A distributed TDMA slot scheduling algorithm for spatially correlated contention in WSNs

In wireless sensor networks (WSNs) the communication traffic is often time and space correlated, where multiple nodes in a proximity start transmitting at the same time. Such a situation is known as spatially correlated contention. The random access methods to resolve such contention suffers from high collision rate, whereas the traditional distributed TDMA scheduling techniques primarily try to improve the network capacity by reducing the schedule length. Usually, the situation of spatially correlated contention persists only for a short duration and therefore generating an optimal or sub-optimal schedule is not very useful. On the other hand, if the algorithm takes very large time to schedule, it will not only introduce additional delay in the data transfer but also consume more energy. To efficiently handle the spatially correlated contention in WSNs, we present a distributed TDMA slot scheduling algorithm, called DTSS algorithm. The DTSS algorithm is designed with the primary objective of reducing the time required to perform scheduling, while restricting the schedule length to maximum degree of interference graph. The algorithm uses randomized TDMA channel access as the mechanism to transmit protocol messages, which bounds the message delay and therefore reduces the time required to get a feasible schedule. The DTSS algorithm supports unicast, multicast and broadcast scheduling, simultaneously without any modification in the protocol. The protocol has been simulated using Castalia simulator to evaluate the run time performance. Simulation results show that our protocol is able to considerably reduce the time required to schedule.

Microstructure Development, Nanomechanical, and Dynamic Compression Properties of Spark Plasma Sintered TiB2-Ti-Based Homogeneous and Bi-layered Composites

The growing threats due to increased use of small-caliber armor piercing projectiles demand the development of new light-weight body armor materials. In this context, TiB2 appears to be a promising ceramic material. However, poor sinterability and low fracture toughness remain two major issues for TiB2. In order to address these issues together, Ti as a sinter-aid is used to develop TiB2-(x wt pct Ti), (x = 10, 20) homogeneous composites and a bi-layered composite (BLC) with each layer having Ti content of 10 and 20 wt pct. The present study uniquely demonstrates the efficacy of two-stage spark plasma sintering route to develop dense TiB2-Ti composites with an excellent combination of nanoscale hardness (similar to 36 GPa) and indentation fracture toughness (similar to 12 MPa m(1/2)). In case of BLC, these properties are not compromised w.r.t. homogeneous composites, suggesting the retention of baseline material properties even in the bi-layer design due to optimal relief of residual stresses. The better indentation toughness of TiB2-(10 wt pct Ti) and TiB2-(20 wt pct Ti) composites can be attributed to the observed crack deflection/arrest, indicating better damage tolerance. Transmission electron microscope investigation reveals the presence of dense dislocation networks and deformation twins in alpha-Ti at the grain boundaries and triple pockets, surrounded by TiB2 grains. The dynamic strength of around 4 GPa has been measured using Split Hopkinson Pressure Bar tests in a reproducible manner at strain rates of the order of 600 s(-1). The damage progression under high strain rate has been investigated by acquiring real time images for the entire test duration using ultra-high speed imaging. An attempt has been made to establish microstructure-property correlation and a simple analysis based on Mohr-Coulomb theory is used to rationalize the measured strength properties.

A Distributed TDMA Slot Scheduling Algorithm for Spatially Correlated Contention in WSNs

In WSNs the communication traffic is often time and space correlated, where multiple nodes in a proximity start transmitting simultaneously. Such a situation is known as spatially correlated contention. The random access method to resolve such contention suffers from high collision rate, whereas the traditional distributed TDMA scheduling techniques primarily try to improve the network capacity by reducing the schedule length. Usually, the situation of spatially correlated contention persists only for a short duration, and therefore generating an optimal or suboptimal schedule is not very useful. Additionally, if an algorithm takes very long time to schedule, it will not only introduce additional delay in the data transfer but also consume more energy. In this paper, we present a distributed TDMA slot scheduling (DTSS) algorithm, which considerably reduces the time required to perform scheduling, while restricting the schedule length to the maximum degree of interference graph. The DTSS algorithm supports unicast, multicast, and broadcast scheduling, simultaneously without any modification in the protocol. We have analyzed the protocol for average case performance and also simulated it using Castalia simulator to evaluate its runtime performance. Both analytical and simulation results show that our protocol is able to considerably reduce the time required for scheduling.

Texture evolution during hot deformation of Moly-TZM

Moly-TZM was deformed at constant strain rate of 1.0 s(-1) to investigate the high strain rate deformation behaviour by microstructural and stress response change within a temperature range of 1400-1700 degrees C. To correlate the deformation behaviour with orientational change, recrystallization and recovery of the material, the microstructural investigation was undertaken using scanning electron microscopy (SEM) of electron back scattered diffraction (EBSD). Depending on the grain size and orientation spread recrystallized grains were identified and texture was calculated. Change in grain boundary characteristics with increasing temperature was determined by the misorientation angle distribution for the deformed and recrystallized grains. Subgrain coalescence and increase in subgrain size with increasing temperature was observed, indicating recrystallization not only occurred from the nucleation of the dislocation free grains in grain boundaries but also from the subgrain rotation and merging of the subgrains by annihilation of the low angle grain boundaries. Detailed studies on the evolution of texture of recrystallized grains showed continuous increase in <001> fiber texture in recrystallised grains, in contrast to a mixed fiber <001> +<111> for the deformed grains.

Chemical-free graphene by unzipping carbon nanotubes using cryo-milling

Current paper reports synthesis of chemical free graphene by unzipping of the carbon nanotubes (CNTs) using high strain rate deformation at 150K. A specially designed cryomill operating at 150 K was used for the experiments. The mechanism of unzipping was further explored using molecular dynamics (MD) simulations. Both experimental and simulation results reveal two modes of unzipping through radial and shear loading. (C) 2015 Elsevier Ltd. All rights reserved.

A Fast and Fault-Tolerant Distributed Algorithm for Near-Optimal TDMA Scheduling in WSNs

The time division multiple access (TDMA) based channel access mechanisms perform better than the contention based channel access mechanisms, in terms of channel utilization, reliability and power consumption, specially for high data rate applications in wireless sensor networks (WSNs). Most of the existing distributed TDMA scheduling techniques can be classified as either static or dynamic. The primary purpose of static TDMA scheduling algorithms is to improve the channel utilization by generating a schedule of smaller length. But, they usually take longer time to schedule, and hence, are not suitable for WSNs, in which the network topology changes dynamically. On the other hand, dynamic TDMA scheduling algorithms generate a schedule quickly, but they are not efficient in terms of generated schedule length. In this paper, we propose a novel scheme for TDMA scheduling in WSNs, which can generate a compact schedule similar to static scheduling algorithms, while its runtime performance can be matched with those of dynamic scheduling algorithms. Furthermore, the proposed distributed TDMA scheduling algorithm has the capability to trade-off schedule length with the time required to generate the schedule. This would allow the developers of WSNs, to tune the performance, as per the requirement of prevalent WSN applications, and the requirement to perform re-scheduling. Finally, the proposed TDMA scheduling is fault-tolerant to packet loss due to erroneous wireless channel. The algorithm has been simulated using the Castalia simulator to compare its performance with those of others in terms of generated schedule length and the time required to generate the TDMA schedule. Simulation results show that the proposed algorithm generates a compact schedule in a very less time.

NANOLOCALIZED SINGLE CELL MEMBRANE NANOELECTROPORATION

Today single cell research is a great interest to analyze cell to cell or cell to environment behavior with their intracellular compounds, where bulk measurement can provide average value. To deliver biomolecules precise and localized way into single living cell with high transfection rate and high cell viability is a challenging and promisible task for biological and therapeutic research. In this report, we present a nano-localized single cell nano-electroporation technique, where electroporation take place in a very precise and localized area on a single cell membrane to achieve high efficient delivery with high cell viability. We fabricated 60nm gap with 40 nm triangular Indium Tin Oxide (ITO) based nano-eletcrode tip, which can intense electric field in a nano-localized area of a single cell to permeabilize cell membrane and deliver exogenous biomolecules from outside to inside of the cell. This device successfully deliver dyes, proteins into single cell with high cell viability (98%). The process not only control precise delivery mechanism into single cell with membrane reversibility, but also it can provide special, temporal and qualitative dosage control, which might be beneficial for therapeutic and biological cell studies.

RD-TDMA: A Randomized and Distributed TDMA Scheduling for Correlated Contention in WSNs

In wireless sensor networks (WSNs), contention occurs when two or more nodes in a proximity simultaneously try to access the channel. The contention causes collisions, which are very likely to occur when traffic is correlated. The excessive collision not only affects the reliability and the QoS of the application, but also the lifetime of the network. It is well-known that random access mechanisms do not efficiently handle correlated-contention, and therefore, suffer from high collision rate. Most of the existing TDMA scheduling techniques try to find an optimal or a sub-optimal schedule. Usually, the situation of correlated-contention persists only for a short duration, and therefore, it is not worthwhile to take a long time to generate an optimal or a sub-optimal schedule. We propose a randomized distributed TDMA scheduling (RD-TDMA) algorithm to quickly generate a feasible schedule (not necessarily optimal) to handle correlated-contention in WSNs. In RD-TDMA, a node in the network negotiates a slot with its neighbors using the message exchange mechanism. The proposed protocol has been simulated using the Castalia simulator to evaluate its runtime performance. Simulation results show that the RD-TDMA algorithm considerably reduces the time required to schedule.

Effect of Ga content in Cu(Ga) on the growth of V3Ga following bronze technique

Diffusion controlled growth rate of V3Ga in the Cu(Ga)/V system changes dramatically because of a small change in Ga content in Cu(Ga). One atomic percent increase from 15 to 16 leads to more than double the product phase layer thickness and a decrease in activation energy from 255 to 142 kJ/mol. Kirkendall marker experiment indicates that V3Ga grows because of diffusion of Ga. Role of different factors influencing the diffusion rate of Ga and high growth rate of V3Ga are discussed. (C) 2015 Elsevier Ltd. All rights reserved.

Material response and failure mechanism of unidirectional metal matrix composites under impulsive shear loading

The material response and failure mechanism of unidirectional metal matrix composite under impulsive shear loading are investigated in this paper. Both experimental and analytical studies were performed. The shear strength of unidirectional C-f/A356.0 composite and A356.0 aluminum alloy at high strain rate were measured with a modified split Hopkinson torsional bar technique. The results indicated that the carbon fibers did not improve the shear strength of aluminum matrix if the fiber orientation aligned with the shear loading axis. The microscopic inspection of the fractured surface showed a multi-scale zigzag feature which implied a complicated shear failure mechanism in the composite. In addition to testing, the micromechanical stress field in the composite was analyzed by the generalized Eshelby equivalent method (GEEM). The influence of cracking in matrix on the micromechanical stress field was investigated as well. The results showed that the stress distribution in the composite is quite nonhomogeneous and very high shear stress concentrations are found in some regions in the matrix. The high shear stress concentration in the matrix induces tensile cracking at 45 degrees to the shear direction. This in turn aggravates the stress concentration at the fiber/matrix interface and finally leads to a catastrophic failure in the composite. From the correlation between the analysis and experimental results, the shear failure mechanism of unidirectional C-f/A356.0 composite can be elucidated qualitatively.

Colliding-pulse modelocked quantum dot lasers

Colliding pulse modelocking is demonstrated for the first time in quantum dot lasers. Using 3.9 mm-long devices with a 245 pm-long central absorber, 7 ps pulses at a repetition rate of 20 GHz is obtained. For Gaussian pulses a time-bandwidth product close to the Fourier transform limit is determined. These results confirm the potential of quantum dot lasers for high repetition rate harmonic modelocking.

Impact response and failure mechanisms of ultrahigh modulus polyethylene fiber composites and polyethylene fiber-carbon fiber hybrid composites

The impact response and failure mechanisms of ultrahigh modulus polyethylene (UHMPE) fiber composites and UHMPE fiber-carbon fiber hybrid composites have been investigated. Charpy impact, drop weight impact and high strain rate impact experiments have been performed in order to study the impact resistance, notch sensitivity, strain rate sensitivity and hybrid effects. Results obtained from dynamic and quasi-static measurements have been compared. Because of the ductility of UHMPE fibers, the impact energy absorption of UHMPE fiber composites is very high, thereby leading to excellent damage tolerance. By hybridizing with UHMPE fibers, the impact properties of carbon fiber composites can be greatly improved. The impact and shock failure mechanisms of these composites are discussed.

Shear Localization In Dynamic Deformation: Microstructural Evolution

Investigations made by the authors and collaborators into the microstructural aspects of adiabatic shear localization are critically reviewed. The materials analyzed are low-carbon steels, 304 stainless steel, monocrystalline Fe-Ni-Cr, Ti and its alloys, Al-Li alloys, Zircaloy, copper, and Al/SiCp composites. The principal findings are the following: (a) there is a strain-rate-dependent critical strain for the development of shear bands; (b) deformed bands and white-etching bands correspond to different stages of deformation; (c) different slip activities occur in different stages of band development; (d) grain refinement and amorphization occur in shear bands; (e) loss of stress-carrying capability is more closely associated with microdefects rather than with localization of strain; (f) both crystalline rotation and slip play important roles; and (g) band development and band structures are material dependent. Additionally, avenues for new research directions are suggested.

Low Spring Constant Regulates P-Selectin-Psgl-1 Bond Rupture

Forced dissociation of selectin-ligand bonds is crucial to such biological processes as leukocyte recruitment, thrombosis formation, and tumor metastasis. Although the bond rupture has been well known at high loading rate r(f) (>= 10(2) pN/s), defined as the product of spring constant k and retract velocity v, how the low r(f) (< 10(2) pN/s) or the low k regulates the bond dissociation remains unclear. Here an optical trap assay was used to quantify the bond rupture at r(f) <= 20 pN/s with low k (similar to 10(-3)-10(-2) pN/nm) when P-selectin and P-selectin glycoprotein ligand 1 (PSGL-1) were respectively coupled onto two glass microbeads. Our data indicated that the bond rupture force f retained the similar values when r(f) increased up to 20 pN/s. It was also found that f varied with different combinations of k and v even at the same r(f). The most probable force, f

Dynamic damage and failure in viscoplastic materials

In this paper, a dynamic damage model in ductile solids under the application of a dynamic mean tensile stress is developed. The proposed model considers void nucleation and growth as parts of the damage process under intense dynamic loading (strain rates epsilon greater than or equal to 10(3) s(-1)). The evolution equation of the ductile void has the closed form, in which work-hardening behavior, rate-dependent contribution and inertial effects are taken into account. Meanwhile, a plate impact test is performed for simulating the dynamic fracture process in LY12 aluminum alloy. The damage model is incorporated in a hydrodynamic computer code, to simulate the first few stress reverberations in the target as it spalls and postimpact porosity in the specimen. Fair agreement between computed and experimental results is obtained. Numerical analysis shows that the influence of inertial resistance on the initial void growth in the case of high loading rate can not be neglected. It is also indicated that the dynamic growth of voids is highly sensitive to the strain rates.