Title: Ultrasonic inspection method and system therefor
Abstract: A method and system for ultrasonically inspecting turbine wheels and other disk-shaped articles having a central opening and multiple secondary openings radially spaced outward from the central opening. At least one ultrasonic transducer is placed in at least one of the secondary openings of the wheel. The transducer is configured and oriented to perform a pulse-echo diagnostic technique on the wheel by emitting ultrasonic signals that intersect radials of the wheel at angles of approximately ninety degrees to the radials. The ultrasonic signals intersect the radials at points so that a plurality of points are located on a plurality of radials. The points define a locus of points through the volume of the wheel between the central opening and the secondary opening, such that the volume is inspected for defects. The transducer receives ultrasonic signals that are reflected from one or more points when a defect is encountered.
Patent Number: 7,017,414 Issued on 03/28/2006 to Falsetti, et al.
| Inventors:
|
Falsetti; Robert V. (Schenectady, NY);
Rose; Curtis Wayne (Mechanicville, NY)
|
| Assignee:
|
General Electric Company (Schenectady, NY)
|
| Appl. No.:
|
604559 |
| Filed:
|
July 30, 2003 |
| Current U.S. Class: |
73/600; 73/602; 73/622; 73/628; 73/641 |
| Current Intern'l Class: |
G01N 29/04 (20060101) |
| Field of Search: |
73/597,598,599,600,618,625,628,660,619,620,622,602, 641
|
References Cited [Referenced By]
U.S. Patent Documents
| 3597963 | Aug., 1971 | Smejkal et al.
| |
| 4441369 | Apr., 1984 | Lessard et al.
| |
| 4660419 | Apr., 1987 | Derkacs et al.
| |
| 4864862 | Sep., 1989 | Nottingham et al.
| |
| 5189915 | Mar., 1993 | Reinhart et al.
| |
| 5618994 | Apr., 1997 | Falsetti.
| |
| 6065344 | May., 2000 | Nolan et al.
| |
| 6082198 | Jul., 2000 | Sabourin et al.
| |
| 6370956 | Apr., 2002 | Bewlay et al.
| |
| 6401537 | Jun., 2002 | Gigliotti et al.
| |
| 6487909 | Dec., 2002 | Harrold et al.
| |
| 6725722 | Apr., 2004 | Murphy et al.
| |
Primary Examiner: Williams; Hezron
Assistant Examiner: Fayyaz; Nashmiya
Attorney, Agent or Firm: Cusick; Ernest, Hartman; Gary M., Hartman; Domenica N. S.
Claims
What is claimed is:
1. A method of ultrasonically inspecting a disk-shaped article having a central
opening and a plurality of secondary openings radially spaced outward from the
central opening and circumferentially spaced from each other, the method comprising
the steps of:
placing at least one ultrasonic transducer in at least a first of the secondary
openings of the article;
performing a pulse-echo diagnostic technique on the article by causing the transducer
to emit ultrasonic signals that intersect radials of the article at angles of approximately
ninety degrees to the radials, the ultrasonic signals intersecting the radials
at points so that a plurality of the points are located on a plurality of the radials;
receiving with the transducer a reflected ultrasonic signal that is returned
from at least one of the plurality of points;
calibrating the amplitude of the reflected ultrasonic signal to a flat bottom
hole standard reflector by causing the transducer to emit a calibration ultrasonic
signal toward the central opening and receiving a reflected calibration ultrasonic
signal from the central opening, and
calculating a relationship between the amplitude of the reflected ultrasonic
signal returned from the one point to the flat bottom hole standard reflector according
to the equation:
Af=λd2Pf(
b/R)
1/2/(2(
R-b)
Pr)
where A
f is the area of the flat bottom hole standard reflector, λ
is the wavelength of sound in the article, d is the distance between the transducer
and the at least one of the points, P
f is the amplitude of the reflected
ultrasonic signal from the at least one of the points, b is the radius of the central
opening, R is the distance between a center of the first secondary opening and
a center of the central opening along one of the radials of the article, and P
r
is the amplitude of the reflected calibration ultrasonic signal from the
central opening.
2. The method according to claim 1, wherein the at least one ultrasonic transducer
is one of an array of ultrasonic transducers placed in the first secondary opening,
the ultrasonic transducers being pulsed simultaneously during the performing step.
3. The method according to claim 1, wherein the at least one ultrasonic transducer
is one of an array of ultrasonic transducers placed in the first secondary opening,
at least some of the ultrasonic transducers being pulsed at different times during
the performing step.
4. The method according to claim 1, wherein the at least one ultrasonic transducer
is one of a phased array of ultrasonic transducers placed in the first secondary
opening, the phased array of ultrasonic transducers being operated to focus the
ultrasonic signals at predetermined depths from the first secondary opening.
5. The method according to claim 1, wherein the central opening is a through-hole
having an axis of symmetry and each of the secondary openings is a through-hole
having an axis of symmetry that is substantially parallel to the axis of the central opening.
6. The method according to claim 1, wherein all of the plurality of points are
located between the central opening and a circle concentric with the central opening
and on which the first secondary opening is located.
7. The method according to claim 1, wherein the at least one ultrasonic transducer
is one of an array of ultrasonic transducers mounted on a body to form a transducer
unit, the placing step comprising placing the transducer unit in the first secondary
opening, the body having a semicircular cross-section with a radius of curvature
approximately equal to a radius of curvature of the first secondary opening.
8. The method according to claim 7, wherein the article and the body on which
the transducer unit is mounted are formed of the same material.
9. The method according to claim 1, wherein the first secondary opening has a
center located a constant distance R from a center of the central opening along
one of the radials of the article, and each of the points is located a distance
d from the center of the secondary opening and a distance r from the center of
the central opening, the distances R, d and r defining sides of a right triangle
with an angle of ninety degrees between the sides corresponding to the distances
d and r, an angle α between the sides corresponding to the distances d and
R, an angle θ between the sides corresponding to the distances r and R, wherein
the plurality of points are located by the equation:
cos
2 θ+cos
2 α=1.
10. The method according to claim 1, wherein all of the points are located within
an intermediate portion of the article between the central opening and the first
secondary opening.
11. The method according to claim 1, wherein the article is a machined gas turbine
wheel, the central opening is an inner hub bore of the wheel, and the secondary
openings are bolt holes of the wheel.
12. A method of ultrasonically inspecting a turbine wheel having a central hub
bore, a plurality of bolt holes radially spaced outward from the central hub bore
and circumferentially spaced from each other along a circle concentric with the
central hub, and an annular-shaped web region between the central hub bore and
the bolt holes, the wheel being formed such that the web region potentially contains
axial-radial oriented defects, the method comprising the steps of:
mounting a plurality of ultrasonic transducers to form at least one transducer unit;
placing the transducer unit in a first of the bolt holes of the turbine wheel;
performing a pulse-echo diagnostic technique on the turbine wheel by causing
the ultrasonic transducers to emit ultrasonic signals that intersect radials of
the turbine wheel at angles of approximately ninety degrees to the radials, the
ultrasonic signals intersecting the radials at points located within the web portion
of the wheel between the central hub bore and the first bolt hole so that each
of a plurality of the points is located on a corresponding one of the radials,
all of the plurality of points being located within the web portion; and
placing the transducer unit in a sufficient additional number of the bolt holes
and performing the pulse-echo diagnostic technique on the wheel to locate additional
points throughout the web portion;
wherein at least one of the ultrasonic transducers receives at least one reflected
ultrasonic signal returned from at least one of the plurality of points at which
an axial-radial oriented defect is present and oriented substantially perpendicular
to the ultrasonic signal that returned the reflected ultrasonic signal; and
wherein each of the bolt holes has a center located a constant distance R from
a center of the central hub bore along one of the radials of the turbine wheel,
each of the plurality of points is located a distance d from the center of the
bolt hole and a distance r from the center of the central hub bore, the distances
R, d and r define sides of a right triangle with an angle of ninety degrees between
the sides corresponding to the distances d and r, an angle α between the
sides corresponding to the distances d and R, and an angle θ between the
sides corresponding to the distances r and R, and the plurality of points are located
within the turbine wheel by the equation:
cos
2 θ+cos
2 α=1.
13. The method according to claim 12, wherein the ultrasonic transducers are
pulsed simultaneously during the performing steps.
14. The method according to claim 12, wherein the ultrasonic transducers are
pulsed at different times during the performing steps.
15. The method according to claim 12, wherein the plurality of ultrasonic transducers
is a phased array and the ultrasonic transducers are operated to focus the ultrasonic
signals at predetermined depths from the bolt holes.
16. The method according to claim 12, wherein the transducer unit comprises the
plurality of ultrasonic transducers and a body on which the plurality of ultrasonic
transducers are mounted, the body having a semicircular cross-section with a radius
of curvature approximately equal to a radius of curvature of the bolt holes.
17. The method according to claim 16, wherein the turbine wheel and the body
on which the plurality of ultrasonic transducers are mounted are formed of the
same material.
18. The method according to claim 12, wherein the step of performing the pulse-echo
diagnostic technique on the turbine wheel is repeated for each of the bolt holes
using the at least one transducer unit or another of the at least one transducer.
19. The method according to claim 12, further comprising the step of calibrating
the amplitude of the reflected ultrasonic signals returned from the plurality of
points to a flat bottom hole standard reflector by causing the transducer to emit
a calibration ultrasonic signal toward the central hub bore and obtaining a reflected
calibration ultrasonic signal from the central hub bore, and then calculating a
relationship between the amplitude of the reflected ultrasonic signals returned
from the plurality of points to the flat bottom hole standard reflector according
to the equation:
Af=λd2Pf(
b/R)
1/2/(2(
R-b)
Pr)
where A
f is the area of the flat bottom hole standard reflector, λ
is the wavelength of sound in the wheel, d is the distance between the transducer
and the at least one of the points, P
f is the amplitude of the reflected
ultrasonic signal from the at least one of the points, b is the radius of the central
hub bore, R is the distance between a center of the first bolt hole and a center
of the central hub bore along one of the radials of the wheel, and P
r is
the amplitude of the reflected calibration ultrasonic signal from the central hub bore.
Description
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention generally relates to ultrasonic inspection methods and
systems. More particularly, this invention relates to a method and system for ultrasonically
inspecting a disk-shaped article having a central opening and a plurality of circumferentially-spaced
secondary openings that are radially spaced outward from the central opening, such
as a turbine wheel or disk, wherein ultrasonic signals are generated and received
by transducers placed in the secondary openings.
2. Description of the Related Art
Ultrasonic inspection techniques have been widely used to perform nondestructive
testing on articles formed of materials having an intrinsically coarse grain structure,
which results in anisotropic and nonuniform acoustic properties. Particular examples
of such articles include forged super-alloy turbine wheels (rotors) used in gas
and steam turbines. In the hostile operating environments of gas and steam turbines,
the structural integrity of the turbine wheel is of great importance in view of
the high mechanical stresses that wheels must be able to continuously withstand
at extremely high temperatures.
Ultrasonic inspection techniques employed with turbine wheels have typically
involved inspecting each forging from a plane perpendicular to the highest operating
stresses, i.e., ultrasonic transducers are placed on the fore and/or aft surfaces
transverse to the rotational axis of a turbine wheel. With this approach, the ultrasonic
energy is generated in a direction substantially perpendicular to the orientation
of the most common defects, which tend to lie in axial-radial planes of a turbine
wheel. Two ultrasonic testing techniques have been conventionally used, a first
of which is a pitch-catch technique using two transducers placed on the fore and
aft surfaces of the wheel. One of the transducers serves to generate an ultrasonic
signal, while ultrasonic signals reflected from acoustical discontinuities being
received by the second transducer. The second common technique is referred to as
pulse-echo and makes use of a single transducer to both generate the ultrasonic
signal and receive reflected signals.
Inspections targeting axial-radial defects are performed routinely on
steam turbine discs with shrunk-on wheels. Stress corrosion cracking typically
develops at the surface of the central bore, and such cracks can be located using
the pitch-catch technique in which the ultrasonic signals are targeted at the bore
and the keyway surfaces at the perimeter of the wheel. This technique is not intended
for full volumetric inspection of the wheel. A nondestructive testing method using
the pitch-catch technique is also disclosed in U.S. Pat. No. 5,189,915 to Reinhart
et al., whereby wheel blade attachment locations can be inspected ultrasonically
from the bore of the wheel. Another pitch-catch technique has been developed for
gas turbine wheels in the as-forged condition, at which time the wheels have parallel
sides so that the technique is relatively simple to execute.
A pitch-catch inspection technique using transducers placed on the fore and aft
faces of a gas turbine wheel following final machining would also seem the most
likely technique to detect flaws having an axial-radial orientation throughout
the wheel volume. However, axial-radial defects are difficult to detect in gas
turbine wheels due to their complex geometry resulting from the final machining
operation, which includes the machining of circumferentially-spaced bolt holes
spaced radially from the central bore. A pitch-catch inspection of such a wheel
would require a complex movement of the transducers and control of transducer angles
to ensure an effective examination. Such an approach would be further exacerbated
by the high sonic noise produced by the large material grain size as well as a
tendency for the acoustic pulse to be steered by flow lines produced during the
forging process.
In view of the above, it would be desirable if an improved method were available
for ultrasonically inspecting articles having complex geometries, such as gas turbine
wheels following final machining.
SUMMARY OF INVENTION
The present invention provides a method and system for ultrasonically inspecting
turbine wheels and other disk-shaped articles having a central opening and a plurality
of secondary openings radially spaced outward from the central opening. The method
and system make use of at least one ultrasonic transducer placed in at least one
of the secondary openings of the article. The ultrasonic transducer is configured
and oriented to perform a pulse-echo diagnostic technique on the article by emitting
ultrasonic signals that intersect radials of the article at angles of approximately
ninety degrees to the radials, with the result that the ultrasonic signals intersect
the radials at points so that a plurality of the points are located on a plurality
of the radials. The points define a locus of points through the volume of the article
between the central opening and the secondary opening, such that the volume is
inspected for defects.
In view of the configuration and operation of the ultrasonic transducer, the
method
of this invention includes placing at least one ultrasonic transducer in at least
a first of the secondary openings of the article, after which the transducer is
caused to emit ultrasonic signals that intersect radials of the article at angles
of approximately ninety degrees to the radials, such that a plurality of intersection
points are located on a plurality of radials. The transducer is then used to receive
reflected ultrasonic signals that return (are reflected) from one or more of the
points, e.g., as a result of the presence of a defect at the point.
In view of the above, it can be seen that the present invention provides an inspection
process that, when applied to a turbine wheel after final machining, makes use
of holes spaced radially outward from the central hub bore. Such holes are commonly
present as bolt holes machined near the rim area of a turbine wheel. When the pulse-echo
ultrasonic technique is conducted from each bolt hole location, the existence of
any axial-radial flaws within the wheel volume between the bolt hole and the central
hub bore can be determined along the locus of points inspected with the ultrasonic
signals, which includes the critical volume of the wheel immediately adjacent the
central hub bore. As such, the present invention provides a method for ultrasonically
inspecting turbine wheels and other articles having complex machined geometries.
The ultrasonic transducer is preferably one of multiple ultrasonic transducers
assembled together as a unit that includes a specially shaped base whose shape
conforms to the diameter of the hole in which the unit is placed. The method of
this invention also preferably encompasses a technique by which the proper orientation
of the transducers can be determined within the hole, and a method of calibrating
the amplitude of reflected sound from a defect to an equivalent reference reflector.
Other objects and advantages of this invention will be better appreciated from
the following detailed description.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 shows a face of a gas turbine wheel in which an ultrasonic transducer
unit is installed for performing a nondestructive ultrasonic inspection of the
wheel in accordance with this invention.
FIG. 2 is a partial cross-sectional view of the wheel of FIG. 1 along lines 22.
FIG. 3 is a view in the direction of line 33 of FIG. 2, showing in greater
detail a transducer unit placed in a bolt hole of the wheel in accordance with
a first embodiment of the invention.
FIG. 4 schematically represents a transducer unit in accordance with a second
embodiment of the invention.
FIGS. 5, 6 and 7 graphically illustrates the ultrasonic inspection
technique of the invention.
DETAILED DESCRIPTION
A gas turbine wheel
10 of a type known in the art is illustrated in FIGS.
1 and 2. The wheel
10 is represented as being in the final-machined condition,
and generally includes a rim
12, a hub
14, and a web
16 between
the rim and hub
12 and
14. The rim
12 is configured for the
attachment of turbine blades (not shown) in accordance with known practice. A hub
bore
18 in the form of a through-hole is centrally located in the hub
14
for mounting the wheel
10 on a turbine axis, and therefore the axis of the
hub bore
18 coincides with the axis of rotation of the wheel
10.
A plurality of bolt holes
20 are machined through the web
16 at locations
that are equal radial distances from the hub bore
18 (center-to-center),
as well as circumferentially equally spaced from each other (center-to-center).
The axes of the bolt holes
20 are shown as parallel to the axis of the hub
bore
18.
From FIGS. 1 and 2, the complexity of the geometry of the wheel
10 is
evident, making any ultrasonic inspection of the wheel
10 using conventional
methods very difficult. According to the present invention, ultrasonic inspection
of the wheel
10 is performed from the bolt holes
20 near the perimeter
of the wheel
10, instead of the fore and aft faces of the wheel
10
or from the hub bore
18, as has been suggested in the past. For this purpose,
a transducer unit
22 is shown in FIGS. 2 and 3 as being configured to fit
within the bolt holes
20 and closely contact the curved surfaces of the
bolt holes
20. As depicted in FIGS. 2 and 3, the unit
22 comprises
several rectangular ultrasonic transducers
24 aligned in a row and mounted
on a semi-cylindrical wedge
26. The wedge
26 provides a flat surface
28 for mounting the transducers
24, and a curved surface
30
whose radius of curvature is closely matched to that of the bolt holes
20.
By forming the wedge
26 of the same (or similar) material as the wheel
10,
the sound velocity in the wedge
10 is the same or nearly the same as in
the wheel
10, so that the effect of the curvature of the bolt hole
20
is minimal. A suitable couplant, such as light weight turbine oil, can be applied
between the curved surface
30 of the wedge
26 and the mating surface
of the bolt hole
20 to further reduce the effect of the interface between
the transducers
24 and the bolt hole
20. The wedge
26 is shown
in FIG. 2 as being equipped with a handle
32 to facilitate placement and
removal of the unit
22 from the bolt holes
20.
The transducers
24 can be of any type suitable for use in ultrasonic inspection
procedures, such as a style RHP gamma series (or equivalent) available from Krautkramer,
Inc. The width of the transducers
24 are preferably maximized to fit into
the bolt holes
20, with minimal separation between elements. The height
of each transducer
24 is preferably equal to or greater than its width in
order to promote the signal-to-noise response from a defect reflector. In the configuration
shown in FIGS. 2 and 3, the transducers
24 are arranged on the transducer
unit
22 as a linear array, and as such can share a single pulser. The transducers
24 produce a longitudinal wave at a fixed angle of zero degrees normal to
the surface of the wedge
26 on which the transducers
24 are mounted.
Groups of the transducers
24 can be pulsed simultaneously or multiplexed
to simulate movement in the axial direction. The entire group of transducers
24
can be rotated to provide radial coverage along a loci
40 of points
36
(FIG. 5).
According to one aspect of the invention, the position of the transducer
unit
22 for proper orientation of the transducers
24 is determined
so that the ultrasonic signals generated by the transducers
24 intersect
axial-radial type defects throughout the volume of the wheel
10 between
the bolt holes
20 and the hub bore
16, as shown in FIGS. 5 and 6.
In FIG. 5, the center-to-center distance between each bolt hole
20 and the
hub bore
18 is identified as a constant distance R along a radial of the
wheel
10. FIG. 5 further represents an ultrasound signal transmitted from
one of the bolt holes
20 along a path
38 that is perpendicular to
a second radial
34 of the wheel
10, intersecting the radial
34
at a point
36 located a distance d from the center of the bolt hole
20
and located a distance r from the center of the hub bore
18. The distances
R, d and r define sides of a right triangle with an angle of ninety degrees between
the sides corresponding to the radial
34 and signal path
38 (distances
r and d, respectively), an angle α between the sides corresponding to the
signal path
38 and the radial between the bolt hole
20 and the hub
bore
18 (the distances d and R, respectively), and an angle θ between
the sides corresponding to the radial
34 intersected by the signal and the
radial between the bolt hole
20 and the hub bore
18 (the distances
r and R, respectively). With this relationship, r=R sin α and d=R sin θ.
The triangle depicted in FIG. 5 is just one of essentially an infinite number of
triangles defined by ultrasonic signals intersecting an infinite number of radials
34 oriented at different angles θ from the radial through the hub
bore
18 and the bolt hole
20 from which the signals are broadcast.
The locus
40 of points
36 where the sound beam from the bolt hole
20 intersects these radials
34 of the wheel
10 at ninety degrees
(perpendicular) is defined by the following equation:
cos
2 θ+cos
2 α=1
The above equation defines the inspection coverage because at these positions
the sound beam is perpendicular to a defect with orientation in the axial-radial
plane. The transducer position within the bolt hole
20 is determined by
physically rotating the transducer unit
22 within the hole
20. For
example, the unit
22 is initially positioned in the hole
20 to maximize
the reflection from the hub bore
18, and then rotated to follow the locus
40 of points
36 identified by the above equation. The maximized reflection
from the bore
18 can be used to calibrate the inspection so indications
can be sized relative to an equivalent flat bottom hole (FBH) reflector, as discussed below.
FIG. 6 plots multiple loci
40 of points
36 resulting from ultrasonic
signals transmitted from over half of the bolt holes
20 of the wheel
10,
and evidences that these loci
40 of points
36 provide coverage of
a substantial annular-shaped volume
42 of the wheel
10 lying between
the hub bore
18 and the circle along which the bolt holes
20 lie.
The volume
42 is defined by only those loci
40 of points
36
established by perpendiculars between the bolt holes
20 and the wheel radials
34. With sound beam spread that inherently occurs, the volume
42
of the wheel
10 covered by the loci
40 can be increased slightly
depending on the particular transducer parameters used. In any event, the volume
42 of material that can be inspected positively coincides with the region
within the wheel
10 in which critical defects typically occur, as represented
in FIGS. 6 and 7.
According to another aspect of this invention, a method is provided for
calibrating the amplitude of reflected sound to standard calibration reflectors,
known as flat bottom holes (FBH). The method involves establishing a predetermined
amplitude response from an ultrasonic signal reflected from the wall of the hub
bore
18, which permits a relationship between the indication amplitude and
the FBH size to be calculated using the following equation:
Af=λd2Pf(
b/R)
1/2/(2(
R-b)
PR)
where A
f is the area of the FBH, λ is the wavelength of sound
in the turbine wheel
10, d is the sound path distance between the transducer
24 and an indication (defect), P
f is the amplitude of the reflected
ultrasonic signal from the indication, b is the radius of the hub bore
18,
R is the distance between the centers of the hub bore
18 and the bolt holes
20, and P
R is the amplitude of the reflected calibration ultrasonic
signal from the hub bore
18. This equation is derived as follows. The sound
pressure along the axis of one of the transducers
24 is:
P=p02 sin(Π((
D/2)
2+d2)
1/2-d)/λ)
where P is the on-axis pressure in the direction of sound travel, p
0
is the original pressure at the material surface, and D is the diameter of the
transducer's disc-shaped oscillator (J. Krautkramer et al., Ultrasonic Testing
of Materials, 4
th edition, Springer-Verlag (1990), p. 70). For large
values of d and D/λ, and for non-circular transducers, this may be simplified as
P=p0A/λd
where A is the area of the oscillator (J. Krautkramer et al. at p. 71). Using
this relationship, the sound pressure measured from the large bore reflection can
be expressed as:
PR=p0AT(
b/R)
1/2/2λ(
R-b)
and the sound pressure measured from a flaw is given by:
Pf=p0(
AT/λd)×(
Af/λd)
where A
T is the area of the oscillator and A
f is the
area of the flaw. Solving the last two equations for A
f yields the above-identified
expression for determining the area of an equivalent flat bottom hole (FBH), namely,
Af=λd2Pf(
b/R)
1/2/(2(
R-b)
PR)
In an alternative embodiment of the invention, the linear array transducer unit
22 represented in FIGS. 2 and 3 is replaced with a phased array transducer
unit
122 represented in FIG. 4. As the term phased array is conventionally
understood in the art, the transducer unit
122 comprises a series of individual
ultrasonic transducers
124 arranged in a row and acoustically isolated from
each other. Contrary to the linear array unit
22 of FIGS. 2 and 3, each
transducer
124 has its own electrical connection and pulser (not shown),
and each produces its own radio-frequency time/amplitude response (A-scan), which
can then be summed and graphically displayed. The angle of the ultrasonic beam
138 generated by each transducer
124 is varied (steered) as well
as the mode and focus of the beam
138 by controlling the timing of the pulse
and reception for each individual transducer
124. The advantage of using
the phased array transducer
122 in the inspection method of this invention
is the ability to focus the ultrasonic signal at a specific target
136 at
a specific depth. This focusing of the sound beam
138 counteracts the negative
effect that the curvature of the bolt holes
20 has on the sound field, which
would disperse the sound beam
138 and reduce the sound amplitude reflected
from a defect.
The phased array transducer unit
122 represented in FIG. 4 comprises eight
transducers
124. The actual number of elements (N) used with the method
of this invention will vary depending on the spacing between elements (k), the
wavelength (λ) of sound in the metal, the sound beam divergence angle (α)
to the target
136, and the active length (L) of the transducer unit
122.
This relationship is given by the following equation:
N=(
L+k) sin α/(0.44λ
+k sin α)
The optimum length (L) of the unit
122 can be determined by solving the
following equation for L using the maximum distance (d) from the center of the
bolt hole
20 to the target
136 and the radius (r) of the bolt hole
20:
d2+r2-2
dr cos(
L/2
r)-
d2
sin
2(
L/2
r)/ sin
2 α=0
The calibration of the phased array transducer unit
122 of FIG. 4 can
be accomplished using a Distance-Gain-Size (DGS) technique that relates the amplitude
of reflected sound from the hub bore
18 of the turbine wheel
10 to
the amplitude response from known size flat bottom holes (FBH) at varying distances
from the unit
122. A DGS diagram can be obtained through computer modeling
of the sound field responses or can be determined empirically using geometrically
equivalent calibration blocks containing machined FBH reflectors.
While the invention has been described in terms of particular embodiments,
it is apparent that other forms could be adopted by one skilled in the art. For
example, the physical configurations of the units
22 and
122 and
of the article (e.g., wheel
10) being inspected with the units
22
and
122 could differ from that shown. Therefore, the scope of the invention
is to be limited only by the following claims.
*
