Precision Flaw Sizing Through Diffracted Waves
Time-of-Flight Diffraction (TOFD) is a high-resolution ultrasonic method designed for the accurate sizing of planar flaws such as cracks and lack of fusion. Unlike conventional UT, which depends on reflected echoes, TOFD uses the time of arrival of diffracted waves from the tips of defects to calculate their exact height and depth.
The system employs a transmitter and receiver probe pair positioned on opposite sides of the weld. The transmitter emits a broad ultrasonic beam, while the receiver records the direct, backwall, and diffracted waves. The resulting time-of-flight data generates grayscale D-scan images that provide a precise representation of flaw geometry.
BQCIS integrates TOFD both as a stand-alone and complementary technique with Phased Array UT (PAUT), particularly in Fitness-for-Service (FFS) and Engineering Critical Assessment (ECA) programs where accurate flaw sizing is vital.
Key TOFD Activities
Key Benefits of TOFD
Accurate Flaw Sizing
Precisely measures through-wall crack height and defect location for engineering assessments.
Sensitive to Planar Defects
Exceptional for detecting and sizing cracks and lack-of-fusion indications, regardless of orientation.
Rapid & Repeatable
Allows fast, encoded scanning over long weld seams with minimal setup time and consistent results.
Digital Traceability
Produces permanent encoded D-scan images for full traceability and audit documentation.
Success Story
TOFD Accurately Sizes Weld Root Crack for FFS
During inspection of a pressure vessel seam weld, PAUT results showed a potential crack at the weld root that required accurate through-wall sizing for Fitness-for-Service evaluation.
BQCIS performed a focused TOFD scan using optimized probe geometry and high-frequency transducers to capture diffracted signals from flaw tips.
TOFD imaging provided clear upper- and lower-tip diffraction patterns, enabling precise crack height measurement. The results supported continued operation until the next shutdown, avoiding unnecessary repairs.