Phased array implementation of high frequency, multi-mode dispersion compensation for ultrasonic guided wave inspections

Abstract

An important advantage of ultrasonic guided wave nondestructive evaluation is the ability of guided waves to propagate large distances and yield more information about flaws than bulk waves. Unfortunately, the multi-modal, dispersive nature of guided waves makes them difficult to interpret. In this work, we present an array based method for removing the deleterious effects of multi-mode dispersion allowing for robust source localization at frequencies comparable to those of bulk waves and independent of the number of modes present.

Presented in this work is a two-step process to remove dispersion in pulse-echo and pitch-catch configurations. Time domain data is obtained using a phased array with elements positioned on the radius of an elastic wedge at angles ranging from 8.9 to 81.1 degrees. By measuring time domain signals over a range of incident angles, data is mapped to the wave number and frequency domain by applying an FFT to measured time traces. By Auld's electro-mechanical reciprocity relation, mode contributions are extracted approximately using a variational method. Once mode contributions have been obtained, the dispersion for each mode is removed via back-propagation techniques. Excepting the presence of artifacts at high frequency-thickness products, experimental data successfully demonstrates the robustness and viability of this approach to guided wave source location for both pulse-echo and pitch-catch measurements.

The remainder of this dissertation is dedicated to examining the source of the artifacts and exploring options for artifact mitigation in the pitch-catch configuration. It was suspected that artifacts originated in uncharacterized mode conversion. Free plate modes incident on the wedge of the array transducer convert to leaky modes beneath the wedge. The effects of evanescent modes in the neighborhood of the interface between the free plate and wedge are ignored. Experimental work focused on observing this mode conversion when single modes were incident beneath the array. After confirming mode conversion experimentally, the phenomena was demonstrated analytically using a boundary integral solution.

From the boundary integral solution a calibration procedure was developed to remove the artifacts. From the boundary integral solution, it was possible to determine a transmission coefficient matrix to approximate the mode conversion beneath the array. The transmission coefficient matrix was improved by extracting mode conversion parameters from a calibration dataset. Using adjusted transmission coefficient matrix, the source location algorithm is applied for sources located at various distances. Final results show the elimination of artifacts after application of the calibration procedure.

Due to unforeseen complexities in the reception of multi-mode signals, it is left as future work to extend these concepts to characterizing defects in terms of size, and orientation.