Showing papers by "Kin-tak Lau published in 2016"

Mechanical and flammability characterisations of biochar/polypropylene biocomposites

Abstract: Biocomposites were manufactured with biochar and polypropylene at five loading levels (0, 15, 25, 30, and 35 wt%) by compounding and injection moulding. Biocomposites were tested by tension, bending, cone calorimeter, thermogravimetry, differential scanning calorimetry, X–ray diffraction, and infrared spectroscopy. Incorporation of increasing amount of biochar to neat polypropylene continuously improved its tensile modulus and flexural strength/modulus. The peak heat release rate and smoke production of the biocomposites were significantly reduced as a result of biochar addition. The high surface area of biochar allowed polypropylene to flow in creating a mechanical interlocking and improving the mechanical properties. The thermally stable biochar provided a compact char structure during combustion which prevented the heat and mass transfer between the polypropylene and the ambient O2. Thermal stability of polypropylene was increased as a result of biochar inclusion, as observed in thermogravimetry.

Impact properties of glass fiber/epoxy composites at cryogenic environment

Abstract: Glass fiber/epoxy polymer (GFRP) composites are widely used in the aerospace engineering industry due to their high specific strength to weight ratio with non-corrosive properties, however they are very sensitive to foreign object impact. This study aims at investigating the impact response of GFRP composites at different temperature environments, which are encountered in their service at high attitude and low earth orbit (LEO) conditions. 18 GFRP samples were fabricated by vacuum infusion process and 9 of them were post-cured at 353 K for 3 h to ensure that a complete chemical reaction inside the samples was achieved. Low velocity drop weight test was performed for the samples prepared and then stored at room temperature (295 K), dry ice temperature (199 K) and liquid nitrogen temperature (100 K) conditions. The apparent damages and their size were visually examined and measured. Impact parameters such as impact load, deflection and energy absorption of each damage type were also analyzed. Besides, the post-curing effect was also studied to verify its significance to the impact properties of composites. Experimental results showed that GFRP composites at cryogenic condition exhibited smaller apparent damage and were stiffer as compared with other cases. However, they demonstrated relatively poor energy absorbability at low temperature condition. It was also found that post-curing could reduce the apparent damage and increase the energy absorption of GFRP composites.

Mechanical properties of an epoxy-based adhesive under high strain rate loadings at low temperature environment

Abstract: The tensile properties of an epoxy-based adhesive subject to high strain rate loadings at low temperatures were studied. Tensile split Hopkinson bar incorporating with an environmental chamber was utilized to conduct tensile property tests. Mechanical Parameters, like strength, Young's Modulus, strain to failure and fracture energy at low temperatures (−20°C and −40 °C) with different strain rates were investigated. The results show that at room temperature (RT), the strength and stiffness both increase significantly with the increase of strain rate. However, the increment of strength at low temperatures is insignificant. Interestingly, at RT and −20 °C, strain to failure increase with the increase of strain rates. From the stress-strain curves, strain softening phenomenon at high strain rates are observed at RT and −20 °C.This could be explained by the localized temperature rise in the specimens when the strain rate reaches high. At −40 °C, strain to failure decreases with the increase of strain rate and strain softening disappears. The larger temperature difference between the environment and the localized temperature rise in the specimens may reduce the strain softening phenomenon. Effects of high strain rate on tensile properties of epoxy adhesive at low temperatures are different from that on polyurethane-based adhesive in the literature.

Experimental and theoretical studies on the properties of injection moulded glass fibre reinforced mixed plastics composites

Abstract: Recycled mixed post-consumer and post-industrial plastic wastes consisting of HDPE, LDPE and PP were injection moulded with short glass fibre (10–30% by weight) to produce a new generation composite materials. Intensive experimental studies were then performed to characterise the tensile, compression and flexural properties of glass fibre reinforced mixed plastics composites. With the addition of 30 wt.% of glass fibre, the strength properties and elastic modulus increased by as much as 141% and 357%, respectively. The best improvement is seen in the flexural properties due to the better orientation of the glass fibres in the longitudinal direction at the outer layers. The randomness and length of the glass fibre were accounted to modify the existing rule of mixture and fibre model analysis to reliably predict the elastic and strength properties of glass fibre reinforced mixed plastics composites.

Tensile properties of a polymer-based adhesive at low temperature with different strain rates

Abstract: The tensile properties of a polymer-based adhesive subject to different strain rates at low temperature were investigated. Experimental results showed that the strength of adhesive increased remarkably with the increase of strain rate and decrease of temperature. According to the results, the strength of adhesive at low temperature (−40 °C) and high strain rate increases as compared with that at room temperature and the same strain rate. The effect of strain rate and low temperature on strength of adhesive is not simply superimposed. Meanwhile the coupling effects on strain rate and low temperature to the tensile strength, failure strain, Young's modulus, and fracture energy are also discussed in this paper. A constitutive model of adhesive at high strain rate and low temperature is proposed.

Enhancement of interfacial bonding strength of SMA smart composites by using mechanical indented method

Abstract: Shape memory alloy (SMA) wire reinforced polymer-based composites have been recognized as a new type of smart materials. However, their weak interfacial bonding strength between the wire and its surrounding matrix limits its applications in many engineering industries. In this paper, a new method called “mechanical indentation” is introduced to effectively enhance the interfacial bonding strength of SMA composites. According to the results from a wire pullout test, the interfacial bonding strength of an indented SMA wire composite increased by 4.48–8.58 times as compared with a hand-sanded SMA wire composite at a room temperature condition. Two different sizes of indentations, sparse and dense indentations were studied in detail and the results indicated that the dense indented SMA wire had a better performance in terms of tensile and recovery properties, than that of the sparse indented wire. Furthermore, the interface between the dense indented wire and vinyl-ester (VE) matrix demonstrated a better restoration performance during a repeated loading-and-unloading process at room temperature condition.

Stress transfer properties of carbon nanotube reinforced polymer composites at low temperature environments

Abstract: Multi-scale modeling method was used to study tensile properties of single-walled carbon nanotube (SWCNT) and double-walled carbon nanotube (DWCNT) reinforced polymer-based composites at room temperature (RT) and cryogenic temperature (like liquid nitrogen temperature 77 K) conditions. At RT, Young's Modulus of a SWCNT reinforced polymer composite is 1.88 times of a DWCNT reinforced polymer composite due to a weak interfacial bonding between layers of DWCNT, which is bound by the weak Van der Waal interaction. The Young's Modulus of DWCNT reinforced polymer composite is significantly improved with the decrease of temperature and the difference in Young's Modulus between SWCNT and DWCNT reinforced polymer composites has been largely reduced. The radial stress imposed on an outer surface of nanotubes due to the contraction of polymer at liquid nitrogen environment is examined to explain the stress transfer mechanism in a composite system. The extra stresses imposed on the surface of DWCNT increase bonding forces between outer and inner layers of DWCNT. This tendency agreed well with experimental results found in other literature.

Woven hemp fabric reinforced vinyl ester composite: effect of water absorption on the mechanical properties degradation

Abstract: Natural fibre composites are potentially used in manufacturing industries and for that purpose, their water absorption properties are essential to be analysed. In this study, untreated and fire retardant chemical (FR) treated woven hemp fabrics (WHF) were used to reinforce vinyl ester resin. Water absorption tests were utilised to study the behaviour of water penetration into the composites. The diffusion coefficient were measured using Fick’s law while degradations of their mechanical properties were measured after 0, 168, 840 and 2688 hours of water immersion. It was found that the maximum water uptakes, times to saturation point and diffusion coefficient for fabricated composites were ranging 3.27-3.43%, 552-1848 hours and 1.45E-06 - 4.71E-06 mm2/s respectively. In terms of mechanical properties, the presence of water reduced the tensile strength and modulus up to 24% and 39% respectively due to the penetration of water which weakened the adhesion between the fibres and resin. The greater decline in tensile modulus than tensile strength due to the plasticisation of water on the vinyl ester and hemp fibres. Similar reasons can be applicable to the flexural properties for both samples since they show quite similar behaviour with tensile properties.