Browsing by Subject "contact"

Now showing 1 - 7 of 7

Air-Coupled Surface Wave Transmission Measurement Across A Partially Closed Surface-Breaking Crack In Concrete
(2011-06) Kee, S. H.; Zhu, J. Y.; Kee, S. H.; Zhu, J. Y.
Previous researchers have demonstrated that the transmission of surface waves is effective to estimate the depth of a surface-breaking crack in solids. However, most of the results were obtained using a well-defined crack (or notch) in laboratory. In fact, there is a critical gap to apply the theory to surface-breaking cracks in concrete structures subjected to external loadings where the cracks are generally ill-defined, and partially closed. In this study, the authors investigated transmission coefficients of surface waves across a partially closed surface-breaking crack in concrete subjected to monotonically increasing compressive loadings. First, a concrete beam (0.5 X 0.154 X 2.1 m(3)) having two surface-breaking cracks with various crack widths was prepared in laboratory. Second, transmission coefficients of impact-induced surface waves were measured across a surface-breaking crack in the concrete beam with increasing compressive loadings from 0 to 140kN (10% of the ultimate compressive strength of the concrete beam). External post-tensioning was used to apply the compression. For comparison purpose, sensitivity of surface wave velocity to compressive loading was also investigated. As a result, observations in this study reveal that transmission coefficient is a more sensitive acoustic parameter than phase velocity to evaluate a surface-breaking cracking in concrete subjected to compressive loadings.
The Evolution Of Sediment Acoustic Models
(2012-11) Chotiros, N. P.; Isakson, M. J.; Chotiros, Nicholas P.; Isakson, Marcia J.
Sediment acoustic models contain two connected components, the geo-physical description of the sediment and the model of acoustic processes. Geo-physical descriptors are used in the classification of sediments, and they are based on grain size, density and other physical descriptors. Acoustic sediment models were initially fluid approximations that were simple to implement. As the need for accuracy increased, the fluid model was extended to stratified fluid and visco-elastic models. The latter, with five frequency-independent parameters, appeared to be consistent with sediment acoustic data up to the 1980s. More recent experimental data have revealed discrepancies in the frequency-dependence of attenuation and sound speed, particularly in the case of sandy sediments, which cover a large fraction of the continental shelves. Broad-band acoustic measurements of wave speeds and attenuations are more consistent with a poro-elastic model, consisting of Biot's theory with extensions to account for the physics of granular media. Aside from terminology, there is a fundamental difference between viscoelastic and poro-elastic theories. The former is based on two types of waves, a compressional wave and a shear wave, while the latter has an additional compressional wave, often called the Biot wave. There are currently two approaches to the development of sediment acoustic models: (a) visco-elastic models with frequency dependent parameters that mimic the observed behavior, and (b) poro-elastic models that reflect the physical processes. It is shown that (a) would be a significant improvement over existing models, but (b) is the preferred solution.
Letter to Esther E. R. Applin from H.B. Stenzel on 1962-06-13
(1962-06-13) Stenzel, Henryk B.
Letter to H.B. Stenzel from Pliny Gale on 1947-12-03
(1947-12-03) Gale, Pliny
Letter to Thomas L. Bailey from H.B. Stenzel on 1937-05-04
(1937-05-04) Stenzel, Henryk B.
Nanoindentation Of Si Nanostructures: Buckling And Friction At Nanoscales
(2009-11) Huang, H.; Li, B.; Zhao, Q.; Luo, Z. Q.; Im, J.; Kang, M. K.; Allen, R. A.; Cresswell, M. W.; Huang, R.; Ho, P. S.; Huang, H.; Li, B.; Zhao, Q.; Luo, Z. Q.; Im, J.; Kang, M. K.; Allen, R. A.; Cresswell, M. W.; Huang, R.; Ho, P. S.
A nanoindentation system was employed to characterize mechanical properties of silicon nanolines (SiNLs), which were fabricated by an anisotropic wet etching (AWE) process. The SiNLs had the linewidth ranging from 24 nm to 90 nm, having smooth and vertical sidewalls and the aspect ratio (height/linewidth) from 7 to 18. During indentation, a buckling instability was observed at a critical load, followed by a displacement burst without a load increase, then a full recovery of displacement upon unloading. This phenomenon was explained by two bucking modes. It was also found that the difference in friction at the contact between the indenter and SiNLs directly affected buckling response of these nanolines. The friction coefficient was estimated to be in a range of 0.02 to 0.05. For experiments with large indentation displacements, irrecoverable indentation displacements were observed due to fracture of Si nanolines, with the strain to failure estimated to be from 3.8% to 9.7%. These observations indicated that the buckling behavior of SiNLs depended on the combined effects of load, line geometry, and the friction at contact. This study demonstrated a valuable approach to fabrication of well-defined Si nanoline structures and the application of the nanoindentation method for investigation of their mechanical properties at the nanoscale.
Phatic rituals of the liberal democratic polity: Hearing voices in the hearings of the Royal Commission on Aboriginal Peoples
(2015) Slotta, James
The truth and reconciliation commissions of Latin America and Africa are paradigms of transitional justice, often regarded as part of the process of transitioning from authoritarian to democratic rule. But truth commissions are also common in first-world settler states, which raises the question of what “transition” such commissions effectuate in Canada, Australia, and the United States. This paper examines the efforts of Canada’s Royal Commission on Aboriginal Peoples to resolve a controversy over a government relocation of Inuit families in the 1950s for which the relocatees were demanding compensation. Concurrent with historical controversies in the Canadian courts concerning Aboriginal rights and title, the historical controversy over the relocation raised questions about the Canadian state’s ability to “hear the voices” of First Nations people, who objected that their accounts of the past had been disregarded by government-contracted historians and courts alike. I argue that the Royal Commission’s efforts to hear the voices of Inuit relocatees, showcased in nationally televised hearings, was a phatic ritual in which communicative contact between marginalized citizens and the state was ritually established. The ritual was presented as a remedy for failures to achieve phatic communion among citizens and state—a condition of communicative contact held up as essential to the realization of liberal democratic ideals. The work of the Royal Commission and other truth commissions highlights the growing prominence of communication, particularly liberal communicative events construed as “open,” “equal,” and “free,” as a concern of both theory and practice in liberal democratic polities.