Acoustic Emission Transducers Research Articles

Estimation of granule size distribution for batch fluidized bed granulation process using acoustic emission and N‐way PLS

AbstractFluidized bed wet granulation of a pharmaceutical mixture is an unpredictable and complex process. Batch‐to‐batch variation, i.e. output material inconsistency, is still an unmanaged problem in fluidized bed granulation. The variation in outcome with different size distributions and yield arises from process conditions, which until now have proved difficult to control. The good quality batch is usually one with a high yield of the desired narrow granule size distribution. Thus, for successful granulation the size distribution needs to be controlled throughout the process in order to prevent the heterogeneity of different batch runs and inconsistent batch products. This paper introduces a method for determining end‐product granule size distribution in the early phase of granulation, based on the observation that the quality of the early phase nucleation strongly affects the quality of the end product. In the quantitative model, the information gained from the process stream acquired with an acoustic emission (AE) transducer was combined and modeled using an N‐way PLS method. Reference sieve analysis for the granule size distribution was performed off‐line and was used as the response variable. AE monitoring is a non‐invasive technique for evaluating granulation process performance by detecting the sound of particle impacts. AE caused by the particle–chamber wall interaction is influenced by the size of the granules; thus AE contains information about the granule size distribution. This study shows for the first time that the nucleation phase of granulation can be detected using AE techniques, and thus the possibility of predicting end product granule size distribution by means of AE measurement during the nucleation phase. Copyright © 2009 John Wiley & Sons, Ltd.

The influence of temperature on the characteristics of piezoelectric transducers and errors of localization of acoustic-emission signals

The experimental dependences of changes in the output signals of standard and high-temperature acoustic-emission transducers (AETs) during heating and cooling are considered. It is shown that high-temperature AETs have a hysteresis that must be taken into account in high-temperature tests of various materials. The errors in localizing acoustic-emission signals from a simulator during heating and cooling of high-temperature and standard AETs are analyzed using the two-interval method.

On shear band velocity and the detectability of acoustic emission in metallic glasses

The acoustic emission (AE) method is proposed for the assessment of shear band velocity in metallic glasses. The shear band velocity is discussed in light of analysis of real-time AE transient events which occur during inhomogeneous plastic flow. It is estimated that the velocity of shear events triggering shear bands should be at least as high as 1–10 m s−1 to be detected by AE transducers.

저속회전축의 균열 검출을 위한 음향방출기법의 적용

Condition monitoring(CM) is a method based on non-destructive test(NDT). So, recently many kind of NDT were applied for CM. Acoustic emission(AE) is widely used for the early detection of faults in rotating machinery in these days because of high sensitivity than common accelerometers and detectable low energy vibration signals. And crack is considered one of severe fault in the rotating machine. Therefore, in this paper, study on early detection using AE has been accomplished for the crack of the low-speed shaft. There is a seeded initial crack on the shaft then the AE signal had been measured with low-speed rotation as the applied load condition. The signal detected from crack in rotating machine was detected by the AE transducer then the trend of crack growth had found out by using some of feature values such as peak value, skewness, kurtosis, crest factor, frequency center value(FC), variance frequency value(VF) and so on.

Shrinkage processes in standard-size Norway spruce wood specimens with different vulnerability to cavitation.

The aim of this study was to observe the radial shrinkage of Norway spruce [Picea abies (L. Karst.)] trunkwood specimens with different hydraulic vulnerability to cavitation from the fully saturated state until the overall shrinkage reaches a stable value, and to relate wood shrinkage and recovery from shrinkage to cavitations of the water column inside the tracheids. Radial shrinkage processes in standard-size sapwood specimens (6 mm x 6 mm x 100 mm; radial, tangential and longitudinal) obtained at different positions within the trunk, representing different ages of the cambium, were compared. Cavitation events were assessed by acoustic emission (AE) testing, hydraulic vulnerability by the AE feature analysis and shrinkage was calculated from the changes in contact pressure between the 150 kHz AE transducer and the wood specimen. Two shrinkage processes were observed in both juvenile (annual rings 1 and 2) and mature wood (annual rings 17-19), the first one termed tension shrinkage and the second one cell wall shrinkage process, which started when most of the tracheids reached relative water contents below fiber saturation. Maximum tension shrinkage coincided with high-energy AEs, and the periods of shrinkage recovery could be traced to tension release due to cavitation. Juvenile wood, which was less sensitive to cavitation, had lower earlywood tracheid diameters and was less prone to deformation due to tensile strain than mature wood, showed a lower cell wall shrinkage, and thus total shrinkage. Earlywood lumen diameters and maximum tension shrinkage were strongly positively related to each other, meaning that bigger tracheids are more prone to deformation at the same water tension than the smaller tracheids.

Acoustic emission testing in wires from the tensile armour of flexible risers under load

The need for an inspection technique that allows continuous monitoring of flexible risers and wire ropes has led to the development of non-destructive techniques. Among them, the acoustic emission (AE) method is a solution that might provide vital data on wire ruptures. The AE parameters can be used as input for pattern recognition techniques that could lead to automation. This work presents a study on the acquisition settings of an equipment suitable for the application, and compares the result of a resonant and a broadband AE transducer. The results have led to the creation of a reliability range for time-based parameters and show that, although both transducers can be used, the broadband is better for a freguency-based analysis while the resonant works better for an analysis based on hits.

In situ fibre strength determination in metal matrix composites

Single fibre fragmentation tests were performed at room temperature on SiC/Ti-6242 specimens in order to estimate the in situ fibre strength. Tensile specimens were instrumented with two acoustic emission transducers and an extensometer in order to monitor the strain at which fibre breaks occurred. Data analysis utilized Monte Carlo simulations of fibre fragmentation. The fibre/matrix stress transfer profile near a fibre break was derived using a finite element analysis. Cohesive zone model is used to describe damage of the interfacial zone. Thermally induced residual stresses and matrix plastic deformations were accounted for. The results presented in this paper show that the in situ Weibull parameters of the fibre are smaller than the reference obtained on as received fibres. Analysis of data raised questions about the validity of the Monte Carlo simulation method.

웨이블릿변환이 접목된 포락처리를 이용한 저속 회전하는 구름요소베어링 결함 진단

Acoustic Emission (AE) technique is a non-destructive testing method and widely used for the early detection of faults in rotating machines in these days, because the sensitivity of AE transducers is higher than normal accelerometers. So it can detect low energy vibration signals. The faults in the rotating machines are generally occurred at bearings and gearboxes which are the principal parts of the machines. It was studied to detect the bearing faults by envelop analysis in several decade years. And the researches showed that AE had a possibility of the application in condition monitoring system(CMS) using the envelope analysis for the rolling bearing. And peak ratio (PR) was developed for expression of the bearing condition in condition monitoring system using AE. Noise level is needed to reduce to take exact PR value because the PR is calculated from total root mean square (RMS) and the harmonics peak levels of the defect frequencies of the bearing. Therefore, in this paper, the discrete wavelet transform (DWT) was added in the envelope analysis to reduce the noise level in the AE signals. And then, the PR was calculated and compared with general envelope analysis result and the result of envelope analysis added the DWT. In the experiment result about inner fault of bearing, defect frequency was difficult to find about only envelop analysis. But it`s easy to find defect frequency after wavelet transform. Therefore, Envelop analysis added wavelet transform was useful method for early detection of default in signal process.

AN ENHANCED REFLECTION REMOVAL TECHNIQUE AND ITS APPLICATIONS

Electroacoustic transducers using piezoelectric materials are popular in various applications such as underwater acoustics, ultrasound, earthquakes and elastic wave propagations. Especially, they are widely used in non-destructive testing for ultrasonic or acoustic emission transducers. In general, they generate and receive waves through media to find meaningful targets or physical characteristics of materials. However, in most uses, the media are bounded with finite dimensions, therefore there are multiple transmitting paths reflected from the boundaries. Such reflections corrupt the principal path signal to be analyzed. To overcome this problem, gating technique to gate successively transmitting and receiving signals, in other words, tone-burst signal technique, is most representatively used. This basically isolates the direct signal before the arrival of reflected signals in the time domain, and therefore it is also described as time windowing or time-selective windowing techniques without loss of generality. These techniques have inherent overlap problems invoked by long pulse duration, especially slightly damped signals or low frequency waves. An enhancement technique of shortening the pulses by digital filtering is proposed and successively applied in practical uses. It can isolate the principal path signal from reflected signals. Thereafter the signal can be perfectly recovered after removing reflections.

Behavior of Acoustic Emission of Mortar Degraded by Mixing with Expanded Polystyrene Beads

Degraded specimens were prepared by mixing expanded polystyrene (EPS) beads with mortar, and sound specimens include no beads. Measurements of compressional elastic wave velocity (Vp) and acoustic emission (AE) were performed using an AE transducer. The relationships between air content and AE parameters were investigated based on the Vp measurements. The changes in AE behavior with increase in strain were measured by a uniaxial compression test and a splitting test. The AE parameters–rise time, count, frequency, average signal level, and event count rate–were evaluated, on average, every ten seconds. The following results were obtained. (1) In the uniaxial compression test, the volumetric contraction of the degraded specimens was small. This is because large volumetric expansion of the degraded specimens occurs with increase in axial strain. (2) It is difficult to measure the degree of degradation due to increase in porosity using only one AE transducer. The degree of degradation can be evaluated qualitatively using two AE parameters (mean value of count and standard deviation of rise time). (3) It is inferred that the area of observation can be recognized as degraded by increase in cracks when the behavior of average signal level and event count rate for monotonic load is similar to that for cyclic load. (4) It is also inferred that the failure type can be determined as either extensive compression or local tension by long-term observation of reverberation frequency and rise time.

In-situ monitoring of the brush/rotor interface in a homopolar motor with Acoustic Emission

Homopolar motors have been proposed as alternative propulsion means for ships. Such motors operate in a very harsh and demanding environment, making in-situ diagnostics and fault-monitoring systems a key issue for extended operation. In a typical homopolar motor, several thousand brushes are used, which are all in sliding contact with a copper rotor at a velocity of about 10 m/s. Wear and degradation of the brush and rotor are key issues that need to be understood in order to design a reliable motor. In this investigation, Acoustic Emission (AE) transducers were used to study the characteristics of the brush/rotor interface of a homopolar motor as a function of speed, load, materials and other design parameters. The AE sensors were directly coupled to the brush in the homopolar motor test rig, and data were obtained over extended periods of time. The sensor signal demonstrated good sensitivity to brush/rotor contact conditions, brush contact load and motor current polarity. The results show that AE is well suited for in-situ monitoring of the brush and rotor conditions.

A study of the brush/rotor interface of a homopolar motor using acoustic emission

Acoustic emission transducers were used to study the brush/rotor interface of a homopolar motor as a function of design parameters such as speed and load. The results indicate that acoustic emission is well suited for in situ monitoring of brush wear and rotor conditions, and for analyzing the tribological phenomena occurring at the brush/rotor interface.

파형 및 주파수해석에 근거한 굽힘 압전 복합재료 작동기 손상모드의 비파괴적 평가

In this study, various damage modes in bending unimorph piezoelectric composite actuators with a thin sandwiched PZT plate during bending fracture tests have been evaluated by monitoring acoustic emission (AE) signals in terms of waveform and peak frequency as well as AE parameters. Three kinds of actuator specimens consisting of woven fabric fiber skin layers and a PZT ceramic core layer are loaded with a roller and an AE activity from the specimen is monitored during the entire loading using an AE transducer mounted on the specimen. AE characteristics from a monolithic PZT ceramic with a thickness of 250 ㎛ are examined first in order to distinguish different AE signals from various possible damage modes in piezoelectric composite actuators. Post-failure observations and stress analyses in the respective layers of the specimens are conducted to identify particular features in the acoustic emission signal that correspond to specific types of damage modes. As a result, the signal classification based on waveform and peak frequency analyses successfully describes the failure process of the bending piezoelectric composite actuator exhibiting diverse failure mechanisms. Furthermore, it is elucidated that when the PZT ceramic embedded actuators are loaded mechanical bending loads, the failure process of actuator specimens with different lay-up configurations is almost same irrespective of their lay-up configurations.

Identification of failure mechanisms in a smart composite actuator with a thin sandwichedPZT plate based on waveform and primary frequency analyses

In the present work, the monitoring of an acoustic emission (AE) activity has beenemployed to investigate the failure mechanisms of smart piezoelectric composite actuatorswith a thin sandwiched PZT plate during bending fracture tests. Three kinds of specimensconsisting of woven fabric fiber skin layers and a PZT ceramic core layer are loaded with aroller and the AE activity is monitored in real time during the entire loading by using aresonant type AE transducer mounted on the specimen. Post-failure observationsare conducted to identify particular features in the acoustic emission signal thatcorrespond to specific types of failure events. AE characteristics from monolithic PZTceramic are first examined to distinguish different AE signals from various possiblefailure mechanisms in the smart piezoelectric composite actuators. AE signalsare analyzed in terms of AE event count and AE cumulative energy, and areclassified into four different signal types based on the waveform and the primaryfrequency which is obtained by a short time Fourier transform (STFT). As aresult, the behavior of the four classified different AE signal types according to thewaveform and the primary frequency successfully describes the failure process of thesmart piezoelectric composite actuator with a thin sandwiched PZT plate showingvarious and multiple failure mechanisms. Finally, it is elucidated that each classifiedsignal type is related to the specific failure mechanism within the smart compositeactuator, and the AE characteristics are summarized in table 1, in association withthe individual failure mechanism of the smart piezoelectric composite actuator.

Resonant capacitive MEMS acoustic emission transducers

We describe resonant capacitive MEMS transducers developed for use as acoustic emission(AE) detectors, fabricated in the commercial three-layer polysilicon surface micromachiningprocess (MUMPs). The 1 cm square device contains six independent transducers in thefrequency range between 100 and 500 kHz, and a seventh transducer at 1 MHz. Eachtransducer is a parallel plate capacitor with one plate free to vibrate, thereby causing acapacitance change which creates an output signal in the form of a current under a dc biasvoltage. With the geometric proportions we employed, each transducer responds with twodistinct resonant frequencies. In our design the etch hole spacing was chosen to limitsqueeze film damping and thereby produce an underdamped vibration when operated atatmospheric pressure. Characterization experiments obtained by capacitance andadmittance measurements are presented, and transducer responses to physicallysimulated AE source are discussed. Finally, we report our use of the device to detectacoustic emissions associated with crack initiation and growth in weld metal.

Experimental investigation into the capabilities of acoustic emission for the detection of shaft-to-seal rubbing in large power generation turbines: A case study

An experimental investigation was undertaken in order to assess the potential of the acoustic emission (AE) technology for the detection of seal-to-rotor rubbing in steam turbines. Rubbing was induced at various axial locations along a 4.5 ton test rotor rotating at 3000 r/min and supported by 7 in (178 mm) journal bearings. This article examines the capabilities of bearing-mounted AE transducers for the detection of seal-to-rotor rubbing. It was concluded that the AE technology can detect simulated rubs from the hydrodynamic bearings.

Detecting Shaft-to-Seal Rubbing in Power Generation Turbines With the Acoustic Emission Technology

This paper presents results of an investigation to assess the potential of the acoustic emission (AE) technology for detecting seal-to-rotor rubbing in power generation turbines. The experimental program involved inducing rubs at various axial locations along a 412ton test rotor, rotating at 3000rpm and supported by 7in.(178mm) journal bearings. Whilst there was no change in vibration levels throughout the tests, bearing mounted AE transducers successfully detected all rubbing induced on the rotor.

Damage Mechanisms in Loaded Aramid Composites by Means of Embedded PVDF Acoustic Emission Sensors

Cost-effective and reliable damage detection is critical for the utilization of composite materials due to the relatively localised nature of damage formation and the resultant reduction in structural integrity. Of the methods available, Acoustic Emission (AE) is considered as one potential technology for on-line and in situ monitoring of structural degradation of composite materials. Purpose of this work was to study the interaction between embedded PVDF (polyvinylidene fluoride) transducers and composite samples as well as detect and characterize the failure mechanisms in aramid/epoxy flexural test specimens using acoustic emission data obtained by embedded PVDF film sensors. Furthermore, it has been realized a comparison with surface mounted PVDF data. Results of our previous works (Caneva et al., 2005) dealing with monitoring tensile and flexural behaviour of glass/epoxy composites enabled to extend this methodology to aramid/epoxy composites. The use of Acoustic Emission and Scanning Electron Microscopy (SEM) observations enabled to identify and understand the failure mechanisms of the composites tested. Furthermore, satisfactory results of this work highlighted that the application of PVDF shows promise as a suitable acoustic emission transducer for fibrous composite materials.

The effects of progressive wear on the frequency characteristic of acoustic emission acquired during face milling

Acoustic emission (AE) provides a non-intrusive means of monitoring insert flank wear in face milling. Progressive wear tests of carbide inserts in eight-point milling of annealed En24 steel were instrumented with piezoelectric AE transducers and a non-contact optical interferometer, from which AE frequency information could be extracted. Mean AE frequency was found generally to decrease with wear in agreement with other published studies. Tool indexed measurements enabled the time evolution of the frequency content to be studied on the timescale of a single pass of the insert. The results may be explained by a simple analytical model for AE frequency associated with plastic deformation. The observed AE decay time constants following insert entry decreased with cutting speed, consistent with thermal models of the cutting process. Whereas the results of this study alone would not constitute an independent means of tool wear monitoring, they could provide a diagnosis of tool wear when supplemented with other AE measures and with knowledge of the specific cutting process.

Acoustic emission transducers—development of a facility for traceable out-of-plane displacement calibration

Acoustic emission (AE) is a widely used technique that has been employed for the integrity testing of a range of vessels and structures for many years. The last decade has seen advances in signal processing, such that the reliability of AE technology is now being recognised by a wider range of industries. Furthermore, the need for quality control at the manufacturing stage, and requirements of in-service testing, is encouraging the issue of traceable measurements to be addressed. Currently, no independent calibration service for acoustic emission transducers is available within Europe. The UKs National Physical Laboratory (NPL) is undertaking work to develop a measurement facility for the traceable calibration of AE sensors. Such calibrations can contribute to greater acceptance of AE techniques in general, by meeting quality system and other traceability requirements. In this paper the key issues surrounding the development of such a facility are reviewed, including the need to establish repeatable AE sources, select suitable test blocks and to understand the limitations imposed by AE sensors themselves. To provide an absolute measurement of the displacement on the surface of a test block, laser interferometry is employed. In this way the output voltage of an AE sensor can be directly related to the displacement detected at the block surface. A possible calibration methodology is discussed and preliminary calibration results are presented for a commercially available AE sensor, showing its response to longitudinal wave modes.