Time of Flight Diffraction (TOFD)
Time-of-flight-diffraction (TOFD) is an ultrasonic method for the detection, sizing and monitoring of flaws in new construction and in-service welds and components.
TOFD employs two longitudinal wave (L-wave) angle beam transducers arranged diametrically on opposite sides straddling the weld or base material under examination. One transducer acts as a transmitter of ultrasonic energy, while the other receives the returning signals. The transducer, pulse, and amplifier characteristics are selected to generate a broad distribution of energy allowing for full weld coverage with single scans. An electronic encoder records the position of the probe along the weld during the scan which enables a real-time display and recording of the digital images.
Defect detection is identified when there is a diffraction of the ultrasonic wave from the tip(s) of the defect(s). Measurement of the time-of-flight of the pulse allows the depth of a defect tip to be calculated using the TOFD technique. This allows the NDT technician to size the indications to determine acceptance or rejection per the applicable code and/or client specification.
TOFD is often complimented by using other ultrasonic conventional and/or phased-array (PAUT) methods.