RESUMO
Recently, 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.
RESUMO
This paper presents a new method to compute the piezoelastic properties of multidomain single crystals from the single-domain constants. Based on a quasi static assumption, a PMN-chiPT multidomain is defined as a periodic medium with a lattice composed of layers of two domains in a twin structure. Such a structure is assumed to have charged domain walls that imply specific lattice media and boundary conditions. A numerical computation has been performed for a PMN-33PT single crystal in the rhombohedral phase. The effective elastic, piezoelectric, and dielectric constants of the macroscopic structure have been calculated, as well as the wave velocities in different configurations of domain patterns.
RESUMO
The objective of this study is to understand the role of residual stress in piezoelectric layers in order to predict the performance of integrated structures. This is of particular importance in thick or thin film technology. Considering a bulk piezoelectric material, the Christoffel equation for a piezoelectric material is modified to take into account a uniform residual stress on a given cross section. A numerical study of its influence is carried out on the slowness curves and coupling coefficients of a lithium niobate material. In a second part, modified Christoffel tensor is used to calculate the dispersion curves of Lamb waves in a piezoelectric plate. The Lamb modes are found to be sensitive to the residual stress. In particular, it is shown how the behavior of the first Lamb modes is modified with residual stress. In a third part, these results are extended to a piezoelectric film laid down on a substrate in order to model the importance of these phenomena on the behavior of an integrated structure. The numerical study of guided waves in a lithium niobate plate is performed first, then the case of a lithium niobate film laid down on a silicon substrate is considered.
RESUMO
The 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.
