Effect of stress concentrations in flexible matrix composite driveshafts

Flexible matrix composites (FMCs), consisting of a high volume fraction of strong, rigid fibers in a soft, flexible matrix, are attractive candidate materials for one-piece rotorcraft drivelines that can accommodate misalignment without the use of mid-span flex-joints. This paper reports on a preliminary experimental investigation of the effects of circular through-holes on the stiffness and strength of small-scale, filament-wound FMC shafts under axial compression and axial tension. Shaft with holes were also subjected to spin tests with misalignment from end-to-end. It is shown that the hole has a more deleterious effect on the axial compressive strength than on the axial tensile strength, but that the notch sensitivity of FMCs in general is significantly less than that seen in conventional rigid matrix composites. Based on x-ray radiographic inspection, it is shown that FMC shafts spun with representative flexural strains show no signs of damage growth near the hole. This information is a step towards the implementation of FMC materials in advanced rotorcraft drivelines.