Sound propagation in a solid through a screen of cylindrical scatterers

The propagation of SH waves in a solid containing a screen of line-like scatterers is investigated. When the scatterers are uniformly distributed, the amplitudes of the coherent waves inside and outside the screen are evaluated in closed form. In the analysis, multiple scattering effects are taken into account within the context of a first-order approximation. A Global Closure Assumption is proposed, which yields an effective wavenumber identical to that of Waterman and Truell. The scatterers can be fibers of circular or elliptical cross-sections; they can also be two-dimensional cracks with slit-like or elliptical cross-sections. Specific analytical and numerical results are presented for flat cracks and empty cavities of circular cross-sections. In those two cases, figures are presented to illustrate the variations of the reflection and transmission coefficients as functions of frequency and of scatterer concentration. The crack and cavity results, respectively, are compared with those of earlier works

Calculation of ultrasonic fields radiated in a ferromagnetic medium by an EMAT of arbitrary bias field driven by a current of arbitrary intensity

ElectroMagnetic Acoustic Transducers (EMATs) are contactless transducers generating ultrasonic waves in conductive media, notably shear horizontal and torsional waves (in plates and pipes, respectively), possibly in hostile environments. In a ferromagnetic part, the elastic strain and the magnetic field couple through magnetostriction phenomena, so that a magnetostriction and magnetization forces add up to the Lorentz force created in any conductive medium. Here, a model is proposed to predict these forces for an arbitrary bias field due to the EMAT permanent magnet and whatever the current intensity in its electric circuit, whereas the usual assumption of high bias field and low intensity current leads to important model simplifications. To handle the nonlinear behavior of all the three forces when the usual assumption cannot be made, forces are expressed in the time domain. In particular, magnetostriction force generates waves at several harmonic frequencies of the driving current frequency. Forces are then transformed into equivalent surface stresses readily usable as source terms in existing models of ultrasonic radiation, under the assumption that ultrasonic wavelengths are much longer than force penetration depths, (which is generally true in NDT applications of EMATs). Force spectra computed in various EMAT configurations are compared for illustration

Subgingival periodontopathogens and host-response factors in GCF as co-biomarkers for assessment of periodontal diseases

To emphasize the anisotropy induced by damage on a ceramic matrix composite subjected to a tensile solicitation at 30° from one of the fiber directions, the changes of all the stiffness tensor components are determined from wave speed measurements of obliquely incident ultrasonic bulk waves. Using the additive form of the stiffness tensor of the damaged material, the crack orientation is determined from the damage tensor, i.e., the part relative to the loss of stiffness caused by the microcracks. The search for the principal frame of this tensor from wave speed measurements allows the assessment of the microcrack orientation

Measuring the porosity and the tortuosity of porous materials via reflected waves at oblique incidence

International audienceAn ultrasonic reflectivity method is proposed for measuring porosity and tortuosity of porous materials having a rigid frame. Porosity is the relative fraction by volume of the air contained within a material. Tortuosity is a geometrical parameter which intervenes in the description of the inertial effects between the fluid filled the porous material and its structure at high frequency range. It is generally easy to evaluate the tortuosity from transmitted waves, this is not the case for porosity because of its weak sensitivity in transmitted mode. The proposed method is based on measurement of reflected wave by the first interface of a slab of rigid porous material. This method is obtained from a temporal model of the direct and inverse scattering problems for the propagation of transient ultrasonic waves in a homogeneous isotropic slab of porous material having a rigid frame ͓Z. E. A. Fellah, M. Fellah, W. Lauriks, and C. Depollier, J. Acoust. Soc. Am. 113, 61 ͑2003͔͒. Reflection and transmission scattering operators for a slab of porous material are derived from the responses of the medium to an incident acoustic pulse at oblique incidence. The porosity and tortuosity are determined simultaneously from the measurements of reflected waves at two oblique incidence angles. Experimental and numerical validation results of this method are presented

Frequency spectra of magnetostrictive and Lorentz forces generated in ferromagnetic materials by a CW excited EMAT

Magnetostriction arises in ferromagnetic materials subjected to magnetization, e.g., when an EMAT (Electro-Magnetic Acoustic Transducer) is used to generate ultrasonic waves. In such a case, the magnetostriction force must be taken into account as a transduction process that adds up to the Lorentz force. When the static magnetic field is high compared to the dynamic field, both forces are driven by the excitation frequency. For lower static relative values of the magnetic fields, the Lorentz force comprises both the excitation frequency and its first harmonic. In this work, a model is derived to predict the frequency content of the magnetostrictive force that comprises several harmonics. The discrete frequency spectrum strongly depends on both the static field and the relative amplitude of the dynamic field. The only material input data needed to predict it is the curve of macroscopic magnetostrictive strain that can be measured in the direction of an imposed magnetic field. Then, the various frequency-dependent distributions of Lorentz and magnetostriction body forces can be transformed into equivalent surface stresses. Examples of computation are given for different static and dynamic magnetic fields to study their influence on the frequency content of waves generated in ferromagnetic materials

Clinical success with resin bonded bridges

The whole set of the stiffnesses of materials that possess any class of symmetry, and then the higher symmetry coordinate system can be determined from wave speed measurements of obliquely incident ultrasonic bulk waves [1–2]. An immersion ultrasonic tank associated with a tensile machine allows one to measure the load-induced changes of the stiffness tensor [3]. The damage can be defined as the variation of the elasticity tensor [4]. No preliminary knowledge of the microstructure is required to describe the damage process. Concerning former works on off-axis loading [5], the choice of a tetragonal material submitted to a tensible solicitation at 45° from fiber directions , was imposed by the limiting hypothesis required by classical ultrasonic techniques that the material keeps its orthorhombic symmetry in a a priori known coordinate system

Analysis of leachable and insoluble trace metals in air particulate matters by capillary electrophoresis

The orthorhombic symmetry is considered as general enough to describe the anisotropy of most of the composite materials. The measurement of the anisotropic elasticity constants by ultrasonic techniques usually begins with an assumption that the axes of symmetry are known [1–2]: coincidence of the symmetry axes with the observation axes associated with the thin plate sample is assumed