On the mechanical properties of bast fibre reinforced thermosetting polymer matrix composites
Bast fibre reinforced, unsaturated polyester matrix composites were fabricated using non-woven mats of hemp or jute fibre as reinforcement. Composites were also prepared using chopped strand mat glass fibre as reinforcement. The short-term mechanical properties of the laminates were assessed. It was observed that at equivalent fibre volume fractions the stiffness of the glass fibre reinforced material only marginally exceeded that of the two, unmodified bast fibre, reinforced materials. At equivalent fibre volume fractions, however, the strength of the glass fibre reinforced composite was found to be significantly greater than that of the bast fibre reinforced materials. It was noted that in the bast fibre reinforced composites, the onset of non-linear behaviour occurred at relatively low applied stresses. Work of fracture in static three-point flexure and Charpy impact strength tests, indicated that the toughness of the plant fibre reinforced material was as much as an order of magnitude less than that of the glass fibre reinforced material. Fracture mechanics techniques were used to further quantify toughness and confirmed this to be so. Furthermore, these tests indicated that the microstructure of the bast fibre reinforced material should be examined more closely. Microscopy conducted on the fibres revealed that these were often subject to extensive micro-compressive damage. It was postulated that uneven fibre straining characteristics could lead to compromised interfacial properties, which might in turn detrimentally affect the macroscopic behaviour of the composite. A technique known as half fringe photoelasticity was used to investigate the stress-field in the matrix surrounding the fibre defects. It was observed that not only did concentrations of stress occur in the vicinity of these, but also that the shear stress distribution along the length of the fibre was interrupted by the presence of the defects. The implications of fibre defects upon composite properties are discussed.