Locating damages in a complex structure using one or two fixed ultrasonic transducers

Detecting and locating defects in a plate is usually performed using multiple transducers and methods that are based on the measurement of the wave travelling directly from the defect to the transducer. The acoustic signatures of the interaction between the interrogating wave with the defects and the structure itself, such as the reflections from the boundaries, are generally discarded from the signal. This paper deals with the experimental application of the topological imaging method that takes advantage of all the information included in the multiple reflections between the boundaries of the structure and the defects. The application of the method is based on a preliminary measurement of the impulse response of the inspected structure in its undamaged state. The impulse response measurement is restricted to a defined region of interest and the structure is excited with a single fixed transducer. This preliminary measurement can be considered as a general acoustic calibration of the inspected structure that is performed once and for all, in the present case with a Laser Doppler velocimeter. The structure is then inspected with the single or several transducers and, if any, damages are located in the region of interest. The experimental results obtained with the one-channel topological imaging method show accurate location of a single small defect and of multiple small defects. The resolving power obtained is close to the theoretical limit of half a wavelength. These results demonstrate that the method has great potential for complex structures in general and for SHM applications involving a limited number of embedded transducers