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

• US 5,974,889 A
• Filed: 01/02/1998
• Issued: 11/02/1999
• Est. Priority Date: 01/02/1998
• Status: Expired due to Fees

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Trantow, Richard L.
• Primary Examiner(s): Williams, Hezron
• Assistant Examiner(s): MILLER, ROSE MARY
• Application Number:
  US09/002,355
• Time in Patent Office: 669 Days
• Field of Search: 73/597, 73/599, 73/602, 73/620, 73/621, 73/622, 73/624, 73/625, 73/627, 73/628, 73/632, 73/633, 73/634
• US Class Current: 73/624
• CPC Class Codes: G01N 2291/0423 Surface waves, e.g. Rayleigh waves, Love waves
• CPC Class Codes: G01N 2291/0425 Parallel to the surface, e.g. creep waves
• CPC Class Codes: G01N 2291/044 Internal reflections (echoes), e.g. on walls or defects
• CPC Class Codes: G01N 2291/055 Angular incidence, perpendicular propagation
• CPC Class Codes: G01N 2291/056 Angular incidence, angular propagation
• CPC Class Codes: G01N 2291/106 one or more transducer arrays
• CPC Class Codes: G01N 2291/2632 flat
• CPC Class Codes: G01N 29/043 in the interior, e.g. by shear waves
• CPC Class Codes: G01N 29/223 Supports, positioning or alignment in fixed situation mounting transducers per se G10K11/004
• CPC Class Codes: G01N 29/2487 Directing probes, e.g. angle probes directing arrangements G01N29/221
• CPC Class Codes: G01N 29/265 by moving the sensor relative to a stationary material

Abstract

An apparatus for ultrasonically scanning a surface includes a plurality of pairs of 180 degree oppositely disposed transducers mounted in a rotatable head having a central axis about which the head is rotatable. A positioning system for positioning the transducers such that all beam axes of the transducers intersect the central axis at a single point. Axes of each pair are equiangular with respect to the centerline and axes of different pairs have incident and reflective angles between beam axes and the central axis that are different from those of other pairs. Incident and reflective angles of different pairs of transducers are preferably predetermined and in close proximity to a predetermined angle over a range of angles bracketing a predetermined critical angle. A translating system is preferably included for effecting translational motion between the head and the surface such that the single point lies substantially on the surface during scanning. The present method further provides a non-destructive material evaluation technique to determine effective Rayleigh wave critical angles using the apparatus. The effective critical angle may be determined from an angular beam intensity profile generated from the beam intensity data provided by each pair of fixed angle transducers.

Claims

1. An apparatus for ultrasonic scanning a surface, said apparatus comprising:

a plurality of pairs of 180 degree oppositely disposed ultrasonic transducers having beam axes;

said transducers mounted in a rotatable head having a central axis about which said head is rotatable; and

a positioning means for positioning said transducers such that all of said beam axes intersect said central axis at a single point, said beam axes within each pair of said transducers are equiangular with respect to said central axis, and said beam axes of different pairs of transducers have incident and reflective angles respectively of said beam axes that are different from those of other pairs.

2. An apparatus as claimed in claim 1 wherein said positioning means is a fixture having a plurality of angled holes in which said transducers are disposed.

3. An apparatus as claimed in claim 2 wherein distances from said transducers to said point along said transmission axes are equal.

4. An apparatus as claimed in claim 3 further comprising transducer angles between said beam axes and said central axis wherein said transducer angles of said different pairs of transducers are predetermined and in close proximity to a predetermined angle.

5. An apparatus as claimed in claim 4 wherein said transducer angles are set at intervals of about 0.2 of a degree greater than and less than said predetermined angle.

6. An apparatus as claimed in claim 2 further comprising a translating means for effecting translational motion between head and the surface and positioning said head with respect to the surface such that said point lies substantially on the surface during ultrasonic scanning.

7. An apparatus as claimed in claim 2 further comprising a translating means for translating said head over the surface and positioning said head above the surface such that said point lies substantially on the surface during ultrasonic scanning.

8. An apparatus as claimed in claim 1 further comprising a counter-rotatable annular collar encircling said head and counter-rotating motive means to drive said collar in a circular direction counter to that of said head about said central axis when said head is rotated.

9. An apparatus as claimed in claim 8 wherein said positioning means is a fixture having a plurality of angled holes in which said transducers are disposed.

10. An apparatus as claimed in claim 9 wherein distances from said transducers to said point along said transmission axes are equal.

11. An apparatus as claimed in claim 10 further comprising transducer angles between said beam axes and said central axis wherein said transducer angles of said different pairs of transducers are predetermined and in close proximity to a predetermined angle.

12. An apparatus as claimed in claim 11 further comprising a translating means for translating said head over the surface and positioning said head above the surface such that said point lies substantially on the surface during ultrasonic scanning.

13. An apparatus as claimed in claim 12 wherein said transducer angles are set at intervals of about 0.2 of a degree greater than and less than said predetermined angle.

14. 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.

15. A method as claimed in claim 14 wherein said determining the effective critical angle in step (d) further comprises:

(e) generating a null angle profile of data indicating intensity versus corresponding incident angles from measured intensities in step (c);

(f) determining an intensity curve by applying a curve fit to a low intensity portion of the null angle profile; and

(g) determining the effective critical angle by determining a substantially minimum point on the curve, the effective critical angle being equal to a determined angle of incidence at which the energy of the reflected beam is the substantially minimum point on the curve.

16. A method as claimed in claim 15 wherein said curve fit is a polynomial fit.

17. A method as claimed in claim 16 wherein said polynomial curve fit is a second order polynomial fit.

18. 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.

19. A method as claimed in claim 18 wherein said curve fits are polynomial fits.

20. A method as claimed in claim 19 wherein said polynomial curve fits are second order polynomial fits.

21. A method as claimed in claim 20 wherein the scanning comprises a plurality of passes of the rotatable head with the transducers held at a fixed angular orientation around the central axis during each pass, pitch and catch transducers of the plurality of transducers are the positioned to the same angular orientation during each pass, and head is rotated between passes such that different ones of the plurality of transducers are used as the pitch and catch transducers in each of the passes.

22. A method as claimed in claim 21 wherein the pitch and catch transducers are operated in bursts over the points in the area during each pass.

23. A method as claimed in claim 19 wherein the scanning comprises a single pass of the rotatable head with the head stopping at each of the points, at each of the points the head is rotated and step (b) is performed.