Acousto-ultrasonic Waves Research Articles

Direct-write nanocomposite sensor array for ultrasonic imaging of composites

To improve the ultrasonic imaging of composites, an all-printed nanocomposite sensor array (APNSA) is developed using a direct-write approach. Individual sensing elements of APNSA are inkjet printed by directly writing graphene/poly (amic acid) (PAA)-based nanocomposite ink on Kapton film substrates, with an ultra-thin thickness of ∼1 μm only. APNSA is morphologically tuned at a nano scale to be sensitive to acousto-ultrasonic waves of a broad band regime. Each sensing element features a homogenous and consolidated nanostructure, with which transient change of tunneling resistance between adjacent graphene nanoplatelets in the polyimide (PI) matrix can be triggered when the element is loaded with acousto-ultrasonic waves. The triggered quantum tunneling effect endows APNSA with capability to perceive dynamic strains in a broadband regime with high fidelity and accuracy. Compared with a conventional ultrasonic phased array which is of a low degree of compatibility with composites, APNSA can be fully integrated with the inspected composites. In conjunction with the use of the additively manufactured APNSA, ultrasonic imaging of composites can be implemented, spotlighting a nature of full integration of APNSA with composites for in situ structural health monitoring and anomaly detection, yet without degrading the original integrity of the composites.

Temperature effect on all-inkjet-printed nanocomposite piezoresistive sensors for ultrasonics-based health monitoring

The sensing performance of nanocomposite piezoresistive sensors in acquiring broadband acousto-ultrasonic wave signals is scrutinized in an extensive regime of temperature variation from −60 to 150 °C, which spans the thermal extremes undergone by most aircraft and spacecraft. Ultralight and flexible, the sensors are all-inkjet-printed using a drop-on-demand additive manufacturing approach, and then optimized sensitive to the ultraweak disturbance induced by acousto-ultrasonic waves in virtue of quantum tunneling effect. Under high-intensity thermal cycles from −60 to 150 °C, the sensors have proven stability and accuracy in responding to signals in a broad band from static to half a megahertz. Compared with conventional broadband sensors such as piezoelectric wafers, this genre of inkjet-printed nanocomposite sensors avoids the influence of increased dielectric permittivity during the measurement of high-frequency signals at elevated temperatures. Use of the sensors for characterizing undersized cracks in a typical aerospace structural component under acute temperature variation has spotlighted the alluring application potentials of the all-inkjet-printed nanocomposite sensors in implementing in-situ structural health monitoring for key aircraft and spacecraft components.

Theoretical and experimental investigation of Lamb waves excited by partially debonded rectangular piezoelectric transducers

The paper proposes a new hybrid approach technique to simulate acousto-ultrasonic wave excitation and propagation due to operation of the partially debonded piezoelectric transducer attached to a plate-like structure. The semi-analytical boundary integral equation method is applied to calculate guided waves propagation in the unbounded structures and to separate different guided waves in the piezo-induced wave-fields. The obtained model is verified experimentally using the scanning laser Doppler vibrometry. Eigenfrequencies are calculated and analysed for various sizes of the transducer and for different bonding conditions between the transducer and the waveguide. The impact of the transducer’s height, size and debonding area on symmetric and antisymmetric Lamb waves excitation is analysed. The paper demonstrates that one-sided debonding of the transducer exerts intense influence on the distribution of the wave energy among the excited Lamb wave modes, while center debonding has a sizable impact only at relatively high frequencies.

Effect of different loading systems on acousto-ultrasonic characteristics of concrete under axial compression

Abstract In order to study the effects of different loading systems and stress history on the damage evolution of concrete, the acoustic emission characteristic parameters, the acoustic emission release rate, acousto-ultrasonic pulse wave velocity and the Rv value were compared and analyzed in the process of axial compression damage of concrete with different water cement ratios under step and intermittent loading. In this paper, the intermittent period led to a certain degree of “closure” in the internal microcracks of concrete, thus in the same HB to HD holding period, the acoustic emission release rate, mean value of acoustic emission characteristic parameters and mean acousto-ultrasonic pulse wave velocity under intermittent loading were all higher than that of step loading. The acousto-ultrasonic pulse wave velocity decreased with the increasing of the stress level and increased during the IA to ID intermittent period. Among them, with the decreasing of water cement ratio (from 0.6 to 0.4), the acoustic emission cumulative hits and the acoustic emission release rate increased, while the acousto-ultrasonic pulse wave velocity increased, and there were some differences in the acoustic emission release rate of the two loading systems at different water cement ratios. Furthermore, the Rv value decreased with the decreasing of water cement ratio in the IA to IC intermittent period and in the ID intermittent period, the Rv value increased with the decreasing of water cement ratio, thus the Rv value could reflect the evolution law of concrete with different water cement ratios during the intermittent period. The obtained results show that the acoustic emission and acousto-ultrasonic characteristic parameters are not consistent under two different loading systems, so research in this paper can provide a theoretical basis for the damage identification and analysis of concrete structures under cyclic loading based on the acousto-ultrasonic technology in practical engineering.

A Quantitative Approach for the Bone-implant Osseointegration Assessment Based on Ultrasonic Elastic Guided Waves

Quantitative and reliable monitoring of osseointegration will help further evaluate the integrity of the orthopaedic construct to promote novel prosthesis design and allow early mobilisation. Quantitative assessment of the degree or the lack of osseointegration is important for the clinical management with the introduction of prosthetic implants to amputees. Acousto-ultrasonic wave propagation has been used in structural health monitoring as well as human health monitoring but so far has not extended to osseointegrated implants or prostheses. This paper presents an ultrasonic guided wave approach to assess the osseointegration of a novel implant. This study explores the potential of integrating structural health monitoring concepts into a new osseointegrated implant. The aim is to demonstrate the extension of acousto-ultrasonic techniques, which have been widely reported for the structural health monitoring of engineering structures, to assess the state of osseointegration of a bone and implant. To illustrate this potential, this paper will report on the experimental findings which investigated the unification of an aluminium implant and bone-like geometry surrogate. The core of the test specimen is filled with silicone and wrapped with plasticine to simulate the highly damped cancellous bone and soft tissue, respectively. To simulate the osseointegration process, a 2-h adhesive epoxy is used to bond the surrogate implant and a bone-like structure. A series of piezoelectric elements are bonded onto the surrogate implant to serve as actuators and sensors. The actuating piezoelectric element on an extramedullary strut is excited with a 1 MHz pulse signal. The reception of the ultrasonic wave by the sensing elements located on the adjacent and furthest struts is used to assess the integration of this implant to the parent bone structure. The study shows an Osseointegration Index can be formulated by using engineering and acousto-ultrasonic methods to measure the unification of a bone and implant. This also highlights a potential quantitative evaluation technique regardless of bone-implant geometry and soft tissue damping.

Applications of a nanocomposite-inspired in-situ broadband ultrasonic sensor to acousto-ultrasonics-based passive and active structural health monitoring

A novel nanocomposite-inspired in-situ broadband ultrasonic sensor previously developed, with carbon black as the nanofiller and polyvinylidene fluoride as the matrix, was networked for acousto-ultrasonic wave-based passive and active structural health monitoring (SHM). Being lightweight and small, this kind of sensor was proven to be capable of perceiving strain perturbation in virtue of the tunneling effect in the formed nanofiller conductive network when acousto-ultrasonic waves traverse the sensor. Proof-of-concept validation was implemented, to examine the sensor performance in responding to acousto-ultrasonic waves in a broad frequency regime: from acoustic emission (AE) of lower frequencies to guided ultrasonic waves (GUWs) of higher frequencies. Results have demonstrated the high fidelity, ultrafast response and high sensitivity of the sensor to acousto-ultrasonic waves up to 400kHz yet with an ultra-low magnitude (of the order of micro-strain). The sensor is proven to possess sensitivity and accuracy comparable with commercial piezoelectric ultrasonic transducers, whereas with greater flexibility in accommodating curved structural surfaces. Application paradigms of using the sensor for damage evaluation have spotlighted the capability of the sensor in compromising “sensing cost” with “sensing effectiveness” for passive AE- or active GUW-based SHM.

Subsurface pressure profiling: a novel mathematical paradigm for computing colony pressures on substrate during fungal infections.

Colony expansion is an essential feature of fungal infections. Although mechanisms that regulate hyphal forces on the substrate during expansion have been reported previously, there is a critical need of a methodology that can compute the pressure profiles exerted by fungi on substrates during expansion; this will facilitate the validation of therapeutic efficacy of novel antifungals. Here, we introduce an analytical decoding method based on Biot’s incremental stress model, which was used to map the pressure distribution from an expanding mycelium of a popular plant pathogen, Aspergillus parasiticus. Using our recently developed Quantitative acoustic contrast tomography (Q-ACT) we detected that the mycelial growth on the solid agar created multiple surface and subsurface wrinkles with varying wavelengths across the depth of substrate that were computable with acousto-ultrasonic waves between 50 MHz–175 MHz. We derive here the fundamental correlation between these wrinkle wavelengths and the pressure distribution on the colony subsurface. Using our correlation we show that A. parasiticus can exert pressure as high as 300 KPa on the surface of a standard agar growth medium. The study provides a novel mathematical foundation for quantifying fungal pressures on substrate during hyphal invasions under normal and pathophysiological growth conditions.

Acousto-ultrasonics-based fatigue damage characterization: Linear versus nonlinear signal features

Engineering structures are prone to fatigue damage over service lifespan, entailing early detection and continuous monitoring of the fatigue damage from its initiation through growth. A hybrid approach for characterizing fatigue damage was developed, using two genres of damage indices constructed based on the linear and the nonlinear features of acousto-ultrasonic waves. The feasibility, precision and practicability of using linear and nonlinear signal features, for quantitatively evaluating multiple barely visible fatigue cracks in a metallic structure, was compared. Miniaturized piezoelectric elements were networked to actively generate and acquire acousto-ultrasonic waves. The active sensing, in conjunction with a diagnostic imaging algorithm, enabled quantitative evaluation of fatigue damage and facilitated embeddable health monitoring. Results unveiled that the nonlinear features of acousto-ultrasonic waves outperform their linear counterparts in terms of the detectability. Despite the deficiency in perceiving small-scale damage and the possibility of conveying false alarms, linear features show advantages in noise tolerance and therefore superior practicability. The comparison has consequently motivated an amalgamation of linear and nonlinear features of acousto-ultrasonic waves, targeting the prediction of multi-scale damage ranging from microscopic fatigue cracks to macroscopic gross damage.

Evaluation of fatigue cracks using nonlinearities of acousto-ultrasonic waves acquired by an active sensor network

There has been increasing interest in using the nonlinear features of acousto-ultrasonic (AU) waves to detect damage onset (e.g., micro-fatigue cracks) due to their high sensitivity to damage with small dimensions. However, most existing approaches are able to infer the existence of fatigue damage qualitatively, but fail to further ascertain its location and severity. A damage characterization approach, in conjunction with the use of an active piezoelectric sensor network, was established, capable of evaluating fatigue cracks in a quantitative manner (including the co-presence of multiple fatigue cracks, and their individual locations and severities). Fundamental investigations, using both experiment and enhanced finite element analysis dedicated to the simulation of nonlinear AU waves, were carried out to link the accumulation of nonlinearities extracted from high-order AU waves to the characteristic parameters of a fatigue crack. A probability-based diagnostic imaging algorithm was developed, facilitating an intuitive presentation of identification results in images. The approach was verified experimentally by evaluating multi-fatigue cracks near rivet holes of a fatigued aluminum plate, showing satisfactory precision in characterizing real, barely visible fatigue cracks. Compared with existing methods, this approach innovatively (i) uses permanently integrated active sensor networks, conducive to automatic and online health monitoring; (ii) characterizes fatigue cracks at a quantitative level; (iii) allows detection of multiple fatigue cracks; and (iv) visualizes identification results in intuitive images.

Imaging-based quantitative characterization of fatigue crack for structural integrity monitoring using nonlinear acousto-ultrasonics and active sensor networks

The majority of today’s damage detection techniques rely substantially on linear macroscopic changes in either global vibration signatures or local wave scattering phenomena. However, damage in real-world structures often initiates from fatigue cracks at microscopic levels, presenting highly nonlinear characteristics which may not be well evidenced in these linear macroscopic changes. It is of great significance but also a great challenge to quantitatively characterize micro-fatigue cracks without terminating the normal operation of an engineering structure. This is a critical step towards automatic and online structural integrity monitoring (SIM). By exploring the nonlinearities of higher-order acousto-ultrasonic (AU) waves upon interaction with fatigue cracks, a damage characterization approach, in conjunction with use of an active piezoelectric sensor network, was established, with the particular capacity of evaluating multiple fatigue cracks at a quantitative level (including the co-presence of multiple cracks, and their individual locations and severities). Identification results were presented in pixelated images using an imaging algorithm, enabling visualization of fatigue cracks and depiction of overall structural integrity in a quantitative, rapid and automatic manner. The effectiveness of the proposed technique was demonstrated by experimentally characterizing multiple fatigue cracks near rivet holes in aluminium plates.

Design of copper/carbon-coated fiber Bragg grating acoustic sensor net for integrated health monitoring of nuclear power plant

A nuclear power plant (NPP) is a harsh environment that gives rise to age-related degradation of the plant structures, and eventually leads to radiation leakage that threatens humans. Integrated structural health monitoring (ISHM) technology is a strong candidate for the prevention of the NPP accidents during operation. Prior studies have shown that fiber Bragg gratings (FBGs) and metal-coated fibers have good radiation and high temperature resistance. In this study, a FBG acoustic sensor using a metallic adhesive for installation and a relatively economical copper/carbon (Cu/C)-coated fiber is developed for ISHM of high temperature NPP structures. A chemical method is proposed to remove the Cu/C coating. A 5 mm FBG was successfully inscribed in a Ge-doped silica core through a 7 mm-long silica section with the coating removed. The Cu/C-coated fiber with the same core/clad structure as the standard SMF allowed no-loss fusion splicing, and showed good adaptability to the economical standard fiber, adaptor, connector, and instruments. It showed also good thermal resistance (<345 °C) with no degradation in optical power during the optical transmission. The metallic adhesive used to install the FBG in a one-end-free configuration showed superior bonding reliability during temperature cycles ranging from 25 °C to 345 °C. The FBG reflectivity was stabilized at a 58% drop from the initial reflectivity, and the Cu/C-coated FBG sensor using the metallic adhesive could successfully detect the acousto-ultrasonic waves generated by pencil lead breaking and laser beam excitation.

Design of Fiber Bragg Grating Acoustic Sensor for Structural Health Monitoring of Nuclear Power Plant

Nuclear power plant (NPP) is commonly categorized as a harsh environment that gives rise to age-related degradation on the structures of plant and eventually leads to radiation leakage that threatens human. Integrated structural health monitoring (ISHM) technology is a strong candidate for NPP accidents. The emergence of optical fiber technology into SHM system for NPP was greatly interested by researchers, and also prior research works have shown that the fiber Bragg grating (FBG) was able to retain its reflectivity under radiation exposure. In this paper, a metal-coated fiber was newly used to develop a FBG acoustic sensor for ISHM of NPP. The 7 mm length of aluminum, copper/carbon coatings were successfully removed with sodium hydroxide and nitric acid solutions. A 5 mm FBG was successfully inscribed in the silica core through the 7 mm long coating-removed silica section of copper/carbon-coated fiber and the initial reflectivity was 71%. Then, the FBG acoustic sensor was developed in one-end-free FBG configuration on the stainless steel vessel using a high temperature metallic adhesive. The reflective power of the sensor was stabilized at 345°C during the high temperature elevation cyclic process. The FBG acoustic sensor showed good response to the acousto-ultrasonic waves during pencil-lead breaking and laser ultrasonics tests. The high temperature FBG acoustic sensor written in the cost-effective metal/carbon is feasible to be used for ISHM of the NPP.

Wave propagation in elastic solids undergoing damage and growth process

Modeling and analysis of wave propagation in elastic solids undergoing damage and growth process are reported in this paper. Two types of diagnostic problems, (1) the propagation of waves in the presence of a slow growth process and (2) the propagation of waves in the presence of a fast growth process, are considered. The proposed model employs a slow and a fast time scale and a homogenization technique in the wavelength scale. A detailed analysis of wave dispersion is carried out. A spectral analysis reveals certain low-frequency bands, where the interaction between the wave and the growth process produces acoustic metamaterial-like behavior. Various practical issues in designing an efficient method of acousto-ultrasonic wave based diagnostics of the growth process are discussed. Diagnostics of isotropic damage in a ductile or quasi-brittle solid by using a micro-second pulsating signal is considered for computer simulations, which is to illustrate the practical application of the proposed modeling and analysis. The simulated results explain how an estimate of signal spreading can be effectively employed to detect the presence of a steady-state damage or the saturation of a process.

Time-Domain Spectral Element Method for Built-In Piezoelectric-Actuator-Induced Lamb Wave Propagation Analysis

An investigation was performed to develop a computational model based on a spectral element method to simulate piezoelectric-actuator-induced acousto-ultrasonic wave propagation in a metallic structure. The model solves the coupled electromechanical field equations simultaneously in both three-dimensional and two-dimensional plane strain conditions, and so it can accept any arbitrary waveform of electrical voltage as input to any piezoelectric transducer and produce piezoelectric sensor output in voltage as a result of the excitation generated by the transducer. Basically, the model inputs electrical voltage to actuators and outputs electrical signals of sensors. To visualize the transient dynamic wave motions in the structure generated by the transducer, the code is integrated with commercial pre/postprocessing software to provide graphical outputs of the dynamic deformations of the structure. The code was verified by comparison with experimental results. Performance of the model was examined in terms of solution convergence compared with the finite element method.

Impact wave and damage detections using a strain-free fiber Bragg grating ultrasonic receiver

A strain-free mobile fiber Bragg grating (FBG) ultrasonic receiver is applied for the impact-related experiments of carbon fiber reinforced plastic laminates. The strain-free FBG sensor detects an impact-induced acousto-ultrasonic wave and its responses are compared with those of a piezoelectric sensor. Ultrasonic mode wavelength-related averaging effect in FBG ultrasonic sensors is also reported. The mobile FBG sensor can be useful for the acoustic characterization and the sensor placement optimization being required before construction of a built-in FBG network. Finally, the mobility of the strain-free FBG sensor head is extended to ultrasonic scanning application. Based on its high scanning spatial resolution, impact damage sizing is conducted more precisely.

Structural health monitoring using scanning laser vibrometry: II. Lamb waves for damagedetection

Guided acousto-ultrasonic waves have great potential for structural health monitoringapplications. Lamb waves are the most widely used guided waves for damagedetection in metallic and composite structures. These waves, once introduced to astructure, can propagate long distances. Nevertheless the method still requires asignificant number of transducers for monitoring large structures. Recent years haveshown various applications of non-contact generation and sensing of Lamb waves.This paper proposes a new scanning technique based on laser vibrometry. Themethod is applied for damage detection in simple aluminium plates and validatedusing numerical simulations. The study shows the potential of the method forsimple and rapid detection, location and monitoring of damage in structures.

Modelling of Lamb waves for damage detection in metallic structures: Part II. Waveinteractions with damage

Lamb waves have shown great potential for structural health monitoring. The technique isbased on guided ultrasonic waves introduced into a structure at one point and sensed at adifferent location. Damage in a structure is identified by a change in the output signal.However, previous studies show that even simple structural configurations can lead tocomplex response signals. Therefore a knowledge and understanding of wave propagationcan ease the interpretation of damage detection results. This paper reports anapplication of the local interaction simulation approach for Lamb wave propagationmodelling in metallic structures. The focus of the analysis is on two-dimensionalwave interactions with slot-type defects. The method shows the potential forcomplex modelling of acousto-ultrasonic waves in damage detection applications.

Nondestructive Evaluation and Characterization of Microfibers Modified Textile Carbon-Phenolic and Carbon-Carbon Composites

Short carbon microfibers have been added to carbon-phenolic fabric composites of plain, eight-harness satin (8HS) and stretch broken 8HS weave architectures. The resulting composites have been studied for their interlaminar mechanical properties as well as process related material evolution mechanisms that result in carbon-carbon (C-C) composites. Dry coupling ultrasonic A-scan velocity measurements, immersion type ultrasonic C-scan technique, acousto-ultrasonic (AU) stress wave factor measurements and vibration damping ratio based nondestructive evaluation-characterization (NDE-C) test parameters were studied in relation to the carbon-phenolic and C-C composites. Interlaminar shear strength (ILSS) tests were conducted along with acoustic emission (AE) monitoring. From the various NDE techniques accompanied by mechanical tests and processing stages, the dependence of fabric weave architectures with short microfibers has been established.

Spectral analysis of acousto-ultrasonic waves for defect sizing

A study of the feasibility of employing acousto-ultrasonic (AU) testing to size defects has been carried out using a model system consisting of a single defect in a glass matrix. A through-transmission mode was used for the AU testing and the output signals were analysed by a counting technique (cumulative ringdown count, CRDC) and spectral analysis. The CRDC decreased with increasing defect size but the scatter in the data did not allow accurate sizing. The frequency spectra exhibited multiple peaks which occurred at constant frequency intervals. The frequency interval decreased with increasing defect size and this could be attributed to either multiple time delay or a resonance effect or to both phenomena. Most evidence supported the resonance effect and good agreement was found between defect size calculated from AU data using a resonance theory and that measured optically.