Effects of varying strain-rate on the behaviour of FRP yacht hull Structure under Slam Load Conditions

Abstract

The recent progress of the performance of sailing yachts has been driven by the continuing use and development of lightweight sandwich structures made of polymeric composite materials. So far the structural design of sailing yachts has relied on static or quasi-static approaches which usually lead to conservative design. Sailing yachts undergo several diverse dynamic loads in a seaway. Rigs and rigging, deck and hull have to be designed to withstand local and distributed loads whose entity is always difficult to determine. In this respect, the phenomenon of slamming, namely the impact of the hull bottom against the water surface in a rough sea, and its effects on the structure represent a crucial issue.

This implies that when structural optimisation is required it is necessary to better define the external loads and the strain-rate properties of the material utilized. With this in mind, this thesis explores the dynamic response of a FRP (fibre reinforced plastics) sandwich hull panel subject to slam loads.

This is achieved initially by investigating experimentally the dynamic properties of FRP under rates of strain typically experienced by sailing yacht structures. A systematic methodology is then proposed to describe the strain-rate behaviour of the material by LS-DYNA explicit finite element code. This methodology is subsequently applied to examine the response of a hull panel to a slam load. It is shown that the ALE (Arbitrary Lagrangian-Eulerian) method, within LSDYNA code, is suitable to model the fluid-structure interaction slam problem and
to assess the relative entity of the load to be used in the panel analysis. A static finite element analysis of the panel is also carried out based on standard design rules. Results are compared with the dynamic approach presented and the conservativeness of the static method is underpinned.

Developing the knowledge of both the dynamic properties of the materials and the use of tools such as explicit finite element codes is shown to be a valid approach to optimise the design of sailing structures under slam load.

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