Showing papers by "Kin-tak Lau published in 2004"

Low velocity impact on shape memory alloy stitched composite plates

Abstract: Delamination of advanced composite materials due to various scenarios such as low velocity and ballistic impacts and high strain rate is one of the major problems for aerospace and automotive structural applications. The low velocity impact does not immediately induce any visible damage on the surface of structures whilst the stiffness and compressive strength of the structures decrease dramatically. Shape memory alloy (SMA) materials possess many unique mechanical, thermal and thermal–mechanical properties compared with other conventional materials. Many studies have reported that the superelastic and hysteresis properties of the SMA materials can absorb energies coming from external excitations or sudden impacts. Stitching is well recognized as a promising technique to enhance the through-the-thickness reinforcement, in order to improve the delamination properties of composite structures. By stitching SMA wires into the composite structures one is theoretically able to reduce the risk of delamination of the structures during impact. In this paper, the damage resistance properties of SMA stitched glass/epoxy composites after low velocity impact are experimentally and theoretically studied. The results show that the tensile strength of composite plates increased and the number of translaminar cracks decreased after being stitched by SMA wires. Theoretical study also proves that the delamination energy of composite plates after stitching by superelastic SMA wires is smaller than that of an unstitched composite plate because of the energy absorbed by the SMA wires.

Formation and Growth Mechanism of Dissimilar Coiled Carbon Nanotubes by Reduced-Pressure Catalytic Chemical Vapor Deposition

Abstract: Dissimilar coiled carbon nanotubes were prepared by catalytic chemical vapor deposition (CCVD) on finely divided Co nanoparticles supported on silica gel under reduced pressure and at lower gas flow rates. The morphology of the regular coiled carbon nanotubes were examined by TEM, while the polygonization characteristics of the helix were examined by SAED. Observations were made on other forms of irregular coils with various shapes by TEM. On the basis of the heptagon-pentagon construction theory, we proposed a helix formation mechanism, which involves a carbon core formation centering on a catalytic particle followed by carbon helices growth controlled by kinetics.

Impedance spectroscopy studies on doped polyanilines

Abstract: Impedance spectroscopy studies on polyaniline doped by various amounts of HCl are reported. The measurements were performed at room temperature, over a wide range of frequencies [1000 Hz to 45 MHz]. The experimental data shows that the resistance dominates the AC behavior of polyaniline confirming that the charge transport occurs through an one-dimensional hopping process.

Electron Spin Resonance Studies on Electron Beam Irradiated Carbon Nanotubes Dispersed in Styrene-Isoprene-Styrene Block Copolymer

Abstract: Electron spin resonance investigations on the effect of electron bombardment of nanocomposites obtained by dispersing carbon nanotubes within styrene-isoprene-styrene are reported. The experimental results revealed the absence of radiation-induced free radicals and a negligible effect of electron beam irradiation on the electron spin resonance spectra of carbon nanotubes.

Conducting and Antistatic Composites for Space Applications

Abstract: The percolative dependence of the DC conductivity on the volume concentration of fillers for composites obtained by dispersing conducting particles into polymeric matrices is studied in detail. An empirical Boltzmann like dependence is proposed for the modeling of the dependence of DC conductivity versus filler concentration. This expression allows for a more accurate determination of the percolation threshold in the case of broad percolations. It is shown that the loading of the polymeric matrices with conducting fillers produces percolative-like changes of various physical properties (such as the reciprocal of the tensile strength and the reciprocal of the double integral of the resonance spectrum). Experimental mechanical, electrical, and electron spin resonance data on polyvinylchloride-carbon, polyvinylchloride-polyaniline, and polyethylene-polyaniline composites are reported.