Ductile fracture by multiple void growth and interaction ahead of a notch tip in polycrystalline plastic solids

The objectives of this paper are to study the effects of plastic anisotropy and evolution in crystallographic texture with deformation on the ductile fracture behaviour of polycrystalline solids. To this end, numerical simulations of multiple void growth and interaction ahead of a notch tip are performed under mode I, plane strain, small scale yielding conditions using two approaches. The first approach is based on the Hill yield theory, while the second employs crystal plasticity constitutive equations and a Taylor-type homogenization in order to represent the ductile polycrystalline solid. The initial textures pertaining to continuous cast Al-Mg AA5754 sheets in recrystallized and cold rolled conditions are considered. The former is nearly-isotropic, while the latter displays pronounced anisotropy. The results indicate distinct changes in texture in the ligaments bridging the voids ahead of the notch tip with increase in load level which gives rise to retardation in porosity evolution and increase in tearing resistance for both materials.

Effect of Annealing Temperature in Al 1145 Alloy Sheets on Formability, Void Coalescence, and Texture Analysis

This paper deals with a combined forming and fracture limit diagram and void coalescence analysis for the aluminum alloy Al 1145 alloy sheets of 1.8 mm thickness, annealed at four different temperatures, namely 200, 250, 300, and 350 A degrees C. At different annealing temperatures these sheets were examined for their effects on microstructure, tensile properties, formability, void coalescence, and texture. Scanning electron microscope (SEM) images taken from the fractured surfaces were examined. The tensile properties and formability of sheet metals were correlated with fractography features and void analysis. The variation of formability parameters, normal anisotropy of sheet metals, and void coalescence parameters were compared with texture analysis.

Cavitation in materials with distributed weak zones: Implications on the origin of brittle fracture in metallic glasses

Several experimental studies have shown that fracture surfaces in brittle metallic glasses (MGs) generally exhibit nanoscale corrugations which may be attributed to the nucleation and coalescence of nanovoids during crack propagation. Recent atomistic simulations suggest that this phenomenon is due to large spatial fluctuations in material properties in a brittle MG, which leads to void nucleation in regions of low atomic density and then catastrophic fracture through void coalescence. To explain this behavior, we propose a model of a heterogeneous solid containing a distribution of weak zones to represent a brittle MG. Plane strain continuum finite element analysis of cavitation in such an elastic-plastic solid is performed with the weak zones idealized as periodically distributed regions having lower yield strength than the background material. It is found that the presence of weak zones can significantly reduce the critical hydrostatic stress for the onset of cavitation which is controlled uniquely by the local yield properties of these zones. Also, the presence of weak zones diminishes the sensitivity of the cavitation stress to the volume fraction of a preexisting void. These results provide plausible explanations for the observations reported in recent atomistic simulations of brittle MGs. An analytical solution for a composite, incompressible elastic-plastic solid with a weak inner core is used to investigate the effect of volume fraction and yield strength of the core on the nature of cavitation bifurcation. It is shown that snap-cavitation may occur, giving rise to sudden formation of voids with finite size, which does not happen in a homogeneous plastic solid. (c) 2012 Elsevier Ltd. All rights reserved.

Postliquefaction Undrained Shear Behavior of Sand-Silt Mixtures at Constant Void Ratio

A detailed study on the postliquefaction undrained shear behavior of sand-silt mixtures at constant void ratios is presented in this article. The influence of different parameters such as density, amplitude of cyclic shear stress, and drainage conditions on the postliquefaction undrained response of sand-silt mixtures has been investigated, in addition to the effect of fines content. The results showed that the limiting silt content plays a vital role in the strength of the soil under both cyclic and monotonic shear loading. Both the liquefaction resistance and postliquefaction shear strength of the soils are found to decrease with an increase in the fines content until the limiting silt content is reached. However, further increase in the silt content beyond the limiting silt content increases the liquefaction resistance as well as the postliquefaction shear strength of the soils. It is also observed that these variations on the liquefaction and postliquefaction resistance of soils are closely related to the variations in relative density. (C) 2013 American Society of Civil Engineers.

Detailed investigation of contact deformation in ZrN/Zr multiplayer-understanding the role of volume fraction, bilayer spacing, and morphology of interfaces

A systematic study was done to understand the influence of volume fractions and bilayer spacings for metal/nitride multilayer coating using finite element method (FEM). An axisymmetric model was chosen to model the real situation by incorporating metal and substrate plasticity. Combinations of volume fractions and bilayer spacings were chosen for FEM analysis consistent with experimental results. The model was able to predict trends in cracking with respect to layer spacing and volume fraction. Metal layer plasticity is seen to greatly influence the stress field inside nitride. It is seen that the thicker metal induces higher tensile stresses inside nitride and hence leads to lower cracking loads. Thin metal layers < 10 nm were seen to have curved interfaces, and hence, the deformation mode was interfacial delamination in combination with edge cracking. There is an optimum seen with respect to volume fraction similar to 13% and metal layer thickness similar to 30 nm, which give maximum crack resistance.

Estimating the fraction of progeny virions that must incorporate APOBEC3G for suppression of productive HIV-1 infection

The contest between the host factor APOBEC3G (A3G) and the HIV-1 protein Vif presents an attractive target of intervention. The extent to which the A3G-Vif interaction must be suppressed to tilt the balance in favor of A3G remains unknown. We employed stochastic simulations and mathematical modeling of the within-host dynamics and evolution of HIV-1 to estimate the fraction of progeny virions that must incorporate A3G to render productive infection unsustainable. Using three different approaches, we found consistently that a transition from sustained infection to suppression of productive infection occurred when the latter fraction exceeded similar to 0.8. The transition was triggered by A3G-induced hypermutations that led to premature stop codons compromising viral production and was consistent with driving the basic reproductive number, R-o, below unity. The fraction identified may serve as a quantitative guideline for strategies targeting the A3G-Vif axis. (C) 2013 Elsevier Inc. All rights reserved.

A simple model for spatial disaggregation of evaporative fraction: Comparative study with thermal sharpened land surface temperature data over India

[1] Evaporative fraction (EF) is a measure of the amount of available energy at the earth surface that is partitioned into latent heat flux. The currently operational thermal sensors like the Moderate Resolution Imaging Spectroradiometer (MODIS) on satellite platforms provide data only at 1000 m, which constraints the spatial resolution of EF estimates. A simple model (disaggregation of evaporative fraction (DEFrac)) based on the observed relationship between EF and the normalized difference vegetation index is proposed to spatially disaggregate EF. The DEFrac model was tested with EF estimated from the triangle method using 113 clear sky data sets from the MODIS sensor aboard Terra and Aqua satellites. Validation was done using the data at four micrometeorological tower sites across varied agro-climatic zones possessing different land cover conditions in India using Bowen ratio energy balance method. The root-mean-square error (RMSE) of EF estimated at 1000 m resolution using the triangle method was 0.09 for all the four sites put together. The RMSE of DEFrac disaggregated EF was 0.09 for 250 m resolution. Two models of input disaggregation were also tried with thermal data sharpened using two thermal sharpening models DisTrad and TsHARP. The RMSE of disaggregated EF was 0.14 for both the input disaggregation models for 250 m resolution. Moreover, spatial analysis of disaggregation was performed using Landsat-7 (Enhanced Thematic Mapper) ETM+ data over four grids in India for contrasted seasons. It was observed that the DEFrac model performed better than the input disaggregation models under cropped conditions while they were marginally similar under non-cropped conditions.

Filling-in Void and Sparse Regions in Protein Sequence Space by Protein-Like Artificial Sequences Enables Remarkable Enhancement in Remote Homology Detection Capability

Protein functional annotation relies on the identification of accurate relationships, sequence divergence being a key factor. This is especially evident when distant protein relationships are demonstrated only with three-dimensional structures. To address this challenge, we describe a computational approach to purposefully bridge gaps between related protein families through directed design of protein-like ``linker'' sequences. For this, we represented SCOP domain families, integrated with sequence homologues, as multiple profiles and performed HMM-HMM alignments between related domain families. Where convincing alignments were achieved, we applied a roulette wheel-based method to design 3,611,010 protein-like sequences corresponding to 374 SCOP folds. To analyze their ability to link proteins in homology searches, we used 3024 queries to search two databases, one containing only natural sequences and another one additionally containing designed sequences. Our results showed that augmented database searches showed up to 30% improvement in fold coverage for over 74% of the folds, with 52 folds achieving all theoretically possible connections. Although sequences could not be designed between some families, the availability of designed sequences between other families within the fold established the sequence continuum to demonstrate 373 difficult relationships. Ultimately, as a practical and realistic extension, we demonstrate that such protein-like sequences can be ``plugged-into'' routine and generic sequence database searches to empower not only remote homology detection but also fold recognition. Our richly statistically supported findings show that complementary searches in both databases will increase the effectiveness of sequence-based searches in recognizing all homologues sharing a common fold. (C) 2013 Elsevier Ltd. All rights reserved.

Experimental investigation of forming limit, void coalescence and crystallographic textures of aluminum alloy 8011 sheet annealed at various temperatures

In this work, a combined forming and fracture limit diagram, fractured void coalescence and texture analysis have been experimentally evaluated for the commercially available aluminum alloy Al 8011 sheet annealed at different temperatures viz. 200 degrees C, 250 degrees C, 300 degrees C and 350 degrees C. The sheets were examined at different annealing temperatures on microstructure, tensile properties, formability and void coalescence. The fractured surfaces of the formed samples were examined using scanning electron microscope (SEM) and these images were correlated with fracture behavior and formability of sheet metals. Formability of Al 8011 was studied and examined at various annealing temperatures using their bulk X-ray crystallographic textures and ODF plots. Forming limit diagrams, void coalescence parameters and crystallographic textures were correlated with normal anisotropy of the sheet metals annealed at different temperatures. (C) 2013 Politechnika Wroclawska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.

Deformation behaviour of Rheocast A356 Al alloy at microlevel considering approximated RVEs

A micromechanical approach is considered here to predict the deformation behaviour of Rheocast A356 (Al-Si-Mg) alloy. Two representative volume elements (RVEs) are modelled in the finite element (FE) framework. Two dimensional approximated microstructures are generated assuming elliptic grains, based on the grain size, shape factor and area fraction of the primary Al phase of the said alloy at different processing condition. Plastic instability is shown using stress and strain distribution between the Al rich primary and Si rich eutectic phases under different boundary conditions. Boundary conditions are applied on the approximated RVEs in such a manner, so that they represent the real life situation depending on their position on a cylindrical tensile test sample. FE analysis is carried out using commercial finite element code ABAQUS without specifying any damage or failure criteria. Micro-level in-homogeneity leads to incompatible deformation between the constituent phases of the rheocast alloy and steers plastic strain localisation. Plastic stain localised regions within the RVEs are predicted as the favourable sites for void nucleation. Subsequent growth of nucleated voids leads to final failure of the materials under investigation.

Dependence of ``critical cloud fraction' on aerosol composition

Recent studies, over regions influenced by biomass burning aerosol, have shown that it is possible to define a critical cloud fraction' (CCF) at which the aerosol direct radiative forcing switch from a cooling to a warming effect. Using 4 years of multi-satellite data analysis, we show that CCF varies with aerosol composition and changed from 0.28 to 0.13 from postmonsoon to winter as a result of shift from less absorbing to moderately absorbing aerosol. Our results indicate that we can estimate aerosol absorption from space using independently measured top of the atmosphere (TOA) fluxes Cloud Aerosol Lidar with Orthogonal Polarization-Moderate resolution Imaging Spectroradiometer-Clouds and the Earth's Radiant Energy System (CALIPSO-MODIS-CERES)] combined algorithms for example.

Influence of fuel hydrogen fraction on syngas fueled SI engine: Fuel thermo-physical property analysis and in-cylinder experimental investigations

Hydrogen, either in pure form or as a gaseous fuel mixture specie enhances the fuel conversion efficiency and reduce emissions in an internal combustion engine. This is due to the reduction in combustion duration attributed to higher laminar flame speeds. Hydrogen is also expected to increase the engine convective heat flux, attributed (directly or indirectly) to parameters like higher adiabatic flame temperature, laminar flame speed, thermal conductivity and diffusivity and lower flame quenching distance. These factors (adversely) affect the thermo-kinematic response and offset some of the benefits. The current work addresses the influence of mixture hydrogen fraction in syngas on the engine energy balance and the thermo-kinematic response for close to stoichiometric operating conditions. Four different bio-derived syngas compositions with fuel calorific value varying from 3.14 MJ/kg to 7.55 MJ/kg and air fuel mixture hydrogen fraction varying from 7.1% to 14.2% by volume are used. The analysis comprises of (a) use of chemical kinetics simulation package CHEMKIN for quantifying the thermo-physical properties (b) 0-D model for engine in-cylinder analysis and (c) in-cylinder investigations on a two-cylinder engine in open loop cooling mode for quantifying the thermo-kinematic response and engine energy balance. With lower adiabatic flame temperature for Syngas, the in-cylinder heat transfer analysis suggests that temperature has little effect in terms of increasing the heat flux. For typical engine like conditions (700 K and 25 bar at CR of 10), the laminar flame speed for syngas exceeds that of methane (55.5 cm/s) beyond mixture hydrogen fraction of 11% and is attributed to the increase in H based radicals. This leads to a reduction in the effective Lewis number and laminar flame thickness, potentially inducing flame instability and cellularity. Use of a thermodynamic model to assess the isolated influence of thermal conductivity and diffusivity on heat flux suggests an increase in the peak heat flux between 2% and 15% for the lowest (0.420 MW/m(2)) and highest (0.480 MW/m(2)) hydrogen containing syngas over methane (0.415 MW/m(2)) fueled operation. Experimental investigations indicate the engine cooling load for syngas fueled engine is higher by about 7% and 12% as compared to methane fueled operation; the losses are seen to increase with increasing mixture hydrogen fraction. Increase in the gas to electricity efficiency is observed from 18% to 24% as the mixture hydrogen fraction increases from 7.1% to 9.5%. Further increase in mixture hydrogen fraction to 14.2% results in the reduction of efficiency to 23%; argued due to the changes in the initial and terminal stages of combustion. On doubling of mixture hydrogen fraction, the flame kernel development and fast burn phase duration decrease by about 7% and 10% respectively and the terminal combustion duration, corresponding to 90%-98% mass burn, increases by about 23%. This increase in combustion duration arises from the cooling of the near wall mixture in the boundary layer attributed to the presence of hydrogen. The enhancement in engine cooling load and subsequent reduction in the brake thermal efficiency with increasing hydrogen fraction is evident from the engine energy balance along with the cumulative heat release profiles. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Anthropogenic aerosol fraction over the Indian region: model simulations versus multi-satellite data analysis

Anthropogenic aerosols play a crucial role in our environment, climate, and health. Assessment of spatial and temporal variation in anthropogenic aerosols is essential to determine their impact. Aerosols are of natural and anthropogenic origin and together constitute a composite aerosol system. Information about either component needs elimination of the other from the composite aerosol system. In the present work we estimated the anthropogenic aerosol fraction (AF) over the Indian region following two different approaches and inter-compared the estimates. We espouse multi-satellite data analysis and model simulations (using the CHIMERE Chemical transport model) to derive natural aerosol distribution, which was subsequently used to estimate AF over the Indian subcontinent. These two approaches are significantly different from each other. Natural aerosol satellite-derived information was extracted in terms of optical depth while model simulations yielded mass concentration. Anthropogenic aerosol fraction distribution was studied over two periods in 2008: premonsoon (March-May) and winter (November-February) in regard to the known distinct seasonality in aerosol loading and type over the Indian region. Although both techniques have derived the same property, considerable differences were noted in temporal and spatial distribution. Satellite retrieval of AF showed maximum values during the pre-monsoon and summer months while lowest values were observed in winter. On the other hand, model simulations showed the highest concentration of AF in winter and the lowest during pre-monsoon and summer months. Both techniques provided an annual average AF of comparable magnitude (similar to 0.43 +/- 0.06 from the satellite and similar to 0.48 +/- 0.19 from the model). For winter months the model-estimated AF was similar to 0.62 +/- 0.09, significantly higher than that (0.39 +/- 0.05) estimated from the satellite, while during pre-monsoon months satellite-estimated AF was similar to 0.46 +/- 0.06 and the model simulation estimation similar to 0.53 +/- 0.14. Preliminary results from this work indicate that model-simulated results are nearer to the actual variation as compared to satellite estimation in view of general seasonal variation in aerosol concentrations.

The effect of straining mode on void growth

Large strain finite element method is employed to investigate the effect of straining mode on void growth. Axisymmetric cell model embedded with spherical void is controlled by constant triaxiality: loading,while plane-stress model containing a circular void is loaded by constant ratio of straining. Elastic-plastic material is used for the matrix in both cases. It is concluded that, besides the known effect of triaxiality, the straining mode which intensifies the plastic concentration around the void is also a void growth stimulator. Experimental results are cited to justify the computation results.