Scholarship & Research
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Item Investigation of the effect of in-plane fiber waviness in composite materials through multiple scales of testing and finite element modeling(Montana State University - Bozeman, College of Engineering, 2015) Lerman, Michael William; Chairperson, Graduate Committee: Douglas S. CairnsDefects in materials can reduce strengths and lifetimes of manufactured parts. The number of possible defects increase with the complexity inherent in composite materials. The wind industry uses composite wind turbine blades in which the manufacturing process induces a number of defects. In order for the wind industry to continue sustainable expansion, the effects of defects must be better understood. In-plane (IP) fiber waviness is the focus of this work. The three main parts of this work include testing on the coupon level, modeling on the coupon level, and testing of beams in four-point bending (with and without defects). The coupon level testing includes partial IP waves, similar to those in manufactured parts, rather than full width IP waves. This allows investigation into complex interactions and varying failure mechanisms caused by the fiber misalignment gradient. Partial waves are also modeled to both validate testing as well as to increase robustness of a previously developed progressive damage modeling method. Lastly, a sandwich beam test specimen for testing in 4-point bending is developed to investigate the effects of fiber waviness in both tension and compression when loaded in flexure.Item Effects of manufacturing defects on the strength of toughened carbon/epoxy prepreg composites(Montana State University - Bozeman, College of Engineering, 2000) Turoski, Luke EverettItem Development of failure criteria and experimental teseting for composite adhesively bonded scarf repairs utilizing structural paste adhesives(Montana State University - Bozeman, College of Engineering, 2014) Ritchey, Tammy Nicole; Chairperson, Graduate Committee: Douglas S. CairnsRapid growth of composite application in wind energy has strained the wind turbine blade service industry as blades continually fail prior to life cycle expectations. Minimal research surrounding composite repair has led to gaps in the ability to characterize, analyze, and predict composite repair behavior. Certification standards derived from two dimensional simplified geometric configurations are speculated to be conservative. In this study, coupon level adhesively bonded composite scarf repair specimens are developed and tested to failure under static tensile loads. Based on experimental results, a geometry independent failure criteria is proposed and validated through finite element modeling. Specimen failures are compared against current repair standards and analysis techniques. The proposed maximum Von Mises Strain failure criteria has potential to provide a preliminary decision making framework for the service industry reducing time, cost, and resources.Item Development of reliability program for risk assessment of composite structures treating defects as uncertainty variables(Montana State University - Bozeman, College of Engineering, 2013) Riddle, William Wilson, III; Chairperson, Graduate Committee: Douglas S. CairnsIt 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.Item A comparison of continuum and discrete modeling techniques of the effects of manufacturing defects common to composite structures(Montana State University - Bozeman, College of Engineering, 2013) Nelson, Jared William; Chairperson, Graduate Committee: Douglas S. CairnsApplication 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 resulting in approximating defects as a hole or notch in a plate instead of modeling actual flaw geometry. These analytical tools have helped bound composite material and structure capabilities, but do not allow for comprehensive understanding of the effects of defects as the characteristic parameters of the defects vary. In order to develop a tool that will allow for accurate analysis of a complete structure, including defects of different parameters, modeling approaches must be optimized. 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. Parallel to the material testing, each of the three rounds increased in analytical complexity to ensure that models were only as complex as necessary to achieve acceptable correlation. Correlation was compared both qualitatively and quantitatively for an initial case and other cases were investigated only if initial correlation was acceptable. 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.Item Manufacturing process modeling for composite materials(Montana State University - Bozeman, College of Engineering, 2013) Guest, Daniel Aaron; Chairperson, Graduate Committee: Douglas S. CairnsThe increased use and interest in wind energy over the last few years has necessitated an increase in the manufacturing of wind turbine blades. This increase in manufacturing has in many ways out stepped the current understanding of not only the materials used but also the manufacturing methods used to construct composite laminates. The goal of this study is to develop a list of process parameters which influence the quality of composite laminates manufactured using vacuum assisted resin transfer molding and to evaluate how they influence laminate quality. Known to be primary factors for the manufacturing process are resin flow rate and vacuum pressure. An incorrect balance of these parameters will often cause porosity or voids in laminates that ultimately degrade the strength of the composite. Fiber waviness has also been seen as a major contributor to failures in wind turbine blades and is often the effect of mishandling during the lay-up process. Based on laboratory tests conducted, a relationship between these parameters and laminate quality has been established which will be a valuable tool in developing best practices and standard procedures for the manufacture of wind turbine blade composites.