Abstract:
A system for structural assessment is described that comprises a plurality of sensors and a surface covering for at least a portion of a structure. The sensors are arranged in a pattern and attached to the surface covering. The surface covering is attached to a surface of a structure, and the sensors are configured to provide signals relating to the integrity of the structure to an external device.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to assessment and monitoring of structures and more specifically, to methods and systems for using active surface coverings for structural assessment and monitoring. 
         [0002]    Composite materials are increasingly utilized in a wide variety of applications, including for aircraft structures. Multi-functional systems using composite material having complex geometries, however, can be a maintenance burden for servicing personnel. In addition, the maintenance tools, procedures, and practices for metallic aircraft structures generally are not compatible or cost effective with composite aircraft structures. Composite aircraft structures include mixtures of bonded and bolted laminates with a variety of metallic and composite substructures. Other examples include composite sandwiched structures and other adhesive bonded panels including assemblies and structures with contoured surfaces. It is desirable to inspect such structures to identify any defects, such as cracks, discontinuities, voids, or porosity, which could adversely affect the performance of the structure. 
         [0003]    With respect to inspecting aircraft structure, in-service structural health monitoring (SHM) sensors are often part of a permanently installed system that includes other electronic hardware. Such other hardware takes up space, is heavy, and is typically mounted and wired into position on an aircraft. Known SHM sensors typically also require energization from a power source during operation. Space, weight, and power consumption factors are continually examined for reduction in aircraft design and configuration. 
         [0004]    Non-destructive evaluation (NDE) sensors are generally placed by hand onto a structure under inspection by an inspector. Such inspectors typically do not have easy access to all the locations, for example, on an airframe, that might require inspection. Often, at least a portion of such structures has to be removed in order to inspect it, or secondary structures adjacent to the structure to be inspected. In addition to the time consuming and labor intensive aspect of NDE, there is also a danger of damaging an aircraft component or structure in connection with such NDE. NDE may be performed during personnel training sessions, field testing of the composite product, or after the completed structure has been put into service to validate integrity and fitness of the structure. NDE is sometimes referred to as non-destructive inspection (NDI). 
         [0005]    Utilization of composite structures and a continuing shift towards lightweight composite and bonded materials, dictate that devices and processes are made available to ensure structural integrity, production quality, and lifecycle support. However, known non-destructive evaluation and non-destructive inspection methods still utilize sensors that are temporarily placed on to the exterior surface or an interior surface of a structure in order to accomplish the inspection. The time and labor costs associated with placement of these sensors is not insignificant. In addition, temporary placement of sensors and sensor grids may result in incorrect placement for the tests that are to be performed. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In one aspect, a system for structural assessment is provided. The system includes a plurality of sensors and a surface covering for at least a portion of a structure. The sensors are arranged in a pattern and attached to the surface covering which is attached to a surface of a structure. The sensors are configured to provide signals relating to the integrity of the structure to an external device. 
         [0007]    In another aspect, a surface covering for a structure is provided. The surface covering includes a layer of surface covering material, an adhesive backing attached to the layer of surface covering material, a plurality of nondestructive evaluation sensors arranged in a pattern and attached to the layer of surface covering material, and an indicator material configured to indicate one or phenomena occurring with respect to a structure to which the surface covering is attached. 
         [0008]    In still another aspect, a method for structural assessment and monitoring of a structure is provided. The method includes providing at least one layer of material that includes a plurality of sensors attached thereto, utilizing at least one layer of material and plurality of sensors as a coating for the structure, and configuring the plurality of sensors to output signals indicative of some aspect of the integrity of the structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0009]      FIG. 1  is an illustration of an aircraft having several sheets of active appliqué applied thereto. 
           [0010]      FIG. 2  is a diagram illustrating one example of active appliqué, the appliqué containing a grid of piezo-electric sensors. 
           [0011]      FIG. 3  is a diagram of active appliqué containing a grid of piezo-electric sensors, groupings of piezo-electric sensors communicatively coupled to an RFID device. 
           [0012]      FIG. 4  is a diagram of active appliqué that is coated with one or more of an impact sensitive material, a strain sensitive material, and a heat sensitive material. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    Embodiments of an active surface covering, referred to herein as appliqué which, in addition to providing a replacement for paint or other coatings, also provide nondestructive evaluation (NDE) and nondestructive inspection (NDI) testing capabilities are described herein. As a replacement for paint and other coatings, an active surface covering and provides protection capabilities for the structures to which the active surface covering is affixed. 
         [0014]      FIG. 1  is a diagram of an aircraft  10  that includes several sheets of appliqué  12 , sometimes referred to herein as active appliqué, attached thereto. In various embodiments, active appliqué  12  is configured as thin plastic or elastomer sheets having an adhesive backing. While described herein as plastic or elastomer, it is to be understood that the sheets of active appliqué  12  may be fabricated from one or more bonded layers of any organic, metallic, synthetic or polymer based material. Sheets of active appliqué  12  can be cut to fit and affixed to the surface of a structure, for example, aircraft  10 . Generally, sheets of appliqué  12  weigh less than paint, and are easily repaired or replaced with less expense than the expenses associated with paint repair or replacement. 
         [0015]    In one embodiment described herein, appliqué  12  is fabricated to include a grid of piezo-electric sensors. In other embodiments, active appliqué  12  is fabricated to include one or more substrate layers for use in lightning strike protection (LSP) applications by integrating conductive media into these layers or onto the appliqué material. In still other embodiments, active appliqué  12  may be configured to include one or more of RF antennas, RFID tags, materials that change color upon an impact (e.g., bruisable paint), and other sensing devices that may be incorporated into an appliqué while providing a sensing or communication capability. 
         [0016]    Referring again to  FIG. 1 , active appliqué  12  is affixed to portions of aircraft  10  where, for example, monitoring of impacts and other structural monitoring is desired utilizing sensors embedded within appliqué  12 . Such structural monitoring may be a part of the maintenance plan for the structure. The sensors are capable of detecting a flaw on or within the structure of aircraft  10 . In one embodiment, to test the structure of aircraft  10 , an impact mechanism is used to generate stress waves along the surface of the structure and within the structure. In this test, the sensors are capable of detecting the stress waves in order to determine whether a flaw or other anomaly is present within the structure. The structure to be tested could be any number of materials. For example, and with respect to aircraft  10 , structure could be fabricated from a metallic material such as aluminum, or composite materials such as graphite-epoxy. 
         [0017]      FIG. 2  is an illustration of a sheet of active appliqué  12 . In the embodiment illustrated, active appliqué  12  contains a grid of sensors  20 , for example, thin piezo-electric sensors located within a thin sheet of material  12 . The sensors  20  can be utilized to locate and determine a size of impacts on a surface of a structure, for example, using time-based triangulation techniques. In one particular embodiment, an impact location and size of impact is determined by time-of-stress wave travel to the various individual sensors  20 , and by the amplitude of the stress wave each sensor  20  experiences. In such an embodiment, sensors  20  may be electrically connected using a conductive ribbon  14  (not shown) within appliqué  12 , or as further described below, integrated with one or more RFID tags to provide data storage and wireless communication relating to impact location of impact, size of impact, and any internal damages to the structure. In other embodiments, current paths to and from each sensor  20  are formed with one or more of metallic deposition, etching, or bonding. Also, wires  24  may be attached to the material  12  to provide the current paths. 
         [0018]    In any of the above described configurations, active appliqué  12  is utilized to locate and size such damages that may occur with use of composite or metal structures, including, but not limited to, delaminations, disbonds, degradation, corrosion, or cracking on the surface or subsurface of structures such as aircraft  10 . Information from sensors  20  is input into, for example, a portable data acquisition unit (not shown) that is capable of recording electrical inputs produced by stress waves passing through each sensor  20 . Material  22  is typically a non-conductive sheet that is flexible and pliable, for example, a thin polymeric or synthetic material having multiple layers. While illustrated as having sensors  20  in a grid pattern it is to be understood that sensors  20  may be arranged in any number of configurations within the sheet of material  22 , as applicable for the structure being tested or monitored. The number, or arrangement, of sensors  20  within a sheet of active appliqué  12  may be varied depending on the size of the flaw to be detected in or on the structure. The number, or arrangement, of sensors within a sheet of active appliqué may also be varied to achieve a particular resolution for the structural test. 
         [0019]    While described herein in the context of piezo-electric sensors  20 , various embodiments of active appliqué includes one or more other types of thin nondestructive evaluation (NDE) or nondestructive inspection (NDI) sensors including one or more of piezo-electric sensors, strain sensors, eddy current sensors, capacitive sensors, resonance sensors, pulse echo sensors, mechanical impedance sensors, ultrasonic sensors, vibration sensors, temperature sensors, moisture sensors and other similarly configured sensors. As illustrated in  FIG. 1 , active appliqué  12  can be applied to regions having potentially high numbers of impacts from foreign objects, remote or difficult to inspect regions, or regions where degradation (corrosion, cracking, etc.) is expected or has been noted. Also, active appliqué  12  can be placed over structural repairs to allow for future inspection and/or continuing monitoring of the structural repairs. Further active appliqué  12  may also be placed onto structures undergoing mechanical testing, wind tunnel testing, flight testing or other monitoring to provide invaluable test data to designers of such structures. 
         [0020]    As described above, in an alternative embodiment, a wireless form of active appliqué incorporating RFID tags is also provided.  FIG. 3  is an illustration of one embodiment of active appliqué  40  configured for wireless communication. Specifically, active appliqué  40  includes one or more NDE/NDI sensors  42  (i.e., piezo-electric sensors, strain sensors, eddy current sensors, capacitive sensors, vibration sensors, temperature sensors, moisture sensors and others), subsets of which are connected to radio-frequency identification (RFID) tags  44 . The combination of sensors  42  and RFID tags  44  provide both a methodology for in-service NDE/NDI of critical aircraft structures that is based upon the integration of RFID tags  44  with NDE sensors  42 . Further provided is a structure that is configured for utilization as a protective coating for at least a portion of an airframe or other structure. RFID tags  44  store stress wave information received from sensors  42  and are wirelessly interrogated using an RF transceiver (not shown in  FIG. 3 ). RFID tags  44  include a read and write capability and are capable of collecting and holding data until requested by a receiver. 
         [0021]    Additionally, RFID tags  44  provide an additional sensing capability. For example, should one of RFID tags  44  undergo a compression or other type of physical strain, the signal properties of its transmitted signal will change, for example, the signal transmission from an altered RFID tag  44  might occur at a different frequency. When a receiver of signals from RFID tags  44  determines that a frequency of transmission from an individual RFID tag  44  has changes, a user will be alerted to further inspect the area at and around the individual RFID tag  44  for anomalies that might compromise the structure to which the RFID tag  44  is attached. 
         [0022]    RFID tags  44  are integrated into appliqué  40  for wireless sensing, monitoring, and data retrieval of events associated with sensors  42 . Although RFID tags  44  may be disabled when mounted directly to a conductive surface, alternative RFID tag configurations include RFID tags that utilize high magnetic permeability/low electrical conductivity material backing or a backing film made from nanocomposite magnetic material. The various embodiments of RFID tags  44  may be considered depending upon the individual application for appliqué  40 . For example, for wireless metallic structural testing/monitoring, active appliqué may be configured with the nanocomposite magnetic material backing at RFID tag locations. Additionally, active appliqué may be configured with embedded conductive paths that act as wiring for sensors  42 , if a wireless RFID approach is not chosen. 
         [0023]    Besides being configured with discrete sensors, active appliqué can be coated with impact, strain, or heat sensitive material, which can give indications of various phenomena occurring at specific structural locations.  FIG. 4  is an illustration of a sheet of active appliqué  60  that has been coated with one or more of the impact, strain, and heat sensitive material. In an alternative embodiment, rather than coating active appliqué  60  with such a material, the material is included as one of the layers of active appliqué  60 . Active appliqué  60  includes sensors  20  as illustrated, and is further depicted as having an area  62  that has been impacted, strained or overheated. The type of visible information provided by active appliqué  60  is extremely valuable for assessing or monitoring a prototype, or flight test article, as well as for monitoring the fitness of the various structures associated with production aircraft. Also, other embodiments of active appliqué are utilized to control electrical properties at the surface of the structure, for example, not only as lightning strike protection, but also as a low observable coating or as an electrical shielding of vital component hardware or other systems underneath a sheet of such active appliqué. 
         [0024]    The embodiments of active appliqué described herein may be utilized in applications and industries other than those associated with airframe structural testing and monitoring, including, but not limited to, the roadway, bridge, and building infrastructure industries. For example, one or more of the embodiments of active appliqué described herein, with slight modifications, is capable of being wrapped around columns or other load-bearing structure to provide engineers with quick structural assessment capabilities during routine inspections or after a catastrophic event such as an earthquake. In these embodiments, the active appliqué may be coated with or fabricated using a strain sensitive material as described above that changes color or that includes fiducials that move apart under strain. These color changes and fiducials are visually observed or measured with a laser and impact testing is accomplished using the above described sensors within the appliqué. Instead of having to apply individual markers for optical or laser-based dimensional reference measurements on structures, active appliqué with multiple markers (e.g., color changes and fiducials) are applied to, and remain on structures, while being monitored and providing data over time. 
         [0025]    Application of sensors, RFID tags, RF antennas, mechanical markers, and special coatings to aircraft and other structures utilizing appliqué is a practical and workable alternative to paint or other traditional coatings. In these aircraft applications, active appliqué that is more or less permanently affixed to an aircraft structure allows for weight savings and wiring reductions from the existing structural health monitoring and integrated vehicle health monitoring approaches while still providing the capabilities of these structural monitoring approaches. The above described embodiments of active appliqué also reduce corrosion in metallic structures and reduces the cost of inspecting, testing, and monitoring. The active appliqué also provides remote NDE in critical areas, and supports vehicle testing and analysis for new, repaired, or prototype systems. With continuing advances being made with RFID technology, improvements in wireless maintenance approaches will be achieved through utilization of active appliqué. 
         [0026]    The ability to quickly identify, predict, monitor and manage various structural anomalies instills customer confidence in products that incorporate active appliqué. In addition, increased safety and user confidence in the critical structures results, as well as, reduced monitoring and inspection costs of these critical structures. 
         [0027]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.