Editing Windmill (section)

==== Materials ====
{{Main|Wind turbine design}}

In an attempt to make wind turbines more efficient and increase their energy output, they are being built bigger, with taller towers and longer blades, and being increasingly deployed in offshore locations.<ref>Ng C., Ran L. "Offshore Wind Farms: Technologies, Design and Operation" Woodhead Publishing (2016)</ref><ref>Paul Breeze, Chapter 11 - Wind Power,"Power Generation Technologies (Second Edition)", Newnes,2014, Pages 223-242,{{ISBN|9780080983301}}, https://doi.org/10.1016B978-0-08-098330-1.00011-9.</ref> While such changes increase their power output, they subject the components of the windmills to stronger forces and consequently put them at a greater risk of failure. Taller towers and longer blades suffer from higher fatigue, and offshore windfarms are subject to greater forces due to higher wind speeds and accelerated corrosion due to the proximity to seawater.
To ensure a long enough lifetime to make the return on the investment viable, the materials for the components must be chosen appropriately.

The blade of a wind turbine consists of 4 main elements: the root, spar, aerodynamic fairing, and surfacing. The fairing is composed of two shells (one on the pressure side, and one on the suction side), connected by one or more webs linking the upper and lower shells. The webs connect to the spar laminates, which are enclosed within the skins (surfacing) of the blade, and together, the system of the webs and spars resist the flapwise loading. Flapwise loading, one of the two different types of loading that blades are subject to, is caused by the wind pressure, and edgewise loading (the second type of loading) is caused by the gravitational force and torque load. The former loading subjects the spar laminate on the pressure (upwind) side of the blade to cyclic tension-tension loading, while the suction (downwind) side of the blade is subject to cyclic compression-compression loading. Edgewise bending subjects the leading edge to a tensile load, and the trailing edge to a compressive load. The remainder of the shell, not supported by the spars or laminated at the leading and trailing edges, is designed as a sandwiched structure, consisting of multiple layers to prevent elastic buckling.<ref>Mishnaevsky, Leon et al. “Materials for Wind Turbine Blades: An Overview.” Materials vol. 10,11 1285. 9 November 2017, doi:10.3390/ma10111285</ref>

In addition to meeting the stiffness, strength, and toughness requirements determined by the loading, the blade needs to be lightweight, and the weight of the blade scales with the cube of its radius. To determine which materials fit the criteria described above, a parameter known as the beam merit index is defined: Mb = E^1/2 / rho,<ref>H. R. Shercliff, M. F. Ashby,"Elastic Structures in Design", Reference Module in Materials Science and Materials Engineering, Elsevier,2016,{{ISBN|9780128035818}},
https://doi.org/10.1016/B978-0-12-803581-8.02944-1.</ref> where E is [[Young's modulus]] and rho is the density. The best blade materials are [[carbon fiber]] and [[glass fiber]] reinforced [[polymer]]s ([[Cfrp|CFRP]] and [[GFRP]]). Currently, GFRP materials are chosen for their lower cost, despite the much greater figure of merit of CFRP.<ref>Ennis, Kelley, et al. "Optimized Carbon Fiber Composites in Wind Turbine Blade Design" US Department of Energy (2019), https://www.energy.gov/eere/wind/downloads/optimized-carbon-fiber-composites-wind-turbine-blade-design</ref>