Pancasatya Agastra

Analysis of SNL/MSU/DOE fatigue database trends for wind turbine blade materials.

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 resin-infused 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 Thick Ply Drops in Carbon and Glass Fiber Laminates Under Fatigue Loading

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.

Mixed Mode Static and Fatigue Crack Growth in Wind Blade Paste Adhesives

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 GIc 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 GIc and mixed mode fracture, and fatigue crack growth resistance.

Fatigue Trends for Wind Blade Infusion Resins and Fabrics

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.

Effects of Glass Fabric and Laminate Constructions on the Fatigue of Resin Infused Blade Materials

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.

Delamination and Failure at Ply Drops in Carbon Filter Laminates Under Static and Fatigue Loading

Delamination at ply drops in composites with thickness tapering has been a major concern in aerospace applications of carbon fibers, where the plies are typically very thin. This study explored the resistance to delamination in fatigue of hybrid carbon fiber and glass fiber prepreg laminates containing various ply drop geometries, and using thicker plies typical of wind turbine blades. Strain levels to produce significant delamination at both carbon and glass fiber ply drops were determined and compared in terms of a simple delamination model. The carbon fiber laminates with ply drops, while performing reasonably well under static loads, delaminated in fatigue at low maximum strain levels except for the thinnest ply drops. The lower elastic modulus and higher interlaminar toughness of the glass fiber prepreg resulted in much higher strains to produce delamination at equivalent ply drops, compared with carbon fiber prepreg using the same resin system. The results indicate that the thickness of ply drops with carbon fibers should be much less than those commonly used for glass fibers.

Fatigue Results and Analysis for Thick Adhesive Notched Lap Shear Test

Three test methods have been developed for thick paste adhesives typical of wind turbine blades, all adaptations from studies reported in the adhesives literature. Each test can provide important data for thick paste adhesive joints under a full range of loading conditions experienced in the wind turbine blade application. The notched lap shear test results for static loading explore a variety of parameters including: batch to batch variation, loading rate, laminate adherend peel plies, overlap length, adhesive thickness and applied load direction (tension vs. compression). The effects found are largely resolved by finite element analysis using a maximum local tensile strain failure criterion based on neat adhesive tests. Six paste adhesives were tested, showing a range of notched lap shear strength. Fatigue data were obtained for three loading conditions (tension, reversed loading and compression) with two blade adhesives, one relatively brittle and the other relatively tough. Fatigue results were similar for the two adhesives despite their toughness difference, which was evident in the crack propagation phase of the lifetime. Two fracture mechanics based test methods were studied with artificial starter cracks. The mixed mode cracked lap shear test was run under reversed loading to obtain fatigue crack growth data for two adhesives; the data were separated by the GI/GII ratio. The cracks for both adhesives propagated inside the laminate surface, along the top of the reinforcement, with the exception at higher growth rates of some growth inside the adhesive, next to the interface, for the tougher adhesive. Mode I DCB tests produced crack growth down the adhesive mid-thickness for the three adhesives tested. The static GIc values ranged from a low of 581 J/m 2 to 1626 J/m 2 for the three adhesives studied.

Analysis of SNL/MSU/DOE Fatigue Database Trends for Wind Turbine Blade Materials 2010-2015.

Wind turbine blades are designed to several major structural conditions, including tip deflection, strength and buckling during severe loading, as well as very high numbers of fatigue cycles and various service environments. The MSU Database Program has, since 1989, addressed the broad range of properties needed for current and potential blade materials through static and fatigue testing and test development in cooperation with Sandia National Laboratories and wind industry and supplier partners. This report is the latest in a series, giving test results and analysis for the period 2010-2015. Program data are compiled in a public database [1] and other reports and publications given in the cited references. The report begins with an executive summary and introductory material including background discussion of previous related studies. Section 3 describes experimental methods including processing, test methods, instrumentation and test development. Section 4 provides static tension, compression and shear stress-strain properties in three directions using coupons sectioned from a thick infused unidirectional glass/epoxy laminate. The nonlinear, shear dominated static properties were characterized with loading-unloading-reloading (LUR) tests in tension and compression to increasing load levels, for ±45O laminates. Section 5 explores the origins of tensile fatigue sensitivity in glass fiber dominated laminates with variations in fabric architecture including specially prepared fabrics and aligned strand laminates. Several types of resins are considered, with variations in resin toughness and bonding to fibers, as well as cure cycle variations for an epoxy. Conclusions are drawn as to the limits of tensile fatigue resistance