FACTORS INFLUENCING THE STRESS-STRAIN BEHAVIOR OF CERAMIC MATERIALS.

The stress-strain behavior of ceramic materials is greatly influenced by microstructural features ranging from the presence of point defects in single crystals to the size and location of pores and nature of grain boundaries in polycrystals. Several factors may affect the behavior at anyone time, and the analysis of experimental data, particularly for polycrystals, is thus extremely difficult. This review examines the interpretation of mechanical behavior in materials having the rock salt structure, with particular emphasis on the role of impurities, the significance of grain boundary and/or intragranular porosity, and the problems associated with the intersection of slip bands. <br/

Kinetic Enhancement of Raman Backscatter, and Electron Acoustic Thomson Scatter

1-D Eulerian Vlasov-Maxwell simulations are presented which show kinetic enhancement of stimulated Raman backscatter (SRBS) due to electron trapping in regimes of heavy linear Landau damping. The conventional Raman Langmuir wave is transformed into a set of beam acoustic modes [L. Yin et al., Phys. Rev. E 73, 025401 (2006)]. For the first time, a low phase velocity electron acoustic wave (EAW) is seen developing from the self-consistent Raman physics. Backscatter of the pump laser off the EAW fluctuations is reported and referred to as electron acoustic Thomson scatter. This light is similar in wavelength to, although much lower in amplitude than, the reflected light between the pump and SRBS wavelengths observed in single hot spot experiments, and previously interpreted as stimulated electron acoustic scatter [D. S. Montgomery et al., Phys. Rev. Lett. 87, 155001 (2001)]. The EAW is strongest well below the phase-matched frequency for electron acoustic scatter, and therefore the EAW is not produced by it. The beating of different beam acoustic modes is proposed as the EAW excitation mechanism, and is called beam acoustic decay. Supporting evidence for this process, including bispectral analysis, is presented. The linear electrostatic modes, found by projecting the numerical distribution function onto a Gauss-Hermite basis, include beam acoustic modes (some of which are unstable even without parametric coupling to light waves) and a strongly-damped EAW similar to the observed one. This linear EAW results from non-Maxwellian features in the electron distribution, rather than nonlinearity due to electron trapping.Comment: 15 pages, 16 figures, accepted in Physics of Plasmas (2006

Deformation and rupture of armour grade steel under localised blast loading

A series of 30 blast experiments were conducted on monolithic steel panels of two armour grade steels. The two steels evaluated were a high hardness armour (HHA) and a rolled homogenous armour (RHA). Tests were conducted at two standoff distances using a fixed charge diameter. The charge weight was varied to produce specific magnitudes of blast loading and to isolate the rupture threshold of each material. The results indicated that the HHA steel, generally reserved for ballistic protection, outperformed a more ductile RHA steel in terms of both its deformation resistance and rupture threshold. Optical and scanning electron microscopy was utilised for fractographic analysis of the ruptured plates. The failure of the steels in this investigation was found to be initiated by slant shear fracture with little to no localised thinning. This is in contrast to the tensile instability and ductile tearing predicted by established theories of plate rupture for mild steels under blast loading. The deformation and rupture of the candidate steels was analysed for all experimental conditions and compared to current empirical models based on a non-dimensional impulse parameter. While deformation behaviour is well predicted, the blast rupture threshold of the armour grade steels is poorly captured by current empirical modelling approaches. The identified shear fracture mode leads to lower energy absorption capabilities of the material compared to more ductile tensile failure

Evolution of microstructure and hardness in an AZ80 magnesium alloy processed by high-pressure torsion

An AZ80 magnesium alloy with an initial grain size of ?25 ?m and a hardness of Hv ? 63 was processed by high-pressure torsion (HPT) at room temperature for up to 10 turns under an imposed pressure of 6.0 GPa. After processing, the specimens were examined by optical microscopy and transmission electron microscopy and measurements were taken of the Vickers microhardness along diameters of the HPT discs. The results show that the grains are refined to ?200 nm after 5 and 10 turns of HPT and the hardness increases to Hv ? 120 at an equivalent strain of ?30. There is a saturation condition and no further hardening at additional equivalent strains up to &gt;200

N-methylformamide: antitumour activity and metabolism in mice.

The antitumour activities of N-methylformamide, N-ethylformamide and formamide against a number of murine tumours in vivo (Sarcoma 180, M5076 ovarian sarcoma and TLX5 lymphoma) have been estimated. In all cases N-methyl-formamide had significant activity, formamide had marginal or no activity and N-ethylformamide had no significant activity. N-methylformamide and N-ethylformamide were equitoxic to the TLX5 lymphoma in vitro. Formamide was found as a metabolite in the plasma and urine of animals given N-methylformamide and N-ethylformamide, but excretion profiles do not support the hypothesis that formamide is an active antitumour species formed from N-alkylformamides. No appreciable metabolism of N-methylformamide occurred under a variety of conditions with liver preparations in vitro. N-methylformamide, but not N-ethylformamide or formamide, reduced liver soluble non-protein thiols by 59.8% 1 h after administration of an effective antitumour dose

Fundamentals of interface phenomena in advanced bulk nanoscale materials

The review is devoted to a study of interface phenomena influencing advanced properties of nanoscale materials processed by means of severe plastic deformation, high-energy ball milling and their combinations. Interface phenomena include processes of interface defect structure relaxation from a highly nonequilibrium state to an equilibrium condition, grain boundary phase transformations and enhanced grain boundary and triple junction diffusivity. On the basis of an experimental investigation, a theoretical description of the key interfacial phenomena controlling the functional properties of advanced bulk nanoscale materials has been conducted. An interface defect structure investigation has been performed by TEM, high-resolution x-ray diffraction, atomic simulation and modeling. The problem of a transition from highly non-equilibrium state to an equilibrium one, which seems to be responsible for low thermostability of nanoscale materials, was studied. Also enhanced grain boundary diffusivity is addressed. Structure recovery and dislocation emission from grain boundaries in nanocrystalline materials have been investigated by analytical methods and modeling

The response of plates under non-uniform impulsive loads

Near-field blast loads are non-uniformly distributed across the loaded face of a structural element and result in large, localised plastic deformations. Experimental characterisation of both the loading and resultant deformation is rare in the literature, and to date there have been no studies which investigate modes and mechanisms of target deformation with a detailed knowledge of the imparted load. This paper presents results from a collaboration between the University of Sheffield, UK, and the University of Cape Town, South Africa, aimed at measuring the loading imparted to, and subsequent dynamic deformation of blast loaded plates. This paper presents results from these experiments, where the presence of inward and outward travelling flexural waves are observed and discussed in relation to localised variations in the imparted load

Probabilistic analysis of the response of plates subjected to near-field blast loading

Accurate prediction of the response of structures subjected to close proximity blast loads is a pressing engineering concern; the landscape of global terror has shifted away from large and indiscriminate bombings towards much smaller and more targeted attacks (e.g. against critical infrastructure and/or transport). In such close-proximity blast events (in the so-called ‘nearfield’), interaction between the expanding detonation products and air shock gives rise to complex hydrodynamic features which introduce localised variations in the pressure field. The resultant loading (typically defined in terms of specific impulse since loading durations act on timescales considerably shorter than structural response) is therefore highly uncertain, and even nominally identical experiments produce loading distributions with a high degree of local variability. Current predictive approaches either grossly simplify or neglect entirely the inherent ‘fuzziness’ of nearfield blast loading, to the extent where it is currently unknown what effect this has on structural response, how sensitive plate structures are to uncertainties in loading distribution, and how this varies with plate properties and loading condition (e.g. charge mass and stand-off distance). This paper presents a numerical study aimed at answering these questions, where specific impulse distributions are probabilistically simulated with varying degrees of localised variations and mapped onto a range of different plates. This work aims to shed light on the fundamentally stochastic nature of close-proximity blast, with a view to implementing the findings in fast running engineering models for prediction of plate response under near-field blast loading