Department of Mechanical Engineering

Mr. Varun Chenna

Degree Candidacy: Master of Science in Mechanical Engineering
Date: Wednesday, June 25, 2014
Time: 3:30 PM
Location: 108 EIB

Advisor: Dr. Mica Grujicic
Committee Members: Dr. Chenning Tong and Dr. Jay Ochterbeck

Title: Fatigue-Induced Failure in Horizontal-Axis Wind-Turbine (HAWT) Blades and HAWT Drivetrain Gears

ABSTRACT

Wind energy is one of the most promising and the fastest growing installed alternative-energy production technologies. In fact, it is anticipated that by 2030, at least 20% of the U.S. energy needs will be met by various onshore and offshore wind-farms [a collection of wind-turbines (converters of wind energy into electrical energy) at the same location]. A majority of wind turbines nowadays fall into the class of the so-called Horizontal Axis Wind Turbines (HAWTs).

Turbine blades and the gearbox are perhaps the most critical components/subsystems in the present designs of HAWTs. The combination of high failure rates (particularly those associated with turbine-blades and gear-boxes), long downtimes and the high cost of repair remains one of the major problems to the wind-energy industry today.

In the case of HAWT blades, one is typically concerned about the following two quasi-static structural-performance requirements: (a) sufficient “flap-wise” bending strength to withstand highly-rare extreme static-loading conditions (e.g., 50-year return-period gust, a short strong blast of wind); and (b) sufficient turbine blade “flap-wise” bending stiffness in order to ensure that a minimal clearance is maintained between blade tip and the turbine tower at all times during wind turbine operation. If these two structural requirements are not met, HAWT blades typically fail prematurely. In addition to the aforementioned quasi-static structural-performance requirements, one is also concerned about the premature-failure caused by inadequate fatigue-based durability of the HAWT blades. The durability requirement for the turbine blades is typically defined as a minimum of 20-year fatigue life (which corresponds roughly to ca. 108 cycles) when subjected to stochastic wind-loading conditions and cyclic gravity-induced edge-wise bending loads in the presence of thermally-fluctuating and environmentally challenging conditions. In the present work, a computational framework has been developed to address: (a) structural response of HAWT blades subjected to extreme loading conditions; (b) high-cycle-fatigue-controlled durability of the HAWT blades; and (c) methodology for HAWT-blade material selection. To validate the computational approach used, key results are compared with their experimental counterparts available in the public-domain literature.

As far as the HAWT gear-boxes are concerned, while they are designed for the entire life (ca. 20 years) of the HAWT, in practice, most gear-boxes have to be repaired or even overhauled considerably earlier (3–5 years). Typically, a HAWT gear-box fails either due to the bending-fatigue-induced failure of its gears, or by tribo-chemical degradation and failure of its bearings. In the present work, a computational framework has been developed to predict HAWT service-life under extreme loading and unfavorable kinematic conditions, for the case when the gear-box service-life is controlled by gear-tooth bending-fatigue failure. In addition, a preliminary investigation of gear-box bearing kinematics, which can result in undesirable rolling-element skidding conditions, is conducted.