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A zirconium-based bulk metallic glass, Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 ͑Vit 1͒, and its composite, Zr 56.3 Ti 13.8 Cu 6.9 Ni 5.6 Nb 5.0 Be 12.5 ͑␤-Vit͒, were subjected to planar impact loading. A surprisingly low amplitude elastic precursor and bulk wave, corresponding to the elastic response of the ''frozen structure'' of the intact metallic glasses, were observed to precede the rate-dependent large deformation shock wave. A concave downward curvature after the initial increase of the U s -U p shock Hugoniots suggests that a phase-change-like transition occurred during shock compression. Further, compression damage occurred due to the shear localization. The spalling in Vit 1 was induced by shear localization, while in ␤-Vit, it was due to debonding of the ␤-phase boundary from the matrix. The spall strengths at strain rate of 2ϫ10 6 s Ϫ1 were determined to be 2.35 and 2.11 GPa for Vit 1 and ␤-Vit, respectively. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1485300͔ Metallic glasses are amorphous metastable solids and are being processed in bulk form suitable for structural applications including those involving impact. Due to their noncrystalline, fluid-like randomly ordered atomic structures resulting from the controlled solidification processes from liquid to solid state, metallic glasses have many unique mechanical, electrical, magnetic, and corrosion properties which distinguish them from their counterparts of crystalline alloys. 1,2 The mechanical properties and structural performance, as well as the deformation and damage mechanisms of bulk metallic glasses have been extensively studied through different techniques, and considerable amount of experimental data and theoretical modeling are available in literature. However, only a few studies if there is any, can the work be found in the literature addressing the dynamic response and damage of metallic glasses under shock loading. In this letter, we report the first shock compression experimental results for a metallic glass, Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 ͑Vitreloy 1͒ 3 and its particulate composite, Zr 56.3 Ti 13.8 Cu 6.9 Ni 5.6 Nb 5.0 Be 12.5 ͑in situ dendritc ␤-phase reinforced Vitreloy͒, 4 which are to be respectively referred to as Vit 1 and ␤-Vit hereafter.Specimens of 20 mm diameter were impacted by 36 mm diameter planar flyers which were accelerated to a desired velocity by a powder gun loading system. 5 The experimental parameters and mechanical properties of the materials are listed in Tables I and II, respectively. The instance of impact by the flyer on the specimen was detected using electric pins ͑EPs͒ 5,6 flush mounted with the impact surface of specimen. The arrival time of shock wave at the rear surface of specimen was sensed either by EPs which were flush mounted with the rear surface of specimen, or by a time-resolved laser velocity interferometer system, VISAR 5,7 which provides the particle velocity history. Impactors of different materials, polycarbonate ͑PC͒, 6061 aluminum alloy ͑Al͒, 304 stainless steel ͑SS͒, an...

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Ultrasonic wave speed measurements are widely used to infer the elastic properties of solids. In the standard method, longitudinal and shear transducers are used separately to measure the corresponding wave speeds in a material. A new experimental method is introduced for simultaneously measuring the longitudinal and the shear wave speeds using a single set of longitudinal or shear transducers. The method can also be used to measure the wave speeds in situ during deformation by placing the transducers along the loading axis. The transducers are housed in a specially designed fixture such that they are not subjected to loading. The technique is demonstrated by performing uniaxial compression experiments on fully dense isotropic solids (where the wave speeds are not expected to change during deformation) and in polymeric foams (where the wave speeds are affected by damage).

Host-defense peptides (HDPs) are promising next generation of antibiotic agents, as they have the potential to circumvent emerging drug resistance, due to their mechanism of bacterial killing through disruption of their membranes. Nonetheless, HDPs have intrinsic drawbacks such as low-to-moderate activity, susceptibility to enzymatic degradation. In the past few years, we developed a new class of peptidomimetics named ‘γ-AApeptides’, which have superior resistance to proteolysis and a variety of diversification via straightforward synthesis. Our recent studies suggested that γ-AApeptides can mimic the bactericidal mechanism of HDPs and show potent and broad-spectrum activity against both Gram-positive and Gram-negative multidrug-resistant bacteria. In this review, we summarize our current studies of antimicrobial γ-AApeptides and discuss their potential future development as antimicrobial peptidomimetics.

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