Understanding TOFD (Time of Flight Diffraction)

What Is TOFD?
Time of Flight Diffraction (TOFD) is an advanced ultrasonic inspection technique widely used for examining welds and other critical components. Unlike conventional ultrasonic testing, which detects echoes reflected from defects, TOFD identifies ultrasonic waves that are diffracted from the tips of discontinuities.

A typical TOFD setup uses two probes positioned on opposite sides of the weld. One probe transmits longitudinal ultrasonic waves while the other receives them. As the sound travels through the material, any crack or discontinuity causes the wave to diffract from its upper and lower tips. By measuring the arrival time of these signals, the system can accurately determine the defect's location and height.

Inspection data is displayed as a high resolution B Scan, allowing defects to be measured, recorded and reviewed long after the inspection has been completed.

How TOFD Works

The inspection process follows a simple sequence:

• A transmitting probe introduces longitudinal ultrasonic waves into the material.
• The sound travels through the component toward the receiving probe.
• If the wave encounters a discontinuity, diffraction occurs at the defect tips.
• The receiving probe detects these diffracted signals.
• The system calculates the defect position and through wall height from the measured travel times.
• Results are displayed as a B Scan image for interpretation and reporting.

Because TOFD relies on diffraction rather than reflected echoes, it remains highly effective even when defects are not ideally oriented to reflect sound back toward the probe.

Why TOFD Is So Effective

Accurate Defect Sizing
TOFD is recognised for its ability to measure defect height with excellent accuracy, making it one of the preferred techniques for weld integrity assessments and Fitness for Service evaluations.

Detects Defects Regardless of Orientation
Since the technique responds to diffraction from defect tips, it is highly effective at detecting planar discontinuities that may produce weak or no reflections during conventional pulse echo inspections.

Fast Inspection Speeds
Automated scanning enables large weld lengths to be inspected efficiently, reducing inspection time while maintaining high quality results.

Permanent Inspection Records
All scan data is stored digitally, providing a permanent inspection record that can be reviewed, compared with future inspections and included in inspection reports.

Suitable for Critical Assets
TOFD is commonly used on pressure retaining equipment where accurate sizing of discontinuities is essential for engineering assessments and continued safe operation.