Time Of Flight Diffraction Constantia Kloof

What is Time of Flight Diffraction?

Time of Flight Diffraction (TOFD) is an advanced ultrasonic NDT method that detects and accurately
sizes internal flaws (especially planar defects like cracks, lack of fusion, and lack of penetration) by measuring the time difference between diffracted ultrasonic signals from the tip of a defect, not
relying on amplitude like conventional UT.
TOFD = “Ultrasonic Holography” — it maps the entire weld volume in a single scan with high
accuracy (±1 mm in height).

How It Works:

1. Probe Pair
2. Signal Types (in A-scan):
3. Time of Flight Measurement
4. D-Scan Imaging

Applications:

• Weld inspection (pipelines, pressure vessels, structural steel).
• Crack sizing (fatigue, stress corrosion, HIC).
• In-service inspection (CUI, mid-wall flaws).
• Thick welds (>12 mm) — ideal up to 300 mm.
• Austenitic & dissimilar welds (with L-wave probes).

Limitations:

• Dead zones: Near surface (~3 mm) and near back wall.
• Requires two-sided access (or single-side TOFD with creep wave).
• Poor for volumetric defects (porosity, slag — low diffraction).
• High skill level (Level 2/3).
• Sensitive to surface condition (needs clean, parallel).