互易法校准压电型声发射传感器的研究与实现

声发射传感器的校准是实现声发射定量技术的前提,依据电声换能器互易原理,在计算互易常数的基础 上,建立了适用于压电型声发射传感器表面波和纵波互易的校准系统。通过设置特定的激励信号波形,依据接收 电压信号与激励电流信号之间的时间延迟,准确获取电流信号与电压信号对应的特征值,实现了声发射传感器的表面波和纵波互易校准。由于传感器的尺寸效应,传感器在高频时的表面波速度灵敏度低于纵波灵敏度,不确定度评定结果表明,声发射传感器速度灵敏度的互易法校准不确定度为1. 2 dB。     

Self‐Powered Real‐Time Arterial Pulse Monitoring Using Ultrathin Epidermal Piezoelectric Sensors

Continuous monitoring of an arterial pulse using a pressure sensor attached on the epidermis is an important technology for detecting the early onset of cardiovascular disease and assessing personal health status. Conventional pulse sensors have the capability of detecting human biosignals, but have significant drawbacks of power consumption issues that limit sustainable operation of wearable medical devices. Here, a self‐powered piezoelectric pulse sensor is demonstrated to enable in vivo measurement of radial/carotid pulse signals in near‐surface arteries. The inorganic piezoelectric sensor on an ultrathin plastic achieves conformal contact with the complex texture of the rugged skin, which allows to respond to the tiny pulse changes arising on the surface of epidermis. Experimental studies provide characteristics of the sensor with a sensitivity (≈0.018 kPa^?1), response time (≈60 ms), and good mechanical stability. Wireless transmission of detected arterial pressure signals to a smart phone demonstrates the possibility of self‐powered and real‐time pulse monitoring system.     

Recovery of microcrack parameters in mortar using quantitative acoustic emission

A three-dimensional quantitative acoustic emission (AE) analysis of microcracking in unreinforced mortar beams was conducted. In order to facilitate the analysis of the large amounts of data generated by an AE test, a simplified method for the inversion of AE signals was developed. By applying the theoretical Green's function for an infinite space, the multichannel deconvolution normally required of AE data inversion reduces to a nonlinear curve-fitting problem. Using this procedure, microcracking in a mortar specimen was evaluated using a seismic moment tensor representation. Source-time functions for the microcracks were also recovered. The locations of the AE events were calculated, and damage localization was observed. The moment tensor analysis showed the dominant mode of microfracture to be mode II, with a limited number classified as mixed mode. A microstructural mechanism for this behavior is presented.     

Detection of Damaged Tool in Injection Molding Process with Acoustic Emission

ABSTRACT

The measurement of acoustic emission (AE) signals during injection molding of polypropylene with new and damaged mold is presented. The damaged injection mold was fitted with a steel insert with cracks induced by laser surface heat treatment. Two resonant piezoelectric AE sensors were attached to the mold via AE waveguides. To improve the mold integrity prediction with smaller defects, AE signal frequency characteristics and a measure of AE signal amplitude probability distribution are implemented. A 5-dimensional feature vector with real-valued explanatory variables is proposed, providing the defining points in an appropriate multidimensional space to characterize the state of injection molding tool. Feature vectors are classified with neural network pattern recognition. The results confirm that presented AE technique offers characterizing the integrity of molds also with resonant sensors.     

Tailoring Thin‐Film Piezoelectrics for Crash Sensing

Crash sensing and its assessment play a pivotal role in autonomous vehicles for preventing fatal casualties. Existing crash sensors are severely bottlenecked by sluggish response time, rigid mechanical components, and space constraints. Miniaturized sensors embedded with custom‐tailored nanomaterials upholds potential to overcome these limitations. In this article, piezoelectric Zinc‐Oxide thin film as a crash sensing layer is integrated onto a flexible metal‐alloy cantilever. Material characterization studies are conducted to confirm piezoelectric property of sputtered ZnO film. The piezoelectric d ₃₁ coefficient value of ZnO film was 7.2 pm V^–1. The ZnO sensing element is firmly mounted on a scaled car model and used in a crash sensing experimental set‐up. A comprehensive theoretical analysis for two different real scenarios (nearly elastic and nearly inelastic collision) of crash events followed by experimental study is discussed. The crash sensor's output exhibits a linear relationship with magnitude of impact forces experienced at crash events. The response time of ZnO crash sensor is 18.2 ms, and it exhibits a sensitivity of 28.7 mV N^–1. The developed crash sensor has potential to replace bulk material sensors owing to its faster response time, high sensitivity, and compactness as the demand for crash sensors in next‐generation automobile industries is progressively growing.     

基于含金属芯压电纤维与Lamb波的一维结构损伤定位研究

含金属芯压电纤维(Metal-core Piezoelectric Ceramic Fiber,MPF)是一种新型压电功能器件.介绍了MPF的结构及其对圆形压电片激励Lamb波的传感响应模型.利用Gabor小波变换计算损伤反射信号到达时间延迟的原理,把MPF传感单一模式Lamb波在一维结构中进行了损伤定位研究.研究结果表明:MPF可以进行Lamb波的单一模式传感,采用Gabor小波变换计算损伤反射信号到达时间延迟效果较好,损伤定位精度较高.     

Flexible Piezoelectric ZnO–Paper Nanocomposite Strain Sensor

The fabrication of a mechanically flexible, piezoelectric nanocomposite material for strain sensing applications is reported. Nanocomposite material consisting of zinc oxide (ZnO) nanostructures embedded in a stable matrix of paper (cellulose fibers) is prepared by a solvothermal method. The applicability of this material as a strain sensor is demonstrated by studying its real‐time current response under both static and dynamic mechanical loading. The material presented highlights a novel approach to introduce flexibility into strain sensors by embedding crystalline piezoelectric material in a flexible cellulose‐based secondary matrix.     

Contact-Type Vibration Sensors Using Curved Clamped PVDF Film

This paper describes a new type of contact vibration sensor made by bonding a piezoelectric polyvinylidene fluoride (PVDF) film to a curved frame structure. The concave surface of the film is bonded to a rubber piece having a front contact face. Vibration is transmitted from this face through the rubber to the surface of the PVDF film. Pressure normal to the surface of the film is converted to circumferential strain, and an electric field is induced by the piezoelectric effect. The frequency response of the device was measured using an accelerometer mounted between the rubber face and a rigid vibration exciter plate. Sensitivity (voltage per unit displacement) was deduced from the device output and measured acceleration. The sensitivity was flat from 16 Hz to 3 kHz, peaking at 6 kHz due to a structural resonance. A contact vibration sensor theory has been developed, which accounts for the effect of the radiation medium. It has been found that the imaginary part of the radiation impedance has an effect equivalent to the addition of mass to the curved PVDF film structure, which reduces the resonance frequency by about one order. Calculations predicting performance against human tissue (stethoscope or contact microphone) show results similar to data measured against the metal vibrator. This implies that an accelerometer can be used for calibrating a stethoscope or contact microphone. The observed arterial pulse waveform from the new PVDF sensor showed more low-frequency content than a conventional electronic stethoscope.     

1-3型水泥基压电复合材料传感器的性能

以 123型水泥基压电复合材料作为传感元件制备了水泥基压电复合材料传感器。研究了水泥基压电复合材料传感器的频率响应、 线性性能以及应用于混凝土后的传感性能。结果表明: 当加载频率小于 5 Hz时 , 所有载荷下传感器输出电压的幅值均增大 , 但当加载频率大于 5 Hz时 , 所有载荷下传感器输出电压的幅值几乎与输入载荷频率无关 ; 传感器的输出电压幅值和输入载荷幅值之间存在明显的线性关系。水泥基压电传感器在实际混凝土结构中具有良好的传感特性 , 其输出电压与复杂载荷、 随机载荷和脉冲载荷均呈现明显的一一对应关系 ,且与输入载荷基本同步 , 不存在滞后现象 , 试验输出电压值与理论输出电压值也非常吻合。该传感器非常适合于土木工程结构的健康监测。      

Acoustic Emission Sensor Calibration for Absolute Source Measurements

This paper describes sensor calibration and signal analysis techniques applicable to the method of acoustic emission (AE) and ultrasonic testing. They are particularly useful for obtaining absolute measurements of AE wave amplitude and shape, which can be used to constrain the physics and mechanics of the AE source. We illustrate how to perform calibration tests on a thick plate and how to implement two different mechanical calibration sources: ball impact and glass capillary fracture. In this way, the instrument response function can be estimated from theory, without the need for a reference transducer. We demonstrate the methodology by comparing calibration results for four different piezoelectric acoustic emission sensors: Physical Acoustics (PAC) PAC R15, PAC NANO30, DigitalWave B1025, and the Glaser-type conical sensor. From the results of these tests, sensor aperture effects are quantified and the accuracy of calibration source models is verified. Finally, this paper describes how the effects of the sensor can be modeled using an autoregressive-moving average (ARMA) model, and how this technique can be used to effectively remove sensor-induced distortion so that a displacement time history can be retrieved from recorded signals.     

A Self‐Powered Sensor Mimicking Slow‐ and Fast‐Adapting Cutaneous Mechanoreceptors

Highly efficient human skin systems transmit fast adaptive (FA) and slow adaptive (SA) pulses selectively or consolidatively to the brain for a variety of external stimuli. The integrated analysis of these signals determines how humans perceive external physical stimuli. Here, a self‐powered mechanoreceptor sensor based on an artificial ion‐channel system combined with a piezoelectric film is presented, which can simultaneously implement FA and SA pulses like human skin. This device detects stimuli with high sensitivity and broad frequency band without external power. For the feasibility study, various stimuli are measured or detected. Vital signs such as the heart rate and ballistocardiogram can be measured simultaneously in real time. Also, a variety of stimuli such as the mechanical stress, surface roughness, and contact by a moving object can be distinguished and detected. This opens new scientific fields to realize the somatic cutaneous sensor of the real skin. Moreover, this new sensing scheme inspired by natural sensing structures is able to mimic the five senses of living creatures.     

Wearable and Ultrasensitive Strain Sensor Based on High‐Quality GaN pn Junction Microwire Arrays

With the rapid growth in wearable electronics sensing devices, flexible sensing devices that monitor the human body have shown great promise in personalized healthcare. In the study, high‐quality GaN pn junction microwire arrays with different aspect ratios and large‐area uniformity are fabricated through an easy, repeatable fabrication process. The piezoelectric coefficient (d₃₃) of GaN pn junction microwire arrays increases from 7.23 to 14.46 pm V^?1 with the increasing of the aspect ratio, which is several times higher than that of GaN bulk material. Furthermore, flexible ultrasensitive strain sensor based on GaN microwires with the highest d₃₃ is demonstrated to achieve the maximum open circuit voltage of 10.4 V, and presents excellent durability with stable output signals over 10 000 cycles with a response time of 50 ms. As a flexible and wearable sensor attached to the human skin, the GaN microwire pn junction arrays with such a high degree of uniformity can precisely monitor subtle human pulse and motions, which show great promise in future personalized healthcare.     

Localization of a ``Synthetic' Acoustic Emission Source on the Surface of a Fatigue Specimen

The finite geometry of a laboratory specimen influences a measured acoustic emission waveform because of reflections, transmission, and mode conversion at the interface and boundaries of the specimen, thus making it difficult to determine the location of an acoustic emission (AE) source. The objective of this investigation is to develop a model experiment to identifiy the exact source location on the surface using ``synthetic' AE signals. The AE event is generated by a short local thermal expansion. This expansion is produced by the absorption of a short laser pulse which provides a noncontact and broad-band generation of elastic waves. The signals are detected by a noncontact, broad-band, and high-fidelity sensor: a laser interferometer. The triangulation with several detectors is replaced by a single probe laser interferometer located at different coordinates under reproducible conditions. The recorded signals are analyzed by wavelet transform in order to determine the arrival times of waves for several frequency levels. These arrival times are used to quantify the location of the AE source in the surface as well as the velocity of the most dominant feature, the Rayleigh wave, and the time lag between the instant of the AE and the recording of the signal. The accuracy of the method is demonstrated by comparing the identified source location with the exact one.     

基于含金属芯压电纤维与Lamb波的结构损伤定位研究

摘要：含金属芯压电纤维（Metal-core Piezoelectric Ceramic Fiber,MPF）是一种新型压电功能器件。介绍了MPF的结构及其对圆形压电片激励Lamb波的传感响应模型。利用Gabor小波变换计算损伤反射信号到达时间延迟的原理,把MPF传感单一模式Lamb波在一维结构中进行了损伤定位研究。研究结果表明：MPF可以进行Lamb波的单一模式传感,采用Gabor小波变换计算损伤反射信号到达时间延迟效果较好,损伤定位精度较高。     

Damage monitoring of reinforced concrete frames under seismic loading using cement-based piezoelectric sensor

Damage process monitoring of concrete structures using acoustic emission (AE) technology has been drawn more and more attention due to its powerful capability. In this paper, a brand new cement-based piezoelectric composite sensor was introduced with improved performance, particularly concerning its AE signals detection capacity in the high frequency domain. Such sensors were embedded into the foundation of reinforced concrete (RC) frames during construction, and appointed to monitor concrete damage due to shake table excitation. A standard ground motion record of 1999 Taiwan earthquake with 840 gal and 1300 gal peak ground accelerations were employed in the tests as the excitation inputs. The signals detected by the sensors were stored and analyzed by commercial available data acquisition devices. Advanced stochastic signal analysis methods were adopted to effectively interpret the frequency domain components and identify the useful information representing the damage processes of the RC frames. The results were compared with the eye observations of structural damage and corresponding cyclic loading tests results. It is shown that the health monitoring method using cement-based piezoelectric composite sensors and advanced stochastic signal analysis are capable of detecting and evaluating the damage process of RC frames due to seismic loading. An effective damage indicator of the RC frames is possible to be evaluated from extracted AE information.     

Frequency encoding of resonant mass sensors for chemical vapor detection

Chemical vapors can be detected by a resonant mass sensor array with selective absorption coatings implementing a frequency encoding method. The sensor array consists of sensor elements with different frequencies for their identifications in the frequency response obtained with a pulse Fourier transform detection scheme. Zero-loading resonance frequencies are chosen so that frequency shift due to absorption is bounded within a predefined region so that there is no overlap of peaks and all peaks can be assigned to the correct elements at any operation conditions. Mechanical oscillations of all or selected numbers of the sensor elements are excited by application of an excitation signal. Free oscillation decay signals from all or selectively excited sensor elements are detected and digitized. The free oscillation decay signal is subjected to a spectral analysis routine converting into a frequency spectrum, in which frequency shifts due to absorption of chemical vapors can be obtained. The implementation of the frequency encoding method with pulse Fourier transform detection to resonant mass sensors allows simultaneous multisensor detection, fast data acquisition speed, high signal-to-noise ratio by coaddition of raw data, flexible excitation, reduced complexity of electronic hardware, application of advanced data/spectral analysis algorithms, and realization of many other advantages by the introduction of the pulse Fourier transform method. A practical chemical vapor sensing system is demonstrated experimentally by use of nine frequency-encoded and polymer-coated sensors.     

Highly Sensitive and Stretchable Resistive Strain Sensors Based on Microstructured Metal Nanowire/Elastomer Composite Films

High sensitivity and high stretchability are two conflicting characteristics that are difficult to achieve simultaneously in elastic strain sensors. A highly sensitive and stretchable strain sensor comprising a microstructured metal nanowire (mNW)/elastomer composite film is presented. The surface structure is easily prepared by combining an mNW coating and soft‐lithographic replication processes in a simple and reproducible manner. The densely packed microprism‐array architecture of the composite film leads to a large morphological change in the mNW percolation network by efficiently concentrating the strain in the valley regions upon stretching. Meanwhile, the percolation network comprising mNWs with a high aspect ratio is stable enough to prevent electrical failure, even under high strains. This enables the sensor to simultaneously satisfy high sensitivity (gauge factor ≈81 at >130% strain) and high stretchability (150%) while ensuring long‐term reliability (10 000 cycles at 150% strain). The sensor can also detect strain induced by bending and pressure, thus demonstrating its potential as a versatile sensing tool. The sensor is successfully utilized to monitor a wide range of human motions in real time. Furthermore, the unique sensing mechanism is easily extended to detect more complex multiaxial strains by optimizing the surface morphology of the device.     

Optimization of piezoelectric sensor location for delamination detection in composite laminates

The optimal placement of sensors is a critical issue in detecting damage in laminated composite structures. The aim is to use a minimum number of sensors, placed at the correct locations, so that the voltage signals received from the sensor set can be used to detect both the presence and the extent of damage. In this study, an optimization procedure is developed to detect arbitrarily located discrete delamination in composite plates using distributed piezoelectric sensors. The probability of damage distribution in the plate is determined using a statistical model. A genetic algorithm (GA) is used to detect the number and location of the sensors. The analysis uses a Monte Carlo method to generate the initial population. The simulation and signal processing is performed using a finite element procedure based on the refined layer-wise theory, which is capable of modelling ply-level stresses, and seeded delaminations are modelled with Heaviside step functions. A two-way electromechanical coupled field formulation is used to describe the induced strain. The objective function is a damage index which compares the voltage signals from a healthy (no delamination) and a statistically determined delaminated model. The voltage signals are affected by the local changes in the strain induced by the presence of delamination. The optimization solutions are verified by numerical simulation as well as with experiments conducted using customized piezoelectric sensors and a laser scanning vibrometer. The results presented show that the optimum sensor pattern is capable of detecting discrete seeded delaminations in moderately thick composite plates.     

Role of mass accumulation and viscoelastic film properties for the response of acoustic-wave-based chemical sensors

The sensitivity of acoustic-wave microsensors coated with a viscoelastic film to mass changes and film modulus (changes) is examined. The study analyzes the acoustic load at the interface between the acoustic device and the coating. The acoustic load carries information about surface mass and film modulus; its determination has no restrictions in film thickness. Two regimes of film behavior can be distinguished: the gravimetric regime, where the sensor response is mainly mass sensitive, and the nongravimetric regime, where viscoelasticity gains influence on the sensor response. We develop a method, which allows the assignment of the sensor signal to a gravimetric or a nongravimetric response. The critical value can be determined from oscillator measurements. The related limits for the coating thickness are not the same for the coating procedure and mass accumulation during chemical sensing. As an example, we present results from a 10 MHz quartz crystal resonator.     

Evaluation of the use of envelope analysis and DWT on AE signals generated from degrading shafts

Vibration analysis is widely used in machinery diagnosis. Wavelet transforms and envelope analysis, which have been implemented in many applications in the condition monitoring of machinery, are applied in the development of a condition monitoring system for early detection of faults generated in several key components of machinery. Early fault detection is a very important factor in condition monitoring and a basic component for the application of condition-based maintenance (CBM) and predictive maintenance (PM). In addition, acoustic emission (AE) sensors have specific characteristics that are highly sensitive to high-frequency and low-energy signals. Therefore, the AE technique has been applied recently in studies on the early detection of failure. In this paper, AE signals caused by crack growth on a rotating shaft were captured through an AE sensor. The AE signatures were pre-processed using the proposed signal processing method, after which power spectrums were generated from the FFT results. In the power spectrum, some peaks from fault frequencies were presented. According to the results, crack growth in rotating machinery can be considered and detected using an AE sensor and the signal processing method.