Daniel D. Samborsky

DOE/MSU composite material fatigue database: Test methods, materials, and analysis

This report presents a detailed analysis of the results from fatigue studies of wind turbine blade composite materials carried out at Montana State University (MSU) over the last seven years. It is intended to be used in conjunction with the DOE/MSU composite Materials Fatigue Database. The fatigue testing of composite materials requires the adaptation of standard test methods to the particular composite structure of concern. The stranded fabric E-glass reinforcement used by many blade manufacturers has required the development of several test modifications to obtain valid test data for materials with particular reinforcement details, over the required range of tensile and compressive loadings. Additionally, a novel testing approach to high frequency (100 Hz) testing for high cycle fatigue using minicoupons has been developed and validated. The database for standard coupon tests now includes over 4,100 data points for over 110 materials systems. The report analyzes the database for trends and transitions in static and fatigue behavior with various materials parameters. Parameters explored are reinforcement fabric architecture, fiber content, content of fibers oriented in the load direction, matrix material, and loading parameters (tension, compression, and reversed loading). Significant transitions from good fatigue resistance to poor fatigue resistance are evident in the range of materials currently used in many blades. A preliminary evaluation of knockdowns for selected structural details is also presented. The high frequency database provides a significant set of data for various loading conditions in the longitudinal and transverse directions of unidirectional composites out to 10{sup 8} cycles. The results are expressed in stress and strain based Goodman Diagrams suitable for design. A discussion is provided to guide the user of the database in its application to blade design.

Fatigue of Composite Materials and Substructures for Wind Turbine Blades

This report presents the major findings of the Montana State University Composite Materials Fatigue Program from 1997 to 2001, and is intended to be used in conjunction with the DOE/MSU Composite Materials Fatigue Database. Additions of greatest interest to the database in this time period include environmental and time under load effects for various resin systems; large tow carbon fiber laminates and glass/carbon hybrids; new reinforcement architectures varying from large strands to prepreg with well-dispersed fibers; spectrum loading and cumulative damage laws; giga-cycle testing of strands; tough resins for improved structural integrity; static and fatigue data for interply delamination; and design knockdown factors due to flaws and structural details as well as time under load and environmental conditions. The origins of a transition to increased tensile fatigue sensitivity with increasing fiber content are explored in detail for typical stranded reinforcing fabrics. The second focus of the report is on structural details which are prone to delamination failure, including ply terminations, skin-stiffener intersections, and sandwich panel terminations. Finite element based methodologies for predicting delamination initiation and growth in structural details are developed and validated, and simplified design recommendations are presented.

New Fatigue Data for Wind Turbine Blade Materials

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 fi bers. Tensile fatigue results from a specialized high frequency small strand testing facility have been carried out to 10 10 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 t he pr oper models for extrapolating f rom 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 m inimum 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 i nduced flaws on properties continues to be a m ajor concern, particularly with large tow carbon fabrics. The results of a study o f fiber waviness effects on compressive st rength s how 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 car bon fibers. Improve d compressive static and fatigue performance is found with stitched fabrics as compared with woven fabrics.

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.

Spectrum Fatigue Lifetime and Residual Strength for Fiberglass Laminates

This report addresses the effects of spectrum loading on lifetime and residual strength of a typical fiberglass laminate configuration used in wind turbine blade construction. Over 1100 tests have been run on laboratory specimens under a variety of load sequences. Repeated block loading at two or more load levels, either tensile-tensile, compressive-compressive, or reversing, as well as more random standard spectra have been studied. Data have been obtained for residual strength at various stages of the lifetime. Several lifetime prediction theories have been applied to the results. The repeated block loading data show lifetimes that are usually shorter than predicted by the most widely used linear damage accumulation theory, Miners sum. Actual lifetimes are in the range of 10 to 20 percent of predicted lifetime in many cases. Linear and nonlinear residual strength models tend to fit the data better than Miners sum, with the nonlinear providing a better fit of the two. Direct tests of residual strength at various fractions of the lifetime are consistent with the residual strength models. Load sequencing effects are found to be insignificant. The more a spectrum deviates from constant amplitude, the more sensitive predictions are to the damage law used. The nonlinear model provided improved correlation with test data for a modified standard wind turbine spectrum. When a single, relatively high load cycle was removed, all models provided similar, though somewhat non-conservative correlation with the experimental results. Predictions for the full spectrum, including tensile and compressive loads were slightly non-conservative relative to the experimental data, and accurately captured the trend with varying maximum load. The nonlinear residual strength based prediction with a power law S-N curve extrapolation provided the best fit to the data in most cases. The selection of the constant amplitude fatigue regression model becomes important at the lower stress, higher cycle loading cases. The residual strength models may provide a more accurate estimate of blade lifetime than Miners rule for some loads spectra. They have the added advantage of providing an estimate of current blade strength throughout the service life.

Compression Strength of Carbon Fiber Laminates Containing Flaws with Fiber Waviness

Recent studies of carbon fiber and carbon/glass hybrid laminates have reported compression strengths and failure strains which are borderline for wind turbine blade designs, depending upon the reinforcement architecture, matrix resin, and environment. Compressive strength is known to be sensitive to the straightness of the fibers, with even relatively small degrees of waviness or misalignment causing significant decreases in compression properties. The effects of fiber waviness, induced by infusion processes and inherent in fabric architectures, on compressive strength, have been investigated. Structural details such as ply drops and ply joints can cause significant levels of fiber misalignment, depending on parameters such as ply thickness, fraction of plies dropped, ply drop location, ply joint gap, and mold geometry and pressure. These parameters have been varied in the study reported in this paper, with compressive properties determined in each case. The results show that prepreg laminates containing p ly drops and joints can provide adequate compressive strength, but that severe knockdowns can occur for geometries where large misalignments are induced.

Prediction of Delamination in Wind Turbine Blade Structural Details

Delamination between plies is the root cause of many failures of composite materials structures such as wind turbine blades. Design methodologies to prevent such failures have not been widely available for the materials and processes used in blades. This paper presents simplified methodologies for the prediction of delamination under both static and fatigue loading at typical structural details in blades. The methodology is based on fracture mechanics. The critical strain energy release rate, GIC and GIIC , are determined for opening mode (I) and shearing mode (II) delamination cracks; fatigue crack growth in each mode is also characterized. These data can be used directly for matrix selection, and as properties for the prediction of delamination in structural details. The strain energy release rates are then determined for an assumed interlaminar flaw in the structural detail. The flaw is positioned based on finite element analysis (FEA), and the strain energy release rates are calculated using the virtual crack closure feature available in codes like ANSYS. The methodology has been validated for a skin-stiffener intersection. Two prediction methods differing in complexity and data requirements have been explored. Results for both methods show good agreement between predicted and experimental delamination loads under both static and fatigue loading.© 2003 ASME

Comparison of Tensile Fatigue Resistance and Constant Life Diagrams for Several Potential Wind Turbine Blade Laminates

New fatigue test results are presented for four multidirectional laminates of current and potential interest for wind turbine blades, representing three types of fibers: E-glass, WindStrand™ glass, and carbon, all with epoxy resins. A broad range of loading conditions is included for two of the laminates, with the results represented as mean and 95/95 confidence level constant life diagrams. The constant life diagrams are then used to predict the performance under spectrum fatigue loading relative to an earlier material. Comparisons of the materials show significant improvements under tensile fatigue loading for carbon, WindStrand, and one of the E-glass fabrics relative to many E-glass laminates in the 0.5-0.6 fiber volume fraction range. The carbon fiber dominated laminate shows superior fatigue and static strengths, as well as stiffness, for all loading conditions.

Fatigue of Composite Material Beam Elements Representative of Wind Turbine Blade Substructure

The database and analysis methods used to predict wind turbine blade structural performance for stiffness, static strength, dynamic response,and fatigue lifetime are validated through the design, fabrication, and testing of substructural elements. We chose a test specimen representative of wind turbine blade primary substructure to represent the spar area of a typical wind turbine blade. We then designed an I-beam with flanges and web to represent blade structure, using materials typical of many U.S.-manufactured blades. Our study included the fabrication and fatigue testing of 52 beams and many coupons of beam material. Fatigue lifetimes were consistent with predictions based on the coupon database. The final beam specimen proved to be a very useful tool for validating strength and lifetime predictions for a variety of flange and web materials, and is serving as a test bed to ongoing studies of structural details and the interaction between manufacturing and structural performance. Th e beam test results provide a significant validation of the coupon database and the methodologies for predicting fatigue of composite material beam elements.

The SNL/MSU/DOE Fatigue of Composite Materials Database: Recent Trends

Trends of recent test data in three areas are described for wind blade materials in the SNL/MSU/DOE fatigue of composite materials database 1 . First, a complete 3-D set of static elastic constants and strength properties is given for a thick infused glass fabric/epoxy laminate. Second, results are presented which explore the effects of fabric structure and resin type on the tensile fatigue resistance. Using aligned strand structure as a baseline, the efficiency of stitched fabric reinforcement is quantified for static and fatigue properties, and the origins of poor fatigue performance with some resins are identified. Third, an overall comparison is given of the tensile fatigue sensitivity of various blade materials including laminate in-plane and interlaminar failure, epoxy based blade adhesives and core materials. Comparisons of fiber dominated and resin dominated failure modes show clear trends in the fatigue exponent, depending on the resin system.