Effects of moisture absorption on damage progression and strength of unidirectional and cross-ply fiberglass–epoxy composites

Fiber-reinforced-polymer composites (FRPs) possess superior mechanical properties and formability, making them a desirable material for construction of large optimized mechanical structures, such as aircraft, wind turbines, and marine hydrokinetic (MHK) devices. However, exposure to harsh marine environments can result in moisture absorption into the microstructure of the FRPs comprising these structures and often degrading mechanical properties. Specifically, laminate static and fatigue strengths are often significantly reduced, which must be considered in design of FRP structures in marine environments. A study of fiberglass epoxy unidirectional and cross-ply laminates was conducted to investigate hygrothermal effects on the mechanical behavior of a common material system used in wind applications. Several laminates were aged in 50 °C distilled water until maximum saturation was reached. Unconditioned control and the saturated samples were tested in quasi-static tension with the accompaniment of acoustic emission (AE) monitoring. Cross-ply laminates experienced a 54 % reduction in strength due to moisture absorption, while unidirectional laminate strengths were reduced by 40 %. Stress–strain curves and AE activity of the samples were analyzed to identify changes in damage progression due to aging

Fatigue of Composite Material Beam Elements Representative of Wind Turbine Blade Substructure

The database and analysis methods used to predict wind turbine blade structural performance for stiffness, static strength, dynamic response,and fatigue lifetime are validated through the design, fabrication, and testing of substructural elements. We chose a test specimen representative of wind turbine blade primary substructure to represent the spar area of a typical wind turbine blade. We then designed an I-beam with flanges and web to represent blade structure, using materials typical of many U.S.-manufactured blades. Our study included the fabrication and fatigue testing of 52 beams and many coupons of beam material. Fatigue lifetimes were consistent with predictions based on the coupon database. The final beam specimen proved to be a very useful tool for validating strength and lifetime predictions for a variety of flange and web materials, and is serving as a test bed to ongoing studies of structural details and the interaction between manufacturing and structural performance. Th e beam test results provide a significant validation of the coupon database and the methodologies for predicting fatigue of composite material beam elements

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