Abstract:
Methods are provided for determining the presence of cracks in razor blades during manufacturing. The methods may include, for example, providing a plurality of razor blades in a stack, inducing a current in the stack, and measuring a parameter that is related to the current and indicative of the presence or absence of cracks in the razor blades.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates to methods of detecting cracks in razor blades during a razor manufacturing process.  
       BACKGROUND  
       [0002]     Razor blades are in many cases formed by a process involving heat treating and sharpening a continuous strip of perforated blade steel, and then breaking the strip into segments of the desired blade length. The resulting blades are then stacked on a spindle for further treatment, e.g., treating the blade edges with coatings to enhance durability and/or lubricity.  
         [0003]     Such spindles typically contain a great many blades, for example up to 4000 blades, stacked so that their edges are parallel and their adjacent broad faces are in contact with each other. When the blades are arranged in this manner, it is difficult to inspect the blades for cracks. While other types of defects generally may be observed using machine vision or human observation, cracks tend to be difficult to detect due to the tendency of the weight of the stack to close up any cracks. Cracked or broken blades can be extremely difficult to detect visually when the fracture surfaces are aligned to their original position and the crack is closed without the presence of an air gap. In other industries, such cracks can be made visible with the use of dye penetrants. However, dye cannot be applied to the surface of a blade stack because the dye would penetrate between the blades and cause staining. The presence of interfaces between neighboring blades exacerbates the difficulty of detecting cracked blades, as these interfaces are generally indistinguishable from the targeted cracks using conventional nondestructive testing systems such as machine vision, ultrasonic inspection, or eddy current testing.  
       SUMMARY  
       [0004]     The present invention provides automated methods for crack detection in a stack of razor blades.  
         [0005]     In some methods, electrical current is induced with a high-frequency induction coil as an air “knife” fans the blades. Thermal images are recorded with an infrared camera while the blades become exposed to the electromagnetic field. If the proper current density and timing is reached, the presence of a crack will be revealed by an abnormal temperature gradient across the blade.  
         [0006]     In one aspect of the invention, a method of inspecting razor blades for cracks includes providing a plurality of razor blades in a stack, inducing a current in the stack, and measuring a parameter that is related to the current and indicative of the presence or absence of cracks in the razor blades.  
         [0007]     In another aspect of the invention, a method of inspecting razor blades for cracks includes providing a plurality of razor blades in a stack, inducing a current in the stack, and measuring infrared radiation emitted by the stack to determine the presence or absence of cracks in the razor blades.  
         [0008]     The methods can include one or more of the following features.  
         [0009]     In some embodiments, the inducing step includes placing an induction coil adjacent the stack and delivering alternating current to the coil. In certain embodiments, the inducing step includes placing the induction coil around the stack and causing relative movement between the induction coil and the stack in a direction parallel to a long axis of the stack.  
         [0010]     In some embodiments, the measuring step includes measuring the temperature of at least one of the razor blades in the stack. In certain embodiments, the method further includes monitoring the measured temperature for localized areas of relatively higher temperature, indicative of cracks. In some embodiments, the method further includes obtaining a temperature map (e.g., a set of temperature profiles of the blades along the length of the stack).  
         [0011]     In certain embodiments, the measuring step includes measuring infrared radiation emitted by at least one of the blades of the stack. In some embodiments, the measuring step includes using an infrared camera to generate a temperature map (e.g., a set of temperature profiles of the blades along the length of the stack). In certain embodiments, the method includes operating the infrared camera at a speed of at least 250 frames/second. In some embodiments, the razor blades are of uniform blade length and blade thickness, and the method further includes using the infrared camera to generate an infrared image having a length at least equal to the blade length and a width at least equal to twenty times the blade thickness.  
         [0012]     In certain embodiments, the method further includes fanning the blades during the measuring step. In some embodiments, the fanning step includes applying an air jet to a surface of the stack. In certain embodiments, the applying step includes directing an air knife at the surface. In some embodiments, the air knife includes a dual air nozzle assembly.  
         [0013]     In some embodiments, the method further includes recording a sequence of thermal images of the blades along the stack. In certain embodiments, a rate of capture of the images is adjusted with the relative movement of the stack such that each individual blade appears on at least one image.  
         [0014]     In certain embodiments, the measuring step includes measuring the induced current. In some embodiments, the induced current is measured using a second coil. In certain embodiments, the method further includes monitoring the measured current for areas of reduced current, indicative of cracks.  
         [0015]     In some embodiments, the method further includes focusing the induced current. In certain embodiments, the focusing step includes positioning counter-rotating field cancellation coils on both sides of the current-inducing coil. In some embodiments, the ratio of the current in each of the field cancellation coils to the current in the current-inducing coil is from about 1:5 to 3:5.  
         [0016]     In certain embodiments, the providing step includes stacking the blades so that their cutting edges are in parallel alignment.  
         [0017]     In some embodiments, the blades each have a thickness of less than about 85 microns. In certain embodiments, the air jet has a width that is less than or equal to the blade thickness.  
         [0018]     Some of the methods described herein provide one or more of the following advantages.  
         [0019]     Crack inspection can be conducted using automated techniques, minimizing the time and labor required for quality control and thus reducing the cost of the blades. The presence of cracks can be reliably determined, reducing the likelihood that finished razors will be rejected due to blade cracks.  
         [0020]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0021]      FIG. 1  is a partial cross-sectional, top view of a crack detection system.  
         [0022]      FIG. 2  is a cross-sectional view of a blade stack taken along line  2 - 2  in  FIG. 1 .  
         [0023]      FIG. 3  is a graphic display of currents transmitted to the blade stack by an induction coil and counter rotating induction coils of the crack detection system of  FIG. 1 .  
         [0024]      FIG. 4  is a graphic display of a current in the blade stack resulting from the induction coil and the counter-rotating induction coils. 
     
    
       [0025]     Like reference numerals indicate like elements.  
       DETAILED DESCRIPTION  
       [0026]     A number of approaches may be used to detect cracks in blades arranged in a blade stack. For example, electromagnetic energy can be applied to the blade stack, and a sensor, such as an infrared camera, can detect the temperature along one or more blades within the blade stack. When a blade includes a crack or a similar defect, the infrared camera can detect a substantial temperature gradient within the blade.  
         [0027]     Referring to  FIG. 1 , a crack detection system  10  includes a blade stack  12 , an induction coil  14 , counter-rotating induction coils  24 ,  26 , an infrared (IR) camera  16 , and an air knife  18 . The blade stack  12  can be conveyed through the induction coil  14 , which produces a current within one or more blades  20  in the blade stack  12 . As a result of the current in the blades, the temperature of the affected blades generally increases. The air knife  18  forces a stream of air between blades in the blade stack  12  in order to separate one of the blades from adjacent blades. The infrared camera  16  then detects the temperature across the separated blade. A localized temperature spike detected by the infrared camera  16  can indicate the presence of a crack within the blade. An encoder measures the position of the stack (e.g., the position of blades within the stack) throughout the process. When a crack is detected, the position of the faulty blade is recorded, and the recorded position can be used to locate and remove the blade at a later stage of the process.  
         [0028]     The blade stack  12  includes multiple razor blades  20  supported by a support fork  22 . The blades  20  can be arranged in bundles of about 4000 blades/stack. The blade stack  12 , for example, can have a length of about 30 cm. The blades  20  can be formed of any of various materials, such as martensitic stainless steel and/or other types of blade steel. A locking mechanism can be included on one or both ends of the support fork  22  in order to prevent substantial movement of the blades  20  along the longitudinal axis of the fork  22 . The locking mechanisms, for example, can prevent the blades  20  from sliding off one or both ends of the fork  22 . An example of a locking mechanism is a spring-loaded piston that is arranged to apply a controlled pressure on the end of the blade stack.  
         [0029]     As shown in  FIG. 2 , the blades  20  define apertures  21  through which prongs  23  of the support fork  22  can be inserted. The apertures can be sized and shaped to fit securely around the prongs  23  of the fork  22  such that the blades  20  are substantially prevented from rotating about the prongs during the process described herein. This blade stack arrangement can help to stabilize the blades  20  during various manufacturing operations, such as coating operations, and during transportation of the blades  20 . Furthermore, this blade stack arrangement can help to ensure that the blades  20  are properly oriented throughout the imaging process. Adjacent blades are typically contacting each other in the blade stack  12 , but may be separated by small gaps caused by the presence of burrs and/or dust particles for example. Similarly, small gaps may be caused by a lack of flatness of one or more adjacent blades. Each of the blades  20  includes four edges (e.g., one sharp edge and three duller edges). The blades  20  also include two broad faces. The broad face of one blade can abut the broad face of an adjacent blade in the stack  12 . The blades  20  can be arranged such that the sharp edges of the blades  20  are positioned along one side of the blade stack  12 . In this configuration, the infrared camera  16  can detect the temperature across the blades  20  as the blade stack  12  is being conveyed in order to detect cracks within the blades  20  (e.g., cracks within interior regions of the blades, cracks within the sharp edges of the blades, and/or cracks propagating between the apertures and the sharp edges of the blades).  
         [0030]     The blade stack  12  can be conveyed through the detection system  10  in order to identify cracks and/or other defects within the blades  20 , as described in more detail below. The blade stack, for example, can be conveyed with the assistance of a computer-controlled linear motorized stage that includes a position encoder. The blade stack can be linearly displaced at a rate that allows the thermal camera to image each blade. The detection system  10  can inspect the blades for cracks at a relatively rapid rate, as compared to conventional methods (e.g., visual inspection). For example, the blade stack  12  can be conveyed and inspected at a rate of about 0.4 cm/second or greater (e.g., about 2 cm/second or greater, about 5 cm/second or greater). In certain embodiments, the blade stack  12  is inspected at a rate of about 100 blades/second or greater (e.g., about 250 blades/second or greater, about 500 blades/second or greater, about 650 blades/second or greater).  
         [0031]     Referring again to  FIG. 1 , the induction coil  14  encircles the blade stack  12 . The induction coil  14  can be formed of any of various conductive materials, such as copper and/or aluminum. The induction coil  14  can be connected to an energy source that generates a high frequency alternating current. Upon activating the energy source, the induction coil  14  can carry a current of about 30 Amperes to about 400 Amperes (e.g., about 150 Amperes to about 250 Amperes). Upon activating the energy source to which the induction coil  14  is connected, the induction coil  14  applies electromagnetic energy to the blade stack  12 , inducing a circulating current within the blade stack  12 . As a result of resistive losses, the electrical current can increase the temperature of the blade(s)  20 . The temperature of the blade typically increases as the resistivity increases. It is believed that cracks within the blades create an increased resistance, and thus create a localized temperature gradient as current passes through the cracked region of the blade. The temperature differential between cracked and uncracked regions of the blades  20  can be a function of the current in the blades  20 . For example, as the current in the blade increases, the temperature differential generally increases, and vice versa.  
         [0032]     The current induced within the blade(s) of the blade stack  12  generally circulates in a direction perpendicular to the magnetic field and in the same plane as the induction coil  14 . For example, the current can circulate in a plane in which the broad faces of the affected blades lie. Thus, the current does not generally flow across the blade-to-blade interfaces along the stack. This can be beneficial in detecting cracks within the blades  20 , as interference created by the blade-to-blade interfaces is reduced (e.g., eliminated).  
         [0033]     The induction coil can induce a power of about 100 Watts to about 1000 Watts in the blade stack  12 . The current in each blade  20 , for example, can range from about 1 Amperes to about 15 Amperes (e.g., about 1 Ampere to about 10 Amperes, about 5 Amperes). Consequently, the temperature differential between cracked and uncracked regions of the blades  20 , can range from about 1° C. to about 40° C. (e.g., about 10° C. to 20° C.). The overall temperature increase of the blade can be limited to about 50° C. or less. By limiting the temperature increase in the blades  20  to about 50° C. or less, for example, substantial damage to the blades  20  can generally be prevented.  
         [0034]     In some embodiments, as shown in  FIG. 1 , the induction coil is oriented at an angle α relative to a plane B extending substantially perpendicular to a longitudinal axis A of the blade stack  12 . Angle α, for example, can be about 0 degrees to about 30 degrees (e.g., about 10 degrees to about 20 degrees). As the blade stack  12  is being conveyed through the induction coil  14 , air pressure from the air knife  16  can cause some of the blades  20  to tilt as they are fanned, such that they are substantially parallel to the induction coil  14  (e.g., about 0 degrees to about 30 degrees relative to the plane B extending substantially perpendicular to the longitudinal axis A of the blade stack). Adjusting the angle a so that the plane of the induction coil  14  is substantially parallel to the broad surface of the blades can increase (e.g., optimize) the efficiency of the current induction process. Furthermore, as the angle between the broad side of the blades  20  and the infrared camera&#39;s line of view (the dotted arrow in  FIG. 1 ) approaches 90 degrees, the thermal image of cracks within the blades generally becomes more pronounced. Thus, by tilting the blades  20  as they pass through the induction coil  14 , the infrared camera  16  can, in some cases, achieve a better view of the broad-faces of the blades  20  for crack detection purposes.  
         [0035]     Using a device capable of creating non-contact induction, such as the induction coil  14 , can be advantageous because the lack of physical contact between the induction coil  14  and the blades  20  can provide a substantially consistent electrical connection, as compared to contact induction. At high currents, physical contact points of contact induction devices can cause arcing, which can result in an inconsistent electrical contact. Arcing, however, can be substantially reduced or eliminated by using non-contact induction.  
         [0036]     The counter-rotating induction coils  24 ,  26  are positioned on each side of induction coil  14 . The counter-rotating induction coils  24 ,  26  can be formed of any of various conductive materials, such as copper and/or aluminum. Like the induction coil  14 , the counter-rotating induction coils  24 ,  26  can be connected to a power source.  
         [0037]     The counter-rotating induction coils  24 ,  26  deliver energy to the blade stack  12  at the same time as the induction coil  14 . The counter-rotating induction coils  24 ,  26  carry a fraction of the current carried by the induction coil  14 , and the current carried by the counter-rotating induction coils  24 ,  26  flows in a reverse direction relative to the current flow in the induction coil  14 . The counter-rotating induction coils  24 ,  26 , for example, can carry a counter-rotating current of about ⅕ to about ⅗ of the current carried by the main central coil. Because the counter-rotating induction coils  24 ,  26  are positioned on each side of the induction coil  14 , the induction created by the counter-rotating induction coils  24 ,  26  substantially cancels a portion of the induction created by the induction coil  14 . This can be particularly advantageous for non-contact induction because the energy transmitted by non-contact induction devices (e.g., induction coil  14 ) tends to fan out slightly. For example, the energy transmitted to the blade stack  12  by the induction coil  14  generally induces a current in those blades over which the induction coil  14  is positioned, as well as in several neighboring blades. The current induced by the induction coil  14 , for example, can span over a distance of about 5 mm, which can include about 60 to about 100 blades. The counter-rotating induction coils  24 ,  26  can advantageously negate at least a portion of the induction created by the induction coil  14 , such that a smaller area of the blade stack (e.g., fewer blades) experiences a substantial increase in induction. This, as discussed below, can help the infrared camera  16  to more easily identify temperature gradients within the blade stack  12 , and thus more easily identify cracks within the blades  20 .  
         [0038]      FIG. 3  illustrates currents  28 ,  30 ,  32  produced within a portion of the blade stack  12  that is energized by the induction coil  14  and the counter-rotating induction coils  24 ,  26 . Current  28  is produced by the induction coil  14 , and currents  30 ,  32  are produced by counter-rotating induction coils  24 ,  26 , respectively. The current  28  is produced primarily within blade(s) in a central region of the blade stack portion. The currents  30 ,  32  are produced primarily within blade(s) on each side of the blade(s) in the central region. As shown, the currents  30 ,  32  substantially negate the current  28  produced in the blades neighboring the blades in the central region of the blade stack portion. Referring to  FIG. 4 , by negating the current  28  produced in the blades neighboring the central region of the blade stack portion, a resulting current  34  is present in substantially only those blades  20  in the central region of the blade stack portion. In some cases, for example, substantially only the region of the blade stack  12  covered by the induction coil  14  carries the current  34 . Because smaller regions of the blade stack  12  carry the current  34 , the overall period of time that each blade carries the current can be reduced. Thus, the temperature within the blades  20  can be maintained at a relatively low temperature, which can reduce background noise created by neighboring blades and can help to prevent heat damage to the blades  20 .  
         [0039]     Referring again to  FIG. 1 , the air knife  18  includes a pressurized air source and an aperture through which the pressurized air can be expelled. The aperture of the air knife  18  can have a diameter or width of about 0.025 mm to about 0.075 mm. The area of the aperture, for example, can range from about 0.9 mm 2  to about 2.7 mm 2 . The air knife  18  can produce an air stream having a pressure of about 550 kPa or greater (e.g., about 100 kPa to about 1000 kPa, about 500 kPa to about 750 kPa) and a relatively high velocity (e.g., about 25 m/s to about 100 m/s). The air knife  18  can be positioned from about 0.5 cm to about 2.5 cm away from the blade stack  12  in the region of the blade stack  12  in which the blades  20  are exposed to the electromagnetic energy. In some embodiments, the air knife  18  is positioned such that the air stream flows at an angle of approximately 90 degrees relative to the longitudinal axis A of the blade stack  12 .  
         [0040]     By blowing a concentrated stream of air into the blade stack  12 , the air knife  18  can separate adjacent blades  20  to a predetermined distance. For example, as the blade stack  12  is being conveyed, the air knife  18  delivers a stream of air to the stack  12  that systematically flips individual blades from one side of the air stream to the other. Thus, for a period of time, the blade that is flipped from one side of the air stream to the other is separated from both adjacent blades. The blade being flipped can be separated from adjacent blades by a distance of about 2 mm or more (e.g., about 3 mm or more, about 5 mm or more). By separating one blade from adjacent blades, the air knife  18  can provide a better view for the infrared camera  16 . For example, by increasing the distance between adjacent blades, the infrared camera can have a view of the blade that is substantially unobstructed or less obstructed by neighboring blades.  
         [0041]     Alternatively or additionally, the air knife  18  can help to focus the energy from the induction coil  14  on fewer blades. For example, as the separation distance between blades increases, the number of blades affected by the narrowly focused electromagnetic energy can be reduced. In some embodiments, the energy can affect substantially only the blade that is separated from its neighboring blades by the air knife  18 . As a result, interference from adjacent blades, which can negatively affect the detection of temperature gradients resulting from cracks within the separated blade, can be reduced (e.g., substantially eliminated).  
         [0042]     The infrared camera  16  can have a spatial resolution of about 10 μm to about 100 μm (e.g., about 20 μm to about 40 μm), a temperature resolution of about 0.01° C. to about 0.5° C. (e.g., about 0.05° C. to about 0.2° C.), and a frame rate of about 60 frames/second to about 1500 frames/second (e.g., about 500 frames/second to about 1000 frames/second). The infrared camera  16  can be arranged at an angle β relative to a plane C extending perpendicular to the longitudinal axis of the blade stack, and at a distance of about 3 cm to about 30 cm from the blade stack  12 . Angle β, for example, can be about 15 degrees to about 60 degrees. In the even that the field of view of the thermal camera does not cover the entire width of one blade, the measurement can be accomplished in a succession of staggered scans. The arrangement and specifications of the infrared camera  16 , for example, can be selected to provide clear thermal images of the blades  20  as they are conveyed in the blade stack  12 .  
         [0043]     In some embodiments, the infrared camera  16  includes a solid-state cooled detector arranged in a focal plane. However, uncooled microbolometers can also be used. Any of various infrared cameras can be used. Examples of infrared cameras include the Phoenix-Mid imager manufactured by Flir Systems (N. Billerica, MA), Scan IR II manufactured by Ircon, and Mikroline 2700 manufactured by Mikron, and SBF184 focal plane array manufactured by Santa-Barbara Focal Plane (Goleta, Calif.).  
         [0044]     The infrared camera  16  can measure the temperature of the blades  20  as they are flipped from one side of the air stream to the other. The infrared camera  16 , for example, can detect temperature gradients within the blades  20 . Substantially unflawed blades (e.g., blades without cracks) generally have a substantially uniform temperature gradient along the blade. However, as noted above, cracks within the blades can increase the resistance and the temperature in particular regions of the blade. Thus, temperature gradients of blades that have one or more cracks are generally not uniform. For example, such temperature gradients can include a temperature spike (e.g., a substantial increase in temperature) in the cracked region of the blade. In some embodiments, the infrared camera  16  can scan the blade stack  12  at a rate of about  1  cm/s or greater (e.g., about 4 cm/s or greater, about 8 cm/s or greater).  
         [0045]     In certain embodiments, the temperature measurements recorded by the infrared camera can be transmitted to a display. For example, the data (i.e., the temperature measurements) can be plotted on a graph to help the user to more easily identify cracked regions of the blades. Alternatively or additionally, software can be configured to automatically detect the presence of an abnormally high temperature gradient and to record the position of the cracked blade within the stack from the position encoder reading.  
         [0046]     Upon detecting a cracked blade, the position along the stack is read from the position encoder, recorded, and displayed to an operator. The operator can then extract the faulty blade from the stack at the later stage of the process.  
         [0047]     While several embodiments were discussed above, other embodiments are possible.  
         [0048]     In some embodiments, the induction coil  14  and/or the counter-rotating induction coils  24 ,  26  are water-cooled to reduce their active temperature. For example, water or another type of cooling liquid can be cycled through hollow passageways within the coils in order to maintain targeted temperature levels within the coils.  
         [0049]     In some embodiments, the air knife  16  includes a dual nozzle design. For example, one nozzle can direct air to an upper region of the blade stack  12  and another nozzle can direct air to a lower portion of the blade stack  12 . As a result, the air can be delivered to the blade stack  12  in a narrower stream. This can help to prevent multiple blades from being simultaneously flipped by the air stream, for example.  
         [0050]     In certain embodiments, the detection system  10  includes a magnetic separator as an alternative to or in addition to the air knife  18 . The magnetic separator can include magnets positioned adjacent the blade stack  12 . As a result, magnetization can be produced within the blade stack  12 . The magnets can be arranged to cause the blades  20  to repel one another. The magnetic separator can be positioned near the induction coil  14  such that the blades  20  are separated as they are conveyed through the induction coil  14 .  
         [0051]     While the embodiments described above involve transmitting electromagnetic energy to the blade stack  12  with the induction coil  14 , other devices can alternatively or additionally be used to induce current in the blades  20 . In some embodiments, for example, surface coils (sometimes called “pancake coils”) are used to induce current in the blades  20 . Surface coils have flat windings and can be positioned parallel to a side of the blade stack  12 . For example, a rectangular surface coil can be positioned parallel to the side of the blade stack  12  along which the sharp edges of the blades  20  are oriented. The thermal camera can be aimed at the blades either through the aperture in the center of the coil, or through the spacings between the windings in order to image the temperature of the surface of the blades. The induced current in the blades  20  generally lies in a plane parallel to the surface coils, and generally circulates in a direction opposite the direction of the current in the surface coils.  
         [0052]     In certain embodiments, current is induced within the blades  20  using electrically conductive rollers. For example, the detection system  10  can include a pair of conductive rollers electrically connected to a power source. The rollers can be formed of any of various conductive materials, such as copper, brass, copper-tungsten (75%), and/or silver-graphite. The rollers can have a diameter ranging from about 5 mm to about 30 mm. The rollers can be spring-loaded to contact the blade stack  12  with a predetermined force. As the blade stack  12  is conveyed, the rollers can contact one or more of the blades  20 , thereby transmitting energy to the one or more blades  20 . For example, the rollers can roll along opposite sides of the blade stack  12  as the blade stack  12  is being conveyed. In some embodiments, the rollers are sized and shaped to contact about ten blades or fewer (e.g., about 5 blades or fewer, about 1 blade) at a time. Thus, current can be induced in a limited number of blades along the blade stack at a given time. By limiting the number of blades induced with current at a given time, interference resulting from neighboring blades when performing the infrared scan can be substantially reduced or eliminated. Similarly, the blades can be heated for a reduced period of time, thereby reducing the risk of heat related damage.  
         [0053]     In some embodiments, a current detector is used instead of or in addition to the infrared camera  16 . The current detector can be part of a coil that is configured to encircle the blade stack  12 . The encircling coil, for example, can include two series of windings. The first series of windings can be connected to an alternating power source in order to induce an eddy current in the blades  20 . The second series of windings can be positioned nearer the blades than the first series of windings, and can detect the eddy current in the blades  20 . Generally, the amplitude of the current signal detected by the current detector decreases in regions of the blade that are cracked. Thus, decreased amplitude in the current signal can indicate that the blade being scanned is cracked.  
         [0054]     In certain embodiments, the current detector alternatively or additionally includes a planar array of current sensors arranged in relatively close proximity to the blade stack  12 . Like the second series of windings discussed above, the planar array of sensors can detect the eddy current within the blades  20 , and cracks within the blades can be detected by monitoring the current for decreases in amplitude.  
         [0055]     While the embodiments discussed above involve the use of induction coils and/or rolling electrodes to induce current in the blade stack  12 , other types of energy sources (e.g., thermal sources) can be used to impart energy to the blade stack  12 . In such embodiments, the infrared camera can be used to measure the temperature across the blades. As described above, a localized temperature gradient in the blade can indicate the presence of a crack. For example, a crack within the blade may appear as a cold spot. Examples of thermal sources that can be used to impart energy to the blade stack  12  include flash lamps, laser beams, microwave generators, electrical power supplies, induction heaters, and ultrasonic generators. Thermal energy from these sources can be delivered to the blade stack  12  using any of various techniques, such as pulsed delivery, step delivery, and lock-in delivery. For pulsed delivery, the thermal energy is delivered in a short burst and the thermal measurement is performed while the blades  20  are cooling. For step delivery, the thermal energy delivered to the blade stack  12  is abruptly increased and maintained while the temperature of the blades  20  is measured. For lock-in delivery, a series of thermal energy pulses are delivered. The series of pulses can be timed with the temperature detection equipment using a lock-in amplifier to substantially eliminate the effect of noise.  
         [0056]     Other embodiments are within the scope of the following claims.