Depth Measurement of Small Surface-Breaking Cracks Using the Half-Skip Total Focusing Method

The Total Focusing Method is an ultrasonic array post-processing technique which synthetically focuses at every pixel in an image space in both transmission and reception. The Half-Skip Total Focusing Method is a modification to the method in which either the transmitting or receiving ray path contains one reflection from the back surface of the component being inspected. When an array is placed on a parallel-sided component on the opposite side to that from which surface-breaking cracks initiate, the Half-Skip method enables specular reflections to be captured from the faces of the crack. In this paper, the Half-Skip method is applied to experimental array data from notches in a titanium component of 6 mm thickness and from fatigue cracks in a titanium component of 3 mm thickness. An ultrasonic array with a center frequency of 18 MHz is used. The HSTFM is found to be advantageous over the TFM for measuring the depth of the smaller notches and cracks, and vice-versa. The HSTFM is also successfully applied to simulated array data from a real stress corrosion crack shape.