the present invention provides a structure-integrated sensor based on screen printing and a method for monitoring invisible cracks. The present invention uses screen printing technology to deposit low-impedance ink on the surface where cracks occur to form the sensing circuit and deposit high-impedance ink to form the adjusting resistors, and judges the change of the crack through the step-change electrical signal generated by the crack expansion cutting off the branch. The present invention can directly integrate the sensor on the surface of the aircraft structure to achieve the integration of the sensor in a small space and small volume, which is beneficial to solving problems such as low integration efficiency and large additional weight of traditional sensors; at the same time, the sensor in the present invention has high sensitivity, ensuring the monitoring performance.

[0021] In order to make the above - mentioned features and advantages of the invention more obvious and understandable, specific embodiments are given below and will be described in detail in conjunction with the accompanying drawings. Description of the Drawings

[0022] Figure 1 It is a schematic diagram of the conductive structure in the integrated sensor based on the screen - printing structure provided in the first embodiment of the present invention.

[0023] Figure 2 It is a usage state diagram of the integrated sensor based on the screen - printing structure provided in the first embodiment of the present invention.

[0024] Figure 3 It is a curve graph of the resistance change during the fatigue tensile process of the integrated sensor based on the screen - printing structure provided in the first embodiment of the present invention.

[0025] Figure 4 It is a schematic diagram of the conductive structure in the integrated sensor based on the screen - printing structure provided in the second embodiment of the present invention.

[0026] Figure 5 It is a usage state diagram of the integrated sensor based on the screen - printing structure provided in the second embodiment of the present invention.

[0027] Figure 6 It is a curve graph of the resistance change during the fatigue tensile process of the integrated sensor based on the screen - printing structure provided in the second embodiment of the present invention. Description of the Drawings:

[0029] Sensor - 1;

[0030] Conductive structure - 11;

[0031] Sensing circuit - 111; Adjusting resistor - 112; Electrode - 113;

[0032] Substrate - 12;

[0033] Notch - 121; Crack - 122;

[0034] Sensor - 2;

[0035] Conductive structure - 21;

[0036] Sensing circuit - 211; Adjusting resistor - 212; Electrode - 213;

[0037] Substrate - 22;

[0038] Bolt hole - 221; Crack - 222; Bolt - 223. Detailed Description of the Invention

[0039] To make the objectives and technical solutions of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0040] The present invention provides an integrated sensor based on a screen printing structure. The sensor includes a substrate, an insulating layer, a conductive structure, and a protective layer. The insulating layer covers the substrate, the conductive structure is integrated on the insulating layer, and the protective layer covers the conductive structure.

[0041] Embodiment 1

[0042] When performing structural health monitoring, the integrated sensor based on the screen printing structure in the present invention is disposed on the structure to be monitored to monitor cracks, so as to achieve the integrated integration of the sensor and the structure to be monitored. Figure 1 It is a schematic diagram of the conductive structure in the integrated sensor based on the screen printing structure provided in Embodiment 1 of the present invention. As Figure 1 shown, the conductive structure 11 of the present application adopts a parallel resistance configuration, and includes a sensing circuit 111, a regulating resistor 112, and electrodes 113. A plurality of sensing circuits 111 are connected in parallel between the two electrodes 113, and a certain distance is provided between adjacent two sensing circuits 111. The regulating resistor 112 is connected in series alone on the branch of the sensing circuit 111. The sensing circuit 111 is used to sense the propagation of cracks, and the regulating resistor 112 provides a relatively large resistance output for the conductive structure 11.

[0043] The sensing circuit 111 includes n cables spaced at a certain distance, where n is a natural number greater than 1. Based on the shape of the crack to be monitored and the concept of a parallel circuit, the sensing circuit 111 includes, but is not limited to, a linear type or a circular type. Among them, n linear cables spaced at a certain distance are provided in the linear sensing circuit 111; the circular sensing circuit 111 includes concentric circles formed by n cables.

[0044] The adjusting resistors 112 are arranged in parallel, and their resistance values are respectively denoted as R1, R2, ……, Rn. The pattern of the adjusting resistors 112 can be selected as linear, zigzag, serpentine, or other shapes. Based on the calculation rules of parallel circuits and in order to make the electrical signal output step-shaped, preferably, the resistance values of the adjusting resistors 112 are set in a gradually increasing manner, that is, R1 < R2 < …… < Rn, to ensure that the step change of the output of the conductive structure 11 is more obvious. Further, in order to reduce the initial resistance of the conductive structure 11 and improve the sensitivity of the initial crack monitoring of the conductive structure 11, the adjusting resistor 112 is not provided in one branch of the sensing circuit 111. Preferably, the adjusting resistor 112 with a resistance value of R1 is installed, that is, the resistance value R1 can be taken as 0Ω.

[0045] When using the sensor 1 for invisible crack monitoring, the integration of the sensor 1 needs to be carried out first. The integration of the sensor 1 includes pre-treatment, screen printing, and post-treatment.

[0046] As Figure 2 shown, in the pre-treatment step, the substrate 12 can be selected as an aluminum plate structure to simulate the surface structure of the aircraft. A preset notch 121 is provided on the substrate to guide the generation of cracks 122 in the structure, so as to conduct simulation tests on the sensor 1. The surface of the substrate 12 will be insulated, and the treatment processes include but are not limited to depositing an insulating film, anodizing to form an oxide insulating layer, or spin-coating a curable insulating polymer by scraping. After insulation treatment, the surface of the substrate 12 also needs to be modified to improve the surface adhesion performance of the substrate 12. The treatment processes include but are not limited to plasma cleaning, ultraviolet cleaning, ultrasonic cleaning, and corona. In addition, the pre-treatment for the preparation of the sensor 1 also includes the preparation process of printing conductive ink. Among them, the sensing circuit 111 is prepared by using a low-impedance ink material. Specifically, one or several of metal-based conductive ink, carbon-based conductive ink, and polymer conductive ink can be used; the adjusting resistor 112 is prepared by using a high-impedance ink material, and one or several of metal-based conductive ink, carbon-based conductive ink, and polymer conductive ink can be used. In one embodiment, low-impedance conductive silver ink is selected as the printing material for the sensing circuit 111, and high-impedance conductive carbon paste is selected as the printing material for the adjusting resistor 112, and a diluent is added to adjust the viscosities of the conductive silver ink and the conductive carbon paste. The ratio of the conductive silver ink to the diluent is 10:1, and the ratio of the conductive carbon paste to the diluent is 4:1.

[0047] The screen printing steps include the printing and curing of the sensing circuit 111 and the printing and curing of the adjusting resistor 112. During the screen printing process of the sensing circuit 111, the sensing circuit 111 is arranged at the notch 121, and the printing parameters are set for screen printing. The crack 122 passes through at least one branch of the sensing circuit 111. The printing parameters include printing speed, squeegee angle, squeegee pressure, number of printing passes, etc. Among them, the number of printing passes can be selected as 3 times. The distance between the notch 121 and the sensing circuit 111 will directly affect the sensitivity of the conductive structure 11. Therefore, a reasonable layout can be made according to the requirements and the size of the crack 122. After the screen printing of the sensing circuit 111 is completed, it will be thermally cured in a timely manner under the conditions of 120 °C for 40 minutes. Further, the printing parameters are set and the adjusting resistor 112 is screen printed at the preset position of the adjusting resistor 112, and the printed adjusting resistor 112 is thermally cured at 120 °C for 15 minutes.

[0048] In the post-treatment steps, it includes the preparation of the protective layer, the lead wire of the electrode, and the protection of the electrode. During the installation of the electrode 113 and the lead wire connection, the electrode 113 and the sensing circuit 111 are connected by conductive silver paste and fine copper wire. The connection method of the fine copper wire can be welding or other methods. During the protection process of the electrode 113, the electrode 113 is covered with high-temperature tape to achieve the protection of the electrode 113. During the preparation process of the protective layer, the protective layer can be arranged by means of scraping or spin-coating ultraviolet-curable UV epoxy resin. In order to improve the surface hardness, wear resistance, adhesion, etc. of the protective layer, reinforcing phases, coupling agents, etc. will be added to the resin. The reinforcing phase includes one or several of ceramic reinforcing phases or fiber reinforcing phases. The ceramic reinforcing phase can be selected from nano-level alumina, silicon carbide powder, etc.

[0049] After the integration of the sensor 1 is completed, the crack 122 will be monitored. The electrode 113 is connected to an external cable to realize the connection between the sensor 1 and the external acquisition circuit. When a crack occurs at the notch 121 or the existing crack expands, some branches in the sensing circuit 111 will be broken due to the expansion of the crack 122, and the resistance value of the sensor 1 will change with a step increase. Furthermore, the external acquisition circuit will obtain the electrical signal generated due to the change in the resistance or potential of the sensor 1, and judge the generation and expansion of the crack through the step change of the electrical signal.

[0050] Further, in order to verify the feasibility of the structure-integrated sensor based on screen printing for monitoring unilateral cracks in the structure in the first embodiment above, the sensor 1 will be subjected to a fatigue tensile test to simulate the expansion of the crack 121 during the flight of the aircraft. During the fatigue loading process, a high-precision digital multimeter is connected to the sensor to record the change in resistance value, and the test results are as Figure 3 shown. It can be Figure 3 seen that the number of step changes in the resistance output is consistent with the number of branches of the sensing circuit 111, and the sensor 1 can be used to monitor the expansion of the crack.

[0051] Embodiment 2

[0052] In order to improve the adaptability of the sensor 2 to the crack at the hole edge, Figure 4 the ring-shaped conductive structure is shown in. The conductive structure 21 also adopts a parallel resistance configuration. The conductive structure 21 includes a sensing circuit 211, a regulating resistor 212, and electrodes 213. Multiple sensing circuits 211 are connected in parallel between the two electrodes 213, and a certain distance is spaced between adjacent two sensing circuits 211. The regulating resistor 212 is connected in series singly on the branch of the sensing circuit 211, and the regulating resistors 212 are arranged in parallel. The sensing circuit 211 is used to sense the expansion of the crack; the regulating resistor 212 provides a large resistance output for the sensor 2; the electrodes 213 are externally leaded to connect to an external acquisition circuit. Preferably, the sensing circuit 211 includes 7 branches, and the number of the regulating resistors 212 is 6. A section of the sensing circuit 211 adopts a concentric circle-shaped cable design, and the regulating resistor 212 is not provided in the innermost branch. Among the remaining 6 branches from the inside to the outside, the resistance values of the installed regulating resistors 212 are R2, ……, R7 respectively. The printed line width of the sensing circuit 211 can be selected as 0.1 mm, the radius of the innermost branch of the sensing circuit 211 can be selected as 3.5 mm, the distance between the first five branches from the inside to the outside can be selected as 0.5 mm, and the distance between the remaining branches can be selected as 1 mm. The lengths of the regulating resistors 212 with resistance values of R2, R3, R4, R5, R6, and R7 can be selected as 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, and 7 mm respectively.

[0053] In the integrated pre - processing process of the sensor 2, the substrate 22 is selected as an aluminum alloy plate structure to simulate the surface structure of the aircraft. Bolt holes 221 are preset on the substrate 22, and the radius of the bolt holes 221 can be selected as 3 mm. The surface of the substrate 22 is subjected to insulation treatment, which can be carried out by surface anodization or other methods. Then, the surface - modified treatment is carried out on the substrate 22 after the insulation treatment to improve the adhesion effect between the conductive ink and the surface of the aluminum plate structure. The surface - modified treatment can be carried out by plasma cleaning. In addition, Shenzhen Sheng Tian Feng 823SS type conductive silver paste with low impedance can be selected as the printing material for the sensing circuit 211, and Tenryu carbon paste from Japan with high impedance can be selected as the printing material for the adjusting resistor 212. And the viscosity is adjusted by adding a diluent. The mass ratio of the conductive silver paste to the diluent is 10:1, and the mass ratio of the conductive carbon paste to the diluent is 4:1.

[0054] In the screen - printing step, the sensor circuit 211 is arranged concentrically with the bolt holes 221. Cracks 222 on the edge of the bolt holes expand outward from the inside of the sensing circuit 211, and the cracks 222 pass through at least one branch 211 of the sensing circuit. Set the printing speed to 300 mm / s, the squeegee angle to 60°, the off - contact distance to 0.5 mm, and the number of printing times to 3 times for screen - printing. First, screen - print the sensing circuit 211 and perform thermal curing on the printed sensing circuit 211 at 120 °C for 40 min. Then, screen - print the adjusting resistor 212 at the corresponding position and perform thermal curing on the printed adjusting resistor 212 at 120 °C for 15 min. The radius of the bolt holes 221 and the radius of the innermost branch of the sensing circuit 211 determine that the minimum crack length that the sensor 2 can monitor is 0.5 mm.

[0055] In the post - processing step, the protective layer uses Araldite epoxy resin as the base material, adds nano - level alumina powder as the reinforcing phase, and adds alumina powder with a mass fraction of 5%. The protective layer is arranged by scraping and subjected to thermal curing at 120 °C for 1 h. Connect the electrode 213 and the sensing circuit 211 with conductive silver paste and fine copper wire, and use high - temperature tape to protect the electrode 213. As Figure 5 shown, after preparation, install the bolt 223 at the bolt hole 221.

[0056] During the crack monitoring process, a fatigue tensile test will be conducted on the sensor 2 to simulate the crack 222 propagation at the edge of the bolt hole 221 of the aircraft during flight. During the simulation process, a sine wave form is used for fatigue loading, with an amplitude and an offset of 3.6 kN and 4.4 kN respectively, and a frequency of 10 Hz. During the fatigue loading process, a high-precision digital multimeter is connected to the sensor to record the change in the resistance value of the sensor 2. The test results are as Figure 6 shown. The number of step changes in the resistance output of the surface sensor 2 is consistent with the number of branches of the sensing circuit 221, and the sensor 2 can be used to monitor the crack propagation.

[0057] In summary, the present invention provides a structure-integrated sensor based on screen printing and a method for invisible crack monitoring. The present invention uses screen printing technology to deposit low-impedance ink on the surface where cracks occur to form the sensing circuit and deposits high-impedance ink to form the adjustment resistor. The step change electrical signal generated by cutting off the branch due to crack propagation is used to judge the change of the crack. The present invention can directly integrate the sensor on the surface of the aircraft structure to achieve the integration of the sensor in a small space and small volume, which is beneficial to solving problems such as low integration efficiency and large additional weight of traditional sensors; at the same time, the sensor in the present invention has high sensitivity, ensuring the monitoring performance.

[0058] Although the present invention has been disclosed as above by way of embodiments, it is not intended to limit the present invention. Any person with ordinary knowledge in the technical field to which the present invention pertains may make some modifications and refinements without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be subject to that defined by the appended patent application scope.

Claims

1. A structurally integrated sensor based on screen printing, characterized in that: include: substrate; an insulating layer, wherein the insulating layer covers the substrate; A conductive structure, wherein the conductive structure is integrated on the insulating layer; The conductive structure includes a plurality of sensor circuits, a plurality of adjustment resistors and electrodes, wherein the sensor circuits are arranged in parallel, the adjustment resistors are connected in series in a single branch of the sensor circuit, and there is a gap between any two adjacent branches of the sensor circuit; A protective layer covers the conductive structure.

2. The structure-integrated sensor based on screen printing as claimed in claim 1, characterized in that: The regulating resistor is not provided in one branch of the sensing circuit.

3. The structure-integrated sensor based on screen printing as claimed in claim 1, characterized in that: The resistance value of the regulating resistor is set in a gradient.

4. The structure-integrated sensor based on screen printing as claimed in claim 3, characterized in that: The regulating resistors are arranged in parallel.

5. The structure-integrated sensor based on screen printing as claimed in claim 1, characterized in that: The conductive structure further comprises electrodes, and multiple sensing circuits are connected in parallel between two of the electrodes.

6. The structure-integrated sensor based on screen printing as claimed in claim 5, characterized in that: The electrodes and the sensor circuit are connected by conductive silver paste and thin copper wire.

7. The structure-integrated sensor based on screen printing as claimed in claim 6, characterized in that: The regulating resistor and the sensing circuit both use one or more of metal conductive ink, carbon conductive ink, and polymer conductive ink, and the protective layer includes one or more of a ceramic reinforced phase and a fiber reinforced phase.

8. The structure-integrated sensor based on screen printing as claimed in claim 7, characterized in that: The protective layer includes a high temperature tape, and the high temperature tape covers the electrode.

9. A method for monitoring invisible cracks, characterized in that: include: Integration of the sensor according to any one of claims 1 to 8: selecting the structure at the crack as the substrate, covering the substrate with the insulating layer, and performing surface modification treatment; According to the set screen printing parameters, the sensing circuit and the adjustment resistor are screen printed on the insulating layer and cured; Installing the electrode, performing lead processing on the electrode and the sensor circuit to realize the integration of the conductive structure, and covering and fixing the integrated conductive structure with the protective layer; Monitoring of the crack: the sensor is connected to an external acquisition circuit. Based on the damage of the crack to the sensor circuit, the resistance or potential of the sensor changes. The external acquisition circuit obtains the generated step-change electrical signal to determine the generation and expansion of the crack.

10. The invisible crack monitoring method according to claim 9, characterized in that: During the sensor integration step, the crack passes through at least one branch of the sensor circuit.

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