Piezoelectric Acoustic Emission Sensors Research Articles

A new method of using sensor arrays for gas leakage location based on correlation of the time-space domain of continuous ultrasound.

This paper proposes a time-space domain correlation-based method for gas leakage detection and location. It acquires the propagated signal on the skin of the plate by using a piezoelectric acoustic emission (AE) sensor array. The signal generated from the gas leakage hole (which diameter is less than 2 mm) is time continuous. By collecting and analyzing signals from different sensors’ positions in the array, the correlation among those signals in the time-space domain can be achieved. Then, the directional relationship between the sensor array and the leakage source can be calculated. The method successfully solves the real-time orientation problem of continuous ultrasonic signals generated from leakage sources (the orientation time is about 15 s once), and acquires high accuracy location information of leakage sources by the combination of multiple sets of orientation results. According to the experimental results, the mean value of the location absolute error is 5.83 mm on a one square meter plate, and the maximum location error is generally within a ±10 mm interval. Meanwhile, the error variance is less than 20.17.

High-temperature acoustic emission sensing tests using a yttrium calcium oxyborate sensor.

Piezoelectric materials have been broadly utilized in acoustic emission sensors, but are often hindered by the loss of piezoelectric properties at temperatures in the 500°C to 700°C range or higher. In this paper, a piezoelectric acoustic emission sensor was designed and fabricated using yttrium calcium oxyborate (YCOB) single crystals, followed by Hsu-Nielsen tests for high-temperature (>700°C) applications. The sensitivity of the YCOB sensor was found to have minimal degradation with increasing temperature up to 1000°C. During Hsu-Nielsen tests with a steel bar, this YCOB acoustic sensor showed the ability to detect zero-order symmetric and antisymmetric modes at 30 and 120 kHz, respectively, as well as distinguish a first-order antisymmetric mode at 240 kHz at elevated temperatures up to 1000°C. The frequency characteristics of the signal were verified using a finite-element model and wavelet transformation analysis.

Acoustic Emission Measurement by Fiber Bragg Grating Glued to Cylindrical Sensor Holder

Fiber Bragg grating (FBG) is an optical fiber sensor and is suitable for acoustic emission (AE) measurements of structures such as chemical plants that require water resistance, explosion proofing, and so forth, However, directionality has been a problem in conventional usage which glues the grating range of an FBG to the inspection surface. This study proposes a cylinder-FBG (C-FBG) sensor with the FBG glued to a cylindrical sensor holder. Effectiveness of the sensor was evaluated by characterizing directionality and frequency sensitivity of the sensor and by comparing the detected AE signals in a 3-point bending test of a carbon fiber reinforced plastic (CFRP) laminate against a conventional wide-band piezoelectric AE sensor. The features of the detected signals were similar to the signals detected by the wide-band AE sensor. These results show that it is possible to produce a wide-band cylindrical sensor without the directionality limitations of the conventional usage of FBG.

Structural Health Monitoring of Space Vehicle Thermal Protection Systems

The thermal protection systems of spacecraft are vulnerable to damage from impacts by foreign objects moving at high velocities. This paper describes a proposed novel structural health monitoring system that will detect, locate and evaluate the damage resulting from such impacts. This system consists of a network of intelligent local agents, each of which controls a network of piezoelectric acoustic emission sensors to detect and locate an impact, and a network of optical fibre Bragg grating sensors to evaluate the effect of the impact damage by means of a thermographic technique. The paper concentrates on two issues that are critical to the successful implementation of the proposed SHM system: measurement of the elastic properties of the thermal protection material, knowledge of which is essential to the design and operation of the acoustic emission sensor network; and investigation of the practical feasibility of a switched network of optical fibre sensors.

Acoustic Emission Based Tank Bottom Floor Corrosion Detection

The tank bottom floor is a crucial component of large oil storage tanks, and its status has a pivotal impact on the integrity of the entire tank. Acoustic Emission (AE) monitoring is an advanced, in-service, nondestructive testing method internationally recognized to be capable of assessing the tank bottom floor without the necessity of prior tank cleaning. In this paper, the principle of Time Difference of Arrival (TDOA) is illustrated, and the efficiency of piezoelectric AE sensors is verified through a field experiment. The authors then investigate Mach-Zehnder interferometerbased AE sensors with a view to using optical fiber sensors as a substitute for acoustic emission detecting. The results of the experiment indicate that optical fiber AE sensors based on the Mach-Zehnder interferometer can be used as transducers sensitive to acoustic events, so they can serve as indicators of the imminent failure of a structure. In addition, some suggestions are put forward regarding forthcoming actual application.

Acoustic emission techniques for failure characterisation in composite top-hat stiffeners

Modern day aerospace, automotive, marine, mechanical and civil structures rely on the advanced composites for their added benefits over conventional metallic structures. The complex damage process in composites involves various failure modes such as matrix cracking, fibre-matrix debonding, fibre fracture and delamination. This paper presents an acoustic emission technique for the failure characterisation of top-hat stiffener specimens using conventional piezoelectric acoustic emission sensors and modern fibre Bragg Gratings. Strain insensitive fibre Bragg Grating sensors are used in this experimental investigation to compare the performance of piezoelectric sensors for composite structures. This work has contributed to the development of an acoustic emission sensing system based on fibre Bragg Gratings. Main objectives of this work are to compare the sensing characteristics simultaneously in a composite structure with a surface-bonded acoustic emission–fibre Bragg Grating sensor and acoustic emission–piezoelectric sensor. The feasibility of the system is demonstrated in typical applications of in-situ structural health monitoring based on acoustic emission techniques.

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.

AE Monitoring and Analysis of HVOF Thermal Spraying Process

This work presents an in situ monitoring of HVOF thermal spraying process through an acoustic emission (AE) technique in an industrial coating chamber. Single layer thermal spraying on substrate was carried out through slits. Continuous multilayer thermal spraying onto the sample without slit was also conducted. The AE was measured using a broadband piezoelectric AE sensor positioned on the back of the substrate. A mathematical model has been developed to determine the total kinetic energy of particles impacting the substrate through slits. Results of this work demonstrate that AE associated with particle impacts can be used for in situ monitoring of coating process. Results also show that the amplitude and AE energy is related to the spray gun transverse speed and the oxy-fuel pressure. The measured AE energy was found to vary with the number of particles impacting the substrate, determined using the mathematical model.

J0404-5-5 固体ロケットモータケースヘルスモニタリングへのFBGセンサの適用([J0404-5]知的材料・構造システム(5):損傷検知・診断・モニタリングIII)

This report shows the results of the feasibility study to examine the applicability of fiber Bragg gratings (FBG) sensors to structural health monitoring of solid rocket motor cases, especially which are made of CFRP. For the study, a trial apparatus for a few FBG sensors which monitors strains and acoustic emission (AE) signal was designed and manufactured. The verification of the apparatus was carried out by the pressure test of a subscale motor case which was made of CFRP. The apparatus was manufactured by combining with DWDM components and CDWM components for large strain change. In the test, signals of FBG sensors were compared with conventional electrical resistance strain gages and piezoelectric AE sensors. The strain measurement results of FBG sensors and strain gages showed good agreement.

Characterizing Bond Breakages in Cemented Sands Using a MEMS Accelerometer

Abstract In this study, a microelectromechanical systems (MEMS) accelerometer, MEMSA (1–3 kHz), and a commercially available piezoelectric acoustic emission (AE) sensor, PZT (125–750 kHz), were used to capture AE responses in uncemented and cemented sands during triaxial compression tests. The AE rates measured in the cemented sands by both sensors demonstrated a similar trend and showed a strong resemblance to the stress-strain response. The bond breakages and the associated AE activities were mild at small strains and increased afterwards to initiate yielding. After the peak stress, shear banding gradually formed and the AE rate distinctly dropped. These observations suggest that a MEMS accelerometer can function as an effective AE sensor to detect the bond-breakage process in cemented sands. In addition, a PZT is more sensitive to the AE detection for cemented sands but a MEMSA starts earlier to capture AE and also continue to capture AE from the shear band at large strains while the PZT only measures a few or no AE activities.

Response of lead metaniobate acoustic emission sensors to gamma irradiation

Piezoelectric (PE) acoustic emission (AE) sensors were exposed to hundreds of kGy from Co-60 gamma rays. Recalibration of gamma-irradiated sensors showed a sensitivity decline of 8.4% per 100 kGy. Exposure of lead metaniobate disc specimens, the sensor ferroelectric material, to /spl sim/1 MGy revealed a capacitance increase and PE voltage (g/sub 33/) coefficient decrease with gamma dose. The planar coupling factor (k/sub p/) and mechanical quality factor (Q/sub m/) exhibited small reductions. The AE sensors were also subjected to nuclear radiation from a pulse reactor.

A self-calibrating piezoelectric transducer with integral sensor for in situ energy calibration of acoustic emission

Piezoelectric transducers are used as both transmitters and sensors in acoustic emission (AE) testing applications. However, the transducer outputs are significantly affected by the coupling between the transducer and the structure. This means that the coupling must be very carefully controlled or assessed if reliable AE calibration sources are to be generated, and be considered in the AE system calibration if reliable AE measurements are to be made. The paper shows that by integrating a piezoelectric AE sensor to the backing of a conical piezoelectric transducer acting as the transmitter, a self-calibrating AE energy point source can be obtained. There is a negative linear relationship between the AE signal energy from the backing sensor and the coupling efficiency for a constant electrical input to the transmitter over the frequency range of interest in AE measurements (100 kHz to around 1 MHz). This should enable the strength of the simulated AE point source to be determined using the AE measurement from the backing sensor, and hence to facilitate in situ energy calibration of AEs in structures.

Output Characteristics of a Thin-Film Piezoelectric Acoustic Emission Sensor Array on a Si Slider.

Output characteristics of a thin-film piezoelectric acoustic emission (AE) sensor array (sputtered ZnO, 0.1×0.1 mm) fabricated on a 2-mm-long Si slider for detecting head-disk interactions in hard disk drive systems were determined. The output characteristics were compared with those of a bulk AE sensor (0.7×0.7×1.0) attached to the back of the slider. Sensitivity of the thin-film sensor was about 1.7 mV/N, almost half of that of the bulk sensor. The thin-film sensor could detect AE waves caused by the contact force about 1.7 mm away from the sensor. When the slider contacted a bump on the disk, the output waveforms of the thin-film sensor were almost the same as those of the bulk sensor, and the noise level of the thin-film sensor was lower than that of the bulk sensor. Output signals of the array sensor matched the slider flying height at the position of each sensor.

アコースティック・エミッションの放射形式計測によるアングルプライＣＦＲＰ積層板の微視割れモード評価

A new acoustic emission (AE) analysis method for fiber reinforced composite material was developed. Phase combinations of first arrival quasi-longitudinal wave of AE in plate (Lamb wave) was analyzed based on AE spherical radiation pattern in infinite homogeneous medium, plate wave propagation properties and detection properties of piezoelectric AE sensor. Angle ply carbon fiber/epoxy composites were prepared to the AE and Ultrasonic C-scope tests to evaluate the fracture processes of delamination. Three stages characterized both the compliance and the AE properties were observed during tensile tests. The spherical AE radiation pattern analysis could distinguish into three types of fracture mode for detected AE events, such as tensile fracture, shear fracture and mixture fracture of tensile and shear. By analyzing the AE spherical pattern events, C-scope, surface and fracture surface observation, fracture processes of the angle ply CFRP were proved. At stage I of which is linear load-strain region, debondings occurred at notch tip along the fiber orientation of each layer as shear fracture. At stage II of which is transient region between stage I and III, coalescence of the debonding in each layer occurred as tensile fracture. At stage III of which is heavy increase of the compliance and the AE counts region, the debonding and the coalescence occurred at the same time along the fiber orientation.