Technical BulletinTechnical Bulletin

Time of Flight Diffraction Time of Flight Diffraction (TOFD)(TOFD)

The Ultrasonic Time of Flight Diffraction (TOFD) technique provides a means of obtain-ing fast, reliable ultrasonic information that quickly alerts personnel of undesirable condi-tions existing within a weldment. TOFD tech-nique offers the following advantages in com-parison to radiography:

• Radiation hazard-free area – This allows work to be performed by other personnel in the vicinity of the examination area without any radiation safety precautions.

• Fast, accurate results – This process pro-

vides information at the examination area without the need to develop film, deter-mine radiograph quality conformance (i.e. density and sensitivity) and film interpre-tation. Less expensive than radiography per foot of weld.

• Volumetric information – As opposed to

radiography that provides a 2-D view of a weldment, and radiography’s detection dif-ficulty with cracks not directly oriented to the cone of radiation, this process can pro-vide accurate length and depth informa-tion of recorded indications.

• Permanent record – The scan image files

can be saved for future reference and evaluation.

The American Society of Mechanical Engi-neers through code case 2235-4 implemented the use of Ultrasonic TOFD in the Boiler and Pressure Vessel code. It was intended as a method to be used in lieu of radiography for materials greater than 0.5 inch.

Side-wall Non-fusion

OVERVIEW:

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APPLICATIONS:

• FLAW DETECTION ? SIZING ? EVALUATION ? HARD COPY AND ELECTRONIC DOCUMENTATION

• IN– SERVICE FLAW MONITORING

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The Time of Flight Diffraction technique employs the use of two compressional wave transducers using an-gles between 300 and 700 set at a calculated distance apart in the pitch and catch mode. When the com-pressional wave strikes a defect diffracted waves are generated from the extremities of the defect. The dif-fracted waves are detected by the receiving trans-ducer. These waves are captured and presented in the “D –Scan” view in real time. Through the system soft-ware defects can be sized for length, depth from the examination surface and through wall dimension. “A-Scan” views of the defect are also available for the technician to aid in the evaluation of the defects. The first signal received (1) is from the lateral wave. This is a creeping wave at the surface with the same velocity of the compressional or longitudinal waves. The second (2) and third (3) signals are the diffracted waves from the tips of the flaw. The fourth (4) signal is the backwall representing one complete sound path. The signals past the backwall are mode con-verted shearwave.

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TECHNIQUE:

CONCLUSION: Time of Flight Diffraction (TOFD) is a proven ultrasonic examination technique for the rapid detection and sizing of fab-rication and service induced flaws. TOFD has a very high probability of de-tection for planar and spherical defects compared to other nondestructive test-ing methods such as radiography. Defect through wall dimensions can be precisely measured for fitness for service and risk reduction calculations.

LIMITATIONS:

• The weld must be accessible from both sides on the examination surface.

• A dead-zone exist for defect detec-tion slightly below the examination surface.

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Technical BulletinTechnical Bulletin

Time of Flight Diffraction Time of Flight Diffraction (TOFD)(TOFD)