Orientation characterisation of branched crack-like defects in curved components using ultrasonic array vector total focusing method

Crack orientation is an important characteristic in crack evaluation such as predicting crack growth and measuring crack size. However, the curved surface of a component makes the ultrasonic ray path complicated and seriously affects the crack orientation characterisation in ultrasonic detection. To compensate for the effect of curved surfaces, this paper proposes a vector total focusing method (VTFM) for curved components using an immersion ultrasonic array. In this method, the curved surface profile is extracted from the image of the total focusing method (TFM) and combined with the VTFM to construct the vector fields containing defect orientation information. Then, the crack orientation angles are calculated from the vector fields. In simulation, a V-shaped crack-like defect is characterised under curved surfaces of different curvature, and excellent agreement is achieved between the calculated angles and the actual values. Experimentally, two curved samples with branched crack-like defects are manufactured and detected. The orientation angles of the inclined branches are measured to within 5° for large defects, and within 12° for small defects close to one wavelength. For branches perpendicular or parallel to the array, the errors would be larger due to the influence of sound waves reflected from the tip or the junction region of the branches. Through simulation and experiments, this curved surface VTFM algorithm is validated.