What Is Time of Flight Diffraction (TOFD) In NDT? Advantage & Disadvantage Of TOFD.

Time Of Flight Diffraction (TOFD)

Time-of-Flight Diffraction (TOFD) is an advanced ultrasonic non-destructive testing (NDT) technique that accurately locates and sizes material defects, such as cracks, by detecting the ultrasonic waves diffracted from the tips of the defects, rather than reflected signals. 

This method uses a pair of ultrasonic transducers on opposite sides of the inspection area, with one acting as a transmitter and the other as a receiver, and is widely used for inspecting welds in industries like oil and gas, power generation, and petrochemicals. 

Principle of TOFD

• It uses ultrasonic longitudinal waves sent into the material by a transmitter probe.
• A receiver probe is placed on the opposite side of the weld.
• Instead of relying only on reflected echoes (like conventional UT), TOFD mainly uses diffracted sound waves produced at the tips of flaws or cracks.
• By measuring the time of flight (how long it takes the sound to travel from transmitter → flaw tip → receiver), the exact position and size of defects can be determined.

Key Benefits of TOFD

• High Accuracy: Provides very accurate sizing of defects. 
• All Defect Types: Can detect various flaws, including cracks, lack of fusion, and slag inclusions, regardless of their orientation. 
• Permanent Records: Digital data is recorded, allowing for future analysis and comparison. 
• Safety: The method is non-radiative, meaning it does not use radiation during inspection. 
• Fast Inspection: Offers a quick method for weld inspection, with immediate availability of results

How TOFD Works?

Transducer Placement: Two ultrasonic probes are placed on the surface of the material, typically on opposite sides of the weld, with one transmitting and the other receiving. 

Ultrasonic Pulse: The transmitting probe sends an ultrasonic pulse into the material. 

Diffraction and Lateral Wave: Instead of relying on reflected signals, TOFD detects the diffracted sound waves that emanate from the sharp tips of a defect. It also uses the "lateral wave" which travels along the surface of the defect or crack. 

Time of Flight Measurement: The system measures the time it takes for these diffracted signals (and the lateral wave) to travel between the probes. 

Data Analysis: By measuring the "time of flight," the system can precisely determine the position and size of the defect within the material. 

Basic Setup for TOFD?

• Two probes (transmitter and receiver) are placed on opposite sides of the weld.
• A lateral wave, back wall echo, and diffracted signals are recorded.
• Data is displayed as a grayscale image (known as a TOFD scan) showing defect locations.

Common TOFD Applications 

• Weld inspection in pipelines, boilers, pressure vessels, and structural steel.
• Corrosion monitoring.
• Crack depth sizing.

Key advantages of TOFD:

High accuracy: TOFD is highly accurate for sizing the height of defects, especially cracks, making it ideal for tracking their growth over time.

Orientation-independent: It can detect flaws regardless of their orientation, which is a limitation for some other ultrasonic methods.

Speed: TOFD is a rapid scanning technique that can cover large areas, including thick materials, quickly.

Digital records: The inspection provides a permanent, digital record for future reference and comparison.

Safety: It is safer than radiography because it uses ultrasound instead of radiation. 

Disadvantage Of TOFD:

Dead zone: TOFD has a "dead zone" just below the inspection surface where it cannot reliably detect defects. For this reason, it is often combined with other techniques like Phased Array Ultrasonic Testing (PAUT) to ensure complete coverage.

Difficult interpretation: While effective, interpreting the data requires a high degree of training and experience.

Equipment costs: The specialized equipment and training can make the technique more expensive than conventional methods.

Double-sided access: TOFD typically requires access to both sides of the weld for the transmitter and receiver probes.