Fused deposition modelling using direct extrusion

In the present work, a process based on the principle of polymer extrusion is developed: the extruder deposition process (EDP). This system uses a screw extruder to deposit the material on a computer-controlled positioning system to build components. Experiments (Box-Behnken technique is used for experimental design) are carried out to study the influence of three process variables: nozzle temperature, chamber temperature and road gap on bond strength (inter-road and interlayer) and surface finish. Surface roughness and ultimate tensile strength values are measured for test specimens. Analysis of variance (ANOVA) is used to determine the significance of process variables. It is concluded that the developed EDP eliminates many of the shortcomings of the systems developed based on the principles of extrusion and produces components having higher bond strength than that achieved in commercial fused deposition modelling (FDM) systems.     

Processing of In Situ Aluminium Matrix Composites by Micropyretic Reactive Sintering

In situ aluminium matrix composites were processed by the micropyretic reactions between the host aluminium matrix and powder containing nanodimensional Fe₂O₃ crystallites used as the precursor compound. Differential Scanning Calorimetric (DSC), studies have shown, use of nanosized Fe₂O₃ crystallites has decreased the initiation temperature of the in situ micropyretic reaction and the desirable reinforcements; iron-aluminide and alumina are formed during sintering. Microstructural features of the composite consist of well-dispersed reinforcements in the host Al matrix with size less than 1 µm. Wear tests performed on the composite samples have revealed coefficient of friction and wear volume can be reduced significantly by incorporating 20 vol% reinforcement.     

Improvement of Thermal Characteristics of Refractory Castable by Addition of Gel-Route Spinel Nanoparticles

Selected physical and thermal properties of conventional spinel-alumina castables were compared with those of castables produced using spinel fines prepared from a prepared gel. Limited numbers of hydroxyl groups created around the spinel precursor helped to improve thermal shock resistance. Micrographic examination confirmed that retained nanodimensional spinels firmly connected the hibonite and corundum grains in the castable, developing multiple interfaces after densification. Castable-containing spinel with excess alumina powder was found to have the best combination of bulk density, apparent porosity, and hot modulus of rupture. The reactive magnesia fine used to prepare in situ spinel-bonded castable was not found to give satisfactory results, owing to progressive disintegration of brucite-type compounds and abnormally grown spinels. Thermal characteristics of castables were assessed by differential thermal analysis (DTA), measurement of porosity, and percent linear change (PLC). Transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS) analysis of the gel-derived spinel and castable confirmed their differences, particularly with commercial spinel having comparable chemistry but lacking nanocrystalline structure.     

Corrosion-induced bond strength degradation in reinforced concrete—Analytical and empirical models

The present paper aims to investigate the relationship between the bond strength and the reinforcement corrosion in reinforced concrete (RC). Analytical and empirical models are proposed for the bond strength of corroded reinforcing bars. Analytical model proposed by Cairns.and Abdullah [Cairns, J., Abdullah, R.B., 1996. Bond strength of black and epoxy-coated reinforcement—a theoretical approach. ACI Mater. J. 93 (4), 362–369] for splitting bond failure and later modified by Coronelli [Coronelli, D. 2002. Corrosion cracking and bond strength modeling for corroded bars in reinforced concrete. ACI Struct. J. 99 (3), 267–276] to consider the corroded bars, has been adopted. Estimation of the various parameters in the earlier analytical model has been proposed by the present authors. These parameters include corrosion pressure due to expansive action of corrosion products, modeling of tensile behaviour of cracked concrete and adhesion and friction coefficient between the corroded bar and cracked concrete. Simple empirical models are also proposed to evaluate the reduction in bond strength as a function of reinforcement corrosion in RC specimens. These empirical models are proposed by considering a wide range of published experimental investigations related to the bond degradation in RC specimens due to reinforcement corrosion. It has been found that the proposed analytical and empirical bond models are capable of providing the estimates of predicted bond strength of corroded reinforcement that are in reasonably good agreement with the experimentally observed values and with those of the other reported published data on analytical and empirical predictions. An attempt has also been made to evaluate the flexural strength of RC beams with corroded reinforcement failing in bond. It has also been found that the analytical predictions for the flexural strength of RC beams based on the proposed bond degradation models are in agreement with those of the experimentally observed and the empirically predicted values of the reported published data (Mangat, P.S., Elgarf, M.S. 1999a. Flexural strength of concrete beams with corroding reinforcement. ACI Struct. J. 96 (1), 149–158).     

Photoluminescence and field emission properties of ZnS:Mn nanoparticles synthesized by rf-magnetron sputtering technique

Nanoparticles of ZnS:Mn have been grown by radio frequency magnetron sputtering technique on glass and Si substrates at a substrate temperature 300 K. X-ray diffraction patterns and selected area electron diffraction patterns confirmed the nanocrystalline cubic ZnS phase formation. TEM micrographs of the films revealed the manifestation of ZnS:Mn nanoparticles with an average size 6 nm. UV–Vis–NIR spectrophotometric measurement showed that the films are highly transparent (90%) in the wavelength range 400–2600 nm. From the measurements of transmittance spectra of the films the direct allowed bandgap values have been calculated and they lie in the range 3.89–4.12 eV. The bandgap decreased with the increase of Mn concentration in the films. The Mn concentrations in the films have been varied from 0% to 8.9% and was measured by energy dispersive X-ray analysis. The photoluminescence of the Mn doped ZnS nanoparticles was measured. The intensity of the PL peaks at first increased with the increase of Mn concentration in the films up to 3.8% of Mn doping and at a Mn concentration higher than this, the intensity of PL peak decreased. Nanocrystalline ZnS:Mn showed good field emission property with a turn on field lying in the range 5.26–6.78 V/μm for a variation of anode to sample distance from 60 μm to 100 μm.     

Calorimetric studies of 8090 and 1441 Al–Li–Cu–Mg–Zr alloys of conventional and retrogressed and reaged tempers

The differential scanning calorimetry (DSC) technique has been used to examine the solid state reactions of GPB zones formation, precipitation of δ′, T₁, T₂, S′, δ phases and their dissolution occurring in the 8090 and 1441 Al–Li–Cu–Mg–Zr alloys of the water-quenched (WQ), peak aged T8 and T6, over aged T7, retrogressed (R), retrogressed and reaged (RRA) T77 tempers. All the exothermic and endothermic peaks in the DSC thermograms have been identified and discussed. The noticeable differences observed in the thermograms of the 8090 and 1441 alloys have been explained and this is attributed to the variation in the concentrations of the solute elements Li, Cu and Mg in the alloys. The peak temperatures as well as the heat evolved and absorbed during the precipitation and dissolution reactions have been determined with the help of the built-in software of the simultaneous thermal analyzer (STA) used for DSC studies. X-ray diffractograms and a few TEM micrographs have been illustrated to correlate the reactions in these alloys. Further, all the thermograms of the WQ state of the 1441 alloy, taken at four different heating rates, have exhibited overlapping peaks of GPB zones formation and the δ′ phase precipitation. The overlapping peaks cause restrictions in determining the kinetic parameters of activation energy and growth parameter, but, interestingly, the thermograms of retrogressed tempers have showed separate peaks of GPB zones formation and δ′ phase precipitation which will easily enable to find out kinetic parameters, by varying heating rate method.     

Homogeneous dynamical systems theory

We study controlled homogeneous dynamical systems and derive conditions under which the system is perspective controllable. We also derive conditions under which the system is observable in the presence of a control over the complex base field. In the absence of any control input, we derive a necessary and sufficient condition for observability of a homogeneous dynamical system over the real base field. The observability criterion obtained generalizes a well known Popov-Belevitch-Hautus rank criterion to check the observability of a linear dynamical system. Finally, we introduce rational, exponential, interpolation problems as an important step toward solving the problem of realizing homogeneous dynamical systems with minimum state dimensions     

A planar simple shear test and flow behavior in a superplastic Al-Mg alloy

Superplasticity is generally studied by performing tensile and gas-pressure-bulge tests. In formed parts, however, a variety of strain states, including in-plane shear, are encountered. The understanding of the mechanical response in shear is helpful in the study of superplastic metal forming. In this study, a device for a planar simple shear test was designed and used to perform tests on a superplastic Al-Mg alloy sheet at the elevated temperatures of 500 °C (773K) and 550 °C (823K). In such a test, the incremental rotation of the principal strain axes and specimen-end effects during deformation can complicate the determination of true mechanical response. The possible approximations regarding the strain state in the specimen gage have been investigated. The σ _e -ε _e curves developed based on a simple-shear assumption show a lower flow stress than that under uniaxial tension, and strain hardening is related to dynamic grain growth. The rate of strain hardening at a fixed _e level is essentially the same for both uniaxial tension and shear, but the difference in the effective stress between uniaxial tension and shear depends upon strain rate and temperature. This study marks the first known attempt to characterize large strain response for superplastic metals under conditions of simple shear.     

Anisotropic Superconducting Properties of MgB2 and Related Compounds

Anisotropic superconducting properties of inter-metallic compounds MgB₂, Y₂PdGe₃, and CaAlSi with AlB₂ structure are studied by detailed angular dependent transport measurements. MgB₂ and CaAlSi shows appreciable anisotropy of the upper critical field, H _c2, with mass anisotropy parameter =3 and 2, respectively, while Y₂PdGe₃ is almost isotropic. In-plane anisotropy of H _c2 in these hexagonal superconductors is very small, consistent with the prediction based on GL theory.     

Impact of β-Sheet Conformations on the Mechanical Response of Protein in Biocomposites

Proteins in biological nanocomposites play an important role in their mechanical response. Proteins in nacre, the inner layer of seashells, have been shown to have exceptional mechanical properties. One of the important nacre proteins, Lustrin-A, has abundance of polypeptides in zig-zag conformation called β-sheets. β-sheets of protein when present close to each other in multiple numbers could take the shape of a planar β-sheath like structure or a β-barrel to form a domain. In natural proteins both these types of structures are commonly found. However, the conformation of β-sheets in Lustrin-A is not known at this time. Effort has been made through this work to study the mechanical response of these β-planar sheath and β-barrel structures when subjected to external loads. Comparative study of the stress-deformation characteristics of these two types of structures has been made. Both these structures with almost similar number of amino acids have been extracted from one single spinach protein: Ferredoxin Reductase (1FNR). Steered molecular dynamics has been used to conduct these studies. The article deals with the separation of the two domains from the main protein, simulation details, and results comparing the responses.