Low-cost Piezoelectric Transducer Research Articles

Very High Cycle Fatigue and Ultrasonic Testing in Multiaxial Loading

Abstract Fatigue damage has special relevance on the lifespan of mechanical components and structures, as it takes responsibility for the majority of registered structural failures. Most fatigue testing today still uses uniaxial loads, nonetheless it is generally recognized that multiaxial stresses occur in many full-scale structures. Nowadays, the growing need for greater lifespans forced the study of material behavior under what is nowadays very high cycle fatigue (VHCF) regime, fatigue research beyond 10E07 up to 10E10 cycles. However, most classic fatigue machines are often very costly, both in terms of time and energy, for such an elevated number of cycles. The high costs of equipment and time to conduct such experiments have seen a massive improvement with the introduction of high frequency ultrasonic machines. Presently, it is already possible to characterize materials VHCF response under axial and multiaxial loading conditions in a fraction of the time. Ultrasonic testing machines were first introduced for simpler uniaxial loading and more recently adapted for multiaxial loading conditions. The present work reviews the current background of ultrasonic fatigue testing machines working at 20-kHz frequency, with emphasis on multiaxial fatigue and VHCF. A review of a series of fatigue tests are presented, respectively, axial, shear, axial/shear, and in-plane bi-axial fatigue tests with different combining dimension specimens allowing different shear/axial stress ratios. Special attention will be put into the performance of multiaxial classical cylindrical specimens under tension/torsion and flat cruciform specimens under in-plane bi-axial testing using low-cost piezoelectric transducers. By comparing all so far developed and successfully reached multiaxial specimens and respective methodologies, two base approaches were possible to be established. When designing new specimens or ultrasonic machines, or both, one of the two methods must be followed. To have a well-comprehended methodology and successful design approach will undoubtedly guide future multiaxial ultrasonic method ideas.

A New Acoustic-Based Approach for Assessing Induced Adulteration in Bovine Milk.

Milk is an important dietary requirement for many populations due to its high nutritional value. However, increased demand has also made it prone to fraudulent activity. In this sense, scientists have sought to develop simple, low-cost, and portable techniques to achieve quality control of milk in industry and farms as well. This work proposes a new instrumentation system based on acoustic propagation and advanced signal processing techniques to identify milk adulteration by industrial contaminants. A pair of transmitter-receiver low-cost piezoelectric transducers, configured in a pitch-catch mode, propagated acoustic waves in the bovine milk samples contaminated with 0.5% of sodium bicarbonate, urea, and hydrogen peroxide. Signal processing approaches such as chromatic technique and statistical indexes like the correlation coefficient, Euclidian norm and cross-correlation square difference were applied to identify the contaminants. According to the presented results, CCSD and RMSD metrics presented more effectiveness to perform the identification of milk contaminants. However, CCSD was 2.28 × 105 more sensitivity to distinguish adulteration in relation to RMSD. For chromatic clustering technique, the major selectivity was observed between the contamination performed by sodium bicarbonate and urea. Therefore, results indicate that the proposed approach can be an effective and quick alternative to assess the milk condition and classify its contaminants.

Review of Multiaxial Testing for Very High Cycle Fatigue: From ‘Conventional’ to Ultrasonic Machines

Fatigue is one of the main causes for in service failure of mechanical components and structures. With the development of new materials, such as high strength aluminium or titanium alloys with different microstructures from steels, materials no longer have a fatigue limit in the classical sense, where it was accepted that they would have ‘infinite life’ from 10 million (107) cycles. The emergence of new materials used in critical mechanical parts, including parts obtained from metal additive manufacturing (AM), the need for weight reduction and the ambition to travel greater distances in shorter periods of time, have brought many challenges to design engineers, since they demand predictability of material properties and that they are readily available. Most fatigue testing today still uses uniaxial loads. However, it is generally recognised that multiaxial stresses occur in many full-scale structures, being rare the occurrence of pure uniaxial stress states. By combining both Ultrasonic Fatigue Testing with multiaxial testing through Single-Input-Multiple-Output Modal Analysis, the high costs of both equipment and time to conduct experiments have seen a massive improvement. It is presently possible to test materials under multiaxial loading conditions and for a very high number of cycles in a fraction of the time compared to non-ultrasonic fatigue testing methods (days compared to months or years). This work presents the current status of ultrasonic fatigue testing machines working at a frequency of 20 kHz to date, with emphasis on multiaxial fatigue and very high cycle fatigue. Special attention will be put into the performance of multiaxial fatigue tests of classical cylindrical specimens under tension/torsion and flat cruciform specimens under in-plane bi-axial testing using low cost piezoelectric transducers. Together with the description of the testing machines and associated instrumentation, some experimental results of fatigue tests are presented in order to demonstrate how ultrasonic fatigue testing can be used to determine the behaviour of a steel alloy from a railway wheel at very high cycle fatigue regime when subjected to multiaxial tension/torsion loadings.

Tension/torsion ultrasonic fatigue testing on a railway wheel

Fatigue is one of the main causes for in service failure of mechanical components and structures. The need for the weight reduction of components and a growing need for greater lifespans of equipment, forced the understanding of the fatigue behaviour of materials under multiaxial loading cycles and increased number of loading cycles. It is generally recognized that multiaxial stresses occur in many full-scale structures, being rare the occurrence of pure uniaxial stress states. Nevertheless, the high cost of fatigue materials testing under multiaxial loading and for very high number of cycles, was a serious drawback for the research under these testing conditions. This study is a description of the present status of fatigue testing machines working at a frequency of 20 kHz, and the research that can be carried out in order to perform multiaxial fatigue tests under bi-axial stress states and very high cycle fatigue. Special emphasis will be given to the performance of multiaxial fatigue tests under axial/torsion and in plane bi-axial testing using low cost piezo-electric transducers either in classical cylindrical or in cruciform specimens and associated instrumentation. Together with the description of the testing machines, experimental results of fatigue tests will be presented in order to compare the behaviour of a metallic material from a Railway wheel under multiaxial tension/torsion at very high cycle fatigue.

In-Dressing Acoustic Map by Low-Cost Piezoelectric Transducer

The main difficulty in the grinding process is to identify the correct moment to dress the grinding wheel. Therefore, the cutting tool (grinding wheel) must be monitored. In this context, an innovative technique was developed in this article to obtain an image from the surface of the grinding wheel during the dressing process, based on acoustic images acquired through a piezoelectric diaphragm or piezoelectric buzzer. To this end, scratches (faults) are made on a grinding wheel, after which tests are performed at various dressing depths, and signals are collected by an acoustic emission (AE) sensor and a piezoelectric diaphragm. Based on these signals, frequency bands are evaluated to obtain acoustic images that would accurately and clearly represent the scratches imprinted on the grinding wheel. Finally, the performance of the two sensors (AE sensor and piezoelectric diaphragm) are compared, and the results are analyzed in light of the dressing conditions under study. The results indicate that the piezoelectric diaphragm is as efficient in obtaining acoustic maps of the grinding wheel surface as the AE sensor and in some machining conditions, it provides superior results to those obtained when monitoring the tool with the AE sensor.

Low-Cost Piezoelectric Transducer for Ceramic Grinding Monitoring

This paper presents a performance assessment of the low-cost thin-disk piezoelectric diaphragm (PZT) for the surface integrity monitoring in the grinding operation. PZT diaphragms are flexible, low-cost, and smart devices, making them promising for applications in manufacturing fields, such as in the grinding operation. To accomplish this, experimental grinding tests were carried out on workpieces of ceramic material covering from slight to severe machining conditions. A low-cost thin-disk PZT diaphragm was assessed, which was attached to the workpiece holder using a cyanoacrylate glue. In addition, a commercial and consolidated acoustic emission (AE) sensor was used as a baseline for comparative purpose. Likewise, the surface roughness measurements were performed after grinding each workpiece to support the sensor monitoring system. Feature extraction using frequency spectrum and time domain analysis were applied to the sensor signals by means of fast Fourier transform (FFT), digital bandpass filters, root mean square (RMS) value, and power deviation (DPO) metric. The results indicate a clear relationship between the thin-disk PZT diaphragm and the conventional AE sensor for surface integrity monitoring in grinding, which proves the effectiveness and reliability of an innovative sensing device for the present application.

Data-driven semi-supervised and supervised learning algorithms for health monitoring of pipes

• A semi-supervised and a supervised learning algorithm for health monitoring of pipes is proposed. • The proposed approaches are data-driven, hence circumventing the need of computationally prohibitive model-based approaches. • The proposed approaches can perform damage detection in pipes with only two actuator-sensor pairs. • Efficacy of the proposed approaches are demonstrated using experimental data from two cast iron pipes. The use of guided ultrasonic waves (GUWs) for SHM of pipelines has been a popular method for over three decades. The superiority of GUWs over traditional vibration-based techniques lie in its ability to detect small damages (cracks and corrosion) over a satisfactory length of a pipeline. The physics of the system, however, is extremely involved that renders model-based techniques computationally prohibitive. Data-driven approaches, based on statistical learning algorithmsare far more suitable in such scenarios. In this paper, we propose two data-driven techniques, involving a semi-supervised and a supervised learning approach, for damage detection in pipes. In addition to circumventing the use of a model-based approach, the proposed approaches also aid in reducing the number of sensors deployed, leading to reductions in maintenance costs. The semi-supervised learning-based approach detects the presence of damage using a hierarchical clustering-based algorithm. The supervised learning-based approach performs damage localization in a multinomial logistic regression framework. We validate the proposed algorithms by acquiring guided wave responses from experimental pipes in a pitch-catch configuration using low-cost piezoelectric transducers. We demonstrate that our fully data-driven techniques accurately detect and localize cracks on two cast iron pipes of different lengths using a combination of two sensors.

A chromatic technique for structural damage detection under noise effects based on impedance measurements

Structural health monitoring (SHM) based on electromechanical impedance (EMI) measurements has been extensively studied in recent years and stands out as an effective damage detection approach. The EMI method commonly uses low-cost piezoelectric transducers and structural diagnosis can be accomplished by analyzing the changes in the impedance measurements from the transducers attached to the host structure. Although many studies have proved the effectiveness of the EMI method, one of the most reported problems is the variations in the impedance caused by external noise, which can alter the impedance measurements and limit the effectiveness of the detection and quantification of damage using conventional basic damage indices. Therefore, this paper proposes a new approach for structure feature extraction in noisy environments based on the chromatic signal processing technique associated with the Hinkley criterion, thus aiming to expand the applicability of the EMI method to real applications. Experimental tests were carried out on aluminum and carbon fiber composite plates subjected to various signal noise levels. The experimental results indicate that the proposed approach is more effective for damage detection compared to the basic indices commonly reported in the literature.

New signal processing approach for structural health monitoring in noisy environments based on impedance measurements

The electro-mechanical impedance (EMI) technique is one of the most promising structural health monitoring (SHM) approaches for material damage detection, which is based on impedance measurements of low-cost piezoelectric transducers. However, numerous practical issues such as signal noise effects caused by environmental conditions can alter signal measurements and limit the capabilities of the EMI technique when the characterization of damage is performed using conventional basic indices. Therefore, this paper proposes a new index for structure feature extraction based on the cross-correlation signal processing technique that can be applied in real noisy environment. The proposed index was evaluated in the frequency domain, where the damage detection is performed directly on the electrical impedance measurements of the transducer, as well as on the time domain, which is based on the wavelet transform applied to the transducer response signal. Experimental tests were carried out on a damaged aluminium structure subject to various signal noise levels. Experimental results revealed that the proposed approach for material feature extraction under noisy environments proved to be effective for detecting damage, thus enhancing the reliability and expanding the applicability of the EMI technique.

Characterization of Impact Damage in Composite Plates

Low-velocity impact on composites typically produces a barely visible damage at the impacted surface. The internal defects can be complex, consisting of multimode damage and the extent of the impact damage normally spreads across the thickness under the impacted surface. The characterization of impact damage in composites can be very complicated and varies for every different composite structure. In this paper, independent characterization of the low-velocity impact damage on carbon-fiber/epoxy plates using three different non-destructive evaluation methods were used. The goal is to demonstrate the ability of guided ultrasonic waves imaging technique and compared to the more widely employed techniques such as X-ray imaging and ultrasonic immersion C-scan. It was demonstrated that the low frequency A0 guided ultrasonic wave mode generated by a low-cost piezoelectric transducer can be successfully employed to detect impact damage in composite plates and managed to estimate the size and shape of the impact damage.

Electromechanical impedance (EMI) technique as alternative to monitor workpiece surface damages after the grinding operation

Electromechanical impedance (EMI) technique has been employed in detection of structural failure in civil and mechanical structures because of its non-destructive property and easy implementation of small and inexpensive piezoelectric transducers that are attached to the structures, which lead to cost reduction as well as lesser dependence of manual inspection methods. In this technique, the capsule is excited by applying a sinusoidal voltage to generate waves to propagate throughout the structure. From the impedance signature of the structure without any damage, any structural change can be detected by measuring the electrical impedance of the piezoelectric (PZT) patch. Based on its real potentiality and because of its non-destructive characteristics, this work aimed to employ the EMI technique as the first alternative to monitor workpiece surface damages after grinding operation with a conventional abrasive grinding wheel. EMI measurements were performed by using a low-cost PZT transducer and under controlled environmental conditions. Microhardness and surface roughness of the machined surfaces, as well as grinding power, were also measured to detect any damage in the machined surface and to stablish relationship with the EMI technique. From the damage indices root mean square deviation (RMSD) and correlation coefficient deviation metric (CCDM), surface alterations on the ground surfaces were inferred by the EMI method. Also, it was observed a good correlation between the EMI technique and the other output parameters that were investigated in this work, such as surface roughness and power grinding, thereby posing as a non-destructive, low-cost, and viable technique to monitor workpiece surface damages in the grinding operation.

Impedance-based damage detection under noise and vibration effects

The electro-mechanical impedance technique has been extensively studied in recent decades as a non-destructive method for detecting structural damage in structural health monitoring applications using low-cost piezoelectric transducers. Although many studies have reported the effectiveness of this detection method, numerous practical problems, such as the effects of noise and vibration, need to be addressed to enable this method’s effective use in real applications. Therefore, this article presents an experimental analysis of noise and vibration effects on structural damage detection in impedance-based structural health monitoring systems. The experiments were performed on an aluminum bar using two piezoelectric diaphragms, where one diaphragm was used to measure the electrical impedance signatures and the other diaphragm was used as an actuator to generate noise and controlled vibration. The effects of noise and vibration on impedance signatures were evaluated by computing the coherence function and basic damage indices. The results indicate that vibration and noise significantly affect the threshold of the lowest detectable damage, which can be compensated by increasing the excitation signal of the piezoelectric transducer.

Design and initial testing of a piezoelectric sensor to quantify aeolian sand transport

Abstract This paper describes a sensor for measuring the mass flux of aeolian sand transport based on a low-cost piezo-electric transducer. The device is able to measure time series of aeolian sand transport. Maximum fluxes of 27 mg per second can be achieved. The design includes a sand trap, an electronic amplifier circuit and an embedded system for data collection. A field test was performed, where the basis for signal interpretation and the corresponding measurements of aeolian sand transport are presented. The sensor successfully measures fluxes driven by sea breezes of 10 ms −1 , showing the importance of this process for dune-building in the region.

New acoustic instrumentation for the SIMPLE superheated droplet detector

Previous acoustic instrumentation of the SIMPLE detector has relied on the use of a low-cost piezoelectric transducer, which was generally unable to provide discrimination between true bubble nucleation events and ambiental sources of acoustic noise. The use of a high-quality electret microphone and adaptive electronics is shown to provide this capacity through reduced noise levels and distinct fast Fourier transforms of the event registration.

A self-adapting sonar ranging system based on digital polarity correlators

The logic capacity of programmable logic devices has reached a level which allows their useas digital correlators in sonar signal-processing applications. The devices can be modified atalmost any point during use in response to changes in the input data or processingenvironment. This paper reports on the development of a self-adapting ultrasonicsystem consisting of digital polarity correlators for multiple-user ranging. Thedigital signal-processing system is integrated into the single programmable logicdevice. The system is able to provide good performance in different conditionsby automatically changing both the signal that drives the transmitter and therange properties of the matched filter receiver. Such an approach allows distancemeasurements up to a few metres while still eliminating frequent misreadings caused bycrosstalk or external ultrasound sources with the low-cost piezoelectric transducers.