Enhancing Debond Fracture Toughness of Sandwich Composites for Marine Current Turbine Blades

Sandwich composites for marine current turbine blades were investigated. Sandwich with composite face-sheet and polymeric foam core is of particular interest. Foam as core materials can provide high stiffness and buckling strength to the blade which is required under water where the density is 800 times higher than air. In addition to stiffness and strength, simplicity in manufacturing, self-buoyancy, and superior fatigue performance of sandwich composites are other advantages. However, in sandwich composites, two failure modes are dominant; one is core shear, and the other is face-core debonding. To assess this debonding, mode-I fracture toughness, also known as debond-fracture toughness was determined. The sandwich composite was made of carbon/epoxy face sheet with syntactic and polyurethane foam cores. In order to improve upon the fracture toughness, chopped strand mat (CSM) were inserted at the face-core interface. ASTM D5528-01 test method was used in single cantilever beam (SCB) configuration to determine the fracture toughness. Sandwich composites with a syntactic and polyurethane foam core were found to have an average debond toughness of 177 J/m2 and 175 J/m2, respectively. After CSM was introduced at the interface, toughness increased by about 14% in both cases. Details of sandwich construction, their characterization, and source of improvement in fracture toughness is described in the paper.