A comparison of impulse response modification techniques for time reversal with application to crack detection

Abstract

Time reversal (TR) focusing used for nonlinear detection of cracks relies on the ability of the TR process to provide spatially localized, high-amplitude excitation. The high amplitude improves the ability to detect nonlinear features that are a signature of the motion of closed cracks. It follows that a higher peak focal amplitude than what can be generated with the traditional TR process will improve the detection capability. Modifying the time-reversed impulse response to increase the amplitude of later arrivals in the impulse response, while maintaining the phase information of all arrivals, increases the overall focal signal amplitude. A variety of existing techniques for increasing amplitude are discussed, and decay compensation TR, a technique wherein amplitude is increased according to the inverse of the amplitude envelope of the impulse response decay, is identified as the best modification technique for nonlinear crack detection. This technique increases the focal signal amplitude with a minor introduction of harmonic content, a drawback in two other methods studied, one-bit TR and clipping TR. A final study employs both decay compensation TR and traditional TR, focusing on a rod with stress corrosion cracking, and compares the merits of each in detecting nonlinearity from cracks in a real system. V

Damage detection in a laboratory-scale wellbore applying Time Reversal and Nonlinear Elastic Wave Spectroscopy (TR NEWS)

“…Alternatively, chaotic cavities or significant medium scattering accomplishes the same thing. Work has been done by many to improve the acoustic focus in space and time at the focus volume, through changes to the time-reversed signal, like using impulse response to increase the amplitudes of late wave components arriving at the focus point, and deconvolution to scale the frequency content of the reversed signal to an approximate delta function made of the frequency content that arrived in the recorded forward signal [7][8][9][10][11][12].…”