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Metal failure in many applications, such as ballistic impact, containment, shielding, metal forming, and crashworthiness, occurs while the material is in a three-dimensional state of stress. Many previous definitions of triaxiality use two invariants to define the relative stress state in a virtual element, leading to a characterization that can be better thought of as biaxial. In this paper, an additional parameter based upon the third stress invariant is defined, which extends the characterization of the state of stress to three dimensions and to true triaxiality. The relation of the two parameters is explored and limits are found in the failure surface, which is used in defining the critical failure regions. Standard tests are examined to determine if they can provide enough data to construct these regions of interest and new tests are proposed, which envelope the limits and thus define this failure surface. DOI: 10.1061/(ASCE)0893-1321(2009)22:3(280) CE Database subject headings: Metal; Material failures; Stress.

Prior to the Columbia accident, quantitative impact assessment tools were not available to analyze debris impacts onto the Shuttle's thermal protection system. Following the accident, the Columbia Accident Investigation Board recommended changes to increase the safety of future shuttle flights; one component was the development of physics-based analytical capabilities to evaluate damage due to debris impacts. This paper will present an overview of real time debris assessment impact analysis conducted by the Boeing Philadelphia Advanced Structural Analysis Impact Analysis Team in support of Space Shuttle missions since Return to Flight, the first mission after the Columbia accident. Specifically, analyses performed in support of missions STS-114, 121 and 117 will be presented. For each of these cases, an overview of the structural and material model development will be provided, and results of each analysis will be presented followed by a discussion of how the results lead into real time mission decisions. This work illustrates the importance of maintaining a physics-based real-time analysis capability as a vital instrument in supporting the safety of future spaceflight missions. DOI: 10.1061/(ASCE)0893-1321(2009)22:3(222) CE Database subject headings: Space shuttle orbiter; Debris; Impact; NASA.

A study was conducted to measure and compare the coherence properties of 2 * 1 arrays of photonic crystal vertical-cavity surface-emitting lasers. The results show that the amplitude and phase of the complex degree of coherence are correlated, such that coherence is maximized near in-phase and out-of-phase coupling conditions, and controllable by independent current injection to each array element.