Determination of the cutoff frequency of an acoustic circumferential wave using a time–frequency analysis

In this article, time–frequency representation of Wigner–Ville is used to analyse the acoustic signal backscattered by a thin elastic tube of radii ratio b/a. This analysis allows to determine the reduced cutoff frequency of the circumferential antisymmetric wave A1 propagating around the tube. The evolution of this reduced cutoff frequency in function of b/a is reported. The values obtained of reduced cutoff frequency are compared to the values computed from the proper modes method.     

Time–energy density analysis based on wavelet transform

Energy is an important physical variable in signal analysis. The distribution of energy with the change of time and frequency can show the characteristics of a signal. A time–energy density analysis approach based on wavelet transform is proposed in this paper. This method can analyze the energy distribution of signal with the change of time in different frequency bands. Simulation and practical application of the proposed method to roller bearing with faults show that the time–energy density analysis approach can extract the fault characteristics from vibration signal efficiently.     

Lamb and SH waves generated and detected by air-coupled ultrasonic transducers in composite material plates

Electrostatic, air-coupled, ultrasonic transducers are used to generate and detect guided waves in anisotropic solid plates. Waves considered in this study are Lamb-type and SH-type, guided modes. If the plane of propagation coincides with a plane of symmetry of the material, then Lamb modes only are launched and detected by the transducers. If the plane of propagation does not coincide with a plane of symmetry of the material, then Lamb modes are still generated and detected, but guided, SH-like modes are, too. The variation of phase velocity with frequency is measured for several modes propagating in different directions along a glass–epoxy composite plate.A numerical model that takes into account the anisotropy of composite materials is developed to predict the dispersion curves (phase velocity, group velocity or wave-number versus frequency) and the displacement fields of plate waves, the plane of propagation being either a plane of symmetry or not.The experimental phase velocities are in good agreement with the predicted dispersion curves, thus showing that the forward problem concerning the propagation of plate waves in anisotropic, homogeneous, composite material plates is properly solved. The dispersion curves associated with the predicted displacement fields show that guided modes in composite plates have different behaviors depending on their direction of propagation.     

Ultrasonic non-destructive evaluation with spatial combination of Wigner–Ville Transforms

This paper introduces a novel ultrasonic signal combination technique to be applied in detection systems based on multiple transducers. The technique uses a spatial combination approach that considers the specimen inspection from several apertures located in different planes. Information received from transducers is fused in a common integrated pattern with a signal to noise ratio (SNR) improvement. The result of the combination is a high quality image of the inspected material obtained from simple A-scans.The method is based on digital signal processing techniques, more concretely time–frequency analysis. Combination is performed by means of the Wigner–Ville Transform preserving temporal and frequencial information. Temporal techniques for combination are presented and the results obtained from both techniques are compared using the SNR.     

Thermal–mechanical analysis of the surface waves in laser cleaning

The work described in this paper is to investigate the particle removal mechanism by surface wave in laser cleaning. A mathematical model of laser cleaning processes was established for a flat plate to remove micro-particles from the surfaces by surface waves. The software ABAQUS, based on finite element method, was used to simulate the non-linear stress wave problem for an uncoupled thermal–mechanical system in a two-dimensional domain. The result shows that the cleaned area increases with the laser energy, which led to surface waves with large acceleration in the vertical direction, but the effective cleaned area decreases with an increase of the laser spot size.     

Non-contact estimation of thickness and elastic properties of metallic foils by laser-generated Lamb waves

Non-contact estimation of the thickness and elastic properties of metallic foils was attempted by quantitative analysis of velocity dispersion of laser-generated Lamb waves. Lamb waves were generated in stainless steel (AISI304) foils with a thickness of less than 40 μm by a Q-switched Nd:YAG laser. Both the zeroth order symmetric S₀ and anti-symmetric A₀ waves were monitored using a heterodyne-type laser interferometer. Dispersion of group velocity of the A₀ mode was obtained by the wavelet transformation, and was found to agree well with the numerical solution of the Rayleigh–Lamb equation. A modified method to estimate both the thickness and acoustic (or elastic) properties from the sheet wave velocity and the group velocity dispersion of the A₀ mode was proposed. The modified method was found to provide a correct estimate for stainless steel foils thinner than 30 μm.     

IE–SASW method for nondestructive evaluation of concrete structure

The Impact–Echo (IE) method has been widely used to evaluate the integrity of concrete structures. In this method, the P-wave velocity of concrete is a crucial parameter in determining the thickness of concrete slab and the location of cracks or other defects. To determine P-wave velocity of concrete, in this paper, Spectral Analysis of Surface Wave (SASW) method was employed, and IE–SASW method was suggested by combining two nondestructive testing methods. IE method was used for the detailed nondestructive evaluation of concrete whereas SASW method was employed for the measurement of the average P-wave velocity and for the status evaluation of concrete. The feasibility study of SASW method was performed by using finite element method. Experimental studies were also performed in the slab type concrete model specimens in which various types of defects or boundaries were included at known locations. SASW tests showed the potential of determining the P-wave velocity of concrete accurately and IE tests were able to determine the thickness of structures and locations of defects. Based on both experimental and numerical studies, the feasibility of the proposed method was verified.     

The Zn-rich corner of the 450 °C isothermal section of the Zn–Bi–Fe–Ni quaternary system

Following up on recent studies of the isothermal section of the Zn–Fe–Ni, Zn–Fe–Bi and Zn–Bi–Ni ternary systems at 450 °C, the Zn-rich corner of the 450 °C isothermal section of the Zn–Bi–Fe–Ni quaternary system with the Zn being fixed at 93 at.% was determined experimentally using the equilibrated alloys approach. The specimens were investigated by means of scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). It was found there exist 4 two-phase regions, 5 three-phase regions and 2 four-phase regions. Two liquid L (Zn) and L (Bi) can coexist with T, ζ and δ-Ni in this isothermal section, no new phase was found in this study.     

Fault detection and location in gears by the smoothed instantaneous power spectrum distribution

Time–frequency methods are effective tools for analysing diagnostics signals and have been widely used to describe machine condition. This paper introduces a time–frequency distribution, called the smoothed instantaneous power spectrum (SIPS) distribution, and demonstrates its use in the detection and location of local tooth defects in gears. The SIPS distribution is derived from the frequency domain definition of the instantaneous power spectrum (IPS) distribution, but has the added advantage that provides a considerable reduction in the ringing effect of the IPS transform, which results in a smoother and clearer time–frequency representation. A simulated gear vibration signal is used to show the capabilities of the proposed method over the IPS distribution and spectrogram. Healthy and faulty vibration signals monitored from a gear test rig are analysed, the results of which show that a local gear tooth defect can clearly be detected by the SIPS distribution.     

Experimental study of ultrasound propagation at a liquid–solid composite interface for inspection of liquid–metal cooled nuclear reactors

The influence of unfilled cavities at a liquid–solid interface on ultrasound propagation is investigated. This kind of interface exists only when the surface is rough and the liquid is non-wetting. Normally incident compression waves are used. Possible modelling approaches are discussed, showing that no model is able to efficiently describe this kind of interface. We demonstrate that wave transmission drops dramatically. It is suggested that the incoming ultrasonic energy induces the growth and the coalescence of the vapour phase contained in the unfilled cavities under ultrasound field. A major result of this paper is to provide proof that difficult experiments in metal–liquid can be replaced by easier experiments in water.     

Glass–rubber phase transformation detected in polymers by means of ultrasonic waves

Ultrasonic attenuation and velocity measurements were carried out in order to study the kinetics of water sorption process in hydrogels characterized by strong structural changes occurring in the material. Hydrogel, or gel contains water, can exist in two forms: a solid glassy phase when it is dry, a rubber phase in equilibrium with water. Scanning laser acoustic microscope (SLAM) technique has been used to monitor the change in the ultrasonic attenuation, during water sorption in crosslinked poly(hydroxyethylmethacrylate) and poly-vinyl alcohol (PVA) hydrogels of different thickness at the frequencies of 10 and 30 MHz. The pulse-echo technique has been applied to the measurement of the longitudinal velocity and ultrasonic attenuation and to monitor the advancement of the swollen–unswollen fronts. During the hydrogel water sorption a peak in the ultrasonic attenuation and a decrease in the longitudinal velocity have been detected. The increment in the ultrasonic attenuation has been analysed in terms of reflections from the swollen–unswollen boundary, microvoids scattering and absorption of the ultrasonic waves due to the glass to rubber transformation.     

Application of the ultrasonic pulse-echo technique for quality control of the multi-layered plastic materials

The main aim of the ultrasonic pulse-echo technique application for quality control is to characterize the internal structure of the object under investigation. The advantage of such technique is a possibility to perform non-contact and one-side access measurements, and to investigate the internal structure of multi-layered materials as well. The presented novel application of the ultrasonic pulse-echo technique for characterization of the multi-layered plastic materials covers the complete attenuation measurement in the frequency domain and is based on ill-posed Tikhonov regularization task for each layer separately. The law of the frequency-dependent attenuation has been estimated from the inverse transfer function approximation in the frequency domain. Phase velocity dispersion curves have been estimated in two ways: from the experimental signal phase spectra and from the causal Kramers–Kronig relations. The developed method enables to predict waveforms of the reflected signals from the interfaces of the individual layers in on-line mode.According to this approach, the step-by-step iterative analysis has been performed for each layer using the information about the previous layers. During each step, the acoustic properties of an individual layer, such as density, absorption, ultrasound velocity and phase velocity dispersion, have been recovered using numerical optimization. Optimization has been performed comparing the real ultrasonic signal, reflected by multi-layered object, with the simulated response of the model. The comparison of the predicted waveforms with the experimental ones has shown a good correspondence.     

Effect of melt convection on the secondary dendritic arm spacing in peritectic Nd–Fe–B alloy

Dendrites are one of the major microstructural constituents of peritectic alloys. In the present work, the effect of melt convection on the secondary dendritic arm spacing (SDAS) and volume fraction of properitectic α-Fe was investigated during solidification of stoichiometric Nd–Fe–B alloys using the forced crucible rotation technique. The resulting microstructure of the alloy in consideration of melt convection has been investigated using scanning electron microscopy and optical microscopy. The average SDAS was determined for each sample from the whole cross-section of the cylindrical test samples using image analyzing software LEICA QWIN. A detailed statistical analysis of the spacing distribution was performed on the basis of the variation of SDAS values, averaged from about 80 to 120 dendrites in different zones. The α-Fe volume fraction, measured by vibrating sample magnetometer (VSM), reduces with increasing crucible rotation frequency. Similarly, the SDAS values decrease with increasing rotation frequency. These results are explained from the viewpoint of a reduced melt convection state under steady forced crucible rotation leading to a reduced effective mass transfer coefficient.     

Oxidation of a quaternary two-phase Cu–40Ni–17.5Cr–2.5Al alloy at 973–1073 K in 101 kPa O2

Oxidation of a quaternary two-phase Cu–40Ni–17.5Cr–2.5Al (at.%) alloy was investigated at 973–1073 K in 101 kPa O₂. The alloy is composed of two phases. One light phase with lower Cr content forms the matrix of the alloy, and the other medium gray phase richer in Cr is presented in the form of continuous islands. At 973 and 1073 K, the kinetic curves for the present alloy deviate evidently from the parabolic rate law. They show a large mass gain in initial stage, and then their oxidation rates decrease evidently with time until they become very small up to 24 h. Cross sectional morphologies show the present alloy is able to form continuous external scales of chromia over the alloy surface with a gradual decrease in the oxidation rate. However, the previous studies showed that a ternary two-phase Cu–40Ni–20Cr alloy is unable to form protective external scales of chromia over the alloy surface, but is able to form a thin and very irregularly continuous layer of chromia at the top of the mixed internal oxidation region. Therefore, substituting Cr in Cu–40Ni–20Cr alloy with 2.5 at.% Al is able to decrease the critical content required to form Cr oxide and help to form continuous external scales of chromia under lower Cr content in two-phase alloys.     

Structure and optical properties of nanocrystalline NiO thin film synthesized by sol–gel spin-coating method

NiO thin film was prepared by sol–gel spin-coating method. This thin film annealed at T = 600 °C. The structure of NiO thin film was investigated by means of X-ray diffraction (XRD) technique and scanning electron microscopy (SEM). The optical properties of the deposited film were characterized from the analysis of the experimentally recorded transmittance and reflectance data in the spectral wavelength range of 300–800 nm. The values of some important parameters of the studied films are determined, such as refractive index (n), extinction coefficient (k), optical absorption coefficient (α) and band energy gap (E_g). According to the analysis of dispersion curves, it has been found that the dispersion data obeyed the single oscillator of the Wemple–DiDomenico model, from which the dispersion parameters and high-frequency dielectric constant were determined. In such work, from the transmission spectra, the dielectric constant (_∞), the third-order optical nonlinear susceptibility χ⁽³⁾, volume energy loss function (VELF) and surface energy loss function (SELF) were determined.     

Thermodynamic assessment of the Ga–Sc and Ga–Tb systems

The Ga–Sc and Ga–Tb binary systems have been assessed with CALPHAD method. Liquid is treated as substitutional solution phase, of which the excess Gibbs energies are modeled by Redlich–Kister polynomial function. The binary intermetallic compounds are treated as stoichiometric phases. Thermodynamic parameters of various phases have been optimized and the calculated results are in reasonable agreement with experimental data.     

Phase formation and structures of quasicrystals and approximants in the Zn–Mg–(Ti, Zr, Hf) system

The compositional regions for primitive (P) and face-centered (F) icosahedral quasicrystals (iQc) have been determined to be around Zn₈₄Mg₉Zr₇ and Zn₇₅Mg₁₈Zr₆, respectively. A 1/1 approximant of the F-type iQc was found to have a composition around Zn₇₇Mg₁₈Zr₅. A similar tendency has been verified for the Zn–Mg–Hf system. No stable iQc was observed in the Zn–Mg–Ti system. High-resolution X-ray measurements performed with synchrotron radiation showed that the stable iQcs are highly ordered and contain less phason disorder. High-angle annular dark field (HAADF) scanning transmission electron microscopy observation of the 1/1 Zn–Mg–Hf approximant effectively revealed the Hf positions in the structure, whose local contrasts can be reasonably interpreted by a structural model where icosahedral and dodecahedral Hf clusters are mutually interpenetrated. Similar appearances of local contrasts were frequently observed in HAADF images of the F-type iQc, indicating that the iQc structure is also build up of icosahedral clusters that are almost identical to those in the 1/1 approximant.     

Thermodynamic analysis of alloys Ti–Al, Ti–V, Al–V and Ti–Al–V

Thermodynamic analysis of three binary Ti-based alloys: Ti–Al, Ti–V, and Al–V, as well as ternary alloy Ti–Al–V, is shown in this paper. Thermodynamic analysis involved thermodynamic determination of activities, coefficient of activities, partial and integral values for enthalpies and Gibbs energies of mixing and excess energies at four different temperatures: 2000, 2073, 2200 and 2273 K, as well as calculated phase diagrams for the investigated binary and ternary systems. The FactSage is used for all thermodynamic calculations.     

Thermo-optical modulation for improved ultrasonic fatigue crack detection in Ti–6Al–4V

Pulsed infrared laser irradiation was used to positively identify small fatigue cracks on the surface of fatigue damaged Ti–6Al–4V specimens. The resulting transient thermoelastic deformation perceptibly changes the opening of partially closed surface cracks without affecting other scatterers, such as surface grooves, corrosion pits, coarse grains, etc. that might hide the fatigue crack from ultrasonic detection. We found that this method, which was previously shown to be very effective in 2024 aluminum alloy, must be modified in order to successfully adapt it to Ti–6Al–4V titanium alloy, where significant thermo-optical modulation was found even from straight corners or open notches. This spurious modulation is caused by direct thermal modulation of the sound velocity in the intact material rather than thermal stresses via crack closure. Different methods have been developed to distinguish direct thermal modulation from crack-closure modulation due to thermoelastic stresses. It was found that the modified thermo-optical modulation method can increase the detectability of hidden fatigue cracks in Ti–6Al–4V specimens by approximately one order of magnitude.     

The microstructures and mechanical properties of cast Mg–Zn–Al–RE alloys

The microstructures and mechanical properties of cast Mg–Zn–Al–RE alloys with 4 wt.% RE and variable Zn and Al contents were investigated. The results show that the alloys mainly consist of α-Mg, Al₂REZn₂, Al₄RE and τ-Mg₃₂(Al,Zn)₄₉ phases, and a little amount of the β-Mg₁₇Al₁₂ phase will also be formed with certain Zn and Al contents. When increasing the Zn or Al content, the distribution of the Al₂REZn₂ and Al₄RE phases will be changed from cluster to dispersed, and the content of τ-Mg₃₂(Al,Zn)₄₉ phase increased gradually. The distribution of the Al₂REZn₂ and Al₄RE phases, and the content of β- or τ-phase are critical to the mechanical properties of Mg–Zn–Al–RE alloys. The Mg–6Zn–5Al–4RE alloy with cluster Al₂REZn₂ phase and low content of β-phase, exhibits the optimal mechanical properties, and the ultimate tensile strength, yield strength and elongation are 242 MPa, 140 MPa and 6.4% at room temperature, respectively.