Date of Award


Access Type

Thesis - Open Access

Degree Name

Master of Science in Mechanical Engineering


Mechanical Engineering

Committee Chair

Darris White, Ph.D.

First Committee Member

Charles Reinholtz, Ph.D.

Second Committee Member

Marc Compere, Ph.D.

Third Committee Member

Michael Desmond


The installed capacity of wind turbines has grown steadly for the past decade with wind energy now providing 3.6-percent of the U.S.’s electricity supply [13]. This trend can be attributed to advancements in wind turbine technology and the ability to increase wind turbine sizes. As wind turbines grow in size, so do the loads experienced by the turbine. One of the most significant load increases is in the lead-lag direction of the wind turbine blades. The increase in lead-lag load is due to the increase in weight of the larger blades. Current wind turbine blades have lead-lag and flapwise loads on the same order of magnitude. The blades of wind turbines are critical components, and full-scale blade fatigue testing is a necessary step to validate blade designs. A collaborative effort between the National Renewable Energy Laboratory (NREL) and Embry-Riddle Aeronautical University (ERAU) has resulted in the development of the dual-axis resonant Phase-Locked Excitation (PhLEX) fatigue test method. The PhLEX method fatigues wind turbine blades by loading both flapwise and lead-lag directions simultaneously at the leadlag fundamental frequency while controlling the phase between the directional loadings. The PhLEX method offers a load distribution that accurately resembles field operation loads while decreasing test duration. A proof-of-concept test of the PhLEX method was conducted at the National Wind Technology Center (NWTC) in July of 2012. It was found that a dual-axis resonant test running at the lead-lag fundamental frequency could be run in a controlled manner, with both the lead-lag and flapwise directions demonstrating first mode deflections. The PhLEX proof-of-concept test set-up and the results and conclusions of the proof-of-concept test are presented in this thesis.