Abstract:
Apparatus, systems, and methods for inspecting and monitoring bolted joints of metallic and composite structures for defects such as delamination and fatigue cracking are provided that incorporate ultrasonic transducers with load bearing washers. These active washers may be used for inspecting and monitoring a structure beneath such load bearing fasteners as bolts and nuts. Active washers may be used for continuous, periodic, and controlled inspections of bolted joints. Ultrasonic transducers may be permanently applied to a surface of a washer or recessed in a cavity on a surface of the washer. Inspection signals may be transmitted from ultrasonic transducers into a structure and reflected in pulse-echo application or received by another active washer on the opposing side of the structure in a through-transmission application.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to an apparatus, systems, and methods for inspecting a structure and, more particularly, to an apparatus, systems, and methods for using active washers for non-destructive inspection of bolted joints of a structure. 
     BACKGROUND 
     Structural inspections represent a large portion of overall maintenance costs on aircraft and other vehicles and structures. Non-destructive inspection (NDI) of structures involves thoroughly examining a structure without harming the structure or requiring its significant disassembly. Non-destructive inspection is often preferred over visual or destructive inspection methods to avoid the schedule, labor, and costs associated with removal of parts or other disassembly for inspection (with the associated potential for damaging the structure). In the field, access to interior surfaces of the structure is often restricted, requiring disassembly of the structure, introducing additional time and labor. Frequently, inspections are necessary or mandated to be performed in hazardous or difficult-to-access areas, such as in fuel cells, electronics bays, and pressure bulkhead cavities, which may require fuel cell venting and removal of panels, ducts, insulation, and other surrounding structures. Non-destructive inspection is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required, particularly where gaining access to an inspection area is limited. For example, non-destructive inspection is commonly used in the aircraft industry to inspect aircraft structures for damage or defects (flaws) in the structure. Inspection may be performed during manufacturing or after the completed structure has been put into service to validate the integrity and fitness of the structure. 
     Related to the need for performing structural inspections is the ability to determine whether maintenance is required. For example, to decrease the costs of airplane maintenance, the concept of Vehicle Health Management (VHM) can be used to more accurately determine when maintenance is required, in essence by monitoring the health of the vehicle. Central to the concept of Vehicle Health Management for an airplane is a network of sensors installed throughout the airplane. These sensors may be monitored continuously or queried periodically during maintenance checks, when the tools and/or facilities for repairing any problems are immediately available. Such a sensor network may also be used for Condition-Based Maintenance (CBM) in which maintenance checks and maintenance of parts and systems of a vehicle may be initiated by sensor data. For example, maintenance intervals of airplanes are typically conservatively set for routine maintenance, but Condition-Based Maintenance could reduce the need for certain routine maintenance which can be monitored to determine when the maintenance is required, thereby resulting in less frequent maintenance and reduced maintenance overall. 
     A feature of airplanes and other vehicles and structures that is critical to structural integrity is the bolted joint, including the bolted composite joint. Of concern for bolted composite joints is the potential for defects such as delamination and fatigue cracking around the bolt-holes. Currently, one approach for inspecting bolted joints involves an operator gaining access to an inspection area around the bolted joint and inspecting the area immediately surrounding the bolt-hole with a shear wave ultrasonic beam or eddy current. The operator typically scans the inspection area while monitoring a display screen for any signals which may be interpreted as a defect in the structure. Such inspection often requires preparing the inspection area surface, such as scraping away sealant fillets to provide a clean surface for a transducer or probe. Gaining access to the inspection area often involves removing access panels, hydraulic lines, cables, hoses, brackets, and other interfering structures. In addition to the practical impediments to such inspection, the operator must know how to place and orient a transducer or probe to ensure that defects are examined from the optimum angle. Further, the operator must be able to interpret and evaluate the inspection data on the display in real-time and determine if any potential defects are significant or non-significant. Following such inspections, sealants and coatings must be restored and interfering structures replaced. 
     Manual inspection of structures typically is very labor intensive, time consuming, and expensive. Manual inspection is subject to human error in performance and variations of interpretation of results. Noise in inspection signals can be interpreted as defects (false positives), and defects can be missed or overlooked as non-significant (false negatives). Further, shear wave ultrasonic beam and eddy current inspection are limited in that only cracks of particular orientations may be detectable. Many structures may also incorporate numerous bolted joints which require inspection in areas which cannot be accessed or are exceptionally difficult to access. 
     Several approaches have been attempted to inspect bolted joints with sensors. One approach is a smart washer proposed by Innovative Dynamics, Inc, of Ithaca, N.Y. These smart washers incorporate eddy current sensors. However, the sensors cannot be “nulled” or balanced between widely spaced interrogation intervals, so it is not possible to discern crack signals from signals caused by temperature variations, instrument drift, and other noise factors. Furthermore, these smart washers use eddy currents and can only be used on electrically conductive structures. 
     Another approach is using eddy current rosettes produced by Jentek Sensors of Waltham, Mass. These eddy current rosettes are bonded onto the area surrounding a rivet or bolt. The rosettes contain eddy current sensor loops for detection of surface-breaking cracks. The rosettes can be calibrated in air and provide an absolute measurement, unlike the Innovative Dynamics smart washers which only provide relative measurements that depend upon a stable null point over time. However, the Jentek Sensors rosettes depend on a strain gage adhesive to cement the sensor in place, and these adhesives are subject to failure over time. Furthermore, the Jentek Sensors rosettes are expensive, can be difficult to use and understand, and require new computer models for different applications. 
     Yet another approach is comparative vacuum monitoring (CVM) sensors produced by Structural Monitoring Systems Ltd. of Perth, Australia. Comparative vacuum monitoring sensors measure the pressure differential between small recesses containing a low vacuum alternating with small recesses at atmosphere, where the alternating series of low vacuum and atmosphere recesses are located in a simple manifold. If no surface-breaking crack is present, the low vacuum will remain at a stable level. If a crack develops, air will flow through the crack from the atmosphere recesses to the vacuum recesses. Comparative vacuum monitoring sensors only have application to surface-breaking fatigue cracks and are unable to detect delaminations below the surface, or fatigue cracks originating at the far surface of a layer. 
     Although used for a different purpose, a related technology is the permanent mounted transducer (PMT) system by PFW Technologies GmbH of Speyer, Germany. The PFW Technologies permanent mounted transducer system uses an ultrasonic transducer with a bolt to measure elongations of the bolt caused by the clamp load on the bolt, thereby providing a way of measuring the clamp load on the bolt during assembly and providing a way of controlling the torque load exerted by a tightening tool. The PFW Technologies permanent mounted transducer system, however, does not have the capability of detecting defects in the vicinity of the bolt-hole, but only monitors changes in the bolt stress state. 
     Accordingly, improved apparatus, systems, and methods for inspecting bolted joints of structures are desired. 
     BRIEF SUMMARY OF THE INVENTION 
     Provided are improved apparatus, systems, and methods for inspecting and monitoring bolted joints of both metallic and composite structures for defects such as delamination and fatigue cracking. 
     Embodiments of apparatus of the present invention use one or more inspection sensors, typically ultrasonic transducers, incorporated with or into washers, typical of washers used as a bearing surface beneath a nut or bolt head, creating what are referred to herein as active washers. Active washers may be used to inspect and monitor structures under washers used with bolts, nuts, rivets, and similar bearing fasteners. Active washers may be used for continuous, periodic, and controlled inspections of bolted joints. Active washers may be used individually for inspection of a bolted joint and used as sensors of a Vehicle Health Management system. Using active washers may reduce maintenance costs and overall required maintenance by providing a way of passively inspecting and monitoring bolted joints to determine whether maintenance is required or not. Use of active washers eliminates the need for disassembly and related labor intensive activities associated with gaining access to inspection areas and performing manual inspections. Instead, active washers may remotely provide inspection and monitoring data. 
     Embodiments of methods of the present invention provide an active washer with an inspection sensor carried by the washer which is around a bolted joint such that the inspection sensor is capable of inspecting at least a portion of the structure proximate to the washer. An inspection signal may be transmitted from the inspection sensor into the structure, and an inspection signal may be received from the structure, either in a pulse-echo mode of non-destructive inspection by the inspection sensor transmitting the inspection signal into the structure, or in a through-transmission mode of non-destructive inspection by another inspection sensor on the opposing side of the structure from the first washer. 
     These and other characteristics, as well as additional details, of the present invention are further described in the Detailed Description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
       Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  is a cross-sectional schematic diagram of an embodiment of the present invention; 
         FIG. 1A  is an enlarged view of a portion of  FIG. 1 . 
         FIG. 2  is a cross-sectional schematic diagram of another embodiment of the present invention; 
         FIG. 3  is a plan view of yet another embodiment of the present invention; and 
         FIG. 4  is a plan view of yet another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present inventions now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. 
     The term “bolted joint” refers generally to a joint which is held together using a bolt and a nut or similar fastener on opposite sides of a structure, where the bolt passes through a bolt-hole in the structure. As used herein, a bolt also refers to similar fastening mechanisms which rely upon a connector passing through a hole in a structure and having expanded surfaces functioning like a bolt head and a nut on opposing sides of a structure. The bolted structure may be a single layer, or have multiple layers jointed together. However, for simplicity, embodiments of the present invention are generally described below with reference to typical bolted joints using a bolt and a nut, but embodiments of the present invention are not limited to configurations of bolted joints only using bolts and nuts. 
     Although embodiments of the present invention may be used for applications in the aircraft industry, the present invention is applicable to inspecting and monitoring other bolted joints, and may be advantageously used, without limitation, for inspecting and monitoring bolted joints on holding tanks and pipelines. Similarly, although embodiments of the present invention are described with reference to use of ultrasonic transducers, other types of inspection sensors may be used with embodiments of the present invention. 
     Embodiments of the present invention incorporate a non-destructive inspection sensor with or into a washer, typical of washers used as a bearing surface beneath a nut or bolt head, creating what are referred to herein as “active washers.” The non-destructive inspection sensor may be incorporated as part of the washer, for example, using sensor mounting techniques such as used by PFW Technologies for creating a permanent mounted transducer (PMT) system of a bolt as described in U.S. Pat. No. 4,846,001 and which is incorporated herein by reference, except that a non-destructive inspection sensor of an embodiment of the present invention is incorporated into a washer, rather than a bolt or other fastener adapted to be deformed, and is used for defect inspection of a bolted structure, rather than elongation of a bolt. Because the sensor of an active washer is typically permanently mounted or permanently fixed in place in a cavity or similar recess, a stable environment is created between multiple inspections. The stable environment enables previous inspection results to be retrieved and subtracted from current readings to isolate changes in the structure and identify changes due to progression or growth of defects, as explained further below. 
     Because an active washer is situated in close proximity to the bolt hole, an active washer may be able to detect cracks and delaminations at early stated of progression, when they might otherwise be hard to detect or undetectable by other inspection methods. Active washers may be most sensitive for detection of delaminations in composite bolted structures due to the orientation of laminar flaws with regard to the interrogating wave. However, detection of axial fatigue cracks is also possible by measuring diffraction signals from the crack tip and referencing the diffraction signals against signals taken during previous maintenance checks. By subtracting previous signals from later signals, the changes due to crack growth are isolated and the signal-to-noise level increases. 
       FIG. 1  is a cross-sectional schematic diagram of an embodiment of the present invention.  FIG. 1  presents an example configuration for an active washer. The transducer  110  is mounted to the surface of the washer  104  facing away from the structure  120 , typically where the transducer is permanently affixed to the washer, although embodiments of the present invention may use inspection sensors which are only carried by a washer, such as describe below with reference to  FIGS. 2 ,  3 , and  4  which show inspection sensors that may float in cavities defined in the washer. The transducer  110  is mounted radially from the center of the washer  104  just beyond where the head of the bolt  102  terminates. The fact that the washer  104  has additional width beyond the head of the bolt  102 , over which the bearing load of the bolt  102  is dispersed, allows space for the transducer  110  on the top of the washer  104 . Alternatively, if insufficient space is available to accommodate both the head of a bolt and a transducer, the transducer may be segmented and recessed into counterbore chambers occupying space beneath the bearing surface of the head of the bolt. If an active washer with the configuration shown in  FIG. 1  is used in a pulse-echo inspection application, signal echoes from the far side of the washer may interfere with the interpretation of signal echoes originating beyond the washer in the structure under inspection. This would not be a problem, however, if the active washer were used in a through-transmission inspection application because stable, interfering signals would be subtracted out, i.e., each monitoring measurement would be the result of subtracting out the baseline signal so that only changes remain, such as changes produced by propagating damage. 
     The configuration in  FIG. 1  also shows how the bolt  102  includes a shaft with threads  106  at the terminating end of the shaft opposite the head of the bolt  102 . A nut  122  is screwed onto the threads  106  of the bolt  102  to tighten down the bolted joint. A second washer  124 , which may also be an active washer, is located between the nut  122  and the structure  120 . Accordingly, the bolted joint may resemble and function as conventional bolted joint, except that at least one washer used for displacing the bearing load of the fastener is an active washer. As shown in  FIG. 1 , an active washer may include a probe contact  112  for the transducer  110  to which a data connection can be made using a wire  114 . A wired connection to an active washer can be connected to a data control system to inspect and monitor the bolted joint for the active washer, and potentially numerous active washers or similar inspection sensors as part of a Vehicle Health Management system. Embodiments of the present invention may also operate using wireless communications with active washers, although considerations such as space limitations and availability of a power source may restrict the use of wireless communications in some applications. 
       FIG. 1A  is an enlarged view of a portion of  FIG. 1  showing the composition of an example embodiment of a transducer for an active washer. The example transducer  110 A includes a probe contact  112  attached to a metal electrode layer  140  which is attached to a protection layer/corrosion barrier  142  which is attached to a piezoelectric thin-film ceramic crystal  144  which is attached to a washer  104 . The metal electrode layer  140  and washer  104  operate as the two opposing poles for the piezoelectric crystal  144 . 
     In an example operation of an embodiment of the present invention, non-destructive pulse-echo ultrasonic inspection may consist of making contact with two poles across a piezoelectric ultrasonic transducer crystal sensor bonded to the surface of a washer opposite the structure and exciting the crystal with a voltage spike, where one of the poles is the washer material itself, such as where the metal electrode layer  140  and the washer  104  are the poles for the transducer crystal. The voltage spike causes the sensor to launch an ultrasonic compression wave  130  through the washer and into the structure in contact with one face of the washer. As in conventional pulse-echo ultrasonic inspections, the ultrasonic wave launched into the base material may be reflected, and/or altered by flaws in the material and reflected by the far-side of the structure, and a return signal  131  may be detected by the sensor. The presence of a sealant (or couplant) between the washer and the structure may assist (couple) transmission of the ultrasonic wave into the structure, but the presence of a sealant (or couplant) is not necessary for operation of embodiments of the present invention. 
       FIG. 2  is an enlarged view of a cross-sectional schematic diagram of another embodiment of the present invention. By comparison to the embodiment of  FIGS. 1 and 1A , the transducer  210  in the embodiment of  FIG. 2  is located on the face of the washer  204  which is in contact with the structure  120 , essentially with the transducer  210  between the washer  204  and the structure  120 , rather than having the washer  104  between the transducer  110 ,  110 A and the structure  120  as in the embodiment of  FIGS. 1 and 1A . In the configuration of  FIG. 2 , the washer  204  becomes a casing or housing for the transducer  210 , similar to how a typical transducer may have a steel sleeve or case. A damping material  238 , such as a rubber, polymer, or rubber-like substance, may be placed behind the transducer to stop the crystal from ring-down or prolonged vibrating, thereby preventing the ring-down caused by one electrical impulse from interfering with the sound signal returning to the crystal before the next electrical impulse causes the crystal to ring again, and to fill any excess space in the cavity between a recess for a probe contact  212  and the transducer  210 . An electrode lead wire  214  may run from the probe contact  212  through a small hole in the washer  204  to a data collection unit. An alternative embodiment may use wireless communications with an inspection sensor, thereby avoiding the need for a hole for a wire. The configuration of  FIG. 2  permits the transducer  210  to be in direct contact with the structure  120  and avoids any interfering signals from washer  204 . This configuration makes direct measurements of a structure possible without recourse to baseline subtractions. The configuration of  FIG. 2  also permits the transducer to span through an area of the washer which may otherwise be beneath the head and/or bearing surface of a bolt where a transducer as in the embodiment of  FIG. 1  could not be affixed because of potential damage to the transducer and/or physical interference between the bolt and the transducer. In the configuration of  FIG. 2 , the bearing load of the bolt may be transferred to the areas of the washer surfaces surrounding the transducer cavity, such as inner and outer perimeters and areas between segmented transducer crystals as shown in  FIGS. 3 and 4 , while leaving any cavities housing the transducer unaffected. To prevent damage to a transducer recessed in a cavity facing a surface of a structure, the transducer may be covered by a rubber or polymer contact facing  239 . In such a manner, the transducer may “float” within the cavity between the contact facing  239  and the damping material  238 . A similar cavity facing may be used to prevent damage to a transducer in embodiments of active washers where the transducer is recessed in the surface of the washer facing away from the structure and in contact with the head of the bolt. Because of the limited size for a transducer in the configuration of  FIG. 2 , an embodiment may advantageously use a MEMS transducer disposed in one or more cavities of a washer. 
       FIGS. 3 and 4  are plan views of embodiments for active washers in accordance with the present invention. The active washer of both  FIGS. 3 and 4  are similar to the embodiment described with respect to  FIG. 2  where the transducer is located on the surface of the washer facing the structure held together by the bolted joint, rather than on the surface of the washer facing the head of a bolt or a nut. The transducer may be manufactured as a segmented series, such as quadrants of piezoelectric crystals  310  recessed into four cavities in a washer  304 , as shown in  FIG. 3 . Alternatively, a ring-shaped transducer  410  may be recessed into a single ring-shaped cavity in a washer  404 , as shown in  FIG. 4 . Alternatively, an embodiment of the present invention may use multiple concentric ring-shaped cavities to enable a phasing functionality such as to make flaw length progression measurements. Similarly, a transducer affixed to the surface of a washer opposite a structure, as in a configuration like that of  FIGS. 1 and 1A , may be segmented or ring-shaped, similar to the embodiments shown in  FIGS. 3 and 4 , and may be recessed into cavities, similar to the embodiments shown in  FIGS. 2 ,  3 , and  4  although on the opposite surface of the washer from the structure. 
     As with many other non-destructive inspection applications, various types of sensors may be used to perform different non-destructive inspection methods. For example, one embodiment of the present invention may use a single active washer with a pulse-echo transducer for one-sided inspection. An alternate embodiment of the present invention may use a pair of active washers on opposite surfaces of a bolted joint to perform through-transmission inspection. 
     If a bolted joint holds together multiple layers of a structure, a single active washer may inspect both layers of the structure in a pulse-echo inspection if a faying surface sealant or similar material is present between the multiple layers creating an interface layer, even though the interface layer may cause an interference reflection. 
     Data acquired by active washers can be used to detect defects or the onset of structural degradation so as to facilitate Condition-Based Maintenance and Vehicle Health Management techniques. Based on data from active washers, any necessary repairs may be made before defects worsen. 
     Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.