Sharif Digital Repository

A series of herbal extract incorporated into poly(lactic acid) (PLA) composite nanofibrous scaffolds were successfully prepared by using electrospinning technique. Equisetum arvense extract (EE) and nanohydroxyapatite (nHA) in different quantities were loaded into PLA solution to fabricate composite nanofibrous webs under various electrospinning conditions. Uniform nanofibers were obtained with an average diameter of 157 ± 47 nm in the case of those containing the herbal extract. Characterization of the webs was carried out by means of Fourier transform infrared (FTIR) spectroscopy, field emission-scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), and... 

Phase field theory for fracture is developed at large strains with an emphasis on a correct introduction of surface stresses at nanoscale. This is achieved by multiplying the cohesion and gradient energies by the local ratio of the crack surface areas in the deformed and undeformed configurations and with the gradient energy in terms of the gradient of the order parameter in the reference configuration. This results in an expression for the surface stresses which is consistent with the sharp surface approach. Namely, the structural part of the Cauchy surface stress represents an isotropic biaxial tension, with the magnitude of a force per unit length equal to the surface energy. The surface... 

Nanocomposites of poly(ethylene terephthalate) and two different montmorillonite-based organoclays were prepared by a co-rotating twin screw extruder. Dispersion of nanoclays in the polymer matrix was examined by TEM and XRD. Nanocomposites with lower content of organoclay showed exfoliated morphology while by increasing the amount of organoclay the intercalated morphology was more prevalent. Both organoclays had a good intercalation with PET and were uniformly dispersed within the polymer. Oxygen permeability of thin films of nanocomposites showed that the nanocompo-sites had better oxygen barrier properties than the neat PET. Tensile and impact properties of the nanocomposites also were... 

The present study focuses on simultaneous influence of graphene nanoplatelets (GNP) and hydroxyapatite (HAp) nanopowder on microstructural, wear, tensile and biofunctional behavior of UHMWPE based nanocomposites used in biomedical applications, with the aim to utilize GNP's mechanical strength and wear resistance, while benefitting from HAp's biocompatibility at the same time. 0.1, 0.5 and 1 wt% GNP with 10 wt% optimized concentration of HAp were added to the UHMWPE matrix through an easy two-step approach consisting of solvent mixing and ultrasonication in ethanol as a liquid media. The dried nanocomposite samples of powder were then hot pressed at an optimized temperature and pressure to... 

Nowadays, environmental hazards caused by plastic wastes are a major concern in academia and industry. Utilization of biodegradable polymers derived from renewable sources for replacing common petroleum-based plastics is a potential solution for reducing the problem. In this regard, starch has become one of the most promising alternatives to non-biodegradable polymers for depleting plastic waste thanks to its low expense, abundance, renewability and biodegradability. However, the main drawbacks of starch are its poor processability, weak mechanical properties and severe hydrophilicity. In this work, thermoplastic starch (TPS) samples have been prepared using glycerol and sorbitol as... 

In this work, the effect of hot deformation temperature on microstructure and mechanical properties of micro alloyed steel was studied. The results indicated that by decreasing the deformation temperature final microstructure is refined and the volume fraction of grain boundary ferrite is increased and some pearlite is produced. Therefore both the yield strength and ultimate tensile strength is increased, while the toughness is preserved in comparison to a ferritic-pearlitic microstructure. Also a model was developed to relate the deformation condition to the volume fraction of acicular ferrite at mixed microstructure. © 2008 Elsevier Ltd. All rights reserved  

The nonlinear equation of motion for pre-stretched EulerBernoulli beams is derived. The effect of pre-tension and pre-compression in EulerBernoulli beams is studied. It is shown that compressive strain affects the bending stiffness much more than tensile strain. Based on the derived equation, the dynamic model of bimorph piezo-actuated beams, which is accurate, yet simple, is then developed. Afterwards, the critical voltage, which makes the piezo-actuated microbeam unstable, is numerically investigated. It is shown that the strain of the centerline is proportional to the beam's aspect ratio squared. Results show that the more the aspect ratio, the less the critical voltage. In addition, it... 

Highly cold worked (HCW) low carbon steel sheets with cellular structure in the range of 200 to 300 nm are fabricated via constrained groove pressing process. Joining of the sheets is carried out by resistance spot welding process at different welding currents and times. Thereafter, failure behavior of these welded samples during tensile-shear test is investigated. Considered concepts include; failure load, fusion zone size, failure mode, ultimate shear stress, failure absorbed energy, and fracture surface. The results show that HCW low carbon steel spot welds have higher failure peak load with respect to the as-received one at different welding currents and times. Also, current limits for... 

Nanofibrous structures that mimic the native extracellular matrix and promote cell adhesion have attracted considerable interest for biomedical applications. In this study, GO-modified nanofibrous biopolymers (GO) were prepared by electrospinning blended solutions of chitosan (80 vol%), polyvinyl pyrrolidone (15 vol%), polyethylene oxide (5 vol%) containing GO nanosheets (0–2 wt%). It is shown that GO nanosheets significantly change the conductivity and viscosity of highly concentrated chitosan solutions, so that ultrafine and uniform fibers with an average diameter of 60 nm are spinnable. The GO-reinforced nanofibers with controlled pore structure exhibit enhanced elastic modulus and... 

A smeared crack approach has been proposed to model the static and dynamic behavior of mass concrete in three-dimensional space. The proposed model simulates the tensile fracture on the mass concrete and contains pre-softening behavior, softening initiation, fracture energy conservation and strain rate effects under dynamic loads. The validity of the proposed model has been checked using the available experimental results under static and dynamic loads. The direct and indirect displacement control algorithms have been employed under incremental increasing static loads. It was found that the proposed model gives excellent results and crack profiles when compared with the available data under... 

A multilayer back propagation learning algorithm was used as an artificial neural network tool to predict the mechanical properties of porous NiTi shape memory alloys fabricated by press/sintering of the mixed powders. Effects of green porosity, sintering time and the ratio of the average Ti to Ni particle sizes on properties of the product were investigated. Hardness and tensile strength of the compacts were determined by hardness Rockwell B method and shear punch test. Three-fourths of 36 pairs of experimental data were used for training the network within the toolbox of the MATLAB software. Porosity, sintering time and particle size ratios were defined as the input variables of the model.... 

Reinforcement of polydimethylsiloxane (PDMS) was investigated using untreated nanodiamond (uND) and silane functionalized ND (sND) at various degrees of silanization. Scanning electron microscopy revealed an improved dispersion of ND in PDMS by silanization. The incorporation of uND and sNDs enhanced tensile strength up to 45% and 70%, respectively; however, greater improvement was observed for sNDs at very low concentrations, e.g. 0.2 wt%. The much improvement in crosslinking density, analyzed by Flory-Rehner model and Mooney-Rivlin plot, and interfacial interaction, characterized by Kraus and Cunneen-Russell plots, was also observed for sNDs. Interestingly, it was found that the extent of... 

Natural compounds containing polysaccharide ingredients have been employed as candidates for treatment of skin tissue. Herein, for the first time, electrospinning setup was proposed to fabricate an efficient composite nanofibrous structure of Beta vulgaris (obtained from Beet [Chenopodiaceae or Amaranthaceae]) belonged to polysaccharides and an elastic polymer named nylon 66 for skin tissue engineering. Both prepared scaffolds including noncomposite and composite types were studied by Scanning electron microscope (SEM), Fourier transform infrared (FTIR) spectroscopy, mechanical assay, and contact angle. Scanning electron microscope examinations have approved the uniform and homogeneous... 

Commercial purity aluminum sheets are subjected to a severe plastic deformation technique called constrained groove pressing. In this study for the first time by using some technical optimizations, a strain magnitude of 6.9 is imposed to the sheets. The grain size evolution during severe plastic deformation is studied using Williamson-Hall analysis on X-ray diffraction pattern of the deformed samples. These results and transmission electron microscopy observations show that constrained groove pressing process can effectively refine the coarse-grained structure to an ultrafine grain range. The results of mechanical tests show that imposing strain in range of 0-5.75 causes to strengthening of... 

Post-deposition annealing effects on nanomechanical properties of granular TiO2 films on soda-lime glass substrates were studied. In particular, the effects of Na diffusion on the films' mechanical properties were examined. TiO2 photocatalyst films, 330 nm thick, were prepared by dip-coating using a TiO2 sol, and were annealed between 100 °C and 500 °C. Film's morphology, physical and nanomechanical properties were characterized by atomic force microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, differential thermo-gravimetric analysis, and nanoindentation. Contrary to expectations, the maximum film hardness was achieved for 300°C annealing, with a value of 0.69 ± 0.05 GPa.... 

To enhance physicomechanical properties and bioactivity of fibrous membranes for wound dressing and tissue engineering applications, novel composite scaffolds consisting of fibrous mats and thermosensitive hydrogel particles were prepared by concurrent electrospinning and electrospraying technique. The composite scaffolds were composed of keratin/bacterial cellulose fibers (150 ± 43 nm) which are hybridized with hydrogel particles (500 nm to 2 μm) based on nonionic triblock copolymers conjugated with Tragacanth gum (TG). FTIR and H-NMR studies indicated ester reactions between carboxylated copolymers and TG through carbodiimide crosslinker chemistry. The hydrogel particles were uniformly... 

To improve the strength and toughness of AISI 4340 steel, different microstructures, containing full bainite, bainite-ferrite, martensite-ferrite and full martensite were produced by different heat treatment cycles. Tensile, impact and hardness tests were carried out at room temperature. The ductile-brittle transition temperature was determined from impact data at different temperatures. The results showed that steel with bainite - 0.34 ferrite microstructure has the highest elongation and charpy impact energy, while its tensile strength and yield stress decreased in comparison to other microstructures. This increment was noticeable when bainite - 0.34 ferrite steel was tempered. The... 

The variation of morphology and mechanical properties of Al6061 automotive aluminum alloy due to friction stir welding (FSW) and gas tungsten arc welding (GTAW) was investigated by optical metallography, scanning electron microscopy, microhardness measurement, X-ray diffraction, tensile testing, and fractography. The center-line dendrite emergence and microhardness reduction in the heat-affected zone were observed in the GTAW process. Although similar microhardness reduction with respect to the base metal was observed in the FSW samples, higher HVs were obtained for the FSW rather than the GTAW process at almost all heat-affected locations. Ultimate tensile strengths of the FSW and the GTAW... 

The microstructure and mechanical properties of the ultra-fine grained (UFG) Al6063 alloy reinforced with nanometric aluminum oxide nanoparticles (25 nm) were investigated and compared with the coarse-grained (CG) Al6063 alloy (∼2 μm). The UFG materials were prepared by mechanical alloying (MA) under high-purity Ar and Ar-5 vol pct O2 atmospheres followed by hot powder extrusion (HPE). The CG alloy was produced by HPE of the gas-atomized Al6063 powder without applying MA. Electron backscatter diffraction under scanning electron microscopy together with transmission electron microscopy studies revealed that the microstructure of the milled powders after HPE consisted of ultra-fine grains... 

Three-dimensional porous scaffolds are essential in tissue engineering applications. One of the most conventional methods to form porosity in scaffolds is freeze-drying, which is not energy efficient and cost effective. Therefore in this work, it was experimentally investigated whether gelatin, with its unique mechanical properties and cell binding applications, could be used as a comprising polymer of scaffolds with porous structure made by the freeze-gelation method. Chitosan, gelatin and chitosan/gelatin scaffolds were fabricated by the freeze-gelation method and their behaviors, determined by analysis of scanning electron microscopy images, Fourier transform infrared spectroscopy,...