The tubular components are commonly used in marine and offshore structures on which the fatigue cracks are easily induced by the initiation and propagation of surface flaws. Compared with other types of defects such as internal voids, surface cracks often impose more serious threats to structural integrity. This study presents an approach to detect and size surface cracks on tubular samples through the use of ultrasonic phased array technology with surface acoustic waves (SAWs).

As one of the non-destructive techniques, ultrasonic phased array has been widely used for the inspection of defects in engineering structures, with the main advantages of increased inspection coverage and sensitivity. Although the presence and location of surface cracks have been confirmed with phased array with bulk waves, it is challenging to accurately measure the crack depth and length for structural health evaluation. Compared with bulk waves, surface waves have most of the energy concentrated near the surface and they can propagate along curved surface such as cylindrical hollow components with fairly large diameters. Thus, surface wave inspection has become one of the most useful techniques and been frequently applied for the evaluation of surface cracks. A Rayleigh wave which carries most of the energy within a wavelength is essentially a guided surface wave and is highly sensitive to shallow surface cracks.

In this work, the transmitted and reflected Rayleigh waves are used to calculate the transmission and reflection coefficient regarding the ratio of crack depth to the wavelength.  In order to achieve a higher sensitivity and spatial resolution, phased array can be used to focus the Rayleigh waves at an arbitrary location on the surface by exciting each array element with a delay law. When there is a surface crack in the vicinity of the focal point, the scattered Rayleigh wave from the crack can then be received by the array. The delay-and-sum processing of received signals is subsequently conducted for the signal enhancement. To accurately measure surface crack depth as well as length, a full matrix capture (FMC) data acquisition strategy is utilized in this work to collect phased array data. The depth of the crack can then be obtained by transmission and reflection coefficients of Rayleigh waves scattered at the surface crack while the crack length can be estimated by phased array imaging method. Due to the complicated surface of tubular sample and multiple wave modes generated, the generation and propagation of Rayleigh waves as well as the interaction with the defect are discussed for the optimization of array transducer configuration.