The critically refracted longitudinal (LCR) waves are used in various domains of
nondestructive evaluation, especially for residual stress measurements. However, few works
have characterized the associated ultrasonic beam. In this article, the characterization of the
LCR ultrasonic beam, both numerically and experimentally, is first clarified in order to provide
some answers to questions that arise about its behavior in elastic solids. The aim of the
second part of this work is to investigate the use of the LCR waves for the detection of surface
defects of different sizes in aluminum. For that, the effect of defects at fixed depth (5 mm)
with various diameters (2, 4, 6, and 8 mm) and fixed diameter (8 mm) with various depths
(5, 7, and 10 mm) in an aluminum sample have been investigated at frequencies around
1 MHz. It has been experimentally found that the amplitude of the (LCR) wave decreases
and its frequency spectrum changes with a given defect. The rate of the decrease of the
amplitude and the change in the spectrum is related to the increase of the defect depth.
The study shows that the effect on the propagating of the LCR waves is larger as the defect
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