The principle consists of transmitting an ultrasonic wave (in practice a pulse wave) via a transmitter and observing the reflection at the receiver. If this ultrasonic wave encounters two cracking planes delineating different acoustic impedances, it will reflect rather than continue its path and thus create an echo. Measuring the time between the pulse and the echo indicates whether the echo is due to the reflection of the wave on the other side of the part to be inspected or to an internal discontinuity in the part.
By scanning the part correctly, it is possible to interpret the signals and position the defect while determining the relative dimensions. This so-called interpretation operation is quite simple for geometrically and materially simple parts. For complex geometries, the technique requires a great amount of experience.
Other techniques associated with or derived from ultrasound nowadays make it possible to characterise defects in a component with great precision. The TOFD (Time of flight diffraction) method uses the principle of ultrasonic wave diffraction to detect and characterize defects. The PA (Phased Array) method places several transmitters and receivers next to each other to check the "shape" of the emitted wave train. This makes it possible to reproduce a three-dimensional image of the component with its defects.
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