New Fatigue Data for Wind Turbine Blade Materials

Abstract

This paper reports on recent fatigue data of interest to the wind turbine industry in several areas: (a) very high-cycle fatigue data; (b) refined Goodman Diagram; (c) effects of fiber waviness; and (d) large-tow carbon fibers. Tensile fatigue results from a specialized high-frequency small strand testing facility have been carried out to 1010 cycles in some cases, beyond the expected cycle range for turbines. While the data cannot be used directly in design due to the specialized test specimen, the data trends help to clarify the proper models for extrapolating from standard coupons to higher cycles. The results for various fiber and matrix systems also provide insight into basic failure mechanisms. For spectrum loading predictions, a more detailed Goodman Diagram has been developed with additional R-values (R is the ratio of minimum to maximum stress in a cycle). The data of greatest interest were obtained for tensile fatigue with low cyclic amplitudes, close to R=1.0, to clarify the shape of the diagram as the cyclic amplitude approaches zero. These data may significantly shorten lifetime predictions compared with traditional Goodman Diagram constructions based on more limited data. The effects of material/process induced flaws on properties continues to be a major concern, particularly with large-tow carbon fabrics. The results of a study of fiber waviness effects on compressive strength show significant strength reductions for severe waviness which can be introduced in resin infusion processes. The final section presents new fatigue results for large-tow carbon/fiberglass hybrid composites. Epoxy resin laminates show marginally higher compressive strength and fatigue resistance with carbon fibers. Improved compressive static and fatigue performance is found with stitched fabrics as compared with woven fabrics.