Conventional Pulse-echo Method Research Articles

Separation of multi-echo overlapping ultrasonic signals for increasing the axial resolution using a neural network

Abstract The ultrasonic pulse-echo method plays a significant role in ultrasonic thickness measurement. However, when the measured thickness is relatively thin, it is universal for the reflected ultrasonic echoes to overlap in the time domain which constrains the achievable resolution using the conventional pulse-echo method. A deep learning network for multi-echo overlapping ultrasonic signals separation under the constraint of an ultrasonic transducer with specific center frequency and bandwidth was proposed to enhance the axial resolution. With this model, the adaptive separation of overlapping signals with an unknown number of echoes is achieved by extracting detailed features of the overlapping signals. An overlapping signal training dataset based on truncated Nakagami functions is constructed to train the network. Simulation experiments under different degrees of overlap and noise levels were conducted for the performance analysis. Simulation results manifested the performance advantage of proposed separation method over some traditional separation methods. Further, ultrasonic thickness measurement experiments were performed on thin-walled aluminum alloy plates with various thicknesses, demonstrating feasibility and practicability of the network over traditional separation methods.

Measurement of Adhesive Bonding Strength With an EMAT in the Resonant Mode

Abstract The electromagnetic acoustic resonance (EMAR) method with shear wave is sensitive to boundary conditions and plate thickness. In this paper, a new noncontact ultrasonic testing method based on the electromagnetic acoustic transducer (EMAT) in the resonant mode is proposed for the bonding strength evaluation in metal-based adhesive structures. Different from the conventional pulse-echo method using short-burst excitation for bonding inspection, the attenuation coefficient feature of the resonant ultrasonic signal with long-burst excitation is applied to increase the signal-to-noise ratio (SNR) and detecting sensitivity of the EMAT for adhesive bonding strength evaluation. A theoretical model for adhesive bonding testing with EMAT signals in the resonant mode is established. To extract the signal feature representing the reflection coefficient, the time-domain signal was processed by Hilbert transformation and exponential curve fitting. Through the simulation on the received signal, the correlation between the attenuation coefficient of the exponent fitted curve and the strength on the adhesive imperfect interface were confirmed. Finally, the proposed correlation is verified by an experiment on stainless steel plates bonded with polymethyl methacrylate plates by epoxy adhesion via a permanent magnetic EMAT.

Adhesive debonding inspection with a small EMAT in resonant mode

In this paper, a noncontact ultrasonic testing method with using a small electromagnetic acoustic transducer (EMAT) in resonant mode is proposed for metal-based adhesive debonding inspection. The small EMAT with a new magnet configuration and a small coil is developed firstly to improve the spatial resolution and detecting ability for minute defects. Different from conventional pulse echo method using short pulse excitation for debonding inspection, a new method based on power spectrum estimation of the resonant ultrasonic signal with long burst excitation is applied to increase the signal-to-noise ratio (SNR) and detecting ability of the small EMAT. Finally, ultrasonic C-scan images of specimens with various adhesive bonding are obtained to demonstrate the capability of this technique.

A Hybrid Ultrasonic Guided Wave-Fiber Optic System for Flaw Detection in Pipe

The work presented in this paper shows that Fiber Bragg Grating (FBG) optical fiber sensors can potentially be used as receivers in a long-range guided wave torsional-mode pipe inspection system. Benefits over the conventional pulse-echo method arise due to reduced total travel distance of the ultrasonic guided wave reflections, since reflections from defects and structural features do not need to propagate a full round trip back to the transmitting collar. This is especially important in pipe configurations with high attenuation, such as coated and buried pipelines. The use of FBGs as receivers instead of conventional piezoelectric or magnetostrictive elements also significantly reduces cabling, since multiple FBG receivers can be placed along a single optical fiber which has a diameter on the order of only around 100 μm. The basic approach and sample results are presented in the paper. Additionally, a brief overview of some topics in ultrasonic guided waves is presented as a background to understand the inspection problem presented here.

Fabrication of a PMN-PT single crystal-based transcranial Doppler transducer and the power regulation of its detection system.

Doppler sonographic measurement of flow velocity in the basal cerebral arteries through the intact skull was developed using a pulsed Doppler technique and 2 MHz emitting frequency. Relaxor-based ferroelectric single crystals Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) were chosen to be the piezoelectric transducer material due to their ultrahigh piezoelectric coefficients, high electromechanical coupling coefficients and low dielectric loss. The pulse-echo response of the transducer was measured using the conventional pulse-echo method in a water bath at room temperature. The −6 dB bandwidth of the transducer is 68.4% and the sensitivity is −17.4 dB. In order to get a good match between transducer and detection system, different transmission powers have been regulated by changing the impedance of the transmitting electric circuit. In the middle cerebral artery (MCA) measurement photograph results, as the transmission power is increasing, the detection results become clearer and clearer. A comparison at the same transmission power for different transducers shows that the detection photograph obtained by the crystal transducer was clearer than that obtained with a commercial transducer, which should make it easier for doctors to find the cerebral arteries.

Elastic properties of coarse grained lead-free solder alloys

Because of health and environmental concerns about lead, lead-free solder alloys in most consumer electronics have been required in the European Union since 2006. Many of these alloys are prone to mechanical failure over time, leading to less reliable circuitry. The source of these failures is not well known and many have conjectured that the coarse grained alloys become more brittle over time when exposed to elevated temperatures (~100 °C). Our group, in collaboration with Cisco Systems, has recently studied the effects of aging on the mechanical properties of Sn-Ag-Cu (SAC) solder alloys using using both resonant ultrasound spectroscopy (RUS) and conventional pulse-echo methods. With grain sizes on the order of 100's of microns, the heterogeneity of these alloys present a particular problem for RUS and interpretation of pulse-echo data. Resonance data exhibiting the effect of the heterogeneity will be presented and discussed. Elastic moduli derived from pulse-echo methods as a function of temperature and isothermal aging time will also be shown.

Magneto-elastic attenuation in austenitic phase of Ni–Mn–Ga alloy investigated by ultrasonic methods

Magneto-elastic ultrasound attenuation in single crystals of a Ni–Mn–Ga is studied by ultrasonic methods at different temperatures, ranging from temperatures above the Curie point down to the vicinity of the premartensitic transition temperature. The conventional pulse–echo method is used for analysis of the attenuation of longitudinal waves only, whereas the attenuation related to shear motion is investigated by the resonant ultrasound spectroscopy (RUS). Because of the strong attenuation (increasing with the distance from the Curie point), the analysis of the resonant spectra obtained from RUS measurements must be done in successive steps, tracing individual resonances from above the Curie point into the region of higher attenuation. This method is also verified as appropriate for accurate determination of the c ′ elastic coefficient. The overall character of the attenuation is shown to be strongly anisotropic and different for longitudinal and shear waves.

Detection of Impact Damage in Composite Plates Using Surface Contact Method

It is a well known fact that the fundamental causes of most failures in composite structures are in the forms of incipient damages such as delaminations and cracks which usually remain undetected until they grow to levels large enough to cause failure. In this study, unidirectional carbon/epoxy composite plates with known defects are investigated. The known defects are generated by impacting the composite specimens simulating external collision. A pair of transmitter and receiver was used for generation of Lamb waves and reception of signals. The received signals were monitored by scanning the receiver toward internal defect or by scanning both transmitter and receiver with confined distance over the surface of the composite plates which have known defects lie beneath them. The proper selection of incident angle and frequency are also considered. The characteristics of received signals such as amplitude, energy and wave form are analyzed. The acquired information is used to locate and to measure the size of the impact damage. The suggested method is very effective if the internal damage is presented closed to surface of the plate where the conventional pulse-echo method has problems. The proposed technique can be used widely for the real time and online monitoring of composite structures.

Analysis of Propagation of Ultrasonic Pulses in Dispersive Media Using Group Delay Concept

A new analytical technique of the propagation of ultrasonic pulses in dispersive or nondispersive media is proposed. First, on the basis of the concept of group delay, we develop the theory as follows: (1) group delay is deduced directly from the Fourier transform of waveforms, and (2) cross-correlation theory is extended to a total measurement system which may include, for example, a sensor, couplant, amplifier, and A/D converter. Then, a fundamental result that the group delay between wave motions generated in a medium is exactly the same as that of corresponding waveforms observed by an observer is obtained. Next, this result is applied to improve the conventional pulse-echo method, and it is shown that by preparing two specimens of the same material but with different thicknesses, correct group delay in propagation can be obtained even though no physical or mechanical constants of the couplant or sensor may be known. In order to apply the results to the pulse-echo method, measurements of phase and group velocity in synthetic fused silica were also attempted.

Free of speckle ultrasonic imaging of soft tissue with account of second harmonic signal

The Born approximation deconvolved inverse scattering imaging technique is an alternative to the conventional pulse-echo method. This novel technique deconvolves the incident pulse from the reflected pulse, and uses the resulting impulse response to produce an image of the acoustic impedance distribution. It is applicable mainly to structures that resemble a layered medium. The images captured by this method prove to have improved resolution and are free of speckle. With this method one can use ultrasound of lower frequencies than would be required by the pulse-echo method to achieve the same resolution. To provide further improvement of images the second harmonic signals can be employed. Here we describe the deconvolved inverse scattering imaging technique with account of the second harmonic signal. For this purpose the hybrid transducer by Krautkramer Branson Co., which consists of a cylindrical 5 MHz transducer wrapped in an annulus-shaped 2.5 MHz transducer, has been used. The phantom and soft tissue were imaged and in both cases the account of the second harmonic reflection data provides an improvement of the image quality.

Application of time delay spectrometry for rough surface characterization

This paper describes the design and performance of an ultrasound measurement system, utilizing a swept frequency excitation signal, for analyzing the backscattered signal from planar rough surfaces. The implementation is in the form of a time delay spectrometry (TDS) system operating in reflection mode whose advantages are improved signal-to-noise ratio even with low peak power relative to conventional pulse-echo methods. Because of simultaneous transmission and reception, the TDS system requires two transducers. The TDS measurement system uses a swept frequency spectrum analyzer as the central analog processing unit. Both planar piston and focused transducers in the low MHz range were used for the measurements. Due to the statistical nature of rough surface backscatter, the mean of several statistically uncorrelated measurements is required to characterize the scattering behavior of a given rough surface. Backscatter data are obtained for a series of planar rough surfaces, in the form of scattering magnitude vs. frequency and vs. incident (=backscattered) angle. Analysis of the results reveals a good correlation between the root-mean-square (RMS) height and mean backscatter magnitude at 0° incident angle, and between the ratio of RMS height to correlation length and the difference in mean backscatter magnitude between 0° and 5°.

Vibrating sample method for amplitude and phase measurements with the acoustic microscope

The letter describes how the scanning acoustic microscope may be operated in a continuous wave mode. By oscillating the sample by approximately a few hundred angstroms, single frequency amplitude and phase measurements may be obtained with the acoustic microscope. We also show that the method should allow very high frequency quantitative operation of the microscope, where conventional pulse echo methods would fail. A system implementation and preliminary results are presented.

Direct ultrasonic visualization of fatigue cracks

The advantages of direct ultrasonic visualization over conventional pulse-echo methods in detecting and characterizing fatigue cracks is demonstrated. It is shown that visualization is especially advantageous when the plane of the crack is not perpendicular to the ultrasound beam. Angles of inclination of about 20° can be clearly discerned by visualization whereas the pulse-echo pattern becomes difficult to interpret.

Simple Measurement Technique for Small Relative Velocity Changes of Gigahertz Acoustic Waves

A simple method for measuring small relative acoustic velocity changes of gigahertz acoustic waves over a temperature range from 1.7° to 50°K has been developed. It has been possible to extend the conventional pulse-echo method to the gigahertz frequency range by displaying the intermediate frequency (60 MHz) on an oscilloscope and recording the changes in the phase of the i.f. associated with a chosen echo as a function of temperature. Using a Tektronix 454 oscilloscope sweeping at 5 nsec/cm, a precision of about 10 ppm can be achieved. Results on KCl:Li+ and KCl:CN− illustrate the usefulness of the technique.