TIME OF FLIGHT DIFFRACTION (TOFD)

TOFD works on the principle of diffracted ultrasonic waves. Two ultrasonic probes are positioned on opposite sides of the weld, with one probe emitting an ultrasonic pulse and the other probe receiving the diffracted signals. The diffracted signals arise from the edges of any flaws present in the weld, and their time of flight is measured to determine the flaw's position and size. 

TOFD is commonly used for weld inspection in various industries, including oil and gas, power generation, construction, and manufacturing. It is particularly effective for detecting and sizing planar flaws, such as cracks, lack of fusion, and lack of penetration in welds. TOFD is especially useful for detecting critical flaws that could lead to structural failure or compromise the integrity of welded components. 

Several international standards provide guidelines for TOFD, including 

1. ASME Section V, Article 4 - "Ultrasonic Examination"; 
2. ISO 10863 - "Non-destructive testing of welds - Ultrasonic testing - Use of time-of-flight diffraction technique (TOFD)"; and 
3. EN 583-6 - "Non-destructive testing - Ultrasonic examination - Part 6: Time-of-flight diffraction technique as a method for detection and sizing of discontinuities." 

1. Accurate Flaw Sizing: TOFD provides accurate sizing of flaws, enabling quantification of flaw length and height.
2. Full Coverage: The technique provides full coverage of the weld volume between the two probes, ensuring comprehensive flaw detection.
3. Real-time Imaging: TOFD produces a cross-sectional image of the weld, facilitating immediate visualization and analysis of flaws.
4. No Flaw Orientation Dependency: TOFD is not sensitive to the orientation of flaws, allowing for reliable flaw detection regardless of their alignment.
5. Quantitative Data: The TOFD technique provides measurements of flaw dimensions, allowing for accurate assessment and comparison over time.

1. Skilled Operator Required: TOFD requires skilled operators who are proficient in probe positioning, alignment, and interpretation of the obtained data.
2. Surface Conditions: The quality of the ultrasonic signals can be affected by surface roughness, coatings, and other factors, requiring proper surface preparation.
3. Limited Detection of Non-Planar Flaws: TOFD is most effective for planar flaws and may have limitations in detecting non-planar or irregularly shaped flaws.
4. Coupling Medium: Adequate coupling between the probes and the inspected surface is crucial for optimal signal transmission, requiring attention and care.

It's essential to follow applicable standards, guidelines, and best practices when conducting TOFD inspections. Proper training, calibration, and validation of equipment are necessary to ensure reliable and accurate flaw detection and sizing. Additionally, TOFD is often used in combination with other inspection techniques to obtain a comprehensive assessment of weld integrity.