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

Abstract

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. 1 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. 4

Influence of microstructural defects and hydrostatic pressure on water absorption in composite materials for tidal energy

“…Although a previous investigation has shown little effect on load bearing capacity of composite laminates due to freeze-thaw action [59], it should be noted that this study involved samples with a negligible void content. The current work suggests that the effect of freeze-thaw action on composites with fully saturated voids and void contents representative of actual blade structures (estimated to be in the range of 5% to 20% for composite wind turbine blades [60]) may warrant further study.…”

Numerical and experimental investigation of out-of-plane fiber waviness on the mechanical properties of composite materials

“…40. A simplified analytical consideration using the Euler-Bernoulli beam theory (neutral axis coincides with the beam centroid line, bending moment M B = FA = σ x b t A, where A is the amplitude, the moment of inertia for rectangular cross section J y ¼ b t 3 12 and y ¼ t 2 ) leads to…”

“…Fiber waviness is denoted as a wave-formed ply and/or fiber deviation from a straight alignment in a unidirectional laminate. Wavy plies can appear in arbitrary shapes and locations and can principally be classified into in-plane and out-of-plane waves, whereas Nelson et al [3] stated, that both show similar strength degradations.…”

Mechanisms of Origin and Classification of Out-of-Plane Fiber Waviness in Composite Materials—A Review

“…If the out-of-plane fiber waviness occurs due to stability issues when the ply is loaded under compression, it is also referred to as buckles or fiber buckling. Wavy plies can appear in arbitrary shapes and locations and can principally be classified into in-plane and out-of-plane waves, whereas Nelson et al [6] stated, that both show similar strength degradations. This paper aims to investigate out-of-plane waviness because of its more frequent occurrence compared to in-plane waviness.…”

“…Fiber misalignment is the general description for the angular deviation of nominal, intended fiber directions and; therefore, not corresponding to the designed fiber path. Misaligned fibers remain straight compared to curved fibers in waves, wrinkles, Wavy plies can appear in arbitrary shapes and locations and can principally be classified into in-plane and out-of-plane waves, whereas Nelson et al [6] stated, that both show similar strength degradations. This paper aims to investigate out-of-plane waviness because of its more frequent occurrence compared to in-plane waviness.…”