Flexural and shear properties of unidirectional pultruded composites

Abstract

Three point bend tests were performed on pultruded half-round cross-section rods of glass fibre-reinforced polyester and glass fibre-reinforced epoxy, the tests being arranged so that the specimen was unloaded as soon as the deflection corresponding to the maximum load was reached. This testing method enabled the fracture mechanisms at maximum loads to be mapped as a function of the loading rate and the span-to-depth ratios. An effect highlighted by this map is that a specimen tested at intermediate span-to-depth ratios may result in a purely tensile fracture at low loading rates and a purely shear fracture at high loading rates. This behaviour is interpreted as being due to the fact that the increase in loading rate increases the brittleness of the material, subsequently increasing the defect sensitivity and leading to a shear fracture. This explanation is supported by fracture surface observations. In addition, it was found that the shear strength increases significantly with increasing load rate up to 5000 mm min −1 and then starts to decrease slightly. The decrease in shear strength at very high loading rates was found to correspond to multiple cracking behaviour rather than to a longitudinal fracture plane as for intermediate loading rates.