Abstract:
A method and apparatus for detecting structural anomalies in the body of rotating members of an engine involves establishing an electric circuit in the body of the rotating members by embedding a network of electrically conductive fibers. Upon rotation of the members in the presence of a magnetic field, an electric current flows in the electrically conductive fibers of the rotating members. Detection of a structural anomaly in any of the rotating members results from detecting a break of the electric circuit.

Description:
BACKGROUND OF THE INVENTION 
   Rotating components in large machines such as gas turbines, steam turbines, or aircraft engines, operate under large centrifugal stresses and may suffer breaks or cracks located at various points in the body of the rotating component. For example, engines have rotating compressor units that move and compress air. The compressor has blades around the periphery of a rotating disk, and the high speeds that the blades rotate induce high levels of stresses that tend to pull the components apart and cause cracks. In addition, through problems of excessive wear, the rotating assembly components may become unbalanced, which imposes unacceptable loads on the unit, possibly resulting in the creation of cracks. Once a crack is formed, it may grow in size until the rotating component is separated from the main assembly, thus resulting in a catastrophic failure. The problem is of gravest importance with passenger-carrying jet airliner engines, where upon such failure, separated blade components penetrate the engine compartment and/or the aircraft itself, thus impairing the air worthiness of the aircraft. 
   The most common method of detecting anomalies, such as cracks, in a rotating component of an engine, is to electronically monitor vibrations emanating from the rotating component. The idea is that a rotating assembly that lacks any cracks or similar imperfections is optimally balanced during the manufacturing and testing process, therefore vibrations due to asymmetrical rotational forces are minimal. Monitoring of dangerously high vibration amplitudes, or at least, large incremental changes over otherwise normal patterns of vibration, provides a good indication of a break or excessive wear present anywhere in the rotating unit. 
   Sonnichsen and Milatovic (U.S. Pat. No. 6,456,945) discloses a method of detecting cracks in a rotor, by measuring the rotational speed and vibration of the rotor. A processor subtracts a baseline vibration signal from the detected vibration measurement to produce a vibration difference signal. Subsequently, the amplitude and the phase of the vibration difference signal is measured to determine if anomaly, such as a crack, has occurred. Similarly, Gasch and Mingfu (U.S. Pat. No. 5,533,400), discloses a method for sensing cracks formed in rotating shafts, wherein the harmonic vibration components of flexural vibrations of the shaft are measured. 
   The vibration sensing method even though it offers the advantage of real time detection of anomalies in the body of the rotating unit, lacks sensitivity to the exact or even the approximate location of the crack. Since the method is based on sensing the vibration caused by unbalance during the operation of the unit, and said unbalance is created by the presence of cracks anywhere in the body of the unit, which produce an aggregate result, the method can not be tailored to sense small individual cracks in high-risk areas, such as the tip of the blade. Furthermore, the vibration sensing method is relatively complex, as it requires several peripheral electronic components, such as filters, speed sensors, etc. 
   In addition to the above real time monitoring of the occurrence of cracks in rotating units in engines, manual inspection may also be performed. More specifically, the engine is taken apart and the unit under inspection is immersed in a fluorescent solution. The fluorescent material will penetrate any cracks in the area and make them visible under ultraviolet light. Manual inspections are costly and time consuming, since the engine must be taken out of service and be disassembled. 
   BRIEF DESCRIPTION OF THE INVENTION 
   Described herein is an apparatus and a method for detecting the occurrence of structural anomalies, such as cracks, in rotating compressor blades of an engine during operation of the rotating unit. The detection system can be used with great sensitivity both in terms of the magnitude of the crack as well as its location within the rotating part. Furthermore, the system can be used during testing of the rotating unit or during actual operation. 
   In one embodiment, the detection apparatus includes a network of carbon fibers embedded in the body of the rotating blade. The fibers are connected and are in the proximity of a magnetic member. An electric circuit is established in the carbon fiber network, when the blade rotates. Upon the occurrence of a crack anywhere along the carbon fiber circuit, the fibers separate and break the electric circuit. This, in turn, creates a detection signal. 
   In another embodiment, a method for detecting structural anomalies in the body of rotating members of an engine includes establishing a network of embedded electrically conductive fibers in the members, positioning a magnetic member relative to the rotating members, detecting a break in the electric circuit created along the embedded fibers due to the presence of the magnetic member, the break in the electric circuit being caused by the occurrence of a structural anomaly in the body of a rotating member and generating warning signals upon detection of the break in the electric circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a crack-detection system according to one embodiment of the invention, showing one of the blades. 
       FIG. 2  is an enlarged view of a portion of the crack-detection system of  FIG. 1 . 
       FIG. 3  is a schematic diagram of a crack-detection system according to another embodiment of the invention showing the creation of an electric circuit established in the rotating blades. 
       FIG. 4  is a schematic diagram showing the detection stage of the crack-sensor. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows an exemplary embodiment incorporating carbon fibers for the detection of cracks formed in rotating blades. The system includes a rotating disk  1 , which may be part of a rotating element, such as a compressor of an engine, including, one of a gas turbine, a steam turbine or an aircraft turbine. Along the circumference of the disk  1 , there are radial grooves  2  machined or formed at the blade attachment slots, i.e., locations corresponding to the locations of the blades  3 . A single blade  3  is shown in  FIG. 1 . It comprises a dovetail  4  that conforms in size and shape with the size and shape of the groove  2 , and an airfoil  5 , which constitutes the main body of the blade  3 . The shape of part  5  is typically polygonal and depends on the specific type of the engine. 
   A pre-impregnated carbon tow  6 , the tow comprising a bundle of individual carbon filaments impregnated with a polymer resin and subjected to partial curing, is inlaid into a trough machined or formed into the airfoil at critical stress location(s), i.e., areas where cracks  7  are likely to initiate. Referring to  FIG. 1 , such areas include, the tips of the blade, the leading edge of the blade, and the dovetail of the blade. The prepreg (pre-impregnated with matrix) carbon fibers are co-cured (bonded) to the airfoil using non-conductive matrix and/or a non-conductive adhesive at the chosen areas. The fiber volume of the inlaid prepreg carbon tow material is less then 30%, so that the whole sensing component is uncoupled from the response of the blade itself, in other words, it behaves as an electrical component rather than a part of the structural medium. Preferably, the matrix material is an epoxy, but it can also be a polymer. The carbon fibers are commercially available and are chosen based on their strength relative to the airfoil material. Preferably, they have low strain-to-failure relative to other ductile airfoil materials. In addition, carbon fibers have a high modulus, and are much stronger and typically stiffer that the metallic materials of compressor blades, hence must be used in low fiber volumes when they are in the composite tow form. 
   As shown in  FIG. 1 , each blade element includes a carbon fiber tow  6  that is inlaid around the periphery of the blade. Connecting points  8  in the groove  2  provide electrical contact between the circuit located in the blade  3  and the part of the circuit located in the disk  1 . An expanded view of the connection area between a blade and the disk is shown in  FIG. 2 . 
   In a further embodiment, carbon fibers tows are embedded in other blade elements of the disk in a similar way as described above. Subsequently, the various carbon fiber inlays from all the blades are connected in series to form a circuit. This can be seen in  FIG. 3 , where two adjacent blades  9  and  10 , with their embedded carbon fiber tow networks, are shown. At least one magnet  11  is then assembled on the compressor casing  12  and axially aligned with the mid-chord of the blade. This arrangement will generate an electric current, I, in the carbon fiber circuit when the blades rotate under the influence of magnetic field B. If a crack occurs anywhere within the area of the blades and propagates through the locations where the carbon fibers are tightly bonded to the airfoil, then the strain energy that is released due to the crack will cause the low strain-to-failure fibers to fail (separate) and break the (series) electric circuit. This, in turn, creates a response signal that provides a feed signal to an audible alarm or a visual alarm for the plant operator, or the aircraft pilot. 
     FIG. 4  is a schematic of the detection stage of the crack-sensor. The carbon fibers embedded in the airfoil of each of the blades are connected in series, so that when a current is generated due to rotation in the presence of the magnetic field, the series circuit of  FIG. 4  is created. Each fiber inlay in a blade is represented by a resistance (only three  13 ,  14 , and  15  are labeled in  FIG. 4 ). A current sensor  16  is connected in series with the fiber inlays. In a preferred embodiment, there is a current sensor dedicated to each rotating disk/stage. Typically, there are 10 to 12 stages in a land-based power generating gas turbine unit. The current sensor, which can be any state-of-the art commercially available current sensor, is located away from the disk, electrically connected to it, in a stationary station. When a crack anywhere in the blades of a stage occurs that causes a break in the circuit, then the current sensor transmits a signal to one or more of control units  17 , through either hard-wired or wireless (radio frequency, infrared, etc.) circuitry. The control unit, typically located within 100 to 300 feet from the current sensors, usually has a separate channel or zone for an alarm sensor  18 . The alarm sensor produces an appropriate output signal  19 , when the crack-sensor senses a crack that triggers (fractures carbon composite tow-wire) the system. The alarm output signal can be used to create an audible tone (high frequency, typically around 3.2 KHz, at a high volume-around 85 dB at appropriate distance), or a visible signal such as a strobe light with at least 110 candela output, or a combination of the two. 
   The crack detection capability of the disclosed system can be tailored to the specific type of compressor used, since the carbon fiber inlays can be designed to correspond only to the high-risk areas of the blade. For example, if the compressor is used in applications where cracks at the tips of the blades are most common, then the carbon fibers are embedded in such a way so that they include the high-risk areas around the tips. This is an advantage over the vibration sensing method, which is insensitive to the exact location of the crack within the body of the rotating blade. 
   The detection system described herein is used for detecting cracks in compressor blades before they grow in critical size and cause blade separation. Furthermore, the embedded carbon fibers are very light and they are uncoupled from the response of the blade itself. In addition, the fiber selection is tailored to the fiber&#39;s strain capability so that the response does not become hyper-sensitive, resulting in false alarms. 
   The written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.