Chee Yoon Yue

Tensile properties of short-glass-fiber- and short-carbon-fiber-reinforced polypropylene composites

Composites of polypropylene (PP) reinforced with short glass fibers (SGF) and short carbon fibers (SCF) were prepared with extrusion compounding and injection molding techniques. The tensile properties of these composites were investigated. It was noted that an increase in fiber volume fraction led to a decrease in mean fiber length as observed previously. The relationship between mean fiber length and fiber volume fraction was described by a proper exponential function with an offset. The tensile strength and modulus of SGF/PP and SCF/PP composites were studied taking into account the combined effect of fiber volume fraction and mean fiber length. The results about the composite strength and modulus were interpreted using the modified rule of mixtures equations by introducing two fiber efficiency factors, respectively, for the composite strength and modulus. It was found that for both types of composites the fiber efficiency factors decreased with increasing fiber volume fraction and the more brittle fiber namely carbon fiber corresponded to the lower fiber efficiency factors than glass fiber. Meanwhile, it was noted that the fiber efficiency factor for the composite modulus was much higher than that for the composite strength. Moreover, it was observed that the tensile failure strain of the composites decreased with the increase of fiber volume fraction. An empirical but good relationship of the composite failure strain with fiber volume fraction, fiber length and fiber radius was established.

Fracture resistance of short-glass-fiber-reinforced and short-carbon-fiber-reinforced polypropylene under Charpy impact load and its dependence on processing

Abstract In this paper, short-glass-fiber-reinforced and short-carbon-fiber-reinforced polypropylene composites were investigated with respect to the work of fracture (WOF), here viz the notched Charpy impact energy. Short glass fibers and short carbon fibers were incorporated into polypropylene with a twin-screw extruder and all of the specimens were injection molded into dumbbell-shaped tensile bars in a twin-screw injection-molding machine. Rectangular impact bars were obtained from tensile bars by removing the two clamping parts. Charpy impact tests were performed with a Charpy impact tester on specimens with V-notches. The values of the notched Charpy impact energy were obtained from each group of eight specimens. The effects of glass and carbon fiber volume fractions on their lengths in single and hybrid short fiber composites were investigated. The composite WOF was studied qualitatively by taking into account the effects of fiber volume fraction and fiber length distributions using the total WOF theory. Fiber length distributions have been previously shown to depend on the processing conditions. On the other hand, the composite impact resistance was shown to depend on fiber length and hence on processing. The dependence of the composite impact energy on processing is discussed.

Assessment of fibre-matrix adhesion and interfacial properties using the pull-out test

Abstract Fibre--matrix adhesion is assessed using a technique based on the fibre pull-out test. The fibre-matrix adhesion can be characterized in terms of the interfacial shear (bond) strength, r;. The non-uniform shear stress distribution at the fibre-matrix interface is taken into account. The technique also allows other interfacial properties such as the matrix shrinkage pressure Po and the interfacial coefficient of friction μ to be evaluated from a single set of pull-out test data. Analysis of new experimental data for glass fibre-nylon 66 and glass fibre-nylon 11 composite systems is presented to illustrate application of the technique. The issue of the effect of loading configuration and specimen geometry on the pull-out test is then considered. This is to facilitate proper analysis of experimental data, to ensure a high success rate in the pull-out experiment and to minimize specimen failure by fibre fracture and other non pull-out modes.

Hybrid effects on tensile properties of hybrid short-glass-fiber- and short-carbon-fiber-reinforced polypropylene composites

Hybrid composites of polypropylene reinforced with short glass fibers and short carbon fibers were prepared using extrusion compounding and injection molding techniques. The tensile properties of these composites were investigated taking into account the effect of the hybridization by these two types of short fibers. It was noted that the tensile strength and modulus of the hybrid composites increase while the failure strain of the hybrid composites decreases with increasing the relative carbon fiber volume fraction in the mixture. The hybrid effects for the tensile strength and modulus were studied by the rule of hybrid mixtures (RoHM) using the tensile strength and modulus of single-fiber composites, respectively. It was observed that the strength shows a positive deviation from that predicted by the RoHM and hence exhibits a positive hybrid effect. However, the values of the tensile modulus are close to those predicted by the RoHM and thus the modulus shows no existence of a hybrid effect. Moreover, the failure strains of the hybrid composites were found to be higher than the failure strain of the single carbon fiber-reinforced composite, indicating that a positive hybrid effect exists. Explanations for the hybrid effects on the tensile strength and failure strain were finally presented.

Synergistic effect on the fracture toughness of hybrid short glass fiber and short carbon fiber reinforced polypropylene composites

Abstract The fracture behavior of injection-molded polypropylene (PP) composites reinforced with hybrid short glass fibers (SGF) and short carbon fibers (SCF) was studied. The fracture toughness ( K c ) of the composites was measured using compact tension specimens made from the manufactured plaques. It was of interest to note that the fracture toughness of hybrid SGF/SCF/PP composites was higher than that predicted by the rule of hybrid mixtures (RoHM), showing a positive hybrid (synergistic) effect. Also, the fracture toughness of the hybrid composites was higher than that of both single SGF/PP and SCF/PP composites, thus showing that the synergistic effect was remarkable. SEM observations on the fracture surfaces of the materials were performed. A three-layer structure, that is, a skin–core–skin layer structure, of the composites was observed for all the investigated composites. For the single short-fiber composites, the three-layer structure was the same as that reported previously. In the skin layers, the fibers lay parallel to the mold flow direction (MFD); in the core layer, the fibers were perpendicular to the MFD. However, for the hybrid short-fiber composites, the three-layer structure was different. In the skin layers, the fibers were aligned parallel to the MFD, but in the core layer, the fibers were aligned oblique to the MFD. The oblique fibers would result in a higher pullout toughness contribution to the hybrid composites. Moreover, for the single SCF/PP composite, the matrix in the core layer fractured in a brittle fashion. In contrast, for the hybrid SGF/SCF/PP composites, the matrix in the core layer fractured in a ductile manner, imparting an additional contribution to the total toughness of the hybrid composites. Consequently, the positive hybrid (synergistic) effect on the fracture toughness of hybrid SGF/SCF/PP composites could be explained.

Manufacture and evaluation of bioactive and biodegradable materials and scaffolds for tissue engineering

For tissue regeneration and tissue engineering applications, a number of bioactive and biodegradable composites, either porous or non-porous, were fabricated. The newly developed materials included tricalcium phosphate reinforced polyhydroxybutyrate and its copolymer, poorly crystallized hydroxyapatite reinforced chitin, and plasma sprayed hydroxyapatite reinforced poly(L-lactic acid). It was shown that these new materials could be successfully produced using the manufacturing techniques adopted. In vitro experiments revealed that the incorporation of bioceramic particles in biodegradable polymers rendered the composites bioactive and significantly improved the ability of composites to induce the formation of bone-like apatite on their surfaces. Degradation of composite scaffolds in simulated body fluid was observed and could be due to the simultaneous degradation of polymer matrix and dissolution of bioceramic particles.© 2001 Kluwer Academic Publishers

Analyses of the micromechanics of stress transfer in single- and multi-fiber pull-out tests

Analyses have been carried out on the micromechanics of elastic stress transfer taking place across the fiber/matrix interface in both single- and multi-fiber pull-out tests. A two-cylinder model for the single-fiber pull-out test and a three-cylinder model for the multi-fiber pull-out test were employed in order to study the fiber pull-out problems. The difference in the stress transfer between the two models is clearly shown. Moreover, since real multi-fiber composites are inhomogeneous, the local fiber volume fraction would undoubtedly influence the stress transfer between the pulled-out fiber and the neighbouring matrix. The effect of the local fiber volume fraction on the stress transfer is discussed. When the fibers in the composite medium cylinder of the three-cylinder model are short, a fiber length factor for the composite modulus is introduced and its effect on the stress transfer is discussed. Furthermore, special attention is given to how the neighboring and remote fibers affect stress transfer. The results show that the neighboring fibers play a major role in determining the stress transfer while the influence of the remote fibers on the stress transfer is negligible. In addition, the present model for the single-fiber pull-out test is compared with other existing theories.

Effects of heat treatment on the mechanical properties of Kevlar-29 fibre

Thermal aging treatments under both atmosphere and vacuum environments at temperatures of 100°C to 300°C for durations from 2 to 8 h have been made on Kevlar-29 yarns. The effects of thermal aging on tensile strength, tensile strain and the Youngs modulus of the single fibre have been evaluated. It was established that both the tensile strength and the tensile strain decreased with increase in treatment temperature. For treatment at constant temperature, the heating duration did not appear to have any effect on the tensile strength and the tensile strain. It was also found that the Youngs modulus of the single fibre was not affected by heat treatment under the conditions described above. Heat treatment in vacuum did not have any effect on the tensile strength.

Interfacial properties of fibre-reinforced composites

Factors which control the strength and toughness of a fibre-reinforced composite in terms of the role of interfacial parameters such as the interfacial shear strength, the interfacial coefficient of friction and the matrix shrinkage pressure are reviewed. Techniques for determining the interfacial parameters, the stress distribution along the embedded fibre, and the mode of failure of the pull-out tests utilized in these techniques are considered.

Synthesis of polyaniline nanotubes using the self-assembly behavior of vitamin C: a mechanistic study and application in electrochemical supercapacitors

Herein, we report the discovery of an unprecedented behavior of vitamin C to form a rod-like assembly through hydrogen-bonding in water, which, upon the addition of aniline monomer, produces polyaniline (PANI) nanotubes via the oxidative polymerization method. It was observed that tubular growth of PANI at the nanometer scale can be controlled by varying the molar ratio of vitamin C to aniline. At a molar ratio of 0.25 (i.e. [Vitamin C]/[Aniline] = 0.25), long and uniform nanotubes which extended to several micrometers with an outer diameter (OD) in the range of 80–120 nm were observed. We have also demonstrated the efficient electrochemical properties of novel PANI nanotube based electrodes which showed higher capacitance and energy density values of 714.68 and 99.34 W h kg−1 at 0.5 mA, respectively. The observed results were compared and justified with the theoretical capacitance value.