Prediction of the twisting moment and axial force in a circular rubber cylinder for combined extension and torsion based on the logarithmic strain approach

The scope of the present work is the application of a particular class of strain energy function, based on the logarithmic strain, for the prediction of the twisting moment and axial force of a rubber circular cylinder under combined extension and torsion. The strain energy function involves four material parameters three of which are determined by fitting published experimental data from simple tensile and compression tests of natural rubber. One of the parameters of the proposed model has physical meaning, and it is equal to one ninth of the initial modulus of elasticity of the material. Hence, the number of unknown parameters is reduced to three. The logarithmic strain energy function is then applied to a combined extension and torsion problem of a rubber circular cylinder to check its performance for more complicated deformations. The results are compared with corresponding experimental and theoretical solutions available in the literature to validate the proposed model. It is found that the proposed strain energy function apart from predicting the common modes of deformations is also capable to determine more complicated types of deformation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Size-dependent non-linear mechanical properties of graphene nanoribbons

An atomistic, spring-based, non-linear finite element method is implemented in order to predict the non-linear mechanical behavior of graphene nanoribbons. According this method, appropriate non-linear springs are utilized to simulate each interatomic interaction. Their force–displacement curve follows the relation between the first differentiation of the potential energy of the corresponding interaction-bond deformation. The potential which corresponds to the bond angle variation is simulated by a torsional spring, while the bond stretching is simulated by a uniaxial compression/extension spring. The linear approximation, commonly made in the literature for the bond angle bending interaction, is not followed here and thus the overall non-linear response of the specific interaction is accurately introduced into the model. Following the proposed formulation, the tensile uniaxial stress–strain behavior for various graphene nanoribbons, of zigzag as well as armchair orientation, arise. The results demonstrate that the linear and non-linear mechanical properties are strongly dependent on the structure as well as on the size of the graphene strip tested.     

An extension of the penalty function formulation to incompressible hyperelastic solids described by general measure of strain

The penalty function formulation for incompressible hyperelastic solids was first proposed about 30 years ago. Since then all studies have been limited to invariant type formulation of the strain energy function, although it is well known that this formulation does not correctly describe the behavior of a real material. On the other hand more realistic constitutive equations, based on general measures of the strain only, have been incorporated to mixed finite element algorithms. In this article, a penalty function formulation is proposed for the analysis of stress field in materials with constitutive equations based on the general measure of strain. The reduced integration method is used to weaken the penalty constraint in order to obtain meaningful numerical results. The incremental equilibrium equations are solved using the regular Newton–Raphson algorithm. The method is applied to evaluate the stress field in materials subjected to plane strain conditions. Satisfactory agreements have been obtained with analytical solutions when available. © 1995 John Wiley & Sons, Inc.     

Experimental quadrotor flight performance using computationally efficient and recursively feasible linear model predictive control

A new linear model predictive control (MPC) algorithm in a state-space framework is presented based on the fusion of two past MPC control laws: steady-state optimal MPC (SSOMPC) and Laguerre optimal MPC (LOMPC). The new controller, SSLOMPC, is demonstrated to have improved feasibility, tracking performance and computation time than its predecessors. This is verified in both simulation and practical experimentation on a quadrotor unmanned air vehicle in an indoor motion-capture testbed. The performance of the control law is experimentally compared with proportional-integral-derivative (PID) and linear quadratic regulator (LQR) controllers in an unconstrained square manoeuvre. The use of soft control output and hard control input constraints is also examined in single and dual constrained manoeuvres.     

Study of the solubility,antioxidant activity and structure of inclusion complex of vanillin with β-cyclodextrin

In the present study, complexes of vanillin with β-cyclodextrin (β-CD) were prepared by the freeze-drying method. The formation of the inclusion complex was confirmed by differential scanning calorimetry (DSC). DSC thermograms indicated that the endothermic peak of vanillin and the physical mixture of vanillin with β-CD, due to the melting of vanillin crystals, were absent in DSC thermograms obtained for the freeze-dried inclusion complex. Moreover, the DSC studies under oxidation conditions indicate that the complex of vanillin with β-CD is protected towards oxidation since it remains intact at temperatures where the free vanillin is oxidising. The structure of the complex in aqueous solutions was established by nuclear magnetic resonance (NMR) studies and specifically by two-dimensional rotational frame NOE spectra. NMR studies showed inclusion of the entire vanillin molecule in the β-CD in a tilted manner and with the aldehyde group in the primary side. The stoichiometry of the complex was 1:1. A phase solubility study was performed by mixing an excess amount of vanillin with aqueous solutions containing increasing amounts of β-CD. The results indicated that the complex of a vanillin/β-CD inclusion is more soluble in water than vanillin alone.     

On the performance of two-hop message spreading in DTNs

In this paper, a Delay Tolerant Network environment is considered where the source is in full control of the two-hop spreading mechanism by setting key parameters such as the number of copies allowed to be spread in the network and the delay bound of the messages. The introduced analysis allows for a differentiation between the source of the message and the intermediate nodes (in terms of e.g. transmission power, speed or cooperation degree). Analytical expressions for the cumulative distribution function (cdf) of the delivery delay and the induced overhead are extracted, taking into account the fact that the source node may continue spreading copies after the message delivery. In addition, a fairly accurate approximate expression for the cdf of the delivery delay is also derived and validated through simulations.     

COLLAPSE OF STRUCTURE DURING DRYING OF CELERY

Collapse of structure of foodstuffs during air drying affects quality. In many materials the soluble components, mainly sugars, are an important part of the tissue in which case collapse may be related to their glass transition temperature (T_g). It has been speculated that collapse occurs at a temperature (T_c) related to, but greater than, T_g. Plant tissues with high moisture contents, such as celery, have low T_gS. Therefore considerable collapse is expected at drying temperatures.

The aim of this study was to determine how air drying temperature affected the quantity characteristics of the tissue. Celery, air dried at temperatures between 5 and 80°C, was examined for volumetric shrinkage, rehydration characteristics and porosity changes. significant shrinkage occurred at all drying conditions. At low water content collapse was limited, probably due to a higher collapse temperature. porosity development was insignificant during drying until the sample was very dry. Lower air-drying temperatures gave a product with improved quality characteristics.