Basic Principles of Ultrasonic Time Reversal Processing in Non-Destructive Testing

Ultrasonic non destructive testing needs large focusing apertures in order to detect small defects in solid media. Current ultrasonic inspections use scanned immersion techniques. In recent years, two different approaches have been extensively studied in order to obtain focused beams through non-planar liquid-solid interfaces. The first technique uses one or several transducers whose geometry is matched to the liquid-solid interface and to the desired focal point. The second technique is based on the combination of a multi-elements array and on a set of electronic delay lines whose values are matched to produce focusing. These two techniques suffer from important limitations. They are based on an exact a priori knowledge of the interface geometry and require highly precise positioning of the defect