Fatigue Life Prediction of GFRPs under Random Ocean Current Loading

Abstract

A comprehensive fatigue characterization of Glass Fiber Reinforced Plastics (GFRPs) under random ocean current loading is performed. A procedure to calculate random loading from the time history data of ocean current velocity is described. Two sources of randomness were included in the calculation, namely, i) turbulence in ocean current and ii) wake flow behind the support structure of the Ocean Current Turbine (OCT). The random ocean current velocity was first converted into hydrodynamic load and subsequently to tensile stress of the OCT blade using Blade Element Momentum (BEM) theory and Finite Element (FE) method. The random ocean current loading had high stress ratio (R), which was inherently different from that of the wind loading. The random ocean current loading was applied to GFRP coupons using a materials testing machine. A code was developed with LabView and was interfaced with the testing machine to implement random loading. Fatigue experiments were then conducted under this random loading. Experimental results indicated that fatigue life increased exponentially with the decrease in mean velocity. Fatigue life of GFRP was also predicted by using Constant Life Diagram (CLD) and Palmgren-Miner’s rule. CLD was developed from S-N curves of various stress ratios between R = 0.1 and 0.8. A good agreement was observed between the experimental and the predicted fatigue lives when the mean velocity was high. Comparing with traditional Goodman diagram, it was found that results obtained from CLD prediction were more conservative. Details of random load calculations, fatigue experiments, and life prediction methodologies are described in the paper.