The three-dimensional ultrasonic phased array technique with a circular array probe was investigated for application to longitudinal angle beam ultrasonic testing in all directions for inspection of a thick material. Conventional 2D array probes have ultrasonic transducer elements of
a similar size, whereas a circular array probe has ultrasonic transducer elements that increase in size at positions away from the centre of the array. It is shown that, for a fixed number of total elements on the array, it is possible for a circular array probe to generate a much larger aperture
than conventional 2D array probes, allowing them to focus much deeper inside the component. However, the increasing size of the outer elements of a circular array can give rise to increased noise. A technique was proposed to reduce noise by limiting the active elements on the outside of a
circular array probe to those elements whose pitch projected in the direction of propagation and were small enough not to generate the grating lobe. This technique has two main advantages compared to conventional phased array techniques: a larger inspection range is possible and noise caused
by the grating lobe is decreased.