Digital Age in the By Kevin
Arc P h o t o I l l u s t r a t i o n b y S t r o b o P h o t o , © 1 1 / 0 1
Challenges and Opportunities Presented by Archiving in the Electronic Era
With the rapid development of electronic and network technologies, there is considerable and justifiable concern in the academic community about the challenges associated with protecting electronic information for future generations of scholars and students. As a not-for-profit organization founded specifically to build and provide a trusted central repository of important journal literature in electronic form, JSTOR has been working on the problem of electronic archiving since its founding. Unfortunately, there are no simple or easy solutions to this problem and this brief essay does not attempt to provide them. Rather, the purpose of this paper is to share JSTOR's experience and reflect upon the key principles we have identified that must be addressed if one is to maintain reliable long-term access to electronic content. The scope of this archiving discussion is limited to published journal literature, admittedly a small subset of the digital materials that require attention. Still, many of the issues raised in the journals context apply more generally as well.
Thermal incompatibility analysis of metal-ceramic systems based on flexural displacement data.
The feasibility of simple tests or analytical methods for prediction of residual stress states in metal-ceramic (MC) prostheses has not been demonstrated. Biomaterial metal-ceramic strips have been proposed to provide sensitive measures of transient and residual stress states through the measurement of midpoint deflection after cooling from the ceramic sintering temperature. The objective of this study was to apply the elastic-viscoelastic analogy to calculate transient and residual midpoint deflections in MC biomaterial strips and to compare these values with deflections measured with a beam-bending viscometer (BBV). Calculations and measurements were made for five MC systems that were found from a clinical study to be "thermally compatible" systems. Metal strips, 64 mm in length, 3 mm wide, and either 0.5 mm, 1.0 mm, or 2.0 mm in thickness, were veneered with four 0.25-mm thick layers of opaque porcelain. Midpoint deflection of the MC strips (ceramic oriented in the posterior position) was measured during cooling from an initial temperature of 700 degrees C. In general, the directions of the measured residual deflections did not agree with the "textbook" convention that negative deflections are associated with positive thermal contraction mismatch (alpha(M) - alpha(c) > 0) regardless of metal thickness. For a metal thickness of 0.5 mm, the residual midpoint deflection for all thermal contraction mismatch cases, except one, was positive (upward deflection) whereas the residual midpoint deflections were all negative when the metal thickness was increased to 1 or 2 mm, independent of the thermal contraction mismatch. The best agreement between calculated and measured values of residual midpoint deflection (+16 microns vs. +14 +/- 2.3 microns, respectively was obtained for MC biomaterial strips with a Ni-Cr alloy (0.5 mm thick) while the largest difference (+346 microns vs. +61 +/- 43.8 microns) was obtained for MC bimaterial strips with a Au-Pd allow (0.5 mm thick). In all but one case, changes in deflection direction as a function of metal thickness were correctly predicted by the viscoelastic analysis. The results of this study indicate that a viscoelastic model is useful for estimating thermal compatibility conditions of MC systems.