Presentation Description: Numerous pretensioned flange bolts are used in wind turbines to join different wind tower segments. Consequently, to maintain the structural integrity and stability of a wind tower, it is imperative that the proper torque be applied to the flange bolts. To maintain proper pretension conditions, current methods require the periodic re-tightening of 100% of the flange bolts in a wind tower by mechanical tools. However, this method is labor, time, and cost intensive due to the involvement of a hydraulic jack and torque wrench. A need exists in the industry for a nondestructive method that can identify bolts with unacceptable tension while minimizing the workload of mechanical re-tightening. An ultrasonic nondestructive method based on the acoustoelastic effect is proposed to determine residual torque on wind turbine tower flange bolts. The initial static stress affects the effective elastic moduli of the material, through which the dynamic elastic wave propagates, resulting in corresponding changes to the ultrasonic wave speed. The impact of static stress on ultrasound velocity is material and direction dependent. Extensive lab tests indicate consistent changes in both longitudinal and shear wave velocities corresponding to applied torque on M36 steel flange bolts. A practical NDT methodology applicable to the field inspection was developed. Given the relationship between bolt torque and ultrasonic wave speed, one intuitive NDT method for determining bolt torque is to measure the sound speed of an axial longitudinal wave and retrieve the bolt torque based on their linear relation. This NDT approach requires accurate measurement of bolt length using a big jaw caliper, which is time consuming and not practical in the field.
Methodology: To develop a practical NDT methodology for field deployment, a combined approach using longitudinal and shear waves is identified to achieve a compact NDT instrument and high inspection efficiency.
Learning Objectives:
Upon completion, participant will be able to predict existing torque on flange bolts using the nondestructive testing method developed in this study with an accuracy of 200 lb-ft.
