Determination of mode I & II strain energy release rates in composite foam core sandwiches. An experimental study of the composite foam core interfacial fracture resistance

Abstract : The use of composite materials is on the rise in different engineering fields. Following this trend the wind turbine industry has adopted composites as their primary material of choice. For wind turbine blades having large unsupported functional aerodynamic surfaces; the structural stiffness is very important. Stiffness is required to keep the deformations to a minimum under aerodynamic forces. The blade is thus stiffened using sandwich structures at high strain locations within the structure. The lightweight foam cored sandwiches though add stiffness, at the same time pose a challenge for design as the difference in stiffness of both the face-plate and the foam core is very high. The resistance to fracture in any part of the structure is an important design parameter to be determined. The determination of fracture resistance quantified here as the Strain Energy Release Rate (SERR) poses some unique challenges when dealing with highly heterogeneous materials in terms of stiffness. In this study some approaches have been analyzed while others are developed to tackle this problem and to measure the Mode I & II SERR of the face-plate foam-core interface. The sandwich core varies in both thickness and density depending on the loading and thus the location along the blade length. However for this study we have used a single density of foam core for the most part of the turbine blade. Different thicknesses of the foam cores are used to determine the effect of scale on the calculated SERR.