John F. Mandell

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.

Effect of mean stress on the damage of wind turbine blades

In many analyses of composite wind turbine blades, the effects of mean stress on the determination of damage are either ignored completely or they are characterized inadequately. An updated Goodman diagram for the fiberglass materials that are typically used in wind turbine blades has been released recently. This diagram, which is based on the MSU/DOE Fatigue Database, contains detailed information at thirteen Rvalues. This diagram is the most detailed to date, and it includes several loading conditions that have been poorly represented in earlier studies. This formulation allows the effects of mean stress on damage calculations to be evaluated. The evaluation presented here uses four formulations for the S-N behavior of the fiberglass. In the first analysis, the S-N curve for the composite is assumed to be independent of mean stress and to have a constant slope. The second is a linear Goodman diagram, the third is a bi-linear Goodman diagram and the fourth is the full Goodman diagram. Two sets of load spectra, obtained by the LIST (Long term Inflow and Structural Test) program, are used for this evaluation. The results of the analyses, equivalent fatigue loads and damage predictions, are compared to one another. These results illustrate a significant overestimation of the equivalent fatigue loads when the mean stress is not considered in the calculation. And, the results from the updated Goodman diagram illustrate that there are a significant differences in accumulated damage when the Goodman diagram

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.

Modeling of resin transfer molding of composite materials with oriented unidirectional plies

Abstract One of the limiting factors in the application of composites to primary structural applications has been the relatively high cost of converting basic material forms to structural configurations. One potential manufacturing method is Resin Transfer Molding (RTM). In RTM, resin is injected into a fibrous preform. In this study, the processing parameters are defined and investigated analytically and experimentally. A basic model is proposed that incorporates Darcys law in fibrous bundle regions and channel flow equations between bundles. A model description is provided along with experimental procedures. The model results are compared to experimental data for unidirectional, stitched preforms, and multi-layer configurations with good experimental agreement. It was found that incorporating channel flow is an important feature for properly modeling the RTM process. Pressure profiles, resin velocities, and resin flow fronts are predicted accurately, and are available for manufacturing process development. The same materials were compared to fully stitched preforms. It was found that although the shapes for resin flow are similar between experimental and analytical results, the stitching affects the permeability such that unidirectional ply data do not accurately capture the times for resin flow.

Design and manufacturing considerations for ply drops in composite structures

Thickness variations are required to optimize the design of modern laminated composite structures. These thickness variations are accomplished by dropping plies along the length to match varying in-plane and bending loads. This results in a structure which is matched to stiffness and loading requirements. Unfortunately, these ply drops produce internal and local stress concentrations as a consequence of geometric discontinuities and shear lag. In this study, we explore various factors for design of composite structures with ply drops. These factors include: thicknesses, ply stacking sequences, ply drop geometries and manufacturing considerations. In addition, fatigue loading is considered with respect to delamination initiation and growth. A strong sensitivity to the position and the manufacturing details of ply drops is shown for fatigue damage initiation and growth. All studies were conducted on a low-cost E-glass/polyester composite system. The results indicate that it will be difficult to completely suppress damage and delamination initiation in service. However, it was found that, in many cases, there is a threshold loading under which there is little growth after initiation is noted. Factors affecting this threshold are analyzed via the virtual crack closure method in Finite Element Analysis and verified experimentally. Design rules for ply dropping are presented on the basis of these results.

Analysis of a Composite Blade Design for the AOC 15/50 Wind Turbine using a Finite Element Model

A fiberglass blade was designed for the Atlantic Orient Corporation (AOC) H/50 wind turbine through the use of finite element (FE) modeling techniques. In this initial design phase, the goals were: 1) make the blade as stiff as the previously designed laminated wood blade, 2) minimize resonant operating conditions, 3) design the blade to withstand extreme wind conditions, and 4) make the blade compatible with reasonable manufacturing techniques. The modeling assumptions used are discussed and the final results, for this initial design phase, are presented. Based on the J?E model, the designed blade will be able to withstand extreme wind conditions through elastic deformation, and resonant operating conditions will be minimized. This document is an overview of the design and manufacturing synthesis data of composite wind turbine blades for applications to the Sandia National Laboratories’ NuMAD wind turbine blade design tool.

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.