Nonlinear ultrasonic phased array imaging of partially-closed fatigue cracks
Abstract
Phased array ultrasound is a highly advanced technique that is not only used
in the nondestructive evaluation (NDE) of defects in materials but also in the medical
diagnostic, for instance, e.g. for a noninvasive examination of the heart. However,
di culties occur in NDE applications when it comes to monitoring some challenging
(nonlinear) defects such as small fatigue cracks in the early stages or partially- or
fully- closed cracks. Small cracks may be undetected or sizes of closed cracks may
be underestimated. The aim of this research is to obtain images of the closed part
of a fatigue crack using the nonlinear ultrasonic phased array method. The present
technique is based on a subtraction of response signals received at a su ciently high
and low energy level.
In this study, the essential experiments are performed on an aluminum CT specimen
of 15 mm thickness that has a fatigue crack of 25 mm length and a Plexiglas
specimen of 50 mm thickness. The crack in the Plexiglas specimen was produced by
stretching the initial notch width of 2 mm to a nal width of 2.5 mm. In order to
investigate and evaluate these cracks, a commercial phased array system, with a 16
element probe operating at a frequency of 1.5 MHz, is used. The image of the crack
tip is successfully generated by post-processing of the received signals at two excitation
levels. The results clearly demonstrate the e ciency of the presented method to
detect and image nonlinear defects.