This report presents an analysis of trends in fatigue results from the Montana State University program on the fatigue of composite materials for wind turbine blades for the period 2005-2009. Test data can be found in the SNL/MSU/DOE Fatigue of Composite Materials Database which is updated annually. This is the fifth report in this series, which summarizes progress of the overall program since its inception in 1989. The primary thrust of this program has been research and testing of a broad range of structural laminate materials of interest to blade structures. The report is focused on current types of infused and prepreg blade materials, either processed in-house or by industry partners. Trends in static and fatigue performance are analyzed for a range of materials, geometries and loading conditions. Materials include: sixteen resins of three general types, five epoxy based paste adhesives, fifteen reinforcing fabrics including three fiber types, three prepregs, many laminate lay-ups and process variations. Significant differences in static and fatigue performance and delamination resistance are quantified for particular materials and process conditions. When blades do fail, the likely cause is fatigue in the structural detail areas or at major flaws. The program is focused strongly on these issues in addition to standard laminates. Structural detail tests allow evaluation of various blade materials options in the context of more realistic representations of blade structure than do the standard test methods. Types of structural details addressed in this report include ply drops used in thickness tapering, and adhesive joints, each tested over a range of fatigue loading conditions. Ply drop studies were in two areas: (1) a combined experimental and finite element study of basic ply drop delamination parameters for glass and carbon prepreg laminates, and (2) the development of a complex structured resininfused coupon including ply drops, for comparison studies of various resins, fabrics and pry drop thicknesses. Adhesive joint tests using typical blade adhesives included both generic testing of materials parameters using a notched-lap-shear test geometry developed in this study, and also a series of simulated blade web joint geometries fabricated by an industry partner.
Delamination at ply drops in composites with thickness tapering has been a concern in applications of carbon fibers. This study explored the resistance to delamination under fatigue loading of carbon and glass fiber prepreg laminates with the same resin system, containing various ply drop geometries, and using thicker plies typical of wind turbine blades. Applied stress and strain levels to produce significant delamination at ply drops have been determined, and the experimental results correlated through finite element and analytical models. Carbon fiber laminates with ply drops, while performing adequately under static loads, delaminated in fatigue at low maximum strain levels except for the thinnest ply drops. The lower elastic modulus of the glass fiber laminates resulted in much higher strains to produce delamination for equivalent ply drop geometries. The results indicate that ply drops for carbon fibers should be much thinner than those commonly used for glass fibers in wind turbine blades.
This paper presents recently expanded test data for resin infused glass fiber laminates of interest for wind turbine blades. The new static and fatigue data extend and clarify trends reported in References 3-7 for the relative performance of epoxy, vinyl ester and polyester resins, and various unidirectional (UD) and biaxial (±45) fabrics, and multidirectional (MD) combinations, in standard laminate tests. Significant resin, fabric and process interactions are identified and explored. A second part of the study involves characterizing the performance of the various fabrics and resins in the context of a recently developed complex structured coupon geometry including ply drops. This coupon provides a simplified approach to exploring the relative performance of blade materials in the context of the complex structural details typical of infused blades. This testing approach highlights the significance of resin toughness differences in a representation which can conveniently be applied to blade design as strain knockdown factors.
New tensile fatigue test results are presented for infusion molded laminates, providing a comparison of several commercial E-glass reinforcing fabrics with epoxy resins over a range of fiber contents. Significant improvements in tensile fatigue resistance are demonstrated for some of the laminates relative to baseline materials, apparently depending on fabric architecture and stitching details. All stitched fabric laminates show a transition to lower fatigue resistance as the fiber content is increased, with the transition occurring at higher fiber content for the more fatigue resistant fabrics. The best fabric tested approached the performance of uniformly dispersed fiber prepreg molded laminates of similar construction. Differences in fatigue resistance appear to derive from the distortion and packing of fabric strands associated with increasing mold pressures, as demonstrated by finite element modeling and molding experiments at increasing pressures.
This study has explored static and fatigue crack growth in thick adhesive joints with fiberglass laminate adherends, for three adhesive systems with a broad range of G Ic values. Test methods include a relatively stiff non-symmetrical cracked lap shear (CLS) geometry as well as more conventional flexural geometries. The several versions of the CLS test geometry allow fully reversed and compression loading, in addition to tension. Flexural test geometries (DCB, MMB, and ENF) have been used to obtain static crack growth properties and as a baseline for comparison to the CLS test results, as well as for comparison to interlaminar growth in the adherends. Crack paths and damage characteristics have been explored using microscopy, for CLS and flexural geometries. Test results are presented for static and fatigue crack growth rates, the latter under tension-tension and reversed loading. Comparisons of the three adhesives are given in terms of crack growth characteristics, static G Ic and mixed mode fracture, and fatigue crack growth resistance.
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