Time of Flight Diffraction (TOFD) is an advanced form of Ultrasonic Testing used for the detection and sizing of defects. In TOFD, a pair of ultrasonic probes sits on opposite sides of a weld. One probe, the transmitter, emits an ultrasonic pulse which is picked up by the probe on the other side, the receiver. When there are no defects, the sound waves travel in a straight line and reach the receiver in a predictable amount of time. But if there's a defect or crack in the weld, the sound waves will hit it and get diffracted, changing their path and the time it takes for them to reach the receiver.
By measuring this "time of flight" and the amount of diffraction, you can determine the size and location of the defect.
So, in simple terms, Time of Flight Diffraction is like using the speed of sound to find and measure hidden defects in a material. It's a powerful tool that's used in many industries, especially where safety and quality are critical.