Daniel L. Laird

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.

Modeling of blades as equivalent beams for aeroelastic analysis

A procedure has been developed and tested to derive a set of one-dimensional beam properties that will duplicate the behavior of a full three-dimensional finite element model of a wind turbine blade. This allows the full features of a detailed model to be transferred to an aeroelastic code for dynamic simulation of the complete wind turbine. The process uses the NuMAD interface to generate an ANSYS® finite element model to which a set of six unit loads are applied at the tip. The displacement results are used in a series of MATLAB routines to extract the stiffness matrices of the desired beam elements. Tests have been carried out on a number of blades and the stiffness matrices incorporated into ADAMS® models of the blades and complete wind turbines.

Finite Element Modeling of Wind Turbine Blades

*† ‡ The typical approach to modeling a full wind turbine blade with the finite element method (FEM) has been to use layered shell elements with the nodes offset to the exterior surface. Though the bending response of the blade is of primary interest, the previous few years have seen increased interest in blades incorporating bend-twist coupling. In the analysis of such blades, the torsional response is critical. Unfortunately, recent FEM analyses (layered shell elements, offset nodes) have shown the results for a full-blade model with a torsional load to be erroneous. This paper describes the pitfalls associated with offset nodes for a layered shell element, the issues associated with modeling a wind turbine blade with mid-thickness shells, and a discussion of the feasibility of migrating from shell elements to either layered or non-layered solid (brick) elements.

Introduction to NuMAD: A numerical manufacturing and design tool

Given the complex geometry of most wind turbine blades, structural modeling using the finite element method is generally performed using a unique model for each particular blade analysis. Development time (often considerable) spent creating a model for one blade may not aid in the development of a model for a different blade. In an effort to reduce model development time and increase the usability of advanced finite element analysis capabilities, a new software tool, NuMAD, is being developed.

Identification and Use of Blade Physical Properties

* * * § The paper describes a comprehensive method of determining the equivalent beam properties of a full finite element model of a wind turbine blade. The approach uses the displacements of the model to a series of unit loads applied at the blade tip to calculate the stiffness matrices of the equivalent beam elements. The method includes all aspects of the deformation such as shear and warping. A series of test cases are presented for verification and the method is applied to a blade section of constant section as well as to a model of a full blade.

Estimation of uncertain material parameters using modal test data

Analytical models of wind turbine blades have many uncertainties, particularly with composite construction where material properties and cross-sectional dimension may not be known or precisely controllable. In this paper the authors demonstrate how modal testing can be used to estimate important material parameters and to update and improve a finite-element (FE) model of a prototype wind turbine blade. An example of prototype blade is used here to demonstrate how model parameters can be identified. The starting point is an FE model of the blade, using best estimates for the material constants. Frequencies of the lowest fourteen modes are used as the basis for comparisons between model predictions and test data. Natural frequencies and mode shapes calculated with the FE model are used in an optimal test design code to select instrumentation (accelerometer) and excitation locations that capture all the desired mode shapes. The FE model is also used to calculate sensitivities of the modal frequencies to each of the uncertain material parameters. These parameters are estimated, or updated, using a weighted least-squares technique to minimize the difference between test frequencies and predicted results. Updated material properties are determined for axial, transverse, and shear moduli in two separate regions of the blade cross section: in the central box, and in the leading and trailing panels. Static FE analyses are then conducted with the updated material parameters to determine changes in effective beam stiffness and buckling loads.

Mechanical Response of X-Ray Masks

Overlay accuracy remains one of the major challenges in 1× lithography. In the case of X-ray masks, it is necessary to have a precise pattern on the mask itself and ensure that the pattern is not distorted due to mounting. A detailed study has been performed of the distortions induced in the membrane when the mask is held kinematically in various configurations, e.g., horizontal, vertical and inverted orientations (for e-beam writing, X-ray exposure and point source exposure, respectively). The new proposed X-ray mask standard has been used for all results presented. Finite element models were used to simulate alignment techniques and magnification correction procedures and subsequently determine maximum resultant distortions. Results show it necessary to optimize the mounting strategy to meet the stringent error budget required for 0.25 µm technology (and below).

Alternative Materials for Megawatt-Scale Wind Turbine Blades: Coupon and Subscale Testing of Carbon Fiber Composites

*† ‡ As part of the U.S. Department of Energy’s Low Wind Speed Turbine program, Global Energy Concepts LLC (GEC) is studying alternative composite materials for wind turbine blades in the multi-megawatt size range. Earlier in this project, GEC assessed candidate innovations in composite materials, manufacturing processes, and structural configurations; GEC also made recommendations for testing composite coupons, details, assemblies, and blade sub-structures. The current paper summarizes the results from this test program which was initiated in June 2004. Composite materials evaluated include carbon fiber in both pre-impregnated and vacuum-assisted resin transfer molding (VARTM) forms. Results include static and fatigue testing of thin coupons, and static testing of thick coupons. Efforts to obtain satisfactory test results for thick-coupon fatigue and panels with carbon-tofiberglass ply transitions are ongoing. In addition to the test program results, this paper discusses some of the challenges encountered in the processing and testing of these carbon composite materials.

Structured Innovation of High-Performance Wave Energy Converter Technology: Preprint

Wave energy converter (WEC) technology development has neither reached the desired commercial maturity nor, and more importantly, the techno-economic performance to achieve economic viability. The reasons for this delay in development success have been recognized and fundamental requirements for successful WEC technology development have been identified in [1] and [2]. This paper describes a multiyear project pursued in collaboration by the National Renewable Energy Laboratory and Sandia National Laboratories to innovate and develop new WEC technology. It specifies the project strategy, shows how this differs from the state of the art approach, and presents some early project results. Based on the specification of fundamental functional requirements of WEC technology, structured innovation and systemic problem-solving methodologies are applied to invent and identify new WEC technology concepts. Using technology performance level (TPL) as an assessment metric of the techno-economic performance potential, high-performance technology concepts are identified and selected for further development. System performance is numerically modeled and optimized and key performance aspects are empirically validated. The project deliverables are WEC technology specifications of high techno-economic performance technologies of TPL 7 or higher at a technology readiness level 3 (TRL 3) with some key technology challenges investigated at higher TRLs. These wave energy converter technology specifications will be made available to industry for completion of the technology development and commercialisation (TRL 4–TRL 9). Keywords— Wave energy converter, technology development, high techno-economic performance, structured innovation, techniques of inventive problem solving, TIPS, TRIZ, technology performance level, TPL, technology readiness level, TRL, DOE, NREL, SNL

Stability and stiffness characteristics of the national x-ray mask standard

Finite element analyses have been performed to investigate the stability and stiffness characteristics of the ARPA-NIST National X-ray Mask Standard. The use of different materials (such as silicon carbide and Pyrex) for the support ring has been studied to identify the effects on the maximum in-plane mounting distortions (within a 50 mm by 50 mm patterned area). Additional finite element calculations have been completed to determine the out-of-plane distortions (or bowing) of the mask due to the fabrication process. Parametric studies have been performed to identify the stiffness characteristics of the mask as the overall ring thickness is reduced while the wafer thickness is increased. Results show how various design parameters can be controlled to repeatedly fabricate masks that fulfill requirements for sub-0.13 micrometer technology.