Time of Flight Diffraction (ToFD) is a high sensitivity, advanced ultrasonic non-destructive testing (NDT) method used to detect and accurately size weld defects, such as cracks, by measuring the time of flight of diffracted waves.
ToFD uses a pair of ultrasonic testing probes that are placed on opposite sides of a weld-joint or area of interest in a pitch-catch configuration. A transmitter probe emits an ultrasonic pulse that is picked up by a receiver probe on the opposite side. In an undamaged part, the signals picked up by the receiver probe are from two waves; one that travels along the surface (lateral wave) and one that reflects off the far wall (back-wall reflection). When a discontinuity such as a crack is present, there is a diffraction of the ultrasonic sound wave from the top and bottom tips of the crack. Using the measured time of flight of the pulse, the depth of the crack tips can be calculated using trigonometry. This method is even more reliable than traditional radiographic, pulse echo manual UT and automated UT weld testing methods, providing great accuracy for measuring the critical through-wall size of crack-like-defects. The accuracy of greater than ±1mm can be obtained in a wide range of material thicknesses.
ToFD was initially developed as a method of accurate monitoring and sizing of through-wall height of in-service discontinuities in the nuclear industry, but time of flight diffraction techniques have now been independently validated as among the most effective for locating and sizing discontinuities in ferritic welds. By utilising wide, diverging ultrasonic beams, a single scan with TOFD can cover a large area of material, making it ideal for rapid, comprehensive, structural integrity assessments.
ToFD is highly valued for its speed, digital data storage, and ability to detect flaws regardless of their orientation, making it standard process in the nuclear, oil and gas, and petrochemical industries.