Efficient modular exponentiation-based puzzles for denial-of-service protection

Client puzzles are moderately-hard cryptographic problems neither easy nor impossible to solve that can be used as a counter-measure against denial of service attacks on network protocols. Puzzles based on modular exponentiation are attractive as they provide important properties such as non-parallelisability, deterministic solving time, and linear granularity. We propose an efficient client puzzle based on modular exponentiation. Our puzzle requires only a few modular multiplications for puzzle generation and verification. For a server under denial of service attack, this is a significant improvement as the best known non-parallelisable puzzle proposed by Karame and Capkun (ESORICS 2010) requires at least 2k-bit modular exponentiation, where k is a security parameter. We show that our puzzle satisfies the unforgeability and difficulty properties defined by Chen et al. (Asiacrypt 2009). We present experimental results which show that, for 1024-bit moduli, our proposed puzzle can be up to 30 times faster to verify than the Karame-Capkun puzzle and 99 times faster than the Rivest et al.'s time-lock puzzle.

Dynamic compression improves biosynthesis of human zonal chondrocytes from osteoarthritis patients

Objective: We hypothesize that chondrocytes from distinct zones of articular cartilage respond differently to compressive loading, and that zonal chondrocytes from osteoarthritis (OA) patients can benefit from optimized compressive stimulation. Therefore, we aimed to determine the transcriptional response of superficial (S) and middle/deep (MD) zone chondrocytes to varying dynamic compressive strain and loading duration. To confirm effects of compressive stimulation on overall matrix production, we subjected zonal chondrocytes to compression for 2 weeks. Design: Human S and MD chondrocytes from osteoarthritic joints were encapsulated in 2% alginate, pre-cultured, and subjected to compression with varying dynamic strain (5, 15, 50% at 1 Hz) and loading duration (1, 3, 12 h). Temporal changes in cartilage-specific, zonal, and dedifferentiation genes following compression were evaluated using quantitative real-time reverse transcriptase polymerase chain reaction (qRT-PCR). The benefits of long-term compression (50% strain, 3 h/day, for 2 weeks) were assessed by measuring construct glycosaminoglycan (GAG) content and compressive moduli, as well as immunostaining. Results: Compressive stimulation significantly induced aggrecan (ACAN), COL2A1, COL1A1, proteoglycan 4 (PRG4), and COL10A1 gene expression after 2 h of unloading, in a zone-dependent manner (P < 0.05). ACAN and PRG4 mRNA levels depended on strain and load duration, with 50% and 3 h loading resulting in highest levels (P < 0.05). Long-term compression increased collagen type II and ACAN immunostaining and total GAG (P < 0.05), but only S constructs showed more PRG4 stain, retained more GAG (P < 0.01), and developed higher compressive moduli than non-loaded controls. Conclusions: The biosynthetic activity of zonal chondrocytes from osteoarthritis joints can be enhanced with selected compression regimes, indicating the potential for cartilage tissue engineering applications. © 2012 Osteoarthritis Research Society International.

Electrospinning and crosslinking of low-molecular-weight poly(trimethylene carbonate-co-L-lactide) as an elastomeric scaffold for vascular engineering

The growth of suitable tissue to replace natural blood vessels requires a degradable scaffold material that is processable into porous structures with appropriate mechanical and cell growth properties. This study investigates the fabrication of degradable, crosslinkable prepolymers of l-lactide-co-trimethylene carbonate into porous scaffolds by electrospinning. After crosslinking by γ-radiation, dimensionally stable scaffolds were obtained with up to 56% trimethylene carbonate incorporation. The fibrous mats showed Young’s moduli closely matching human arteries (0.4–0.8 MPa). Repeated cyclic extension yielded negligible change in mechanical properties, demonstrating the potential for use under dynamic physiological conditions. The scaffolds remained elastic and resilient at 30% strain after 84 days of degradation in phosphate buffer, while the modulus and ultimate stress and strain progressively decreased. The electrospun mats are mechanically superior to solid films of the same materials. In vitro, human mesenchymal stem cells adhered to and readily proliferated on the three-dimensional fiber network, demonstrating that these polymers may find use in growing artificial blood vessels in vivo.

Strength of stiffened plates with openings

The load-deflection and ultimate strength behaviour of longitudinally stiffened plates with openings was studied using a second-order elastic post-buckling analysis and a rigid-plastic analysis. The ultimate strength was predicted from the intersection point of elastic and rigid-plastic curves and the Perry strut formula. Comparison with experimental results shows that satisfactory prediction of ultimate strength can be obtained by this simple method. Effects of the size of opening, the initial geometrical imperfections and the plate slenderness ratio on the strength of perforated stiffened plates were also studied.

The modulus of elasticity of steel - is it 200 GPa?

In cold-formed steel construction, the use of a range of thin, high strength steels (0.35 mm thickness and 550 MPa yield stress) has increased significantly in recent times. A good knowledge of the basic mechanical properties of these steels is needed for a satisfactory use of them. In relation to the modulus of elasticity, the current practice is to assume it to be about 200 GPa for all steel grades. However, tensile tests of these steels have consistently shown that the modulus of elasticity varies with grade of steel and thickness. It was found that it increases to values as high as 240 GPa for smaller thicknesses and higher grades of steel. This paper discusses this topic, presents the tensile test results for a number of steel grades and thicknesses, and attempts to develop a relationship between modulus of elasticity, yield stress and thickness for the steel grades considered in this investigation.

The impact of spatial scales and spatial smoothing on the outcome of Bayesian spatial model

Discretization of a geographical region is quite common in spatial analysis. There have been few studies into the impact of different geographical scales on the outcome of spatial models for different spatial patterns. This study aims to investigate the impact of spatial scales and spatial smoothing on the outcomes of modelling spatial point-based data. Given a spatial point-based dataset (such as occurrence of a disease), we study the geographical variation of residual disease risk using regular grid cells. The individual disease risk is modelled using a logistic model with the inclusion of spatially unstructured and/or spatially structured random effects. Three spatial smoothness priors for the spatially structured component are employed in modelling, namely an intrinsic Gaussian Markov random field, a second-order random walk on a lattice, and a Gaussian field with Matern correlation function. We investigate how changes in grid cell size affect model outcomes under different spatial structures and different smoothness priors for the spatial component. A realistic example (the Humberside data) is analyzed and a simulation study is described. Bayesian computation is carried out using an integrated nested Laplace approximation. The results suggest that the performance and predictive capacity of the spatial models improve as the grid cell size decreases for certain spatial structures. It also appears that different spatial smoothness priors should be applied for different patterns of point data.

Design, characterisation and in vivo testing of a new, adjustable stiffness, external fixator for the rat femur

Very little is known about the infl uence of the mechanical environment on the healing of large segmental defects. This partly reflects the lack of standardised, well characterised technologies to enable such studies. Here we report the design, construction and characterisation of a novel external fixator for use in conjunction with rat femoral defects. This device not only imposes a predetermined axial stiffness on the lesion, but also enables the stiffness to be changed during the healing process. The main frame of the fi xator consists of polyethylethylketone with titanium alloy mounting pins. The stiffness of the fi xator is determined by interchangeable connection elements of different thicknesses. Fixators were shown to stabilise 5 mm femoral defects in rats in vivo for at least 8 weeks during unrestricted cage activity. No distortion or infections, including pin infections, were noted. The healing process was simulated in vitro by inserting into a 5 mm femoral defect, materials whose Young’s moduli approximated those of the different tissues present in regenerating bone. These studies confirmed that, although the external fixator is the major determinant of axial stiffness during the early phase of healing, the regenerate within the lesion subsequently dominates this property. There is much clinical interest in altering the mechanics of the defect to enhance bone healing. Our data suggest that, if alteration of the mechanical environment is to be used to modulate the healing of large segmental defects, this needs to be performed before the tissue properties become dominant.

Preparation and characterization of composites from starch with sugarcane bagasse nanofibres

This paper reports on the results of using unbleached sugar cane bagasse nanofibres (average diameter 26.5 nm; aspect ratio 247 assuming a dry fibre density of 1,500 kg/m3) to improve the physico-chemical properties of starch-based films. The addition of bagasse nanofibres (2.5 to 20 wt%) to modified potato starch (i.e. soluble starch) reduced the moisture uptake by up to 17 % at 58 % relative humidity. The film’s tensile strength and Young’s modulus increased by up to 100 % (3.1 to 6.2 MPa) and 300 % (66.3 to 198.3 MPa) respectively with 10 and 20 wt% fibre addition. However, the strain at yield dropped by 50 % for the film containing 10 wt% fibre. Models for composite materials were used to account for the strong interactions between the nanofibres and the starch matrix. The storage and loss moduli as well as the glass transition temperature (Tg) obtained from dynamic mechanical thermal analysis, were increased with the starch-nanofibre films indicating decreased starch chain mobility due to the interacting effect of the nanofibres. Evidence of the existence of strong interactions between the starch matrix and the nanofibres was revealed from detailed Fourier transform infra-red and scanning electron microscopic evaluation.

Characterisation of thin layer polymer cement mortared concrete masonry bond

Masonry bond is affected by many parameters such as the type of mortar used, the techniques of dispersion of mortar and the surface texture of the concrete blocks. Additionally it is understood from the studies on conventional masonry that the bond characteristics are also influenced by the curing methods as well as the age of the bond at the time of testing. These effects on thin layer mortared masonry employing polymer cement mortars are not well understood. Therefore, the effect of curing methods and age to the bond strength and deformation of masonry containing thin layered polymer cement mortar was investigated as part of an ongoing research program at the Queensland University of Technology. This paper presents an experimental investigation of the flexural and shear bond characteristics of the thin layer mortared concrete masonry. The parameters examined include the effects curing and ageing to the bond development over a period from 14 days to 56 days after fabrication. The results exhibit that dry cured thin layer mortared masonry exhibits higher bond strength and Young’s and shear moduli compared to the wet cured specimens.

Climbing skill and complexity of climbing wall design: Assessment of jerk as a novel indicator of performance fluency

This study investigated a new performance indicator to assess climbing fluency (smoothness of the hip trajectory and orientation of a climber using normalized jerk coefficients) to explore effects of practice and hold design on performance. Eight experienced climbers completed four repetitions of two, 10-m high routes with similar difficulty levels, but varying in hold graspability (holds with one edge vs holds with two edges). An inertial measurement unit was attached to the hips of each climber to collect 3D acceleration and 3D orientation data to compute jerk coefficients. Results showed high correlations (r = .99, P < .05) between the normalized jerk coefficient of hip trajectory and orientation. Results showed higher normalized jerk coefficients for the route with two graspable edges, perhaps due to more complex route finding and action regulation behaviors. This effect decreased with practice. Jerk coefficient of hip trajectory and orientation could be a useful indicator of climbing fluency for coaches as its computation takes into account both spatial and temporal parameters (ie, changes in both climbing trajectory and time to travel this trajectory)

Elevated temperature mechanical properties of hollow flange channel sections

The fire performance of cold-formed steel members is an important criterion to be verified for their successful use in structural applications. However, lack of clear design guidance on their fire performance has inhibited their usage in buildings. Their elevated temperature mechanical properties, i.e., yield strengths, elastic moduli and stress–strain relationships, are imperative for the fire design. In the past many researchers have proposed elevated temperature mechanical property reduction factors for cold-formed steels, however, large variations exist among them. The LiteSteel Beam (LSB), a hollow flange channel section, is manufactured by a combined cold-forming and electric resistance welding process. Its web, inner and outer flange elements have different yield strengths due to varying levels of cold-working caused by their manufacturing process. Elevated temperature mechanical properties of LSBs are not the same even within their cross-sections. Therefore an experimental study was undertaken to determine the elevated temperature mechanical properties of steel plate elements in LSBs. Elevated temperature tensile tests were performed on web, inner and outer flange specimens taken from LSBs, and their results are presented in this paper including their comparisons with previous studies. Based on the test results and the proposed values from previous studies and fire design standards, suitable predictive equations are proposed for the determination of elevated temperature mechanical properties of LSB web and flange elements. Suitable stress–strain models are also proposed for the plate elements of this cold-formed and welded hollow flange channel section.

Elastic Properties Of Sodium Borovanadate Glasses

The elastic properties of sodium borovanadate glasses have been studied over a wide range of composition using ultrasonic measurements. It is found that variation of different elastic moduli is very similar in any given series of composition. The bulk and shear moduli show a monotonic variation with the covalent bond energy densities calculated from the proposed structural model for these glasses. The bulk moduli also vary as a negative power function of the mean atomic volume. The Debye temperature varies linearly with the glass transition temperature. The implications of the observed behavior have been discussed.

Multiscale modeling of lamellar mesophases

The mesoscale simulation of a lamellar mesophase based on a free energy functional is examined with the objective of determining the relationship between the parameters in the model and molecular parameters. Attention is restricted to a symmetric lamellar phase with equal volumes of hydrophilic and hydrophobic components. Apart from the lamellar spacing, there are two parameters in the free energy functional. One of the parameters, r, determines the sharpness of the interface, and it is shown how this parameter can be obtained from the interface profile in a molecular simulation. The other parameter, A, provides an energy scale. Analytical expressions are derived to relate these parameters to r and A to the bending and compression moduli and the permeation constant in the macroscopic equation to the Onsager coefficient in the concentration diffusion equation. The linear hydrodynamic response predicted by the theory is verified by carrying out a mesoscale simulation using the lattice-Boltzmann technique and verifying that the analytical predictions are in agreement with simulation results. A macroscale model based on the layer thickness field and the layer normal field is proposed, and the relationship between the parameters in the macroscale model from the parameters in the mesoscale free energy functional is obtained.

Influence of surface stress on elastic constants of nanohoneycombs

Surface effect on the four independent elastic constants of nanohoneycombs is investigated in this paper. The axial deformation of the horizontal cell wall is included, comparing to the Gibson's method, and the contributions of the two components of surface stress (i.e. surface residual stress and surface elasticity) are discussed. The result shows that the regular hexagonal honeycomb is not isotropic but orthotropic. An increase in the cell-wall thickness t leads to an increase in the discrepancy of the Young's moduli in both directions. Furthermore, the surface residual stress dominates the surface effect on the elastic constants when t < 15 nm (or the relative density <0.17), which is in contrast to that the surface elasticity does when t > 15 nm (or the relative density > 0.17) for metal Al. The present structure and theory may be useful in the design of future nanodevices.

Analysis Of Curved Laminated Beams Of Bimodulus Composite-Materials

Composite materials exhibiting different moduli in tension and in compression, commonly called as bimodular composites are being used in many engineering fields. A finite element analysis is carried out for small deflection static behavior of laminated curved beams of bi modulus materials for both solid and hollow circular cross-sections using an iterative procedure. The finite element has 16 d.o.f. and uses the displacement field in terms of first order Hermite in terpolation polynomials. The neutral surface, i.e. the locus of points having zero axial strain is found to vary drastically depending on the loading, lay up schemes and radius of curvature. As il lustrations, plots of the cross-sections of the ruled neutral-surface are presented for some of the investigated cases. Using this element a few problems of curved laminated beams of bimodulus materials are solved for both solid and hollow circular cross-sections.