Time of Flight Diffraction
High-precision ultrasonic technique for accurate flaw detection, sizing, and weld integrity assessment.
Overview
Time of Flight Diffraction (TOFD) is one of the most accurate ultrasonic inspection methods in modern NDT. Instead of relying on the amplitude of reflected sound, TOFD measures the time taken by ultrasonic waves diffracted from the tips of flaws — yielding sizing accuracy that conventional pulse-echo techniques simply cannot match.
How the method works
TOFD uses a pair of angled probes — one transmitter, one receiver — placed on opposite sides of a weld. Diffracted signals from upper and lower flaw tips are captured and time-gated, producing a grey-scale D-scan image of the entire weld volume in a single pass.
Because TOFD is sizing-based rather than amplitude-based, it is largely independent of flaw orientation. This makes it exceptionally reliable for detecting planar defects such as lack of fusion, lack of penetration, and crack-like indications that traditional radiography or pulse-echo UT can miss.
Every TOFD scan produces a permanent digital record. This enables true engineering-grade reporting, repeat inspections for trending, and full traceability for fitness-for-service assessments under codes like API 579 and BS 7910.
Why AUTRESONUS for TOFD
- 29+ years of field-proven TOFD experience across oil & gas, marine, and power industries.
- Level II and Level III TOFD-certified technicians compliant with ISO 9712 and ASNT SNT-TC-1A.
- Industry-leading scanners and calibrated wedges tailored to each weld geometry — no compromises on data quality.
- Engineering-grade reports with full A-scan, B-scan, and D-scan data delivered alongside flaw assessments.
- Combined TOFD + PAUT inspections in a single mobilisation for maximum coverage and efficiency.
Key Benefits
- Highly accurate flaw sizing and depth measurement (±1 mm typical)
- Single-pass volumetric inspection of complete welds
- Permanent digital records for traceability and trending
- Effective on thick-walled components from 6 mm to 300 mm+
- Fast scanning speeds reduce plant downtime significantly
- Radiation-free alternative to radiographic testing (RT)
Our Inspection Process
- 1
Scope & calibration
Review weld drawings, set probe spacing, and calibrate against a representative reference block.
- 2
Surface preparation
Light cleaning of scan paths and couplant application for stable acoustic contact.
- 3
Encoded scanning
Mechanised scanner pass with synchronised position encoding for accurate flaw mapping.
- 4
Data analysis
Offline review of D-scan, B-scan, and lateral wave data by Level II/III analysts.
- 5
Engineering report
Defect characterisation, sizing, and code-compliant accept/reject with raw data attached.
Applications
- Pressure vessel and storage tank welds
- Pipeline girth and longitudinal welds
- Refinery and petrochemical reactors
- Offshore structural and riser welds
- Power generation steam piping and headers
- In-service weld monitoring and trending
Codes & Standards
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