1. The principles of Time of flight diffraction (TOFD):

TOFD is usually performed using longitudinal waves as the primary detection method. Ultrasonic sensors are placed on each side of the weld. One sensor sends the ultrasonic beam into the material and the other sensor receives reflected and diffracted ultrasound from anomalies and geometric reflectors. 

TOFD provides a wide area of coverage with a single beam by exploiting ultrasonic beam spread theory inside the wedge and the inspected material. 

When the beam comes into contact with the tip of a flaw, or crack, diffracted energy is cast in all directions. 

Measuring the time of flight of the diffracted beams enables accurate and reliable flaw detection and sizing, even if the crack is off-oriented to the initial beam direction. 

During typical TOFD inspections, A-scans are collected and used to create B-scan (side view) images of the weld. Analysis is done on the acquisition unit or in post-analysis software, positioning cursors to measure the length and through-wall height of flaws.

TOFD offers great accuracy for measuring the critical through-wall size of crack-like-defects. The accuracy of greater than ±1mm can be obtained in a wide range of material thickness.

Picture 1: The principles of Time of flight diffraction (TOFD).

2. Advantages and disadvantages of TOFD

- Advantages:

• The accuracy of flaw location, position and sizing.
• Not dependent on defect orientation relative to nominal probe angle.
• Semi-automatic or automatic inspection with digitized results and permanent records.
• Less safety issues than RT.
• Equipment is portable.
• Highly sensitive to all weld flaw types.

- Disadvantages:

• Dead zone close to the surface.
• Not effective for defects lying parallel to the inspection surface.
• Gain must be very high.
• Minimum thickness: 6mm with a diameter of 4”. No limitation on maximum thickness.
• Some geometries impossible to be inspected.
• Higher initial cost of equipment.
• Need of experienced and skilled operator.