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 S-N 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.

Compression Strength of Carbon Fiber Laminates Containing Flaws with Fiber Waviness

“…This is an example of extreme waviness unintentionally introduced during a resin infusion process. Waviness of the severity in Figure 4 would be expected to reduce strength properties by a factor of at least two to three relative to a laminate with straight 0° fibers [3,4]; less severe waviness as in woven fabrics can also cause serious reductions in strength properties. Misorientation is also produced in otherwise straight-fiber laminates, such as those made from prepreg, at details such as ply-drops [3].…”

“…load direction (0°) (Figure 2). The second factor may also affect tensile strength in severe cases [4,5]. Compared with typical aerospace prepreg laminates, blade prepregs are thicker, and blade fabrics tend to be both thicker and more heterogeneous, with strands of fiber either woven (usually over a small strand) or stitched together ( Figure 3).…”

“…Fiber misorientation due to ply waviness has been the subject of recent studies of blade materials [1][2][3][4][5] and earlier studies of aerospace prepreg laminates [5][6][7][8][9]. Waviness is expected to reduce compressive strength due to two factors: (1) the fibers may be put into a geometry which exacerbates the basic fiber, strand, or layer buckling mode of failure (Figure 1), and (2) the waviness shifts the fiber orientation off the axis of the ply longitudinal direction, producing matrix dominated failures for plies nominally orientated in the primary _______________ This research was supported by Sandia National Laboratories under subcontract BC7159.…”

“…Recent interest in blade materials has shown a significant shift toward carbon fiber and hybrids. Laminates with carbon fibers as the main, 0°, reinforcement are likely to be limited by their compressive properties [2][3][4]. Carbon laminates constructed from relatively low-cost forms of carbon fiber show compressive ultimate strains in the 1.0 to 1.2 % range when the fibers are nominally straight [3,4] (there is always some fiber misalignment within individual tows, which may be greater for larger tows [11]).…”

“…Carbon laminates constructed from relatively low-cost forms of carbon fiber show compressive ultimate strains in the 1.0 to 1.2 % range when the fibers are nominally straight [3,4] (there is always some fiber misalignment within individual tows, which may be greater for larger tows [11]). Laminates fabricated by infusion methods, using stitched or woven carbon fabrics, tend to have ultimate compressive strains in the 0.6 to 0.8 % range, which, with statistical treatment and safety factors, could limit blade designs [3].…”