The impact toughness of discontinuous boron-reinforced epoxy composites

Previous theories for the impact strength of discontinuously-reinforced composites predict that the toughness is a maximum when critical transfer length fibres are used. Experiments utilizing mini-Charpy specimens of unidirectional boron-fibre-reinforced epoxy composites have been conducted which corroborate this prediction. However, calculations of the fracture energy, based on a uniform interfacial shear stress during fibre pull-out, proved inadequate for the reinforced epoxy composites. Revisions to existing theories are presented to take into account the non-uniformity of the interfacial shear stress distribution along the fibre length and catastrophic failure of the interfacial bond.Nomenclature A _f fibre cross-sectional area - E _f fibre Young's modulus - G _m matrix shear modulus - l fibre length - L fibre pull-out length - l _c fibre critical length - r fibre radius - R half fibre centre-to-centre spacing - V _f fibre volume fraction - W mean work of fracture per unit area of specimen cross-section - x distance from fibre end - y dummy variable of integration - surface energy - strain in composite - tensile stress on fibre - _f fibre fracture strength - interfacial shear stress