Browsing by Subject "Acoustic imaging"

Now showing 1 - 7 of 7

Open Access
Generation of focused surface waves with a solid wedge
(IEEE, 1987) Atalar, Abdullah; Köymen, Hayrettin
Focusing of surface acoustic waves on material surfaces was achieved by axicons in liquid immersion. The system consisted of a plane acoustic wave generator obliquely insonifying a cylindrical mirror placed perpendicularly on the surface to be examined. A similar axicon can be implemented with a solid cylindrical structure which does not require an immersion liquid. The lower cap of a solid cylinder is placed on the surface of the solid object, while the upper cap, where a shear-wave transducer is placed, is inclined with respect to the axis of the cylinder. Shear waves generated by the transducer are reflected from the free cylindrical surface of the solid. The reflected waves are incident on the bottom surface at a critical angle and couple to the object surface as surface waves to converge to a diffraction limited focus. The cylindrical solid material must be chosen from low-shear-wave-velocity materials. The critical angle determines the inclination of the top surface; it is found from the solution of reflection problem for shear waves at a solid-solid interface with a slippery boundary condition. The focusing system is implemented successfully with lucite as the solid and aluminum as the object material.
Open Access
Imaging flaws close to surface using focused surface acoustic waves
(IEEE, 1986-11) Köymen, Hayrettin; Atalar, Abdullah; Çiloğlu, T.; Önder, Murat; Uzel, Ç.; Yavuz, H.
The resolving power and detection ability of the focused surface acoustic wave (SAW) imaging modality is investigated in this paper. In this mode of imaging, conical bulk acoustic waves are used to generate and focus leaky surface acoustic waves on smooth surfaces of materials. Imaging systems built using this technique has diffraction limited focusing property. An imaging system using this focusing principle has been built, operating at 1.5 and 20 MHz. A slow mechanical scanning system controlled by a personal computer scans the surface of the object, and the data is acquired by the computer to generate a color or a black and white image on its graphic screen. The results of the initial experiments show that the inaging system is very sensitive to the grain structure and possible residual stresses on the surface of the object. It can resolve subsurface gratings of spacing less than a SAW wavelength very close to surface. The imaging system is inherently zero background, providing a high sensitivity not found in similar systems.
Open Access
A Lamb Wave Lens for Acoustic Microscopy
(1992) Atalar, Abdullah; Köymen, Hayrettin; Değertekin, F. Levent
In a conventional scanning acoustic microscope the excited leaky modes contributes significantly to the high contrast obtained in the images. However, all such modes exist simultaneously, and the interpretation of the images is not straightforward, especially in layered media. A new lens geometry is proposed that can be used with acoustic microscopes to image layered solid structures. This new lens can focus the acoustic waves in only one of the Lamb wave modes of the layered solid with a high efficiency. V(Z) curves obtained from this lens are more sensitive to material properties compared to that obtained from conventional lens. Measuring the return signal as a function of frequency results in another characteristic curve, V(f). The Lamb wave lens and the associated characterization methods for the layered structures are described. The results presented show that the Lamb wave lens is at least an order of magnitude more sensitive than the conventional lens and can differentiate between a good bond and a disbond in a layered structure easily. © 1992 IEEE
Open Access
Location and curvature estimation of spherical targets using multiple sonar time-of-flight measurements
(Institute of Electrical and Electronics Engineers, 1999-12) Barshan, B.
A novel, flexible, three-dimensional multisensor sonar system is described to localize the center of a generalized spherical target and estimate its radius of curvature. Point, line, and planar targets are included as limiting cases which are important for the characterization of a mobile robot's environment. Sensitivity analysis of the curvature estimate with respect to measurement errors and some of the system parameters is provided. The analysis is verified experimentally for specularly reflecting cylindrical and planar targets. Typical accuracies in range and azimuth are 0.17 mm and 0.1°, respectively. Accuracy of the curvature estimate depends on the target type and system parameters such as transducer separation and operating range.
Open Access
Response of acoustic imaging systems using convergent leaky waves to cylindrical flaws
(IEEE, 1989) Gonalp, N.; Atalar, Abdullah
Characterization of near-surface properties of materials by acoustic means is most convenienth done by surface-acoustic-waves (SAW) that are spatially confined to the surface. There are several techniques available to excite focused SAW's on the surfaces of non-piezoelectric materials for imaging purposes. A conventional acoustic microscope lens excites focused SANI's on the object surface, but with a low efficiency. SAW can be excited with a high efficiency using conical wavefronts as obtained from a conical axicon. Such methods can be used in a scanning arrangement to generate images of surface inho-mogeneities. In this paper, an analysis of imaging systems making use of convergent SAW's is given using angular spectrum approach and T-matrix formulation. Inhomogeneities on the object surface are assumed to be circular cylinders whose axes are perpendicular to the surface. The response of imaging systems to such defects is found as a function of defect position with respect to the focus point. Theoretical and experimental results are compared and are found to be in good agreement. © 1989 IEEE
Open Access
Response of surface acoustic wave imaging systems to cylindrical inhomogeneities
(IEEE, 1988-10) Gunalp, N.; Baygun, B.; Surucu, F.; Atalar, Abdullah
The authors present a theory to predict the response of a SAW (surface acoustic wave) imaging system. In particular, they calculate the response to cylindrical cavities as a function of cavity position with respect to the focus point. The calculations indicated a high leak rate material will result in a better resolution in the y-direction. The theory also predicts an enhancement in the received signal level when the cylinder circumference equals the SAW wavelength. The images of cylinders obtained with such systems will indicate the size of the cylinders correctly at 20 dB below peak value as long as the cylinder diameter is larger than about a Rayleigh wavelength. For small cylinders the received peak amplitude can be used to determine the size of the cylinder.
Open Access
A synthetic aperture imaging system using surface wave modes
(IEEE, 1995) Bozkurt, Ayhan; Arıkan, Orhan; Atalar, Abdullah
A synthetic aperture acoustic imaging system with a novel inversion algorithm is described. Data is obtained by using a transducer insonifying the sample surface at a critical angle which is excited by a short electrical pulse. The critical angle is chosen for a suitable surface wave or Lamb wave mode that exists on the object. The transducer is mechanically scanned in only one direction during which many pulse excitations and subsequent recordings are realized. The received signal is sampled in time and digitized to be processed by using the new inversion approach providing an optimal 2-D image of the surface reflectivity.