Andri Andriyana

Electrospinning of polymethyl methacrylate nanofibers: optimization of processing parameters using the Taguchi design of experiments

The effects of polymer concentration and electrospinning parameters on the diameter of electrospun polymethyl methacrylate (PMMA) fibers were experimentally investigated. It was also studied how the controlled factors would affect the output with the intention of finding the optimal electrospinning settings in order to obtain the smallest PMMA fiber diameter. Subsequently the solution feed rate, needle gauge diameter, supply voltage, polymer concentration and tip-to-collector distance were considered as the control factors. To achieve these aims, Taguchi’s mixed-level parameter design (L18) was employed for the experimental design. Optimal electrospinning conditions were determined using the signal-to-noise (S/N) ratio that was calculated from the electrospun PMMA fiber diameter according to “the-smaller-the-better” approach. Accordingly, the smallest fiber diameter observed was 228 (±76) nm and it was yielded at 15 wt% polymer concentration, 20 kV of supply voltage, 1 ml/h feed rate, 15 cm tip-to-distance and 19 needle gauge. Moreover, the S/N ratio response showed that the polymer concentration was the most effective parameter on determination of fiber diameter followed by feed rate, tip-to distance, needle gauge and voltage, respectively. The Taguchi design of experiments method has been found to be an effective approach to statistically optimize the critical parameters used in electrospinning so as to effectively tailor the resulting electrospun fiber diameters and morphology.

Synthesis, characterisation and stability of superparamagnetic maghemite nanoparticle suspension

Abstract Maghemite nanoparticles were synthesised using the co-precipitation method and characterised by various techniques, including X-ray diffraction, transmission electron microscopy, alternating gradient magnetometry, dynamic light scattering and zeta potential. The stability of the suspension was monitored by measuring the particle size distribution and zeta potential using dynamic light scattering over a period of few months. The pattern obtained from X-ray diffraction confirmed that the particles were maghemite with crystallite size of 9·4 nm. Transmission electron microscopy observations and analyses showed that the mean physical size of the nanoparticles was 9·5 nm. The nanoparticles show superparamagnetic behaviour with magnetisation value at ±10 kOe of 32·18 emu g−1. The intensity averaged particle size of as-synthesised maghemite nanoparticles was 45·3 nm. The suspension was stored for periods of 2, 4 and 8 months. The intensity averaged sizes were 47·1, 50·5 and 52·1 nm respectively. No sedimentation was observed. The suspensions zeta potential value was 44·6 mV for as-synthesised sample and 43·3, 42·7 and 41·8 mV for sample after storage period of 2, 4 and 8 months, respectively. This indicated that the suspension was very stable.

The role of infarct transmural extent in infarct extension: A computational study

Infarct extension, a process involving progressive extension of the infarct zone (IZ) into the normally perfused border zone (BZ), leads to continuous degradation of the myocardial function and adverse remodelling. Despite carrying a high risk of mortality, detailed understanding of the mechanisms leading to BZ hypoxia and infarct extension remains unexplored. In the present study, we developed a 3D truncated ellipsoidal left ventricular model incorporating realistic electromechanical properties and fibre orientation to examine the mechanical interaction among the remote, infarct and BZs in the presence of varying infarct transmural extent (TME). Localized highly abnormal systolic fibre stress was observed at the BZ, owing to the simultaneous presence of moderately increased stiffness and fibre strain at this region, caused by the mechanical tethering effect imposed by the overstretched IZ. Our simulations also demonstrated the greatest tethering effect and stress in BZ regions with fibre direction tangential to the BZ-remote zone boundary. This can be explained by the lower stiffness in the cross-fibre direction, which gave rise to a greater stretching of the IZ in this direction. The average fibre strain of the IZ, as well as the maximum stress in the sub-endocardial layer, increased steeply from 10% to 50% infarct TME, and slower thereafter. Based on our stress-strain loop analysis, we found impairment in the myocardial energy efficiency and elevated energy expenditure with increasing infarct TME, which we believe to place the BZ at further risk of hypoxia. Copyright

Numerical Simulation of a Corner Crack Growth in Metals under Multiaxial Fatigue Loading

In practical engineering applications, many components especially rotating members are frequently subjected to fluctuating multiaxial loading. In this case, fatigue failure is recognized as a major cause of failure of engineering components in service. Hence it is important to characterize fatigue behaviour of the material.

Effects of Carbon Black and the Presence of Static Mechanical Strain on the Swelling of Elastomers in Solvent

The effect of carbon black on the mechanical properties of elastomers is of great interest, because the filler is one of principal ingredients for the manufacturing of rubber products. While fillers can be used to enhance the properties of elastomers, including stress-free swelling resistance in solvent, it is widely known that the introduction of fillers yields significant inelastic responses of elastomers under cyclic mechanical loading, such as stress-softening, hysteresis and permanent set. When a filled elastomer is under mechanical deformation, the filler acts as a strain amplifier in the rubber matrix. Since the matrix local strain has a profound effect on the materials ability to absorb solvent, the study of the effect of carbon black content on the swelling characteristics of elastomeric components exposed to solvent in the presence of mechanical deformation is a prerequisite for durability analysis. The aim of this study is to investigate the effect of carbon black content on the swelling of elastomers in solvent in the presence of static mechanical strains: simple extension and simple torsion. Three different types of elastomers are considered: unfilled, filled with 33 phr (parts per hundred) and 66 phr of carbon black. The peculiar role of carbon black on the swelling characteristics of elastomers in solvent in the presence of mechanical strain is explored.

An extended two-phase model for Mullins effect in swollen rubber

Abstract It is well-known that dry rubber exhibits Mullins effect under cyclic loading condition. For swollen rubber, this response is found to strongly depend on the degree of swelling. In the present work, the modelling of the Mullins effect in swollen rubber under cyclic compressive loading is addressed. Different degrees of swelling are obtained by immersing initially dry rubber in biodiesel. The two-phase model of Mullins and Tobin [L. Mullins and N. R. Tobin: ‘Theoretical model for the elastic behaviour of filler reinforced vulcanized rubbers’, Rubber Chem. Technol. 1957, 30, 551–571] and Qi and Boyce [H. Qi and M. C. Boyce: ‘Constitutive model for stretch-°©-induced softening of the stress-stretch behavior of elastomeric materials’, J. Mech. Phys. Solids, 2004, 52, (10), 2187–2205] is considered and extended in order to capture the effect of swelling on the Mullins effect. Results show that the proposed model agree well with the experimental findings.

Diffusion of Palm Biodiesel in Elastomers Undergoing Multiaxial Large Deformations

Petroleum-based fuel is facing significant depletion issue due to its limited reserves and increasing demand from various industries. Thus, various considerations from economical, environmental and political concerns have motivated researchers to develop alternative energy sources such as biofuel to decrease dependence on petroleum-based fuel. However, the changes in the fuel composition of biofuel affect the material compatibility. In engineering applications where elastomeric components are exposed to hostile environment such as palm biodiesel medium, at least two aspects contribute to the degradation of the materials during the service: diffusion of the liquids leading to swelling and fluctuating multiaxial mechanical loading leading to fatigue failure. Therefore, it is of utmost importance to study the mechanical responses of elastomers under this coupled diffusion-mechanical loading in order to predict accurately their fatigue failure. The present work investigates the swelling of elastomers under simultaneous diffusion of palm biodiesel and multiaxial large deformations.

Surface Morphology Analysis and Mechanical Characterization of Electrospun Nanofibrous Structure

The extraordinary properties of nanofibrous structure have gained ever-increasing appeal as an attractive candidate in a myriad of applications, especially for the water filtration. This type of structure has permeability due to its porous structure. Electrospinning is one of the most viable approaches in fabricating nanofibrous web that exhibits novel and outstanding performance in membrane separation as compared to those produced through conventional methods. Thus, the optimization of electrospinning processes is actively pursued by many researchers in order to obtain the best structure suitable for real-life applications. In this study, the surface morphology analysis and mechanical characterization of nanofibrous structure are addressed. For this purpose, polyvinylidene fluoride (PVDF) is considered. The polymeric nanofibrous structures are fabricated through electrospinning technique. Parameters such as polymer concentration and applied voltage for electrospinning process were varied and the resulting morphology of the structure were observed using SEM. In addition, the macroscopic mechanical responses of the structure were probed by means of tensile tests with special attention given to membrane anisotropy and behavior under cyclic loading.

Statistical analysis of imperfection effect on cylindrical buckling response

It is widely reported that no efficient guidelines for modelling imperfections in composite structures are available. In response, this work evaluates the imperfection factors of axially compressed Carbon Fibre Reinforced Polymer (CFRP) cylinder with different ply angles through finite element (FE) analysis. The sensitivity of imperfection factors were analysed using design of experiment: factorial design approach. From the analysis it identified three critical factors that sensitively reacted towards buckling load. Furthermore empirical equation is proposed according to each type of cylinder. Eventually, critical buckling loads estimated by empirical equation showed good agreements with FE analysis. The design of experiment methodology is useful in identifying parameters that lead to structures imperfection tolerance.

Effect of carbon black content on the swelling of elastomers in solvent in the presence of static mechanical loading

Abstract The effect of carbon black on the mechanical properties of elastomers is of great interest because filler is one of the necessity items for manufacturing of rubber products. While fillers can be used to enhance the properties of elastomers, it is known that filler yields to significant inelastic responses such as stress-softening, hysteresis and permanent set. Moreover, under tensile loading, the filler acts as a strain amplifier in the rubber matrix resulting in a higher local strain state in the matrix as compared to the applied macroscopic strain. The study of the effect of carbon black content on the swelling characteristics of elastomeric components undergoing mechanical loading and simultaneously exposed to aggressive solvent is a prerequisite for durability analysis. The present study investigates the effect of carbon black content on the swelling of elastomers in solvent in the absence and in the presence of static tensile loadings.