Abstract
It is of great significance to grasp the ultrasonic attenuation characteristics of materials for the nondestructive testing of materials. The dynamic properties of a piezoelectric composite material (OPCM) with self-developed transverse anisotropy have been analyzed using the experiment in this work. The OPCM sensor is attached to the surface of the iron plate and concrete structures to sense the surface waves generated by acoustic emission (AE). The experiment results show that OPCM sensor elements have unique advantages compared to piezoelectric ceramic materials (PZT). Further, by comparing the signals of isotropic and anisotropic materials, the attenuation characteristics of surface waves propagating in different materials are studied, and a new method for measuring the attenuation coefficient of surface waves is demonstrated.
1. Introduction
Surface waves have been widely used for damage detection in many engineering structures [1–3]. The sensors, which receive the surface waves, play an important role in damage detection. In 1885, Lord Rayleigh discovered surface acoustic waves in semi-infinite isotropic solid bodies and has since been used for signal processing techniques [4], including filtering [5], delay [6], pulse compression [7], correlation, and convolution [8]. It has been used in such fields as radar, aerospace, communications, and damage detection [9, 10]. Moreover, with the development of SAW technology, various kinds of new low-loss sensors such as Rayleigh-type SAW, shear-type SAW, and BG wave were developed. The generation and emergence of various sensor simulation theories are the conditions for the development and application of new fields of SAW sensors with excellent performance [11]. It is due to the essential characteristic of SAW that sound waves propagate along the substrate surface, making SAW sensitive to physical, chemical, or other mechanical perturbations of its surface, thereby making it possible to produce a wide variety of highly sensitive sensors [12]. The basic principle is that when the physical or chemical parameters act on the surface of a SAW transducer is changed, the disturbance of SAW propagation will cause the variation of velocity and amplitude. The corresponding damage detection signals will be recorded by SAW sensors.
In addition to good flexibility, high strength, large ultimate strain, light weight, and other advantages provided by piezoelectric materials, OPCM sensor elements have a flat cross-anisotropic characteristic that can distinguish the planar structure of various components of a stress wave [13–15]. In this study, the feasibility of frequency chosen by AE broken pencil signals was verified by testing natural frequency ranges of SAW sensors. The attenuation characteristics of surface wave generated by AE simulation source in iron and concrete plate is tested by broken lead experiments. When the sample thickness is more than 2 to 3 wavelengths, the stress waves excited on the surface of the sample are mainly Rayleigh waves, which propagate longer distances. This is because the spread of the wave attenuation is much smaller than the longitudinal and transverse waves. Therefore, Rayleigh waves generated by the broken lead can be used to assess the performance of the sensor element and to analyze the attenuation characteristics effectively.
2. Dynamic Performance Analysis of 1-1 OPCM Sensor Element
The first step to construct OPCM elements is to design the piezoelectric phase of the element. Additionally, the volume ratio of the piezoelectric in the composite should be taken into account, including the effects of the elastic constants of the polymer material to impact the OPCM elements’ mechanical and electric characteristics. Taking the above factors into account, the OPCM sensor element was developed [16, 17], as shown in Figure 1. The length of the OPCM sensor element l is 13.8 mm, its width b is 4 mm, and is thickness h is 0.7 mm. The electrode interval a is 1 mm, and the electrode width d is 0.1 mm. The piezoelectric phases of the OPCM sensor are cut from the same monolithic and already polarized P-51 piezoelectric ceramic plate to ensure that the piezoelectric material properties are consistent. Table 1 shows the material properties of P-51 PZT. The substrate material of DAD-40 is conductive epoxy adhesive. The shear strength at normal temperature is 14.7 MPa, and the volume resistivity is less than 1.0 × 10−3 Ω cm. The interdigitated electrodes were shown in Figure 1, and two sides of the electrode can generate opposite electric polarity. The manufacturing process of two electrodes with opposite polarities was provided. It can form a good insulation tape to prevent electrical leakage. The resistance of the insulated region, between the two electrodes, is measured, and it is found to be more than 200 MΩ.
In order to obtain the frequency response of OPCM sensor element and set the range of frequency, the oscilloscope was used to record the waveform from two different directions in the experiment. When the excitation frequency was changed point by point, the corresponding variation voltage was recorded by the oscilloscope, and then the resonant frequency of two component voltages from directions x and y with OPCM sensor element can be gotten. When the test frequency changes from 20 kHz to 400 kHz, the OPCM sensing element receives the amplitude and encrypts the measuring point when the amplitude suddenly increases. The frequency is the resonant response frequency of the sensing element in this direction. Figure 2 shows the amplitude-frequency characteristic curve obtained from the frequency and voltage testing experiment.
The resonance frequency of the OPCM sensor in direction 3 is 173 kHz. The maximum amplitude at the resonant frequency is 41 mV, while the 1-direction amplitude is low, which shows the orthotropic performance.
3. Attenuation Characteristic Experiment
3.1. Specimen Preparation
The iron plate used in the experiment is of size 1000 mm × 1000 mm × 20 mm with density of 7.8 × 103 kg/m3, whereas the size of the standard curing concrete specimen is 700 mm × 700 mm × 200 mm, and the density is 2452 kg/m3. The concrete specimen is prepared with the mass mixing proportion (cement : gravel : sand : water) of 1 : 2.86 : 1.25 : 0.49, and the maximum size of gravel is 16 mm, and the water-cement ratio is 0.49.
The OPCM elements should be polished first before pasting it to the surfaces of the iron plate and concrete specimen with AB glue. The interval from the border is 200 mm. The HB pencil with a diameter of 0.5 mm and length of 2.5 mm is used in the broken lead experiments. The signal from the pencil broken at 30° at each measuring point simulates the AE signal in the structures.
3.2. Experiment Equipment and Experiment Content
Figure 3 shows the experimental setup used for this work. The dashed lines indicate the test lines of the polarization direction of sensing elements of the OPCM sensor. The interval of the pencile-broken points is 10 mm. The first OPCM sensor is used to test the directional performance of polarization and wave attenuation characteristic. By using a second OPCM sensor, the surface wave velocities of two materials can be calculated from the measured time difference. In this experiment, the velocity error depends on the sampling length. The Agilent54820A waveform generator, whose sampling length is set as 32768 pts and sampling frequency is set to 2.5 MHz, is used to break the pencil with band frequency from 50 kHz to 400 kHz [18] at given points along the direction of the OPCM sensor. The experimental testing setups of two kinds of plate are as shown in Figure 4.
(a) Attenuation characteristic testing experimental setup of the metal plate
(b) Attenuation characteristic testing experimental setup of the concrete plate
In the test, the signals are collected from the iron plate and concrete along the direction of polarization of the OPCM sensing element and at every 15° along the direction from 0° to 90° for different lead lengths. The velocity of the surface wave was tested in the iron plate and concrete, separately.
4. Experimental Results and Discussion
4.1. Surface Wave Propagation Characteristics in Different Materials
Figure 5 shows the waveform generated from the surface of the iron and concrete specimens with SAW sensors, respectively. Three typical waveforms tested by OPCM SAW sensors were selected to analyze. The results show that the wave amplitudes were attenuated on the iron plate more regularly than that on the concrete. This is because the concrete is an anisotropic material, and particles of different sizes cause reflection and scattering. Next, the amplitudes of the concrete are smaller than those of the iron plate, and this indicates that the attenuation of the wave in concrete is faster. Figure 6 shows the spectrums of the typical waves in concrete plate that were analyzed, and it is analogous in the iron plate. The resonance frequency testing results indicates that the SAW sensor can receive pencil-broken signals with it’s special frequency. The velocity of SAW can be calculated from the signal travel time difference. It is found that the surface velocity is 2958 m/s in the iron plate and 2103 m/s in the concrete material. The surface velocity values validate the correctness and feasibility to test the Rayleigh wave velocity. Before recording the signals, the pencil core must be broken at least three times to get the stable signals. The experiments are repeated for a number of times, and the surface velocity values obtained from each run are consistent with the nondispersion characteristics of Rayleigh wave, and the recorded signals are considered mainly to be Rayleigh waves.
(a) Different distance waveforms in metal plate
(b) Different distance waveforms in concrete plate
The OPCM sensor was stuck on the surface of iron plate and concrete specimen. The signals from different angle points can be collected 200 mm away from the center of the senor. Only 20 mV amplitude peak can be received in the polarization direction (90°), and no signals are received in other directions. This phenomenon further verifies the good cross-anisotropy of the developed OPCM sensor elements.
4.2. Wave Attenuation Characteristics of Different Materials
In the experiment, the signal generated by the by broken pencil is very weak. A lead signal was measured every 10 mm away from the center of the sensing element in the polarization direction of the sensing element, and the corresponding surface wave amplitude is calculated. Figure 7 shows the distance-amplitude curve. The results show that the maximum propagation distance measured in the iron plate is 460 mm, and the attenuation is observed up to a distance pf 260 mm. In case of the concrete specimen, the maximum distance measured is 300 mm, and the attenuation is observed up to a maximum distance of 150 mm. The results show that the amplitude of the wave decreases with propagation distance for both concrete and iron plates. However, the wave attenuates quickly, with propagation distance, in concrete compared to iron. In order to study the attenuation performance of surface wave, a mathematical fit is obtained from the amplitude data of Figure 7 and the propagation distance. The results show that the amplitude peak shows an exponential decline with the propagation distance x as shown in Figure 7. The attenuation law of the surface wave is an exponential decay, which can be expressed as , where A is the constant related to the material, and n is the attenuation coefficient that controls the surface waves decay rate. Figure 7 shows that the attenuation coefficients of SAW are 0.0403 and 0.3064 in iron and concrete specimen, respectively. It has provided a method to test the attenuation coefficients.
(a) The attenuation curve of the iron plate
(b) The attenuation curve of the concrete specimen
5. Conclusions
The developed AE tests are successfully conducted to study the wave propagation phenomena in the iron plate and concrete specimens using the 1-1 OPCM sensor. The single-point frequency-amplitude test results in its resonant frequency, which proves the feasibility of the AE broken pencil signal test and can be used in subsequent experiments to provide excitation frequency selection. The experiment is conducted multiple times on the same material using different OPCM sheets. The attenuation characteristics of the surface wave and wave speeds are obtained from the experiments. The attenuation characteristics of surface wave generated by the AE simulation source in iron and concrete materials are studied. The characteristics of the surface wave and its propagation velocity in different materials are analyzed. The waveforms recorded at different positions on iron and concrete plates show that the increasing peak values of the surface wave deviated from the sensing element are decreased rapidly exponentially and decayed rapidly in concrete. According to the material attenuation coefficient obtained experimentally, it can provide guidance for the study of amplitude compensation and impedance matching in multielement transducers.
Conflicts of Interest
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Grant nos. 11402101 and 11520101001) and Jiangsu University Foundation (14JDG022).