Optimisation of the Half-Skip Total Focusing Method (HSTFM) parameters for sizing surface-breaking cracks

The Half-Skip Total Focusing Method (HSTFM) is an ultrasonic array post-processing (imaging) technique. The method allows surface-breaking cracks (SBCs) to be imaged and also sized using 6 dB drop rule. The HSTFM has previously been used to size SBCs initiating from flat and horizontal walls. However, there has been limited research to optimise the method in a rigours manner and quantitatively determine the performance of the HSTFM to size cracks accurately. In this paper, the SBCs characterization capability of the HSTFM was tested and applied on both numerically simulated and experimentally generated data. Under numerical simulation, considering the travel time of ultrasonic signals as per the ray theory, first, a mathematical framework was developed for the HSTFM imaging algorithm. Then, a quantitative parametric study, using the finite element method and point reflector model, was performed to evaluate the sizing capability of the HSTFM under different scenarios. These scenarios include the position of the ultrasonic array relative to the crack, the accuracy of the acoustic velocity, the height of the sample, the slope of the back wall, and the tilting angle of the defect. For each of the array configuration, the crack-sizing range (the smallest and largest Through Wall Extent (TWE) of a crack) that can be measured using the HSTFM was calculated. Thereafter, experimental validation was performed, where an excellent match with the numerical results was observed.