ToFD, or time-of-flight diffraction, is a type of ultrasonic testing method used to detect and evaluate defects in materials. ToFD is an advanced form of ultrasonic testing that uses multiple transducers to generate and receive ultrasonic waves.

The time it takes for the ultrasonic waves to travel from the transmitting transducer to the receiving transducer is measured, and any changes in the time-of-flight, such as those caused by defects in the material, are detected and evaluated. ToFD is commonly used to inspect large or complex structures, such as bridges, pipelines, and pressure vessels, for defects that could affect their strength and performance.