Prediction of the flexural modulus of fibre reinforced thermoplastics for use as kayak paddle blades

It can be shown that there is a strong link between paddler performance and paddle stiffness. Despite the fact that composite materials are often chosen to enable design of more rigid sporting equipment, readily available flexural modulus values for composites other than proprietary blends are not easily obtained. Therefore, when composition deviates from proprietary blends, accurate prediction of material properties becomes necessary. A mathematical model for predicting the flexural modulus of short fibre reinforced composite materials is developed based on the application of simple beam theory. The flexural modulus can be modelled by considering a small section of the composite comprising a finite number of polymer and reinforcement layers. Simple beam theory assumes that there is perfect bonding between these layers, but it is well known that interfacial adhesion plays a significant role in composite properties. To account for the interfacial layer and interfacial bonding the second moment of area of the composite beam element was modified by assuming that the contribution to the second moment of area from the matrix layer is reduced by an amount representing the nonbonded interfacial layer. The flexural modulus values obtained from the model are compared to experimental values for glass-fibre reinforced linear low density polyethylene. It has been found that the use of non-contact regions in the model resulted in improved accuracy over the model with perfect bonding for short fibre reinforced LLDPE.