Time of Flight Diffraction (TOFD)

First introduced in the 1970s, Time of Flight Diffraction (TOFD) is a highly accurate way to size planar flaws (cracks) for the nuclear and chemical industries. Advances in technology have made TOFD an extremely accurate and reliable defect detection tool, which can be used to gain data quickly with a positive result.

The Technology

Time of Flight Diffraction uses two ultrasonic transducers in a pitch-catch method to introduce ultrasonic sound waves into the test piece for inspection. The transducers produce three distinct waves with different characteristics, paths and velocities.

Both longitudinal (compression) waves and shear waves are produced, which spread and give full volumetric coverage of the test piece. There are two different longitudinal waves that are introduced into the material, each with a different sound path.

The first L-wave travels between the transducers just under the surface of the material and is called the Lateral wave.

The second is angled at the back wall and skips to the receiving transducer in a full-V path.

From these L-waves, shear waves are formed and propagate through the volume of the material. When defects are present, diffracted energy from the edges or tips are reflected and picked up by the receiving transducer.

Defect Detection

The sensitivity of the ultrasonics involved in TOFD make this inspection much easier to evaluate and size defects for length and depth.

Applications

TOFD is generally used on piping and vessel welds. It is a useful tool in finding defects from equipment production, as well as service-induced defects. Weld washout due to process, cracking in coke drum welds; general existing or new weld production defects as well as surface corrosion can be detected and measured using the TOFD technique. Since there is no need to raster the transducers, TOFD is preformed in a single parallel line scans which makes data collection fast. Up to 250 linear feet of weld can be inspected a day using this technique and equipment.