Propagation of ultrasonic pulses in flawed media : applications to nondestructive testing and evaluation

A detailed analysis of the propagation of elastic waves in flawed materials is presented in this dissertation.

After a thorough review of various analytical and numerical procedures, a computer code, JUNA, has been developed for the computation of wave propagation and interaction in two-dimensional closed systems with flaws. Care has been exercised to validate the code by comparison with analytical and experimental results. Upon performance of these tests, the code was found to be stable, robust and consistent with the expected features associated with elastic wave propagation. In particular, excellent results were obtained in the detection of cracks.

The code validation step also led to the conclusion that its output does in fact provide a convenient way for the visualization and analysis of wave propagation in flawed systems.

One of the first applications of the code JUNA, has been the modeling of finite- size transducers.

Parallel to the above development, new, modern signal analysis methods, of immediate application to Nondestructive Evaluation (NDE) techniques, were implemented for the evaluation and understanding of the JUNA output, by means of the Gabor Short Time Fourier Transform (STFT) and Continuous Wavelet Transform (CWT). Based on these methods, a procedure for the measurement of the dispersion law for ultrasonic pulses was developed and applied to specific measurements.

This particular development was made possible by the introduction of wavelet-like pulses as excitation sources, containing a wide spectrum of input frequencies.

The combination of the JUNA code, the Gabor STFT and CWT allowed the study of Lamb waves in thin plates.

The basis for the extension of the code to cylindrical geometry was developed; however, both its numerical implementation and that of the propagation of elastic waves along interfaces, await future developments.