Time-of-Flight Diffraction (ToFD)

Time-of-flight diffraction (ToFD) is an advanced ultrasonic testing (UT) method used for detecting and sizing discontinuities in welded structures and components. It is recognized for its exceptional accuracy, speed, and reliability, making it a preferred choice over traditional radiographic and pulse-echo UT techniques.

How ToFD Works ?

ToFD systems employ a pair of ultrasonic probes placed on opposite sides of a weld joint or area of interest. One probe acts as a transmitter, emitting ultrasonic pulses, while the other serves as a receiver, capturing the transmitted signals.

Signal Interpretation in ToFD

1. Lateral Wave – A portion of the ultrasonic wave travels directly along the surface between the probes.
2. Back-Wall Reflection – Some of the energy travels through the material, reflects off the far wall, and is received by the opposite probe.
3. Diffraction from Defects – If a discontinuity (e.g., a crack) is present, the ultrasonic waves diffract from its top and bottom edges, producing additional signals. By analyzing the time of flight of these diffracted signals, the depth and size of the defect can be accurately determined.

ToFD’s ability to measure the through-wall height of crack-like defects with an accuracy of ±1mm makes it highly effective for structural integrity assessments.

Why ToFD is Superior to Other Testing Methods ?

ToFD was originally developed for monitoring defect growth in the nuclear industry and has since been independently validated as one of the most accurate techniques for detecting and sizing discontinuities in ferritic welds. Compared to conventional methods like radiography and manual UT, ToFD offers:

• Higher Accuracy – Precise measurement of defect size and depth using trigonometry.
• Faster Inspection – Large areas can be scanned quickly with minimal disruption.
• Permanent Digital Records – Provides high-resolution images and data for future reference.
• Independence from Defect Orientation – Unlike conventional UT, which relies on wave reflection angles, ToFD detects defects regardless of their orientation.
• Compliance with Industry Standards – Meets ASME Code Case 2235 requirements.

TWI’s Expertise in ToFD

TWI has extensive research, development, and field experience in applying ToFD across various industries. Key applications include:

• Defect Growth Monitoring – Long-term assessment of crack propagation.
• Gas Storage Bullet Weld Inspections – Ensuring the integrity of critical storage vessels.
• Pressure Vessel and Piping Inspections – Used in lieu of radiography testing (RT).
• Thermal Fatigue Crack Detection – Evaluating turbine and generator components.
• In-Service Inspections – Detecting and sizing flaws for fitness-for-service (FFS) assessments.

Advanced Equipment Used by TWI

TWI utilizes state-of-the-art ToFD equipment, including:

• EddyFi Gekko
• Zetec Topaz
• Evident MX2, SX, and X3

Additionally, TWI has a range of versatile scanners designed for both manual and automated site applications.

Benefits of ToFD

• Fast and Accurate Defect Detection – Locates and sizes defects with precision.
• High Reproducibility – Proven accuracy of <0.5mm.
• Improved Safety – No radiation required, allowing inspections without evacuation.
• Immediate Inspection Results – Real-time data analysis improves decision-making.
• Works at High Temperatures – Can be performed on components above 200°C.
• Better Risk Management – Higher probability of defect detection (POD).
• Enhanced Data Collection – B-scan and D-scan formats provide comprehensive imaging.

Industries Benefiting from ToFD

ToFD is widely used across multiple sectors, including:

• Oil, Gas, and Petrochemical – Inspection of pipelines, pressure vessels, and storage tanks.
• Offshore and Marine – Evaluating the integrity of offshore structures and ship welds.
• Nuclear Industry – Ensuring the safety of critical nuclear components.
• Manufacturing – Quality control of welded structures.
• Aerospace and Defense – Inspecting aircraft components and military equipment.
• Infrastructure and Bridges – Assessing the structural health of bridges and heavy civil structures.

Conclusion

ToFD has revolutionized ultrasonic inspection by providing a highly accurate, reliable, and efficient method for detecting and sizing discontinuities. Its ability to produce digital records, work at high temperatures, and deliver real-time results makes it a preferred choice in industries where structural integrity is critical. With its extensive expertise and advanced ToFD equipment, TWI continues to lead the way in ensuring safe and reliable operations across multiple sectors.