Abstract:
A method and apparatus for non-destructive inspection is provided. An automatic couplant delivery system that delivers and maintains a constant water supply to a single path chamber located at a transducer head. Any couplant run-off is removed and reclaimed by vacuum recovery. A transducer holder block that supports two transducers is used, and mounts to a manifold and brush subassembly. A manifold and brush subassembly includes a manifold block, a couplant containment/removal block, and gimbal. The manifold and brush subassembly has three sets of brushes that define three chambers for water or vacuum. An innermost chamber sets the water path, the middle chamber may provide water or regulated vacuum, and the outer chamber provides full vacuum for water removal. Couplant flow rate and vacuum are selectable and can be adjusted by an operator during initial set up.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method and apparatus for ultrasonic non-destructive inspection. More particularly, the present invention is directed to a method and apparatus for ultrasonic inspection having improved electronic coupling between a transducer and the surface of the object under inspection.  
           [0003]    2. Description of the Prior Art  
           [0004]    Many machines and vehicles experience wear and part degradation during their useful lives, which gets worse as they age. Ultimately such wear can result in complete failure. Depending upon the machine or vehicle, such failure can be catastrophic. For example, for aircraft, a failure in mid-flight may be deadly.  
           [0005]    In order to counter the effects of aging and prolong the effective lives of such machines and vehicles, regimens of regular inspection and maintenance have been put in to practice. While some machines and parts of some vehicles can be inspected and maintained easily, wear and fatigue to some parts and vehicles is difficult to discover, as it may not be visible to the naked eye, and/or be located in an easily accessible location.  
           [0006]    Consequently, inspection of such parts and vehicles may be very laborious, expensive and time-consuming. Significant disassembly and reassembly may be required, and the inspection may require considerable test equipment. Previously, regular inspection has often been destructive in the sense that portions of the device or vehicle had to be expended in order to perform a complete inspection. Such destructive examination is extremely expensive and can often be very time consuming.  
           [0007]    In an attempt to overcome the drawbacks of such destructive inspection, non-destructive inspection (NDI) has been developed. NDI is typically performed by subjecting the device to be inspected with acoustic waves and then analyzing the reflected waves to determine the state of the device without causing damage to the device. Acoustic NDI is particularly suited for determining the integrity of airplane components. Conventional acoustic NDI equipment emits acoustic waves through a transducer. The reflected acoustic waves are received by the transducer, which produces an electrical signal that is subsequently analyzed to rate the status of the workpiece.  
           [0008]    Over time aircraft experience fatigue as a result of normal use of the aircraft. Such fatigue often manifests itself as cracking. In order to prevent failure of the aircraft, which may result in the loss of life, the aircraft are regularly inspected to determine the integrity of the aircraft and to assess the extent of any fatigue experienced by any parts of the aircraft.  
           [0009]    An apparatus for acoustically inspecting a workpiece is taught in U.S. Pat. No. 5,469,744 (Patton et al.). The apparatus disclosed in the Patton et al. patent is known as a contact adaptive bubbler. As noted in the Patton et al. patent, ultrasonic NDI can improve the inspection spatial resolution and signal to noise ratio by using a focused acoustic beam. However, to be reasonably effective an ultrasonic transducer needs a good acoustic coupling between the transducer and the workpiece. Water is typically used as a coupling fluid between the workpiece and the transducer, and is sent through a perforated membrane to come in contact with the workpiece.  
           [0010]    In order to better control the flow of coupling fluid, the Patton et al. apparatus uses a non-perforated membrane, and two fluid chambers. The non-perforated membrane separates the two chambers. The lower chamber is formed when the apparatus is placed adjacent the workpiece, and can conform somewhat to the shape of the workpiece. The lower chamber is smaller than the upper chamber to provide control of the water flow as compared to an apparatus having a perorated membrane, since apparatus with perforated membranes cannot control the amount of fluid leaking through the membrane.  
           [0011]    Conventional bubbler systems including the Patton et al. apparatus utilize a membrane between the transducer and the workpiece to contain the water and reduce air bubble activity. The use of a membrane in such systems creates a drawback, namely the introduction of additional attenuation to the ultrasonic signal.  
           [0012]    Furthermore, conventional bubbler systems require a relatively high water flow rate to maintain the water path requirements of automated canning. Such high flow rates are not conducive to efficient couplant removal.  
           [0013]    Another drawback, in particular for the inspection of the upper surfaces of aircraft, is that conventional bubblers do not have sufficient couplant recovery and leave significant water residue, which can be a safety hazard.  
         SUMMARY OF THE INVENTION  
         [0014]    The deficiencies of the conventional methods are addressed by the present invention that is directed to a method and apparatus for non-destructive inspection. In particular, the method and apparatus of the present invention use an automatic couplant delivery system that delivers and maintains a constant water supply to a single path chamber located at the transducer head. Any couplant run-off is removed and reclaimed by vacuum recovery. A transducer holder block that supports two transducers is used, and mounts to a manifold and brush subassembly.  
           [0015]    The manifold and brush subassembly includes a manifold block, a couplant containment/removal block, and gimbal. The manifold and brush subassembly has three sets of brushes that in turn provide three chambers for water or vacuum. An innermost chamber sets the water path, the middle chamber may provide water or regulated vacuum, and the outer chamber provides full vacuum for water removal. The bottoms of the transducers are positioned below the upper surface of the water chamber to allow air bubbles to naturally migrate to the highest point and then evacuate through a vacuum port. Such a configuration eliminates the need for a membrane containment area and then an additional containment chamber. Couplant flow rate and vacuum are selectable and can be adjusted by an operator during initial set up.  
           [0016]    An advantage of the method and apparatus of the present invention is that no membrane is needed to contain the couplant in the acoustic transducer apparatus.  
           [0017]    Another advantage of the method and apparatus according to the present invention is that additional attenuation is reduced.  
           [0018]    Yet another advantage of the method and apparatus according to the present invention is that a majority of any excess couplant is removed.  
           [0019]    Still another advantage of the method and apparatus according to the present invention is that the apparatus can be configured for operation above and below a workpiece.  
           [0020]    Another advantage of the method and apparatus according to the present invention is that interference from upwardly migrating bubbles is reduced. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    These and other attributes of the present invention will be described with respect to the following drawings in which:  
         [0022]    [0022]FIG. 1 is a partially exploded side cross-sectional view of a couplant/vacuum head of the ultrasonic inspection apparatus of the present invention;  
         [0023]    [0023]FIG. 2 is a side view of a couplant/vacuum head of the ultrasonic inspection apparatus of the present invention;  
         [0024]    [0024]FIG. 3 is a side view of an alternative configuration for the transducer holder block, according to the present invention;  
         [0025]    [0025]FIG. 4 is a bottom view of the couplant/vacuum head of the ultrasonic inspection apparatus of the present invention;  
         [0026]    [0026]FIG. 5 is a cross-sectional side view of the ultrasonic inspection apparatus of the present invention configured for lower surface scanning;  
         [0027]    [0027]FIG. 6 is a top view of the ultrasonic inspection apparatus of the present invention configured for lower surface scanning;  
         [0028]    [0028]FIG. 7 is a cross-sectional side view of the ultrasonic inspection apparatus of the present invention configured for upper surface scanning;  
         [0029]    [0029]FIG. 8 is a top view of the ultrasonic inspection apparatus of the present invention configured for upper surface scanning;  
         [0030]    [0030]FIG. 9 is a cross-sectional side view of the couplant/vacuum head, flow paths, valves, couplant source, and vacuum, according to the present invention, configured for lower surface scanning;  
         [0031]    [0031]FIG. 10 is a cross-sectional side view of the couplant/vacuum head, flow paths, valves, couplant source, and vacuum, according to the present invention, configured for upper surface scanning;  
         [0032]    [0032]FIG. 11 is a block diagram illustrating the various components of the ultrasonic inspection apparatus according to the present invention;  
         [0033]    [0033]FIG. 12 is a graph of the probability of detecting a flaw according to the present invention;  
         [0034]    [0034]FIG. 13 is an upper view of a portion of a wing having splice joints shown in phantom;  
         [0035]    [0035]FIG. 14 is a partial side view of a splice joint shown in FIG. 13;  
         [0036]    [0036]FIG. 15 is a partial cross-sectional view of the splice joint shown in FIGS. 13 and 14; and  
         [0037]    [0037]FIG. 16 is a graph of the probability of detecting a flaw in a splice joint of a wing of aC-141 airplane, produced using the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]    The ultrasonic inspection apparatus  100  of the present invention, shown in FIG. 11, is designed to inspect machines such as airplanes. One area of an airplane that can be inspected using the ultrasonic inspection apparatus  100 , for example, is the wingspan fasteners (i.e. splice joints) used to connect various section of the wing assembly of an airplane. The ultrasonic inspection apparatus  100  of the present invention can inspect and detect fatigue cracks at or near the fastener sites. In addition, the ultrasonic inspection apparatus of the present invention can be used for corrosion inspection.  
         [0039]    The ultrasonic inspection apparatus  100  is designed to operate in conjunction with an automated scanner  62 , such as one manufactured by TIEDM. Such automatic scanners can move the ultrasonic inspection apparatus through a prescribed path over the surface of the workpiece. Typically the path is linear and the automatic scanner includes guide rails that are held to the surface of the workpiece with suction cups. If the ultrasonic inspection apparatus  100  is being used to inspect the wingspan fasteners of an aircraft, the automatic track is deployed so that the ultrasonic inspection apparatus  100  moves over the length of the splice joint.  
         [0040]    Referring to the side cross-sectional view shown in FIG. 1, the ultrasonic inspection apparatus of the present invention includes a transducer holder block  20 , one or two acoustic transducers  22 , and a couplant/vacuum head  24  that includes a manifold  26  and brush subassembly  28 . The transducer holder block  20 , shown in phantom, fits into the manifold  26 , as shown in the side view of FIG. 2.  
         [0041]    While only one transducer  22  is shown in FIGS. 1 and 2, the transducer holder block  20  is configured to support two transducers  22 . In the embodiment shown in FIGS. 1 and 2, the transducers  22  are supported at an angle by the transducer holder block  20 . A preferred angle for the transducers  22  is 20°. In this manner the two transducers  22  have converging focal points so that the two transducers  22  simultaneously inspect the same point. In a preferred embodiment, the angled configuration is used for fastener inspection, and the transducers  22  are immersion transducers.  
         [0042]    [0042]FIG. 3 shows an alternative configuration for the transducer holder block  20 . Instead of being supported at an angle like the embodiment shown in FIGS. 1 and 2, the transducer holder block  20  shown in FIG. 3 supports the two transducers perpendicular to the surface being scanned. The transducers are preferably ultrasonic transducers. The configuration shown in FIG. 3 is designed to inspect for corrosion.  
         [0043]    The couplant/vacuum head  24  is designed to provide a three-region system for automatic couplant delivery and removal. FIG. 4 is a bottom view of the ultrasonic inspection apparatus  100  shown in FIGS. 1 and 2. The couplant (e.g. water) is provided as the transmission medium between the test surface and the transducer(s)  22 . Referring to FIGS. 2 and 4, an innermost transducer region  30  is created by an elongated annular inner brush seal  32 ; An intermediate region  34  is created by a larger, elongated annular, middle brush seal  36  spaced outwardly from the inner brush seal  32 ; and an outermost recovery region  38  is created by a still larger, elongated annular, outer brush seal  40  spaced outwardly from the middle brush seal  36 .  
         [0044]    The brush seals  32 ,  36 , and  40  have a thickness and length, and are made from a material that provides sufficient sealing with a test surface to minimize couplant leakage while accommodating surface variations.  
         [0045]    The innermost transducer region  30  determines the couplant flow path. The intermediate region  34  may provide water or regulated vacuum, and the outermost recovery region  38  provides full vacuum for couplant removal.  
         [0046]    The flow path is selected based upon whether the surface being inspected is an upper or lower surface. Referring to FIGS. 5 and 6, a cross-sectional side view and a top view of the couplant/vacuum head  24 , respectively, are shown and illustrate the couplant flow path for lower surface inspection. Couplant flows into the port  42  at the bottom of the transducer holder block  20 , from a pump  56 , shown in FIGS. 9 and 10.  
         [0047]    The couplant rises to fill the cavity of the innermost transducer region  30  and then flows outwards over the elongated annular inner brush seal  32  into the intermediate region  34 . The intermediate region  34  is connected to a vacuum  54  so that the overflowing couplant from the innermost transducer region  30  is drawn from the surface and is returned. The tubes  44  through which the couplant is drawn have a relatively small diameter compared to the diameter of the tubes  46  connected to the outermost recovery region  38 , discussed below.  
         [0048]    Any couplant that flows over the intermediate brush seal  36 , spaced outwardly from the inner brush seal  32 , enters the outermost recovery region  38  formed by the outer brush seal  40  spaced outwardly from the intermediate brush seal  36 . Like the intermediate region  34 , the outermost recovery region  38  is connected to a vacuum source  54 . The tubes  46  leading from the outermost recovery region  38  have a larger diameter than the diameter of the tubes  44  leading form the intermediate region  34  in order to provide high airflow. The airflow carries the excess couplant back to the supply tank  58  and dries the surface of the workpiece being inspected.  
         [0049]    As a result of the foregoing configuration, any couplant that overflows the inner elongated annular inner brush seal  32  or the intermediate brush seal  36  due to surface irregularities or raised fastener heads is recovered in both the intermediate and outermost regions that are connected to the vacuum  54 .  
         [0050]    Referring to FIGS. 7 and 8, a cross-sectional side view and a top view of the couplant/vacuum head  24 , respectively, are shown and illustrate the couplant flow path for upper surface inspection. Couplant is supplied through the tubes  44  to the intermediate region  34 . Both the innermost transducer region  30  and the outermost recovery region  38  are connected to a vacuum  54 . Excess couplant flowing into the outermost recovery region  38  is drawn back through the tubes  46  to the couplant supply source  58  by the high airflow created by the vacuum  54 . Similarly, couplant that flows inwards under the inner brush seal  32  is drawn through the port  42  in the transducer holding block  20  by the vacuum  54 .  
         [0051]    For upper surface scanning the transducers  22  are set below the upper surface of the couplant so that bubbles will migrate naturally to the upper surface where they are evacuated through the vacuum port  42 .  
         [0052]    [0052]FIG. 9 shows a cross-sectional side view of the couplant/vacuum head  24 , similar to FIG. 5, with additional flow paths, valves, couplant source, and a vacuum. Valve  50  is positioned so that water flows to port  42  at the bottom of the transducer holding block  20 . Valve  52  is moved so that a vacuum created by the vacuum  54  is connected to the intermediate region  34 . No valve is connected to the outermost recovery region  38 , since this region always is subjected to the vacuum from the blower  54 . The couplant is drawn through a pump  56  and is supplied from the couplant supply source  58  to the port  42  at the bottom of the transducer holder block  20 . Any couplant that flows outwards over the elongated annular inner brush seal  32  into the intermediate region  34  is drawn back through tubes  44  and valve  54  into the couplant supply source  58 .  
         [0053]    Any couplant that flows over the intermediate brush seal  36 , enters the outermost recovery region  38  formed by the outer brush seal  40  spaced outwardly from the intermediate brush seal  36 . The vacuum created by the blower  54  draws the couplant back through the tubes  46  to the couplant supply source  58  from the outermost recovery region  38 . As a result of the foregoing configuration, any couplant that overflows the elongated annular inner brush seal  32  or the intermediate brush seal  36  due to surface irregularities or raised fastener heads is recovered in both the intermediate and outermost recovery regions  34  and  38  that are both connected to a vacuum  54 .  
         [0054]    Referring to FIG. 10, a cross-sectional side view of the couplant/vacuum head  24 , similar to FIG. 7, with additional flow paths, valves, couplant source, and a vacuum is shown configured for upper surface inspection. The couplant is supplied from the couplant supply source  58  through valve  52  and tubes  44  to the intermediate region  34 . The innermost transducer region  30  and the outermost recovery region  38  are connected to blower  54  and to the vacuum created thereby. Excess couplant flowing into the outermost recovery region  38  is drawn back to the couplant supply source  58  by the high airflow created by the blower  54 . Similarly, couplant that flows inwards under the inner brush seal  32  is drawn through the port  42  in the transducer holding block  20  through the valve  50  by the vacuum  54 .  
         [0055]    The various components of the ultrasonic inspection apparatus  100  are illustrated in the block diagram shown in FIG. 11. The cart  60  contains a data acquisition/computer system, motion control system, and an emergency stop. The automatic scanner assembly  62  supports the couplant/vacuum head  24 , and is connected to the cart  60 . The cart  60  sends signals to the scanner assembly  62  to control the position of the couplant couplant/vacuum head  24 . These signals include a two dimensional position control signals including an x-axis and a y-axis position control signals. The couplant/vacuum head  24  also receives a vacuum control signal and extend and retract signals from the cart  60 .  
         [0056]    A vacuum source  54  is provided and is connected to the couplant supply source  58 , shown in greater detail in FIGS. 9 and 11. The vacuum source  54  is also connected to the cart  60  to enable the control of the scanner assembly  62 . Three flow paths to the couplant delivery/recovery head are provided, one each for the recovery region  38 , the intermediate region  34 , and the innermost transducer region  30 . Two channels connect the two transducers  22  in the transducer holder block  20  of the couplant/vacuum head  24  to the cart  60 .  
         [0057]    Testing of the ultrasonic inspection apparatus  100  can be used to produce a graph, as shown in FIG. 12, of a probability of detecting a flaw. The plotted probability of detection is a function of the size of the flaw. The probability of detecting flaws increases, as the flaws get larger. The probability of detection graph can be used to provide a level of confidence that a certain defect or flaw size can be detected.  
         [0058]    Aircraft are commonly assembled using thousands of fasteners. For example, approximately 8,800 fasteners are used in the construction of an upper wing surface, and over 10,000 fasteners are used in the construction of a lower wing surface of a C-130 airplane. Over the life of the aircraft these fasteners must be regularly inspected to evaluate the integrity and air-worthiness of the aircraft. The ultrasonic inspection apparatus  100  can be used to perform non-invasive flaw and corrosion inspection of many portions of an airplane, such as the first and second layers of wing splice joints, center wing stringers, rainbow fitting attach areas, and thin and thick multi-layer structures.  
         [0059]    The operation of the ultrasonic inspection apparatus  100  will now be described with regard to wing spanwise splice joints for a C-141 airplane. Cracks in the second layer  80  of the spanwise splice joint are considered a life-limiting feature, i.e. in-flight failure of the splice joint could be catastrophic. FIG. 13 shows an upper view of a portion of the C-141 wing  70  with splice joints  72  shown in phantom. FIGS. 14 and 15 are a side and cross-sectional view of the splice joint  72 , respectively. The fasteners  74  extend through the first layer  76  of one wing plank  78  and a second layer  80  of an adjacent wing plank  82 .  
         [0060]    For lower surface inspection, the ultrasonic inspection apparatus is configured so that couplant flows into the port  42  at the bottom of the transducer holder block  20 , from a pump  56 , as shown in FIG. 9. The couplant rises filling the cavity of the innermost transducer region  30  and then flows outwards over the elongated annular inner brush seal  32  into the intermediate region  34 . The vacuum  54  draws the overflowing couplant from the innermost transducer region  30  is drawn from the surface being inspected and returns it to the couplant supply source  58  through the tubes  44 .  
         [0061]    Couplant flowing over the intermediate brush seal  36  enters the outermost recovery region  38 , as shown in FIG. 9. The vacuum  54  draws the couplant flowing into the outermost recovery region back to the couplant supply source  58  through tubes  46 . The tubes  46  leading from the outermost recovery region  38  have a larger diameter than the diameter of the tubes  44  leading form the intermediate region  34 . The high airflow through the tubes  46  also helps dry the surface being inspected.  
         [0062]    For upper surface inspection, couplant is supplied through the tubes  44  to the intermediate region  34 . The vacuum  54  is connected to both the innermost transducer region  30  and the outermost recovery region  38 . Excess couplant flowing into the outermost recovery region  38  is drawn back through the tubes  46  to the couplant supply source  58  by the high airflow created by the vacuum  54 . Similarly, couplant flowing inwards under the inner brush seal  32  is drawn through the port  42  in the transducer holding block  20  by the vacuum  54 .  
         [0063]    In use the ultrasonic inspection apparatus  100  is attached to the wing of the C-141 so that it travels along the juncture of two adjacent wing planks  78  and  82 . Referring to the probability of detection graph shown in FIG. 16, the ultrasonic inspection apparatus  100  of the present invention has a 90% probability of detecting flaws in the second layer  80  of 0.073 inches or greater, and a 90% probability of detecting flaws in the first layer  76  of 0.040 inches or greater. Furthermore, the ultrasonic inspection apparatus  100  falsely detected a flaw less than 1% of the time. As a result the time interval between inspections can be increased, thereby producing significant cost savings for both the inspection and maintenance and the downtime of the airplane.  
         [0064]    The operation of the ultrasonic inspection apparatus  100  can be tailored to the part being inspected. This is accomplished by adjusting a number of variables, including the gain of the transducers, the time and gate delays for the transducers, the head pressure for the couplant supply  58 , and the surface being inspected. The gain, time delay and gate delay appear to be the most important for achieving good inspection in both layers of a wingspan splice joint. In order to assure a good inspection, the gain should be maintained at a nominal or higher level, and the time delay and gate delay values should not deviate from the nominal level in the same direction, i.e., both high or both low.  
         [0065]    As an example, to assure good inspection results, allowable deviations from the nominal procedure levels (denoted as 0) are for the Gain to be in the interval [0, +6 dB] combined with one of two conditions for the Time Delay and the Gate Delay. The first condition is that the Time Delay be in the interval [−0.06 inch, 0] and the Gate Delay be in the interval [0, +0.09 inch]. Condition 2 is that Time Base Delay be in the interval [0, 0.06 inch] and Gate Delay be in the interval [−0.09 inch, 0]. These results are made with regard to the C-141 second layer inspections on the wing spanwise splices. The dimensions and materials of other inspection sites may yield different desired values for the foregoing variables.  
         [0066]    The length and width of the brush seals, as well as what they are made of, significantly affects the size of the fasteners and the surface irregularities that the ultrasonic inspection system can be used on and still provide satisfactory couplant removal.  
         [0067]    Having described several embodiments of the method and apparatus for non-destructive ultrasonic inspection in accordance with the present invention, it is believed that other modifications, variations and changes will be suggested to those skilled in the art in view of the description set forth above. For example, the couplant need not be water, but could be another fluid. It is therefore to be understood that all such variations, modifications and changes are believed to fall within the scope of the invention as defined in the appended claims.