FIELD OF THE INVENTION
The invention relates to a device for ultrasonic inspection of structural failures for detecting disbonds or delami nations, in particular relative lack of bond of brazed joints at a plurality of locations along the brazed interface. More particularly, the invention relates to a through transmission ultrasonic testing device for detecting disbands or delaminations along brazed joints of top and bottom coils of stator.
BACKGROUND OF THE INVENTION
An electric generator produces electricity according to the principles of generator action of a dynamoelectric machine, in response to a turning torque provided by a combustion or steam-driven turbine. The generator is a mechanically massive structure and electrically complex, with typical output power ratings up to 1,500 MVA at voltages up to 26 kilovolts (kV).
Conventionally, the electric generator comprises a rotor carrying axial field windings (also referred to as rotor windings) for producing a magnetic flux field in response to an input current, which is typically direct current supplied from a separate exciter. One end of the rotor shaft is drivingly coupled to a steam or gas-driven turbine for providing rotational energy to turn the rotor. Rotation of the rotor within stationary stator windings (also referred to as armature windings) causes the rotor magnetic field to induce an output current in the stator windings.
As shown in FIG. 1, conventionally an electric generator 10 comprises a rotor 12 carrying axial field or rotor windings 13 for producing a magnetic flux field that rotates within a stationary stator 14. One end 15 of the rotor 12 is drivingly coupled to a steam or gas driven turbine (not shown in FIG. 1) for providing rotational torque to turn the rotor 12. An opposing end 16 is coupled to a separate exciter (not shown) for providing direct current supplied to the rotor windings 13.

The stator 14 comprises a core 17 including a plurality of thin, high-permeability circumferential slotted laminations placed in a side-by-side orientation and insulated from each other to reduce eddy current losses. Stator coils 18 are disposed within inwardly directed slots of the stator core 17, and interconnected to form one or more dosed-circuit stator windings. Rotation of the axial field windings causes the magnetic field produced thereby to induce alternating current in the stator coils 18. The generated current is carried to the main leads 19 for connection to an external electrical load. Three-phase alternating current is supplied from a generator having three independent stator phase windings, formed by appropriate interconnection of a plurality of stator coils 18, and spaced at 120° around the stator core. Single-phase alternating current is supplied from a single stator coil extending 360° around the stator core.
The rotor 12 and the stator 14 are enclosed within a frame 20. Each rotor end comprises a bearing journal (not shown) for mating with bearings 30 attached to the frame 20. The rotor 12 further carries a blower 32 for forcing cooling fluid through the generator elements. The cooling fluid is retained within the generator 10 by seals 34 located where the rotor ends penetrate the frame 20. The cooling fluid is supplied to coolers 36 for releasing the heat absorbed from the generator components, after which the coolant is recirculated back through the generator elements.
FIG. 2 is a cross-sectional view of the stator 14, illustrating a face 60 of one stator core lamination and inwardly directed slots 62 carrying a top coil 18A and a bottom coil 18B. The individual core laminations are coupled by damp structures 64 to form the stator core 17.
FIGS. 3A and 3B illustrate one end of the top coil ISA and the bottom coil 18B, each comprising two groups or columns of conductive strands 66 and a plurality of cooling ducts 67 disposed between each strand group. The cooling ducts 67 remove heat energy produced by current flow through the top and bottom coils 18A and 18B. As shown, the top and bottom coils 18A and 18B are separated by a void 68.

Consolidation dips 70, typically constructed from copper, enclrde and capture a conductive strand group at the end region of the conductive strands 66. Thus four consolidation dips 70 are shown in FIG. 3A. A similar arrangement of conductive strands, cooling ducts and consolidation dips is present at the opposing end of the top coil ISA and the bottom coil 18B.
It is known by those skilled in the art that other generator configurations comprise a stator coil induding only a single coil such as the top coil 18A or the bottom coil 18B. In such a configuration only two consolidation clips are required, one consolidation clip for each strand group, with the two groups separated by cooling ducts. In still another configuration, a stator coil comprises only a single group of conductive strands, absent cooling ducts, with the strand group retained by one consolidation dip.
To form closed-circuit phase windings of the stator 14, the conductive strands 66 of the top coil 18A are electrically connected to the conductive strands 66 of the bottom coil 18B. Connected top and bottom coils 18A and 18B are then further connected to other interconnected top and bottom coils to form the closed-circuit stator phase windings. One known technique for effecting this connection between the top coil 18A and the bottom coil 18B brazes or solders an interconnecting copper bar 74 to opposing sides of both the top and bottom coils 18A and 18B. See FIGS. 4 and 5. An overlap region between the consolidation dip 70 and the copper bar 74 is indicated generally by reference character 76 in FIG. 4. Note from FIG. 5 that there are four such overlap regions, one on each opposing side of both the top coil 18A and the bottom coil 18B.
In certain coil embodiments, the overlap width is about one inch to about 1.25 inches, and the overlap length (designated "L" in FIG. 4) is dependent upon the coil height, (i.e., the distance between top coil 18A and the bottom coil 18B), which is typically in the range of about 3 inches to 5 inches. Assuming a coil height of 4 inches, each overlap region 76 presents an area of about 4 square inches. In certain other stator embodiments, the consolidation dip is replaced by a copper block that encircles the coil

strands. Generally, the overlap region is larger in the embodiment employing the copper block.
After the brazing operation, the overlap region must be inspected to ensure that a high quality braze joint has been formed between the copper bar 74 and the consolidation dips 70. Inspection of the copper bar braze joints at the end of each stator coil is a critical element of generator installation. The inspection is advisable to determine the integrity of the braze joint and ensure that the performance of the generator will not be compromised by a braze joint failure. In addition to conducting an inspection during construction of the generator, the brazed joint is also inspected when a stator coil is rewound. An overlap inspection is also performed in those generator embodiments employing a copper block in lieu of a consolidation dip.
One prior art inspection process utilizes a stencil template in the form of a grid with quarter-inch grid squares for identifying individual inspection sites. An inspector places the stencil over the copper bar 74 in the overlap region 76, and using the grid squares as a guide, manually marks each inspection site to guide the subsequent inspection process. The stencil is removed and a coupiant material (typically a gel-like substance) is applied to the copper bar 74 in the overlap region 70. An ultrasonic transducer is then manually positioned over each inspection site, as marked on the copper bar 74, for inspecting the quality of the braze joint at that site. The ultrasonic transducer emits ultrasonic energy (in one embodiment at about 2.25 MHz) and reads the echo return in each grid region. Differently sized transducers are available depending upon the area of the inspection region. For example, ultrasonic transducers having a diameter of 0.250" and 0375" are available. Prior to beginning the inspection process, the surface of the copper bar 76 must be dean and free of any contaminants that can adversely affect the transmitted and received ultrasonic test signals.
If the copper bar 74 is not adequately brazed to the consolidation dip 70, an air pocket or void, caused by inhomogeneous wetting by the filler metal of the parts to be brazed,

will be present between the mating surfaces. Since the void distorts the echo return, comparison of the actual return with a normal return from a properly mated surface allows void detection. Generally, a greater magnitude echo return indicates a void between the mating surfaces. The ultrasonic inspection process Is based on a physical material property referred to as the acoustic impedance. Air has very high acoustic impedance and therefore incident ultrasonic energy is almost totally reflected (about 99.7% reflection) by air. A high quality brazed joint with no air voids between the mating surfaces produces a small echo return signal as most of the energy is absorbed by the brazed materials.
As ultrasonic energy is transmitted at each inspection site, the technician manually records the parameters of the echo return. After an entire overlap region 76 has been inspected, the number of problematic sites or the ratio of the problematic sites to the total number of sites is determined. In one inspection process, each inspection site is determined to either pass or fail the inspection based on the relationship between the return magnitude and a predetermined return threshold. The number of failed sites or ttie percent of failed sites to the total number of inspection sites is compared to a predetermined threshold, above which the brazed joint in that overlap region is considered unsatisfactory.
Infact, these days, this is particularly characteristic of the up-to-date generators with the direct/indirect ventilation system of winding that the stator windings consist of two layers made of individual bars. To minimize losses, bars are composed of separately insulated strands. To minimize stray losses in end windings, strands of top and bottom bars are separately brazed and insulated from each other.
The generator reliable operation, predominantly, lies in the trouble-free operation of winding, which depends upon the quality of the bars brazed. The standard design of the powerful generator stator winding bars comprises the copper clamps wherein the stripped from insulation strands of the upper and lower bars are brazed up and a solid

copper forK/clip where the abovementiorted copper clamps are brazed. The influence upon the generator serviceability become apparent in the increased heat release in the zone of the brazed connection, thus leading to the temperature increase and, finally, to the connection failure, that causes emergency breakdown in the generator.
Therefore, due to the extreme operating conditions and stringent safety requirements, there is as strong demand for quality assurance by non-destructive evaluation of brazed components. This method allows one to detect voids in the brazing inter face such as voids, which are caused by inhomogeneous wetting by the filler metal of the parts to be brazed.
Ultrasonic Testing (UT), a very useful and versatile Non-Destructive Testing (NDT) method, uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and more.
A typical UT inspection system consists of several functional units, such as the pulser/receiver, transducer, and display devices. A pulser/receiver is an electronic device that can produce high voltage electrical pulses. Driven by the pulser, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen. Signal travel time can be directly related to the distance that the signal traveled. From the signal, information about the reflector location, size, orientation and other features can sometimes be gained.
The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy is the basis for ultrasonic testing. The active element is the heart of the transducer as it converts the electrical

energy to acoustic energy, and vice versa. The active element is basically a piece of polarized material (i.e. some parts of the molecule are positively charged, while other parts of the molecule are negatively charged) with electrodes attached to two of its opposite faces. When an electric field is applied across the material, the polarized molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material. This alignment of molecules will cause the material to change dimensions. This phenomenon is known as electrostriction. In addition, a permanently-polarized material such as quartz (SiO2) or barium titanate (BaTi03) will produce an electric field when the material changes dimensions as a result an imposed mechanical force. This phenomenon is known as the piezoelectric effect.
The outstanding sensitivity of ultrasonic inspection for detecting material discontinuities is due to the large difference in elastic modulus between the structural material to be inspected and the air usually filling the discontinuities to be detected. This remarkable sensitivity to material discontinuities also represents the most severe limitation for ultrasonic nondestructive testing because of the requirement that good coupling be maintained between the transducer and the object to be inspected without the slightest discontinuity between them. This goal can be achieved on flat, smooth surfaces by using some kind of fluid coupling to fill the inevitable thin gap between the transducer and the specimen. The same technique is much less effective on curved surfaces, where the width of the gap between the transducer and the specimen is inherently wider.
For assembled brazed bars it would be advantageous to do the inspection from the sides. However, ultrasonic inspection of the brazed joints from the side is much more difficult than from the top or the bottom of individual rods where easy access is available to the region of interest. There are three main disadvantages of inspection from the side.

First, no additional spatial information is obtained on the lateral location of detected flaw with respect to the inspection plane (i.e., on the through-thickness position). In comparison, when the inspection is done from the top, through inspection we can also determine whether the defect is closer to the top or the bottom, or positioned approximately at the center.
Second, there is the need for using different wedges for greatly different specimen sizes. In the case of inspection from the top, different thicknesses can be accommodated by the same wedge using different gating times, which can be easily adjusted electronically by the computer. In the case of inspection from the side, different widths cannot be accommodated simply by changing the gating function as full coverage of the brazed interface requires different wedge angles.
Third, dusters of essentially spherical or ellipsoidal pores present in most brazes appear to cover a larger fraction of the cross section when the inspection is done at oblique incidence. This presents a problem in the case of side inspection because of the necessarily low inspection angles (typically about 15°).
In general, with respected to the three major issues outlined hereinabove, the present invention overcomes these difficulties encountered with prior art by proposing an ultrasonic device that shall provide spatial approach to the braze joint site that has to be inspected and through which a noticeable improvement in the pulse echo response can be obtained.
OBJECTS OF THE INVENTION
it is therefore an object of the present invention to propose a through transmission ultrasonic testing device for detecting disbonds or detaminations along brazed joints of top and bottom coils of stator.

Another object of the present invention is to propose a through transmission ultrasonic testing device for detecting disbonds or delaminations along brazed joints of top and bottom coils of stator which is capable of isolating defect signals from high level noise signals and sounding an alarm upon the occurrence of such a defect signal.
A further object of the present invention is to propose a through transmission ultrasonic testing device for detecting disbands or delaminations along brazed joints of top and bottom coils of stator, which can be applied for inspections at locations that are tough to approach having low inspection angles.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The foregoing and other features of the invention will be apparent from the following more particular description of the invention, as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
FIG. 1 is a cross-sectional view of an electric generator.
FIG. 2 is a cross-sectional view through the stator of FIG. 1.
FIGS. 3A and 3B illustrate two views of an end region of a stator top and bottom coil of FIG. 2.
FIGS. 4 and 5 illustrate a copper bar interconnecting the top and bottom coils.
FIG. 6 illustrates the proposed ultrasonic device
FIG.7 illustrates the probe elements assembled in the device of Figure 6.

DETAILED DESCRIPTION OF THE INVENTION
Before describing in detail the automated ultrasonic inspection tool in accordance with the present invention, it should be observed that the present invention resides primarily in a novel and non-obvious combination of hardware elements and method steps. Accordingly, these elements and steps have been represented by conventional elements and steps in the drawings, showing only those specific details that are pertinent to the present invention so as not to obscure the disclosure with details that will be readily apparent to those skilled in the art having the benefit of the description herein.
It is proposed to provide an ambidextrous through transmission ultrasonic device having a spring mechanism for urging the handles in the open position, thereby exposing the device scanning head having transducer wherein such transducer have inwardly positioned receiver and transmitter probe. To reduce the likelihood of inadvertent operation, a spring-loaded mechanism to provide a rebound force is provided for maintaining the transducers in the dosed position when the test device is not in use.
Turning to Fig. 6 and Fig. 7, which illustrates the through transmission ultrasonic device assembled wherein the device consists of a hollow symmetric pair of left and a right handle 100 which serve as a frame to carry the more active components such as pivotal means 200, pair of scanning heads 300 having provisions to carry in them signal cable and elastic components 230, 250 to provide required rebound forces and to ensure that transducer element is in maximum contact area with the inspection surface.
The said ultrasonic test device consists of a symmetric pair of a left and a right hollow handle 100A, 100B with two implement ends 300A, 300B capable of being articulated apart and together by pivoting are formed at the front portion of the handle 100A, 100B through weld 160 and two hand grips 110 with knurled peripheral surface 120 on which

a dose palm style grip may be employed for manual operation are disposed through weld 501 at the rear portion of the assembly positioning.
Disposed on its inner faces 130A,130B handle 100A, 100B also include pivotal means having inward facing upper and lower pivotal links 200A, 200B that are overlaid and pivotably connected to each other and wherein each links 200A, 200B has aligned pivot hole 210 in which pivot pin 220 is secured.
Through welds 290, 280, handle 100A, 100B include a pair of opposing projections (posts) 240 and a pair of opposing welded C-clip 260 disposed on inner faces 130A, 130B near the pivotable linkage area on which opposite ends of a compression spring 230 and tension spring 250 is respectively mounted to provide a rebound force when the two hand grips 110 are pivoted by manual squeezing;
When the user or operator wants to utilize the pivotal linkage 210, welded to handle 100A, 100B through weld 140, for distension or constriction, pressure is first applied or released in an axial orientation to achieve pivoting around the said linkage 210 while the direction control components 270 is manipulated to axially disengage the said linkage for rotation.
While test probe elements are referred as transducer elements, reference is sometimes also made herein to transmitter elements 330A and to receiver elements 330B to distinguish them on the basis of their functions. In scanning head 300, a transducer 320A with transducer probe 330A, acting as transmitter for transmitting an ultrasonic signal, and a separate transducer 320B with transducer probe 330B, acting as receiver receives an ultrasonic signal from the transmitting transducer and sends a signal through signal cable 400, having cable connections 410, to a data acquisition system (not shown in figure), are hingedty mounted through hinge 310 on transducer mounting brackets 350A and 350B respectively.

Hinge 310 helps transducer elements 330A, 330B to swivel around axis A-A to accommodate surface variations with conducting the inspection and to have tendency to rotate about its axis A-A or to deflect laterally to a limited extent during its movement over test surface. Stopper 340 limits such swivel movement.
As such, the said two scanning head 300 and the linkage about pivot pin 220 are configured to have a relationship of interconnected leverage such that when the two hand grips 110 are squeezed, the two implement ends 300A, 300B at their opposite extremities open outward at an alterable angle, naturally causing the two scanning head 300 to simultaneously proceed outward to execute the distension operation.
For conducting ultrasonic testing of the inspection object such as a brazed joint, while the scanning head 300 are in an outwardly open position, the test object is required to be placed between receiver probe 330A and transmitter probe 330A in such a way that the elastic components provide enough rebound force to ensure that both transducer element is tightly in maximum contact area with the inspection surface
Conversely, the said two scanning head 300 and the linkage about pivot pin 220 are configured to have a relationship of interconnected leverage such that when the squeeze pressure is released from two hand grips 110, the two implement ends 300A, 300B at their opposite extremities close, naturally causing both scanning head 300 to simultaneously proceed inward to execute the constriction operation.
Although the present discussion relates to a ultrasonic inspection of structural failures particularly for detecting disbonds or delaminations and, more particularly, to an relative lack of bond of brazed joints at a plurality of locations along the brazed interface, it will be appreciated that the device and methods discussed may be readily adapted for use with other similar procedures. Not only the device will help in easy inspection of brazed joint but is cheap and easy to manufacture and shall provide safer and ergonomic means for the operators to deal with the inspection operation.

It will be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its scope.
Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Also, the various embodiments described herein are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

WE CLAIM:
1. A through transmission ultrasonic testing device having atleast two hollow handle members each having a head end, a handle end, and an intermediate body portion, further comprising:
- the at least two hollow handle members 100A, 100B telescoped over of signal cable 400 along its length, and having two implement ends 300A, 300B capable of being articulated spaced-apart and together by pivoting means 220 formed at a front portion of the handle through weld 160, two hand grips 110 with knurled peripheral surface 120 attached through weld 150 at the rear portion of the assembly;
- the pivotal means (220) disposed on handle inner faces 130A, 130B, through weld 140, and having inward facing overlaid pivotal links 200A, 200B pivotally connected to each other and wherein each links (200A, 200B) has aligned pivot hole 210 in which a pivot pin 220 can be secured;
- a plurality of direction control components 270 to restrict said linkage from rotation;
- a pair of opposing projections 240 and a pair of C-clip 260 disposed on handle inner faces, through welds 290, 280, on which opposite ends of a compression spring 230 and a tension spring 250 can be is mounted to provide a rebound force when the two hand grips (110) manually pivoted;
- a scanning head 300 having a transducer 320A with transmitter probe 330A and a separate transducer 320B with receiver probe 330B;

- a signal cable 400 having cable connections 410 to transmit inspection hingedly mounted transducer mounting brackets 350A and 350B respectively; and
- a hinge means 310 to swivel transducer elements and accommodate surface variations during the inspection process; and a stopper 340 which said limits swivel movement.

2. The through transmission ultrasonic testing device as claimed in daim 1, wherein the elastic components 230, 250 are selected in such a way to provide the desired rebound forces and ensure that the transducer element is in maximum contact area with the inspection surface.
3. The through transmission ultrasonic testing device as claimed in daim 1, comprising at least two hand grips 110 having knurled peripheral surface 120 on which a close palm style grip may be employed for manual operation.
4. A method of carrying out through an ultrasonic testing on a welded job in the device as claimed in daim 1, the method comprising :-

- squeezing the two hand grips 110 so that the two implement ends 300A, 300B at their opposite extremities open outward at a variable angle,
- causing the two scanning head 300 to simultaneously proceed outward to execute the distension operation based on the advantageous configurations of said two scanning head 300 and the linkage about pivot pin 220 which has a relationship of interconnected leverage.

- placing the test object between receiver probe 330A and transmitter probe
330A while the scanning heads 300 remaining in an outwardly
open position in such a way that the elastic components provide enough rebound force to ensure that both the transducer element tightly remain in maximum contact area with the inspect surface;
- releasing the squeezed pressure from said two hand grips 110, the two implement ends 300A, 300B at their opposite extremities close, causing both the scanning head 300 to simultaneously proceed inward to execute the construction operation, and
- positioning the inspection device manually at a plurality of inspection locations serially on the surface of the brazed joint