Abstract:
A hand-held thermography system ( 8 ). A generator ( 10 ) supplies current to a transformer ( 15 ) in a handle ( 16 ). An induction coil ( 20 ) connected to the transformer ( 15 ) extends from the handle ( 16 ). The induction coil ( 20 ) induces eddy currents in a test object ( 50 ), producing a thermal topography on a surface ( 52 ) of the object ( 50 ) that reveals structural features including defects in the object. An infrared camera ( 24 ) mounted on the transformer ( 16 ) digitizes images of the thermal topography. A controller ( 12 ) processes the images, displays them on a monitor ( 14 ), and stores them in a digital memory ( 11 ) for evaluation. Digitized positional data relating the position of the image to the surface may also be stored. An operator ( 40 ) presses a trigger ( 17 ), signaling the controller ( 12 ) to start current to the induction coil ( 20 ) and simultaneously to acquire and process one or more images from the camera ( 24 ). The images may be evaluated visually and/or by computerized analysis techniques for analyzing defects in the object.

Description:
FIELD OF THE INVENTION 
   The invention relates to the field of non-destructive evaluation of articles of manufacture by stimulating an article with electromagnetic energy, then imaging and evaluating a resulting topography of differential inductive heating on a surface of the article. 
   BACKGROUND OF THE INVENTION 
   Active thermography is a non-destructive evaluation (NDE) technique in which a non-destructive stimulation such as acoustic or electromagnetic energy is applied to a test object. The applied energy induces mechanical vibrations or electromagnetic currents (respectively) in the object, thereby producing an uneven temperature distribution in the object. Structural features and flaws in the object generate localized heat under such stimulation. A resulting temperature topography on a surface of the object is imaged with an infrared camera. Information about defects and the inner structure of the object can be obtained by evaluating the images individually or a time series of such images. Each image may be digitized into picture elements, or pixels, with each pixel representing a small unit area on the surface. These digitized images can then be used for digital displays and for computer analyses, in which a temperature/time series of images may be processed and analyzed by pixel over time and in patterns of pixels over time and/or space. Time series information improves overall sensitivity of the technique, and facilitates the determination of geometric quantities like local coating thickness, wall thickness, or depth of a defect. 
   Stationary inspection systems are generally used to test articles of manufacture during their production. Mobile systems are often used for field inspections of operational apparatus such as aircraft, power plant equipment, transportation equipment, and the like. Current NDE techniques such as dye penetrant, magnetic particle coatings, ultrasonic stimulation, and eddy current stimulation have various disadvantages in speed, flexibility and/or potential contamination to the articles tested. Improved NDE devices and techniques are needed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in following description in view of the drawings that show: 
       FIG. 1  schematically illustrates a hand-held thermography system in accordance with the invention. 
       FIG. 2  illustrates additional induction coil spacers for 3-point spacing. 
       FIG. 3  illustrates a method of operation of the invention. 
       FIG. 4  illustrates an offset induction coil shape and a spacer/digitizer. 
       FIG. 5  is a schematic diagram of components. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a thermography system  8  with an electric current generator  10  electrically connected  32  to a processor or controller  12  with a display  14 . The connection  32  provides control and status signals between the controller and the generator  10 . The generator  10  provides pulsed alternating current (10 ms-1,000 ms bursts) to a transformer  15  in a handle  16  via an electrical cord  30 . A digital infrared camera  24  may be mounted on the handle  16  on a support  22  and may be electrically connected  34  to the controller  12 . The camera support  22  may have a camera position adjustment mechanism  23 . An induction coil  20  extends from the transformer  15 , and may have a generally planar frame shape. The induced current is highest directly underneath the coil winding and decreases with increasing lateral distance from the winding. Therefore, an inner and outer border area with a width defined by the distance from the winding where the current decays to a certain extent (for example by 50%) can be used for thermographic inspection. The effective test area is a function of many variables, such as the amount of current in the coil  20 , the properties of the test object  50 , and the sensitivity of the camera  24 . The test area surrounding the coil  20  is transparent to infrared radiation emitted from the test object  50 . The entire inner zone of the coil  20  may be used for test purposes if the induced currents are sufficiently high. A trigger  17  connected  31  to the controller  12  may be provided on the handle  16  to start current flow in the induction coil  30 . The trigger  17  connected  31  to the controller  12  may also be used to start the imaging sequence from camera  24 . A second handle (not shown) may be provided for two-handed operation. If so, for additional safety a trigger  17  may be provided on both handles, and the controller  12  may require both triggers to be pressed to energize the induction coil  30 . 
   The transformer  15  transforms electrical current provided by the generator  10  into current suitable for the induction coil  20 . The transformer may be a step-down type with a voltage ratio such as 10:1 and a corresponding amperage ratio such as 1:10. For example, the generator  10  may provide an alternating current of about 100-1,000 volts, 10-100 amps, and frequency of about 10 to 1000 kHz. The transformer  15  may convert the current to about 10-100 volts, and 100-1,000 amps for the induction coil  20 . A coaxial transformer design is especially suitable for hand-held operation due to its size and weight. 
   Placing a transformer  15  in the handle  16  reduces current in the cable  30  that would otherwise be needed between the generator  10  and the handle  16 . This reduces resistive heating in the cable  30 , which avoids damage to the cable  30 . The current and transformer parameters above are provided as examples only. The generator  10  may provide current with user-selectable characteristics to the induction coil  20 , as selected from a user input device  13  on the generator  10  or the controller  12  or the transformer  15 , as known in electronics. For example, the user input device  13  may be a keyboard, keypad, or dial interfaced to the controller  12 . Electric current parameters for the induction coil  20  may be selected based on the application or type of test object. For test objects with a high electric conductivity (copper, aluminum, etc.), the resistive heating and thus the temperature rise is low. In this case, the amplitude of the excitation current must be chosen to be sufficiently high to obtain meaningful results. The excitation frequency determines the “skin depth”, a parameter that describes the penetration depth of the induced current. Resistive heating occurs only in the skin depth layer immediately adjacent to the surface, and may vary from only a few micrometers for magnetic materials to some meters for materials with low electrical conductivity, such as carbon composite materials. 
     FIG. 1  shows a generally rectangular induction coil  20  with the camera  24  pointing to an area  21  surrounded by the induction coil  20 . However, other coil shapes may also be used, such as rectangular, circular, etc. For curved test objects a bending of the coil may be appropriate. Coils with only one winding may be used; however, multiple windings may be appropriate in some applications. The inductance of the coil  20  should match the output properties of the excitation device.  FIG. 4  shows an induction coil  20 ′ formed with a shape corresponding to the cross-sectional shape of a blade root  51  of a turbine blade  53 , with the camera field of view  38  including the critical blade root region. The orientation of the induction coil  20 ′ is particularly useful in this embodiment because it contains sections oriented approximately perpendicular to the orientation of cracks that may be expected to develop in this region of the turbine. The perpendicular orientation provides a maximum sensitivity for the detection of such flaws. The embodiment of  FIG. 4  is operated by moving the induction coil  20 ′ along a surface  52  of the turbine disk  50 , or holding the coil  20 ′ still while the turbine disk  50  is rotated, while taking a series of images. The images may be triggered manually by the operator pressing a trigger  17  or automatically by the controller  12 , based on a time or space interval. The induction coil  20 ′ is thermally insulated to reduce its impact on the thermographic images. 
   To provide consistent spacing between the induction coil  20  and the test surface  52 , at least one spacer  18  may be attached to a side of the handle  16  to provide contact with the surface  52 . As shown in  FIG. 2 , additional spacers  19  may be provided on the induction coil  20  or on the handle  16  to separate the test surface  52  from the induction coil  20  along a line or plane of contact with the test surface  52  that is generally parallel to the induction coil  20 . Two points of contact can be used to establish a line of contact, or three points of contact may be used to establish a plane of contact. For stationary testing, the spacer  18  may be stationary. In this case, the operator  40  may hold the induction coil  20  in one position on the surface  52  after starting the stimulation, while the controller  12  processes one or more images. For moving testing, the spacer  18  may be in the form of a low friction skid or a rotating ball or wheel.  FIG. 4  shows the spacer  18 ′ as a position detector such as a digitizer ball used on a computer mouse, that is electronically connected  30  to the controller  12  to track the motion of the induction coil  20 ′ over the surface  52 . The controller  12  may use this input to operate the camera  24  to acquire images with a known spatial relationship. With digitizer input the controller  12  may also dynamically adjust the current in the induction coil  20 ′ in proportion to the speed of induction coil motion. Alternately, the controller  12  may start the induction coil current upon triggering, then input a time series of images, and then display a message prompting the operator to move the induction coil  20 ′. The controller  12  may be a programmable logic controller, or it may be a circuit card interfaced to a computer, for example. The controller  12  may include a clock circuit and/or may use a clock signal provided by a computer to calculate time intervals and speeds of digitizer  18 ′ motion. 
   Referring now to  FIG. 3 , to operate  60  the system  8  to inspect an article of manufacture  50 , a user selects  62  electrical parameters via the input device  13 , then holds  64  the induction coil  20  adjacent to a surface  52  on an article  50  to be tested, and presses  66  the trigger  17 . In one embodiment, triggering causes the controller to start current pulse in the induction coil  20 , and contemporaneously (i.e. simultaneously or immediately thereafter) to input one or more images from the camera  24 . In this embodiment, the user then moves  68  the induction coil  20  to another position and repeats. On an embodiment with a digitizer  18 ′, pressing the trigger may cause the controller  12  to initiate repeated current bursts (repetition rate typically 0.5-50 hertz), automatic image acquisition, and positional input by the digitizer  18 ′. In a digitizer embodiment, the user may move  68  the induction coil over the test surface  52  continuously, while the controller  12  controls image acquisition and digitizer position input. The images may then be evaluated  70  visually by display on the monitor  14  and/or by computerized image analysis techniques. The images may be stored  72  on a digital memory device  11  such as a disc drive. 
   The images acquired for each current pulse may optionally be post-processed, such as by background subtraction or pulse-phase analysis. Background subtraction is a technique used in thermography wherein the first image of the recorded infrared sequence corresponds to the initial status of the test sample before heating and is subtracted from the following images. This eliminates a potential non-uniform infrared emissivity of the sample surface due to inhomogeneous material properties, dirt, etc. Pulse-phase analysis is used to evaluate not only the amplitude but also the time behavior of the temperature signal. A sinusoidal signal (e.g. with a period in the order of two pulse lengths) is correlated with the measured time signal. The calculated phase of the sinusoidal signal corresponds to the time delay of the induced heat flow and the amplitude to the temperature rise. From the time delay, the depth of a defect can be evaluated. Both techniques provide lateral resolved information because they are applied to each pixel of a series of images. 
   In tests of a prototype, detection of discontinuities in a test object  50  was more sensitive when the induction coil  20  was closer to the test surface  52 . Distances up to about 20 mm provided sufficient sensitivity to detect fatigue cracks in metal superalloy parts. The hand-held design allows an operator to make continuous adjustments in the angle of the induction coil  20  in order to test parts with various curvatures and shapes. The induction coil  20  may be optimally sized for a particular application. For example, to inspect a turbine disc the induction coil  20  may be made about the size of a blade attachment slot. 
   The induction coil  20  may be actively cooled, although active cooling may not be required when operating with pulsed current. For active cooling, the winding of the induction coil may be hollow and attached to a water circulation system with a heat exchanger. The coil winding may be insulated to minimize heat emission that would produce “noise” on the camera image, and could thus mask defect indications. 
   While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.