Loading...
ADVANCED NDT

Time of Flight Diffraction (TOFD)

The most accurate ultrasonic technique for through-wall sizing of planar defects — delivering the defect height measurement precision required for fitness-for-service assessment and fracture mechanics evaluation on critical pressure-containing welds.

 What Is Time of Flight Diffraction Testing?


Time of Flight Diffraction (TOFD) is a pulse-echo ultrasonic technique that detects and sizes defects by measuring the diffraction of ultrasonic energy at defect tips — rather than relying on specular reflection as in conventional UT. Two transducers are positioned symmetrically either side of the weld — a transmitter and a receiver — and the time of flight of signals diffracted from the top and bottom tips of any planar defect is used to calculate its precise through-wall height. 

This tip-diffraction principle gives TOFD its defining advantage over all other ultrasonic techniques: unmatched accuracy in through-wall defect sizing. Where conventional UT can approximate defect height from amplitude-based methods, TOFD measures it directly from the physics of tip diffraction — providing the accurate defect sizing data that fracture mechanics and f itness-for-service (FFS) calculations require.

What TOFD detects and sizes: 

  • Planar defects: lack of fusion, incomplete penetration, cracks, and fatigue cracks 
  • Through-wall height (depth extent) — the critical parameter for FFS assessment 
  • Depth to defect top-tip — enabling precise location within the weld cross-section 
  • Weld volume scanning from a single probe pair pass — without angular adjustment 

TOFD limitations to understand: 

  • Reduced near-surface sensitivity (lateral wave dead zone) — typically the top 2–3mm 
  • Requires supplementary technique (PAUT or MT/PT) for surface-breaking indication assessment 
  • Not suitable as a standalone technique for rough or highly attenuating materials without prior characterisation

Where We Apply TOFD


  • Pressure vessel and heat exchanger girth and seam weld inspection — fabrication and in service 
  • High-integrity piping weld inspection in Oil & Gas and Petrochemical facilities 
  • Power generation component weld inspection — steam drums, headers, and pressure boundary welds 
  • Structural weld inspection where accurate defect characterisation is required 
  • Defect sizing for fitness-for-service assessment under API 579 / BS 7910 In-service crack growth monitoring in critical welds 
  • Replacement or supplementation of radiographic testing on thick-section weld inspection

 Applicable Codes and Standards


  • ASME Section V — Article 4 (TOFD as a supplemental examination technique) 
  • ASME Code Case 2235 — TOFD as an alternative to RT for pressure vessels 
  • EN ISO 10863 — Non-destructive testing of welds — Ultrasonic testing using TOFD technique 
  • EN ISO 15626 — Non-destructive testing of welds — TOFD technique — Acceptance levels 
  • BS 7910 — Guide to methods for assessing the acceptability of flaws in metallic structures — TOFD sizing input for 
  • ECA API 579-1 / ASME FFS-1 — Fitness-for-service — TOFD defect sizing input for Level 2 and Level 3 assessments 
  • DNV-OS-F101 and operator-specific TOFD procedure qualification requirements
Advanced Techniques
Contact Us

Email:
info@altairengg.com

Phone:
+91 98765 43210