Mast Design Performance - Influence of the Non-Linear Elastic Behaviour of Carbon Fibres on the Accuracy of Fluid/Structure Interaction Models for Estimating Mast Bending

Abstract

_ The masts of racing yachts are generally made of carbon fibre reinforced plastics (CFRP) and are subject to two main loads: dock tuning and the forces exerted by the sails. Both result in compression and bending of the mast. The mast design must ensure that for any given sailing configuration, the mast deflection is matched to the sail design to maximise the boat’s performance. Fluid/structure interaction models are generally employed by sail designers who use a stick model for the structural definition of the mast. The elastic properties of the CFRP plies involved in mast lamination are assumed to be constant in this latter model. However, stiffness increases under tension and decreases under compression (up to 30% at failure depending on the fibre type, standard modulus, intermediate modulus, etc) due to some reversible changes in the microstructure of the carbon fibres. As a result, the actual curvature of the mast can be very different from that calculated with the stick model, leading to a significant error in the actual shape of the sail during sailing. In this work, we use a series of three critical loading cases corresponding to those of an IMOCA mast. The influence of the elastic non-linearity of the CFRP plies on the performance of the racing yacht mast is quantified. It is concluded clearly that this elastic non-linear behaviour of carbon fibres is to be accounted for by sails designers to optimise performance. Such an influence is expected to be even more pronounced on spreader masts such as those used for America’s Cup. Keywords Yacht design; IMOCA mast; Carbon fibre composites; Non-linear elasticity