Abstract:
A damage locating system also provides thermal protection. An array of sensors substantially tiles an area of interest. Each sensor is a reflective-surface conductor having operatively coupled inductance and capacitance. A magnetic field response recorder is provided to interrogate each sensor before and after a damage condition. Changes in response are indicative of damage and a corresponding location thereof.

Description:
Pursuant to 35 U.S.C. §119, the benefit of priority from provisional application 60/774,802, with a filing date of Feb. 6, 2006, is claimed for this non-provisional application. 

   ORIGIN OF THE INVENTION 
   This invention was made in part by employees of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to the detection and localization of damage to a material. More specifically, the invention is a system that can be used to detect and localize damage to a material while simultaneously providing thermal protection characteristics. 
   2. Description of the Related Art 
   A variety of garments and structures are made from specialty fabrics designed to withstand harsh environmental conditions. For example, the space industry fabricates space suits, space and surface habitats, airlocks, and a variety of inflatable structures from multi-layered fabrics designed to withstand a certain amount of impacts and provide thermal protection. Similarly, earth-based protective clothing (e.g., “HazMat” suits) and some earth-based temporary structures (e.g., military structures) are fabricated with damage-resistant and thermally protective multi-layer fabrics. However, all multi-layer fabrics can be damaged (e.g., penetrated, punctured, cut, torn, etc.) if the right amount of force is applied, if the fabric is breached by a sharp or pointed implement, or if the fabric receives an impact from a high velocity object such as a micrometeorite. While large damages are readily noticed, small damages can easily be unrecognizable. However, when occupant(s) of a slightly damaged structure or clothing are in a harsh environment (e.g., space, toxic gas, etc.), even small scale (i.e., unrecognized) damages to a protective structure or clothing can cause serious injury or death. Furthermore, even if only outer layers experience damage while internal layers are not damaged, it is still advantageous to identify damage to outer layers. Current damage detection methods known in the art require electrical sensors/circuits, the integrity of which must be maintained for damage detection. However, the failure probability of these circuits increases proportionally to the number of sensors used. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a system that can be used to detect and locate damage in or on a material. 
   Another object of the present invention is to provide a damage detection and localization system that can be incorporated into a fabric. 
   Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
   In accordance with the present invention, a damage locating system providing thermal protection includes an array of sensors substantially tiling an area of interest. Each sensor is defined by capacitance operatively coupled to inductance such that energy can be exchanged between the magnetic field stored via inductance and the electric field stored via capacitance. Each sensor is a reflective-surface conductor. A magnetic field response recorder is used to interrogate each sensor prior to damage thereof to establish a baseline frequency response therefore. The recorder is also used to interrogate each sensor after damage thereto. Changes from the baseline frequency response for any of the sensors are indicative of damage thereto and a corresponding location thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of a system for detecting and locating damage while also providing thermal protection in accordance with the present invention; 
       FIG. 2  is an electrical circuit equivalent of one of the damage detection/localization chains illustrated in  FIG. 1 ; 
       FIG. 3  is a schematic view of an embodiment of a magnetic field response recorder used in the present invention; and 
       FIG. 4  is a schematic view of another embodiment of a damage sensing arrangement in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings and more particularly to  FIG. 1 , a system for detecting and locating damage in accordance with an embodiment of the present invention is shown and is referenced generally by numeral  10 . In general, system  10  includes a damage sensing arrangement  20  and a magnetic field response recorder  30 . Damage sensing arrangement  20  will typically be incorporated into a multi-layer fabric (i.e., as one of the fabric&#39;s layers) that will be used in the fabrication of some type of protective clothing or protective structure. It is to be understood that the particular type of fabric, clothing, and/or structure incorporating damage sensing arrangement  20  are not limitations of the present invention. Furthermore, as will be explained further below, when damage sensing arrangement  20  is constructed in accordance with the teachings provided herein, damage sensing arrangement  20  also provides thermal protection. 
   Damage sensing arrangement  20  can include a substrate material  22  that is typically electrically non-conductive and flexible. Substrate material  22  could also be a non-conductive material or structure that is to be monitored for damage. The particular choice of substrate material  22  will vary depending on the ultimate application incorporating damage sensing arrangement  20 . Accordingly, the choice of substrate material  22  is not a limitation of the present invention. 
   Deposited on substrate material  22  are one or more series or chains of electrically-conductive runs or traces, each of which defines an electrical circuit. Techniques used to deposit the metal conductor can be any conventional metal deposition process to include thin-film fabrication techniques. In the illustrated embodiment, two such chains or electrical circuits are shown. However, as will become evident from the description, a single chain/electrical circuit or more than two thereof could be used without departing from the scope of the present invention. Regardless of how many chains are used, they are laid out to substantially cover or tile an area of interest, i.e., an area that is to be monitored for damage. 
   Referring now specifically to the illustrated embodiment, first and second separate electrical circuits  24  and  26  are shown. For simplicity of illustration, circuits  24  and  26  are identically constructed so that only circuit  24  will be described in further detail. However, the present invention is not so limited as each circuit could be constructed differently to provide different response characteristics depending on the requirements of a particular application. 
   The electrically-conductive material forming each circuit defines an alternating arrangement of capacitors and inductors that are connected in parallel. In the illustrated embodiment, circuit  24  is defined by a capacitor portion  24 A, an inductor portion  24 B, a capacitor portion  24 C, and an inductor portion  24 D. Single solid or single dashed lines are used to indicate electrical connections between adjoining capacitor and inductor portions. Actual connections between these electrical connections and an inductor or capacitor portion are indicated by solid “dots”. The resulting electrical circuit equivalent is shown in  FIG. 2 . 
   To provide thermal protection, circuits  24  and  26  should be made from a reflective material or coated with a reflective material. For example, as is known in the art, excellent reflectivity can be achieved by depositing the circuit traces in a way that they are convex in cross-section, heating the traces, and then depositing a very thin layer of aluminum or silver on the circuit traces. The circuit traces on their non-conductive substrate material would essentially take the place of a conventional insulation system (e.g., a continuous metal layer deposited on a layer of insulative material). Then, in use, the reflective material would face the environment from which thermal protection is sought. Since the area of interest is substantially tiled by circuits  24  and  26 , a reflective form of circuits  24  and  26  can provide substantial thermal protection. 
   Each capacitor portion  24 A and  24 C is created by interdigitated electrodes, and each inductor portion  24 B and  24 D is created by a spiral run or trace. Each adjoining capacitor portion and inductor portion (connected in parallel) of a circuit defines a magnetic field response sensor. In the presence of an oscillating magnetic field, each such sensor electrically oscillates at a resonant frequency that is dependent upon the capacitance and inductance of the particular capacitor portion and inductor portion, respectively. This oscillation occurs as the energy is harmonically transferred between the inductor (as magnetic energy) and capacitor (as electrical energy). In the present invention, each adjoining capacitor-inductor is designed/constructed to resonate over a unique frequency band. 
   The application of the magnetic field to the inductor portions of damage sensing arrangement  20  as well as the reading of the induced resonant frequencies is accomplished by magnetic field response recorder  30 . The operating principles and construction details of recorder  30  are provided in U.S. Pat. Nos. 7,086,593 and 7,159,774, the contents of which are hereby incorporated by reference. Briefly, as shown in  FIG. 3 , magnetic field response recorder  30  includes a processor  32  and a broadband radio frequency (RE) antenna  34  capable of transmitting and receiving RF energy. Processor  32  includes algorithms embodied in software for controlling antenna  34  and for analyzing the RF signals received from the magnetic field response sensors defined by damage sensing arrangement  20 . On the transmission side, processor  32  modulates an input signal that is then supplied to antenna  34  so that antenna  34  produces either a broadband time-varying magnetic field or a single harmonic field. On the reception side, antenna  34  receives harmonic magnetic responses produced by damage sensing arrangement  20 . Antenna  34  can be realized by two separate antennas or a single antenna that is switched between transmission and reception. 
   In terms of damage detection and localization, the circuits of damage sensing arrangement  20  must be interrogated by magnetic field response recorder  30 . Such interrogation must first be done to establish a baseline or calibration response when the circuits are undamaged. This will typically be done when damage sensing arrangement  20  is incorporated into a fabric (not shown) that, in turn, has been used to fabricate clothing or a structure. With the baseline response established, damage sensing arrangement  20  can then be interrogated by recorder  30  on a continual, periodic, or on-demand basis to see if any damage has occurred. 
   Since each capacitor-inductor pair has a unique frequency response, the entire sensing arrangement  20  can be interrogated at the same time using a broadband time-varying magnetic field. If a damage (e.g., cut, break, etc.) occurs in some part of circuits  24  or  26 , the electrical circuit will be broken and a “pre-damage” single capacitor-inductor pair essentially becomes two “post-damage” capacitor-inductor pairs that will resonate differently than before the damage. For example, if a break occurred in capacitor portion  24 C, one capacitor-inductor pair is formed by inductor portion  24 B and the portion of capacitor portion  24 C that is still electrically coupled thereto. The other capacitor-inductor pair is formed by inductor portion  24 D and the portion of capacitor portion  24 C still electrically coupled thereto. Since the location of the pre-damaged capacitor-inductor pair and its pre-damaged baseline frequency response are already known, the change in the frequency response resulting from the damage is indicative of the damage location. In addition, once a damage has been detected, the present invention can be used to establish new baseline resonant frequencies which could be used to detect subsequent damage. Location of damage can be determined by identifying the location of the highest frequency shift. Another method of identifying the damage location is to determine where the largest difference exists between a baseline frequency response and the present response. 
   The advantages of the present invention are numerous. Damages to fabric-based protective clothing or structures can be readily detected even when the damages are extremely small. The wireless nature of the system eliminates the need for cumbersome electrical connections or electrical arcing problems associated therewith. The system can simultaneously provide a level of thermal protection. If multiple layers of the damage sensing arrangement are used and the circuit traces from adjacent layers are positioned to be out of registration with one another, the thermal protection properties provided by the present invention are further enhanced over a single-layer damage sensing arrangement. 
   Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. For example, the damage sensing arrangement of the present invention could also be realized by tiling a non-conductive substrate material with open-circuit electrically-conductive spiral-trace sensors, the details of which are disclosed in the co-pending application entitled “WIRELESS SENSING SYSTEM USING OPEN-CIRCUIT, ELECTRICALLY-CONDUCTIVE SPIRAL-TRACE SENSOR” (Ser. No. 11/671,089), the contents of which are hereby incorporated by reference. One such damage sensing arrangement is shown in  FIG. 4  where substrate material  22  is tiled with an array of open-circuit spiral trace sensors  44 . Each spiral trace sensor  44  is constructed to have inductance and capacitance characteristics necessary to generate a detectable response harmonic when interrogated by the magnetic field response recorder described above. In this way, each inductor  44  resembles an inductor and capacitor electrically coupled to one another in parallel. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.