Ultrasonic NDE of Green-State Ceramics by Focused Through-Transmission

Reliable NDE techniques for green-state (unfired) ceramics are needed (1) to evaluate ceramic powder processing and compaction methods and (2) to screen out defective ceramic components prior to the costly densification process. Past work in the application of ultrasonic NDE to green-state ceramics has been hampered by the lack of an efficient yet safe means to obtain ultrasonic coupling, since conventional coupling fluids (water, gels, oils, etc.) have a detrimental effect on fragile green-state materials. In early work, direct contact pressure was used to obtain dry coupling between transducer and specimen [1]. This approach was later improved upon by placing an elastomer membrane between the transducer and specimen; this method provided efficient coupling at significantly lower contact pressures [2]. In the study presented here, an acoustically transparent plastic membrane was held against the ceramic specimen by atmospheric pressure [3]. The advantage of this technique is that it allows the use of ultrasonic immersion techniques as well as contact transducers

Finite Element Modeling of Ultrasonic Inspection of Weldments

High performance weldments for critical service applications require 100% inspection. Balanced against the adaptability of the ultrasonic method for automated inspection are the difficulties encountered with nonhomogeneous and anisotropic materials. This research utilizes crystals and bicrystals of nickel to model austenitic weld metal, where the anisotropy produces scattering and mode conversion, making detection and measurement of actual defects difficult. Well characterized samples of Ni are produced in a levitation zone melting facility. Crystals in excess of 25 mm diameter and length are large enough to permit ultrasonic measurements of attenuation, wave speed, and spectral content. At the same time, the experiments are duplicated as finite element models for comparison purposes. Finite element models permit easy description of boundary conditions, geometry, and loading. Direct integration of the wave equation is done with the Newmark-Beta and Wilson-Theta Methods. The usual problem with the large number of degress of freedom can be alleviated with the use of Guyan reduction. Two-dimensional comparisons showing mode conversion and a plate with a flaw are made. The continued development of this computational tool should increase understanding of quantitative ultrasonic inspection

Urban Regeneration in Hong Kong: Problems and Prospects

During the last ten years, the nation has become increasingly aware of the potential for environmental damage from leaks in underground and aboveground storage tanks, and related pipelines. Federal and state regulations have been developed that mandate inspections, require regular testing and set out design standards of construction for underground storage systems. More specifically, existing federal regulations (40 CFR Parts 280 and 281, September 1988) require that underground tanks and pipelines containing petroleum products and other hazardous substances be tested for leaks on a regular basis and that once a leak has been detected, it be corrected. While this requirement appears to be relatively simple, it, in fact, presents substantial difficulty in implementation. The difficulties are of two types

Ashes of the Glacial Crescent - for amplified violin

(Musical composition, 5) Commissioned by the Hong Kong Composers Guild and the Red House Editions (Australia); score completed in December 199

Ultrasonic Beam Propagation in Cast Stainless Steel

The ultrasonic examination of cast stainless steel components found in nuclear reactors has been plagued by problems such as difficulties in achieving sufficient penetration, poor signal-to-noise ratios, false indications, and mislocated flaws. One factor which plays an important role in these problems is anisotropy of the material, whereas many metal components can be viewed as isotropic, having randomly oriented, equi-axed grains, such is not the case for cast austenitic steels, in which the structure tends to crystallize with the [100]-axis of each grain parallel to the local thermal gradient. A consequence is that the ultrasonic wave speeds vary with direction, which in turn leads to such phenomena as beam skewing and excess beam divergence. The materials, anisotropic or not, can also exhibit large grain sizes which can lead to excess attenuation and background noise. Much effort has been placed on classifying the various microstructures and determining their elastic properties as well as studying beam propagation through them [1–8]. The theoretical modeling of beam propagation in anisotropic and inhomogeneous materials has also received much attention recently [9–16].</p

Dancing with my mistress: embezzlement and the professionalization of banking employees in Shanghai.

Jointly organized by Chinese University of Hong Kong, Universite de Paris IV and French Centre for Research on Contemporary China

Ecosystem Services and the Value of Places

In the US Environmental Protection Agency, the World Wide Fund for Nature and many other environmental organisations, it is standard practice to evaluate particular woods, wetlands and other such places on the basis of the ‘ecosystem services’ they are thought to provide. I argue that this practice cannot account for one important way in which places are of value to human beings. When they play integral roles in our lives, particular places have a kind of value which cannot be adequately conceived in terms of service provision. Since it is in this respect limited, the ecosystem services framework can, I suggest, be criticised on grounds of justice