NDT Technique
Time of Flight
Diffraction
TOFD
Accurate detection and sizing of cracks, slag, and lack of fusion in true wall direction — enabling precise life assessment of your critical assets.
Overview
What is Time of Flight Diffraction?
Welding defects can compromise the stability and integrity of a welded structure. When you need confidence in testing of welds, Time of Flight Diffraction (TOFD), a high-performance non-destructive testing (NDT) technique, offers accurate detection and sizing of cracks, slag and lack of fusion in true wall direction — enabling you to assess the life of the vessel.
U-SONIX utilizes TOFD to inspect weld volume including heat-affected zones and in-service components to determine asset integrity. We accompany you throughout your testing project and offer individual solutions tailored to your application.
Material Thickness
13 – 450 mm
Operating Temperature
Up to 400°C
Technique Type
Longitudinal Wave
Data Format
RF Digital (A-Scan)
Technique
How TOFD Works
TOFD uses two probes and relies on diffracted — not reflected — energy for flaw detection, making it orientation-independent and highly accurate.
In TOFD, longitudinal waves propagate between two ultrasonic sensors to detect, locate, and size flaws based on the time of flight of diffracted beams — not their amplitude. This fundamental difference eliminates probe-orientation dependency that limits conventional pulse-echo techniques.
The technique depends on low-energy diffracted signals from the tips of flaws, allowing an exact absolute position and size to be measured and imaged. Because TOFD uses longitudinal waves only, mode-converted signals can be screened out, simplifying flaw characterization.
Data from the scan is collected in raw form and digitized in RF format (A-Scan), then assembled into a B-scan image for direct visual interpretation of flaw depth, height, and extent.
Transmitter Fires
The transmitter probe emits a longitudinal ultrasonic wave at an angle into the material under test.
Wave Propagation
The wave travels through the weld volume, including heat-affected zones, at the speed of sound in the material.
Diffraction at Flaw Tips
When the wave encounters a defect, the tips of the flaw diffract energy in all directions — unlike simple reflection.
Receiver Captures
The receiver probe on the opposite side captures the diffracted signals along with the lateral wave and backwall echo.
Time Measurement
Precise time-of-flight differences between signals reveal exact flaw depth, height, and position.
Digital B-Scan Image
Raw RF A-scan data is digitized and assembled into a B-scan image for direct visual interpretation.
Imaging
B-Scan: Visual Interpretation
Unlike conventional techniques that display single A-scans, TOFD produces a continuous B-scan image that shows the full cross-section of the weld. The lateral wave (top), backwall echo (bottom), and flaw tip signals appear as distinct horizontal bands.
The vertical separation between upper and lower flaw tip signals directly corresponds to the flaw height in true wall direction — enabling precise, code-compliant fitness-for-service assessments.
Lateral Wave — Reference signal along top surface
Back-Wall Echo — Full-skip reflection from far surface
Flaw Tips — Upper and lower tip diffraction signals
Advantages
Merits of TOFD
🎯
High POD
High Probability of Detection across all flaw orientations.
🌡️
High Temp Capable
On-stream inspection at temperatures exceeding 400°C.
📏
Wide Thickness Range
Applicable on material thicknesses from 13 to 450 mm.
☢️
No Radiation
No harmful ionizing radiation — safe for all environments.
🏭
No Production Stop
Inspect without interrupting your production processes.
📐
Exact Sizing
Exact indication length and depth — true wall direction.
💾
Digital Data
All inspection data captured and available in digital format.
🖼️
Visual Results
Inspection data represented visually for direct interpretation.
🔧
Wide Material Range
Suitable for a wide range of materials including HDPE pipes.
⏱️
Time-Based Sizing
Absolute flaw position and size measured from diffracted signals.
Comparison
TOFD vs Conventional Ultrasonics
| Feature | TOFD | Conventional UT |
|---|---|---|
| Detection basis | Time of flight (diffraction) | Amplitude (reflection) |
| Flaw orientation dependency | None — orientation independent | High — must align to flaw |
| Sizing accuracy | Exact height in true wall | Estimated from amplitude drop |
| Wave mode | Longitudinal only | Longitudinal + shear |
| Surface condition sensitivity | Low | High |
| Coupling dependency | Low | High |
| Data output | RF digital B-scan image | A-scan trace |
Capabilities
Applications
⚙️
Weld Inspection
Detection of cracks, slag, and lack of fusion in weld volume and heat-affected zones.
🏗️
Pressure Vessels
In-service inspection to assess vessel integrity without production shutdown.
🔩
Pipelines
Corrosion and crack sizing in pipelines including HDPE material types.
🛢️
Storage Tanks
Bottom and shell inspection for corrosion and fabrication defects.
🌡️
High-Temp Assets
On-stream inspection at operating temperatures up to 400°C.
📊
Fitness-for-Service
Accurate height sizing data to support ECA and remaining life calculations.
Get Started
Ready to Inspect with Confidence?
Our TOFD experts will help you design the right inspection solution for your weld and asset integrity requirements.