ABSTRACT--An experimental technique that modifies the conventional split Hopkinson pressure bar has been developed for measuring the compressive stress-strain responses of materials with low mechanical impedance and low compressive strengths such as elastomers at high strain rates. A high-strength aluminum alloy was used for the bar materials instead of steel, and the transmission bar was hollow. The lower Young's modulus of the aluminum alloy and the smaller cross-sectional area of the hollow bar increased the amplitude of the transmitted strain signal by an order of magnitude as compared to a conventional steel bar. In addition, a pulse shaper lengthened the rise time of the incident pulse to ensure stress equilibrium and homogeneous deformation in the low-impedance specimen. Experimental results show that the high strain rate, compressive stress-strain behavior of an elastomeric material can be determined accurately and reliably using this technique.

The effects of various microconstituents on crack initiation and propagation in high-cycle fatigue (HCF) were investigated in an aluminum casting alloy (A356.2). Fatigue cracking was induced in both axial and bending loading conditions at strain/stress ratios of Ϫ1, 0.1, and 0.2. The secondary dendrite arm spacing (SDAS) and porosity (maximum size and density distribution) were quantified in the directionally solidified casting alloy. Using scanning electron microscopy, we observed that cracks initiate at near-surface porosity, at oxides, and within the eutectic microconstituents, depending on the SDAS. When the SDAS is greater than ϳ 25 to 28 m, the fatigue cracks initiate from surface and subsurface porosity. When the SDAS is less than ϳ 25 to 28 m, the fatigue cracks initiate from the interdendritic eutectic constituents, where the silicon particles are segregated. Fatigue cracks initiated at oxide inclusions whenever they were near the surface, regardless of the SDAS. The fatigue life of a specimen whose crack initiated at a large eutectic constituent was about equal to that when the crack initiated at a pore or oxide of comparable size.

Abstract-Internet-scale dissemination of streaming contents (e.g., live sports games) can be achieved by infrastructure-based overlay multicast networks, where multicast service providers deliver the contents via dedicated servers strategically placed over the Internet. Given the huge amount of data traffic, one of the major operation costs is the ISP cost for network access. However, existing overlay multicast protocols only consider network performance metrics in building dissemination trees without taking into account the potentially high ISP cost they may incur. This paper presents a scheme, Revenue-driven Overlay Multicast Networks (ROMaN), to assign users to different servers in order to maximize the profit derived from providing multicast services. ROMaN exploits the fact that ISP charging functions are concave by assigning users to the cheapest available servers, and dynamically adjusts the assignment to accommodate the churns of group membership. The evaluation shows that ROMaN not only can reduce ISP cost substantially, but also has shorter end-to-end delay due to smaller overlay size, and the longer a user stays in the group the better the service it will receive.