Ultrasonic inspection of long steel pipes using Lamb waves

An ultrasonic inspection technique using Lamb waves was evaluated to detect and determine the exact location of flaws present in long steel pipes. Since multiple modes of Lamb waves are generated due to their dispersive characteristics in the inspected pipes, the selection of a specific Lamb wave mode is very important for inspection of flaws. Experimental studies of flaw detectability with the use of each Lamb wave mode, namely, the A₀ S₀, A₁, and S₁ modes and their ultrasonic attenuation characteristics were conducted. The experimental results showed that the A₀ mode is the most effective for detection and exact determination of the location flaws. A lucite wedge containing a water column that generates the A₀ Lamb wave mode was developed and used in the present inspection study. It was found that the ultrasonic beam divergence after wrapping around the inspected pipe once interferes with exact determination of the location of flaws and that the maximum reflection signals are obtained when the transducer is offset axially from the straight line with the position of the flaw. The present study showed feasibilities of ultrasonic inspection with the use of Lamb waves for detection of flaws in insulated or inaccessible steel pipes.     

Optimization of lamb wave inspection techniques

Lamb waves can propagate over long distances which means that they are attractive for the quick, long range inspection of large structures, and they can also be useful for localized inspection, particularly in thin structures. This paper discusses the selection of the appropriate mode and frequency range for different inspection requirements and reviews the possible methods of excitation, response measurement and signal processing. It is usually desirable to transmit a single, non-dispersive mode, and excitation methods to achieve this are discussed. A variety of signal processing techniques from simple time domain to relatively complex two-dimensional Fourier analysis are available. Time domain processing can often be applied satisfactorily in low frequency-thickness regions where only two modes can propagate, but tends to be unreliable above the cut-off frequency of the a₁ mode. As an example of the design of a Lamb wave testing regime, a set of tests on a butt-welded steel plate with simulated weld defects of different depths is described. It is shown that by operating below the a₁ cut-off frequency with judicious selection of testing technique, the presence of defects can be detected reliably from changes in the shape of the received waveform.     

Evaluation of dispersion characteristics of multimodal guided waves using slant stack transform

In this paper slant stack (SL) transform is presented and its application for processing of multi-modal dispersive Lamb waves snapshots is proposed. The SL transform can facilitate the evaluation of dispersion curves based on a set of signals captured at the structure׳s surface. The presented technique leads explicitly to the frequency-phase-velocity representation of the processed signals. Theory behind the technique is presented and the SL results are compared to those obtained using the 2D discrete Fourier transform. The SL is used to process data acquired from an aluminum plate and to investigate anisotropic properties of a composite plate.     

Air coupled Lamb waves evaluation of the long-term thermo-oxidative ageing of carbon-epoxy plates

Thermo-oxidative ageing of carbon-epoxy plates has been done for up to 3500 h at two temperatures. Non-destructive evaluation by means of non-contact ultrasonic methods has been tested to characterize the aged plates. Experimental results obtained using Lamb waves propagation in the plates are presented here. Experimental and theoretical dispersion curves are compared. It is shown that the phase velocity of the A₀ and S₀ Lamb modes is decreasing when ageing duration increases. Assuming the plate to be homogeneous and orthotropic, we demonstrate that this phenomenon is related to an important decrease of the C₅₅ elastic parameter of the plate.     

Ultrasonic Lamb wave based monitoring of corrosion type of damage in plate using a circular array of piezoelectric transducers

In this paper we propose a concept and report experimental results based on a circular array of Piezoelectric Wafer Active Sensors (PWASs) for rapid localization and parametric identification of corrosion type damage in metallic plates. Implementation of this circular array of PWASs combines the use of ultrasonic Lamb wave propagation technique and an algorithm based on symmetry breaking in the signal pattern to locate and monitor the growth of a corrosion pit on a metallic plate. Wavelet time-frequency maps of the sensor signals are employed to obtain an insight regarding the effect of corrosion growth on the Lamb wave transmission in time-frequency scale. We present here a method to eliminate the time scale, which helps in identifying easily the signature of damage in the measured signals. The proposed method becomes useful in determining the approximate location of the damage with respect to the location of three neighboring sensors in the circular array. A cumulative damage index is computed from the wavelet coefficients for varying damage sizes and the results appear promising. Damage index is plotted against the damage parameters for frequency sweep of the excitation signal (a windowed sine signal). Results of corrosion damage are compared with circular holes of various sizes to demonstrate the applicability of present method to different types of damage.     

Detection of a straight groove in a metal plate by acoustic scattering in water with applications to marine current turbines

The developed method is based on acoustic underwater scattering measurements and can be applied to characterize a defect in a blade of a marine current turbine. To simplify the study, the blade is replaced by a rectangular plate immersed in water having a groove opening out. The measurements are made on horizontal plane perpendicular to the long axis of the stainless steel plate. The frequency range of signal is between 50 and 400 kHz. The transducer is remote from the plate. The measurements are recorded on 360° with a 1° step. The experimental trajectories of the scattering signals are compared to the trajectories calculated from the group velocities of Lamb waves A and S₀. Spectra give also an information on groove position. The present study is the first step toward remote monitoring of blades on a marine current turbine.     

Wavelet analysis based deconvolution to improve the resolution of scanning acoustic microscope images for the inspection of thin die layer in semiconductor

Scanning acoustic microscopy (SAM) is used as an important non-destructive test tool in semiconductor reliability evaluation and failure analysis. However, inspection of thin die layer has proven difficult as the reflected signals from the die top and bottom are superimposed. Conventional deconvolution techniques have been used for the improvement of time resolution in A-scan signal, however, they are not effective for SAM signal because the waveform of the reflected echo is quite different from the incident waveform due to the frequency dependent attenuation and the focal effect. In this paper, in order to overcome this difficulty, a new signal processing method, wavelet analysis based deconvolution technique is proposed. Its validity is approved by computer simulations and practical performances are demonstrated by experiments for the fabricated semiconductor sample.     

In-situ magnetic inspection of the part fixture and the residual stress in micromilled hot-work tool steel

A magnetic method for the in-situ inspection of the combined effects of both, clamping force (CF) and the distribution of the surface residual stress (RS) on a face milled workpiece are presented. FE simulations have been conducted and evaluated, illustrating the impact of clamping on the workpiece surface deflection and thus, the machining accuracy. Furthermore, magnetic Barkhausen noise (MBN) signals were acquired on a hot-work tool steel workpiece after three different milling procedures: Face milling, one pass of micro-milling and 38 passes of micro-milling. Finally, the MBN signal on the unclamped workpiece was obtained. The MBN signal energy MBN_energy presented the highest values with the probe oriented in the pick (clamping) direction for all stages of milling process where the CF was applied. After unclamping the workpiece, the higher value of the MBN_energy was obtained with the probe oriented in feed direction of the milling processes revealing the true effect of the RS. These results were in agreement with the literature and confirmed by numerical simulations and X-ray diffraction based residual stress measurements.     

Development of ultrasonic waveguide sensor for under-sodium inspection in a sodium-cooled fast reactor

The reactor core and in-vessel structures of a sodium-cooled fast reactor (SFR) cannot be examined visually due to the opaque sodium. The examination of the in-vessel structures is possible using ultrasonics to penetrate the sodium. A plate-type ultrasonic waveguide sensor using a leaky Lamb wave (A₀ mode) has been developed for under-sodium visual inspection of the reactor core and in-vessel structures. In the plate waveguide sensor, the A₀ leaky Lamb wave is utilized for the single mode generation and the effective radiation capability in a fluid. The liquid wedge is applied for the generation of the A₀ mode in the low frequency range. The long pulse tone-burst excitation should be applied to minimize the dispersion effect in 10 m long distance propagation of the A₀ Lamb wave. And a novel technique which is capable of steering a radiation beam of a waveguide sensor without a mechanical movement of the waveguide sensor has been suggested. The control of the beam angle can be achieved by a frequency tuning method of the excitation pulse in the dispersive low frequency range of the A₀ Lamb wave. A 10 m long ultrasonic waveguide sensor module which consists of a plate waveguide, a liquid wedge, an ultrasonic sensor, and an acoustical shielding protection tube has been designed and manufactured. The possibility of applying the ultrasonic waveguide sensor module to an under-sodium visual inspection has been investigated. The experimental tests such as the long distance propagation test of A₀ Lamb wave, the beam profile measurements, and C-scanning experiments in water have been carried out for the performance of the ultrasonic waveguide sensor. The feasibility of the ultrasonic waveguide sensor technique has been successfully demonstrated.     

Quantitative assessment of through-thickness crack size based on Lamb wave scattering in aluminium plates

The interaction of Lamb wave modes at varying frequencies with a through-thickness crack of different lengths in aluminium plates was analysed in terms of finite element method and experimental study. For oblique-wave incidence, both numerical and experimental results showed that the wave scattering from a crack leads to complicated transmission, reflection and diffraction accompanied by possible wave-mode conversion. A dual-PZT actuation scheme was therefore applied to generate the fundamental symmetrical mode (S₀) with enhanced energy to facilitate the identification of crack-scattered wave components. The relationship between crack length and the reflection/transmission coefficient obtained with the aid of the Hilbert transform was established, through which the crack length was quantitatively evaluated. The effects of wavelength of Lamb waves and wave diffraction on the properties of the reflection and transmission coefficients were analysed.     

Monitoring of the moisture content in carbon-epoxy plates using Lamb waves

Non-destructive material health monitoring is of great importance for aerospace industries, in particular for tanks made of composite materials, the mechanical properties of which can be altered by hydrothermal environment. In this context, the sensitivity of Lamb waves to various levels of moisture in a carbon-epoxy plate has been investigated. The plate has been manufactured and submitted to different cycles of hydrothermal aging and drying. For various steps of these cycles, both changes in weight and ultrasonic data are measured. Low-order A₀, S₀, S₁ Lamb modes are generated and detected using air-coupled transducers, and signal processing allows both real and imaginary parts of the wave-numbers to be measured. The real parts of the wave-numbers of all the generated modes are shown to be not sensitive to the moisture level of the material. However the imaginary part of the wave-number, i.e. the attenuation, of the A₀ mode is very sensitive to the moisture content, and its changes follow those of the plate weight, thus making it a promising mode for NDT of composite materials used in humid environments.     

Investigation of carbon fiber reinforced polymer (CFRP) sheet with subsurface defects inspection using thermal-wave radar imaging (TWRI) based on the multi-transform technique

A combined theoretical and experimental approach is reported using thermal-wave radar imaging (TRWI) for carbon fiber reinforced polymer (CFRP) with subsurface defects inspection. The multi-transform technique (Fourier transform, FT; Hilbert transform, HT; and cross-correlation, CC) is applied to extract the characteristics of thermal-wave signal. Experimental results indicate that the multi-transform technique of thermal-wave signal is available for detecting the subsurface defect. For the shallow defect (defect depth ≤1 mm), the delay time image of CC exhibits high contrast, and the phase image of FT has high SNR at the right frequency component. For the deep defect (defect depth 2.0 mm), the phase images of HT have both high contrast and large SNR value.     

Pendant droplet oscillation during GMAW

Abstract

This paper deals with pendant droplet oscillation in pulsed current gas metal arc welding (GMAW). Using a constant current power source, bead on plate welds were made on mild steel plates. The frequency of the pendant droplet oscillation was monitored visually (using a high speed video camera) and electronically (by analysis of the voltage signals). The results obtained are compared with the results predicted by a model based on a mass spring system. It was found that the oscillation frequency of small droplets revealed by both monitoring techniques matches that predicted by the model. For larger droplets, the oscillation frequency measured by voltage signal analysis tends to be higher than that revealed by video analysis and that predicted by the model. This implies that during droplet oscillation in (GMAW) the arc behaviour may change and as a result the arc voltage is not any more proportionally related to the arc length.     

金属薄板兰姆波检验技术

从兰姆波检验参考曲线的绘制与参考、对比试块设计与制造、探头的制作、反射回波与质点振动方向关系等方面,对金属薄板兰姆波检验进行了探索,得出了一些新的试验结果和认识.     

An evaluation of the electrochemical frequency modulation (EFM) technique

The EFM technique has been used to calculate anodic and cathodic Tafel slopes as well as corrosion current densities for the systems Cu/NaCl, Al 5083/NaCl, stainless steel/NaCl and mild steel in H₂SO₄ and NaCl. These values have been compared with those obtained from the analysis of polarization curves determined in the vicinity of the corrosion potential. In addition, polarization resistance values calculated based on the analysis of EFM data have been compared with values obtained from analysis of impedance data collected for the same systems. In most cases the corrosion rates determined with the EFM technique were much higher than the values determined with the other two techniques. This result is mainly due to the fact that for systems with low corrosion rates the modulation frequencies that are used in the EFM technique are in the capacitive region of the impedance spectra. Good agreement of corrosion rates obtained with the EFM and the polarization techniques was only observed for the mild steel/H₂SO₄ system that has very high corrosion rates. Analysis of EFM data assuming charge transfer or diffusion control produced different Tafel slopes and corrosion rates, but the same polarization resistance (R_p) values. When the EFM technique was applied to a resistor, the calculated R_p value equaled the value of the resistance which is further proof that the EFM technique measures the impedance of a corroding system in a narrow frequency band. The difference to the traditional EIS technique is that the amplitude of the ac signal can be different at different frequencies.     

Photoelectrochemistry and spectroscopy of substituted polyanilines

The properties of four substituted polyanilines, poly(N-methylaniline), poly(m-methylaniline), poly(o-methoxyaniline), and poly(o-ethoxyaniline) have been examined by photoelectrochemical and spectroscopic techniques (Raman and XPS). The middle peak in cyclic voltammograms for poly(N-methylaniline) was ascribed to hydrolysis products such as benzoquinone, and in the case of poly(o-methoxyaniline) to branched forms. The photocurrent profiles of the substituted polyanilines resembled those of polyaniline to a large extent, with prolonged cathodic and anodic photocurrents given for the polymer in a state of high conductivity. With poly(o-ethoxyaniline) only short-lived photocurrents were obtained pointing to a more highly insulating material, also shown by the limited growth of this polymer. Raman spectra of the substituted polyanilines taken in situ revealed typical polyaniline Raman bands, while the largest shift in the υ_CN band (35 cm⁻¹) occurred with poly(N-methylaniline). A deconvolution of the N 1s signal in XPS scans showed nitrogens consistent with oxidised forms of the polymers: a high proportion of positively charged nitrogens for poly(N-methylaniline) and of imine nitrogens for poly(m-methylaniline).     

Damage diagnostics on a welded zone of a steel truss member using an active sensing network system

In this paper, a sensor network system based on an active sensing scheme is introduced to identify cracks, which may occur at a welded zone of a steel truss member. The active sensing network system offers special potential for real world applications, as it is light, cheap, and useful as a built-in system. Four pairs of pitch–catch Lamb wave signals are utilized from the active sensing network system. In order to extract damage-sensitive features from the dispersive Lamb waves, a robust wavelet transform is applied to the original response signals. Peak values in the wavelet coefficients corresponding to the A₀ Lamb mode are only considered for application to the damage index. The root-mean-square change of the peak values due to damage is proposed as a damage index. In addition, a least-squares curve-fitting algorithm is applied to the damage indices in order to obtain a practical damage indicator with a threshold value that presents the damage tolerance. Finally, damage localization is carried out by investigating the change rates of the damage index according to each damage step. The applicability of the proposed methods has been demonstrated by an experimental study.     

Stress dependence of the spontaneous stray field signals of ferromagnetic steel

Measuring spontaneous stray field signals provides a promising tool to analyze the stress in ferromagnetic materials. However, strong initial stray field signals on the surface of ferromagnetic materials originating from various manufacturing processes can disturb stress-induced stray field signals. Consequently, it is necessary to conduct a study that will clarify the stress dependence of spontaneous stray field signals by eliminating initial random signals. In the present work, the focus is placed on sheet specimens that have a clean initial magnetic state by means of vacuum heat treatment. Measurements of the normal component H_p(y) signals of stray field were performed during the whole tensile test. The results showed that the stress-induced H_p(y) signal curve had good linearity after loading, i.e., the slope coefficient K_s increased continuously in the elastic deformation stage but decreased slightly during the plastic deformation stage. This phenomenon was discussed and explained from both the stress-induced effective magnetic field and residual compressive stress viewpoints.     

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.