Tone-burst Excitation Research Articles

Application of coded excitation in the Rayleigh wave detection of aluminum plates with narrow-magnet EMAT

Abstract The poor generation efficiency causes the low signal-to-noise ratio (SNR) of the electromagnetic acoustic transducers (EMATs). The magnetic fields at the edges of the magnet are fully utilized in the narrow-magnet EMAT to generate Rayleigh waves of higher amplitudes than the conventional EMAT. This paper applies the coded excitation technology to narrow-magnet EMAT to further improve the SNR and the lift-off performance in aluminum plate detection. The main phase encoding methods are Barker code and Golay code. Compared with the conventional tone-burst excitation method, both coded excitation technologies can effectively improve the SNR of the Rayleigh wave. Theoretically, the received Rayleigh waves using the Golay-coded excitation method have no sidelobe, which is different from the case using the Barker code. Hence the Golay-coded excitation achieves higher SNR in the received ultrasonic waves. In addition, the application of the Golay-coded excitation can also effectively improve the lift-off performance of the narrow-magnet EMAT.

Generation of bidirectional shear horizontal guided waves by size optimization of face-shear piezoelectric transducers

Generating directional guided waves helps to avoid unwanted reflections while providing location information for defects. Bidirectional waves focus wave beams in two opposite directions, which can be applied for generating circumferential zero-order shear horizontal (SH0) waves for large-diameter pipe inspection. In this paper, a novel method for generating bidirectional SH0 waves through size optimization using face-shear (d24) lead zirconate titanate (PZT) transducer is proposed. A theoretical model is established based on the shear-lag model and the Huygens principle, which describes the generation and propagation of SH0 waves in thin plate by bonded d24 PZT transducer. The theoretical model is validated by multi-physics finite element (FE) simulation results in terms of SH0 displacement wavefield. Based on the theoretical model, the performance of bidirectional SH0 wave generation is studied. The maximum bidirectional ratio that can be achieved by d24 PZT within a size range (1–25 mm) under a five-cycle sinusoid tone-burst excitation at a center frequency range (50–300 kHz) is studied. Results show that over 20 dB effect can be achieved at the PZT with a specific aspect ratio. Laboratory experiments are conducted using d24 PZT transducers at 50–300 kHz to validate the effect of bidirectional wave generation in practical detection scenarios. Experimental results show effects of over 20 dB within a certain frequency range can be achieved when using a PZT with a large aspect ratio, which agree well with the theoretical predictions.

Bolt tightness monitoring using multiple reconstructed narrowband Lamb waves combined with piezoelectric ultrasonic transducer

Narrowband tone burst excitation with a carefully selected frequency is generally utilized to generate Lamb waves for mode purity, dispersion diminution and signal interpretability. Narrowband excitation exhibits saturation phenomenon and even non-monotonic trends with the development of bolt looseness. In this research, a piezoelectric ultrasonic transducer with high signal-to-noise ratio is designed and fabricated for bolt tightness monitoring. The chirp signal is utilized to drive the actuator to generate broadband Lamb waves in the connected plate, and a transfer function-based signal reconstruction algorithm is innovatively proposed to extract narrowband tone burst responses with different center frequencies from the broadband chirp response for bolt tightness monitoring. The leaked wave energy-based bolt tightness indexes (TIs) are calculated based on the extracted multiple tone burst responses and a feature-level data fusion strategy is proposed to combine the TIs for exploiting the merits of different-frequency inspecting waves for different bolt tightness conditions. The fused TI not only presents a monotonic tendency with the increasing of bolt tightness, but also tackles the low sensitivity of narrowband Lamb waves for embryo bolt looseness. The proposed method contributes a novel and stable scheme for bolt tightness monitoring and opens a new perspective for damage evaluation of structures.

High-Temperature Piezoelectric Resonators: Electroacoustic Properties and Applications of Langasite-Type Crystals

Piezoelectric devices based on high-temperature stable single crystals are of increasing interest due to a wide range of potential applications, such as actuators and resonant gravimetric sensors. The latter require highly accurate resonant-frequency measurements at temperatures as high as 1000 °C, and this can only be achieved with narrow resonance peaks and correspondingly low acoustic loss at these temperatures. Relevant atomistic defects in langasite (La3Ga5SiO14, LGS) and catangasite (Ca3TaGa3Si2O14, CTGS) are reviewed with respect to their impact on electrical conductivity and acoustic loss. The focus on these crystals also enables a comparison of electroacoustic properties of partially disordered (LGS) and ordered (CTGS) crystals and the formulation of approaches for reducing acoustic loss and enhancing device performance.Starting with defect chemistry models for LGS and CTGS, electrical conductivities as a function of temperature and oxygen partial pressure are considered. One result of this analysis is that contributions of oxygen ions to the total conductivity are determined to be relatively small in the studied crystals. Temperature-dependent acoustic loss of LGS and CTGS crystals is also studied. Two independent techniques, contact resonant piezoelectric spectroscopy, and a contactless tone-burst excitation technique, are applied. Contributions to the measured acoustic loss are determined by fitting loss-related functions for point defects and conductivity-related relaxations. The extracted fit parameters, in particular the activation energies of the processes, are discussed and compared with that of the electrical conductivity. The losses are caused by a superposition of several mechanisms including conductivity-related relaxations, point-defect relaxations and intrinsic phonon-phonon loss.The long-term stability of the crystals is also evaluated at 1000 °C with respect to fundamental materials properties including electrical conductivity and resonant frequency.After one year of thermal treatment, the resonance frequency of CTGS resonators from different crystal sources is found to decrease by no more than 0.4 %. Thus, the applicability of CTGS as a piezoelectric resonator that operates for long periods at extreme temperatures, such as a resonant nanobalance, is demonstrated.Application examples to be presented include resonant nanobalances for in-situ monitoring of non-stoichiometry and phase transformations in thin films during growth at high temperatures. In this context, the key application properties of the sensors at high temperatures, such as mass sensitivity and resolution, are discussed. As an application example, the attached figure shows the oxygen non-stoichiometry x of thin praseodymium-cerium oxide films (Pr0.1Ce0.9O2-x) at 700 °C as a function of oxygen partial pressure determined by LGS and CTGS nanobalances, which agrees very well with literature data (chemical capacitance: D. Chen et al., Chem. Mater. 2014, 26, 22, 6622–6627). Figure 1

Lamb wave based damage imaging under nonlinear chirp excitation

Considering a trade-off between temporal-spatial resolution and multi-mode nature of Lamb waves, tone bursts with short durations are usually used as excitations in Lamb wave based damage detection. A short-duration excitation usually requires a large amplitude to carry sufficient energy so as to obtain response signals with enough signal-to-noise ratio and cover a large inspection area. In this paper, an alternative Lamb wave damage imaging method using nonlinear chirp (nonlinear frequency modulation, NLFM) excitation with a long duration and a small amplitude is proposed. The signal processing techniques of pulse compression and dispersion compensation are adopted to compress the long-duration wave packets of response signals into short ones. Compared with conventional tone burst excitations with short durations and small amplitudes, due to the long duration of the nonlinear chirp excitation and the use of pulse compression, sufficient energy can be applied to transducers under small amplitude excitations so the image contrast in imaging will not degrade. Furthermore, as large amplitude excitations are no longer required, high voltage amplifiers are not necessary so the hardware of the Lamb wave testing system is simplified. Experiments on a carbon steel plate with an artificial crack are carried out and Lamb wave signals are collected using a linear array consisting of nine PZTs. Experimental results under the NLFM signal and conventional tone bursts are provided. Experimental results show that under the condition of the same excitation amplitude, the proposed method under the NLFM excitation can achieve better imaging quality compared with methods under conventional tone bursts.

Highly Sensitive Detection of Microstructure Variation Using a Thickness Resonant Transducer and Pulse-Echo Third Harmonic Generation.

In nonlinear ultrasound testing, the relative nonlinear parameter is conveniently measured as a sensitive means of detecting and imaging overall variation of microstructures and damages. Compared to the quadratic nonlinear parameter (β'), the cubic nonlinear parameter (γ'), calculated as the third harmonic amplitude divided by the cube of the fundamental amplitude, has generally a higher value, providing better sensitivity in nonlinear parameter mapping. Since the third harmonic amplitude is about two orders of magnitude lower than the fundamental amplitude, efficient excitation and highly sensitive reception of third harmonic is very important. In this paper, we explore an odd harmonic thickness resonant transducer that meets the requirements for pulse-echo third harmonic generation (THG) measurements. We also address the problem of source nonlinearity that may be present in the measured amplitude of the third harmonic and propose a method to properly correct it. First, we measure γ' for a series of aluminum specimens using the through-transmission method to observe the behavior of γ' as a function of specimen thickness and input voltage, and examine the effects of various corrections such as attenuation, diffraction and source nonlinearity. Next, we apply the odd harmonic resonant transducer to pulse-echo THG measurements of precipitation heat-treated specimens. It is shown that such transducer is very effective in generation and detection of fundamental and third harmonics under finite amplitude toneburst excitation. The highly sensitive detectability of γ' are presented as a function of aging time, and the sensitivity of γ' is compared with that of β' and β'2.

Wave Attenuation in Additively Manufactured Polymer Acoustic Black Hole Structures Considering the Viscoelastic Effect.

The acoustic black hole (ABH) is a feature commonly found in thin-walled structures that is characterized by a diminishing thickness and damping layer with an efficient wave energy dissipation effect, which has been extensively studied. The additive manufacture of polymer ABH structures has shown promise as a low-cost method to manufacture ABHs with complex geometries, exhibiting even more effective dissipation. However, the commonly used elastic model with viscous damping for both the damping layer and polymer ignores the viscoelastic changes that occur due to variations in frequency. To address this, we used Prony exponential series expansion to describe the viscoelastic behavior of the material, where the modulus is represented by a summation of decaying exponential functions. The parameters of the Prony model were obtained through experimental dynamic mechanical analysis and applied to finite element models to simulate wave attenuation characteristics in polymer ABH structures. The numerical results were validated by experiments, where the out-of-plane displacement response under a tone burst excitation was measured by a scanning laser doppler vibrometer system. The experimental results illustrated good consistency with the simulations, demonstrating the effectiveness of the Prony series model in predicting wave attenuation in polymer ABH structures. Finally, the effect of loading frequency on wave attenuation was studied. The findings of this study have implications for the design of ABH structures with improved wave attenuation characteristics.

Imaging phonon eigenstates and elucidating the energy storage characteristics of a honeycomb-lattice phononic crystal cavity

We extend gigahertz time-domain imaging to a wideband investigation of the eigenstates of a phononic crystal cavity. Using omnidirectionally excited phonon wave vectors, we implement an ultrafast technique to experimentally probe the two-dimensional acoustic field inside and outside a hexagonal cavity in a honeycomb-lattice phononic crystal formed in a microscopic crystalline silicon slab, thereby revealing the confinement and mode volumes of phonon eigenstates—some of which are clearly hexapole in character—lying both inside and outside the phononic-crystal band gap. This allows us to obtain a quantitative measure of the spatial acoustic energy storage characteristics of a phononic crystal cavity. We also introduce a numerical approach involving toneburst excitation and the monitoring of the acoustic energy decay together with the integral of the Poynting vector to calculate the Q factor of the principal in-gap eigenmode, showing it to be limited by ultrasonic attenuation rather than by phonon leakage to the surrounding region.

Dynamic Behavior of Polymer Microbubbles During Long Ultrasound Tone-Burst Excitation and Its Application for Sonoreperfusion Therapy.

Ultrasound (US)-targeted microbubble (MB) cavitation (UTMC)-mediated therapies have been found to restore perfusion and enhance drug/gene delivery. Because of the potentially longer circulation time and relative ease of storage and reconstitution of polymer-shelled MBs compared with lipid MBs, we investigated the dynamic behavior of polymer microbubbles and their therapeutic potential for sonoreperfusion (SRP) therapy. The fate of polymer MBs during a single long tone-burst exposure (1 MHz, 5 ms) at various acoustic pressures and MB concentrations was recorded via high-speed microscopy and passive cavitation detection (PCD). SRP efficacy of the polymer MBs was investigated in an in vitro flow system and compared with that of lipid MBs. Microscopy videos indicated that polymer MBs formed gas-filled clusters that continued to oscillate, fragment and form new gas-filled clusters during the single US burst. PCD confirmed continued acoustic activity throughout the 5-ms US excitation. SRP efficacy with polymer MBs increased with pulse duration and acoustic pressure similarly to that with lipid MBs but no significant differences were found between polymer and lipid MBs. These data suggest that persistent cavitation activity from polymer MBs during long tone-burst US excitation confers excellent reperfusion efficacy.

Acoustically driven translation of a single bubble in pulsed traveling ultrasonic waves

The acoustic radiation force has been proven as an effective mechanism for displacing particles and bubbles, but it has been mainly applied in a standing wave mode in microfluidics. Alternatively, the use of pulsed traveling acoustic waves could enable new options, but its transient dynamic, which entails the additional complexities of pulse timing, reflections, and the type of waveform, has not yet been fully investigated. To better understand these transient effects, a transient numerical solution and an experimental testbed were developed to gain insights into the displacement of microbubbles when exposed to on- and off-periods of pulsed traveling waves. In this study, a practical sinusoid tone burst excitation at a driving frequency of 0.5 MHz is investigated. Our numerical and experimental results were found to be in good agreement, with only a 13% deviation in the acoustically driven velocity. With greater detail from the numerical solution at a sampling rate of 1 GHz, the fundamental mechanism for the bubble translation was revealed. It was found that the added mass force, gained through the on-period of the pulse, continued to drive the bubble throughout the off-period, enabling a large total displacement, even in the case of low duty-cycle (2%) pulsing. In addition, the results showed greater translational velocity is possible with a lower number of cycles for the same input acoustic energy (constant duty cycle and acoustic pressure amplitude). Overall, this study proposes a new, practical, and scalable approach for the acoustic manipulation of microbubbles for scientific, biomedical, and industrial applications.

Application of Pulse Compression Technique in High-Temperature Carbon Steel Forgings Crack Detection with Angled SV-Wave EMATs.

In order to solve the difficulty in localization and poor signal-to-noise ratio (SNR) of the angled shear vertical wave (SV wave) electromagnetic acoustic transducer (EMAT) in cracks detection of high-temperature carbon steel forgings, a finite element (FE) model of the angled SV wave EMAT detection process was established, and the influence of specimen temperature on the EMAT excitation, propagation, and reception processes was analyzed. A high-temperature resistant angled SV wave EMAT was designed to detect carbon steel from 20 °C to 500 °C, and the influence law of the angled SV wave at different temperatures was analyzed. Then a circuit-field coupled FE model of angled SV wave EMAT in the carbon steel detection process based on the Barker code pulse compression technique was established, and the effects of the Barker code element length, impedance matching method, and matching component parameters on the pulse compression effect were analyzed. In addition, the noise suppression effect and the SNR of the crack-reflected wave in the tone-burst excitation method and the Barker code pulse compression technique were compared. The results show that the amplitude of the block-corner reflected wave decreases from 556 mV to 195 mV, and the SNR decreases from 34.9 dB to 23.5 dB when the specimen temperature increases from 20 °C to 500 °C. When the temperature is 500 °C, the SNR of the crack-reflected wave obtained by the Barker code pulse compression technique can be improved by 9.2 dB compared to the tone-burst excitation method with 16 synchronous averages. The study can provide technical and theoretical guidance for online crack detection for high-temperature carbon steel forgings.

Characterization of high-power mechanical quality factor of piezoelectric ceramic discs under self-heating condition

We have developed a method to characterize high-power mechanical quality factor Qm of a thickness-poled piezoelectric disc under self-heating condition. In order to obtain Qm from temperature and vibration measurements, a heat transfer model for a thin piezoelectric disc vibrating in its fundamental radial mode was built, and a numerical solution method based on the finite difference approach was developed to investigate the temperature rise of the piezoelectric disc as a function of position and time. Numerical simulations were performed to verify the validity of the model and to quantify the influence of different material parameters on the temperature rise. A theoretical formula for characterizing Qm of a piezoelectric disc was deduced according to the definition of the mechanical quality factor, in which the relationship among the self-heating parameters (hg), the sample vibration level (v0) and Qm was established. Furthermore, using a self-designed experimental equipment, we have characterized the high-power mechanical quality factor Qm of a PZT8 disc sample under different vibration levels using a tone-burst excitation, which can increase the operating electric field strength limits. The disc results show that Qm degrades drastically with increasing vibration amplitude. Especially, the Qm values measured under self-heating conditions have lower magnitudes than those measured using transient/burst method, indicating self-heating effects do significantly contribute to Qm characterization.

In-Situ Estimation of Bolt Clamping Force by Utilization of Its Effects on Flexural Vibration Propagation

After a bolt assembling process, non-destructive identification of the clamping force is important to minimize unexpected failure of the assembled joint structures. For in-situ inspection for every parts, a detachable device which allows generation and detection of vibration propagation was proposed in this study. Tone burst excitation was transmitted through the bolted-joint structure. The vibration propagation characteristics depended on the clamping force. With an increase in the clamping force, the surface vibration propagation speed increased. The vibration magnitude decreased due to increasing constraint. A band-pass filter was used to remove noise component. The filtered vibration propagation responses were used in convolutional long short-term memory (CLSTM) training. The performance of CLSTM model was verified through comparison with actual clamping force. With minimal physical influence on the assembled parts, the clamping force of bolted-joint was measured using the surface vibration propagation characteristics.

Abstract 15305: The Dynamic Behavior of Polymer Microbubbles During Long Ultrasound Tone-Burst Excitation and Its Application for Sonoreperfusion Therapy

Objective: Ultrasound (US)-targeted microbubble (MB) cavitation (UTMC) mediated therapies have been shown to restore perfusion and enhance drug/gene delivery. Due to the potentially longer circulation time and relative ease of storage and reconstitution of polymer shelled MBs compared with lipid MBs, we investigated the dynamic behavior of polymer MBs and its therapeutic potential for sonoreperfusion (SRP) therapy. Methods: We explored the fate of polymer MBs during a single long tone-burst exposure (1 MHz, 5 ms) at various acoustic pressures (0.25-1.5 MPa) and MB concentrations (2х10 6 -2х10 8 MB/mL) via direct high-speed microscopy observation and passive cavitation detection (PCD) and compared with lipid MBs. SRP efficacy of these MBs was investigated in an in vitro constant flow system at various pulse durations (1 MHz, 1-5 ms every 3 sec for 20 min) and acoustic pressures (0.6-1.5 MPa). The pressure differential across a porous mesh partially occluded with microthrombi was recorded as a function of time during treatment and used as a measure of thrombus burden. The terminal pressure drop and the initial rate of pressure drop were derived and compared. Results: Both polymer and lipid MBs first underwent stable or inertial cavitation depending on the acoustic pressure, and then formed gas-filled clusters that continued to oscillate, fragment, and form new gas-filled clusters during the single US burst. PCD confirmed continued acoustic activity throughout the 5-ms US excitation. SRP efficacy with polymer MBs increased with the pulse duration and acoustic pressure ( p <0.05, ANOVA), similarly to that of lipid MBs, but no significant differences were found between polymer and lipid MBs ( Figure 1 ). Conclusion: Under the acoustic conditions tested, both polymer and lipid MBs demonstrate similar thrombolytic effects in vitro suggesting that persistent cavitation activity from polymer MBs during long tone-burst US confers excellent reperfusion efficacy.

Optimize multilayer matching layer design for tone-burst underwater acoustic transducers.

In this study, a theoretical model has been constructed to address the optimization of matching layers for tone burst excitation underwater acoustic transducers and to investigate the functional mechanism change of matching layers. Single, double, and triple matching layers are attached to piezoelectric composite plates, and tone burst signals with a different number of burst-cycles are applied. Acoustic pressure function (Sp) and acoustic pressure transient response (PTR) are compared among these transducers. The results demonstrated that when the cycle number M ≤ 2, more matching layers lead to shorter ringdown with similar PTR amplitude. Since the objective is for better axial resolution, three matching layers have noticeable advantage, whereas for the case of M > 5, far detection distance is the main objective, and more matching layers caused the decrease in PTR amplitude. Besides, matching layers become part of the resonance structure; they do not serve as a transmission medium anymore. Therefore, the transducer with single matching is optimal for the long tone burst excitation. For the case of 2-5 cycle excitation, one may use double matching layers, which has a balance between the transmitting voltage response amplitude and bandwidth. With the increase in M, the role of matching layers changes from a bridging transmission medium to amplifying vibration velocity and introducing additional vibration modes to broaden the bandwidth.

An efficient Lamb wave-based virtual refined time-reversal method for damage localization in plates using broadband measurements

This study proposes a modified virtual time-reversal (VTR) algorithm for baseline signal-free damage detection in plate-like structures. The physical actuation and sensing of Lamb waves are performed using a broadband Gaussian excitation instead of the conventional narrowband modulated tone burst excitations. The forward response and the reconstructed signal due to the time-reversal process for a narrowband input signal are then constructed virtually using the broadband transfer function. The method eliminates the possibility of numerical errors encountered in the conventional VTR method based on narrowband excitations. It is also more efficient than the traditional VTR algorithm because it can probe at multiple excitation frequencies using a single measurement for each sensing path. This modified VTR algorithm is employed in the recently developed refined time-reversal method (RTRM), which uses an extended signal length of the reconstructed signal for computing damage index (DI) and probes the structure at the best reconstruction frequency. The new method is termed the virtual refined time-reversal method (VRTRM). The DIs based on the VRTRM are used in the reconstruction algorithm for probabilistic inspection of defects to achieve baseline signal-free localization of damages. The efficacy of the proposed VRTRM for damage localization is experimentally verified with the established technique RTRM. Experiments are performed in an aluminium plate equipped with a network of surface-bonded piezoelectric patch transducers to illustrate the conventional VTR’s pitfalls and the modified VTR’s accuracy for a single mass damage scenario. The results show that the proposed VRTRM is as accurate as the established technique RTRM in estimating the reconstructed signals and localizing a block mass damage. Finally, the VRTRM is shown to localize in a dual damage scenario with excellent accuracy. In contrast, the conventional main mode-based VTR method fails to localize the damages with or without single-mode tuning.

Application of pulse compression technique in metal materials cracks detection with LF-EMATs

ABSTRACT To address the problem of low signal-to-noise ratio (SNR) and poor spatial resolution of cracks detection of the angled SV wave electromagnetic acoustic transducer (EMAT), pulse compression technique (PCT) and line-focusing technique are introduced into the angled SV wave EMAT. A finite element (FE) model of the line-focusing (LF) EMAT detection process based on the Barker code excitation was established, and the effects of the design parameters of magnet and meander-line coil (MLC) on the main lobe amplitude and width of the defect echo after pulse compression were analysed. The optimal combination of LF-EMAT design parameters and excitation parameters was obtained, and it was compared with the traditional angled SV wave EMAT and the tone-burst excitation method by experimental measurements. The results shows that compared with angled SV wave EMAT with tone-burst excitation, LF-EMAT can improve the SNR by 14.9 dB with no synchronous average and be processed by PCT. The SNR from LF-EMAT with PCT is10 dB at 1.5 mm lift-off, while the SNR of the angled SV wave EMAT with tone-burst excitation is less than 0 dB at 1 mm lift-off. When the Barker code PCT is applied to the LF-EMAT detection, we can achieve the purpose of increasing EMAT lift-off and reducing the synchronous averages to meet the demand of online, rapid, and high-temperature detection.

Damage severity assessment in composite structures using multi-frequency lamb waves

When a narrowband tone burst excitation signal is used to generate Lamb waves in the thin composite plates, a suitable frequency of the excitation signal is typically chosen to excite a solitary mode of Lamb wave in the structure. However, if and as the damage severity changes in the structure, it may be possible that the frequency of the excitation signal may influence the response metric. In the present work, the use of a chirp signal as an excitation signal for damage severity assessment is proposed. The use of a chirp signal as an excitation signal may prove suitable for damage severity assessment because the chirp response contains a large frequency bandwidth, and the cumulative effect of all the frequencies with the delamination is reflected in the damage index. The sensitivity of the chirp response to varying delamination sizes is investigated through numerical simulations and experiments. The damage index calculated using the chirp response showed a monotonic trend with an increase in damage severity, in contrast to a bidirectional trend of the damage index when conventional narrow excitation signals are used.

Study and comparison of pulse compression techniques with three types of excitation for high-temperature EMAT detection

To address the low conversion efficiency and high ultrasonic attenuation in the detection of hot castings and forgings, the pulse compression technique (PCT) is an important and efficient method for improving the signal-to-noise ratio (SNR) and range resolution of ultrasonic echoes. A finite element model was established for the process of detection using a spiral coil electromagnetic acoustic transducer (EMAT) with three types of excitation, these being chirp signals, Barkercoded signals and combined Barker-coded signals, and the PCT processions were also provided. The SNR and range resolution of the PCTs with three types of excitation were compared using simulations and verified by experiments. The self-developed and water-cooled EMAT with three PCTs was used to measure a high-temperature forging and the pulse compression effect was also compared. The results indicate that the PCT with Barker-coded excitation has the highest SNR and the PCT with chirp excitation and combined Barker-coded excitation processes a higher range resolution. When the tested forging is at 710°C and compared with the traditional tone-burst excitation, the SNR from the PCT with combined Barker-coded excitation can be improved by 8.5 dB and the ultrasonic echo width can be reduced by 100%.

Nonlinear reverberation spectroscopy with phase-sensitive superheterodyne reception

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).