Abstract
Composite materials are being considered for marine energy structures for their potential low weight and durability benefits. However, composites are susceptible to moisture intrusion when submerged, which can degrade their mechanical properties. Combining moisture intrusion with repeated loading from currents and waves could further diminish those properties. In previous studies, water absorption testing was followed by fatigue testing, which may not represent in-service conditions— when submerged composites are loaded, water may enter open fatigue cracks, and when the composites are unloaded, the water could further propagate cracks.
To better represent in-service conditions, a submerged fatigue test fixture is being developed. Prior to designing the test fixture, a composite beam was designed that met these design criteria: (1) has a load scenario like a tidal blade, (2) has evenly distributed stresses, (3) fails in the gauge section, (4) withstands tidal turbine blade design loads, and (5) can be tested in submerged fatigue conditions.
Stress value comparisons of tapered beam geometries with an isotropic material were performed. In addition to comparing beam geometries, comparisons were made between Ansys finite element models and cross-section analyses in MATLAB. The selected beam was a quadratically tapered-to-constant I-beam with a taper that ends at the mid-span. The composite beam was then developed in Ansys Composite PrepPost using the base geometry. Eigenvalue buckling load, Tsai-Wu failure indices, and strain values were measured in Ansys to inform composite beam failure load, location, and strain value reasonability, respectively. The eigenvalue buckling load was 33% higher than the design load. Tsai-Wu failure indices indicated the beam will likely fail in the gauge section. Strain values were 20% higher than those recommended in a wind turbine blade standard and 127% higher than those in the tidal blade of interest. Overall, all five design criteria were met. The composite beam designed in this study will be manufactured and tested in submerged fatigue conditions. Results from testing will be used to estimate the durability of composite marine energy structures under in-service conditions.