Guillaume Robin

In situ elaboration of a binary Ti-26Nb alloy by selective laser melting of elemental titanium and niobium mixed powders.

Ti-Nb alloys are excellent candidates for biomedical applications such as implantology and joint replacement because of their very low elastic modulus, their excellent biocompatibility and their high strength. A low elastic modulus, close to that of the cortical bone minimizes the stress shielding effect that appears subsequent to the insertion of an implant. The objective of this study is to investigate the microstructural and mechanical properties of a Ti-Nb alloy elaborated by selective laser melting on powder bed of a mixture of Ti and Nb elemental powders (26 at.%). The influence of operating parameters on porosity of manufactured samples and on efficacy of dissolving Nb particles in Ti was studied. The results obtained by optical microscopy, SEM analysis and X-ray microtomography show that the laser energy has a significant effect on the compactness and homogeneity of the manufactured parts. Homogeneous and compact samples were obtained for high energy levels. Microstructure of these samples has been further characterized. Their mechanical properties were assessed by ultrasonic measures and the Youngs modulus found is close to that of classically elaborated Ti-26 Nbingot.

Phase Transformation States of Loose and Dense Granular Materials under Proportional Strain Loading

AbstractDepending on the rate of external loading, the permeability, and the boundary conditions, different drainage conditions are expected during the loading of sands. These drainage conditions are thus neither fully drained nor fully undrained as usually assumed for saturated soils, which implies simultaneous changes in pore volume and in pore water pressure. For a given material, the transition between contractive behavior (increase of the pore water pressure) and dilative behavior (decrease of the pore water pressure) defines the phase transformation state. It is found that the position of this phase transformation state, which is known to be dependent on the relative density, depends on the rate of dilatancy (e˙v/e˙1) imposed on the sample. Constraining a loose sample to dilate leads to an unstable state earlier than observed during undrained tests. Even dense samples, which do not exhibit a nonlocalized unstable state during undrained tests, are prone to collapse if the rate to dilatancy is high en...

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.

Elastic properties in a clayey material under mechanical loading – an estimation through ultrasonic propagations

The aim of this research was to characterise the elastic properties of a clayey material Kaolinite K13 and its change during a mechanical loading. An experimental test system, based on the ultrasonic propagation method, was developed. The ultrasonic tests were performed on samples recovered after oedometric loading. The experimental approach consists in investigating, through acoustic local measurements, the evolution of the elastic parameters along the oedometric path. Measurement of wave velocities at different frequencies was performed using an insertion–substitution method. Afterwards, the dynamic elastic modulus deduced from the wave velocities was compared to the static modulus obtained in the oedometric test.

Non linear parametric imaging using an ultrasonic focused transducer

In this work, a quantitative ultrasonic non linear parameter imaging technique in soft materials is developed. Using a self calibrated method in pulse echo mode, absolute local measurements of fundamental and harmonic pressures are determined and the non-linear parameter is locally measured with a focused transducer. The set up is coupled with an XY scanning system to produce images and the potential of the technique is demonstrated.

Monitoring of the nonlinear coefficient during silica gel formation using an ultrasonic self-calibrated method

The sol-gel process is currently used to develop new materials with a wide range of applications in chemical and food industries. An ultrasonic self-calibrated method is used to evaluate the behavior of the nonlinear coefficient during the sol to gel reaction. The method uses a single piezoelectric transducer in burst echo-mode. Assuming a classical non linearity, the propagation of ultrasonic waves in the sol-gel material can be modeled in a quasi-linear regime using the KZK equation. The temporal evolution of the nonlinear coefficient of silica gel is analyzed. A variation of around 6% of the nonlinear coefficient is observed during the formation of a sol-gel with an initial hydrolysis ratio of 4 and a surrounding temperature at 30°C. Nonlinear results are compared to linear measurements and to literature data, showing a good sensitivity of the nonlinear coefficient with the conventional linear approach of gelation process monitoring.