Biomedical subjects

AG Every

Publications and source records attributed to AG Every.

At least 19 recordsLinked to original sources

PubMed2000

Wave propagation in an anisotropic nickel-based superalloy

The effects of elastic anisotropy on ultrasound propagation in a nickel-based single crystal test component are studied using a 25 MHz focused probe in a water immersion system. Anisotropy gives rise to directionally dependent acoustic wavespeeds, beam steering, acoustic energy focusing and mode conversion for normal incidence. Transverse mode echoes are particularly strong in the vicinity of crystallographic directions in which the Gaussian curvature of the slowness surface is zero and divergence of the echo amplitude is predicted on the basis of the stationary phase approximation. There are other directions where the transverse mode echoes vanish for symmetry reasons. The longitudinal mode echo amplitude also shows significant variation with direction. Overall there is good agreement between the echo signal arrival times and amplitudes we measure and calculation. Progress in applying this technique to gas turbine blades is reported.

Journal Article↗

PubMed2000

Elastic measurements of layered nanocomposite materials by Brillouin spectroscopy

Surface Brillouin spectroscopy makes it possible to measure surface elastic wave propagation parameters at frequencies up to 20 GHz or more. This enables us to measure the elastic properties of surface layers only a small fraction of a micrometre thick. The wavelength and incident angle of the light determine the wavenumber of surface elastic waves (SAW) that scatter the light inelastically, and their frequency can be found by measuring the change in wavelength of the scattered light. By analysing the elastic wave modes present in the surface, the elastic properties can be deduced. We have used this technique to measure the elastic properties of layered nanocomposite materials, which are widely used in the packaging industry. 12 microns polymer films (PET) were coated with glass oxide layers of thickness as little as 25 nm, to give transparent nanocomposite structures with excellent gas barrier properties. In order to understand and model the behaviour of these films under deformation, it is necessary to determine the elastic properties of the different layers. Evaluation of the elastic properties presents several challenges. First, the oxide layers are much thinner than the wavelengths of the surface phonons in surface Brillouin spectroscopy (and hence the depth probed), which usually lie in the range 250-500 nm. The anisotropic elastic properties of the PET substrate must therefore be measured accurately, and this can be done using bulk Brillouin spectroscopy. Second, a thin layer of metal (usually 10-20 nm) must be deposited on the glass surface so that the surface phonons scatter the light effectively. The elastic properties of the glass layer can then be deduced from surface Brillouin spectroscopy measurements, by simulating the surface wave modes of the metal/glass/polymer composite, and adjusting the parameters to give the best fit. In this way it is possible to observe how the properties of the glass vary as a function of thickness, and in turn to understand how to improve systematically the properties under deformation.

Journal Article↗

PubMed2000

Surface Brillouin scattering of opaque solids and thin supported films

Surface Brillouin scattering (SBS) has been used successfully for the study of acoustic excitations in opaque solids and thin supported films, at both ambient and high temperatures. A number of different systems have been investigated recently by SBS including crystalline silicon, amorphous silicon layers produced by ion bombardment and their high temperature recrystallisation, vanadium carbides, and a nickel-based superalloy. The most recent development includes the measurement of a supported gold film at high pressure. The extraction of the elastic constants is successfully accomplished by a combination of the angular dependence of surface wave velocities and the longitudinal wave threshold within the Lamb shoulder. The application of surface Green's function methods successfully reproduces the experimental SBS spectra. The discrepancies often observed between surface wave velocities and by ultrasonics measurements have been investigated and a detailed correction procedure for the SBS measurements has been developed.

Journal Article↗

PubMed2000

Observation of Scholte-like waves on the liquid-loaded surfaces of periodic structures

The observation of Scholte-like ultrasonic waves travelling along the water-loaded surfaces of solids with periodically varying properties is reported. Results are presented for two 2D superlattices that intersect the surface normally: a laminated solid of alternating 0.5 mm thick layers of aluminium and a polymer, and a hexagonal array of polymer rods of lattice spacing 1 mm in an aluminium matrix. The surface waves are generated and detected by line-focus acoustic lenses aligned parallel to each other, and separated by varying distances. For homogeneous solids, phase matching constraints do not allow the Scholte wave to be coupled into with an experimental configuration of this type, and this is demonstrated with results on a uni-directional carbon-fibre/epoxy composite. These constraints are relaxed for a periodic solid, where coupling takes place through Umklapp processes. In our experiments, the source pulse is fairly broadband, extending up to about 6 MHz, whereas the spectrum of the observed Scholte arrival is peaked at around 4 MHz. We attribute this to a resonance in the surface response of the solid associated with the superlattice structure. On rotating the solid about its surface normal, the Scholte wave displays a characteristic variation in phase arrival time and, to a lesser extent, also group arrival time. This variation is well accounted for with a model that incorporates Umklapp processes in the solid's surface response.

Journal Article↗

PubMed2000

Transverse curvature of the acoustic slowness surface in crystal symmetry planes and associated phonon focusing cusps

Conditions are derived for the existence of focusing cusps in ballistic phonon intensity patterns for propagation directions in crystal symmetry planes. Line caustics are known to be associated with lines of vanishing Gaussian curvature (parabolic lines) on the acoustic slowness surface, while cusps are associated specifically with points where the direction of vanishing principal curvature is parallel to the parabolic line. A parabolic line meets a crystal symmetry plane sigma at a right angle, and so it is the vanishing of the slowness-surface curvature transverse to sigma that conditions the existence of a cusp. A relation for the transverse curvature is derived and analyzed. It is shown that in an arbitrary symmetry plane sigma there may be up to four pairs of inversion-equivalent cuspidal points for SH (out-of-plane polarized) waves, and up to eight pairs of cuspidal points associated with the in-plane polarized (usually quasi-transverse) waves. In tetragonal crystals, the symmetry planes containing the four-fold axis can have at most two pairs of cusps for the SH waves and up to six pairs of cusps for the in-plane waves. In cubic crystals, the face symmetry planes sigma cannot have cuspidal points for SH waves, as is known, while four pairs of cusps for in-plane waves exist in sigma if and only if the outer-most slowness sheet has a concave region embracing the four-fold axis. The points of vanishing transverse curvature on the slowness surface in symmetry planes of tetragonal and cubic media are identified by concise relations, facilitating their explicit analysis.

Journal Article↗

PubMed1996