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.