Time of Flight Diffraction (TOFD)

TIME OF FLIGHT DIFFRACTION (TOFD)

Time of flight diffraction is an advanced non-destructive testing technique used for weld inspection

TOFD uses the concept of Diffraction unlike reflection as in the case of PAUT. Typically, two probes that are single angled, highly damped, with wide beam spread are placed on both sides of the weld at a precalculated distance. One transducer sends the ultrasonic signal into the material and the other receives reflected and diffracted ultrasound from discontinuities and geometric reflectors. When the ultrasound signal comes in contact with the tip of a flaw, or crack, diffracted energy travels in all directions. By measuring the time of flight of the diffracted beams accurate and reliable flaw detection and sizing, even if the crack is off-oriented to the initial beam direction is possible.

Advantages and Limitations of Time of flight diffraction (TOFD)

•  TOFD is sensitive to virtually all types of defects regardless of orientation.
•  TOFD provides high speed inspection of heavy walled vessels even up to 250 mm thickness unlike radiography which would require longer exposure times and slower films for sensitivity.
•  The tip diffraction technique used in TOFD is so far the most accurate height sizing technique available.
•  When used for monitoring purpose of embedded cracks, TOFD gives reading within 0.3mm, which makes it suitable for in-service defect analysis, monitoring, crack propagation study.
•  Major disadvantage is the dead zone associated with the lateral wave ring-time and similarly with the back-wall. The near surface resolution is poor and therefore requires a combination of PAUT.
•  The diffracted signals associated with TOFD are 20-30 dB lower than pulse echo signals used in PAUT, therefore pre-amplifiers are required.
•  Access to both sides of the weld is required since it uses two probes in pitch-catch mode.
•  Interpretation of the data.

Application of Time of flight diffraction / TOFD Inspection

•  In-service inspection at ambient or high temperatures up to 200 oC is possible for the detection of damage mechanisms such as:
  •  Root corrosion
  •  Inter-granular stress corrosion cracking
  •  High Temperature Hydrogen Attack
  •  Fatigue cracking
  •  Monitoring of indications detected during earlier inspections
•  TOFD profiling of the back wall provides an effective means when looking for erosion of process and transport piping systems subjected to chemical attack.
•  With 80 deg wedges, a 15 MHz probe pair, TOFD can be used for inspection from 5mm and above thickness carbon steel welds.
•  TOFD insepction technique perfom high speed examination of heavy walled vessels even up to 250 mm thickness.