Publications (20)36.49 Total impact
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Article: Strain-controlled fatigue life and modeling of conduit polymers
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ABSTRACT: Strain-controlled fatigue lives of conduit polymers, viz., nylon 6, polypropylene (PP) and calcium carbonate filled black colored polypropylene (PP-blk) were studied. Thermal and mechanical analyses were conducted before fatigue tests. Thermal characteristics, such as the degree of molecular degradation, glass transition temperatures, and melting points were determined. Tensile strength, elastic modulus, and Poisson’s ratio were obtained from tests under quasi-static loading. Fatigue lives were measured under different displacement ranges and temperature conditions. Four different temperatures were selected to represent low (−40°C), room (25°C), and high (65 and 125°C) temperature conditions. Hysteretic heating was found to be significantly operative in PP specimens. By optimizing the previously developed unified strain model [1], strain fatigue lives were predicted based on the studied materials.Journal of Materials Science 04/2012; 46(6):1902-1912. · 2.02 Impact Factor -
Article: Measurement of Adhesion Work of Electrospun Polymer Membrane by Shaft-Loaded Blister Test.
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ABSTRACT: The work of adhesion at the interface of electrospun membrane and rigid substrate is measured by a shaft-loaded blister test (SLBT). Poly(vinylidene fluoride) (PVDF) were electrospun with an average fiber diameter of 333 ± 59 nm. Commercial cardboard with inorganic coating was used to provide a model substrate for adhesion tests. In SLBT, the elastic response PVDF was analyzed and its adhesion energy measured. The average value of the adhesion work is 206 ± 26 mJ/m(2). Elastic modulus of electrospun membrane obtained by SLBT is found to be 23.42 ± 2.69 MPa, which is consistent with the value obtained from standard tensile tests. The results show SLBT presented a viable methodology for evaluating the adhesion energy of electrospun polymer fabrics.Langmuir 04/2012; · 4.19 Impact Factor -
Article: One-dimensional multiferroic bismuth ferrite fibers obtained by electrospinning techniques.
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ABSTRACT: We report the fabrication of novel multiferroic nanostructured bismuth ferrite (BiFeO(3)) fibers using the sol-gel based electrospinning technique. Phase pure BiFeO(3) fibers were prepared by thermally annealing the electrospun BiFeO(3)/polyvinylpyrrolidone composite fibers in air for 1 h at 600 °C. The x-ray diffraction pattern of the fibers (BiFeO(3)) obtained showed that their crystalline structures were rhombohedral perovskite structures. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images revealed that the BiFeO(3) fibers were composed of fine grained microstructures. The grains were self-assembled and self-organized to yield dense and continuous fibrous structures. The magnetic hysteresis loops of these nanostructured fibers displayed the expected ferromagnetic behavior, whereby a coercivity of ∼ 250 Oe and a saturation magnetization of ∼ 1.34 emu g(-1) were obtained. The ferroelectricity and ferroelectric domain structures of the fibers were confirmed using piezoresponse force microscopy (PFM). The piezoelectric hysteresis loops and polar domain switching behavior of the fibers were examined. Such multiferroic fibers are significant for electroactive applications and nano-scale devices.Nanotechnology 06/2011; 22(23):235702. · 3.98 Impact Factor -
Article: Effect of Fiber Diameter on the Deformation Behavior of Self-Assembled Carbon Nanotube Reinforced Electrospun Polyamide 6,6, Fibers
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ABSTRACT: High precision electrospinning technique was used to obtain self-assembled carbon nano-tube (CNT) reinforced polyamide (PA) 6,6 fibers. The reinforcement factors were critically evaluated with respect to the effects of fiber diameter and inclusion of CNTs.The average fiber diameter ranged from 240 to 1400 nm and the CNT contents were 0, 1 and 2.5 wt%. A sharp increase in modulus and strength of the fibers was demonstrated when the size of the fiber was decreased below 500 nm, which could be attributed to ordered arrangement of crystals and the spatial confinement effect of the fibers. Also, investigation of the deformation behavior of fibers as a function of CNT content revealed that tensile fiber modulus and strength improved significantly with increase of CNTs. Addition of CNTs restricted the segmental motion of polymer chains and provided the confinement effect to the neighboring molecules. (C) 2011 Elsevier B.V. All rights reserved.Materials Science and Engineering A 01/2011; 528(21):6565-6572. · 2.00 Impact Factor -
Article: Do electrospun polymer fibers stick?
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ABSTRACT: Adhesion between electrospun polycaprolactone (PCL) fibers was directly measured in a cross-cylinder geometry using a nanoforce tensile tester. The surface roughness of fibers was determined by an atomic force microscope (AFM), and the structural factors were characterized by differential scanning calorimeter (DSC) and wide-angle X-ray diffraction (WAXD). "Pull-off" force was found to be in the order of 10(-6) N, and the adhesion energy was 190 +/- 7 mJ/m(2). Adhesion increases with decreasing fiber radius. The experimental data are analyzed by the classical Johnson-Kendall-Roberts (JKR) contact mechanics model. The study provides fruitful insights into future development of bio-inspired adhesives and devices.Langmuir 09/2010; 26(17):14188-93. · 4.19 Impact Factor -
Article: Electrospinning of polymer nanofibers: Effects on oriented morphology, structures and tensile properties
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ABSTRACT: The interest in fabrication of nanofibers using electrospinning method has attracted considerable attention due to its versatile maneuverability of producing controlled fiber structures, porosity, orientations and dimensions. Although the process appears to be simple and straightforward, an understanding of the technique and its influence on the morphology, structural and mechanical properties is still not completely clear. Recently, the size effect on the mechanical properties was reported for fibers across different length scales. Both modulus and strength of poly(ε-capro-lactone) (PCL) fibers were found to increase significantly when the diameter of the fibers was reduced to below ∼500 nm. In this article, for the first time, we critically review and evaluate the role of the microstructures on the fiber deformation behavior and present possible explanations for the enhanced properties of the nanofibers. Our discussions are focused on the techniques to obtain controlled structures and the mechanisms behind the size effect in electronspun fibers are given. In-depth understanding of these mechanisms can provide fruitful outcomes in the development of advanced nanomaterials for devices and miniaturized load-bearing applications.Composites Science and Technology 03/2010; · 3.14 Impact Factor -
Article: How super is supercontraction? Persistent versus cyclic responses to humidity in spider dragline silk.
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ABSTRACT: Spider dragline silk has enormous potential for the development of biomimetic fibers that combine strength and elasticity in low density polymers. These applications necessitate understanding how silk reacts to different environmental conditions. For instance, spider dragline silk ;supercontracts' in high humidity. During supercontraction, unrestrained dragline silk contracts up to 50% of its original length and restrained fibers generate substantial stress. Here we characterize the response of dragline silk to changes in humidity before, during and after supercontraction. Our findings demonstrate that dragline silk exhibits two qualitatively different responses to humidity. First, silk undergoes a previously unknown cyclic relaxation-contraction response to wetting and drying. The direction and magnitude of this cyclic response is identical both before and after supercontraction. By contrast, supercontraction is a ;permanent' tensioning of restrained silk in response to high humidity. Here, water induces stress, rather than relaxation and the uptake of water molecules results in a permanent change in molecular composition of the silk, as demonstrated by thermogravimetric analysis (TGA). Even after drying, silk mass increased by approximately 1% after supercontraction. By contrast, the cyclic response to humidity involves a reversible uptake of water. Dried, post-supercontraction silk also differs mechanically from virgin silk. Post-supercontraction silk exhibits reduced stiffness and stress at yield, as well as changes in dynamic energy storage and dissipation. In addition to advancing understanding supercontraction, our findings open up new applications for synthetic silk analogs. For example, dragline silk emerges as a model for a biomimetic muscle, the contraction of which is precisely controlled by humidity alone.Journal of Experimental Biology 08/2009; 212(Pt 13):1981-9. · 3.00 Impact Factor -
Article: Supercontraction forces in spider dragline silk depend on hydration rate.
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ABSTRACT: Spider dragline silk is a model biological polymer for biomimetic research due to its many desirable and unusual properties. 'Supercontraction' describes the dramatic shrinking of dragline silk fibers when wetted. In restrained silk fibers, supercontraction generates substantial stresses of 40-50 MPa above a critical humidity of approximately 70% relative humidity (RH). This stress may maintain tension in webs under the weight of rain or dew and could be used in industry for robotics, sensor technology, and other applications. Our own findings indicate that supercontraction can generate stress over a much broader range than previously reported, from 10 to 140 MPa. Here we show that this variation in supercontraction stress depends upon the rate at which the environment reaches the critical level of humidity causing supercontraction. Slow humidity increase, over several minutes, leads to relatively low supercontraction stress, while fast humidity increase, over a few seconds, typically results in higher supercontraction stress. Slowly supercontracted fibers take up less water and differ in thermostability from rapidly supercontracted fibers, as shown by thermogravimetric analysis. This suggests that spider silk achieves different molecular configurations depending upon the speed at which supercontraction occurs. Ultimately, rate-dependent supercontraction may provide a mechanism to tailor the properties of silk or biomimetic fibers for various applications.Zoology 06/2009; 112(5):325-31. · 1.50 Impact Factor -
Article: Modelling of mechanical properties of electrospun nanofibre network
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ABSTRACT: Electrospun nanofibres are widely investigated as extra-cellular matrix for tissue engineering and biomedical applications. Little is understood on the deformation mechanics of spun fibre mats. A model is developed to predict the deformation behaviour of randomly-oriented electrospun nanofibre network/mats with the fibre-fibre fusion and van der Waals interaction. The nanofibres in the mat are represented by chains of beads; the interactions between the beads are described by bonded (stretch, bending and torsion) and non-bonded (van der Waals) potentials. Stress-strain curves and dynamics fracture are predicted by this model. The results show that the fibre-fibre fusion has a significant effect on the tensile strength of the mats. Increasing the number of fusion points in the mat results in an increase in strength, but over-fusion may lead to lower fracture energy. The predicted stress-strain relationships are consistent with the experimental results. of Akron. His research interests include structural analysis, FEA, stochastic analysis and optimisation. His recent research focus is on modelling of mechanical properties of electrospun nanofibres.Int. J. Experimental and Computational Biomechanics Int. J. Experimental and Computational Biomechanics. 01/2009; 1(1):45-57. -
Article: Fracture strength and adhesive strength of hydroxyapatite-filled polycaprolactone.
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ABSTRACT: Fracture toughness and tear strength of hydroxyapatite (HAP)-filled poly(epsilon-caprolactone) (PCL) with increasing HAP concentration were studied. The toughness was assessed in terms of essential work of fracture (EWF). Adhesive strength between HAP and PCL interfaces was evaluated using T-peel testing. The adhesion between the two components was found to be relatively strong. Double edge notched tension (DENT) and trousers test specimens were used for the EWF tests. The effect of HAP phase in PCL on the fracture and tearing toughness was investigated. The results obtained from the EWF tests for the HAP-filled PCL complied with the validity criteria of the EWF concept, namely, (1) geometric similarity for all ligament lengths; (2) fully yielded ligament and (3) plane-stress fracture condition. Values for specific essential work of fracture (w ( e )) and specific plastic work of fracture (betaw ( p )) were found to decrease with increase in HAP concentration. The testing procedure showed promise in quantifying the tearing resistance and rising R-curve behavior common in natural materials and it can be extended to other biomaterials that exhibit post-yield deformation. A quantitative assessment based on fracture mechanics of the adhesive strength between the bioactive interfaces plays an important role for continued development of tissue replacement and tissue regeneration materials.Journal of Materials Science Materials in Medicine 03/2008; 19(2):929-36. · 2.32 Impact Factor -
Article: Morphological and X-ray diffraction studies of crystalline hydroxyapatite-reinforced polycaprolactone.
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ABSTRACT: Morphological and mechanical properties of hydroxyapatite (HAP)-reinforced polycaprolactone (PCL) were studied. The objective was to examine how morphological features alter the bulk mechanical properties in our laboratory-synthesized HAP-reinforced PCL. HAP crystals were synthesized by hydrolysis of mixtures of calcium and phosphate salts in the laboratory with wet chemical methods. The properties of the commercially available hydroxyapatite (HAP(1)) are compared with that of laboratory-synthesized hydroxyapatite (HAP(2)). The HAP crystals and composition were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Fourier transform infrared spectrometry (FTIR). The HAP(1) and HAP(2) crystals were dispersed into polymers to examine the mechanical behavior of bioactive composites, and the interfacial interactions between the polymer and HAP crystals are addressed. The FTIR results confirmed that the two forms of HAP crystals are consistent in terms of the functional chemical groups. The wide angle X-ray diffraction study was performed to determine the crystallinity of the bioactive composites. It was observed that the crystallinty of HAP-filled PCL steadily increased as the filler concentration increased. Generally, HAP(2) has a particle size considerably smaller than HAP(1) and the composite derived had higher modulus than conventional HAP-filled polymers. This increase in modulus is attributed to better interfacial interaction. Bioresorbability tests performed on HAP particles found that the synthesized HAP had higher resorption rates. It is clear that the mechanical properties are influenced by the particle size and therefore by the processing method used.Journal of Biomedical Materials Research Part B Applied Biomaterials 06/2007; 81(2):343-50. · 2.15 Impact Factor -
Article: Embrittlement Mechanisms of Nylon 66/Organoclay Nanocomposites Prepared by Melt-Compounding Process
Materials and Manufacturing Processes 01/2006; 21(2):153-158. · 1.06 Impact Factor -
Article: Deformation mechanisms of nanoclay‐reinforced maleic anhydride‐modified polypropylene
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ABSTRACT: Fracture properties and deformation mechanisms of nanoclay-reinforced maleic anhydride-modified polypropylene (MAPP) were investigated. Elastic–plastic fracture mechanics was employed to characterize the toughness in light of substantial postyield deformation for the reinforced MAPP. Upon introduction of 2.5 wt % clay loading in maleated MAPP, it was observed that tensile strength, modulus, and fracture initiation toughness concomitantly increased substantially. Continued increase in clay loading thereafter only led to stiffening and strengthening effects to the detriment of fracture toughness. A plot of the J-integral initiation fracture toughness versus the plastic zone size demonstrated that toughening arose from plastic deformation in the reinforced matrix. Careful examination of deformed tensile specimens using small angle X-ray scattering (SAXS) showed 2.5 wt % clay gave rise to the highest equatorial scattering, which indicates the presence of microvoids in the matrix. The SAXS results were consistent with that shown in subcritically loaded crack-tip deformation zone using transmission electron microscopy. Thus, both macroscale three-point bend fracture data and SAXS results led us to consistent findings and conclusions. Further increase in clay loading above 2.5 wt % reduced the scattering the matrix plasticity and thus the fracture toughness. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2759–2768, 2004Journal of Polymer Science Part B Polymer Physics 07/2004; 42(14):2759 - 2768. · 1.53 Impact Factor -
Article: Essential fracture work of short fiber reinforced polymer blends
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ABSTRACT: Short glass fiber reinforced (SGFR) PA6,6/PP blends with 20 wt% styrene-ethylene/butylene-styrene triblock copolymers (SEBS) grafted with different levels of maleic anhydride (MA) were studied using both the essential work of fracture (EWF) and J-integral fracture toughness techniques. Good linearity was found between the plane strain specific fracture work, Wf, and the ligament length, l, in single-edge notched bend (SENB) specimens. The two fracture mechanics parameters were compared and there was good agreement between the J-integral fracture initiation toughness, JIC, and the specific essential fracture work, WIe. The skin-core morphology characteristic of injection molded short fiber reinforced thermoplastics (SFRT) was also revealed using the EWF approach.Polymer Engineering and Science 04/2004; 39(2):356 - 364. · 1.30 Impact Factor -
Article: Polypropylene modified with elastomeric metallocene‐catalyzed polyolefin blends: Fracture behavior and development of damage mechanisms
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ABSTRACT: The fracture behavior and deformation mechanisms of polypropylene modified by elastomeric metallocene-catalyzed polyolefin blends were investigated under both static and dynamic loading conditions. The fracture toughness was evaluated with the J integral approach. The development of damage mechanisms was studied by the examination of fracture surfaces with scanning electron microscopy and by the examination of single-edge, double-notch, four-point-bending or low-impact-energy fractured samples with optical microscopy. In addition, tensile dilatometry measurements were carried out to determine the nature of the deformation micromechanisms. The fracture behavior and the size and shape of the damage zones were drastically influenced by the elastomeric particles and the imposed constraint. The role of the elastomeric particles was different, depending on the strain rate. Under impact loading, particle pullout and crazing were responsible for the increased fracture toughness of polypropylene. Under quasistatic loading, stable fracture growth was caused by particle cavitation, which promoted ductile tearing of polypropylene before failure continued in an unstable fashion via crazing. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1075–1089, 2004Journal of Polymer Science Part B Polymer Physics 02/2004; 42(6):1075 - 1089. · 1.53 Impact Factor -
Article: Electrical and mechanical properties of expanded graphite‐reinforced high‐density polyethylene
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ABSTRACT: High-density polyethylene (HDPE) was reinforced with expanded and untreated graphite in a melt-compounding process. Viscosity increased upon addition of graphite phase, with the expanded graphite (EG) showing more dramatic rise than the untreated graphite (UG) in viscosity. The increase in viscosity was attributed to the increased surface-to-volume ratio for the EG filler after acid treatment. Electrical conductivity also increased from that pertaining to an insulator to one characteristic of a semiconductor. The EG system showed a lower percolation threshold for transition in conductivity compared to that in the UG system. DSC results indicated that the fillers acted as a nucleating agent in inducing the crystallization of HDPE in the composites. However, the overall degree of crystallinity and melting temperature of HDPE decreased with the addition of EG and UG. Mechanical properties improved as a function of filler content but the overall enhancement was not impressive. It was conjectured that the filler–matrix interface was not optimized in the melt-mixing process. However, the role of EG as a reinforcement phase for both electrical and mechanical properties was unambiguously established. The EG composites demonstrated potentially useful attributes for antistatic, barrier, mechanical, electrical, and cost-effective applications. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91:2781–2788, 2004Journal of Applied Polymer Science 01/2004; 91(5):2781 - 2788. · 1.29 Impact Factor -
Article: Fiber reinforcement and fracture resistance of PC/PBT/LCP ternary in situ composite
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ABSTRACT: The microstructures, mechanical properties, and fracture toughness of LCP (Vectra B950) reinforced PC/PBT blend with a 60/40 weight ratio have been studied. LCP of varying concentrations were investigated as rigid fillers in matrices of multiphase polymer blends. In this study, differences in microstructures and morphology between samples of two thicknesses (4 mm thick and 6 mm thick) and two geometries (dumbbell and rectangular) were compared using scanning electron microscopy (SEM). Given identical processing conditions, fibrous LCP structures were evident in the 4-mm-thick injection molded, dumbbell-shaped samples, whereas the 6-mm-thick rectangular samples displayed spherical dispersion of LCP aggregates that embrittled the preblended ductile matrix. Tensile properties of the dumbbell specimens showed superior strengthening and stiffening whereby the tensile strength increased twofold and the modulus increased fourfold. Plane strain fracture toughness was slightly enhanced as the LCP content increased because of the fiber strengthening effect but the overall fracture performance of the in situ composites was relatively poor compared with PC/PBT. Experimental results were compared with those predicted in composite theory. Simplified micromechanics equations were developed to describe the tensile moduli of injection molded LCP reinforced blends that exhibited a strong skin-core morphology.Polymer Engineering and Science 12/1997; 38(1):156 - 168. · 1.30 Impact Factor -
Article: Mechanical behavior of self-assembled carbon nanotube reinforced nylon 6,6 fibers
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ABSTRACT: The versatile electrospinning technique was used to successfully align and disperse multiwalled carbon nanotubes (MWCNT) in nylon 6,6 matrix to obtain composite fibers. The morphology of the composite fibers and the dispersion of the CNTs within the fibers were analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. TEM analysis revealed that the CNTs were well-dispersed, separated and aligned along the fiber axis. The thermal and mechanical properties of the composite fibers were characterized as a function of weight fraction of the CNTs. Incorporation of the CNTs in the fibers resulted in an increase in glass-transition temperature (Tg) by ∼7 °C, indicating that the addition of CNTs has restricted the mobility of the polymer chains and provided confinement to neighboring molecular chains. Tensile and nanoindentation experiments were performed to investigate the mechanical deformation behavior of the composite fibers. The results suggested that incorporation of high strength and high aspect ratio CNTs into the fiber matrix enhanced significantly the stiffness and strength of nylon 6,6 fibers. An understanding of the structure–property relationships can provide fruitful insights to develop electrospun fibers with superior properties for miniaturized and load-bearing applications.Composites Science and Technology. -
Article: Effect of specimen thickness on fracture toughness and adhesive properties of hydroxyapatite-filled polycaprolactone
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ABSTRACT: The fracture toughness of hydroxyapatite (HAP)-filled polycaprolactone (PCL) biocomposites was investigated using the technique of essential work of fracture (EWF). The influence of specimen thickness on the toughness parameters was investigated. The specific essential work of fracture (we) was found to decrease with the increase in filler loading and with the thickness of the specimen. The testing procedure used for the EWF measurement obeyed the validity criteria of the concept and we determined can be considered to be a good measure of the plane-stress toughness of the composite. The adhesive strength between the HAP and the PCL phase was determined using T-peel tests. The layer of the laminates in the PCL–HAP–PCL peel test specimens was varied to study the influence of laminate thickness on the interfacial work of fracture. The effect of temperature on the interfacial work of fracture was also evaluated. The parameters were varied to determine the effect of the plastic deformation on the peel strength. The interfacial work of fracture between HAP and PCL was found to be relatively strong; however, the effect of temperature and PCL laminate thickness did not significantly vary the adhesion strength.Composites Part A: Applied Science and Manufacturing. -
Article: Effect of fiber diameter on tensile properties of electrospun poly(ɛ-caprolactone)
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ABSTRACT: The tensile properties of electrospun fibers have not been widely investigated due to the difficulties in handling nanofibers and measuring low load for deformation. In this study, the effect of dimensional confinement on free standing biodegradable poly(ɛ-caprolactone) (PCL) is investigated using electrospinning-enabled techniques and a nanoforce tensile tester. The structural properties such as crystallinity and molecular orientation of the spun fibers are examined using wide angle X-ray diffraction (WAXD). The degree of crystallinity and molecular orientation of fibers are enhanced when the diameter of spun fibers is reduced, resulting in improved mechanical strength and stiffness. It is evident that PCL fibers with decreasing fiber diameter exhibit an abrupt shift in tensile performance in comparison to those derived from non-spun systems. The abrupt shift in tensile strength and stiffness of electrospun PCL fibers occurs at around 700 nm in diameter and illustrates the importance of studying the mechanical behavior of the nanofibers, for the first time, systematically with the aid from electrospinning techniques. This shift cannot be otherwise explained by a noticeable change in Tg, and the gradual increase in crystallinity and molecular orientation.Graphical abstractPolymer. 49(21):4713-4722.
Top co-authors
Top Journals
Institutions
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2006–2008
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University of Akron
- Department of Mechanical Engineering
Akron, OH, USA
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2004
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Nanyang Technological University
- School of Materials Science and Engineering
Singapore, Singapore -
North Dakota State University
- Department of Mechanical Engineering
Fargo, ND, USA
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1997
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University of Sydney
- School of Aerospace, Mechanical and Mechatronic Engineering (AMME)
Sydney, New South Wales, Australia
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