Dale W. Fitting

Micromachining for improvement of integrated ultrasonic transducer sensitivity

A micromachined diaphragm structure for integrated ultrasonic transducers is discussed. The micromachining significantly reduces a large parasitic capacitance, resulting in improved sensitivity compared with identical transducers on solid substrates. For the particular geometry tested, sensitivity improved 8.9 dB; this result matches a prediction of 10.9 dB, based on a simple model, reasonably well. This level of improvement is considered significant for the applications under consideration (medical imaging and nondestructive evaluation). >

Speckle reduction in pulse-echo ultrasonic imaging using a two-dimensional receiving array

An experimental pulse-echo imager was developed for the purpose of reducing speckle in ultrasonic images. The system utilized a 64-element spherically focused segmented annuli array receiver with a common transmitter. Compounded images were formed using subapertures of varying size, shape, and overlap, and the speckle and resolution characteristics of the final images were observed. A pointlike scatterer was imaged to determine the resolution, point spread function, and sensitivity of the system along with a new measure called the resolution cell size. The response of the system was also simulated for comparisons. It was found that lateral resolution, and resolution cell sizes only gradually increased with a decrease in subaperture size and system sensitivity was not greatly diminished. Incoherent summation of signals from small groups of elements decreased the speckle noise by a factor of four while maintaining enough resolution to improve the image quality as measured by the CSR/d by a factor of almost two.<<ETX>>

A Process Sensor for Locating the Liquid-Solid Boundary through the Mold of a Casting

Accurate process control of single-crystal and directionally-solidified castings requires knowledge of the exact location of the solidifying front. If the front advances too rapidly, single crystal growth in a preferred orientation degenerates into the formation of polycrystals. A solidification front which moves more slowly than necessary is wasteful of the casting resources. A sensing technology is being developed which determines the location of the boundary between a solidifying crystal and liquid metal. The sensing method utilizes the ordered pattern of x-rays diffracted from the solid as an absolute indicator of the liquid-crystal interface.

High Energy X-Ray Diffraction Technique for Monitoring Solidification of Single Crystal Castings

X-ray diffraction has been used successfully to study metal solidification and temperature-dependent phase changes1–3. However, this research used very thin specimens (a few mm at most), furnaces with low attenuation x-ray windows (beryllium, graphite, or polyimide), and low x-ray energies (< 50 keV). Since the penetration depth of low-energy x-rays is shallow, traditional x-ray diffraction is thus unable to probe the interior of thicker structures. Others have used higher energies to study thicker samples. Work, including that by Green4 and by Kopinek, et al5, extended x-ray diffraction investigations to energies exceeding 150 keV.

Through-Thickness Elastic Constant for Aramid-Epoxy/Aluminum Composite Materials

The objectives in this study were to determine the through-thickness elastic constant of fiber-epoxy/metal composites with ultrasonic measurements and determine the elastic constants of the constituent layers. The specimens that we studied consist of alternating layers of aluminum (2024 T3) and fiber-epoxy [1], all of which are 200 to 300 μm thick. As Figure 1 indicates the outer two layers of the composite are aluminum, so that there is always one additional layer of aluminum in the composite. The fiber-epoxy layers consist of a unidirectional layup of aramid fibers for the aramid-epoxy/aluminum specimens. For the glass-epoxylaluminum specimen, the fiber-epoxy layers consist of two sublayers of unidirectional fibers where the fibers of one sublayer lie at 90° with respect to the other sublayer. Because the fibers of both sublayers are perpendicular to the transmitting transducer, the ultrasound perceives the two sublayers as a single layer.

Transmission x-ray diffraction for real-time sensing of the solidification of a casting

A transmission x-ray diffraction (XRD) technique has been developed which may be used, in real time, to locate the molten metal-solidifying metal interface in single-crystal castings. We proved feasibility of the sensor on samples of aluminum and copper in a gradient furnace, and found that the liquid-solid boundary could easily be identified even though the samples were surrounded by a mold wall and encased in a furnace. Subsequently, we have successfully attached the XRD sensor to a small industrial turbine blade casting furnace. The liquid-solid interface can be identified and followed, in real-time during the single- crystal casting process. The high energy x-rays permit in- furnace transmission XRD to be performed on a 6 mm thick nickel alloy specimen. The diffraction pattern was clearly seen, even though the entrance x-ray and diffraction paths through the furnace included 20 mm of Pyrex, 3.2 mm of molybdenum, 9 mm of aluminum oxide, and 12.8 mm of mold material. The x-ray source to imager distance was 1180 mm. An analytical model for transmission XRD has been developed. It is useful for assessing the feasibility of particular sensing applications.

Monitoring of Anisotropic Material Elastic Properties Using Ultrasonic Receiving Arrays

A robust technique has been developed for determining the elastic constants of an anisotropic material and for on-line monitoring of changes in the the elastic properties. Use of an array of small piezoelectric elements as the receiver permits gathering information on the angle-of-arrival of the ultrasonic beam as well as the arrival time. Combining the array with a variable-angle transmitter, phase velocity measurements in an anisotropic material may be made over a range of propagation directions. The measuring and monitoring devices may be directly coupled to the material, immersion is not required.