From the NASA/DOE Mod OA wind energy research program came the finding that laminated wood/epoxy composite is a low-cost construction material that may withstand the long-term severe fatigue environment of the advanced wind turbine blade. Increased efficiency and reliability requirements placed on the next generation of wind machines will challenge designers, demanding their understanding of the mechanisms by which failures originate and propagate in such composites. Previous testing of wood/epoxy laminates has shown that the joints between the individual veneer sheets within the laminate act as failure initiation sites which limit their load-carrying capacity. The detrimental effect of these joints is most often felt in high-cycle fatigue, which is of key importance for reliable, longlife wind turbine components such as blades. The study of how a material behaves under repeated stress around the site of a well-defined, built-in defect is a traditional method in materials science for gaining insight into fatigue-related processes. The Phase I research advanced this technique through implementation of a most highly integrated fatigue test matrix. This unique matrix allows analysis to focus solely on the effects of well-defined sample defects in the form of several technicauy feasible veneer splice joint styles. The Phase II research will include aspects of micromechanics in work aimed at identifying, incorporating, and proving those joint details which are desirable for substantially improving the fatigue life of bulk wood/epoxy composite. The Phase II effort will also explore the static and fatigue strength of bulk wood/epoxy laminate, augmented in the grain direction by the presence of graphite fibers between the veneer layers, and the role of the veneer joint defects in limiting strength in this augmented larranate. There will be a parallel study to gain a fundamental knowledge of the role played by moisture in the high-cycle fatigue damage accumulation process as well as separate groundwork studies of base laminate properties in two important secondary loading modes: radial crossgrain tension fatigue and rolling shear fatigue.Anticapated Results Potential Commercial Applications as described by the awardee:The Phase I research laid the groundwork towards realization of the full structural potential of wood/epoxy laminate material. Phase II work is organized to take maximum advantage of the Phase I findings in the development of new, superior joint styles and laminate reinforcing techniques. Inteuigent optimization and exploitation of wood/epoxy composite could culminate in a significant increase in power generating efficiency through lighter, larger, yet lower cost wind turbines.
