Stress states in individual Si particles of a cast Al-Si alloy: Micro-Raman analysis and microstructure based modeling

The stress states in Si particles of cast Al-Si based alloys depend on its morphology and the heat treatment given to the alloy. The Si particles fracture less on modification and fracture more in the heat treated condition. An attempt has been made in this work to study the effect of heat treatment and Si modification on the stress states of the particles. Such understanding will be valuable for predicting the ductility of the alloy. The stress states of Si particles are estimated by Raman technique and compared with the microstructure-based FEM simulations. Combination of Electron Back-Scattered Diffraction (EBSD) and frequency shift, polarized micro-Raman technique is applied to determine the stress states in Si particles with (111) orientations. Stress states are measured in the as-received state and under uniaxial compression. The residual stress, the stress in the elastic-plastic regime and the stress which causes fracture of the particles is estimated by Raman technique. FEM study demonstrates that the stress distribution is uniform in modified Si, whereas the unmodified Si shows higher and more complex stress states. The onset of plastic flow is observed at sharp corners of the particles and is followed by localization of strain between particles. Clustering of particles generates more inhomogeneous plastic strain in the matrix. Particle stress estimated by Raman technique is in agreement with FEM calculations. (C) 2014 Elsevier B.V. All rights reserved.

Stress Engineering using Si3N4 for stiction free release of SOI beams

We report on the effect of thin silicon nitride (Si3N4) induced tensile stress on the structural release of 200nm thick SOI beam, in the surface micro-machining process. A thin (20nm / 100nm) LPCVD grown Si3N4 is shown to significantly enhance the yield of released beam in wet release technique. This is especially prominent with increase in beam length, where the beams have higher tendency for stiction. We attribute this yield enhancement to the nitride induced tensile stress, as verified by buckling tendency and resonance frequency data obtained from optical profilometry and laser doppler vibrometry.

Probabilistic stress variation studies on composite single lap joint using Monte Carlo simulation

The work presented in this paper involves the stochastic finite element analysis of composite-epoxy adhesive lap joints using Monte Carlo simulation. A set of composite adhesive lap joints were prepared and loaded till failure to obtain their strength. The peel and shear strain in the bond line region at different levels of load were obtained using digital image correlation (DIC). The corresponding stresses were computed assuming a plane strain condition. The finite element model was verified by comparing the numerical and experimental stresses. The stresses exhibited a similar behavior and a good correlation was obtained. Further, the finite element model was used to perform the stochastic analysis using Monte Carlo simulation. The parameters influencing stress distribution were provided as a random input variable and the resulting probabilistic variation of maximum peel and shear stresses were studied. It was found that the adhesive modulus and bond line thickness had significant influence on the maximum stress variation. While the adherend thickness had a major influence, the effect of variation in longitudinal and shear modulus on the stresses was found to be little. (C) 2014 Elsevier Ltd. All rights reserved.

Effect of Anisotropy of Fibers on the Stress-Strain Response of Fiber-Reinforced Soil

Reinforcing soil with fibers is a useful method for improving the strength and settlement response of soil. The soil and fiber characteristics and their interaction are some of the major factors affecting the strength of reinforced soil. The fibers are usually randomly distributed in the soil, and their orientation has a significant effect on the behavior of the reinforced soil. In the paper, a study of the effect of anisotropic distribution of fibers on the stress-strain response is presented. Based on the concept of the modified Cam clay model, an analytical model was formulated for the fiber-reinforced soil, and the effect of fiber orientation on the stress-strain behavior of soil was studied in detail. The results show that, as the inclination of fibers with the horizontal plane increased, the contribution of fibers in improving the strength of fiber-reinforced soil decreased. The effect of fibers is maximum when they are in the direction of extension, and vice versa. (C) 2014 American Society of Civil Engineers.

Reply to ``Comment on `Strongly magnetized cold degenerate electron gas: Mass-radius relation of the magnetized white dwarf'''

We show that the upper bound for the central magnetic field of a super-Chandrasekhar white dwarf calculated by Nityananda and Konar Phys. Rev. D 89, 103017 (2014)] and in the concerned comment, by the same authors, against our work U. Das and B. Mukhopadhyay, Phys. Rev. D 86, 042001 (2012)] is erroneous. This in turn strengthens the argument in favor of the stability of the recently proposed magnetized super-Chandrasekhar white dwarfs. We also point out several other numerical errors in their work. Overall we conclude that the arguments put forth by Nityananda and Konar are misleading.

Temporally distinct roles of ATM and ROS in genotoxic- stress-dependent induction and maintenance of cellular senescence

Cells exposed to genotoxic stress induce cellular senescence through a DNA damage response (DDR) pathway regulated by ATM kinase and reactive oxygen species (ROS). Here, we show that the regulatory roles for ATM kinase and ROS differ during induction and maintenance of cellular senescence. Cells treated with different genotoxic agents were analyzed using specific pathway markers and inhibitors to determine that ATM kinase activation is directly proportional to the dose of the genotoxic stress and that senescence initiation is not dependent on ROS or the p53 status of cells. Cells in which ROS was quenched still activated ATM and initiated the DDR when insulted, and progressed normally to senescence. By contrast, maintenance of a viable senescent state required the presence of ROS as well as activated ATM. Inhibition or removal of either of the components caused cell death in senescent cells, through a deregulated ATM-ROS axis. Overall, our work demonstrates existence of an intricate temporal hierarchy between genotoxic stress, DDR and ROS in cellular senescence. Our model reports the existence of different stages of cellular senescence with distinct regulatory networks.

Unraveling the nature of electric field- and stress- induced structural transformations in soft PZT by a new powder poling technique

A `powder-poling' technique was developed to study electric field induced structural transformations in ferroelectrics exhibiting a morphotropic phase boundary (MPB). The technique was employed on soft PZT exhibiting a large longitudinal piezoelectric response (d(33) similar to 650 pCN(-1)). It was found that electric poling brings about a considerable degree of irreversible tetragonal to monoclinic transformation. The same transformation was achieved after subjecting the specimen to mechanical stress, which suggests an equivalence of stress and electric field with regard to the structural mechanism in MPB compositions. The electric field induced structural transformation was also found to be accompanied by a decrease in the spatial coherence of polarization.

The cold mode: a phenomenological model for the evolution of density perturbations in the intracluster medium

Cool cluster cores are in global thermal equilibrium but are locally thermally unstable. We study a non-linear phenomenological model for the evolution of density perturbations in the intracluster medium (ICM) due to local thermal instability and gravity. We have analysed and extended a model for the evolution of an overdense blob in the ICM. We find two regimes in which the overdense blobs can cool to thermally stable low temperatures. One for large t(cool)/t(ff) (t(cool) is the cooling time and t(ff) is the free-fall time), where a large initial overdensity is required for thermal runaway to occur; this is the regime which was previously analysed in detail. We discover a second regime for t(cool)/t(ff) less than or similar to 1 (in agreement with Cartesian simulations of local thermal instability in an external gravitational field), where runaway cooling happens for arbitrarily small amplitudes. Numerical simulations have shown that cold gas condenses out more easily in a spherical geometry. We extend the analysis to include geometrical compression in weakly stratified atmospheres such as the ICM. With a single parameter, analogous to the mixing length, we are able to reproduce the results from numerical simulations; namely, small density perturbations lead to the condensation of extended cold filaments only if t(cool)/t(ff) less than or similar to 10.

Experimental studies on the influence of intermediate principal stress and inclination on the mechanical behaviour of angular sands

A comprehensive experimental study has been made on angular sand to investigate various aspects of mechanical behavior. A hollow cylinder torsion testing apparatus is used in this program to apply a range of stress conditions on this angular quartzitic fine sand under monotonic drained shear. The effect of the magnitude and inclination of the principal stresses on an element of sand is studied through these experiments. This magnitude and inclination of the principal stresses are presented as an ``ensemble measure of fabric in sands''. This ensemble measure of fabric in the sands evolves through the shearing process, and reaches the final state, which indeed has a unique fabric. The sand shows significant variation in strength with changing inclination of the principal stresses. The locus of the final stress state in principal stress space is also mapped from these series of experiments. Additional aspects of non-coaxiality, a benchmarking exercise with a few constitutive models is presented here. This experimental approach albeit indirect shows that a unique state which is dependent on the fabric, density and confining stress exists. This suite of experiments provides a well-controlled data set for a clear understanding on the mechanical behavior of sands.

Structural and functional analysis of two universal stress proteins YdaA and YnaF from Salmonella typhimurium: possible roles in microbial stress tolerance

In many organisms ``Universal Stress Proteins'' CUSPS) are induced in response to a variety of environmental stresses. Here we report the structures of two USPs, YnaF and YdaA from Salmonella typhimurium determined at 1.8 angstrom and 2.4 angstrom resolutions, respectively. YnaF consists of a single USP domain and forms a tetrameric organization stabilized by interactions mediated through chloride ions. YdaA is a larger protein consisting of two tandem USP domains. Two protomers of YdaA associate to form a structure similar to the YnaF tetramer. YdaA showed ATPase activity and an ATP binding motif G-2X-G-9X-G(S/T/N) was found in its C-terminal domain. The residues corresponding to this motif were not conserved in YnaF although YnaF could bind ATP. However, unlike YdaA, YnaF did not hydrolyse ATP in vitro. Disruption of interactions mediated through chloride ions by selected mutations converted YnaF into an ATPase. Residues that might be important for ATP hydrolysis could be identified by comparing the active sites of native and mutant structures. Only the C-terminal domain of YdaA appears to be involved in ATP hydrolysis. The structurally similar N-terminal domain was found to bind a zinc ion near the segment equivalent to the phosphate binding loop of the C-terminal domain. Mass spectrometric analysis showed that YdaA might bind a ligand of approximate molecular weight 800 daltons. Structural comparisons suggest that the ligand, probably related to an intermediate in lipid A biosynthesis, might bind at a site close to the zinc ion. Therefore, the N-terminal domain of YdaA binds zinc and might play a role in lipid metabolism. Thus, USPs appear to perform several distinct functions such as ATP hydrolysis, altering membrane properties and chloride sensing. (C) 2015 Elsevier Inc. All rights reserved.

Maximum likelihood analysis of multi-stress accelerated life test data of series systems with competing log-normal causes of failure

This article presents frequentist inference of accelerated life test data of series systems with independent log-normal component lifetimes. The means of the component log-lifetimes are assumed to depend on the stress variables through a linear stress translation function that can accommodate the standard stress translation functions in the literature. An expectation-maximization algorithm is developed to obtain the maximum likelihood estimates of model parameters. The maximum likelihood estimates are then further refined by bootstrap, which is also used to infer about the component and system reliability metrics at usage stresses. The developed methodology is illustrated by analyzing a real as well as a simulated dataset. A simulation study is also carried out to judge the effectiveness of the bootstrap. It is found that in this model, application of bootstrap results in significant improvement over the simple maximum likelihood estimates.

Thermal Response on the Microstructure and Texture of ECAP and Cold-Rolled Pure Magnesium

This paper deals with dynamic recrystallization (DRX), static recrystallization, and grain growth phenomena of pure magnesium after equal channel angular pressing (ECAP) by route A and B-C at 523 K (250 A degrees C) followed by 80 pct cold rolling. The ECAP-deformed and the subsequently rolled samples were annealed at 373 K and 773 K (100 A degrees C and 500 A degrees C). The associated changes in the microstructure and texture were studied using electron back-scattered diffraction. ECAP produced an average grain size of 12 to 18 A mu m with B and C-2 fiber textures. Subsequent rolling led to an average grain size 8 to 10 A mu m with basal texture fiber parallel to ND. There was no noticeable increase in the average grain size on annealing at 373 K (100 A degrees C). However, significant increase in the average grain size occurred at 773 K (500 A degrees C). The occurrence of different DRX mechanisms was detected: discontinuous dynamic recrystallization was attributed to basal slip activity and continuous dynamic recovery and recrystallization to prismatic/pyramidal slip systems. Only continuous static recrystallization could be observed on annealing.

A Universal Stress Protein (USP) in Mycobacteria Binds cAMP

Mycobacteria are endowed with rich and diverse machinery for the synthesis, utilization, and degradation of cAMP. The actions of cyclic nucleotides are generally mediated by binding of cAMP to conserved and well characterized cyclic nucleotide binding domains or structurally distinct cGMP-specific and -regulated cyclic nucleotide phosphodiesterase, adenylyl cyclase, and E. coli transcription factor FhlA (GAF) domain-containing proteins. Proteins with cyclic nucleotide binding and GAF domains can be identified in the genome of mycobacterial species, and some of them have been characterized. Here, we show that a significant fraction of intracellular cAMP is bound to protein in mycobacterial species, and by using affinity chromatography techniques, we identify specific universal stress proteins (USP) as abundantly expressed cAMP-binding proteins in slow growing as well as fast growing mycobacteria. We have characterized the biochemical and thermodynamic parameters for binding of cAMP, and we show that these USPs bind cAMP with a higher affinity than ATP, an established ligand for other USPs. We determined the structure of the USP MSMEG_3811 bound to cAMP, and we confirmed through structure-guided mutagenesis, the residues important for cAMP binding. This family of USPs is conserved in all mycobacteria, and we suggest that they serve as ``sinks'' for cAMP, making this second messenger available for downstream effectors as and when ATP levels are altered in the cell.

A Mycobacterial Phosphoribosyltransferase Promotes Bacillary Survival by Inhibiting Oxidative Stress and Autophagy Pathways in Macrophages and Zebrafish

Mycobacterium tuberculosis employs various strategies to modulate host immune responses to facilitate its persistence in macrophages. The M. tuberculosis cell wall contains numerous glycoproteins with unknown roles in pathogenesis. Here, by using Concanavalin A and LC-MS analysis, we identified a novel mannosylated glycoprotein phosphoribosyltransferase, encoded by Rv3242c from M. tuberculosis cell walls. Homology modeling, bioinformatic analyses, and an assay of phosphoribosyltransferase activity in Mycobacterium smegmatis expressing recombinant Rv3242c (MsmRv3242c) confirmed the mass spectrometry data. Using Mycobacterium marinum-zebrafish and the surrogate MsmRv3242c infection models, we proved that phosphoribosyltransferase is involved in mycobacterial virulence. Histological and infection assays showed that the M. marinum mimG mutant, an Rv3242c orthologue in a pathogenic M. marinum strain, was strongly attenuated in adult zebrafish and also survived less in macrophages. In contrast, infection with wild type and the complemented Delta mimG: Rv3242c M. marinum strains showed prominent pathological features, such as severe emaciation, skin lesions, hemorrhaging, and more zebrafish death. Similarly, recombinant Msm Rv3242c bacteria showed increased invasion in non-phagocytic epithelial cells and longer intracellular survival in macrophages as compared with wild type and vector control M. smegmatis strains. Further mechanistic studies revealed that the Rv3242c- and mimG-mediated enhancement of intramacrophagic survival was due to inhibition of autophagy, reactive oxygen species, and reduced activities of superoxide dismutase and catalase enzymes. Infection with MsmRv3242c also activated the MAPK pathway, NF-kappa B, and inflammatory cytokines. In summary, we show that a novel mycobacterial mannosylated phosphoribosyltransferase acts as a virulence and immunomodulatory factor, suggesting that it may constitute a novel target for antimycobacterial drugs.

Mycobacterium tuberculosis has diminished capacity to counteract redox stress induced by elevated levels of endogenous superoxide

Mycobacterium tuberculosis (Mtb) has evolved protective and detoxification mechanisms to maintain cytoplasmic redox balance in response to exogenous oxidative stress encountered inside host phagocytes. In contrast, little is known about the dynamic response of this pathogen to endogenous oxidative stress generated within Mtb. Using a noninvasive and specific biosensor of cytoplasmic redox state of Mtb, we for first time discovered a surprisingly high sensitivity of this pathogen to perturbation in redox homeostasis induced by elevated endogenous reactive oxygen species (ROS). We synthesized a series of hydroquinone-based small molecule ROS generators and found that ATD-3169 permeated mycobacteria to reliably enhance endogenous ROS including superoxide radicals. When Mtb strains including multidrug-resistant (MDR) and extensively drug-resistant (XDR) patient isolates were exposed to this compound, a dose-dependent, long-lasting, and irreversible oxidative shift in intramycobacterial redox potential was detected. Dynamic redox potential measurements revealed that Mtb had diminished capacity to restore cytoplasmic redox balance in comparison with Mycobacterium smegmatis (Msm), a fast growing nonpathogenic mycobacterial species. Accordingly, Mtb strains were extremely susceptible to inhibition by ATD-3169 but not Msm, suggesting a functional linkage between dynamic redox changes and survival. Microarray analysis showed major realignment of pathways involved in redox homeostasis, central metabolism, DNA repair, and cell wall lipid biosynthesis in response to ATD-3169, all consistent with enhanced endogenous ROS contributing to lethality induced by this compound. This work provides empirical evidence that the cytoplasmic redox poise of Mtb is uniquely sensitive to manipulation in steady-state endogenous ROS levels, thus revealing the importance of targeting intramycobacterial redox metabolism for controlling TB infection. (C) 2015 The Authors. Published by Elsevier Inc.