Throughout its operating life, a wind turbine blade is subjected to considerable environmental forces to help ensure blade assembly, strength and durability, and that the top and bottom fiberglass blade shells are bonded together around a shear web. Because of its laminar layup structure, turbine blades are subject to cracks running parallel to the surface. Cracks can be caused by naturally-occurring stresses or by weaknesses resulting from manufacturing anomalies and can have a significant impact on the blade’s overall structural integrity. Since internal cracking is not detectable by radiography, nondestructive ultrasonic flaw detection offers a simple method for locating internal voids.
Understanding nondestructive ultrasonic testing
Ultrasonic flaw detection is a comparative process in which the echo pattern generated by a good part is compared with the echo pattern from a test piece. By observing echo patterns on a display screen, a trained operator can quickly and reliably verify material integrity. Since sound waves will reflect from voids or cracks, changes in the echo pattern indicate changes in the internal structure. In testing fiberglass, the operator typically looks for the presence of echoes within a marked gate or window representing the interior of the test piece. While the inhomogeneous nature of fiberglass frequently generates scatter noise reflections even from solid material, a crack whose area approaches the diameter of the sound beam typically returns strong localized indications easily recognizable by a trained operator.
With a good part, sound waves travel to the bottom surface and create a reflection (Figure 1). When a crack is present, however, the sound will reflect from the crack and create a peak within the zone representing the middle of the part, indicating a large discontinuity in the part (Figure 2).
Up-Tower: Elevate your inspections
With powerful analytical capabilities and exceptional portability, modern instruments, such as Olympus’ EPOCH® 6LT ultra-portable flaw detector, provide fast, accurate analysis of failure points to help keep wind turbines on-line and productive.
A specially-designed rope access kit attaches the 1.95-pound (890 g) instrument to a user’s leg or harness connection points, while an ergonomic design, large rotary knob, and simple menu buttons enable one-handed manipulation (even when wearing gloves) of the unit’s intuitive menu. A bright, 5.7-inch display with outdoor mode helps users view A-scans clearly in difficult lighting conditions, and optional Wi-Fi enables users to wirelessly export data for easy file archiving and remote data access.
EPOCH is a registered trademark of Olympus Corporation.
For additional information, call (800) 225-8330 or visit www.olympus-ims.com/EPOCH-6LT