Elastic Characterization of Orthotropic Composite Materials from Ultrasonic Inspection through Non-Principal Planes

Transmission of bulk ultrasonic waves through materials immersed in water is a well appropriated method to measure the stiffness matrix of anisotropic composite materials. This matrix can be deduced from velocities measurements by simple [1,2,3] or double transmission [4,5] or from amplitudes of double reflected bulk waves [4]. All these methods are working very well for unidirectional composites when transverse isotropy is assumed and the stiffness matrix has only five independent elastic constants.</p

Numerical results and Biot theory in anisotropic porous/fibrous media

Biot theory [l] is a basic tool when studying the propagation of elastic waves in porous/fibrous solids. Due to the simplicity of the assumptions, isotropic materials are most often utilized [2,3]. However, in some cases, anisotropy has to be included in the modelization, e.g. with some fibrous materials. In this work, we have developped routines, based on the standard Biot model, which compute the various characteristic surfaces (slowness, phase and group velocities) [4] in a given plane for anisotropic porous media. By increasing the weight of the coupling constants, one can continuously vary from the limit case of anisotropic solids with no porosity to a number of different porous media. When the coupling terms are not equal to zero, one finds four instead of three propagating modes, the two other eigenvalues being complex, meaning in turns that the two remaining solutions do correspond to some evanescent modes. The slow transverse mode is unchanged. The fourth mode often called the Biot mode [5], which is very similar to the pseudo-thermal wave [6,7] observed in dynamic thermoelasticity, has always a very small wavespeed and is very highly damped

Génération d'ondes acoustiques dans un milieu anisotrope par impact laser

The Lamb's problem for an elastic, anisotropic and homogeneous half space is solved. The source is linear and contains in the free surface of this half space. A numerical calculation based on the Cagniard de Hoop method is performed, in the case of cubic symmetry. The free surface is parallel to a plane of symmetry. In addition, the line source may be parallel, or not, to an axis of symmetry. The displacement field is then calculated at any point of this half space, for various directions of this line source. This theoretical study may be applied to the generation of acoustic waves when using a laser beam focused by a cylindrical lens on a sample. The impact is then linear

Acoustic second-harmonic generation with shear to longitudinal mode conversion in granular media

Excitation of longitudinal acoustic wave at the 2nd harmonic by sinusoidal shear acoustic wave in a granular material is reported. The amplitude of the 2nd harmonic exhibits beatings (typical of nonlinear processes in dispersive media), which are observed not as a function of the distance from emitter, but with increasing amplitude of the primary (pump) shear wave. The effect is attributed to varying contribution of clapping intergrain contacts to the total nonlinearity of the medium with increasing pump amplitude, which modifies the effective length of the nonlinear interaction. This is consistent with the idea that in granular assemblages there is an important amount of contacts loaded much weaker than in average

Wave self-modulation in an acoustic resonator due to self-induced transparency

Observations and interpretation of modulation instability (MI) in an acoustic resonator with a crack-like defect are presented. This MI is similar in appearance to the conventionally disscussed MI, which arises due to the interplay between the reactive nonlinearity and velocity dispersion. However, the observed MI is due to an essentially different, purely dissipative, mechanism that can be common for waves of different physical nature. The mechanism exhibits interesting peculiarities, for example, the MI threshold can only be reached when the excitation frequency does not coincide with the resonator eigenfrequencies

Determination of Angular Parallaxes between the Geometric Coordinate System and the Material Symmetry Coordinate System of Anisotropic Materials

The hypothesis of an orthorhombic symmetry and the knowledge of the material symmetry axes is usually necessary to characterize a medium by ultrasonic techniques [1–5]. However a wrong setting up of the sample or the strata’s stacking defects in industrial composite material lead to the non-superposition between the material symmetry coordinate system and the observation coordinate system. The development of a procedure to identify elasticity coefficients for materials that have no more symmetry planes, is necessary.</p

Luxemburg-Gorky effect in a granular medium: Probing perturbations of the material state via cross-modulation of elastic waves

A nonlinear effect consisting of transfer of modulation from one amplitude-modulated elastic wave, the “pump” wave, to the second initially monochromatic probe wave has been recently introduced in nonlinear acoustics by analogy with radio waves. For the first time, this effect is applied to probe perturbations of the state of a granular material induced by shocks, “seismic events”. The experiments indicate a much stronger variability of the nonlinearity-induced modulation sidelobes in comparison with changes in the components at the fundamental frequencies of the probe and pump waves. Another new feature revealed in the experiments is that the transitional shock-induced variations in the amplitudes of the modulation sidelobes are several times stronger than the relaxed, residual values of the changes. The effects observed suggest interesting possibilities to application in active acoustic/seismic monitoring schemes

Roles of yeast peroxiredoxins in cellular defense against DNA damage

The LASER ultrasonic technique is a powerful mean to generate and detect ultrasonic waves at a distance, without any contact to the specimen. However, it is now well known that in anisotropic media, acoustic waves generated by a line or point source propagate at group velocities which are different from the phase ones. The recovering of phase velocities from such generated acoustic waves is a non trivial matter that requires a numerical analysis [1, 2] of a set of measured group velocities. An alternative solution is proposed: the signal such as it is generated by a plane wave refracted through an anisotropic plate is numerically built. For that purpose, a set of signals are recorded by scanning the line source in a direction normal to it with a constant step. Each measured signal is then shift in time with a constant delay. The summation of the obtained signals yields a waveform similar to the one recorded when a plane wave is refracted through the material. The phase velocity can thus be easily obtained in various directions of propagation

Contribution to Guided Waves Analysis at Low Frequency in Thin Absorbing Plates with Application to the Non Destructive Testing of Paper

The paper is a cellulose fiber base material and consequently it is heterogeneous at different scales. In addition, the paper exhibits two preferential axes of fiber orientation, from which it is easy to define the characteristics of an elementary bi-dimensional fiber network. A third axis, perpendicular to the first, corresponds to the stacking direction of individual networks. Following the example of composite layered media modeling, the paper presents three perpendicular planes of material symmetry. In other words, the paper is commonly assumed to be an orthotropic material [1]. The paper presents also various mechanical behaviors such as elasticity, viscoelasticity, non-linear elasticity. Furthermore the paper properties depend on both the temperature and the hygrometry [2]. At last, paper materials exist in the form of sheets; their thickness (100 µm on average) is very small beside their other dimensions