Abstract
A preliminary experimental characterization of the longitudinal, transverse, and shear strains in wind turbine blades under fatigue loading is presented in this study. For that, strains at different cross-section regions of a 14.3 m blade under different uni-axial and biaxial fatigue tests are recorded and analyzed. The results from this analysis provide initial indications on how current testing methods used to certify and characterize the fatigue response of wind turbine blade materials can be improved to better represent what the blades experience during testing. It is shown, for example, how some of the blade regions can experience high biaxiality strain ratios, and how some of these regions can also experience broad internal loading conditions, in terms of strain levels and load orientation (i.e., compression, tension-compression, and tension), depending on the test type.
Highlights
Different testing methods are currently used to certify and characterize the fatigue response of wind turbine blades and their materials, which are mainly carried out at the full-length scale [1, 2] and at the coupon-length scale [3, 4]
The percentage of number of cycles, N/Ntotal, for a given Q-ratio and strain level, amp,i/ amp,x,max, for different cross-section regions and under flapwise, edgewise, and chaotic loading conditions are presented in Figs. 2, 3, and 5, respectively
Under uni-axial loading, the time series from all strain components and at all evaluated cross-section regions could be simplified as a few blocks of cycles with similar Q-ratio and strain levels, being most of them under C-T and T-C
Summary
Different testing methods are currently used to certify and characterize the fatigue response of wind turbine blades and their materials, which are mainly carried out at the full-length scale [1, 2] and at the coupon-length scale [3, 4]. Are the loads applied during the coupon-scale fatigue tests representing the internal material loads of the blade during full-scale fatigue tests? At the full-length scale, only one or two blades are normally tested according to the standards [1, 2]. In these tests, the blades are commonly evaluated consecutively under flapwise and edgewise fatigue loading, these structures are subjected to combined loads in real operational conditions [5, 6]. Target bending moments in the flapwise and edgewise directions are usually used alone to design the certification tests, which may disregard the actual effects of the loads on the material response [6]
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