Elastic parameters characterization of multilayered structures by air-coupled ultrasonic transmission and genetic algorithm.

This paper describes a non-contact method to characterize isotropic and anisotropic planar multilayer structures using a genetic algorithm. The method is based on the determination of critical angles, where the maxima of the modulus of transmission coefficient of the structure appear, and which correspond to the generation of guided waves. The optimization process minimizes the error between the reference critical angles and associated amplitudes of the transmission coefficient, with the corresponding estimated ones. The estimation of elastic parameters is demonstrated for acrylic and oak plates as well as for a bi-layered structure composed of oak and a thin layer of gesso. It is shown that to obtain satisfactory optimization results, it is necessary for guided modes of higher order than the zero ones to be taken into account. Results also show that some elastic constants such as C33 and C55 retrieved from the transmission coefficient are very sensitive to the optimization.

Efficient algorithm for discrimination of overlapping ultrasonic echoes.

We propose a method to identify the different echoes of an overlapped ultrasonic signal. This method is based on an iterative algorithm that compares the experimental signal to a realistic dictionary of trial functions and allows identification of one overlapped echo at each iteration. Adding physical parameters to the dictionary such as sample attenuation and ultrasound beam diffraction allows the method to be applied to various materials and sample geometries. Measurements at 500kHz and 5MHz on a ABS material and a copper plate are reported. The effectiveness and the robustness of the method are studied as a function of time delay between the different echoes. We show that taking into account the experimental set-up and material properties in the development of the dictionary are critical to identifying a round-trip signal when overlapping occurs.

Ultrasonic self-calibrated method applied to monitoring of sol-gel transition.

In many industrial processes where online control is necessary such as in the food industry, the real time monitoring of visco-elastic properties is essential to ensure the quantity of production. Acoustic methods have shown that reliable properties could be obtained from measurements of velocity and attenuation. This paper proposes a simple, real time ultrasound method for monitoring linear medium properties (phase velocity and attenuation) that vary in time. The method is based on a pulse echo measurement and is self-calibrated. Results on a silica gel are reported and the importance of taking into account the changes of the mechanical loading on the front face of the transducer will be shown. This is done through a modification of the emission and reception transfer parameters. The simultaneous measurement of the input and output currents and voltages enables these parameters to be calculated during the reaction. The variations of the transfer parameters are in the order of 6% and predominate other effects. The evolution of the ultrasonic longitudinal wave phase velocity and attenuation as a function of time allows the characteristic times of the chemical reaction to be determined. The results are well correlated with the gelation time measured by rheological method at low frequency.