Time-of-flight diffraction (TOFD) is a sensitive and accurate ultrasonic method for the nondestructive testing of welds.
TOFD implements tip diffraction techniques to size defects. Tip diffraction has been designed so that the tips of a crack are identified when struck by an ultrasonic wave and will diffract the signals back to the other probe. The depth of these tips can be determined from the diffracted energy.
TOFD uses the time of flight from an ultrasonic pulse to determine the position of a reflector.
A pair of ultrasonic probes are positioned on either side of a weld and coupled to a TOFD system via an encoded scanner. One of the probes emits an ultrasonic pulse and is in turn picked up by the probe on the other side. When a crack is present there is a diffraction of the ultrasonic wave from the tip(s) of the crack(s). Measuring the time of flight of the pulse allows the depth of a crack tip to be calculated by technician using a TOFD system. Subsequently the technician sizes the indications to determine if they are acceptable or rejectable according to code. Typically TOFD is complimented by Conventional or PAUT methods.