Degree of cure-dependent modelling for polymer curing processes at small-strain. Part I: consistent reformulation

A physically-based small strain curing model has been developed and discussed in our previous contribution (Hossain et al. in Comput Mech 43:769–779, 2009a) which was extended later for finite strain elasticity and viscoelasticity including shrinkage in Hossain et al. (Comput Mech 44(5):621–630, 2009b) and in Hossain et al. (Comput Mech 46(3):363–375, 2010), respectively. The previously proposed constitutive models for curing processes are based on the temporal evolution of the material parameters, namely the shear modulus and the relaxation time (in the case of viscoelasticity). In the current paper, a thermodynamically consistent small strain constitutive model is formulated that is directly based on the degree of cure, a key parameter in the curing (reaction) kinetics. The new formulation is also in line with the earlier proposed hypoelastic approach. The curing process of polymers is a complex phenomenon involving a series of chemical reactions which transform a viscoelastic fluid into a viscoelastic solid during which the temperature, the chemistry and the mechanics are coupled. Part I of this work will deal with an isothermal viscoelastic formulation including shrinkage effects whereas the following Part II will give emphasis on the thermomechanical coupled approach. Some representative numerical examples conclude the paper and show the capability of the newly proposed constitutive formulation to capture major phenomena observed during the curing processes of polymers.     

Peptide Amphiphilic Supramolecular Nanogels: Competent Host for Notably Efficient Lipase-Catalyzed Hydrolysis of Water-Insoluble Substrates

The present work depicts the development of stable nanogels in an aqueous medium that were exploited for efficient surface-active lipase-catalyzed hydrolysis of water-insoluble substrates. Surfactant-coated gel nanoparticles (neutral NG1 , anionic NG2 , and cationic NG3 ) were prepared from peptide amphiphilic hydrogelator ( G1 , G2 , and G3 , respectively) at different hydrophilic and lipophilic balance (HLB). Chromobacterium viscosum (CV) lipase activity towards hydrolysis of water-insoluble substrates (p-nitrophyenyl-n-alkanoates (C4–C10)) in the presence of nanogels got remarkably improved by ~1.7–8.0 fold in comparison to that in aqueous buffer and other self-aggregates. An increase in hydrophobicity of the substrate led to a notable improvement in lipase activity in the hydrophilic domain (HLB>8.0) of nanogels. The micro-heterogeneous interface of small-sized (10–65 nm) nanogel was found to be an appropriate scaffold for immobilizing surface-active lipase to exhibit superior catalytic efficiency. Concurrently, the flexible conformation of lipase immobilized in nanogels was reflected in its secondary structure having the highest α-helix content from the circular dichroism spectra.     

Call-Level and Packet-Level Quality of Service and User Utility in Rate-Adaptive Cellular CDMA Networks: A Queuing Analysis

A queuing analytical model is presented to evaluate call-level and packet-level quality of service (QoS) metrics in the uplink of a voice/data cellular code division multiple access (CDMA) network. In this model, a threshold-based call admission control (CAC) is used to limit the number of admitted calls in a cell and also to prioritize handoff calls over new calls. The transmission rates for data calls can be adjusted to accommodate more voice and/or data calls while satisfying the minimum signal-to-interference ratio (SIR)/ transmission rate requirement. Also, automatic repeat request (ARQ)-based error control is used for improved reliability of data packets. Call-level performance measures for both voice and data calls and packet-level performance measures specifically for data calls can be obtained from the analytical model. The interdependencies among call-level and packet-level QoS metrics are investigated under different CAC, rate adaptation, and error control parameter settings. To this end, the level of users' satisfaction (or user utility) is formulated as a function of the QoS metrics and an optimization formulation is presented to obtain the local-optimal system parameters     

A cascaded deep learning framework for iris centre localization in facial image

Accurate iris centre localization is crucial in many computer vision and facial biometric applications such as gaze estimation, human–computer interaction, iris recognition, and liveness detection. However, it is challenging in an uncontrolled environment due to variations like pose, scale, rotation, specular reflection, and image quality. Therefore, a cascaded deep learning framework for iris centre localization in facial images is proposed that is robust to the abovementioned variations. The proposed approach consists of (i) YOLOv3 for eye detection, (ii) UNet for iris segmentation, and (iii) statistical modelling for iris centre localization. The eyes are first detected using the YOLOv3, and subsequently, iris segmentation is performed within the detected eyes using the UNet. Following iris segmentation, statistical modelling is employed to enhance the localization accuracy of the iris centre. Experiments were performed on benchmark databases, resulting in a standardized error measure S_ED of 3.405 pixels for BioID and 3.259 pixels for GI4E databases. In addition, the robustness of the proposed eye detection model was further evaluated on the Yale B for illumination variations and the CAS-PEAL for pose variations.     

Optical and electrical properties of SnO2 films prepared by chemical vapour deposition

Transparent and conducting SnO₂ films are prepared at 500°C on quartz substrates by chemical vapour deposition technique, involving oxidation of SnCl₂. The effect of oxygen gas flow rate on the properties of SnO₂ films is reported. Oxygen with a flow rate from 0·8–1·35 lmin⁻¹ was used as both carrier and oxidizing gas. Electrical and optical properties are studied for 150 nm thick films. The films obtained have a resistivity between 1·72 × 10⁻³ and 4·95 × 10⁻³ ohm cm and the average transmission in the visible region ranges 86–90%. The performance of these films was checked and the maximum figure of merit value of 2·03 × 10⁻³ ohm⁻¹ was obtained with the films deposited at the flow rate of 1·16 lmin⁻¹.     

Triaxial extension and tension tests on lime-cement-improved clay

This paper presents the results of a series of undrained and drained isotropic consolidated triaxial extension, tension and compression laboratory tests on lime-cement-improved very soft clay. The main objective of these tests was to investigate the material strength and stiffness properties for stress conditions similar to those expected on the passive side of excavations where a retaining structure is supported by Deep Mixing columns. The different stress paths to failure were obtained by varying the directions of the major and minor principal stresses in a conventional triaxial test cell. The undrained tests conducted at low consolidation stresses, corresponding to depths of approximately 0–10 m below the ground surface, revealed significant differences in undrained strength depending on the directions of the major and minor principal stresses, indicating anisotropic material behavior. Based on the undrained triaxial test results, the relationship among the undrained strength, the effective consolidation stress and the over-consolidation ratio (OCR) is presented for different stress paths to failure.The experimental data from the drained tests show that a failure surface comprised of a shear failure function based on the Mohr-Coulomb failure criterion and a tensile failure function based on the tensile strength and the confining stress can be applied for lime-cement-stabilized clay.     

THIN-LAYER DRYING CHARACTERISTICS FOR GREEN CHILLI

Thin-layer drying experiments under overflow-underflow and through flow conditions of green chilli were conducted with air temperature ranging from 40 to 65°C, relative humidity ranging from 10 to 60% and air velocity ranging from 0.10 to 1.0 m/s. The single exponential equation and the Page equation were used to determine the thin-layer drying characteristics for green chilli. Both the equations fitted well to the experimental data. The Page equation was found to describe the thin-layer drying of chilli better than the single exponential equation. The parameters of the Page equation and the single exponential equation were expressed as a function of drying air temperature, relative humidity and air velocity.     

Effect of SiC content,additives and process parameters on densification and structure–property relations of pressureless sintered ZrB2–SiC composites

The effect of SiC content, additives, and process parameters on densification and structure–property relations of pressureless sintered ZrB₂–(10–40 vol%) SiC particulate composites have been studied. The ZrB₂–SiC composite powders mixed by ball-milling with 1.2 wt% C (added as phenolic resin) and 3 wt% B₄C have been uniaxially cold-compacted and sintered in argon environment at 1950–2050 °C for 2 h, or at 2000 °C for durations between 1/2 and 3 h. The amount of densification is found to increase with sintering duration, and by prior holding at 1250 and 1600 °C for reduction of oxide impurities (ZrO₂, B₂O₃ and SiO₂) on powder particle surfaces by the aforementioned additives. Presence of SiC with average size smaller than that of ZrB₂ appears to aid in densification by enhancing green density, increasing WC content by erosion of milling media, and inhibiting matrix grain growth. Both SiC and WC appear to aid in reduction of oxide impurities. Furthermore, the impurities enriched in W, Fe and Co obtained from milling media are found to be segregated at ZrB₂ grain boundaries, and appear to assist in densification by forming liquid phase, which completely wets the ZrB₂ grains. Hardness increases with SiC content or with sintering duration till 1 h, but decreases for periods ≥2 h due to grain growth. The experimentally measured elastic moduli approaches corresponding theoretically predicted values with increasing SiC content due to reduction in porosity.     

Solvent Extraction Separation of Trivalent Americium from Curium and the Lanthanides

The sterically constrained, macrocyclic, aqueous soluble ligand N,N′-bis[(6-carboxy-2-pyridyl)methyl]-1,10-diaza-18-crown-6 (H₂BP18C6) was investigated for separating americium from curium and all the lanthanides by solvent extraction. Pairing H₂BP18C6, which favors complexation of larger f-element cations, with acidic organophosphorus extractants that favor extraction of smaller f-element cations, such as bis-(2-ethylhexyl)phosphoric acid (HDEHP) or (2-ethylhexyl)phosphonic acid mono(2-ethylhexyl) ester (HEH[EHP]), created solvent extraction systems with good Cm/Am selectivity, excellent trans-lanthanide selectivity (K_ex,Lu/K_ex,La = 10⁸), but poor selectivity for Am against the lightest lanthanides. However, using an organic phase containing both a neutral extractant, N,N,N’,N’-tetra(2-ethylhexyl)diglycolamide (TEHDGA), and HEH[EHP] enabled rejection of the lightest lanthanides during loading of the organic phase from aqueous nitric acid, eliminating their interference in the americium stripping stages. In addition, although it is a macrocyclic ligand, H₂BP18C6 does not significantly impede the mass transfer kinetics of the HDEHP solvent extraction system.     

Mixed convection flow of non‐Newtonian fluid from a slotted vertical surface with uniform surface heat flux

In the present paper, the combined convection flow of an Ostwald–de Waele type power‐law non‐Newtonian fluid past a vertical slotted surface has been investigated numerically. The boundary condition of uniform surface heat flux is considered. The equations governing the flow and the heat transfer are reduced to local non‐similarity form. The transformed boundary layer equations are solved numerically using implicit finite difference method. Solutions for the heat transfer rate obtained for the rigid surface compare well with those documented in the published literature. From the present analysis, it is observed that, an increase in χ leads to increase in skin friction as well as reduction in heat transfer at the surface. As the power‐law index n increases, the friction factor as well as heat transfer increase.