Effect of material design parameters on the forced vibration response of composite hydrofoils in air and in water

The ability to tailor the properties of structures made from composite materials gives designers new ways to improve their functionality. For example, the layup of a composite can be designed in such a way that elastic (e.g. bendtwist) couplings advantageously control how the shape and vibration characteristics of the structure changes under load. This ability to tailor composite materials is increasingly being used in marine propeller applications to improve their performance and control their vibration characteristics. However, designers face additional challenges when using these materials. The density of composites approaches that of water so fluid inertial effects become equal to or even greater than solid inertial effects. Moreover, the increased compliance of adaptive composite structures means that they can no longer be considered as approximately rigid, and flow-induced vibrations may develop. This means that load-dependent fluid-structure interaction effects must be understood to enable design optimisation and to avoid unwanted hydroelastic instabilities during operation. In this paper, the forced vibration behaviour of composite hydrofoils designed to have different bending stiffness and bendtwist coupling behaviour were experimentally measured in air and in water tests. Arrays of fibre Bragg gratings (FBG) bonded to the surface of the hydrofoils recorded their dynamic strain response. The effect of added mass on the hydrofoils vibration behaviour is discussed.