CN116242510A - Health monitoring method for the service process of composite laminated connection structures - Google Patents

Abstract

The invention discloses a health monitoring method for the service process of the composite material laminated plate connection structure, comprising the following steps: obtaining the internal stress and strain distribution of the simplified model of the composite material connection structure during the loading process, and determining the embedded position of the sensor; During the laying process of the plywood prepreg, the sensor is fixed at the embedding position, and the whole is cured and formed to prepare a functional composite material capable of real-time collection (monitoring) of internal damage; the composite material is processed and connected and assembled; the collection system and the sensor circuit It receives the signal of the sensor, and through signal analysis, realizes the real-time monitoring of the whole life cycle of the connection structure. By adopting the method of the invention, the damage initiation, development and structural failure of the composite material connection structure can be monitored and identified in real time.

Description

Health monitoring method for the service process of composite laminated connection structures

technical field

The invention relates to a composite material connection structure, in particular to a method for monitoring the service process of a composite material connection structure.

Background technique

Due to the excellent mechanical properties of fiber-reinforced polymer composites, their use in aviation, aerospace, marine, automotive and other fields is rapidly increasing. During the assembly of composite parts, high-quality joint requirements must be addressed. Among the commonly used joining techniques for fiber-reinforced polymer composites, mechanical fastening is the most widely used for critical load-bearing structures. Since most failures of composite structures occur at fastened connections, it is important to have an accurate understanding of damage initiation and development in the region of the connection to decide when to take it out of service for repair or replacement. At the same time, it is very difficult to directly observe or check the state at the joints of the connecting structure because most of the damage always occurs inside the connecting structure. In order to solve the above problems, the structural health monitoring of the service state of the composite material connection structure is of great significance 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 method of using piezoelectric ceramic sensors to monitor the health of composite laminate veneers during service, but the application of piezoelectric ceramic sensors is limited to a single laminate and has not been extended to the entire in the connection structure. Among the currently commonly used health monitoring methods for composite material connection structures, most of them require the machine to be stopped for a period of time, resulting in high time delays and increased maintenance costs. In addition, the external sensor mode is used to ensure the sensitivity of the sensor itself. The requirements for the external environment are more stringent. Therefore, it is necessary to propose a simple and feasible method to monitor the health of the composite laminate connection structure during service.

Contents of the invention

The purpose of the present invention is to provide a method for monitoring the service process of a composite material connection structure, which can monitor and identify the damage initiation, development and structural failure of the composite material connection structure in real time.

For reaching this purpose, the present invention adopts following technical scheme:

The health monitoring method aimed at the service process of the composite laminated plate connection structure of the present invention comprises the following steps:

S1: Select simulation software, use numerical analysis methods to establish the finite element model of the composite laminated plate connection structure during service, obtain the internal stress and strain distribution cloud map of the composite laminated plate connection structure, and change the stress and strain in the cloud map of the stress and strain distribution significantly And the uniform area is used as the embedding position of the sensor after insulation;

S2: Apply insulating glue evenly on the upper and lower surfaces of the sensor connected to the signal transmission line, and then cure the insulating glue on the upper and lower surfaces of the sensor to form an insulated sensor;

S3: During the process of laminating composite material laminates with prepregs, the insulated sensor is fixed at the embedding position simulated in step S1, and the sensor is located between the middle layers of the prepreg stacked layers , and then curing the prepreg to form a composite laminate equipped with sensors, the size of the composite laminate equipped with sensors is the same as that of the composite laminate connected in the finite element model in step S1 The size of the sensor is consistent; the upper and lower surfaces of the sensor coated with insulating glue are the contact surfaces with the prepreg;

S4: Process and assemble two composite material laminates into a connection structure, the connection structure is consistent with the size of the finite element model in step S1, and at least one composite material laminate in the two composite material laminates is used in step S3 Composite laminates with embedded sensors;

S5: Connect the sensor to the 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. damage.

Beneficial effects of the present invention:

1. Optimization of laying position: Embed the sensor inside the composite material, optimize the ordinary composite material into an intelligent material that can monitor and feedback damage, and avoid low monitoring accuracy and poor monitoring effect caused by external sensors affected by environmental factors. defect. It has no effect on the size of the material, and it has been verified by experiments that the mechanical properties of the composite laminate after embedding the sensor are not reduced during stretching and bending.

2. Optimization of the sensor processing method during the laying process: the sensor is processed with double-layer colloid, which respectively realizes the fixation of the signal transmission line and the electrical insulation of the sensor, avoiding the unevenness of the sensor when the data transmission line is connected by the method of welding wire. Composite laminates are crushed during preparation.

3. Simple operation: no additional molds and other equipment are required during 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 material sheet. No other sensors are needed to realize the health monitoring of the connection structure during service, and to detect the initiation of internal damage of the composite laminate without shutting down for inspection.

Description of drawings

Figure 1 is the simulation model of the bolted connection structure in the finite element software;

Fig. 2 is a side view of the piezoelectric ceramic sensor after insulation treatment;

Fig. 3 is the front view of the piezoelectric ceramic sensor after insulation treatment;

Fig. 4 is the relative position of the sensor in the composite material plate;

Fig. 5 is a schematic diagram of a single lap bolt connection structure.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, but not to limit the present invention. In addition, it should be noted that, for the convenience of description, only some structures related to the present invention are shown in the drawings but not all structures.

In the description of the present invention, unless otherwise clearly specified and limited, the terms "connected", "connected" and "fixed" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integrated ; It can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediary, and it can be the internal communication of two components or the interaction relationship between two components. Those of ordinary skill in the art can understand the specific meanings of the above terms in the present invention in specific situations.

In the present invention, unless otherwise clearly specified and limited, a first feature being "on" or "under" a second feature may include direct contact between the first and second features, and may also include the first and second features Not in direct contact but through another characteristic contact between them. Moreover, "above", "above" and "above" the first feature on the second feature include that the first feature is directly above and obliquely above the second feature, or simply means that the first feature is horizontally higher than the second feature. "Below", "beneath" and "under" the first feature to the second feature include that the first feature is directly below and obliquely below the second feature, or simply means that the first feature has a lower level than the second feature.

In the description of this embodiment, the terms "up", "down", "right", and other orientations or positional relationships are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of description and simplification of operations, rather than indicating Or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore should not be construed as limiting the invention. In addition, the terms "first" and "second" are only used to distinguish in description, and have no special meaning.

The method for monitoring the service process of the connection structure of the composite material provided by the present invention embeds the sensor inside the material, and can monitor and identify the damage initiation, development and structural failure of the connection structure of the composite material in real time. The method of the present invention will be described below with specific examples.

The health monitoring method aimed at the service process of the composite laminated plate connection structure of the present invention comprises the following steps:

S1: Select simulation software, use numerical analysis methods to establish the finite element model of the composite laminated plate connection structure during service, obtain the internal stress and strain distribution cloud map of the composite laminated plate connection structure, and change the stress and strain in the cloud map of the stress and strain distribution significantly And the uniform area is used as the embedding position of the sensor 1 after insulation.

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

Preferably, the two boundary lines of the embedding position are respectively 20mm and 40mm away from the central axis of the connector 8 used to connect the composite material laminate connection structure. The distribution of stress and strain at this position is uniform, and the collection of stress and strain changes of the connecting structure can be realized on the basis of protecting the integrity of the sensor.

In this step, ABAQUS software is selected as the simulation software, and the numerical analysis method is used to establish the finite element model of the composite material bolt connection structure in service. Stress inside the material. In other embodiments, the finite element model can be established by common commercial finite element software. Composite material connection structure can include various forms of bolt connection, riveting, glue-screw hybrid connection, etc.

S2: Apply insulating glue 3 evenly on the upper and lower surfaces of the sensor 1 connected with the signal transmission line 4, and then cure the insulating glue 3 on the upper and lower surfaces of the sensor 1 to form an insulated sensor;

In this step, the sensor 1 to be installed inside the composite laminated board 7 can be a piezoelectric ceramic sensor, which can achieve a good monitoring effect by taking advantage of the advantages of piezoelectric ceramics such as small size and sensitive piezoelectric effect. In other embodiments, the sensor can also choose a piezoelectric fiber film sensor or a grating sensor.

S3: During the process of laminating the composite material laminate 7 with the prepreg material, the insulated sensor 1 is fixed at the embedding position simulated in step S1, and the sensor is located in the middle layer of the prepreg material stacking layer In between, the prepreg is then cured to form a sensor-equipped composite laminate 7, the size of the sensor-equipped composite laminate 7 is the same as that in the composite laminate connection structure in the finite element model in step S1 The dimensions of the composite laminates are consistent; the upper and lower surfaces of the sensor 1 coated with the insulating glue 3 are the contact surfaces with the prepreg.

After the composite laminate is cured, the laminate together with the sensor constitutes an intelligent material with the functions of identifying and feedbacking damage.

S4: Process and assemble two composite material laminates into a connection structure, the connection structure is consistent with the size of the finite element model in step S1, and at least one composite material laminate 7 of the two composite material laminates adopts step S3 Composite laminates with sensors embedded in them.

S5: Connect the sensor 1 to the signal acquisition system, the signal acquisition system is used to receive the internal strain of the material output by the sensor 1, observe the collected voltage signal, and when the voltage signal changes suddenly, it indicates the connection structure of the composite material Internal damage.

Example 1

In this embodiment, the composite material prepreg is selected as T700 grade carbon fiber unidirectional prepreg, which has the advantages of good dimensional stability, high specific strength and high specific modulus.

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

In this embodiment, the size of a single composite laminate 7 is 189mm×36mm×2.5mm, and the connector 8 is a Φ6mm bolt. The simulated model after assembly is shown in Figure 1, and the relevant dimensions are in accordance with the ASTM-D5961 standard. The model provides two areas with fine grids, and subdivides the grids for the positions expected to embed PZT piezoelectric ceramics, so as to judge the stress and strain of the same position between different layers. In other embodiments, the size of the bolted connection structure model may vary according to different analysis objectives.

In this embodiment, the motion parameter setting of the simulation model during the experiment can refer to the corresponding test standards. In this model, the ASTM-D5961 standard is used as the standard in the tensile test, and the ASTM-D7264 standard is used in the bending test. The simulation results can be obtained in the post-processing stage of ABAQUS. Observe the stress-strain distribution cloud images of different layers of carbon fiber during service, and select the position with uniform and obvious strain distribution as the embedding position of the sensor.

In this embodiment, it is determined that the sensor 1 is embedded in the middle layer of the composite material laminate 7, which can ensure that the sensor 1 has a high sensitivity to the damage of the connection structure without being damaged. At the same time, according to the signal excitation mode of the piezoelectric ceramic sensor embedded in the composite material laminate 7, an appropriate lateral distance between the sensor and the connector 8 is selected, that is, it is located in the overlapping area of the two composite material laminates 7 and away from the connection position. Preferably, the overlapping area is determined to be 20-40 mm away from the connection hole in this model.

It can be understood that, in other embodiments, the embedding position of the sensor can also be determined through a test method.

S2: Apply insulating glue 3 evenly on the upper and lower surfaces of the sensor 1 connected to the signal transmission line 4, and then use high temperature curing on the

The upper and lower surfaces of the sensor 1;

In this step, the internal sensor of the composite material laminate is a piezoelectric ceramic sensor, and the diameter of the piezoelectric ceramic disc is 7mm and the thickness is 0.2mm. The signal transmission line 4 is fixed on both ends of the piezoelectric ceramic disc by using the fixing glue 2 to form a closed circuit and become a simple acquisition device. Its structure is shown in Figure 2. Apply insulating glue 3 evenly on both sides of the collection device to insulate its surface to adapt to the weak conductivity inside the carbon fiber composite material. After curing at high temperature, an insulated sensor 1 that can be embedded in the composite material is obtained. Its structure is shown in the figure 3. Preferably, the fixing adhesive 2 is a resin-type pressure-sensitive adhesive. The resin-type pressure-sensitive adhesive is low in cost, easy to obtain, and is still effective under high temperature conditions, and can better play the role of fixing the signal transmission line. In other embodiments, it is also Elastomeric pressure-sensitive adhesives of the same or similar properties can be used. The insulating glue 3 is one of high-temperature resin glues, which will solidify after high temperature, and can protect the sensor from brittle damage while insulating the sensor. Exemplarily, the insulating glue is j133 two-component epoxy structural glue. After the insulating glue is evenly spread on both sides of the simple collection device, start from room temperature in the incubator and raise the temperature to 80°C at a rate of 3°C/min. 180min, the colloid is cured to complete the insulation treatment. It is understandable that after the sensor is insulated, it should be ensured that its surface is smooth, that the internal piezoelectric ceramic disc has no cracks, and that the signal transmission line is not broken.

S3: During the process of laminating the composite material laminate 7 with prepreg materials, the insulated sensor 1 is fixed at the sensor embedding position and the sensor is located between the middle layers of the prepreg material laminate layers During the process, the prepreg is then cured to form a composite material laminate 7 equipped with a sensor 1, and the size of the composite material laminate 7 equipped with a sensor is consistent with the size of the composite material laminate in the finite element model; The upper and lower surfaces of the sensor 1 coated with insulating glue 3 are the contact surfaces with the prepreg.

In this embodiment, the preparation process of the composite material laminate is:

Firstly, the lay-up process of the composite laminates selects the standard orthogonal lamination sequence of resin-based carbon fiber composites widely used in the aerospace field, and its dimensions are consistent with the simulation model. The final composite laminate includes 20 T700 grade carbon fiber unidirectional prepregs with a single layer thickness of 0.125 mm, which are laid alternately with 0° fiber direction prepregs 5 and 90° fiber direction prepregs 6, in the middle layer (The 10th and 11th layers) are buried with a sensor 1, the sensor is a piezoelectric ceramic sensor, and its structure is shown in FIG. 4 . The lead-out direction of the signal transmission line of the sensor should be the same as the fiber direction of the layer of prepreg. After the sensor is laid, it should be ensured that there are no cracks inside and the insulation of the signal transmission line does not fall off.

Then, the autoclave forming method is selected as the preparation method of fiber-reinforced thermosetting composite materials. The parameters such as the curing pressure and temperature of the autoclave can be adjusted evenly, and the molding process is stable and reliable. 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 2.5mm is selected. During the sample preparation process, the temperature in the autoclave was first heated from room temperature to 125°C at a rate of 1.5°C/min, held for 2.5 hours, and finally cooled at a rate of 3°C/min. During the whole cycle, a pressure of 3 bar is applied and the average vacuum is 98KPa. When the sample is placed in the autoclave, it should be ensured that the signal transmission line and the plate are placed flat and do not overlap. During the curing process of the sample, the signal transmission line exposed on the outside of the board should be wrapped with a plastic film to prevent the thermal flow of the resin from sticking to the signal transmission line and causing damage to the transmission line.

S4: Process and assemble two composite material laminates into a connection structure, the connection structure is consistent with the size of the finite element model in step S1, and at least one composite material laminate 7 of the two composite material laminates adopts step S3 Composite laminates with sensors embedded in them.

In this embodiment, the single lap bolt connection structure is taken as an example:

Firstly, edge milling and hole making are performed on the composite material laminated board 7 by using a numerical control machine tool. During the process of processing samples with CNC machine tools, it should be ensured that the samples do not vibrate violently. The three sides of the sample that do not lead to the data transmission line are processed by a CNC machine tool to ensure that the board size of a single composite laminate is 189mm×36mm×2.5mm. Use a CNC machine tool to make a through hole with a diameter of 6mm at a position 18mm away from the narrow edge of the composite laminate to ensure that the embedded PZT sensor is at a position 20-40mm away from the through hole. Composite materials should be avoided during the hole making process. The laminate shows delamination damage where the sensor sinks. Then, the two composite material laminates 7 are connected in series by using connectors 8, such as: 12.9-grade high-strength hexagon socket bolts in series to form a single lap bolt connection structure, and the final structure is shown in FIG. 5 .

S5: Connect the sensor 1 to the signal acquisition system. The signal acquisition system is used to receive the internal strain of the material output by the sensor 1 and observe the collected voltage signal. When the voltage signal changes suddenly, it means that the composite material is connected. Damage occurs within the structure.

In this embodiment, the sensor 1 is connected in series with the acquisition card, signal amplifier, and computer, and the electrical signal generated by the sensor is collected using LABVIEW software. In the tensile test, the damage of the connection structure is judged by observing the change of the voltage signal amplitude. When the voltage amplitude When a mutation occurs, it represents damage within the junction structure.

It can be seen from the above embodiments that the method for monitoring the service process of the composite material connection structure provided by this embodiment is prepared into a functional composite material capable of real-time collection (monitoring) of internal damage by embedding sensors into the composite material, so as to achieve accurate monitoring of the service process. The effects of real-time monitoring of damage initiation and development of connected structures during the process.

Apparently, the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, rather than limiting the implementation of the present invention. Various obvious changes, readjustments, and substitutions will occur to those skilled in the art without departing from the scope of the present invention. It is not necessary and impossible to exhaustively list all the implementation manners here. All modifications, equivalent replacements and improvements made within the spirit and principles of the present invention shall be included within the protection scope of the claims of the present invention.

Claims (5)

1. The health monitoring method for the service process of the composite material laminated plate connecting structure is characterized by comprising the following steps of:

s1, selecting simulation software, establishing a finite element model of a composite material laminated plate connecting structure in the service process by using a numerical analysis method, acquiring an internal stress strain distribution cloud picture of the composite material laminated plate connecting structure, and taking a region with obvious and uniform stress strain change in the stress strain distribution cloud picture as an embedded position of an insulated sensor;

s2, uniformly coating insulating glue on the upper surface and the lower surface of the sensor connected with the signal transmission line respectively, and then curing the insulating glue on the upper surface and the lower surface of the sensor to form an insulated sensor;

s3, fixing the insulated sensor at the embedded position obtained by simulation in the step S1 in the process of paving the composite material laminated plate by adopting the prepreg laminated layers, enabling the sensor to be positioned between the middle layers of the prepreg laminated layers, and then curing the prepreg to form the composite material laminated plate provided with the sensor, wherein the size of the composite material laminated plate provided with the sensor is consistent with that of the composite material laminated plate in the composite material laminated plate connecting structure in the finite element model in the step S1; the upper surface and the lower surface of the sensor coated with the insulating glue are contact surfaces with the prepreg;

s4: processing and assembling two composite material laminated plates into a connecting structure, wherein the connecting structure is consistent with the size of the finite element model in the step S1, and at least one composite material laminated plate in the two composite material laminated plates adopts the composite material laminated plate embedded with the sensor in the step S3;

s5: the sensor is connected with the signal acquisition system, the signal acquisition system is used for receiving the internal strain condition of the material output by the sensor, observing the acquired voltage signal, and when the voltage signal is suddenly changed, indicating that the inside of the composite material connecting structure is damaged.

2. The method for health monitoring of a composite laminate joint structure service process of claim 1, wherein: the sensor adopts piezoceramics sensor, piezoceramics sensor includes piezoceramics piece and two signal transmission lines, two signal transmission lines's one end is fixed in piezoceramics piece upper and lower two sides respectively through fixed glue, two signal transmission lines's the other end is connected with signal acquisition system.

3. The health monitoring method for a composite laminate joint structure service process of claim 2, wherein: the fixing adhesive is an elastomer pressure-sensitive adhesive or a resin pressure-sensitive adhesive.

4. A method of health monitoring for a composite laminate joint structure service process as claimed in claim 3, wherein: the composite material is a carbon fiber composite material or a glass fiber composite material.

5. The method for health monitoring of a composite laminate joint structure in service according to one of claims 1-4, wherein: the two borderlines of the embedded position are 20mm and 40mm, respectively, from the central axis of the connecting piece for connecting the composite material laminated plate connecting structure.

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