ABSTRACT
When the blade is in position 1 in Figure 13.1 (down-rotating) the blade root at the trailing edge side is exposed to tensile stress and the leading edge side of the blade root is exposed to compressive stress. In position 2 (up-rotating) the trailing edge side of the blade root is exposed to compressive stress and the leading edge side of the blade root is exposed to tensile stress. Thus gravity is responsible for a sinusoidal loading of the blades with a frequency
corresponding to the rotation of the rotor often denoted by 1P. This loading is easily recognized in Figure 10.2 in the time series of the edgewise bending moment. Note that a wind turbine is designed to operate for 20 years, which means that a machine operating at 25 rpm will be exposed to 20 365 24 60 25 = 2.6 108 stress cycles from gravity. Since a wind turbine blade might weigh several tons and be more than 30m long, the stresses from the gravity loading are very important in the fatigue analysis.
