Trey W. Riddle

Characterization of Manufacturing Defects Common to Composite Wind Turbine Blades: Flaw Characterization

The preliminary results from a survey of wind turbine blade manufactures, repair companies, wind farm operators and third party investigators has directed the focus of this investigation on types of flaws commonly found in wind turbine blades; waviness, and porosity/voids. A variety of flaw geometries as defined by in-field collection of production scale blade data has been investigated and complied. Basic statistical analysis has shown that the data generally follows standard distributions. The preliminary results from this effort and coupon level testing have established a protocol by which a defect in a blade can be characterized quantifiably. With this data and other parameters it is possible to develop criticality models that can be used in the field to evaluate the risk of leaving an as manufactured flawed structure in service. The basic metrics for this model have been developed and are described herein.

Wind Turbine Composite Blade Manufacturing: The Need for Understanding Defect Origins, Prevalence, Implications and Reliability

Renewable energy is an important element in the US strategy for mitigating our dependence on non-domestic oil. Wind energy has emerged as a viable and commercially successful renewable energy source. This is the impetus for the 20% wind energy by 2030 initiative in the US. Furthermore, wind energy is important on to enable a global economy. This is the impetus for such rapid, recent growth. Wind turbine blades are a major structural element of a wind turbine blade. Wind turbine blades have near aerospace quality demands at commodity prices; often two orders of magnitude less cost than a comparable aerospace structure. Blade failures are currently as the second most critical concern for wind turbine reliability. Early blade failures typically occur at manufacturing defects. There is a need to understand how to quantify, disposition, and mitigate manufacturing defects to protect the current wind turbine fleet, and for the future. This report is an overview of the needs, approaches, and strategies for addressing the effect of defects in wind turbine blades. The overall goal is to provide the wind turbine industry with a hierarchical procedure for addressing blade manufacturing defects relative to wind turbine reliability.

Effects of Defects Part A: Stochastic Finite Element Modeling of Wind Turbine Blades with Manufacturing Defects for Reliability Estimation

A comprehensive protocol addressing the impact of manufacture flaws on the reliability wind turbines blades has been proposed. The main points of this framework can be summarized in four disciplines: Effects of Defects, Probabilistic Structural Reliability Modeling, Criticality Analysis, and In-Field Evaluations. The majority of the work discussed herein has focused on the development Stochastic Finite Element Analysis (SFEA) model to the probability of failure and address variation in the performance of a composite structure with defects. This analysis is integral to the generation of the Criticality Analysis, a risk assessment tool which can disposition a flawed composite structure.

Effects of Defects: Part A - Development of a Protocol for Defect Risk Management & Improved Reliability of Composite Structures

The Effects of Defects in composites materials common to wind turbine blades is presented in this work. A comprehensive approach has been taken by the authors for the development of a wind turbine blade reliability infrastructure. The framework for a defect risk management and composite structure reliability protocol has been developed. Specific applications to wind turbine blades are discussed. However, the approach has broader implications to composite structures in general. This framework consists of four interrelated, yet distinct efforts: 1.) Effects of Defects which are focused on characterization of flaws physically and mechanically. 2.) Probabilistic Modeling which asses the probability of failure while taking in account variables at all levels of the structure. 3.) Criticality Assessment is used for evaluating the risk of operating an as built flawed structured. 4.) In-Field Evaluations provide feedback and validation. This general framework is described herein along with the data generated to date in support of an application. Testing and analysis on a specific material system are presented but are applicable to other materials and structures.

Effects of Defects: Part B—Progressive Damage Modeling of Fiberglass/Epoxy Composite Structures with Manufacturing Induced Flaws Utilizing Cohesive Zone Elements

The Effects of Defects is important for the reliability of modern composite wind turbine blades. The DOE has sponsored a comprehensive study to characterize common manufacturing defects and establish damage growth and validation tools to contribute to a wind turbine blade reliability infrastructure. To support this development of a reliability infrastructure, progressive damage modeling of blade representative coupons has been performed. A model has been developed that is combined of both continuum and discrete modeling approaches. A user-defined material subroutine has been created and used with material properties established from experimental testing for the former, while cohesive zone elements were placed between each fiber tow to capture the fracture that occurs in these areas for the latter. Two-dimensional (2D), four-layer coupon level models were generated based on the geometry of the as-tested specimens. Results from this model were compared to the visual damage progression and stress-strain data from in-plane wave tests in tension and compression. While the uniformity of the models resulted in more dramatic changes at failure points, the progression was found to match in these cases, thereby verifying the combined continuum and discrete modeling approach.

Composite Wind Turbine Blade Effects of Defects: Part B— Progressive Damage Modeling of Fiberglass/Epoxy Laminates with Manufacturing Induced Flaws

*† ‡ § The Effects of Defects is important for the reliability of modern composite wind turbine blades. The DOE has sponsored a comprehensive study to characterize common manufacturing defects and establish damage growth and validation tools to contribute to a wind turbine blade reliability infrastructure. To support this development of a reliability infrastructure, progressive damage modeling of blade representative coupons has been performed. Initial modeling efforts have been established and correlated to existing experimental results. Two types of analytical models have been investigated and developed with larger scale testing and analytical correlation in mind. A progressive damage routine utilizing combined maximum stress/maximum strain failure criteria was implemented allowing for different types of damage to be estimated. The material properties for damaged elements were then degraded for the appropriate damage mode and type. Two-dimensional (2D), four-layer coupon level models were utilized in increasing complexity from Controls to a flawed specimen containing an In-Plane Wave. Initial models were used to develop convergence in compression and tension before developing a model for an In-Plane Wave group. The results indicate that reasonable correlation was achieved between all groups and their respective experimental data. While some minor discrepancies exist, it appears that they will be an initial point for modeling of larger scale coupons or subsections.

Effects of defects in composite wind turbine blades. Round 1.

The Wind Turbine Blade Reliability Collaborative (BRC) was formed with the goal of improving wind turbine blade reliability. Led by Sandia National Laboratories (SNL), the BRC is made up of wind farm maintenance companies, turbine manufacturers and third party investigators. Specific tasks have been assigned by SNL to the various parties. The portion of the BRC that will focus on establishing the criticality of manufacturing defects (aka, Effect of Defects) is being researched by the Montana State University Composites Group (MSUCG). The research described herein compiles the second round of a multi-year year plan consisting of three rounds of work performed by Montana State University (MSU) within two areas—Flaw Characterization and Effects of Defects. The first round was published in SAND2012-8110. The purpose of this document is to capture and convey the progress which has been made by the MSUCG team since submission of the Round 1 Report. The work presented has built upon the established framework for quantitative categorization and analysis of flaws. Additional physical testing has been performed utilizing improved manufacturing techniques that have reduced manufacturing time and materials, while providing samples with flaws common to wind turbine blades. This additional physical testing was performed to fill two needs: additional data points to further understand the Effects of Defects; and, to increase in scale away from simple coupons toward larger structures. These data were also analyzed to allow for continued analytical/experimental correlation occurring in this and future portions of this study. Manufactured specimens were evaluated non-destructively to verify flaw parameters and visualize damage utilizing computed tomography and digital image correlation, respectively. A further understanding of the changes in the material properties associated with characterized flaws has been achieved on a coupon level with this additional physical testing. This testing further indicated that fiber misalignment and porosity resulted in decreased material performance . Modeling efforts toward matching these results have been performed utilizing two distinct and fundamentally different approaches. These are modeling damage as a degradation of properties, and modeling the actual damage in the material via finite element analysis with progressive damage. Reasonable agreement has been achieved when compared to the test data, though there is room for improvement. Work will continue with efforts to increase size and scale of tested samples. In addition, research will be continued into reliability methods suitable for wind turbine composites. Finally, analytical/experimental correlation will be improved through the use of improved and more accurate models. This is leading to the goals of establishing the severity and criticality of manufacturing defects, and to provide a cogent approach to accept/reject criteria, and the need for repair and/or maintenance in the field.

Effects of Defects Part A: Treatment of Manufacturing Defects as Uncertainty Variables in a Wind Blade Probabilistic Design Framework

It has been reported that a leading cause of repairs and failures in wind turbine blades is attributable to manufacturing defects. The size, weight, shape and economic considerations of wind turbine blades have dictated the use of low cost composite materials. Composite structure manufacturing quality is a critical issue for reliability. While significant research has been performed to better understand what is needed to improve blade reliability, a comprehensive study to characterize and understand the manufacturing flaws commonly found in blades has not been performed. The work presented herein is focused on performing mechanical testing of flawed composite specimen and developing probabilistic models to assess the reliability of a wind blade with defects. The analysis postulates that one should assess defects as a design parameter in a parametric probabilistic analysis. A consistent framework has been established and validated for quantitative categorization and analysis of flaws. Results from this effort have shown that the probability of failure of a wind turbine blade with defects, can be adequately described through the use of Monte Carlo simulation. The two approaches detailed in this analysis have shown that, by treating defects as random variables, one can reduce the design conservatism of a wind blade in fatigue. Reduction in the safe operating lifetime of a blade with defects, compared to one without has shown that the inclusion of defects is critical for proper reliability assessment. If one assumes that defects account for some of the uncertainty in the blade design and these defects are analyzed with application specific data, then safety factors can be reduced. It has been shown that characterization of defects common to wind turbine blades and reduction of design uncertainty is possible. However, it relies on accurate and statistically significant data.

Effects of Defects: Part B—A Comparison of Progressive Damage Modeling of Fiberglass/Epoxy Composite Structures with Manufacturing Induced Flaws

Application of different damage modeling approaches for use with composite materials and composite material structures has grown with increasing computational ability. However, assumptions are often made for “worst case” scenarios with these modeling techniques In order to develop a tool that will allow for accurate analysis of a complete structure, modeling approaches must be optimized by including defects of different parameters. It was the optimization of these approaches that was investigated herein with specific application toward establishing a protocol to understand and quantify the effects of defects in composite wind turbine blades. A systematic, three-round study of increasing complexity was performed to understand the effects of three typical blade manufacturing defects while investigating continuum, discrete, and combined damage modeling. Through the three rounds of the benchmark material testing, significant coupon level testing was performed to generalize the effects of these defects. In addition, material properties and responses were analyzed and then utilized as material inputs and correlation criteria for each analytical technique. A standard defect case was initially used for each modeling technique and correlation was compared both qualitatively and quantitatively. While each modeling type offered certain attributes, a combined approach yielded the most accurate analytical/experimental correlation. Thus, a unique comparison of several different analytical approaches to composites with respect to manufacturing for consistency, accuracy, and predictive capability allowing for improved blade reliability and composite structural assessment.

Use of Statistical Learning in a Reliability Program for Risk Assessment of Composite Structures with Defects

A comprehensive protocol addressing the impact of manufacture flaws on the reliability wind turbines blades has been proposed. The main points of this framework can be summarized in four disciplines: Effects of Defects, Probabilistic Structural Reliability Modeling, Criticality Analysis, and In-Field Evaluations. The majority of the work discussed herein has focused on the development statistical learning models to use as surrogate models to complicated stochastic finite element analysis. This analysis is integral to the generation of the Criticality Analysis, a risk assessment tool which can disposition a flawed composite structure.