Damage development in an armor ceramic under quasi-static indentation

The objective of the present study is to assess the capabilities of a recently developed mechanism-based model for inelastic deformation and damage in structural ceramics. In addition to conventional lattice plasticity, the model accounts for microcrack growth and coalescence as well as granular flow following comminution. The assessment is made through a coupled experimental/computational study of the indentation response of a commercial armor ceramic. The experiments include examinations of subsurface damage zones along with measurements of residual surface profiles and residual near-surface stresses. Extensive finite element computations are conducted in parallel. Comparisons between experiment and simulation indicate that the most discriminating metric in the assessment is the spatial extent of subsurface damage following indentation. Residual stresses provide additional validation. In contrast, surface profiles of indents are dictated largely by lattice plasticity and thus provide minimal additional insight into the inelastic deformation resulting from microcracking or granular flow. A satisfactory level of correlation is obtained using property values that are either measured directly or estimated from physically based arguments, without undue reliance on adjustable (nonphysical) parameters. © 2011 The American Ceramic Society.

Multi-hit ballistic damage characterisation of 304 stainless steel plates with finite elements

Multi-impact of projectiles on thin 304 stainless steel plates is investigated to assess the degradation of ballistic performance, and to characterise the inherent mechanisms. Assessment of ballistic degradation is by means of a double-impact of rigid spheres at the same site on a circular clamped plate. The limiting velocity of the second impact, will be altered by the velocity of the antecedent impact. Finite element analyses were used to elucidate experimental results and understand the underlying mechanisms that give rise to the performance degradation. The effect of strength and ductility on the single and multi-impact performance was also considered. The model captured the experimental results with excellent agreement. Moreover, the material parameters used within the model were exclusively obtained from published works with no fitting or calibration required. An attempt is made to quantify the elevation of the ballistic limit of thin plates by the dynamic mechanism of travelling hinges. Key conclusions: The multi-hit performance scales linearly with the single-hit performance; and strength is a significantly greater effector of increased ballistic limit than ductility, even at the expense of toughness. © 2014 Elsevier Ltd.

Doping mechanism in tetrahedral amorphous carbon

Doping in hydrogenated amorphous silicon occurs by a process of an ionised donor atom partially compensated by a charged dangling bond. The total energies of various dopant and dopant/bonding combinations are calculated for tetrahedral amorphous carbon. It is found that charged dangling bonds are less favoured because of the stronger Coulombic repulsion in ta-C. Instead the dopants can be compensated by weak bond states in the lower gap associated with odd-membered π-rings or odd-numbered π-chains. The effect is that the doping efficiency is low but there are not charged midgap recombination centres, to reduce photoconductivity or photoluminescence with doping, as occurs in a-Si:H.

Doping mechanism in tetrahedral amorphous carbon

Doping in hydrogenated amorphous silicon occurs by a process of an ionized donor atom partially compensated by a charged dangling bond. The total energies of various dopant and dopant/bonding combinations are calculated for tetrahedral amorphous carbon. It is found that charged dangling bonds are less favored because of the stronger Coulombic repulsion in ta-C. Instead the dopants can be compensated by weak bond states in the lower gap associated with odd-membered π-rings or odd-numbered π-chains. The effect is that the doping efficiency is low but there are not charged midgap recombination centres, to reduce photoconductivity or photoluminescence with doping, as occurs in a-Si:H.

Mechanism And Selectivity In The Epoxidation Of Ethylene Over Silver: The Results Of Single Crystal Measurements

The silver-catalysed oxidation of ethylene has been examined on the (III) face of a single crystal by a combination of electron spectroscopy and kinetic measurements at pressures of up to 50 Torr. The necessary and sufficient conditions for ethylene oxide formation are established, reaction intermediates are identified, kinetic isotope effects are observed and the role of Cs in modifying reaction selectivity is examined. It is shown that surface alkali exhibits opposite effects on the reactions which lead to the further oxidation of ethylene oxide and on the direct combustion of ethylene. © 1984.