Study On The Positioning Method Of Damaged Materials With Ultrasonic Time Reversal Techniques

There is a problem in the traditional ultrasonic inspection, that is the ultrasonic pulse will be broaden, distorted and defocused. The reason to the above problem is the multipath effect, which is caused by the multiple reflection from the upper and lower interfaces of solid and defects in the materials. Time reversal technique can overcome the multipath effect and compress the broaden pulse, finally make acoustic beam focused.Time reversal method is popularized by the phase conjugation method of the monochromatic continuous wave, which can be summaried as follows: a time-reversal mirror(TRM) composed by inversible transducer array receives an acoustic signal in time from the target place, keeps it in memory after converting it into electrical signal, and sends the reversed electrical signal back into the transducer array to remotivate ultrasonic wave. The divergent acoustics wave will be converged by using time reversal method and generate the self-focusing of longitudinal and shear waves at the same time without prior knowledge of the propagating medium.This paper applied the time reversal method to the study of defects' localization and it set up an experiment system, which included transmitters, a receiver, transducers and a computer used for processing signals. The basic experimental process is as follows: First, receiver receives acoustic emission signals sent from sound source, then the relevant PZT sends the time reversed signals, that is to say PZT sends the fist received signal at last and sends the final received signals firstly. The divergent acoustics wave generates the self-focusing of longitudinal and shear waves at the same time. It designed a series of experiments which were classified into four types, such as experiments with a single transmitter-receiver pair or multiple transmitters with a receiver, experiments with different waves produced by arbitrary waveform gengrator, experiments with various frequencies and experiments with different amplitude. By analysing focused signals got in the above experiment and calculating focus gain and main flap and side flap ratio, it ascertained ultrasonic wave type, amplitude, frequency and PZT layout of better focus effect.This paper performed a large amount of one channel experiments and multichannel experiments in the aluminium plate. By contrasting the tests' results of aluminium plate with crack and without crack and analysising both receiving signals and focusing signals, it finally found out the method to locate dimension and size of the crack in the aluminium plate.