Fluid-assisted Cracking in Ceramics
J. K. Shang,* D. Yao, D. Zhu
U.S. Air Force Office of Scientific Research, F49620-93-1-0241

Kinetics and mechanisms of fluid-assisted cracking are examined in Si3N4 and Al2O3 ceramics to elucidate the synergistic effect of stress and surface chemistry on microfracture. Three different techniques are being used to measure this effect: the repeated indentation technique, which follows the development of indentation cracks leading to surface microfracture, small crack experiments where the growth of penny-shaped surface cracks is monitored, and fracture mechanics technique with through-thickness cracks. Possible influences of fluid viscosity, temperature, grain size, grain boundary phase, and loading rate on cracking kinetics are investigated.

Structural Relaxations in Glass-forming Melts
J. Kieffer,* A. Kisluik
National Science Foundation, DMR 93-15779

Novel glassy materials are needed for photonics applications. Glass formation has conveniently been attributed to a kinetic arrest in structural relaxation, before crystallization can occur. There is, however, increasing evidence for irreversible transitions in the molecular configurations to take place. The decisive processes which need to be understood occur on the time scale of nanoseconds, whether the origin of glass formation is purely kinetic or involves structural phase transitions. This time regime has so far been little investigated. We use Brillouin light scattering to observe structural dynamics in supercooled liquids. This allows us to determine directly the coefficients of momentum transport on the molecular scale, as well as to monitor the structural assembly upon cooling.