Abstract:
A composite structure contains crystalline and/or polycrystalline triboluminescent elements distributed therein externally and/or internally, totally and/or regionally. The structure is instrumented with at least one optical fiber which is coupled therewith penetratingly and/or superficially/tangentially. Each optical fiber is exteriorly light transparent/translucent along at least a longitudinal portion thereof which is situate in the vicinity of at least one triboluminescent element. Concomitant with the occurrence of damage in and/or on the structure is the occurrence of mechanical action with respect to at least one triboluminescent element, a consequence of which is the occurrence of triboluminescence which, to at least some degree, passes radially into at least one optical fiber so as to reach the optical fiber&#39;s transmissive axial core and thereby be transmitted to remotely located photosensitive equipment. The triboluminescent elements can exist wholely and/or partly in various capacities, e.g., as fiber reinforcement and/or particle reinforcement and/or particle filler.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein 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 therefor. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to methods and apparatuses for sensing or detecting structural damage in objects, more particularly to methods and apparatuses for sensing or detecting fractures, delamination and other forms of mechanical damage in structures such as composites. 
     Presently there is no completely satisfactory method or mature sensing technology to detect composite damage in-situ and in real-time, or to remotely interrogate such occurrences. Sage et al. U.S. Pat. No. 5,905,260 issued May 18, 1999, incorporated herein by reference, disclose a sensing technology to make such measurements; however, the technology disclosed by Sage et al. has its limitations. The sensor proposed by Sage et al. is a discrete sensor (described by Sage et al. as including “a small piece of a triboluminescent material”) that will locate damage only when damage occurs within the sensor&#39;s triboluminescent material itself. This approach by Sage et al. is problematic for various reasons. Firstly, the area (areas) of interest is (are) very specifically targeted according to the Sage et al. methodology, which assumes that the user of the composite structure knows exactly where damage will occur. Secondly, the Sage et al. methodology assumes that any damage in the composite is transferred to cracking in the sensor&#39;s triboluminescent material. Thirdly, measurements made at or near the outside surface of the composite may be impossible to make in accordance with the Sage et al. methodology. 
     In addition to Sage et al., other United States patents disclose triboluminescence or triboluminescent material in some context or capacity, including the following which are incorporated herein by reference: Qiu et al. U.S. Pat. No. 6,281,617 B1 issued Aug. 18, 2001; Storey U.S. Pat. No. 6,270,117 B1 issued Aug. 7, 2001, Akiyama et al. U.S. Pat. No. 6,159,394 issued Dec. 12, 2000; Watanabe et al. U.S. Pat. No. 6,117,574 issued Sep. 12, 2000; Hall-Goulle U.S. Pat. No. 6,071,632 issued Jun. 6, 2000; Hansma et al. U.S. Pat. No. 5,581,082 issued Dec. 3, 1996; Pappalardo et al. U.S. Pat. No. 4,772,417 issued Sep. 20, 1988; Dante U.S. Pat. No. 4,372,211 issued Feb. 8, 1983; Glass, deceased et al. U.S. Pat. No. 4,020,765 issued May 3, 1977. 
     “Triboluminescent” (sometimes called “mechanoluminescent”) material is a substance (usually, a crystalline substance) that, when fractured or otherwise subjected to some form of mechanical action, releases optical radiation in the visible spectrum (about 400-700 nm). Although the consistency of some triboluminescent materials may be roughly compared to that of sand, this is not a valid comparison for many triboluminescent materials. There are numerous different triboluminescent substances which share certain attributes and thus exhibit triboluminescent properties. A triboluminescent substance can be comprised of any of various organic and inorganic materials. As used herein, the terms triboluminescence (or triboluminescent) and mechanoluminescence (or mechanoluminescent) are synonymous. Both terms refer to luminescence (light emission) resulting from mechanical action, such as friction (rubbing), pressure, fracturing, scratching, striking, sawing, crushing, pulverizing, smashing or tearing. 
     Derivationally, triboluminescence comes from “tribo” or the Greek term “tribein,” meaning “friction” or “rubbing.” Notwithstanding that the prefix “tribo” seems to connote a more specific meaning of “friction” or “rubbing,” the terms triboluminescence and triboluminescent are intended herein to denote luminescence resulting from any and all forms of mechanical action. The prefix “mechano” in “mechanoluminescence” and “mechanoluminescent” seems to have a broader connotation more in keeping with the broadest concept of luminescence resulting from any and all forms of mechanical action. Nevertheless, the terms triboluminescence and mechanoluminescence are used fairly interchangeably in conventional usage. Thus, the terms triboluminescence and mechanoluminescence are intended herein to have the same broadest meaning, viz., luminescence resulting from any and all forms of mechanical action. Consistent with conventional usage, the terms fractoluminescence and fractoluminescent are intended herein to more specifically denote luminescence resulting from fracturing, since the prefix “fracto” suggests the more specific meaning of “fracture.” 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide method and apparatus for remotely detecting mechanical damage (e.g., fracture or delamination) occurring in a structure of any kind, especially to provide such method and apparatus in relation to a structure which is a composite structure such as a matrix composite or a laminar composite. 
     It is another object of the present invention to provide method and apparatus for remotely monitoring the mechanical damage condition of a structure which is known to be susceptible to certain types of damage such as involving fracture or delamination. 
     In accordance with typical practice of the present invention, a combination is provided which is suitable for detecting damage in an object. The combination comprises fiber optic means and triboluminescent means. The fiber optic means and the triboluminescent means are each adaptable to association with the object so that a mechanical event attendant the damage is capable of causing the triboluminescent means to emit light at least some of which is transmissible by the fiber optic means. 
     According to many embodiments of the present invention, “damage-autosensitive” apparatus (e.g., apparatus which, in functional effect, is capable of automatically sensing damage to itself) is provided. The damage-autosensitive apparatus comprises a structure, at least one fiber optic line, and at least one triboluminescent element. Each fiber optic line is connectable to a photodetector and is situated so that a portion of the fiber optic line is in communication with the structure. Each triboluminescent element is integrated with the structure and is sufficiently proximate a fiber optic line so that, upon an occurrence of damage to the structure: (i) an accompanying mechanical action upon the triboluminescent element results in a luminescent emission of light by the triboluminescent element; and, (ii) at least a portion of the luminescently emitted light is transmissible to the photodetector via the fiber optic line. 
     According to frequent inventive practice, a method for sensing mechanical damage comprises triboluminescently radiating light in response to the damage, and fiber optically conveying at least some of the triboluminescently radiated light so as be informative about the mechanical damage. 
     Many inventive embodiments provide a method of sensing the damage condition of an object. The method comprises (a) integrating triboluminescent material with the object, and (b) associating at least one fiber optic line with the object and with a photosensitive device. The at least one fiber optic line is associated with the object and with a photosensitive device so that, following a damage-causing event accompanied by a mechanical action upon at least some of the integrated triboluminescent material, a quantity of a resultant triboluminescent light emanation is transmitted by at least one fiber optic line to the photosensitive device. 
     Featured by typical embodiments of the present invention is a sequence of events including a damage-related mechanical action with respect to triboluminescent material, followed by a triboluminescent emission of light, followed by a fiber optic admission of at least some of the emitted light, followed by a fiber optic transmission of at least some of the admitted light, followed by an electronic indication (e.g., including an identification, a registration, a recordation, a representation, a readout, a signal, a digitization, a processing and/or a display) of at least some of the transmitted light. 
     According to many inventive embodiments, the present invention&#39;s fiber optic sensor senses fracture, delamination or similar mechanical damage in composites. The composite structure is doped with triboluminescent crystals, which release light upon fracture. The optical fiber then detects the light along its length, and transmits the light to a photodetector—thereby indicating mechanical damage to the composite structure. The present invention thus affords a methodology for monitoring the physical condition of composite and other structures. 
     According to many embodiments of the present invention, a distributed fiber optic composite damage sensor utilizes triboluminescent material and optical fiber to detect mechanical damage (such as cracking or delamination) in a composite, and to transmit this information to a remote location. An important premise of typical embodiments of the present invention is that an optical fiber (sometimes referred to as an optical thread, line, fiber, filament or strand) can be embedded in a composite structure, and—anywhere along the optical fiber&#39;s length—detect damage occurring within the composite structure, wherein the composite structure has been impregnated (e.g., in its matrix phase) with triboluminescent crystals. The present invention can be applied to any composite structure, including but not limited to edifices and other main structures, support structures, piping, propellers, hull sections and machinery components. The present invention can also be applied to non-composite structures meeting these and other descriptions. 
     The present invention admits of application to any structure which permits the association therewith (e.g., embedment therein) of a fiber optic line. The present invention is especially suitable for application to composite structures. The term “fiber optic line,” as used herein, refers to any discrete fiber optic member, such as an optical fiber, optical thread, optical filament, optical strand or a pluralized form thereof such as represented by a fiber optic cable. In accordance with the present invention, the association of a fiber optic line can be accomplished in any of several ways, such as by means of insertion of the fiber optic line after the structure has been made (for example, by machining the structure and adhesively bonding the fiber optic line in place), or by means of molding the fiber optic line into the structure during construction of the structure. 
     Fiber optic lines in general have certain qualities which are especially suited for inventive practice. They can be made to be relatively small in diameter and weight, yet relatively strong. They are immune to electromagnetic interference (EMI), and can provide distributed or multiplexed measurements. Moreover, fiber optic lines lend themselves to being embedded into rubbers, plastics, composites and even metals. Many inventive embodiments involve composite structures. In inventive application to fiber-reinforced matrix composites, one or more fiber optic lines can each serve as a reinforcement fiber in the fiber-matrix system of the structure. 
     Incorporated herein by reference are the following United States patents which disclose fiber optic technology: Cohen U.S. Pat. No. 6,080,982 issued Jun 27, 2000; E1-Sherif U.S. Pat. No. 5,060,307 issued Oct. 22, 1991; Rode et al U.S. Pat. No. 4,348,665 issued Sep. 7, 1982; Rouam U.S. Pat. No. 4,143,319 issued Mar. 6, 1979; Schutz et al. U.S. Pat. No. 4,509,364 issued Apr. 9, 1985; Jensen U.S. Pat. No. 4,328,462 issued May 4, 1982; Purvis et al. U.S. Pat. No. 4,655,077 issued Apr. 7, 1987; Riegler et al. U.S. Pat. No. 4,092,053 issued May 30, 1978; Slough U.S. Pat. No. 4,107,603 issued Aug. 15, 1978; Considine U.S. Pat. No. 3,981,621 issued Sep. 21, 1976; Fukuyoshi et al. U.S. Pat. No. 5,258,930 issued Nov. 2, 1993; Satake et al. U.S. Pat. No. 4,884,434 issued Dec. 5, 1989; Uejio U.S. Pat. No. 5,015,859 issued May 14, 1991. 
     Various fiber optic sensor technologies involving structural analysis or wear determination have been known or considered. Fiber optic sensor types which have been embedded for structural analysis include: fiber Bragg grating; long period grating; micro-bend interferometer; Fabry-Periot interferometer. Fiber optic measurement of strain, temperature, pressure, torque, vibration and acoustic fields have all been demonstrated. Structural health monitoring systems employing sacrificial, embedded optical filaments have been proposed for some time, primarily for composite structures. For instance, a fiber optic filament can be embedded into a structure in such a way that, if during its life cycle the loading exceeds the strength at the location of attachment, the optical filament breaks and the excessive load condition can be detected. These and more complex embedded sensor systems are now in use on air and space vehicles and civilian structures. See Udd, Eric, Fiber  Optic Smart Structures , John Wiley and Sons, Inc., New York, 1995, incorporated herein by reference. 
     A composite is a combination of two or more materials which differ at the macroscopic level, each different material being a constituent of the composite. The following two references, incorporated herein by reference, are instructive regarding composites: John W. Weeton, Dean M. Peters and Karyn L. Thomas,  Engineers&#39; Guide to Composite Materials , American Society for Metals, Metals Park, Ohio, 1987 (See, esp., Section 1, entitled “Introduction to Composite Materials”); George Lubin,  Handbook of Composites , Van Nostrand Reinhold Company, New York, 1982 (See, esp., Chapter 1, entitled “An Overview of Composites”). 
     A matrix composite comprises (i) a filler or reinforcing agent (e.g., fibers, flakes or particles) and (ii) a matrix binder (e.g., a resin). The matrix is the principal phase or aggregate in which the filler or reinforcing agent is embedded or surrounded. Generally, the matrix serves two functions, viz., (i) it holds the reinforcement phase in place, and (ii) under an applied force, it deforms and distributes the stress to the reinforcement constituents. Types of matrix composite materials include metal matrix composite (MMC) materials, ceramic matrix composite (CMC) materials, and polymeric matrix composite (PMC) materials. 
     Examples of metals (metal elements, or alloys of two or more metal elements) conventionally used as matrices in metal matrix composites are aluminum, titanium, bronze and magnesium. A broad range of fillers or reinforcing agents (e.g, fibers) can be used with lower-melting point matrices in metal matrix composites. For instance, most metals, ceramics and compounds can be used as fillers or reinforcing agents in an aluminum or magnesium matrix. The choice of fillers or reinforcing agents for metal matrix composites becomes increasingly limited as the melting point of the metal matrix material increases. 
     Ceramic compounds (such as those used as matrices in ceramic matrix composites) are formed by the combination of one or more metallic elements with one or more nonmetallic elements. Examples of ceramic materials include aluminum oxide, magnesium oxide and silica. 
     There are two main types of polymers, viz., thermoplastics and thermosets. Examples of thermoplastics which can be used as matrix resins include polycarbonate, polyethylene, polystyrene, polypropylene, polyamide, fluoropolymer, thermoplastic polyester, nylon, vinyl, acetal, polycarbonate, polyphenylene oxide, polyetheretherketone (PEEK), polyphenylene sulfide (PPS), polyetherketone ketone (PEKK) and polyetherketone (PEK). Examples of thermosets which can be used as matrix resins include epoxy, polyester, vinyl ester, phenolic, polyimide and bismaleide. 
     Conventional types of fillers and reinforcing agents (e.g., reinforcing fibers or particles) used in the fabrication of matrix composites include glass, cotton, aramid, carbon, graphite, polyethylene, boron, steel, polyamide, alumina, silicon carbide and aluminaboria-silica. 
     A distinction may be drawn between the kinds of constituents in matrix composites which are “reinforcements” and those which are “fillers.” The primary function of a “reinforcement” is to be a structural constituent—e.g., to afford strength and/or stiffness. For instance, in a fiber-reinforced plastic composite (FRP), the plastic functions as the matrix, while the fibers function mainly as reinforcements. In contrast, a “filler” is a constituent which is added to a matrix for reasons other than structural (e.g., strength and/or stiffness), such as for increasing electrical conductivity, increasing thermal conductivity, improving fire resistance/retardance, cost reduction, control of shrinkage, increasing volume (e.g., acting as a bulking agent so that less resin is required) or other nonstructural purposes. Typically, fillers are particles that extend rather than reinforce the material. Such systems are generally referred to as “filled” systems. Although the main purpose of fillers is not reinforcement, in some cases the filler may also, to some degree, serve to reinforce the matrix material. 
     Reinforcements and fillers may come in various forms. Fiber reinforcements are basically fibers, wherein their lengths are significantly greater than their diameters. Fiber reinforced composites have been classified as either “continuous” fiber composites or “discontinuous” fiber composites. Non-fiber reinforcements (and fillers) are characterized by dimensions that are roughly equal along all axes. Non-fiber reinforcements can be spheres, rods, plates, flakes and many other shapes. According to terminology adopted by many, the two categories of matrix reinforcement/filler shapes are (i) fiber and (ii) particle, wherein the term “particle” is synonymous with the term “non-fiber” and refers to any shape other than a fiber shape (which is characterized by length being considerably greater than diameter). The terms “particle” and “non-fiber” are used herein interchangeably to refer to any shape other than a fiber shape. 
     Generally, fibers are used as reinforcements; however, not all reinforcements are fibers, since particles are also commonly used as reinforcements. Although it is unusual for fibers to serve primarily as fillers, it is much more common for particles to serve primarily as fillers. Generally, matrix-containing constituents which are primarily intended for filler purposes are particles. On the other hand, matrix-containing constituents which are primarily intended for reinforcement purposes may be fibers or particles. 
     Diverse combinations of matrices along with fillers and/or reinforcements are possible within the basic matrix composite structure. For example, a matrix composite can include, within the same matrix, two or more kinds of reinforcing fibers. Additionally or alternatively, a matrix composite can include, within the same matrix, two or more kinds of reinforcements of various forms among fiber, particle, flake, etc. Additionally or alternatively, a matrix composite can include, within the same matrix, two or more kinds of fillers. Additionally or alternatively, a matrix composite can include, within the same matrix, both reinforcements and fillers. The term “hybrid” has been used to refer to these types of mixed filer and/or reinforcement schemes. 
     Matrix composites have one or more discontinuous phases surrounded by a three-dimensional continuous phase. There are also various types of non-matrix composites, which lack these attributes. For instance, felts and fabrics have no body matrix, consisting of different kinds of fibers held together by interweaving or entanglement, and perhaps together with a small amount of binder. Laminar composites are not matrix composites, but rather are characterized by flat, layered profiles. Examples of lamanar composites are a sandwich structure, plywood and a metal coated with another metal (e.g., to prevent corrosion). “Sandwich” structures typically have a bulky, lightweight core which is situated between two thin, strong facings. 
     In accordance with the present invention, the triboluminescent crystals can be integrated with the composite structure in any of various ways or combinations of ways. For instance, according to many embodiments of the present invention, the present invention detects damage occurring within a matrix composite structure which has been impregnated with triboluminescent crystals in the matrix composite&#39;s matrix phase. According to some inventive embodiments, the present invention detects damage occurring within a matrix composite which has been impregnated with triboluminescent crystals in its reinforcement phase and/or its filler phase. According to some inventive embodiments, the present invention detects damage occurring within a composite laminate structure which has been impregnated, within its lamina, with triboluminescent crystals. 
     In inventive practice, triboluminescent elements (e.g., triboluminescent crystals or polycrystals or aggregations thereof) can be integrated with the structure of interest in any of various ways in composites of all kinds, including fiber matrix composites, in particle matrix sit composites, reinforcement-plus-filler hybrid composites, fiber-plus-particle hybrid composites, composite laminates, or combinations thereof. For instance, triboluminescent elements can be, or be included in, some or all of the particle (and/or fiber) reinforcement constituents of a reinforcement-matrix system. Or, triboluminescent elements can be, or be included in, some or all of the particle filler constituents of a filler-matrix system. Or, triboluminescent elements can be, or be included in, some or all of the particle (and/or fiber) reinforcement constituents and/or some or all of the particle filler constituents of a reinforcement-plus-filler-matrix system. Or, triboluminescent elements can be mixed or blended or otherwise combined with the matrix material itself so as to constitute a fully integrated component of such matrix material. The triboluminescent elements can be distributed homogeneously (uniformly) or nonhomogeneously (nonuniformly) within the structure or structural portion with which they are united. 
     In accordance with the present invention, the triboluminescent elements can be generally distributed (e.g., in homogeneous fashion) at least substantially throughout the structure, or can be more specifically placed in one or more selected locations or zones of interest in the structure. The triboluminescent elements can be situated at or near a surface of the structure, or more deeply inside the structure. A key aspect of inventive practice is that the fiber optic line which is associated with the structure—more particularly, a light-permeable portion of the fiber optic line—be disposed in the vicinity of one or more triboluminescent elements; in other words, a light-permeable fiber optic portion must be positioned sufficiently near a triboluminescent element to permit a sufficient amount of luminescence-generated light to pass through its light-permeable axial exterior (outer covering, e.g., cladding) and reach its axial interior (centrally located optical fiber or fibers) in a sufficient amount or to a sufficient degree that its optical fiber(s) transmit an appreciable amount or degree of light indicative of the luminescence-generated light. 
     Advantageously, the present invention&#39;s fiber optic composite damage sensor affords remote detection of damage on the surface of or within a composite structure or other structure such as a machinery component. The structure of interest can thereby be monitored for damage. The present invention is especially advantageous vis&#39;-a-vis the current state-of-the-art because the present invention: (i) does not presuppose knowledge of the exact location of damage; (i) is a distributed sensor able to detect damage through great lengths; (iii) is a versatile sensor having great flexibility in reaching many areas within a composite or other structure. 
    
    
     Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein like numbers indicate the same or similar components, and wherein: 
     FIG. 1 is a schematic diagram illustrative of triboluminescence and its relationship to an optical fiber in accordance with typical practice of the present invention. 
     FIG. 2 is a diagrammatic plan view of an embodiment of damage sensing in accordance with the present invention, particularly showing a composite structure, the association with the composite structure of triboluminescent material elements and an optical fiber, and the connection of a photosensitive device with the optical fiber. The composite structure can be considered to be viewed either from above (wherein the optical fiber is coupled with the composite structure so as to be on a surface of the composite structure), or sectionally (wherein the optical fiber is coupled with the composite structure so as to be inside of or beneath the surface of the composite structure), or both from above and sectionally. 
     FIG. 3 is a diagrammatic plan view, similar to the view shown in FIG. 2 but slightly enlarged, of an inventive embodiment such as shown in FIG. 2, particularly illustrating the characteristic of light permeability of the exterior covering of the optical fiber intermittently along the length of the optical fiber. 
     FIG. 4 is a diagrammatic plan view, similar to the view shown in FIG. 3, of an inventive embodiment such as shown in FIG. 2, particularly illustrating the characteristic of light permeability of the exterior covering of the optical fiber at least substantially continuously along at least substantially the entire length of the optical fiber. 
     FIG. 5 is a diagrammatic perspective view of an embodiment of the present invention, particularly illustrating how a composite structure (shown, for exemplary purposes, to approximately describe a rectangular parallelepiped shape) can be infiltrated with triboluminescent elements and be coupled with optical fibers in various ways. 
     FIG. 6 is a partial elevation view of an embodiment of the present invention, particularly illustrating how a composite laminate structure (three lamina shown) can be infiltrated with triboluminescent elements and be coupled with optical fibers (two optical fibers shown in a longitudinal perspective) situated between the lamina. 
     FIG. 7 is a diagrammatic plan view of an embodiment of damage sensing in accordance with the present invention, particularly showing a composite structure, the association with the composite structure of triboluminescent material elements and optical fibers, and the connection of a photosensitive device with the optical fibers. The composite structure can be considered to be viewed either from above (wherein the optical fibers are coupled with the composite structure so as to be on a surface of the composite structure), or sectionally (wherein the optical fiber is coupled with the composite structure so as to be inside of or beneath the surface of the composite structure), or both from above and sectionally. 
     FIG. 8 is a partial elevation view of an embodiment of the present invention, particularly illustrating, similarly as illustrated in FIG. 6, how a composite laminate structure (two lamina shown) can be infiltrated with triboluminescent elements and be coupled with optical fibers (six optical fibers shown in an axial perspective) situated between the lamina. 
     FIG. 9 is a partial elevation view of an embodiment of the present invention, particularly illustrating how a composite laminate structure can be infiltrated with triboluminescent elements and be coupled with (i) optical fibers (six optical fibers shown in an axial perspective) situated between the lamina, and/or (ii) optical fibers (twelve fibers shown in an axial perspective) situated inside the lamina. 
     FIG. 10 a  through FIG. 10 d  are detailed partial views of various embodiments of the present invention, particularly showing triboluminescent elements and/or particle reinforcements and/or fiber reinforcements and/or optical fibers and the relationships therebetween. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 summarizes the triboluminescent effect as related to typical practice of the present invention. Referring to FIG. 1, a crystal or polycrystal element  11  of triboluminescent material is fractured (indicated as stage “I”), and splits into triboluminescent element parts  11   a  and  11   b  (indicated as stage “II”), thereby creating charged faces  111   a  and  111   b , respectively, of parts  11   a  and  11   b . When energy is created between the two faces  111   a  and  111   b , it is then reabsorbed into the triboluminescent material of parts  11   a  and  11   b  (indicated as stage “III”), which will then glow. It is this luminescent glow that appears when triboluminescent material is fractured. 
     Reference is now made to FIG. 2, which describes the general setup and operation of a typical composite damage sensor in accordance with the present invention. A composite structure  100  is impregnated with triboluminescent crystals or polycrystals  11 . The triboluminescent material  11  may be incorporated into the matrix (of a matrix composite), the reinforcement (of a matrix composite), the filler (of a matrix composite), or an interface (e.g., between two layers of a composite laminate), depending on the inventive application. This composite can be of any composition, including large choices of materials for the reinforcement/filler phase and the matrix phase. 
     Still with reference to FIG.  2  and again with reference to FIG. 1, the main prerequisite for the operation of the present invention&#39;s sensor is that light coming from the fracture of the triboluminescent crystals  11  (which have been incorporated into the composite  100 ) be able to be transmitted to the optical fiber  12 . The optical fiber  12  is embedded into the composite  100  in any area of composite  100  where detection of mechanical damage is desired. Optical fiber  12  includes cladding  13  and fiber core  14 . Upon the occurrence of damage to composite  100 , the light resulting from such damage to composite  100  will be transmitted through optical fiber cladding  13  to optical fiber core  14 , and/or will enter optical fiber core  14  from the end tip  22  of optical fiber  12 . This light will then be transmitted to a photodetector  15 , which will consequently indicate a light signal and, therefore, the coincidental mechanical damage to composite  100 . In the present invention&#39;s fiber optic sensor system there is no traditional light source; the sensor is passive. The light is created within the composite structure  100  when (i) the composite structure  100  incurs damage and (ii) the triboluminescent material  11  present also (e.g., concomitantly) fractures, thereby causing a light signal to be absorbed (taken in) by and transmitted via optical fiber  12 . 
     As shown generally in the figures, optical fiber  12  includes an outer covering (optical fiber cladding  13 ) and a light-transmittable inner core (optical fiber core  14 ). The present invention&#39;s optical fiber  12  can be constructed or configured in various ways, generally falling into two categorical “light-permeable” approaches, viz., “continuous” and “discrete.” A continuous optical fiber  12   C  has a cladding  13   P  which is permeable (e.g., transparent or translucent) to light throughout (or nearly throughout) the entire length of optical fiber  12 . A discrete optical fiber  12   D  includes: (i) at least one light-permeable fiber optic section  120   P  having a light-permeable cladding, section  130   P ; and, (ii) at least one light-impermeable fiber optic section  120   I  having a light-impermeable cladding, section  130   I . Thus, cladding  13  is permeable to light (e.g., transparent or translucent) in at least one section  120   P  of optical fiber  12 , and is impermeable to light (e.g., opaque) in at least one section  120   I  of optical fiber  12 . If there are two or more light-permeable optical fiber sections  120   P  of a discrete optical fiber  12   D , they will generally be unconnected (distinct) sections. 
     Typically, the “sections”  120  of a discrete optical fiber  12   D  are axially sequential segments (arranged side-to-side along the length of discrete optical fiber  12   D ), such as shown in FIG. 3; however, inventive practice of a discrete optical fiber  12   D  can involve any configuration wherein at least one portion/section is a light-permeable section  120   P  and at least one portion/section is a light-impermeable section  120   I . For instance, such sections can be contradistinguished and contraposed as upper and lower sections of discrete optical fiber  12   D , rather than as side-to-side sections of discrete optical fiber  12   D , such as shown in FIG.  3 . Usually, in side-to-side sectional configurations, if there are plural light-permeable optical fiber sections  120   P , then at least two light-permeable optical fiber sections  120   P  will be separated by light-impermeable optical fiber sections  120   I . Generally in inventive practice, a “discrete” fiber optic approach is superior to a “continuous” fiber optic approach for purposes of more definitively determining the location or locations of detected damage to composite  100 . 
     Hence, the optical fiber  12  used in typical inventive practice differs significantly from a standard optical fiber used in conventional practice. To further explain, in a standard optical fiber the outer layer (also referred to herein as the “cladding”) of the optical fiber traps light to be channeled only in the core (i.e., the inner or central area) of the optical fiber. Essentially, the optical fiber cladding is opaque; light does not pass through the optical fiber cladding, but is only transmitted by the optical fiber core. The optical fiber cladding is characterized by a lower index of refraction, whereas the optical fiber core is characterized by a higher index of refraction. 
     In contrast, according to the present invention, light is taken in from outside the optical fiber  12  so as to pass through the optical fiber cladding  13  and into the optical fiber core  14 , so that the light may be detected elsewhere (e.g., by means of electronic photodetection equipment). In order to accomplish this, a new kind of optical fiber—more specifically, a new kind of optical fiber cladding—is provided by the present invention. According to the present invention, at least a portion of the optical fiber cladding is effectively stripped away and replaced with a different, “alternative” cladding material, so that light may pass, from outside the optical fiber  12 , through the cladding  13  and into the optical fiber core  14 . The present invention&#39;s alternative cladding material is fight-permeable, e.g., transparent or translucent to light. Two inventive methodologies according to which this kind of light-permeability can be effectuated are illustrated in FIG.  3  and FIG. 4, which show, respectively, “discrete” fiber optic practice and “continuous” fiber optic practice. 
     As shown in FIG.  3  and FIG.  4  and other figures herein, composite  100  is rich in triboluminescent material, composite  100  having a multiplicity of triboluminescent elements  11  distributed therein. Discrete optical fiber  12   D  and continuous optical fiber  10   C  are shown in FIG.  3  and FIG. 4, respectively. With reference to FIG. 3 (and again with reference to FIG.  1 ), a crack  44  forms in composite  100 , causing triboluminescent light emission  55 . The light would normally be prevented from entering the fiber core by a standard cladding. However, according to the present invention, the installation of a special, light-permeable cladding  13   P  in one or more certain segments (light-permeable fiber optic sections  120   P ) allows the light  55  to enter fiber core  14 . This alternative cladding  13   P  is transparent to the wavelength or wavelengths (at least one wavelength) of light coming from crack  44 . The light  55  (or a portion thereof) which gains admission into optical fiber  12  via cladding  13   P  is transmitted via core  14  to photodetector  15 . 
     According to some inventive embodiments, the light-permeable cladding  13   P  is attributed with a wavelength-determinative quality, such as by having a particular color, tint or hue. Thus, for instance, in addition to allowing propagation of the light  55  to the optical fiber core  14 , alternative cladding  13   P  could contain a photosensitive dye, thereby selectively changing the wavelength of the light  55  transmitted therethrough to the fiber core  14 . In this manner, wavelength-determinative light-permeable cladding  13   P  can inform the photodetector  15  as to which location along optical fiber  12  the light  55  entered. For example, wavelength-determinative light-permeable cladding section  130   P-RED , encompassed by optical fiber section  120   P-RED , can impart information that red light entered cladding section  130   P-RED , thereby suggesting that damage to structure  100  occurred in the vicinity of cladding section  130   P-RED . Similarly, wavelength-determinative cladding  130   P-BLUE , encompassed by optical fiber section  120   P-BLUE , can impart information that blue light entered cladding  130   P-BLUE , thereby suggesting that damage to structure  100  occurred in the vicinity thereof In fact, the present invention can be practiced so that a continuous optical fiber or a discrete optical fiber contains serial or contiguous wavelength-determinative sections, such as shown in FIG. 4, wherein wavelength-determinative red light-permeable cladding section  130   P-RED  and wavelength-determinative blue light-permeable cladding section  130   P-BLUE  are adjacent to each other. 
     Photodetector  15  is intended herein to be broadly representative of any of a variety of known electronic apparatus which may serve the purpose of somehow indicating the presence or existence of a degree of light which has originally radiated from triboluminescent material and which has arrived at or culminated in the electronic apparatus. Hence, terms such as “photodetector” or “photosensor” or “photosensitive device” are synonymously intended herein to generally refer to apparatus capable of identifying, registering, recording, representing, presenting a readout, signaling, digitizing, processing, displaying, etc., or some combination thereof in a manner indicative of the light received by the apparatus. 
     With reference to FIG. 4, in some inventive applications the location of crack  44  along the length of optical fiber  12  is not important. If, instead, cognizance is desired of simply the presence of a crack  44  anywhere in the composite  100 , an arrangement such as depicted in FIG. 4 can be effected according to this invention. As shown in FIG. 4, the cladding  13  along the entire length of optical fiber  12  is light-permeable cladding  13   P . That is, cladding  13  is altered to accept the light  55  from the crack  44  in the composite  100 , so that the light  55  proceeds into the fiber core  14  and is transmitted to the photodetector  15 . 
     Sometimes a relatively small problem area—one which is deemed to be especially susceptible to a certain kind of damage, for example—is identified in a structure such as composite  100 . Perhaps the inventive practitioner wishes merely to “keep an eye” specifically on this vulnerable location. For such objectives, inventive practice might suitably provide for implementation of a completely “standard” optical fiber  12   S  (i.e., an optical fiber having cladding  13   I , which is impermeable to light, throughout the length of the optical fiber). Optical fiber  12  shown in FIG. 2 is envisionable as either a standard optical fiber  12   S  or a discrete optical fiber  12   D  or a continuous optical fiber  12   C , each of which can be practiced according to the present invention. Let us assume, for example, that optical fiber  12  shown in FIG. 2 is a standard optical fiber  12   S ; according to some inventive embodiments, standard optical fiber  12   S  can have an optical fiber tip  22  which can admit triboluminescently emitted light at a location of interest in composite  100 . Inventive use of an optical fiber tip  22  can be made in addition to or as an alternative to inventive use of light-permeable cladding  13   P , regardless of whether such cladding  13   P  is in a discrete mode or a continuous mode. Moreover, a wavelength-determinative window-like optical fiber tip member (such as cap  220   P-GREEN  shown in FIG.  3 ), transparent only to a particular wavelength (e.g., green), can be provided at tip  22  of optical fiber  12 . A light-permeable cap  220   P  or light-impermeable cap  220   I  can be coupled with (e.g., fitted on) an optical fiber  12  at its tip  22  whether it be a standard optical fiber  12   S , a discrete optical fiber  12   D  or a continuous optical fiber  12   C . 
     Using at least one optical fiber  12  as a sensing element in any of diverse inventive fashions such as described herein can permit maximum flexibility and versatility in detecting mechanical damage to any of diverse structures such as composite  100 . With reference to FIG. 5, various configurations in monitoring a structure such as composite  10  for mechanical damage are possible in accordance with the present invention. The optical fiber  12  can be rectilinear and/or curvilinear, and can be disposed, relative to a structure such as composite  100 , in any direction or directions in one, two or three dimensions. 
     As shown in FIG. 5, one-dimensional optical fiber  12 , describes a straight line (shown in FIG. 5 for illustrative purposes as being disposed in a geometric x-y or x-z plane). Two-dimensional optical fiber  12   2  describes a curve which lies in a single geometric plane (shown in FIG. 5 for illustrative purposes as being disposed in an x-y or x-z plane). Three-dimensional optical fiber  12   3  describes a curve disposed in all three (x, y and z) directions in geometric space. As shown in FIG. 5, composite structure  100  has six side faces  60 . Optical fiber  12   1  enters an end or side face  60  of composite  100  at point  600   a  and is run in a straight line through at least some of composite  100 . Optical fiber  12   2  enters an end or side face  60  of composite  100  at point  600   b  and effectively “covers” an imaginary plane by varying direction within the plane in a sinusoidal pattern. Optical fiber  12   3  enters an end or side face  60  of composite  100  at point  600   c  and proceeds in a combination x-y-z direction; thus, by virtue of optical fiber  12   3  alone, composite  100  can be monitored for damage in all three directions in Cartesian space. 
     It is reemphasized that, although the one-dimensional optical fiber  12   1  mode and the two-dimensional optical fiber  12   2  mode are each shown as lying in a horizontal plane, inventive practice is limitless with regard to the locations, extents and orientations of such one and two-dimensional optical fiber modes and of three-dimensional optical fiber modes, as well. The limitlessness in these regards applies regardless of the nature of the structure  100  of interest, whether it be non-composite, laminar composite, matrix composite, or some combination thereof According to this invention, an optical fiber  12  can be oriented in any direction or directions, situated anywhere on and/or in a structure, and thus associated with the structure to any of greater or lesser extents of optical fiber  12 . For instance, after entering a structure such as composite  10 , optical fiber  12  can be threaded in any desired direction or pattern. 
     A noteworthy principle of inventive practice is the selective association, with the structure  10  of interest, of either the triboluminescent material  11  or the optical fiber(s)  12  or both the triboluminescent material  11  and the optical fiber(s)  12 . As previously noted herein, a fiber optic tip  22  can be strategically positioned so as to pinpoint a specific spot on or in a structure  100 . More generally, any fiber optic line such as optical fiber  12  can be strategically positioned for sensing damage at one or more particular locations. A fiber optic line  12  can be placed exteriorly (e.g., tangentially or superficially, such as upon or embedded at a surface of structure  100 ) with respect to the structure  100 . Alternatively, a fiber optic line  12  can be placed interiorly with respect to structure  100 . Alternatively, a fiber optic line  12  can be placed, at different locations along its length, both exteriorly and interiorly with respect to structure  100 . The location or locations of light-permeable optical fiber cladding  13   P  and/or optical fiber tip  22  are especially important in these respects. 
     The locating of triboluminescent elements  11  can similarly serve strategical purposes pertaining to damage detection of structure  100 . As shown in FIG. 5, composite structure  100  is shown to be a matrix composite structure  100   M  which is “globally” infiltrated with triboluminescent elements  11 . However, depending on the inventive application, it may be unnecessary to distribute triboluminescent elements  11  in a global fashion, i.e., throughout matrix composite structure  100   M ; hence, as portrayed by composite regions  200   a  and  200   b , a composite structure can be regionally infiltrated with triboluminescent elements  11  (in one or more selected regions such as regions  200   a  and  200   b ), rather than globally infiltrated with triboluminescent elements  11 . Moreover, triboluminescent elements  11  can be located at or near a surface area (such as a side or end face  60  shown in FIG. 5) of composite structure  100   M , and or more interiorly within composite structure  10   M . 
     In accordance with the present invention, a fiber optic line such as optical fiber  12  can be embedded in any composite  100 , regardless of shape, size or type. Fiber optic line  12 , for instance, can be incorporated with a fibrous reinforcement, or can be molded into a non-fibrous composite. Still referring to FIG.  5  and also referring to FIG.  6  through FIG. 9, various composite structures are fully or partially depicted. Two popular kinds of composite structures are matrix composites and laminar composites. As shown in FIG.  6  through FIG. 9, an optical fiber  12  can be placed intermediate two adjacent laminas  70 , at least one of which is at least partially infiltrated with triboluminescent elements  11 . FIG. 6 shows a continuous optical fiber  12   C  and a discrete optical fiber  12   D , each situated between two laminas (layers)  70  of laminar composite  100   L . 
     According to some inventive embodiments, at least one lamina  70  can itself represent a matrix composite, having fiber or particle reinforcement/filler distributed therein. Shown in FIG. 9 is a sort of hybrid composite structure  100   LM  which has both laminar and matrix indicia. FIG. 9 shows two adjacent laminas  70  wherein optical fibers  12   a  are placed between laminas  70 , while optical fibers  12   b  are placed inside a lamina  70 . Each optical fiber  12   b  shown in FIG. 9 can be all or part of a fiber reinforcement  80  wherein the lamina  70  containing it represents a fiber-reinforced matrix composite  100   M , wherein fiber-reinforced matrix composite  100   M  will usually have one or more triboluminescent elements  11  distributed therein according to this invention. FIG. 7 can be considered to represent either a laminar composite  100   L  or a matrix composite  100   M  or a combination laminar-matrix composite  100   LM . By way of illustration, optical fiber  12   a  is shown to be situated interstructurally similarly as optical fiber  12   a  is situated in FIG. 9; optical fiber  12   b  is shown to be situated intrastructurally similarly as optical fiber  12   b  is situated in FIG.  9 . 
     Reference now being made to FIG. 10 a  through FIG. 10 d , crystal or polycrystal triboluminescent elements  11  can be associated with a matrix composite structure  100  in any of various ways. Matrix composite structure  100  includes a matrix (e.g., resinous) phase  300  and one or more constituent phases selected from among a particulate reinforcement phase  400   RP , a fibrous reinforcement phase  400   RF  and a particulate filler phase  400   FP . As shown in FIG. 10 a , particle reinforcements  400   RP  do not contain any triboluminescent material  11 ; rather, the triboluminescent material  11  is, or is included in, particle fillers  400   FP  and/or matrix  300 . FIG. 10 b  shows only particle reinforcements  400   RP  each of which is totally or partially made of triboluminescent material  11 . As shown in FIG. 10 c , both particle reinforcements  400   RP  and particle fillers  400   FP  contain triboluminescent material  11 . Some particle fillers  400   FP  do not contain any triboluminescent material  11 . Several particles  400   P  each represent a triboluminescent element  11  constituting triboluminescent material  11  in whole or in part. 
     FIG. 10 d  shows a veritable medley of possibilities for incorporating triboluminescent elements  11  into a matrix composite  100   M . Optical fiber  12  is or is made part of a reinforcement fiber  400   a   RF , which lies adjacent to and in parallel with another reinforcement fiber  400   b   RF  having a quantity of triboluminescent material, triboluminescent element  11 , incorporated therein as a section or portion thereof. Also shown are triboluminescent elements  11  which are, or are part of, particle fillers  400   FP  and particle reinforcements  400   RP . Further shown are triboluminescent elements  11  which are incorporated into the matrix  300  material itself In accordance with the present invention, any and all of these and similar approaches to incorporation of triboluminescent material  11  are possible, whether practiced individually or combinationally in any manner. Practically any amount of triboluminescent material  11  and any number of fiber optic lines  12  may be used in accordance with the present invention. 
     Other embodiments of this invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. Various omissions, modifications and changes to the principles described may be made by one skilled in the art without departing from the true scope and spirit of the invention which is indicated by the following claims.