Wind turbines are fatigue-critical machines used to produce electrical energy from the wind. These rotating machines are subjected to combinations of wind, gravity, and gyroscopic loadings that are highly irregular in nature. Historical examples of fatigue problems in both research and commercial wind-turbine development are presented. Some example data on wind-turbine environments, loadings, and material properties are also shown. Before a description of how the authors have chosen to attack the cumulative damage assessment, questions are presented for the reader's reflection. The solution technique used by the authors to define the loading spectrum for wind turbines is then presented. Special emphasis is placed on the development of a loading spectrum for use in the fatigue analysis. Less attention is paid to methods of cumulative damage assessment; Miner's rule and constant amplitude S-n data are used. A case study then applies the procedures to an actual wind-turbine blade joint. The wind turbine is the 34-m diameter vertical axis wind turbine (VAWT) erected by Sandia National Laboratories near Bushland, TX. The case study examines parameter sensitivities for realistic uncertainties in inputs defining the turbine environment, stress response, and material properties. The fatigue lifetimes are calculated using a fatigue analysis program, called LIFE2, which was developed at Sandia.
The LIFE2 code, described in some detail in the Appendix, is a PC-based, menu-driven package that leads the user through the steps required to characterize the loading and material properties, then uses Miner's rule or a linear-crack propagation rule to numerically calculate the time to failure. Only S-n based cumulative damage applications are illustrated here. The LIFE2 code is available to educational institutions for use as a case study in describing complicated loading histories and for use by students in examining, hands on, parameter sensitivity of fatigue life analysis