Laminated plate debonding monitoring method

Abstract

The invention relates to a laminated plate debonding monitoring method, which comprises the following steps: acquiring frequencies corresponding to zero group velocity modes of single-layer plates with different thicknesses as reference frequencies through numerical simulation, the single-layer plates and single-layer plates included in a laminated plate to be monitored being the same in thickness and material; attaching at least one active piezoelectric sheet-shaped sensor on the to-be-monitored laminated plate, connecting a measurement module to obtain an electromechanical impedance spectrum, and identifying and extracting a formant corresponding to the zero group velocity mode in the electromechanical impedance spectrum according to the oscillation characteristics of the zero group velocity mode; and comparing the zero group velocity mode with the reference frequency, and when the zero group velocity mode deviates, disappears or generates a new formant relative to the reference frequency under the same thickness, determining that the monitoring point has debonding. According to the method, only one active piezoelectric sensor needs to be arranged to serve as an excitation source and a receiving end at the same time, frequency sweeping operation is carried out, the corresponding mode is recognized from the electromechanical impedance spectrum through the oscillation characteristics of the zero group velocity mode, and laminated plate debonding monitoring is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of structure monitoring and nondestructive testing, and in particular relates to a laminate debonding monitoring method. Background Art

[0002] Electromechanical Impedance is a non-invasive technique for structural health monitoring, designed to detect and evaluate the health status of engineering structures. This technique is often used to monitor and identify damage, fatigue, cracks, corrosion and other problems in materials and structures. Electromechanical Impedance uses sensors (usually piezoelectric sensors) attached to the surface of the structure, applies electrical signals to excite the sensors, and measures the voltage response of the sensors to couple the mechanical vibration response of the structure, thereby using the resonance information of the structure to evaluate the health status of the structure. Traditional electromechanical impedance methods will blur the resonance information in the impedance spectrum and use these resonance peaks directly. Such fuzzy processing will reduce the accuracy and sensitivity of damage monitoring, and reduce the capture and identification of damage information (type, severity, location, etc. of damage). Zero Group Velocity Mode is a special local resonance mode of ultrasonic guided waves, in which the propagation group velocity is equal to zero and the phase velocity maintains a non-zero value. When the group velocity is zero, it means that in this mode, the energy of the vibration or wave does not propagate over time, but is locally concentrated in a specific area of ​​the structure and does not propagate to other places.

[0003] For the monitoring of debonding of laminates, the vibration modes used by the traditional electromechanical impedance method are mainly the resonant modes of sensors and structures. The offset of these resonant modes is mainly caused by local stiffness and density. The debonding of laminates has little effect on the changes in the local stiffness and local density of the structure, so the traditional electromechanical impedance method has low sensitivity for monitoring debonding. The zero group velocity mode has extremely high sensitivity to anomalies in the thickness direction when the material properties remain unchanged. Once interlayer debonding occurs, the equivalent thickness of the laminate will change, resulting in the frequency corresponding to the zero group velocity mode shifting, so it has high sensitivity. Furthermore, the existing technology for the excitation and capture of zero group velocity modes often requires the use of precision optical instruments for full-field scanning, which has high requirements for equipment, complex processes, and high costs. Some scholars combine the electromechanical impedance method with spatial measurements to identify and extract zero group velocity modes, but this method still requires the assistance of other measurement methods, rather than using the electromechanical impedance method alone to identify and extract. Summary of the invention

[0004] The objects of the present invention are: Aiming at the debonding damage existing in laminated structures and composite materials, a laminate debonding monitoring method based on electromechanical impedance method and using zero group velocity mode is proposed. This method is the first to achieve low cost through electromechanical impedance method, using a single sensor to identify, extract and utilize zero group velocity mode. On the basis of traditional electromechanical impedance method, the monitoring of debonding damage is realized, which greatly improves the sensitivity and reliability.

[0005] In order to achieve the above technical objectives, the present invention adopts the following technical solutions: A method for monitoring debonding of laminated plates, comprising: The dispersion curves of single-layer plates of different thicknesses and materials are obtained by numerical simulation. The frequency corresponding to the zero group velocity in the dispersion curve is used as the reference frequency. The single-layer plate and the single-layer plate included in the laminate to be monitored have the same thickness and material. The reference frequencies f1, f2, f3...fn are recorded. At least one active piezoelectric sheet sensor is attached to the laminate to be monitored, and an impedance measurement module is used to obtain an electromechanical impedance spectrum. According to the oscillation characteristics of the zero-group velocity mode, the frequency corresponding to each zero-group velocity mode is identified and extracted in the electromechanical impedance spectrum, and recorded as F1, F2, F3...Fn; By comparing the frequencies F1, F2, F3...Fn of the zero group velocity modes in the electromechanical impedance spectrum with the reference frequencies f1, f2, f3...fn, when the zero group velocity modes in the electromechanical impedance spectrum are offset, disappear, or new resonance peaks are generated relative to the reference frequencies at the same thickness, it is confirmed that debonding exists at the monitoring point.

[0006] As a preferred implementation, the numerical simulation is calculated using a semi-finite element and semi-analytical method.

[0007] As a preferred embodiment, the thickness of the piezoelectric sensor is less than 1 / 20 of the thickness of the laminate structure. Furthermore, the diameter of the piezoelectric sensor is less than 10 mm. If the thickness and size of the sensor are out of range, the collected signal will include the thickness of the sensor, which will affect the extraction accuracy of the zero group velocity mode.

[0008] As a preferred implementation, the piezoelectric sensor is adhered to the upper and lower surfaces of the laminate to be monitored at the location prone to debonding by means of structural adhesive or quick-drying adhesive.

[0009] As a preferred implementation, the bonding surface between the piezoelectric sensor and the laminate to be monitored is polished smooth, the bonding thickness is less than 1 / 5 of the thickness of the sensor, and the pressure is maintained for 1 minute before being released.

[0010] As a preferred implementation, the impedance measurement module is an impedance analyzer, and the electromechanical impedance spectrum of the coupling between the piezoelectric sensor and the laminate to be monitored is measured by connecting the positive and negative electrodes of the piezoelectric sensor to the impedance analyzer respectively.

[0011] As a preferred implementation, a damage factor based on the zero group velocity mode is used to quantify the presence and severity of damage.

[0012] As a preferred implementation, the damage factor is characterized by a root mean square deviation value of an impedance spectrum; The damage factor formula is: ,in Represents the impedance spectrum in a healthy state. The measured impedance spectrum is shown in Figure 1. The quantified frequency range is the frequency band of the zero group velocity used. The present invention has the following beneficial effects: (1) It breaks through the technical dimensions and measurement limitations of traditional zero-group velocity mode measurement and extraction methods.

[0013] The identification of traditional zero group velocity modes requires the use of dispersion curves, and the calculation of dispersion curves requires the combination of signals in the space domain and time domain of the structure to be measured, that is, the measurement points are densely distributed in the two-dimensional spatial range of a certain area of ​​the structure according to the frequency requirements, and each measurement point needs to measure the time domain response signal under a specific excitation, and then undergo a two-dimensional Fourier transform, and obtain the corresponding dispersion curve through a large number of calculations, thereby determining the frequency of the zero group velocity mode. Among them, the step of measuring the time domain signal at a spatial point often requires precise control instruments and optical measuring instruments, and the equipment is expensive, the process is complicated, and the precision requirements are high. The zero group velocity mode measurement method based on the electromechanical impedance method proposed in the present invention only needs to arrange an active piezoelectric sensor, which serves as an excitation source and a receiving end at the same time, and performs a frequency sweep operation, and the corresponding mode can be identified from the obtained electromechanical impedance spectrum through the unique oscillation characteristics of the zero group velocity mode. In order to ensure the reliability of this method, a semi-analytical semi-finite element method with high computational efficiency can be used to calculate the zero group velocity mode of the structure in advance for mutual verification, but this calculation is used as an optimization item. Different from the electromechanical impedance method used by existing scholars to confirm the zero group velocity mode in combination with spatial point measurement, the present invention identifies the zero group velocity mode only through the unique characteristics of the zero group velocity mode in the electromechanical impedance spectrum.

[0014] (2) It enriches the monitoring mechanism of the traditional electromechanical impedance method and expands the application dimension of resonance information.

[0015] The traditional electromechanical impedance method uses fuzzy processing and resonance information of the structure. These resonances are mainly the bending mode, torsion mode, tension and compression mode of the structure, and the bending mode of the sensor. The resonance frequencies corresponding to these modes are usually low, the structural information reflected is limited, and the sensitivity to damage is also low. The zero group velocity mode is determined by the material properties and thickness size of the plate structure, and is mainly a resonance in the thickness direction. The frequency is higher, so it has a higher sensitivity and is easier to reflect changes in the thickness direction of the structure and thus monitor damage in the thickness direction.

[0016] (3) The sensitivity and accuracy of the electromechanical impedance method for monitoring debonding of laminates have been improved.

[0017] For monitoring the debonding of laminates, the vibration modes used by the traditional electromechanical impedance method are mainly the resonant modes of the sensor and the structure. The offset of these resonant modes is mainly caused by the local stiffness and density. The debonding of laminates has little effect on the changes in the local stiffness and local density of the structure, so the traditional electromechanical impedance method has low sensitivity for monitoring debonding. The zero group velocity mode has a very high sensitivity to anomalies in the thickness direction of the plate when the material properties remain unchanged. Once interlayer debonding occurs, the equivalent thickness of the laminate will change, causing the frequency corresponding to the zero group velocity mode to shift, so it has a higher sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of the structure of single-layer boards and multi-layer bonded boards (laminated boards) of different thicknesses.

[0019] Figure 2 These are the dispersion curves of single-layer plates (ac) and laminates (d) with different thicknesses.

[0020] Figure 3 These are the modal vibration shapes corresponding to the zero group velocity modes of single-layer plates with different thicknesses.

[0021] Figure 4 is the mode shape corresponding to the zero group velocity mode of the laminate.

[0022] Figure 5 It is a schematic diagram of the aluminum plate surface and Teflon sheet.

[0023] Figure 6 This is a schematic diagram of the bonding state of the piezoelectric sensor.

[0024] Figure 7 The electromechanical impedance spectra of single-layer boards with different thicknesses (left) and laminated boards in healthy conditions (right).

[0025] Figure 8 It is the electromechanical impedance spectrum at different positions of the 4mm board in the laminate.

[0026] Fig. 9 It is the comparison of electromechanical impedance spectra of zero group velocity mode, sensor vibration mode and structural vibration mode. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and specific implementation methods.

[0028] (1) The specific structures used in the embodiment are single-layer aluminum plates of 500mm*500mm*4mm, 500mm*500mm*6mm, 500mm*500mm*10mm, and a multi-layer adhesive plate in which the aforementioned 4mm and 6mm single-layer aluminum plates are bonded by 0.3mm thick resin glue in the middle. The structural schematic diagram is shown in FIG. Figure 1 shown.

[0029] (2) The dispersion curves of 4mm, 6mm, 10mm single-layer plates and 4mm+0.3mm+6mm laminates are calculated using the semi-analytical semi-finite element method, as shown in Figure 2. Figure 2 As shown. The frequencies corresponding to the first zero group velocity mode of the single-layer plate can be obtained: 707.5kHz, 471kHz, 283kHz, ( Figure 2 ac) and the frequencies of the zero-group velocity modes of the laminate are 198.5kHz, 506.5 kHz, 735.5 kHz, 786 kHz ( Figure 2 (d)

[0030] (3) The modal vibration shapes corresponding to the zero group velocity modes of single-layer plates and laminated plates are calculated using the semi-analytical semi-finite element method, which is given by Figure 3 It can be seen that the mode of the single-layer plate corresponds to the first-order symmetric zero-group velocity mode, which is given by Figure 4 It can be seen that the vibration modes of the first three zero-group velocity modes of the laminate in the thickness direction correspond to the mode vibration modes of the 10 mm plate, 6 mm plate and 4 mm plate, respectively, which are marked by green shadows.

[0031] (4) The actual structure is the same as that of the theoretical analysis. In the actual implementation, the following preparations are made: Figure 1 The single-layer board and the laminated board shown (the laminated board is bonded with 0.3mm structural adhesive in a healthy state). For these structures, a 7mm*0.2mm active piezoelectric sheet sensor (PWAS sensor) is selected and pasted on the surface of the structure. The bonding of the piezoelectric sensor is as follows Figure 6 shown.

[0032] (5) Prepare another laminate with interlayer debonding. The laminate with interlayer debonding is prepared by attaching a Teflon sheet to the normal laminate to simulate the interlayer debonding area, such as Figure 5In the middle, in area 3 and area 4, as a comparison, area 1 and area 2 are ideal bonding surfaces (0.3mm structural adhesive bonding), and a piezoelectric sensor is attached to each area. Figure 5 As shown, damping rubber (high damping viscoelastic material) is bonded around the aluminum plate to reduce boundary reflection and eliminate overall structural resonance at low frequencies.

[0033] (6) Using an impedance analyzer, connect the positive and negative electrodes of the piezoelectric sensor to the impedance analyzer respectively, and measure the electromechanical impedance spectrum after the piezoelectric sensor is coupled to the aluminum plate.

[0034] (7) The electromechanical impedance spectra of each single-layer aluminum plate (4mm, 6mm, 10mm) and the laminated plate in a healthy state (4mm+0.3mm+6mm) were measured respectively. For the measurement of the laminated plate, the piezoelectric sensor was attached to the front and back sides of the laminated plate, corresponding to the 4mm and 6mm plates respectively. The measured impedance spectra are shown in the figure below: Figure 7 The measured zero group velocity mode of the single-layer plate is consistent with the result of the semi-analytical and semi-finite element solution. It is worth noting that for the actual measurement of the laminate, not all zero group velocity modes can be captured, but only the zero group velocity mode of the single-layer plate on the surface bonded by the sensor can be captured, which corresponds to the zero group velocity mode of 471kHz for the 6mm plate and the zero group velocity mode of 709kHz for the 4mm plate.

[0035] (8) The zero group velocity mode captured in the laminate is used to monitor the debonding damage of the laminate. The monitoring object is Figure 5 The laminate mentioned in .

[0036] (9) Use an impedance analyzer to measure the impedance spectra at 1, 2, 3, and 4 (corresponding to the sensors on the 4 mm board surface in the laminate), as shown in Figure 1. Figure 8 Positions 1 and 2 correspond to the impedance spectra of the ideal bonding state. Figure 8 The impedance spectra obtained by the sensor pasted on the 4mm surface of the laminate are the same, and both can capture the zero group velocity mode of 709kHz corresponding to the 4mm plate.

[0037] (10) For the debonding of Teflon, the impedance spectrum collected shows a big difference. The zero group velocity mode (709kHz) corresponding to the original 4mm plate disappears, and many new resonance peaks appear in the range of 300-600kHz. The disappearance of the zero group velocity mode and the appearance of new resonance peaks are caused by the debonding between layers. Therefore, we can monitor the occurrence of debonding between layers.

[0038] Therefore, when monitoring the debonding of laminates, piezoelectric sensors can be attached to hot spots prone to damage / debonding. One piezoelectric sensor can measure a small area. The offset and disappearance of the zero group velocity mode and the generation of new resonance peaks can all be used as characteristic phenomena of debonding.

[0039] (11) The zero-group velocity mode is different from the traditional sensor vibration mode and the structural vibration mode, and has a uniquely identifiable feature in the impedance spectrum. The ordinary resonance peak is a relatively smooth peak, with no subsequent oscillations or mutations. However, the zero-group velocity mode is followed by many spectral oscillations. Figure 7 As can be seen in Fig. 9 The vibration is caused by the local drastic mode superposition change when the zero group velocity mode is generated.

Claims

1. A method for monitoring debonding of laminated plates, characterized in that: include: The dispersion curves of single-layer plates of different thicknesses and materials are obtained by numerical simulation. The frequency corresponding to the zero group velocity in the dispersion curve is used as the reference frequency. The single-layer plate and the single-layer plate included in the laminate to be monitored have the same thickness and material. The reference frequencies f1, f2, f3...fn are recorded. At least one active piezoelectric sheet sensor is attached to the laminate to be monitored, and an impedance measurement module is used to obtain an electromechanical impedance spectrum. According to the oscillation characteristics of the zero-group velocity mode, the frequency corresponding to each zero-group velocity mode is identified and extracted in the electromechanical impedance spectrum, and recorded as F1, F2, F3...Fn; By comparing the frequencies F1, F2, F3...Fn of the zero group velocity modes in the electromechanical impedance spectrum with the reference frequencies f1, f2, f3...fn, when the zero group velocity modes in the electromechanical impedance spectrum are offset, disappear, or new resonance peaks are generated relative to the reference frequencies at the same thickness, it is confirmed that debonding exists at the monitoring point.

2. A laminate debonding monitoring method according to claim 1, characterized in that: The numerical simulation is calculated using a semi-finite element and semi-analytical method.

3. A laminate debonding monitoring method according to claim 1, characterized in that: The thickness of the piezoelectric sensor is less than 1 / 20 of the thickness of the laminate structure.

4. A laminate debonding monitoring method according to claim 1, characterized in that: The piezoelectric sensor has a diameter less than 10 mm.

5. A laminate debonding monitoring method according to claim 1, characterized in that: The piezoelectric sensor is adhered to the upper surface or lower surface of the easily debonded part of the laminate to be monitored by structural adhesive or quick-drying adhesive.

6. A laminate debonding monitoring method according to claim 5, characterized in that: The bonding surface between the piezoelectric sensor and the laminate to be monitored is polished and smooth, the bonding thickness is less than 1 / 5 of the thickness of the sensor, and the pressure is maintained for 1 minute before being released.

7. A laminate debonding monitoring method according to claim 1, characterized in that: The impedance measurement module is an impedance analyzer, which measures the electromechanical impedance spectrum of the piezoelectric sensor coupled with the laminate to be monitored by connecting the positive and negative electrodes of the piezoelectric sensor to the impedance analyzer respectively.

8. A laminate debonding monitoring method according to claim 1, characterized in that: The presence and severity of damage are quantified using damage factors based on the zero group velocity mode.

9. A laminate debonding monitoring method according to claim 9, characterized in that: The damage factor is characterized by the root mean square deviation value of the impedance spectrum; The damage factor formula is: ,in Represents the impedance spectrum in a healthy state. The measured impedance spectrum is shown, and the frequency range of quantization is selected from the frequency band of zero group velocity used.

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