Papers by Author: V.P.W. Shim

Abstract: AZ31-based magnesium nanocomposites were produced by a disintegrated melt deposition technique, whereby different volume fractions of 50-nm Al₂O₃ nanoparticles (1.0v%, 1.4v% and 3.0v%) were used as reinforcement and added to AZ31 Mg alloy. A monolithic counterpart was also produced by the same process for comparison. Samples of these materials were subjected to dynamic tension at strain rates up to 1.2 10³ s^-1, using a split-Hopkinson Bar device. Compared to the quasi-static response, the monolithic and composite materials showed significantly increased yield stress and ductility under dynamic loading. The enhancement in yield stress with strain rate indicates rate sensitivity of the critical resolved shear stress for slip systems under tension. The addition of nanoparticles was found to reduce the grain size of the resulting material and increase the yield stress and ductility simultaneously, for both low and high rate loading.

Abstract: An experimental investigation into projectile penetration of pre-tensioned woven fabric samples – Tawron^® CT 717 (plain weave) – is undertaken. Specimens are clamped at their top and bottom edges, and pre-stretched by forces of 200N, 1000N, 2000N, 3000N and 4000N respectively. They are then oriented at various angles (0, 7.5°, 15° and 30°) from the vertical (perpendicular to the horizontal axis of a gas gun), and subjected to impact by a 12mm diameter steel sphere. A high-speed camera is utilized to observe the deformation and failure of the fabric, and to determine the residual projectile velocity. For normal incidence, the ballistic limit initially increases, then decreases with pre-tension, displaying a maximum resistance for a pre-tension of about 2000N. With respect to the angle of incidence (obliquity of impact), for target inclinations of 7.5° and 15°, pre-tension has minimal influence on the ballistic limit, while for an inclination of 30°, the effect of pre-tension on the ballistic limit is similar to that for normal impact.

Abstract: The deformation/failure modes and dynamic response of fully clamped cylindrical sandwich shells with aluminum foam cores subjected to air blast loading were investigated experimentally. A four-cable ballistic pendulum system was employed to measure the impulse imparted to the blast-loaded specimen. The deformation/failure modes of specimens were classified and analyzed, the effects of face-sheet thickness, core relative density, specimen curvature and mass of charge on the structural response of metallic sandwich shells were examined. Experimental results indicate that both the deformation/failure modes and the dynamic response of the sandwich shells are sensitive to the structural configurations and blast impulse. The experimental results are useful for validating theoretical predictions, as well as in engineering applications of cellular metal sandwich structures.

Abstract: Silica-nylon6 composites were fabricated using two types of silica nanoparticle fillers with different surface modifications. Type A particles (treated with hexamethyldisilazane) were uniformly dispersed but only displayed weak long-range interaction with the nylon6 matrix; in contrast, type B particles (modified with 3-aminopropyltriethoxysilane) formed covalent bonds with the nylon6 chains but their dispersion is not good. The silica-nylon6 composites synthesized were subjected to quasi-static and dynamic tension to study the effects of strain rate and nanoparticle fraction. Results show that compared to quasi-static loading, both pure nylon6 and the composites exhibit a higher strength but lower ductility under dynamic loading. With respect to the influence of the nanoparticles, both particle types cause an increase in the elastic modulus and tensile strength. The effect of the two particles on ductility differs – particle A reduces ductility, while particle B decreases ductility under quasi-static loading but enhances it noticeably for dynamic loading. Particle B enhances the mechanical properties more significantly, especially in terms of ductility. These results suggest that ensuring strong particle-matrix bonding is more crucial than good particle dispersion.

Abstract: In this study, a Mg-6wt%Al alloy and its composite containing 0.22vol% Al₂O₃ nanoparticles are fabricated using a disintegrated melt deposition technique, and samples are subjected to quasi-static and dynamic tension. Compared to quasi-static loading, both materials exhibit significantly higher yield stresses and tensile strengths, much better ductility, and thus a higher energy absorption capacity under dynamic tension. In terms of nanoparticle addition, its influence on the mechanical properties are not notable; enhancement of the elastic modulus, yield stress and tensile strength are negligible, and there is a small reduction in ductility. The tensile behaviour obtained in this investigation was compared with results of previous compression tests, and significant tension-compression asymmetry in the response is observed. The tensile yield stress is noticeably larger than that in compression, and the profile of the stress-strain curve for tension differs from that for compression – it is convex upwards for tension, but concave upwards for compression. A possible reason for this asymmetry is the occurrence of twinning in compression and its absence in tension.

Abstract: A fracture criterion for rigid polyurethane foam is developed based on idealization of constituent cells by elongated tetrakaidecahedra. The ability of the proposed geometrical model to mimic the fracture characteristics of actual rigid polyurethane foam is examined and a fracture criterion derived analytically. In tandem, the fracture properties of an actual rigid polyurethane foam are obtained from mechanical tests. The fracture criterion based on the model exhibits correspondence with the behavior of actual foam. Consequently, this model constitutes a suitable basis for further investigation into the mechanical properties of actual polymeric foams.