Long term behaviour of FRP structural foam cored sandwich beams

The work is concerned with the investigation of the long term flexural behaviour of structural FRP foam cored sandwich beams due to sustained constant (creep) and cyclic (fatigue) loads. An extensive review of both experimental and theoretical aspects has highlighted many uncertainties in long term behaviour that need to quantified for sandwich structure design to be further optimised. An extensive testing programme has been carried out and two main failure modes, core shear and skin tensile/compressive failure have been identified. It has been demonstrated that the fatigue performance of sandwich beams can be characterised in terms of the component skin and core behaviour. Using this approach, an S-N 'master' curve for sandwich core shear failures can be created, whilst skin failures can be characterised from raw skin coupon fatigue tests.Under creep loads, global beam behaviour can be attributed purely to core shear mechanisms. A compatible modulus (stiffness) degradation approach for both creep and fatigue has been proposed which allows the separation of creep and fatigue components from a waveform where the elements of creep and fatigue are intimately mixed. By separating these components, a better understanding of how 'pure' fatigue damage accumulates is gained. A study of creep-fatigue interaction is also outlined. The sandwiched beam data have finally been used in context of the design of a lifeboat with particular reference to the fatigue limit-state.