Nonlinear ultrasonic phased array with fixed-voltage fundamental wave amplitude difference for high-selectivity imaging of closed cracks
Abstract
The nondestructive evaluation of closed cracks is a challenging subject in ultrasonic testing.
Recently, nonlinear ultrasonic phased array with fixed-voltage fundamental wave amplitude differ-
ence (fixed-voltage FAD) has been proposed as a practical approach. In this study, the maximum
incident wave amplitude, which is one of the most critical parameters in closed-crack imaging, was
investigated. First, a theoretical model was formulated to explicitly show the essence of the funda-
mental principle of FAD and the advantage of fixed-voltage FAD over different-voltage FAD. In
experiments, the authors imaged a closed fatigue crack using a nonlinear ultrasonic phased array
with fixed-voltage FAD while varying the incident wave amplitude. It was found that when the inci-
dent wave amplitude was sufficiently high, the nonlinear image visualized the closed crack tip,
which could not be visualized in linear images. In addition, the incident-wave-amplitude depen-
dence of the nonlinear responses was quantified. It was found that different parts within a single
fatigue crack showed different nonlinear behaviors. This suggests that fixed-voltage FAD is useful
not only for practical application of closed crack imaging but also for examining the nonlinear
dynamics at various parts of closed cracks with a high spatial resolution.