Wahiba Djerir

The first section of this study is devoted to characterizing the lateral wave generated by a laser along the air-aluminum interface. This includes determining its propagation velocity, analyzing its spectrum, and evaluating the variation of its amplitude in relation to the generation/detection distance (d). The obtained results have shown that the lateral wave propagates at the speed of the longitudinal volume wave, following the d^–n law, where n equals 2.46. Its spectrum exhibits a wide bandwidth, with a cutoff frequency of fc = 3 MHz. The second part of the paper focuses on utilizing these waves for surface defect detection, with a comparative analysis of results obtained with Rayleigh waves. Various tests were conducted to analyze the impact of defects on the lateral wave on transmitted or reflected modes. The outcomes illustrate modifications in the temporal signals and frequency spectra of the lateral wave in the presence of defects.

Graphene has attracted extensive attention in various fields due to its intriguing properties. In this work, nanocomposite films based on poly(lactic acid) (PLA and PLLA) polymers filled with graphene oxide (GO) were developed. The impact of treating GO with the anionic surfactant dioctyl sulfosuccinate sodium salt (AOT) on the properties of the resulting nanocomposites was investigated. To determine the morphological, optical, and structural properties of the obtained materials, physicochemical analyses were performed, including scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD) analysis. Additionally, the thermal properties and wettability of neat polymers and nanocomposites were thoroughly investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and contact angle analysis. It was observed that GO was well dispersed throughout the PLA and PLLA matrix, leading to stronger interface bonding. The results demonstrate that the untreated and treated GO improved the crystallinity and thermal stability properties of the PLA and PLLA. However, the AOT-treated GO has significantly higher performance compared to the untreated GO in terms of crystallinity, melting temperature (increased by ~15 °C), and wettability (the contact angle decreased by ~30°). These findings reveal the high performance of the developed novel composite, which could be applied in tissue engineering as a scaffold.

Due to its advantageous properties, the critically refracted longitudinal waves (L_CR) are used in various fields of nondestructive evaluation, especially for residual stress measurements. However, although many researches use these waves, the characteristics of the L_CR beams are not completely understood to date. This paper contributes to clarify in a complete way and gives numerical and experimental analysis of the ultrasonic beam of the L_CR waves generated by a transducer. It also answers questions about the parameters that influence its beam profile and therefore allow performance optimization when used for NDE and NDT applications. First, beam particle displacement components are numerically calculated in both tangential (surface) and normal (depth) directions; the reflectivity diagrams are then deduced. The experimental study is also used to confirm the numerical results. Numerical and experimental studies have shown that the displacement of the L_CR waves at the surface follows a constant law, which does not depend on the frequencies of the sensor. On the other hand, it is shown that the incidents angles, the aperture diameter and the center frequency are the most important factors in controlling the directivity and subsequently the position of the main lobe of the L_CR waves.

The aim of this work is the detection of surface defects by the refracted longitudinal wave and the study of the parameters that influence the use of this wave in transmission and reflection modes in order to optimize the non-destructive testing (NDT) conditions with the L_CR waves. For this, two types of defects are considered, cylindrical defects of different lengths 5 and 8 mm with depth of 10 mm and a rectangular groove of length 8 mm, width 2 mm and depth of 10 mm. The L_CR wave was generated by transducers of center frequencies f_c = 1 MHz and f_c = 3.5 MHz with an inclination at a fixed-angle equal to 27° in aluminum. This study is divided into two parts. The first explains the changes in the signals caused by this interaction from a temporal point of view. The second part analyzes the energy losses induced on the spectrum. The reference signal is acquired in a defect-free sample. By comparison between the time signals and the respective reference signals, it has been found experimentally a decrease in the amplitude and a time shift of the L_CR wave due to the presence of the defect. The sample spectra with a defect are modified relative to the spectrum obtained for the sample without defect, these are narrowed particularly for high frequencies. A shift of the centre frequency to lower values can also be noticed. The influences of the sensor frequency and the transmitter/receiver distance have been studied. When the frequency-sensor increases, the effects on the L_CR wave by the defect increase. The study showed, that the increase in frequency allows the detection of smaller defects. Also, because of the inclination of the L_CR beam, the choice of the distance between the transmitter and the receiver affects the defect detection. The reducing of the distance between the transmitter and the receiver improves the defects detection. The natures of the defects influence also the defects detection. In reflection mode, it is also possible to detected defect and the results found are satisfactory. Therefore this method is very effective because it identifies the defect and its location.

The aim of this work is to propose an experimental method to evaluate the steel carbon content by ultrasound. The sample is immersed in a water tank in order to analyze it under various incidences of sound waves. Longitudinal wave velocities are measured by immersion by using a 5-MHz frequency probe. Transverse wave velocities are measured in a contact mode by using a 4-MHz transverse wave transducer. The attenuation coefficients of ultrasonic longitudinal and transverse waves are deduced from three successive basic echoes through the sample. The effects of some heat treatments on ultrasonic parameters are also studied. The measurement of ultrasonic parameters in steel offers an interesting possibility of tracing the carbon content and, at the same time, provides information on the steel structure and its elasticity.

The critically refracted longitudinal (L_CR) waves are used in various domains of nondestructive evaluation, especially for residual stress measurements. However, few works have characterized the associated ultrasonic beam. In this article, the characterization of the L_CR ultrasonic beam, both numerically and experimentally, is first clarified in order to provide some answers to questions that arise about its behavior in elastic solids. The aim of the second part of this work is to investigate the use of the L_CR waves for the detection of surface defects of different sizes in aluminum. For that, the effect of defects at fixed depth (5 mm) with various diameters (2, 4, 6, and 8 mm) and fixed diameter (8 mm) with various depths (5, 7, and 10 mm) in an aluminum sample have been investigated at frequencies around 1 MHz. It has been experimentally found that the amplitude of the (L_CR) wave decreases and its frequency spectrum changes with a given defect. The rate of the decrease of the amplitude and the change in the spectrum is related to the increase of the defect depth. The study shows that the effect on the propagating of the L_CR waves is larger as the defect depth increases.

A mechanical or thermal treatment of a material can change, among other things, the average grains size. It depends on temperature, holding time, cooling condition or rolling stress. The average grain size, as well as its influence on the propagation velocity and attenuation coefficient of ultrasonic waves, also affects the wave frequency propagating through the material. Grain size is an indicator of material fatigue. It can therefore be used in monitoring or fatigue damage prevention. In this paper, we study the effect of various heat treatments hence different steel average grain sizes on the ultrasonic wave frequency after crossing the material. We have performed the same experimental study on aluminum samples. The different grain sizes are obtained by rolling. The frequency shift measurement of longitudinal waves is achieved by immersion with two probes of different frequencies 2.25 and 5 MHz. The experimental results are shown as curves giving the frequencies depending on the grain size. Heat treatments on steel and aluminum rolling performed on the samples have yielded a grain sizes gradient. Our results are consistent with the theory because of the important path in the sample and in this case the down shift frequency is paramount. They show a direct relationship between the frequency shift and the average grain size. It is therefore possible to trace quantitatively to an average grains size from the frequency of an ultrasonic wave that has passed through this material and hence its thermal or mechanical fatigue state.

When measuring the ultrasound field, the signal provided by the receiving transducer is affected by its spatial properties. Particularly, the displacement normal to its surface is spatially averaged because of the receiver finite size. In this study, we show using a numerical simulation, the effectiveness of the spatial deconvolution of these effects for a rectangular transducer. For that, three methods allowing the inversion of the aperture effect are tested 1) Wiener’s method; 2) the power spectral equalization (PSE) method, and 3) the maximum a posteriori (MAP) method. The obtained results show that the three methods are able to reconstruct the ultrasound field from the spatially averaged values and the quality of the reconstruction depends strongly upon the signal to noise ratio (SNR) and the spatial frequencies of the ultrasound field investigated