Time-of-Flight — Definition & NDT Use
The elapsed time for an ultrasonic wave to travel from the transducer to a reflector and return. Time-of-flight is used with sound velocity to calculate the distance (depth) to the reflector: distance = (velocity × time-of-flight) / 2. Accurate time-of-flight measurement is critical for precise defect depth determination. Digital systems measure time-of-flight with high precision, enabling accurate flaw characterization.
From the inspector's bench, Time-of-Flight is run as a defined sequence: equipment verification on a known reference, scan setup against the procedure, scanning the part, and writing the indications into the report. A piezoelectric element converts the electrical pulse into a mechanical wave at the chosen frequency, transmits it into the part through couplant, and then converts the returning echo back into a voltage that the flaw detector digitises and displays on the screen. Procedure writing, inspector qualification, and the reference block establish the chain that lets a remote engineer trust an indication called a kilometre away from the office.
Time-of-Flight is selected when the failure mode the engineer cares about — surface crack, internal void, wall loss, lack of fusion — lines up with what the technique is physically capable of detecting.
ASME Section V Article 4
Ultrasonic examination methods for welds and components.
ASTM E114 / E164 / E2375
ASTM straight-beam, contact, and wrought-product UT practices.
ISO 16810 / ISO 16811
General principles and sensitivity setting for industrial UT.
The most expensive mistake with Time-of-Flight is treating it as a yes/no test rather than a characterisation — an indication called without a sizing strategy forces a repair where a fitness-for-service review might have left the part in service.
What does "Time-of-Flight" mean in NDT?
The elapsed time for an ultrasonic wave to travel from the transducer to a reflector and return. Time-of-flight is used with sound velocity to calculate the distance (depth) to the reflector: distance = (velocity × time-of-flight) / 2
Which standards govern the use of Time-of-Flight?
Time-of-Flight is most often referenced under ASME Section V together with the relevant ASTM practice or the matching ISO standard for the method; the contract or purchase order will name the controlling document and edition for any specific job.
What other NDT concepts should I read alongside Time-of-Flight?
The most directly related entries in this glossary are "echo", "sound velocity", "distance amplitude correction"; reading those together gives you the surrounding vocabulary used in inspection reports and procedures.