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A mechanics approach is presented to study the intercellular delamination resistance and mechanical behavior of stratum comeum (SC) tissue in the direction normal to the skin surface. The effects of temperature and hydration on debonding behavior were also explored. Such understanding, which includes the relationship of mechanical behavior to the underlying SC cellular structure, is essential for emerging transdermal drug delivery technologies. Fracture mechanics-based cantilever-beam specimens were used to determine reproducibly the energy release rates to quantify the cohesive strength of human SC. The debond resistance of fully hydrated SC was found to decrease with increasing temperature, while dehydrated SC exhibited a more complex variation with temperature. Stress-separation tests showed that fracture energies and peak separation stresses decreased with increasing temperature and hydration, although the SC modulus varied only marginally with temperature and hydration. Results are described in terms of microstructural changes associated with hydrophilic regions and intercellular lipid phase transitions., Symposium held April 1-5 2002, San Francisco, Ca. This article is from ADA418623 Materials Research Society Symposium Proceedings. Volume 724. Biological and Biomimetic Materials - Properties to Function

The recent development of bulk metallic glasses (BMG's) offers the potential for metallic material systems with dramatically improved mechanical properties. Tensile strengths of 2 Gpa and toughnesses of up to 55 Mpav.m have been reported. These high strength and toughness values are accompanied by remarkably little plastic deformation. Until recently, little work has been undertaken to elucidate the fundamental deformation and fracture mechanisms of these alloys. This technical report describes a three year program of study designed to characterize the mechanisms of deformation, fracture and fatigue of a Zr-based BMG under a range of stress state, environmental and fatigue loading conditions.

The research on the fracture and fatigue crack-growth behavior of single crystal intermetallics was motivated by the need for improved high temperature engineering materials. The approach using a first principles total energy calculation of the energies of ?001! and ?110! surfaces in NiAl, by Yoo and Fu predicts very low fracture toughness vs. experimental findings. As the program has developed, the focus has shifted to the use of thermal barrier and wears resistant coatings to improve material behavior at high temperature and other extreme service conditions. The present emphasis therefore to develop thermal barrier and wear resistant coating systems with improved integrity and long term reliability. The major accomplishment of this work has been the development of fracture-mechanics based techniques to accurately and reproducibly measure the fracture resistance (or adhesion) of coating systems. We believe the data presented to be unique for the two coating systems characterized. Interface toughness, K, data was obtained for several specimens containing multi-layer systems of PSZ-NiAl-Pt- Superalloy and TiAlN-Cermet. The interfaces prone to fracture were identified. Experiments were also completed to analyze crack-growth under cyclic fatigue loading of these interfaces.

The main objective of this research has been to develop a fundamental understanding of the high temperature deformation, fracture, and fatigue crack growth resistance of single crystals of NiAl and NiAl-based intermetallic alloys. The approach we have taken involves high resolution mechanical property studies of well-characterized and well-oriented single crystals of NiAl containing various alloying elements (Ti, Fe, Zr) in the form of both solid solutions and two phase alloys. Our studies of the high temperature deformation of the solid-solution alloy Ni-Al(Ti) revealed a significant solute strengthening effect which we studied, characterized and modeled. The creep rates for these alloys were observed to be independent of crystal orientation and were as much as three to four orders of magnitude lower than for unalloyed NiAl. We have also studied the unusual creep properties of Ni3Al, a related intermetallic alloy, at intermediate temperatures. We have also conducted the first-ever studies of the cyclic fatigue crack-growth behavior of NiAl single crystals at both room temperature and 800 deg C. Room temperature crack growths ranged over two orders of magnitude from 10-8 to 10-10 m/cycle with a fatigue threshold value DKTH of approximately 2.3 MPaOem. Similar crack growths at 800 deg C required DK of about 8 MPaOem.