Jacques Goyette

Wavelet-transform-based method of analysis for Lamb-wave ultrasonic NDE signals

A new acoustic nondestructive method using Lamb waves as a probe is presented. These waves are generated and received by an ElectroMagnetic Acoustic Transducer (EMAT). The position of flaws in the structure under test is computed from the time of arrival of the main peak of the reflected signal. Due to the noisy nature of the received signal, we use a wavelet transform algorithm to extract the required time information. The main advantage of such a multi-scale method of signal analysis is to be suitable for peak detection problems especially in highly noisy environments. We explain how we proceed to do the feature extraction, and we propose two methods for reconstructing the image of the inspected structure. Results of real-world ultrasonic Lamb waves signal analysis are presented. In addition, to test the noise robustness of the method, the case of synthetic noisy signals is also treated.

A numerical dispersion compensation technique for time recompression of Lamb wave signals.

A Fourier domain numerical reconstruction technique has been created in order to eliminate the time spread of Lamb wave signals caused by their dispersive nature. This method allows a good time compaction of the echoes obtained from a Lamb wave inspection. In a pulse-echo setup, reflection peaks coming from targets located close one from each other that could not be separated or seen within raw signals are identified using this procedure. The utility of this new technique goes from simple signal analysis to imaging purposes such as the improvement of B-scan images or SAFT processing. It has been tested in three different situations with the S0 mode generated in a frequency bandwidth where it is highly dispersive. The reconstruction of a pure reflection coming from the edge of a plate, the separation of the echoes resulting from reflections on two targets near one each other and the effects of the presence of an obstacle between the emitter and the receiver are treated. Good results are obtained for every case studied.

A SAFT algorithm for lamb wave imaging of isotropic plate-like structures

Abstract Synthetic aperture focusing technique (SAFT) algorithm in the Fourier domain (F-SAFT) has been modified to take into account the dispersive nature of Lamb waves. This new algorithm can be used to produce good ultrasonic images of a plate structure for both dispersive and non-dispersive parts of the Lamb wave dispersion curve. This imaging technique has been tested by performing an inspection in a highly dispersive area of the fundamental flexural S0 mode in order to demonstrate the usefulness of the new algorithm. Results are presented as a comparison between standard B-scan, original F-SAFT algorithm and new Lamb-SAFT algorithm.

The catalytic effect of single-wall carbon nanotubes on the hydrogen sorption properties of sodium alanates

Single-wall carbon nanotubes (SWNTs) were examined as catalysts for improving the hydrogen absorption and desorption properties of Ti/Zr-doped NaAlH4 hydride, proposed as a reversible hydrogen storage material. We studied the hydrogen charge and discharge characteristics and stability of sodium aluminium composites ball milled with carbon additives such as SWNTs, graphite or activated carbon (AX-21). The SWNT–NaAlH4 system was tested at 160 °C for up to 200 cycles, and the sorption kinetics were enhanced by a factor of four. Also, the catalyzed NaAlH4 hydride with graphite and activated carbon additives shows fast absorption and desorption kinetics. Our results indicate that by creating new hydrogen transition sites, the structure of carbon in the composites plays an important role in enhancing the hydrogen absorption and release rates.

Ultrasonic NDE of composite material structures using wavelet coefficients

Abstract A wavelet-based method is proposed to perform the analysis of NDE ultrasonic signals received during the inspection of reinforced composite materials. The non-homogenous nature of such materials induces a very high level of structural noise which greatly complicates the interpretation of the NDE signals. By combining the time domain and the classical Fourier analysis, the wavelet transform provides simultaneously spectral representation and temporal order of the signal decomposition components. To construct a C-scan image from the wavelet transform of the A-scan signals, we propose a selection process of the wavelet coefficients, followed by an interpretation procedure based on a windowing process in the time–frequency domain. The proposed NDE method is tested on cryogenic glass/epoxy hydrogen reservoir samples.

Moisture effect on hydrogen storage properties of nanostructured MgH2–V–Ti composite

Abstract In this paper, we present our recent results on the effect of moisture during prolonged cycling on hydrogen storage properties of nanostructured MgH 2 hydride with V and Ti catalysts additions synthesized by ball milling. The hydrogen charge and discharge stability of the nanocomposite hydride has been tested at 300° C for up to 1000 cycles under hydrogen containing 101 ppm moisture. Between the first and the 500th or 1000th cycle, an increase of hydrogen storage capacity by about 5% is observed in dynamic and PCT measurements. This could be due to structural relaxations. While absorption kinetics remain fast, the results show a significant and systematic slow-down of the desorption rates by about a factor of two during cycling. The X-ray diffraction patterns of nanocomposite performed before and after 1000 cycles reveal that the peak shape for magnesium remains unchanged indicating that the crystal growth is negligible. This microstructural stability during cycling suggests that the decreasing desorption rate of the nanocrystalline magnesium-based composite is not induced by any internal structural modification. On the other hand, the presence of moisture in the hydrogen gas during cycling induces surface effects which most likely cause the decrease of the hydrogen discharge flow rate.

Neural classification of Lamb wave ultrasonic weld testing signals using wavelet coefficients

This paper presents an ultrasonic nondestructive weld testing method based on the wavelet transform (WT) of inspection signals and their classification by a neural network (NN). The use of Lamb waves generated by an electromagnetic acoustic transducer (EMAT) as a probe allows us to test metallic welds. In this work, the case of an aluminum weld is treated. The feature extraction is made by using a method of analysis based on the WT of the ultrasonic testing signals; a classification process of the features based on a neural classifier to interpret the results in terms of weld quality concludes the process. The aim of this complete process of analysis and classification of the testing ultrasonic signals is to lead to an automated system of weld or structure testing. Results of real-world ultrasonic Lamb wave signal analysis and classifications for an aluminum weld are presented; these demonstrate the feasibility and efficiency of the proposed method.

FREEZE DEHYDRATION OF FOAMED MILK BY MICROWAVES

ABSTRACT Previously published experiments on freeze-drying of foamed milk by microwaves are explained with an alternative theoretical model. The frozen milk foam is assumed to consist of parallel thin plates. A low-pressure gas stream consisting of air and water vapor mixture is believed to circulate freely in voids between plates. Air stream provides additional energy for the freeze-drying, helps to remove the sublimating water vapor and prevents an excessive icrowave heating of the dry milk foam.

Determination of the compressibility factor and measurement of traces of water in methane by a microwave method

Abstract We have studied a new microwave method to measure the compressibility factor and the water content of humid gaseous samples. This method uses a resonant cavity to measure the dielectric permittivity of the gas filling it. That measurement permits us to deduce the quantity of interest from the pressure and the dielectric constant virial expansions. By comparison of our measured compressibility factor of methane with the values computed from different equations of state, we have found that the microwave method is at least 0.1% accurate. We can also detect water vapor in methane down to a concentration of 75 ppm at 2 MPa.

Optimization study of a microwave differential technique for humidity measurement in gases

We present a simulation study of precision humidity sensing in gases by a microwave differential technique. We investigate the conditions for the optimization of the system capabilities. The small quantity at the parts per million (ppm) level of water vapor in gases is determined by measuring a shift of the resonant frequency of a measuring resonator relative to a reference one. Because of some unavoidable differences in the physical dimensions and electrical characteristics of the setup components, we studied the variation of the circuit parameters that could significantly have an impact on the sensitivity of the method. We also considered possible asymmetries between each branch of the circuit. It is concluded that our circuit design can tolerate imperfect symmetry of the components and still provide good sensitivity, as shown by our experimental results.