Ultrasonic multi-transducer rotatable scanning apparatus and method of use thereof

A method for determining Rayleigh wave effective critical angles at a surface of a metallic object, said method comprising the steps of:

• (a) rotating a plurality of pairs of 180 degree oppositely disposed ultrasonic transducers mounted in a rotatable head around a central axis wherein the transducers have beam axes set at fixed angles with respect to the central axis such that the fixed angles of each pair of transducers are equiangular with respect to said central axis, and the fixed angles of different pairs of transducers are different with respect to the central axis;
• (b) directing incident beams of ultrasonic waves onto at least one point of the surface at a plurality of incident angles along the beam axes of the different pairs of transducers at the different fixed angles;
• (c) simultaneously measuring intensity of a combination of corresponding directly reflected and re-radiated beams from the incident beams;
• and
• (d) determining the effective critical angle from intensity data gathered in step (c) wherein the effective critical angle is a determined angle of incidence at which the energy of the reflected beam is indicated to be a substantially minimum amount based on the intensity data at the point of the surface.

A method for mapping Rayleigh wave effective critical angles on a surface area of a metallic object, said method comprising the steps of:

• (a) scanning incident beams of ultrasonic waves at a plurality of points on the area at a plurality of incident angles by rotating a plurality of pairs of 180 degree oppositely disposed ultrasonic transducers mounted in a rotatable head around a central axis wherein the transducers have beam axes set at fixed angles with respect to the central axis such that the fixed angles of each pair of transducers are equiangular with respect to said central axis, the fixed angles of different pairs of transducers are different with respect to the central axis, and the fixed angles are in close proximity to at least one predetermined angle;
• (b) simultaneously measuring intensity of a combination of corresponding directly reflected and re-radiated beams at a plurality of corresponding reflected angles equal in magnitude to said incident angles with respect to a line normal to the surface;
• (c) generating a plurality of null angle profiles of data indicating intensity versus corresponding incident angles from measured intensities in step (b) at the plurality of points on the area;
• (d) determining intensity curves for said points by applying curve fits to low intensity portions of the null angle profiles at their respective points on the surface;
• and
• (e) determining the effective critical angles by determining substantially minimum points on the curves, said effective critical angles being equal to determined angles of incidence at which the energy of the reflected beams are the substantially minimum points on the respective curves.