Fatigue modeling for small wind systems: basic theory

Abstract

This report discusses the theory of fatigue as applied to horizontal-axis wind systems and reviews current techniques for predicting fatigue loads and fatigue life. Wind systems are subjected to a complex and diverse variety of loads, ranging from simple gravity and centrifugal loads to complex and random wind loads. The categories of loads described here include transient, periodic, and control-induced loads, as well as loads induced by severe aeroelastic instabilities. Within these categories, load types include centrifugal, aerodynamic thrust and torque, parked rotor survival loads, gravity loads, gyroscopic loads, and aerodynamic loads induced by crossflow or yaw motion. Control systems can impose loads whenever they are activated. Catastrophic failure can result from severe aeroelastically-induced loads caused by flutter. Techniques available for predicting fatigue loads include various aerodynamic performance codes (such as PROP) and simplified methods for calculating the convection of the vortex wake. MOSTAB, an aeroelasticity computer code, treats all major load sources. Results using this code are shown to be fairly comparable to experimental data in the case of a rigid-bladed wind system. Fatigue life of a machine component can be predicted during design by several techniques, including the strength-of-materials method, the fatigue curve method, and the fatigue eventmore » method. The theories of Miner, Manson, and Corten-Dolan can be used to predict fatigue life based on test data and operational experience. A fatigue research project to be performed at Rocky Flats will attempt to resolve whether any of these theories or a combination of theories can be used with an acceptable degree of accuracy in predicting fatigue loads and life for wind systems. An appendix to the report provides sample calculations of fatigue load magnitudes made using available methods.« less