Health monitoring method for composite laminated plate connecting structure in service process

Abstract

The application discloses a health monitoring method for a composite material laminated plate connecting structure during service, and comprises the following steps: obtaining the internal stress and strain distribution of a simplified model of the composite material connecting structure during loading, and determining the embedding position of a sensor; fixing the sensor at the embedding position during the laying of the laminated plate prepreg of the composite material, integrally solidifying and forming, and preparing the functional composite material with the function of real-time collection (monitoring) of internal damage; processing and connecting and assembling the composite material; electrically connecting the collection system with the sensor, receiving the signal of the sensor, and realizing real-time monitoring of the whole life cycle of the connecting structure through signal analysis. The method can realize real-time monitoring and identification of the damage initiation, development and structural failure of the composite material connecting structure.

Description

TECHNICAL FIELD

[0001] The present application relates to a composite material connecting structure, in particular to a method for monitoring the service process of a composite material connecting structure. BACKGROUND

[0002] Due to the excellent mechanical properties of fiber reinforced polymer composites, their use in the fields of aviation, aerospace, marine, automotive and other fields is rapidly increasing. In the assembly process of composite parts, high-quality connection requirements must be addressed. Among the commonly used fiber reinforced polymer composite connection technologies, mechanical fastening connection is the most widely used for critical load-bearing structures. Since most composite structure failures occur at the fastening connections, it is very important to accurately understand the initiation and development of damage in the connection area to determine when to stop using for repair or replacement. At the same time, because most damage always occurs inside the connection structure, it is very difficult to directly observe or inspect the state of the connection structure joint. In order to solve the above problems, it is of great significance to monitor the health of the composite material connecting structure to improve the safety of the structure, reduce the maintenance cost of the structure and prolong the service life of the structure. Chinese patent 201710443291.5 discloses a health monitoring method for a single composite material laminate plate during service, but the application of piezoelectric ceramic sensors is limited to a single laminate plate, and has not been extended to the entire connecting structure. Among the commonly used health monitoring methods for composite material connecting structures, most of them require the machine to stop using for a period of time, resulting in high time delay, increasing maintenance cost, and the external sensor pasting mode requires the sensor to be sensitive to the external environment. Therefore, it is necessary to propose a simple and easy way to monitor the health of the composite material laminate connecting structure during service. SUMMARY

[0003] The purpose of the present application is to provide a method for monitoring the service process of a composite material connecting structure, which can monitor and identify the initiation, development and structural failure of the composite material connecting structure in real time.

[0004] To achieve this purpose, the present application adopts the following technical solutions:

[0005] The health monitoring method for the service process of the composite material laminate connecting structure of the present application comprises the following steps:

[0006] S1: Select a simulation software and use a numerical analysis method to establish a finite element model of the composite material laminate connecting structure during service, obtain the internal stress and strain distribution cloud map of the composite material laminate connecting structure, and select the area with obvious and uniform stress and strain changes in the stress and strain distribution cloud map as the embedding position of the insulated sensor;

[0007] S2: uniformly smearing insulating glue on the upper and lower surfaces of the sensor connected with the signal transmission line, and then curing the insulating glue on the upper and lower surfaces of the sensor to form an insulated sensor;

[0008] S3: fixing the insulated sensor to the embedded position simulated in step S1 in the process of laying the prepreg laminated layer to form a composite laminated plate, and the sensor is located between the middle layers of the prepreg laminated layer, then curing the prepreg to form a composite laminated plate with the sensor, the size of the composite laminated plate with the sensor is consistent with the size of the composite laminated plate in the composite laminated plate connecting structure in the finite element model in step S1; the upper and lower surfaces of the sensor smeared with insulating glue are the contact surfaces with the prepreg;

[0009] S4: processing and assembling two composite laminated plates into a connecting structure, the connecting structure is consistent in size with the finite element model in step S1, and at least one of the two composite laminated plates is the composite laminated plate with the sensor embedded in step S3;

[0010] S5: connecting the sensor with a signal acquisition system, the signal acquisition system is used to receive the internal strain of the material output by the sensor, and observe the collected voltage signal, when the voltage signal suddenly changes, it indicates that damage occurs inside the composite connecting structure.

[0011] The beneficial effects of the present application are:

[0012] 1. Laying position optimization: embedding the sensor in the composite material optimizes the ordinary composite material into intelligent material that can monitor and feedback damage, while avoiding the defects of low monitoring accuracy and poor monitoring effect caused by the influence of external sensors on environmental factors. The size of the material is not affected, and experiments have verified that the mechanical properties of the composite laminated plate with the embedded sensor do not decrease during the stretching and bending process.

[0013] 2. Sensor processing method optimization during laying process: the sensor is treated with double-layer glue, which realizes the fixation of the signal transmission line and the electrical insulation of the sensor, respectively, avoiding the crushing of the sensor caused by the unevenness of the sensor when connecting the data transmission line by welding.

[0014] 3. Simple operation: no additional mold and other equipment are needed in the preparation process of the insulating sensor that can be embedded in the composite material, and it is compatible with the preparation process of the composite plate. No other sensor is needed to realize the health monitoring of the connecting structure during service, and the initiation of damage inside the composite laminated plate without the need for shutdown inspection. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 is a simulation model of bolted structure in finite element software;

[0016] Figure 2 is a side view of piezoelectric ceramic sensor after insulation treatment;

[0017] Figure 3 is a front view of piezoelectric ceramic sensor after insulation treatment;

[0018] Figure 4 is a relative position of sensor in composite material plate;

[0019] Figure 5 is a schematic diagram of single lap bolted structure. DETAILED DESCRIPTION

[0020] The application will be further described below in conjunction with the drawings and embodiments. It can be understood that the specific embodiments described herein are only used to explain the application, and not to limit the application. In addition, it should be noted that, for the convenience of description, only the parts related to the application are shown in the drawings, not all the structures.

[0021] In the description of the application, unless otherwise explicitly specified and limited, the terms "connected", "connected", "fixed" should be understood broadly, for example, it can be fixed connection, or detachable connection, or integrated; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through intermediate medium, or the internal communication of two elements or the interaction relationship between two elements. For those skilled in the art, the specific meaning of the above terms in the application can be understood according to the specific circumstances.

[0022] In the present application, unless otherwise explicitly specified and limited, the "upper" or "lower" of the first feature to the second feature can include the direct contact of the first and second features, or the indirect contact of the first and second features through another feature between them. Moreover, the "upper", "above" and "on" of the first feature to the second feature includes the vertical direction of the first feature above and oblique above the second feature, or only indicates that the horizontal height of the first feature is higher than that of the second feature. The "below", "below" and "under" of the first feature to the second feature includes the vertical direction of the first feature below and oblique below the second feature, or only indicates that the horizontal height of the first feature is less than that of the second feature.

[0023] In the description of the embodiments, the terms "upper", "lower", "right", "left", and the like, orientation or positional relationship are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of description and simplification of operation, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first" and "second" are only used to distinguish in the description, and have no special meaning.

[0024] The method for monitoring the service process of the composite material connection structure provided by the application can embed sensors in the material to monitor and identify the damage initiation, development and structural failure of the composite material connection structure in real time.

[0025] The health monitoring method for the service process of the composite material laminate connection structure of the application comprises the following steps:

[0026] S1: Select a simulation software and use a numerical analysis method to establish a finite element model of the composite material laminate connection structure in the service process, obtain the internal stress and strain distribution cloud diagram of the composite material laminate connection structure, and select the area with obvious and uniform stress and strain changes in the stress and strain distribution cloud diagram as the embedding position of the insulated sensor 1.

[0027] Preferably, the material of the composite material laminate in the composite material laminate connection structure is carbon fiber composite material or glass fiber composite material.

[0028] Preferably, the distance between the two boundary lines of the embedding position and the center axis of the connecting piece 8 for connecting the composite material laminate connection structure is 20mm and 40mm respectively. The stress and strain distribution at this position is uniform, and the stress and strain changes of the connection structure can be collected on the basis of protecting the integrity of the sensor.

[0029] In this step, ABAQUS software is selected as the simulation software, and a numerical analysis method is used to establish a finite element model of the composite material bolt connection structure in the service process. The finite element simulation method is widely used, and the internal stress and strain distribution cloud diagram of the material can be obtained, which is beneficial to judge the stress condition of the sensor in the material. In other embodiments, the finite element model can be established by common commercial finite element software. The composite material connection structure can include various forms of bolt connection, riveting, glue-screw hybrid connection, etc.

[0030] S2: Uniformly apply insulating glue 3 on the upper and lower surfaces of the sensor 1 connected with the signal transmission line 4, and then solidify the insulating glue 3 on the upper and lower surfaces of the sensor 1 to form an insulated sensor;

[0031] In this step, the sensor 1 to be installed inside the composite laminate 7 can be a piezoelectric ceramic sensor, which can better play a monitoring effect due to its small size, sensitive piezoelectric effect and other advantages. In other embodiments, the sensor can also be a piezoelectric fiber film sensor or a grating sensor.

[0032] S3: During the process of laying up the pre-preg laminates to form the composite laminate 7, the insulated sensor 1 is fixed at the embedded position simulated in step S1, and the sensor is located between the middle layers of the pre-preg laminates. Then, the pre-preg is cured to form a composite laminate 7 with a sensor, and the size of the composite laminate 7 with a sensor is consistent with the size of the composite laminate in the composite laminate connection structure in the finite element model in step S1. The upper and lower surfaces of the sensor 1 coated with insulating glue 3 are the contact surfaces with the pre-preg.

[0033] After the composite laminate is cured, the laminate and the sensor together form an intelligent material that can recognize and feedback damage.

[0034] S4: Two composite laminates are processed and assembled into a connection structure, and the connection structure is consistent in size with the finite element model in step S1. At least one of the two composite laminates 7 uses the composite laminate with a sensor embedded in step S3.

[0035] S5: The sensor 1 is connected to a signal acquisition system, which is used to receive the internal strain of the material output by the sensor 1, and to observe the collected voltage signal. When the voltage signal suddenly changes, it indicates that damage has occurred inside the composite connection structure.

[0036] Embodiment 1

[0037] In this embodiment, the composite pre-preg is selected as T700 carbon fiber unidirectional pre-preg, which has the advantages of good dimensional stability, high specific strength, and high specific modulus.

[0038] S1: In this step, ABAQUS software is selected as the simulation software, and the damage model is selected as the progressive damage model, which is a commonly used model in composite simulation analysis and is beneficial to the analysis of material damage evolution. In other embodiments, the reinforcing fiber material in the resin-based composite material can be selected, including carbon fiber, glass fiber, etc., and the corresponding damage model.

[0039] In this embodiment, a single composite laminate 7 with a size of 189mm x 36mm x 2.5mm is selected, and the connecting piece 8 is a Φ6mm size bolt. The assembled simulation model is as follows: Figure 1The relevant dimensions are in accordance with the ASTM-D5961 standard. The model provides two areas with fine grids, and the grid is subdivided for the position where the PZT piezoelectric ceramic is expected to be embedded, so as to determine the stress and strain of the same position between different layers. In other embodiments, the bolted joint model size can be changed according to different analysis targets.

[0040] In this embodiment, the simulation model can refer to the corresponding test standard for setting the motion parameters during the experiment. In the tensile test, the ASTM-D5961 standard is used as the standard, and in the bending test, the ASTM-D7264 standard is referred to. The simulation results can be obtained in the ABAQUS post-processing stage, and the stress and strain distribution cloud diagram of the different layers of carbon fibers during service can be observed. The position with uniform and obvious strain distribution is selected as the embedding position of the sensor.

[0041] In this embodiment, the sensor 1 is determined to be embedded in the middle layer of the composite laminate 7, which can ensure that the sensor 1 has high sensitivity to the damage of the connecting structure without being damaged. At the same time, according to the signal excitation mode of the piezoelectric ceramic sensor embedded in the composite laminate 7, the appropriate transverse distance between the sensor and the connecting piece 8 is selected, i.e. located in the overlap area of the two composite laminates 7 and away from the connection position. As a preferred embodiment, the overlap area is determined to be at a distance of 20-40 mm from the connection hole 20 in the model.

[0042] It can be understood that in other embodiments, the embedding position of the sensor can also be determined by experimental methods.

[0043] S2: Uniformly apply insulating glue 3 to the upper and lower surfaces of the sensor 1 connected with the signal transmission line 4, and then use high temperature curing on

[0044] the upper and lower surfaces of the sensor 1;

[0045] In this step, the internal sensor of the composite laminate is a piezoelectric ceramic sensor, and the piezoelectric ceramic disc has a diameter of 7 mm and a thickness of 0.2 mm. The signal transmission line 4 is fixed at both ends of the piezoelectric ceramic disc by using the fixing glue 2, forming a closed loop, which becomes a simple acquisition device, as shown in Figure 2 The insulating glue 3 is uniformly applied to both sides of the acquisition device to insulate the surface, so as to adapt to the weak conductivity of the carbon fiber composite material. After high temperature curing, the insulating sensor 1 can be embedded in the composite material, and its structure is as shown in Figure 3The fixing glue 2 is preferably a resin type pressure sensitive glue, which is low in cost, easy to obtain and still effective under high temperature conditions, and can better play the role of fixing the signal transmission line. In other embodiments, the same or similar nature of the elastic body type pressure sensitive glue can also be used. The insulation glue 3 is one of high temperature resin glues, which can be cured after high temperature, and can insulate the sensor while protecting the sensor from brittle damage. Exemplarily, the insulation glue is a j133 two-component epoxy structure glue, which is uniformly applied on both sides of the simple collection device, and then heated from room temperature to 80°C at a speed of 3°C / min in an incubator, and kept for 180 min to complete the insulation treatment after the glue is cured. It can be understood that after the insulation treatment of the sensor, the surface of the sensor should be smooth, the internal piezoelectric ceramic disc should be crack-free, and the signal transmission line should not be broken.

[0046] S3: In the process of laying the composite laminated plate 7 with the pre-impregnated material stack, the insulated sensor 1 is fixed in the sensor embedding position between the middle layers of the pre-impregnated material stack, and then the pre-impregnated material is cured to form the composite laminated plate 7 with the sensor 1, the size of the composite laminated plate 7 with the sensor is consistent with the size of the composite laminated plate in the finite element model; the upper and lower surfaces of the sensor 1 coated with the insulation glue 3 are the contact surfaces with the pre-impregnated material.

[0047] In this embodiment, the preparation process of the composite laminated plate is as follows:

[0048] Firstly, the laying process of the composite laminated plate selects the standard orthogonal lamination sequence of the resin-based carbon fiber composite material widely used in the aviation field, and the size is consistent with the simulation model. The final composite laminated plate includes 20 single-layer T700 grade carbon fiber unidirectional pre-impregnated materials with a thickness of 0.125 mm, and the 0° fiber direction pre-impregnated material 5 and the 90° fiber direction pre-impregnated material 6 are alternately laid, and the sensor 1 is buried between the middle layers (10th and 11th layers), and the sensor is a piezoelectric ceramic sensor, and the structure is as shown in Figure 4 The signal transmission line of the sensor should be in the same direction as the fiber direction of the layer of pre-impregnated material, and the sensor should be ensured to be crack-free inside and the insulation skin of the signal transmission line should not be detached after the sensor is laid.

[0049] Then, the autoclave molding method is selected as the preparation method of the fiber-reinforced thermosetting composite material, the autoclave curing pressure, temperature and other parameters are uniform and controllable, the molding process is stable and reliable, and it is a commonly used composite material preparation method. During the curing stage of the sample in the autoclave, a standard mold for preparing a thermosetting composite material with a thickness of 2.5 mm is selected. During the sample preparation process, the temperature in the autoclave is first heated from room temperature to 125°C at a speed of 1.5°C / min, and the holding time is 2.5 hours, and finally the temperature is lowered at a speed of 3°C / min. During the entire cycle, a pressure of 3 bar is applied, and the average vacuum degree is 98 KPa. When the sample is placed in the autoclave, the signal transmission line should be placed flat with the plate without overlapping. During the curing process of the sample, plastic film should be used to wrap the signal transmission line exposed outside the plate to prevent the resin from flowing and sticking to the signal transmission line, which may cause damage to the transmission line.

[0050] S4: The two composite laminates are processed and assembled into a connection structure, the connection structure is consistent in size with the finite element model in step S1, and at least one of the two composite laminates 7 uses the composite laminate with the embedded sensor in step S3.

[0051] In this embodiment, a single lap bolt connection structure is taken as an example:

[0052] First, the composite laminate 7 is milled and drilled using a numerical control machine tool. During the processing of the sample using the numerical control machine tool, the sample should not be subjected to severe vibration. By processing the three edges of the sample that do not lead to the data transmission line using the numerical control machine tool, the size of a single composite laminate is ensured to be 189mm x 36mm x 2.5mm. A through hole with a diameter of 6mm is made at a position 18mm away from the narrow edge of the composite laminate using the numerical control machine tool to ensure that the embedded PZT sensor is located at a position 20-40mm away from the through hole. During the drilling process, the composite laminate should not be damaged by delamination at the sensor insertion position. Then, two composite laminates 7 are connected in series using a connecting piece 8, such as a 12.9-grade high-strength hexagonal bolt, to form a single lap bolt connection structure, and the final structure is as shown in Figure 5 .

[0053] S5: The sensor 1 is connected to a signal acquisition system, which is used to receive the internal strain of the material output by the sensor 1, and to observe the collected voltage signal. When the voltage signal suddenly changes, it indicates that damage has occurred inside the composite connection structure.

[0054] In the embodiment, the sensor 1 is connected with a collection card, a signal amplifier and a computer in series, and a LABVIEW software is used to collect the electric signal generated by the sensor, and the damage condition of the connecting structure is judged by observing the amplitude change of the voltage signal in the tensile experiment, and when the voltage amplitude suddenly changes, it represents that the internal damage of the connecting structure appears.

[0055] As can be seen from the above embodiments, the composite connecting structure service process monitoring method provided by the embodiment can realize the effect of real-time monitoring of the damage initiation and development process of the connecting structure in the service process by embedding the sensor into the composite material to prepare the functional composite material with the function of real-time collection (monitoring) of the internal damage.

[0056] Obviously, the above embodiments of the present application are only examples for clearly illustrating the present application, and are not intended to limit the embodiments of the present application. For those skilled in the art, various obvious changes, re-adjustments and substitutions can be made without departing from the scope of the present application. Here, it is not necessary and impossible to enumerate all the embodiments. Any modification, equivalent substitution and improvement made within the spirit and principle of the present application shall be included in the protection scope of the claims of the present application.

Claims

1. A method for health monitoring of a composite laminate joint structure during service, characterized in that The method comprises the following steps: S1: selecting simulation software, using a numerical analysis method to establish a finite element model of a composite laminate connecting structure in a service process, obtaining an internal stress and strain distribution cloud atlas of the composite laminate connecting structure, and taking a region with obvious and uniform stress and strain changes in the stress and strain distribution cloud atlas as an embedding position of an insulated sensor; S2: uniformly applying insulation glue to upper and lower surfaces of a sensor connected with a signal transmission line, and then curing the insulation glue on the upper and lower surfaces of the sensor to form an insulated sensor; S3: fixing the insulated sensor at the embedding position obtained in step S1 in a process of laminating a composite laminate by using prepreg laminated layers, and the sensor is located between intermediate layers of the prepreg laminated layers, then curing the prepreg to form a composite laminate provided with the sensor, a size of the composite laminate provided with the sensor is consistent with that of a composite laminate in a composite laminate connecting structure in the finite element model in step S1, and upper and lower surfaces of the sensor to which the insulation glue is applied are contact surfaces with the prepreg; S4: processing and assembling two composite laminates into a connecting structure, the connecting structure is consistent in size with the finite element model in step S1, and at least one of the two composite laminates is the composite laminate provided with the sensor in step S3; S5: connecting the sensor with a signal acquisition system, the signal acquisition system is used to receive internal strain conditions of the sensor, and a voltage signal collected is observed, when the voltage signal suddenly changes, it indicates that an internal damage of the composite connecting structure occurs.

2. The method for health monitoring of service process of composite laminated plate connecting structure according to claim 1, characterized in that: The sensor is a piezoelectric ceramic sensor, the piezoelectric ceramic sensor comprises a piezoelectric ceramic sheet and two signal transmission lines, one end of each of the two signal transmission lines is fixed to upper and lower surfaces of the piezoelectric ceramic sheet through fixing glue, and the other end of each of the two signal transmission lines is connected with the signal acquisition system.

3. The method for health monitoring of service process of composite laminated plate connecting structure according to claim 2, characterized in that: The fixing glue is an elastomer type pressure sensitive adhesive or a resin type pressure sensitive adhesive.

4. The method for health monitoring of service process of composite laminated plate connecting structure according to claim 3, characterized in that: The composite material is a carbon fiber composite material or a glass fiber composite material.

5. The method for health monitoring of service process of composite laminated plate joint structure according to any one of claims 1-4, characterized in that: Two boundary lines of the embedding position are respectively 20 mm and 40 mm away from a central axis of a connecting piece used to connect the composite laminate connecting structure.

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