Modeling and numerical study of the influence of imperfect interface properties on the reflectance function for anisotropic multilayered structures

International audienc

Non-Contact Ultrasonic Exploration of Ancient Paintings

Artwork are complex structures formed by multiple layers of materials with different properties and thicknesses. They are fragile structures and can be damaged by exposure to temperature, humidity, and vibrations. Techniques such as raking light, x-rays, infrared and UV provide restorers with some information about the condition of the work and its past. The ultrasonic technique, unlike the others, uses elastic waves to provide information about the mechanical characteristics of the materials. In this work we have developed a system that uses focused air-coupled transducers together with continuous motor scanning devices in order to perform experimental tests on several old paintings

Adaptive innate immunity? Responsive-mode prophylaxis in the mealworm beetle, Tenebrio molitor.

A primary infection by a parasite may indicate a higher risk of being reinfected in the near future (since infection may indicate that enemies are becoming more abundant). Acquired immunity does not exist in invertebrates despite the fact that they also face increased risks of reinfection following primary exposure. However, when subjected to immune insult, insects can produce immune responses that persist for long enough to provide prophylaxis. Because these immune responses are costly, persistence must be maintained through a selective advantage. We tested for the possibility that these long-lasting immune responses provided increased resistance to later infections by experimentally mimicking a primary immune insult (pre-challenge) in larvae of the mealworm beetle, Tenebrio molitor, with lipopolysaccharides (LPS) prior to early or late exposure to spores of the entomopathogenic fungus Metarhizium anisopliae. We found that pre-challenged larvae produced a long-lasting antimicrobial response, which provided a survival benefit when the larvae were exposed to fungal infection. These results suggest that the observed response is functionally "adaptive"

Electromechanical Behavior of a Doped Barium Titanate Piezoceramic Under Mechanical Stress: Modeling and Comparison With Experimental Measurements

International audienc

Modelling of hysteretic behaviour of piezoceramic materials under external electrical and mechanical stress

International audienceThe purpose of this study is to model the non-linear hysteretic behaviour of piezoceramic materials under an external (mechanical or electrical) stress. The model is constructed by bridging the characteristics of microscopic domain distribution into the macroscopic behaviour. Choosing the remanent strain and remanent polarisation as internal variables, a domain orientation distribution is used to describe the evolution of these variables and to develop a phenomenological model of ferroelasticity for electromechanical loading histories. This model is able to reproduce the (longitudinal and transversal) strain and electric displacement in function of uniaxial electrical and mechanical loading. This formulation is validated through a comparison with experimental data of the literature. The evolution of the elastic and piezoelectric constants under mechanical stress is successfully compared as well

A non-invasive method of tendon force measurement

International audienceThe ability to measure the forces exerted in vivo on tendons and, consequently, the forces produced by muscles on tendons, offers a unique opportunity to investigate questions in disciplines as varied as physiology, biomechanics, orthopaedics and neuroscience. Until now, tendon loads could be assessed directly only by means of invasive sensors implanted within or attached to these collagenous structures. This study shows that the forces acting on tendons can be measured, in a non-invasive way, from the analysis of the propagation of an acoustic wave. Using the equine superficial digital flexor tendon as a model, it is demonstrated that the velocity of an ultrasonic wave propagating along the main axis of a tendon increases with the force applied to this tendon. Furthermore, we show that this velocity measurement can be performed even in the presence of skin overlying the tendon. To validate this measurement technique in vivo, the ultrasonic velocity plots obtained in the Achilles tendon at the walk were compared to the loads plots reported by other authors using invasive transducers

Modeling of the Electromechanical Behavior of Barium Titanate Under Mechanical Stress and Comparison With Experimental Measurements

International audienc

Theoretical and experimental study of the electroacoustic behavior of lithium niobate under an initial mechanical stress

International audienceRecently, a second-order formalism of piezoelectric structures under an external mechanical stress was developed. Because the yield strength of lithium niobate was unknown, this study was not able to describe and evaluate realistic benefits of a prestress load on electromechanical properties. Therefore, in this study, experimental determination of the yield strength of lithium niobate sample is performed and shows that this limit is close to 110 MPa. Then, the nonlinearities and evolutions of electroacoustic parameters of this piezoelectric material under mechanical stress are numerically studied. By varying the initial prestress, as well as azimuthal and elevation angles, the cut planes in which a prestress induces significant benefits on velocities and coupling coefficient are identified. Finally, approximate relations describing changes between electroacoustic parameters defined in the two coordinate systems of the study are determined

Internal Electric Field in Co-Doped BaTiO 3 With Co 2+/3+ , Nb 5+ , Li + , and F − : Impact on Functional Properties and Charge Compensation With Niobium and Fluorine Ions

International audienc