Search

This paper addresses the critical need for robust quality assurance in commercial refractories, emphasizing the role of Ultrasonic Pulse Velocity (UPV) testing in assessing material integrity. Traditionally, UPV testing has become a standard practice for evaluating refractory, ceramic, and concrete materials after production. However, this study introduces an innovative approach by advocating the integration of UPV tests at an intermediate stage prior to temperature curing. The primary objective is to predict the performance of refractory samples under compressive loading, examine intrinsic damages, and analyse ultrasonic characteristics in both time and frequency domains. Experimental investigations introduce damages, including artificial voids simulating various defect types, both before and during heat treatment. Analysis of UPV, Root Mean Squared (RMS) values of frequency spectra, and the acoustic nonlinearity parameter (ANP) reveals substantial differences between healthy and damaged regions. The mean UPV and RMS values consistently decrease in damaged regions, indicating sensitivity to defect severity. The ANP is sensitive to damage size, especially with higher frequency transducers detecting subtle changes more effectively. These findings emphasise the potential for proactive testing at an intermediate stage in the manufacturing process, enhancing the structural integrity and overall quality of refractory materials. [ABSTRACT FROM AUTHOR]

This study aimed to assess liver fibrosis in rabbits by deep learning models based on acoustic nonlinearity maps. Injection of carbon tetrachloride was used to induce liver fibrosis. Acoustic nonlinearity maps, which were built by data of echo signals, were used as input data for deep learning model. Convolutional neural network (CNN), CNN combined with support vector machine (SVM), CNN combined with random forest and CNN combined with logistic regression were used as deep learning model. Nested 10-fold cross-validation was used to search hyperparameters and evaluate performance of models. Histologic examination of liver specimens of the rabbits was performed to evaluate the fibrosis stage. Receiver operator characteristic curve and area under curve (AUC) were used for estimating the probability of the correct prediction of liver fibrosis stages. A total of 600 acoustic nonlinearity maps were used. Model of CNN combined with SVM demonstrated the best diagnostic performance compared with all other methods for diagnosis of significant fibrosis (≥F2, AUC = 0.82), advanced fibrosis (≥F3, AUC = 0.88) and cirrhosis (F4, AUC = 0.90). Model of CNN showed the second highest AUCs. The deep learning model based on acoustic nonlinearity maps demonstrated potential for evaluation of liver fibrosis.

A nonlinear acoustic response can be an effective indicator of early-stage damage in a material. However, it is challenging to use this approach to assess thermal damage in highly attenuating materials such as polymethyl methacrylate by measurement of the generated second harmonics, which are present at double the fundamental frequency of the primary waves. Using the quasi-static components (QSCs) generated by ultrasonic-wave propagation in damaged structures—an approach that combines the high sensitivity of acoustic nonlinearities with low attenuation—has great potential for evaluation of early-stage damage in highly attenuating materials. In this paper, an experimental approach for directly detecting the QSC pulses from ultrasonic-wave tone bursts is proposed. In this approach, a high-frequency longitudinal-wave transducer is used for exciting the primary ultrasonic-wave tone burst, and the co-propagating QSC pulse is detected using a low-frequency ultrasonic transducer. The phase-reversal technique is employed to enhance the signal-to-noise ratio of the QSC pulse and to counteract the signals of the primary ultrasonic waves. In the experimental results, variations in the amplitudes of the QSC pulse in response to different degrees of thermal damage are clearly illustrated in a repeatable manner, even though the changes in the linear ultrasonic features in these specimens are negligible. These results indicate that the measurement of QSC pulse generation from ultrasonic-wave propagation is a promising alternative for evaluating thermal degradation in highly attenuating materials. [ABSTRACT FROM AUTHOR]

The detection of bolt loosening using vibro-acoustic modulation (VAM) has been increasingly investigated in the past decade. However, conventional nonlinear coefficients, derived from theoretical analysis, are usually based on the assumption of ideal wave–surface interactions at the joint interfaces. Such coefficients show a poor correlation with the tightening torque when the joint is under the combined influences of structural and material nonlinearities. A reliable inspection method of residual bolt torque is proposed in this study using support vector regression (SVR) with acoustic features from VAM. By considering the material intrinsic nonlinearity (MIN) and dissipative nonlinearity (DN), the responses of aluminum–aluminum and composite–composite bolted joints during the VAM test were accurately simulated. The SVRs were subsequently established based on the database built by combining simulated and experimental nonlinear spectral features when the joints were inspected at different scenarios. The results show that the evaluation of residual torque using the SVR models driven by the acoustic nonlinear responses had higher accuracy compared to the conventional nonlinear coefficients. Requiring limited experimental data, the proposed method can achieve a reliable inspection of bolt torque by including the simulated data in the machine training. [ABSTRACT FROM AUTHOR]

This study aimed to assess liver fibrosis in rabbits by deep learning models based on acoustic nonlinearity maps. Injection of carbon tetrachloride was used to induce liver fibrosis. Acoustic nonlinearity maps, which were built by data of echo signals, were used as input data for deep learning model. Convolutional neural network (CNN), CNN combined with support vector machine (SVM), CNN combined with random forest and CNN combined with logistic regression were used as deep learning model. Nested 10-fold cross-validation was used to search hyperparameters and evaluate performance of models. Histologic examination of liver specimens of the rabbits was performed to evaluate the fibrosis stage. Receiver operator characteristic curve and area under curve (AUC) were used for estimating the probability of the correct prediction of liver fibrosis stages. A total of 600 acoustic nonlinearity maps were used. Model of CNN combined with SVM demonstrated the best diagnostic performance compared with all other methods for diagnosis of significant fibrosis (≥F2, AUC = 0.82), advanced fibrosis (≥F3, AUC = 0.88) and cirrhosis (F4, AUC = 0.90). Model of CNN showed the second highest AUCs. The deep learning model based on acoustic nonlinearity maps demonstrated potential for evaluation of liver fibrosis. [ABSTRACT FROM AUTHOR]

The detection of bolt loosening using vibro-acoustic modulation (VAM) has been increasingly investigated in the past decade. However, conventional nonlinear coefficients, derived from theoretical analysis, are usually based on the assumption of ideal wave–surface interactions at the joint interfaces. Such coefficients show a poor correlation with the tightening torque when the joint is under the combined influences of structural and material nonlinearities. A reliable inspection method of residual bolt torque is proposed in this study using support vector regression (SVR) with acoustic features from VAM. By considering the material intrinsic nonlinearity (MIN) and dissipative nonlinearity (DN), the responses of aluminum–aluminum and composite–composite bolted joints during the VAM test were accurately simulated. The SVRs were subsequently established based on the database built by combining simulated and experimental nonlinear spectral features when the joints were inspected at different scenarios. The results show that the evaluation of residual torque using the SVR models driven by the acoustic nonlinear responses had higher accuracy compared to the conventional nonlinear coefficients. Requiring limited experimental data, the proposed method can achieve a reliable inspection of bolt torque by including the simulated data in the machine training.

Background: High‐intensity focused ultrasound (HIFU) has been in clinical use for a variety of solid tumors and cancers. Accurate and reliable calibration is in a great need for clinical applications. An extracorporeal clinical HIFU system applied for the investigational device exemption (IDE) to the Food and Drug Administration (FDA) so that evaluation of its characteristics, performance, and safety was required. Methods: The acoustic pressure and power output was characterized by a fiber optic probe and a radiation force balance, respectively, with the electrical power up to 2000 W. An in situ acoustic energy was established as the clinical protocol at the electrical power up to 500 W. Temperature elevation inside the tissue sample was measured by a thermocouple array. Generated lesion volume at different in situ acoustic energies and pathological examination of the lesions was evaluated ex vivo. Results: Acoustic pressure mapping showed the insignificant presence of side/grating lobes and pre‐ or post‐focal peaks (≤−12 dB). Although distorted acoustic pressure waveform was found in the free field, the nonlinearity was reduced significantly after the beam propagating through tissue samples (i.e., the second harmonic of −11.8 dB at 500 W). Temperature elevation was <10°C at a distance of 10 mm away from a 20‐mm target, which suggests the well‐controlled HIFU energy deposition and no damage to the surrounding tissue. An acoustic energy in the range of 750–1250 J resulted in discrete lesions with an interval space of 5 mm between the treatment spots. Histology confirmed that the lesions represented a region of permanently damaged cells by heat fixation, without causing cell lysis by either cavitation or boiling. Conclusions: Our characterization and ex vivo evaluation protocol met the IDE requirement. The in‐situ acoustic energy model will be used in clinical trials to deliver almost consistent energy to the various targets. [ABSTRACT FROM AUTHOR]

• The relationship between residual stress and surface wave nonlinearity is investigated. • Finite element model for surface wave nonlinearity with residual stress is established. • Residual stress can be detected and qualitatively evaluated by surface wave nonlinearity. Due to the influence of residual stress on material properties, residual stress measurement is particularly important for the optimization of structure designs and the condition monitoring of machines. In this paper, the influence of residual stress on materials nonlinearity was reported. Firstly, the propagation model of ultrasonic surface waves was established by the finite element method and the effect of frequency was studied to determine an appropriate frequency of acoustic nonlinearity parameters. Then, the different residual stresses were produced into specimen by adding internal stress in the material and the impact on acoustic nonlinearity parameter was studied. The results show that the nonlinearity of the material increases monotonously with the increase of residual stress. Experiments were done by using the nonlinear ultrasonic surface wave on three 7075Al alloy specimens under different residual stresses. Experimental data matched well with the simulation results. It has been shown that the nonlinear ultrasonic surface wave method has great potential in the residual stress detection and evaluation. [ABSTRACT FROM AUTHOR]

Acoustoelastic coefficients (AECs) of a material are used for determining third-order elastic constants (TOECs), measuring residual stress, and evaluating damage and deterioration by analyzing the change of wave velocity with stress. However, wave velocity measurements are highly sensitive to ambient or load-induced temperature changes. In this study, we proposed a temperature correction method that uses the measured temperature history and the temperature–velocity relationship of the test specimen. Acoustoelastic tests were performed in various ambient temperature conditions on three types of metals: 6063-T5 aluminum alloy (Al6063), 304 stainless steel (SS304), and 1018 low-carbon steel (S1018). Before temperature correction, the AEC measurements varied significantly, due to changes in the temperature of the specimen caused by both the ambient environment and mechanical loading. The results after the correction demonstrate that the proposed self-temperature correction method reduces temperature induced error, enabling highly accurate AEC measurements. • Temperature change (even minor) affects the acoustoelastic coefficient measurement. • Using a reference specimen cannot fully compensate the temperature effect. • Temperature history and thermal-acoustic relation of specimens are used in correction. • Validated on 6063-T5 aluminum, 304 stainless steel, and 1018 low-carbon steel. [ABSTRACT FROM AUTHOR]

Direct-write piezoelectric transducers are dedicatedly designed and fabricated to generate and detect both fundamental and second harmonic Rayleigh ultrasonic waves for evaluating overload-induced plastic deformation in aluminum (Al) alloy. The Rayleigh ultrasonic signals, generated by concentric focused direct-write transducers and propagating along the Al-alloy specimens, are measured either with other direct-write transducers or laser scanning vibrometer (LSV) to obtain the fundamental and second harmonic ultrasonic signals for analyzing the acoustic nonlinearity. The results show that the acoustic nonlinearity increases over 50% when the plastic strain reaches 7.8%. In comparison with LSV, the direct-write piezoelectric transducers exhibit substantially improved consistency and repeatability in the acoustic nonlinearity measurements. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) are conducted to correlate micro-structural changes with the acoustic nonlinearity in the Al alloy. The results and analyses indicate that the direct-write piezoelectric transducers with the appropriate design have significant technical advantages for acoustic nonlinearity testing and yielding determination. [ABSTRACT FROM AUTHOR]

The strain-induced martensite transformation during the low-cycle fatigue in austenitic AISI 316 stainless steel was investigated by magnetic and acoustic nondestructive methods. The low-cycle fatigue test was performed at various strain amplitudes. The volume fraction of α′-martensite was determined for the fatigue-failed specimens by magnetic property measurements with further microstructure detection. The cyclic hardening behavior was discussed in terms of the α′-martensite transformation. The volume fraction of α′-martensite was growing with the strain amplitude. An increase in the α′-martensite fraction was evaluated with the nonlinear ultrasonic parameter. The α′-martensite fraction may distort the lattice in austenitic stainless steel, resulting in the distortion of an ultrasonic wave. From this distortion, superharmonics may be generated with the α′-martensite nucleation, which strongly depends on the strain amplitude. The relationship between nonlinear acoustic characteristic and the volume fraction of α′-martensite is linear. [ABSTRACT FROM AUTHOR]

The detection of plastic deformation of metallic alloy materials with second-harmonic Rayleigh ultrasonic wave is first investigated using direct-write piezoelectric ultrasonic transducers, in which piezoelectric poly(vinylidenefluoride/ trifluoroethylene) [P(VDF/TrFE)] polymer coatings and electrodes are directly deposited, processed, and patterned on the alloy to be evaluated. Rayleigh ultrasonic signals, generated by the direct-write transducers on titanium alloy specimens, are characterized by a laser scanning vibrometer. The results show that acoustic nonlinearity increases with plastic strain, and an increase of ~40% in the acoustic nonlinearity corresponding to a plastic strain of 5.1%. The measurement data and technical features with the use of the direct-write transducers are compared with the conventional discrete angle beam piezoelectric transducer. The results and analyses show that compared with the conventional discrete angle beam piezoelectric transducers, implementation of the direct-write piezoelectric transducers has significant technical advantages and is promising for applications in determining nonlinear ultrasonic waves and plastic strain of structural materials. [ABSTRACT FROM PUBLISHER]

The structures in high-temperature environments are prone to undergo hardening and embrittlement as a result of thermal aging; this can cause variations in their mechanical properties. Because these changes occur at the microstructural level, it is difficult to evaluate them through linear ultrasonic techniques. In this work, a surface acoustic wave (SAW) was used to measure and compare the acoustic nonlinearity and mechanical properties of Al6061 alloys heat-treated at 220 °C for different durations (0 min, 20 min, 40 min, 1 h, 2 h, 10 h, 100 h, 1000 h). The SAW was generated by a pulsed laser and then received by an interferometer. Moreover, the yield strength, ultimate strength, and elongation to failure were measured by tensile tests. The results demonstrate that the critical variations in the mechanical properties can be detected by monitoring the variation features in the acoustic nonlinearity. Transmission electron microscopy images were captured to observe the microstructural changes, which shows that the acoustic nonlinearity varied according to the change in the precipitation phase. This supports the acoustic nonlinearity measurement using the laser-generated SAW being an effective technique for the fully noncontact nondestructive evaluation of material degradations as well as changes in mechanical properties.

The measurement of acoustic nonlinear response is known as a promising technique to characterize material micro-damages. In this paper, nonlinear ultrasonic approach is used to characterize the evolution of fatigue induced micro-cracks in polymer bonded explosives. The variations of acoustic nonlinearity with respect to fatigue cycles in the specimens are obtained in this investigation. The present results show a significant increase of acoustic nonlinearity with respect to fatigue cycles. The experimental observation of the correlation between the acoustic nonlinearity and fatigue cycles in carbon/epoxy laminates, verifies that an acoustic nonlinear response can be used to evaluate the progressive fatigue damage in the granular polymer bonded explosives. The sensitivity comparison of nonlinear and linear parameters of ultrasonic waves in the specimens shows that nonlinear acoustic parameters are more promising indicators to fatigue induced micro-damage than linear ones. The feasibility study of the micro-damage assessment of polymer bonded explosives by nonlinear ultrasonic technique in this work can be applied to damage identification, material degradation monitoring, and lifetime prediction of the explosive parts. [ABSTRACT FROM AUTHOR]

Acoustic nonlinearity measurements via contact and noncontact generations of surface acoustic waves (SAWs) were performed in order to characterize the thermal aging of aluminum alloy. The experiments were conducted on aluminum alloy samples (Al6061-T6) that were heat-treated at 220°C for different times (0 min, 20 min, 40 min, 1 h, 2 h, 10 h, 100 h, 1,000 h) and thus had the different levels of thermal aging. The acoustic nonlinearity of the specimens in two types of SAWs was observed according to the thermal aging. The fractional changes in the acoustic nonlinearity exhibited similar trends in both contact and noncontact SAWs, showing that the acoustic nonlinearity measurement via SAWs is independent of the SAW-excitation method. Furthermore, the fractional changes agreed well with the variation in the yield strength, which was a minimum when the acoustic nonlinearity reached its first peak. Then, the acoustic nonlinearity drastically dropped while the yield strength increased to its highest value. Thus, the variation in the acoustic nonlinearity can be perceived as an indicator of the aging level. These results demonstrate the potential feasibility of acoustic nonlinearity measurements via SAWs for the nondestructive evaluation of material degradations. [ABSTRACT FROM PUBLISHER]

Acoustic nonlinearity and loss are found to be positively correlated with porosity at industrially relevant levels of less than half a percent in commercially pure aluminum produced by laser powder bed fusion (L-PBF) with several different power levels. The technique employed for acoustic measurements involves nonlinear reverberation spectroscopy (NRS) with noncontacting electromagnetic-acoustic transduction, which offers advantages of adaptability to complex part geometries and short inspection times for industrial qualification of additively manufactured (AM) parts of arbitrary size. Porosity and microstructure are characterized with the Archimedes technique, X-ray computed tomography, and scanning electron microscopy. Fit parameters of nonlinearity and loss vs. porosity are found to vary significantly with the height of material in the build, consistent with an hypothesis that the correlations are indirect and involve dislocations as the principal nonlinear/anelastic elements. Nonlinearity and loss decrease with time under acoustic excitation, while being relatively insensitive to pauses in excitation of similar duration, indicating that acoustic excitation at inspection levels induces changes in nonlinear/anelastic defects without predominant involvement of thermal excitation. This remarkable behavior is not seen as a fundamental impediment for the application of the technique to nondestructive AM part qualification because of the brief time required for a measurement. [ABSTRACT FROM AUTHOR]

The objective of this study is to investigate experimentally the cyclic deformation in a planar slip material using acoustic nonlinearity. Observation and characterization of the dislocation substructure are conducted using an electron microscope. The acoustic nonlinearity was obtained by a superharmonic generation technique for different levels of fatigue-damaged specimens. The microstructural effects on nonlinearity are discussed in terms of the extent of planararray dislocations in the substructures after the low-cycle fatigue. The acoustic nonlinearity of Cu-Zn alloy increased with the number of fatigue cycles due to the evolution of dislocation substructures. Consequently, acoustic nonlinearity could be a potential index to characterize cyclic deformation with respect to dislocation substructures. [ABSTRACT FROM AUTHOR]

The paper presents the results of laboratory measurements of the acoustic nonlinearity parameter for a granite sample from the site of a conducted field experiment. This made it possible to completely confirm the results of the field experiment and explain the occurrence of a large scatter of values for the nonlinearity parameter in the field measurements. The size of the quadratic linearity parameter in granite rocks was determined, normalized to the volumetric concentration of fractures, which can be used for remote estimation of the fracture concentration. [ABSTRACT FROM AUTHOR]

The higher-order acoustic nonlinear parameters are considered effective damage indices in the field of nondestructive evaluation (NDE). They are defined by using the displacement amplitudes of the fundamental frequency and the harmonics, which are called the absolute nonlinear parameters. Generally, however, it is difficult to measure the very small displacement amplitudes of high-frequency harmonics. Therefore, the simplified parameters using the detected wave signal amplitudes, which are known as the relative nonlinear parameters, have been widely used, although their applications are limited to the relative comparison of before and after damage of a single material under consistent experimental circumstances. In this paper, in order to make clear the concept of relative parameter, we presented first that the relative ratio of the simplified parameters is identical to that of the absolute parameters when the detected signal amplitudes are linearly proportional to the actual displacement amplitudes with respect to the fundamental frequency and the harmonics. In addition, the new relationship between the relative ratio of simplified second-order parameter and the relative ratio of simplified third-order parameter was derived from the relationship between the absolute second- and third-order parameters. This new relationship was successfully verified based on experimental results obtained from Al 6061-T6 processed for different heat treatment times, where it was confirmed in advance that the PZT detection signal amplitudes at the fundamental frequency and its second- and third-order harmonics were linearly proportional to the displacement amplitudes. [ABSTRACT FROM AUTHOR]

A noncontact method to evaluate the acoustic nonlinearity of surface waves in a plastically deformed aluminum alloy is proposed. Line-arrayed laser beams modulated with slit masks were used for the generation of narrowband surface waves. A laser-ultrasonic detector using a two-wave mixing (TWM) approach was also employed to detect the surface waves. The specimens were deformed by a stroke-controlled tensile tester so as to generate various degrees of tensile deformation. The experimental results showed that the acoustic nonlinearity of the laser-generated surface waves increased according to the level of tensile deformation. This tendency was in good agreement with our previous results obtained using a contact piezoelectric (PZT)-transducer as the receiver. These results imply that our noncontact technique is suitable for the evaluation of acoustic nonlinearity and can be applied to practical damage assessment. [ABSTRACT FROM PUBLISHER]

A technique is presented for determining elastic nonlinearity of materials from resonant frequency shifts as a function of signal amplitude during free vibrational decay after tone-burst excitation. The technique differs from previous nonlinear reverberation spectroscopy (NRS) techniques in that it employs phase-sensitive superheterodyne reception. Time-dependent amplitudes of in-phase and out-of-phase components of signals, relative to a reference sinusoid at the excitation frequency, are provided through analog hardware processing in the absence of digitization of the signal from the vibrational sensor. The time-dependent phase and amplitude of the signal are determined through software analysis of these in-phase and out-of-phase components, and the instantaneous frequency during free decay is then determined from the time derivative of the phase. With this approach, superheterodyne reception and low-pass filtering of the phase-detector outputs lead to a great reduction in noise and computation effort, relative to direct digitization and software processing of the sensor signal, while retaining information on frequency shifts on a relevant time scale during ringdown. As with other NRS techniques, rapid acquisition of data on amplitude dependence of the resonant frequency during ringdown leads to minimization of systematic errors from temperature drift. The technique is demonstrated with noncontacting electromagnetic-acoustic transduction on custom alloyed Al (0.2 at.% Zn) and commercial Al 7075 cylinders with axial-shear resonant frequencies between 658 kHz and 659 kHz. The precision of measurements of relative frequency shifts is found to be on the order of 0.1 parts per million (ppm), exceeding by two orders of magnitude the best reported precision of nonlinear resonant ultrasound spectroscopy (NRUS). • A new resonance technique for measuring acoustic nonlinearity is presented. • Nonlinear amplitude dependence is determined from resonant phase during ringdown. • Methods are demonstrated on Al cylinders with electromagnetic-acoustic transduction. • The precision of measurements of amplitude dependence exceeds that of NRUS. • Applications include NDE of additively manufactured parts with complex geometries. [ABSTRACT FROM AUTHOR]

Abstract: Thermally induced damage in Westerly granite subjected to heating/cooling cycle at different temperatures was characterized using linear and nonlinear ultrasonic methods. With the increase of material damage, pulse experiments revealed that linear wave velocity decreased down to nearly 50% of its initial value. The method of inverting wave velocities, according to a non-interactive crack effective medium theory, allowed the quantitative evaluation of the crack density, compared to the one obtained with optical observations of the microstructure of the samples. Nonlinear ultrasonic measurements were performed by emitting 1MHz tonebursts at increasing excitation amplitude. With increasing damage, the obtained waveform appeared to transform into the second time derivative of the envelope of the emitted toneburst, revealing a self-demodulation effect. To characterize the changes with increasing damage quantitatively, a self-demodulation index is introduced. This parameter reflects the change in linear attenuation and the nonlinearity of the material in a combined manner and decreases down to 10% of its initial value, showing the high sensitivity of nonlinear ultrasonic methods compared to linear ones. A simple theoretical model, taking into account a nonlinear stress/strain relationship of the medium, provides analytical solutions of the transmitted waveforms and reproduces well the experimental results. [Copyright &y& Elsevier]

Abstract: The creep damage of Ni-based superalloy has been experimentally investigated by acoustic nonlinearity measured by the finite amplitude method. The creep test is conducted and interrupted after a specified fraction of creep life time. The cuboidal γ′ precipitates coarsen preferentially in the direction perpendicular to the applied stress axis. The aspect ratio (a/b) of the γ′ precipitate increases monotonously with creep duration. The increase in acoustic nonlinearity is discussed in relation to the directional coarsening of the γ′ precipitates, which proves to be closely related to the scattering and distortion of the ultrasonic waves. We have determined that acoustic nonlinearity is a potential diagnostic method for determining creep damage in Ni-based superalloys. [Copyright &y& Elsevier]

Abstract: An analysis of the linear and nonlinear acoustic responses from an interface between rough surfaces in elastoplastic contact is presented as a model of the ultrasonic wave interactions with imperfect interfaces and closed cracks. A micromechanical elastoplastic contact model predicts the linear and second order interfacial stiffness from the topographic and mechanical properties of the contacting surfaces during a loading–unloading cycle. The effects of those surface properties on the linear and nonlinear reflection/transmission of elastic longitudinal waves are shown. The second order harmonic amplitudes of reflected/transmitted waves decrease by more than an order of magnitude during the transition from the elastic contact mode to the elastoplastic contact mode. It is observed that under specific loading histories the interface between smooth surfaces generates higher elastoplastic hysteresis in the interfacial stiffness and the acoustic nonlinearity than interfaces between rough surfaces. The results show that when plastic flow in the contacting asperities is significant, the acoustic nonlinearity is insensitive to the asperity peak distribution. A comparison with existing experimental data for the acoustic nonlinearity in the transmitted waves is also given with a discussion on its contact mechanical implication. [Copyright &y& Elsevier]

The paper reports on experimental study of elastic nonlinearity of polymers and glass fibre-reinforced (GFR-) composites in a wide range of tensile stress applied (up to a fracture limit). Focused slanted transmission mode (FSTM) of air-coupled ultrasound is adapted for remote generation and detection of flexural waves in the samples of plastics. Local noncontact measurements of flexural wave velocity as a function of static strain are used to calculate the second-order nonlinearity parameters β2 and study their behaviour through a loading cycle. Molecular untangling and crazing phenomena are identified, respectively, with maxima of positive and negative β2 in thermoplastics. In composites, mechanics of fibre-matrix interaction is considered for brittle and plastic fractures. Hysteresis in velocity variation during loading–unloading cycle is used as an indicator of residual defect accumulation. [Copyright &y& Elsevier]

Because the number of applications for medical ultrasonic devices continue to increase and, hence, the number of diagnostic ultrasound (US) systems increase, there is a need to reliably characterize the sources in terms of their output pressures. Currently, the transducers are characterized by making pressure measurements in water for every voltage range applied to the source and, then, linearly derate the measured pressure values to estimate the derated acoustic pressure levels. The process is time-consuming and inaccuracies are introduced in the derating process due to nonlinear effects. Therefore, there is a need to find an indicator of nonlinearity that could classify the measured pressure waveform as either linear, where the derating procedure would yield an accurate derated acoustic pressure estimate, or nonlinear, where the derating process would fail. Eight different indicators of nonlinearity were evaluated experimentally using spherically focused US transducers. The transducers were selected to test the indicator sensitivity to frequency (3 to 8 MHz), f-number (1 and 2), and transducer diameter (1.905 and 5.08 cm). Sensitivity to drive voltage conditions was also tested by exciting one of the transducers with pulses of different duration and phase. None of the eight nonlinearity indicators yielded consistent results. The lack of consistency resulted from the competing effects of nonlinear absorption and asymmetrical distortion, which have yet to be combined into a unified theory. (E-mail: wdo@uiuc.edu) [Copyright &y& Elsevier]

The propagation of ultrasonic waves is nonlinear. Phenomena associated with the propagation of diagnostic ultrasound pulses cannot be predicted using linear assumptions alone. These include a progressive distortion in waveform, the generation of frequency harmonics and acoustic shocks, excess deposition of energy and acoustic saturation. These effects occur most strongly when ultrasound propagates within liquids with comparatively low acoustic attenuation, such as water, amniotic fluid or urine. Within soft tissues, similar effects occur, although they are limited by absorption and scattering. Nonlinear effects are of considerable importance during acoustic measurements, especially when these are used to predict in situ exposure. Harmonic generation may be used to create images. These offer improvements over conventional B-mode images in spatial resolution and, more significantly, in the suppression of acoustic clutter and side-lobe artifacts. B/A has promise as a parameter for tissue characterisation, but methods for imaging B/A have shown limited success. (E-mail: f.duck@bath.ac.uk) [Copyright &y& Elsevier]

The distortion of a sinusoidal acoustic wave at unbonded interfaces has been determined in terms of the first and second harmonic amplitudes. The results demonstrate for the first time that the second harmonic can reach the theoretically predicted maximum value. As also predcted, the harmonic generation efficiency at unbonded interfaces first increases and then decreases with an externally applied compressive load. The technique has been applied to diffusion bonded specimens in an attempt to quantify their achieved strength. As already demonstrated earlier, the energy reflected from such diffusion bonds is also useful to characterize their strength. Indications are that a combination of reflected energy and harmonic generation data could be a powerful tool to quantify the strength of diffusion bonds, particularly those of nearly perfect strength. A strength determination of diffusion bonds by nondestructive evaluation is a necessity for the qualification of such bonds in critical applications. [ABSTRACT FROM AUTHOR]

This paper describes an attempt to characterize the deterioration of a structural material's mechanical properties by nonlinear acoustics. In this particular case, the damage was caused by “thermal embrittlement” during which the material, here the nickel-based alloy Inconel 718, loses a significant fraction of its fracture toughness. Harmonic generation was the experimental method used to characterize the microstructural changes in the material as a function of exposure time at elevated temperatures. Tests were performed on two heats of Inconel 718 with slightly different chemistries, with one heat showing particular sensitivity of the fracture toughness to the elevated temperature exposure with corresponding higher changes in the nonlinearity parameter. As a mechanical measure of the fracture toughness deterioration, a small specimen punch test was used in which the ductility of a thin slice of material is determined. A clear difference between the two heats was noted in the metallographic examination, which is reflected in the harmonic generation as well as the punch test data. An explanation for the changes of the harmonic generation during the embrittlement process is speculative at the present time. [ABSTRACT FROM AUTHOR]

Acoustic emission by a vortex dipole decreases the dipole energy and changes the vortex velocity distribution. A relative shift of the dipole component occurs. Earlier, such a process of acoustic instability of the vortex was considered as applied to the case of weak acoustic emission described by the wave equation. Features of acoustic instability upon emission of a strongly nonlinear sound by a vortex are considered in this work. [ABSTRACT FROM AUTHOR]

During the service or external loading of the surface coating, the damage accumulation may develop in the coating or at the interface between the substrate and the coating, but it is difficult to measure directly in the early stage, so the acoustic nonlinear parameters are used as the early damage index of the coating. In this paper, the nonlinear wave motion equation is solved by the perturbation method and the new relationship between the relative ratio of second-order parameter and third-order parameter was derived. The nonlinear ultrasonic testing system is used to detect received signals during tensile testing of for the specimen with Al2O3 coatings. It is found that when the stress is less than 260 MPa, the appearance of the coating has no obvious change, but the nonlinear coefficients measured by the experiment increase with the increase of the tensile stress. By comparing the curves of nonlinear coefficients and stress respectively, the fluctuation of curves the second-order nonlinear coefficient A2 and the relative nonlinear coefficient β′ to stress is relatively small, and close to the linear relationship with the tensile stress, which indicates that the two parameters of the specimen with Al2O3 coatings are more sensitive to the bonding conditions, and can be used as an evaluation method to track the coating damage. [ABSTRACT FROM AUTHOR]

• Analytical interpretation of thermal susceptibility of acoustic nonlinearity in elastic waves. • A delicate approach of using thermal susceptibility of acoustic nonlinearity for material characterization. • Demonstrated capacity of advancing elastic wave-based material anomaly imaging. • Proven capacity of improving imaging precision and enhancing accuracy of elastic wave imaging. Despite proven effectiveness in characterizing material degradation and embryonic defects, the use of acoustic nonlinearity is restricted by its intrinsic vulnerability to measurement contamination and to fluctuations in ambient temperature in particular. Analytically, we shed light on the susceptibility of acoustic nonlinearity embodied in elastic waves to ambient temperature. Rather than eliminating or compensating for such thermal susceptibility, we subtly exploit it to advance nonlinear elastic wave imaging. Experimental validation corroborates theoretical prediction, spotlighting the capacity of the approach to improve the precision of material characterization using nonlinear elastic waves and therefore to enhance the accuracy of anomaly imaging when other nonlinearity sources interfere with the extraction of nonlinear attributes of elastic waves. Exploiting the thermal susceptibility of nonlinear attributes of elastic waves, a delicate approach is developed to evaluate material acoustic nonlinearity and visualize material anomalies, with proven capacity of improving imaging precision and enhancing imaging accuracy Image, graphical abstract [ABSTRACT FROM AUTHOR]

• In micro-crack regions, a resonant Lamb wave can be generated when the special resonance condition ω S 0 / ω A 0 = 2 κ / (κ + 1) is satisfied. • The acoustic nonlinear parameter is feasible to characterize micro-cracks. • The resonant wave's signal is capable of locating micro-crack regions. It is crucial to detect and evaluate early material degradation caused by micro-cracks in key engineering structures. This paper investigates the resonant behavior of one-way Lamb mixing waves in thin plates with randomly distributed micro-cracks by numerical simulations. It is found that when the resonance condition ω S 0 / ω A 0 = 2 κ / (κ + 1) is satisfied, a resonant A 0 mode wave can be generated by a pair of A 0 and S 0 mode waves mixed in the micro-crack damage region. Meanwhile, it is manifested that the acoustic nonlinearity parameter increases with the crack density and the length of the micro-crack region, which is also related to the resonant frequency. Furthermore, it is proven that the location of the micro-crack region can be determined by the time-domain signals of the resonant wave. This study numerically reveals that the one-way Lamb mixing method is feasible to quantitatively identify micro-crack damage in thin plates. [ABSTRACT FROM AUTHOR]