CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority benefit of US Provisional Patent Application No. 62/704,522 by Webb et al. and filed on May 14, 2020, which is incorporated herein by reference in its entirety.

FIELD

[0002] This disclosure relates to the field of fiber-reinforced composite strength members for use with overhead electrical cables, and particularly to the field of structural integrity testing of such fiber-reinforced composite strength members.

BACKGROUND

[0003] Fiber-reinforced composite strength members are beneficial for use in a variety of structural applications due to their relatively high ratio of strength to weight and other desirable properties. For example, many elongated fiber-reinforced composite strength members are utilized as tensile members in structures such as bridges and railway messenger wires, replacing previously utilized materials such as steel.

[0004] One such application of fiber-reinforced composite strength members that has recently emerged is their use in bare overhead electrical cables, e.g., bare overhead electrical cables, for the transmission and distribution of electricity. Such electrical cables typically include one or more conductive layers that each include a plurality of conductive metallic strands that are wrapped around and supported by a central strength member. Traditionally, the strength member was fabricated from steel. In recent years, fiber- reinforced composite materials that include high tensile strength fibers (e.g., carbon fibers) in a binding matrix have been utilized for the strength member. Such composite materials offer significant benefits as compared to steel, including lighter weight, smaller size (enabling the use of a greater cross-section of conductor), lower thermal sag and many other advantages. One example of an overhead electrical cable having such a fiber-reinforced strength member is the ACCC ^® bare overhead electrical cable available from CTC Global Corporation of Irvine, CA, USA. See, for example, U.S. Patent No. Pat. No. 7,368,162 by Hiel et al., which is incorporated herein by reference in its entirety.

[0005] Unlike most metallic materials, fiber-reinforced composite materials typically have a low ductility. As a result, concern may arise that the fiber-reinforced composite strength member has been structurally compromised (e.g., fractured) during production, handling and/or installation of the strength member and/or the electrical cable. Small fractures in the strength member may be difficult detect by conventional methods. This is particularly true for overhead electrical cables due to the extreme length of the overhead electrical cable and due to the strength member being visibly obscured by the conductive strands that are wrapped around the strength member.

[0006] Methods and systems for monitoring the conditions of a fiber-reinforced strength member during use of the strength member have been suggested. For example, it has been suggested that the temperature and strain on a strength member in an overhead electrical cable can be monitored during use using complex techniques such as optical time domain reflectometry (OTDR). Such systems and methods require complex equipment, require the use of a coherent light source (e.g., a laser) and require precise alignment of the coherent light source with optical fibers attached to the strength member; all are difficult to implement in the field, e.g., in the outdoor environment through which electrical transmission and distribution lines are constructed.

SUMMARY

[0007] Disclosed herein are composite strength members, systems, methods, components and tools that enable the interrogation of fiber-reinforced composite strength members to ascertain if the composite strength members are structurally intact, e.g., to detect the presence of defects such as fractures in the strength member. The systems, methods, components and tools enable the relatively simple and low-cost interrogation of the strength members, in the manufacturing process, during installation, and/or after installation of the strength members. [0008] In one embodiment, a fiber-reinforced composite strength member is disclosed. The fiber-reinforced strength member includes at least a first strength element, the first strength element having a binding matrix, a plurality of reinforcing fibers operatively disposed within the binding matrix to form a fiber-reinforced composite material, and at least a first sensing strand embedded within the fiber-reinforced composite material and extending from a first end of first strength element to a second end of the strength element. The sensing strand comprises a conductive wire having an electrical resistivity of not greater than about 3x1 O ⁶ W-m.

[0009] In another embodiment, an electrical cable (e.g., an overhead electrical cable) is disclosed. The overhead electrical cable includes a fiber-reinforced composite strength member and an electrical conductor disposed around and supported by the strength member to form an electrical cable. The fiber-reinforced strength member includes at least a first strength element, the first strength element having a binding matrix, a plurality of reinforcing fibers operatively disposed within the binding matrix to form a fiber- reinforced composite material, and at least a first sensing strand embedded within the fiber-reinforced composite material and extending from a first end of first strength element to a second end of the strength element. The sensing strand comprises a conductive wire having an electrical resistivity of not greater than about 3x10-6 W-m.

[0010] In another embodiment, a method for the interrogation of a fiber-reinforced composite strength member is disclosed. The strength member includes at least a first strength element, the strength element comprising a binding matrix and a plurality of reinforcing fibers operatively disposed within the binding matrix to form a fiber-reinforced composite, and at least a first conductive wire embedded in the strength element and extending along a length of the strength element. The interrogation method includes the steps of operatively attaching a first conductor to a first end of the conductive wire, operatively attaching a second conductor to a second end of the conductive wire, applying one of a substantially constant voltage or a substantially constant current to the conductive wire, and measuring at least one of a resistance, a voltage drop or a change in current across the conductive wire to determine if there is an anomaly in the measurement as compared to an expected base value. [0011] In another embodiment, a method for the interrogation of a fiber-reinforced composite strength member is disclosed. The strength member includes at least a first strength element, the strength element comprising a binding matrix and a plurality of reinforcing fibers operatively disposed within the binding matrix to form a fiber-reinforced composite, and at least a first radiopaque fiber or wire embedded in the fiber-reinforced composite and extending along a length of the fiber-reinforced composite. The method includes the steps of operatively moving an x-ray detection device relative to the strength member and interrogating, during the moving step, the radiopaque wire with the x-ray detection device to ascertain the presence of any defects in the radiopaque wire.

[0012] In another embodiment, a method for the manufacture of an elongate fiber- reinforced composite strength element that is configured for use in a tensile strength member is disclosed. The method includes the steps of forming an elongate fiber- reinforced composite having a longitudinal central axis, the fiber-reinforced composite including a binding matrix, and a plurality of reinforcing fibers disposed in the binding matrix. During the step of forming the fiber-reinforced composite, at least a first sensing strand is embedded into the fiber-reinforced composite, the first sensing strand extending from a first end of the fiber-reinforced composite to a second end of the fiber-reinforced composite. The sensing strand is selected from a conductive wire or a radiopaque wire.

DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 illustrates a perspective views of an overhead electrical cable including a single composite strength element according to the prior art.

[0014] FIG. 2 illustrates a perspective view of an overhead electrical cable including multiple composite strength elements according to the prior art.

[0015] FIG. 3 illustrates a cross-sectional view of composite strength element that is configured for use in an overhead electrical cable according to the prior art.

[0016] FIG. 4 illustrates a cross-sectional view of composite strength element that is configured for use in an overhead electrical cable according to the prior art. [0017] FIG. 5 illustrates a cross-sectional view of composite strength element that is configured for use in an overhead electrical cable according to the prior art.

[0018] FIG. 6 illustrates a cross-sectional view of a composite strength element that is configured for use in an overhead electrical cable according to the prior art.

[0019] FIG. 7 illustrates a cross-sectional view of a composite strength element that is configured for use in an overhead electrical cable according to the prior art.

[0020] FIGS. 8 to 15 illustrate cross-sectional views of composite strength elements including sensing strands according to various embodiments of the present disclosure.

[0021] FIG. 16 schematically illustrates an interrogation technique according to one embodiment of the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

[0022] The present disclosure is directed to composite strength members, systems, methods, components, and tools that enable the interrogation of a fiber-reinforced composite strength member, e.g., to determine the structural integrity of the composite strength member, particularly in an overhead electrical cable.

[0023] As the terms are used in this disclosure, a fiber-reinforced composite strength member is a fiber-reinforced composite structure that is used in an application (e.g., a bare overhead electrical cable), such as for its lightweight and good mechanical properties (e.g., high tensile strength) as compared to, for example, steel. The fiber- reinforced composite includes reinforcing fibers in a binding matrix. A strength member may comprise a single (i.e., no more than one) strength element (e.g., a one-piece fiber- reinforced composite strength member), or may be comprised of several composite strength elements that are combined (e.g., twisted, stranded or otherwise bundled together) to form the strength member. As such, the present disclosure may use the terms strength member and strength element interchangeably, particularly where the strength member includes a single strength element.

[0024] The systems and methods for interrogation of a strength member disclosed herein may incorporate a strength member that is uniquely configured for use in the systems and methods, and for use in similar systems and methods. In one embodiment, systems and methods for the interrogation of a fiber-reinforced composite strength member are disclosed where the composite strength member includes a fiber-reinforced composite strength element having at least a first continuous sensing strand disposed within the strength element, e.g., extending from one end of the strength element to the other end (e.g., to the opposite end) of the strength element. One or more devices (e.g., sensing devices) may be brought into proximity with the sensing strand, e.g., may be contacted with or otherwise attached to one or more ends of the sensing strand, where the sensing device is configured to transmit and/or detect a signal and/or property of the sensing strand, the signal and/or property being indicative of the integrity (e.g., the continuity) of the sensing strand. The method and systems are configured such that the integrity of the sensing strand is an indication of the integrity of the strength element, e.g., to determine if the strength element structurally compromised, e.g., fractured. The systems and methods are cost effective and are relatively easy to implement at any point through the manufacture and installation of the strength member.

[0025] The strength members can be utilized in a variety of structural applications, particularly as a tensile strength member. In one embodiment, an elongate fiber- reinforced composite strength member is configured for use in an overhead electrical cable, e.g., a bare overhead electrical cable. As noted above, overhead electrical cables for the transmission and/or distribution of electricity typically include a central strength member and an electrical conductor disposed around and supported by the strength member. Although the strength member has traditionally been fabricated from steel, such steel strength members are increasingly being replaced by strength members fabricated from composite materials, particularly from fiber-reinforced composite materials, which offer many significant benefits such as high tensile strength, low weight and low thermal expansion. Such fiber-reinforced composite strength members may include of a single fiber-reinforced composite strength element (e.g., a single rod) as is illustrated in FIG. 1. An example of such a configuration is disclosed in U.S. Pat. No. 7,368,162 by Hiel et al. , noted above. Alternatively, the composite strength member may be comprised of a plurality of individual fiber-reinforced composite strength elements (e.g., two or more individual rods) that are operatively combined (e.g., twisted or stranded together) to form the strength member, as is illustrated in FIG. 2. Examples of such multi-element composite strength members include, but are not limited to: the multi-element aluminum matrix composite strength member illustrated in U.S. Patent No. 6,245,425 by McCullough et al.; the multi-element carbon fiber strength member illustrated in U.S. Patent No. 6,015,953 by Tosaka et al.; and the multi-element strength member illustrated in U.S. Patent No. 9,685,257 by Daniel et al. Each of these U.S. patents is incorporated herein by reference in its entirety. Other configurations for the fiber-reinforced composite strength member may be used.

[0026] Referring to FIG. 1, the illustrated example of an overhead electrical cable 100 includes an electrical conductor 112 having a first conductive layer 114a and a second conductive layer 114b, each comprising a plurality of conductive strands115a and 115b, respectively. It will be appreciated that such overhead electrical cables may include a single conductive layer, or more than two conductive layers, depending upon the desired electrical properties of the overhead electrical cable. The conductive strands 115a/115b may be fabricated from conductive metals such as copper or aluminum (i.e. , substantially pure copper, substantially pure aluminum, copper alloys or aluminum alloys). For use in bare overhead electrical cables for the distribution and/or transmission of electricity, the conductive strands are typically fabricated from aluminum, e.g., hardened aluminum, annealed aluminum, and/or aluminum alloys. As illustrated, the conductive strands 115a/115b have a substantially trapezoidal cross-section. The use of polygonal cross- sections such as a trapezoidal cross-section advantageously increases the cross- sectional area of conductive metal for the same effective cable diameter, e.g., as compared to strands having a circular cross-section. Nonetheless, the present disclosure is not limited to any particular cross-section or type of conductor strand.

[0027] Because the conductive materials, e.g., aluminum, do not have adequate mechanical properties (e.g., adequate tensile strength) to be self-supporting when strung between support towers to form an overhead electrical line for transmission and/or distribution of electricity, the conductive strands 115a/115b are helically wrapped around a strength member 118 to support the conductive layers 114a/114b when the overhead electrical cable 110 is strung between the support towers under high mechanical tension. As illustrated in FIG. 1, the strength member 118 includes a single strength element 119. Although the strength member 118 may be fabricated from a variety of materials including metals such as steel and/or aluminum, in one aspect of the present disclosure the strength member 118 is fabricated from a fully elastic material, e.g., from a fiber- reinforced composite material.

[0028] FIG. 2 illustrates an example of an overhead electrical cable 210 that is similar to the electrical cable illustrated in FIG. 1. In the example illustrated in FIG. 2, however, the strength member 218 includes a plurality of individual fiber-reinforced strength elements (e.g., strength element 219a) that are stranded or twisted together to form the strength member 218.

[0029] To further illustrate the of strength members to which the present disclosure is applicable, FIGS. 3 to 7 illustrate cross-sectional views of five examples of a fiber- reinforced composite strength member, i.e. , where each strength member includes one or more strength elements. FIG. 3 illustrates an example of a fiber-reinforced strength member 318 that includes a single strength element 319. The strength element 319 comprises a high tensile strength core320 that includes longitudinally extending reinforcing fibers, e.g., reinforcing carbon fibers, in a binding matrix, e.g., a binding matrix such as a thermoplastic polymer or a thermoset polymer. The core 320 is surrounded by a galvanic layer 321 that is selected (e.g., is configured) to form a barrier between the carbon fibers of the core 320 and the outer conductive strands, e.g., outer aluminum strands, as illustrated in FIG. 1. Such a galvanic layer prevents galvanic corrosion of the aluminum as a result of contact with the carbon (e.g., the carbon fibers) in the presence of an electrolyte. As illustrated in FIG. 3, the galvanic layer 321 is fabricated from an insulating (e.g., high dielectric) plastic (e.g., a polymer) The plastic galvanic layer may comprise a high-performance plastic, e.g., having a continuous service temperature of at least about 150°C, such as at least about 180°C, at least about 200°C or even at least about 220°C. The plastic galvanic layer may be a thermoplastic, e.g., a semi-crystalline thermoplastic. For example, the plastic galvanic layer may be formed from a thermoplastic selected from polyetheretherketone (PEEK) and polyphenylene sulfide (PPS). Other plastic materials such as fluorocarbon polymers, e.g., polytetrafluoroethylene, may also be useful, as well as high-performance amorphous plastics such as amorphous polyetherimide (PEI). In the example illustrated in FIG. 4, the single strength element 418 includes a high tensile strength core 420 of longitudinally extending carbon fibers in a polymer binding matrix and an outer galvanic layer 421 of glass fibers, e.g., continuous and/or braided glass fibers, which may also be disposed in a binding matrix. The galvanic layer 421 of glass fibers provides robust protection of the carbon fibers from contact with the aluminum conductive layer and provides enhanced flexibility to the strength member 418 so that the strength member and the electrical cable may be wrapped onto a spool for storage and transportation without damaging the strength member 418. FIG. 5 illustrates an example of a strength member 518 that includes a single strength element 519a high tensile strength core 520 comprising carbon reinforcing fibers in a polymer matrix and a galvanic layer 521 surrounding the core 520. In the example of 5, the galvanic layer 521 comprises a metal layer such as a conformally coated aluminum layer surrounding the inner core 520. When the metal layer is properly coated around the core 520, no air or moisture (e.g., electrolyte) can penetrate the layer 521 and galvanic corrosion of the outer aluminum conductor layers may be prevented.

[0030] FIGS. 6 and 7 illustrate two examples of a strength member including a plurality of strength elements. Typically, when the strength member is constructed from multiple strength elements, the individual strength elements will have a smaller diameter than a single element strength member. Referring to FIG. 6, the strength member 618 includes a plurality of strength elements, e.g., strength element 619a that are wrapped (e.g., stranded) together to form the strength member 618. The individual strength elements may have a construction the same as or similar to the construction of the strength elements illustrated in FIGS. 3 - 5. As illustrated in FIG. 6, each strength element includes a high tensile strength core (e.g., core 620a), e.g., of reinforcing carbon fibers, and a galvanic layer (e.g., galvanic layer 621a) to isolate each high tensile strength core from the outer conductive layer(s) of the electrical cable (see FIG. 2). In the example illustrated in FIG. 7, the strength member 718 includes a plurality of strength elements that each include a high tensile strength core (e.g., core 720a) of carbon reinforcing fibers with no galvanic layer surrounding the individual strength elements. Rather, a galvanic layer 721 surrounds the plurality of strength elements to isolate the strength elements from the surrounding conductive aluminum strands. The galvanic layer 721 may be formed from a tape (e.g., a helically wound tape) comprising glass fibers and/or a high-performance plastic material as is discussed above with respect to FIG. 6.

[0031] It will be appreciated that other configurations of strength members may be utilized with the present disclosure. For example, the examples of FIG. 6 and FIG. 7 may be combined wherein a galvanic layer is placed over each individual strength element as well as over the bundle of strength elements. Further, although the multi-element strength members illustrated in FIG. 6 and FIG. 7 include seven individual strength elements, it will be appreciated that multi-element strength members may include any number of strength elements that is suitable for a particular application.

[0032] As noted above, the fiber-reinforced composite from which the strength elements are constructed may include reinforcing fibers (e.g., high tensile strength fibers) that are operatively disposed in a binding matrix. The reinforcing fibers may be substantially continuous reinforcing fibers that extend along the length of the fiber- reinforced composite, and/or may include short reinforcing fibers (e.g., fiber whiskers or chopped fibers) that are dispersed through the binding matrix. The reinforcing fibers may be selected from a wide range of materials, including but not limited to, carbon, glass, boron, metal oxides such as aluminum oxide, metal carbides, high-strength polymers such as aramid fibers or fluoropolymer fibers, basalt fibers and the like. Carbon fibers are particularly advantageous in many applications due to their very high tensile strength, and/or due to their relatively low coefficient of thermal expansion (CTE).

[0033] The binding matrix may include, for example, a plastic (e.g., polymer) such as a thermoplastic polymer, including semi-crystalline thermoplastics. Specific examples of useful thermoplastics include, but are not limited to, polyether ether ketone (PEEK), polypropylene (PP), polyphenylene sulfide (PPS), polyetherimide (PEI), liquid crystal polymer (LCP), polyoxymethylene (POM, or acetal), polyamide (PA, or nylon), polyethylene (PE), fluoropolymers and thermoplastic polyesters.

[0034] The binding matrix may also include a thermosetting polymer. Examples of useful thermosetting polymers include, but are not limited to, benzoxazine, thermosetting polyimides (PI), polyether amide resin (PEAR), phenolic resins, epoxy-based vinyl ester resins, polycyanate resins and cyanate ester resins. In one exemplary embodiment, a vinyl ester resin is used in the binding matrix. Another embodiment includes the use of an epoxy resin, such as an epoxy resin that is a reaction product of epichlorohydrin and bisphenol A, bisphenol A diglycidyl ether (DGEBA). Curing agents ( e.g ., hardeners) for epoxy resins may be selected according to the desired properties of the fiber-reinforced composite strength member and the processing method. For example, curing agents may be selected from aliphatic polyamines, polyamides and modified versions of these compounds. Anhydrides and isocyanates may also be used as curing agents. Other examples of polymeric materials useful for a binding matrix may include addition cured phenolic resins, e.g., bismaleimides (BMI), polyetheramides, various anhydrides, or imides.

[0035] The binding matrix may also be a metallic matrix, such as an aluminum matrix. One example of an aluminum matrix fiber-reinforced composite is illustrated in U.S. Patent No. 6,245,425 by McCullough et al. , noted above.

[0036] One configuration of a composite strength member for an overhead electrical cable that is particularly advantageous is the ACCC® composite configuration that is available from CTC Global Corporation of Irvine, CA and is illustrated in U.S. Pat. No. 7,368,162 by Hiel et al., noted above. In the commercial embodiment of the ACCC® electrical cable, the strength member is a single element strength member of substantially circular cross-section that includes a high tensile strength core of substantially continuous reinforcing carbon fibers disposed in a polymer matrix. The core of carbon fibers is surrounded by a robust galvanic layer of glass fibers that are also disposed in a polymer matrix and are selected to insulate the carbon fibers from the surrounding conductive aluminum strands. See FIG. 4. The glass fibers also have a higher elastic modulus than the carbon fibers and provide bendability so that the strength member and the electrical cable can be wrapped upon a spool for storage and transportation.

[0037] Fiber-reinforced composite strength members that are useful in overhead electrical cables may be characterized in several ways. One characterization may be the length of the strength member. For example, strength members for overhead electrical cables are typically produced in very long lengths. In certain characterizations, the as- produced strength member may have a length of at least about 1000 meters, such as at least about 3500 meters, such as at least about 5000 meters, or even at least about 7500 meters. Often, immediately upon production of the strength member, the strength member is typically wrapped around a spool (e.g., a bobbin) for storage and/or transport of the strength member, such as for transport to a stranding facility where the strength member is stranded with an electrical conductor to form an electrical cable. Although the strength member of the present disclosure does not have a particular upper limit for its length, as a practical matter the storage/transport spool will have a maximum capacity for the strength member of not greater than about 9000 meters, such as not greater than about 8000 meters. It will be appreciated that the maximum capacity of the spool will depend upon the diameter of the strength member (or, for example, the number of strength elements making up the strength member) that is wrapped around the spool.

[0038] The length of the fiber-reinforced composite strength member may also be characterized when the electrical cable is formed, e.g., when the strength member is stranded with an electrical conductor. As noted above, this typically occurs during a stranding operation where the strength member is pulled from a spool, is stranded with an electrical conductor (e.g., with conductive strands), and the thus-formed electrical cable is wrapped around another spool for storage and/or transport of the electrical cable. Because the conductive strands add volume in addition to the volume of the strength member, the length of the electrical cable that can be stored on a spool is reduced as compared to the strength member alone. Thus, for example, the length of the electrical cable and the length of the strength member in this state will typically be not greater than about 4500 meters, such as not greater than about 4000 meters.

[0039] The length of the electrical cable and of the underlying composite strength member may also be characterized when the electrical cable is installed (e.g., strung) onto support towers to form the electrical line for distribution and/or transmission of electricity. To form the electrical line, the electrical cable is pulled from a spool and is operatively attached to support towers (e.g., pylons) so that the electrical cable is suspended (e.g., overhead) at a safe distance above the ground. The electrical cable must be cut at certain intervals along the path of the electrical line and reconnected using cable hardware such as conductive splices and/or dead-end structures. The length of the electrical cable may be as short as the distance between two adjacent support towers (e.g., when the electrical line makes a turn), or may span several support towers before being cut and reconnected using hardware. Thus, the installed electrical cable and the underlying strength member may have a length of at least about 50 meters, such as at least about 100 meters, such as at least about 250 meters or greater, at least about 500 meters, or even at least about 1000 meters or greater. Some electrical lines may have a length of up to about 3000 meters, for example lines strung across rivers or valleys in a single span, or lines across uninterrupted terrain supported by multiple towers creating multiple spans.

[0040] As is noted above, the fiber-reinforced composite strength member may have a range of diameters, e.g., diameter of a circular cross-section, the effective diameter of a non-circular cross-section, or the effective diameter of a plurality of stranded or twisted composite strength elements that form the strength member. Although there is no lower limit on the diameter of the strength member, the diameter will typically be at least about 1 mm, such as at least about 2 mm or at least about 3 mm. Likewise, although there is no particular upper limit on the diameter of the strength member, the diameter will typically be not greater than about 30 mm, such as not greater than about 20 mm, such as not greater than about 15 mm, such as not greater than about 11 mm.

[0041] Fiber-reinforced composite strength members that are configured for use in an overhead electrical cable may also be characterized in terms of a minimum tensile strength that is necessary to be safely installed onto the support towers under high mechanical tension. In this regard, it is generally desirable that the strength member have a tensile strength of at least about 1900 MPa, such as at least about 2000 MPa, such as at least about 2050 MPa. There is not practical upper limit on the tensile strength of the strength member, as it is typically desired to have as a high of a tensile strength as possible. As a result, the upper limit on the tensile strength is only limited by the available materials, e.g., the tensile strength of available reinforcing fibers. Given the tensile strength of currently available reinforcing fibers, the practical upper limit on the tensile strength is about 3500 MPa.

[0042] Tensile strength is one primary characteristic of the fiber-reinforced composite strength member. Other characteristics may be desirable for use in a selected application, such as in an overhead electrical cable. For example, it is also desirable that the strength member in an overhead electrical cable have a low coefficient of thermal expansion (CTE) to reduce the occurrence of line sag when the electrical line becomes overheated. For example, the strength member may have a CTE of not greater than about 30x10 ⁶ /°C, such as not greater than about 20x10 ^_6 /°C, such as not greater than about 10x10 ⁶ /°C, such as not greater than about 7.5x10 ⁶ /°C, such as not greater than about 5x10 ⁶ /°C, or even not greater than about 2.5x10 ^_6 /°C. In one characterization, the CTE of the strength member is not greater than about 2.0x10 ^_6 /°C. In some embodiments, the strength member may even have a slightly negative CTE, such as down to about - 0.5x10- ⁶ /°C.

[0043] Another important characteristic of the strength member, e.g., for an overhead electrical cable, may be the tensile modulus of the strength member. If the tensile modulus is too low, the overhead electrical cable may be subject to excessive sag when subjected to, for example, an ice-loading event. In this regard, the tensile modulus of the strength member may be at least about 100 GPa, such as at least about 110 GPa, or even at least about 120 GPa.

[0044] Although the foregoing characteristics of a strength member are disclosed as being desirable for use in an overhead electrical cable, similar characteristics may also be desirable when the strength members disclosed herein are used in other structures, such as bridge cables and messenger cables.

[0045] According to a first embodiment, the fiber-reinforced composite strength member includes at least one fiber-reinforced composite strength element, and the strength element includes at least one elongate and continuous sensing strand. The term “sensing strand” as used herein refers to an elongate and continuous element that is configured to provide an indication that the continuity of the element has been disrupted when the sensing strand is interrogated in some manner. For example, the sensing strand may be interrogated by locating the strand and probing through an end of the strand to determine if the sensing strand has maintained its continuous structure. In another example, the sensing strand may be interrogated along its length by relative movement of a sensing device along the length of the sensing strand, e.g., along the length of the strength element. The term “strand” is not intended to limit the configuration (e.g., the structure) of the element, and the element may comprise a wire, thread, cord, string, line or other slender and elongate structure that is capable of being formed within and/or around an elongate strength element.

[0046] The sensing strand may be embedded within the fiber-reinforced composite material and/or may be placed around the fiber-reinforced material (e.g., within or around the binding matrix). The sensing strands may extend substantially completely through the strength element, i.e. , from a first end of the strength element to a second end of the strength element. By proper selection of the sensing strand (e.g., proper selection of the electrical, mechanical properties and/or transmittance of the sensing strand), the sensing strand can be interrogated (e.g., inspected) to assess the structural integrity of the sensing strand, and therefore to assess the structural integrity of the strength element (e.g., of the binding matrix) that includes the sensing strand. Although a single sensing strand may be utilized in a strength element, the efficacy of the systems and methods disclosed herein may be improved by including multiple sensing strands, e.g., in spaced apart relation, within a fiber-reinforced composite strength element, particularly when the strength member includes a single strength element. As such, the present disclosure may refer to the use of sensing strands (e.g., a plurality of sensing strands) even though the present disclosure contemplates the use of a single sensing strand in a strength element.

[0047] The sensing strands may be disposed linearly along the length of the strength element. Stated another way, the sensing strands may be longitudinally oriented and co- linear with a central longitudinal axis of the strength element. In an alternative arrangement, one or more (e.g., all) of the sensing strands may be disposed in a non linear fashion, i.e., may be wrapped (e.g., helically twisted) relative to the central longitudinal axis of the strength element.

[0048] In one configuration, the elongate sensing strand is selected to provide a continuous pathway for the transmission of a signal (e.g., an electrical signal), such as from the first end of the sensing strand to the second end of the sensing strand, e.g., from the first end of the strength element to the second end of the strength element. In another configuration, the elongate sensing strand is selected to enable interrogation from the outside of the strength element and along the length of the strength element, e.g., by relative movement of an interrogation device along the length of the strength element(s), e.g., along a bare strength element or along an electrical cable including the strength element.

[0049] In either of these configurations, it is necessary to ensure that an anomaly in the integrity of the sensing strand is indicative of an anomaly in the integrity of the strength element (e.g., of a fracture in the strength element). That is, it is important that a disruption in the continuity of the sensing strands is indicative of an anomaly in the structure of the strength element, and that there is a high likelihood that an anomaly, such as a fracture in the strength element, also results in detectable disruption (e.g., a fracture) in the sensing strand.

[0050] Thus, the material properties and the construction (e.g., the shape and/or diameter) of the sensing strand should be carefully selected relative to the material properties and construction of the strength element. One material property that may be of importance is the tensile strain to failure value (S/F) of the sensing strand, particularly in relation to the tensile strain to failure value of the strength element. The tensile strain to failure value is an indication of how far a component (e.g., the sensing strand) can elongate (e.g., stretch) before fracturing, and is expressed as a percentage of the length of the component. Thus, if the tensile strain to failure value of the sensing strand closely matches the tensile strain to failure value of the strength element, a fracture of the sensing strand (e.g., under a tensile stress) will be indicative of an anomaly (e.g., a fracture) in the strength element. If the sensing strand has a strain to failure value that is too high, there is a risk that the sensing strand will not fracture when the strength element fractures. If the strain to failure ratio of the sensing strand is too low, there is a risk that the sensing strand will fracture due to a loading event (e.g., a bend in the strength element) when the strength element does not fracture.

[0051] In one characterization, the sensing strand has a strain to failure value that is not greater than about 500% (e.g., 5 times) of the tensile strain to failure value of the strength element (e.g., of the fiber-reinforced composite material), and is not lower than the tensile strain to failure value of the strength element. For example, the tensile strain to failure value may be not greater than about 150% of the tensile strain to failure value of the strength element, such as not greater than about 100% of the tensile strain to failure value of the strength element, such as not greater than about 80% of the tensile strain to failure value of the strength element, such as not greater than about 50% of the tensile strain to failure value of the strength element, such as not greater than about 30% of the tensile strain to failure value of the strength element, such as not greater than about 10% of the tensile strain to failure value of the strength element.

[0052] In one particular characterization, the fiber-reinforced composite strength element includes carbon reinforcing fibers in a polymer (e.g., thermoset or thermoplastic resin) binding matrix. Sensing strands, e.g., conductive sensing strands, are embedded in the matrix and have a tensile strain to failure value of at least about 1.5%, such as at least about 2%, such as at least about 3%, and not greater than about 6%, such as not greater than about 4%. In one characterization, the sensing strands have a tensile strain to failure value of at least about 2% and not greater than about 6%.

[0053] The systems and methods disclosed herein for the interrogation of a strength element can be used to assess the structural integrity of the strength element, e.g., to determine whether the strength element (e.g., the fiber-reinforced composite) includes any fractures or other similar defects, e.g., arising from manufacture and/or handling of the strength element. To provide a reliable assessment, and depending upon the size and configuration (e.g., diameter and/or cross-section) of the strength element, it may be desirable that the strength element include two or more sensing strands embedded within the fiber-reinforced composite strength element. For example, the strength element may include at least a third sensing strand or at least a fourth sensing strand to provide a reliable assessment, e.g., to increase the likelihood that a fracture in the fiber-reinforced composite will also disrupt (e.g., fracture) at least one of the sensing strands. Multiple sensing strands are particularly useful when a single strength element is used as the strength member.

[0054] When the fiber-reinforced composite strength element includes three or more sensing strands, the sensing strands may be concentrically disposed around the longitudinal central axis of the strength element and may be disposed equidistant from the longitudinal axis and/or equidistant from each other. This may increase the ability of the system and method to detect defects in the fiber-reinforced composite by distributing the sensing strands evenly through the cross-section of the strength element. In one construction, the strength element includes at least four sensing strands that are disposed in a concentric manner around the longitudinal axis and are approximately equally spaced relative to each other, e.g., at about 90° angles about the cross-section of the strength element. In one construction, the longitudinal central axis of the strength element is free of sensing strands, e.g., the strength element does not include a centrally disposed sensing strand. It is believed that placing a sensing strand along the central axis will not meaningfully contribute to the efficacy of the interrogation system and method, as there is a very low probability that a fracture will pass near the central axis of the strength element without also passing very near, and disrupting, one or more of the sensing strands that are disposed around and spaced from the central axis.

[0055] In the configurations described above wherein the strength element comprises a central composite core of reinforcing fibers, such as a core comprising carbon fibers, and an insulating layer surrounding the central core, such as a layer of glass fibers in the binding matrix, the sensing strands may advantageously be embedded at different locations within the fiber-reinforced composite, i.e. , at different distances from the central axis and/or at different angles relative to the central axis of the core at the interface of the central core and the insulating layer. Placement at this interface may enhance the probability of the sensing strands being disrupted when a fracture occurs in the strength element.

[0056] Referring to FIGS. 8 to 13, a cross-sectional view of strength elements according to several examples of the present disclosure are illustrated. The configuration of the fiber-reinforced sections of the strength elements illustrated in FIGS. 8 - 13 is similar to the strength element illustrated in FIG. 3 and includes an inner core of high tensile strength fibers surrounded by an outer layer of an insulative material, e.g., an inner core comprising carbon fibers surrounded by an outer layer comprising glass fibers. As illustrated in FIG. 8, the strength member 818 includes a single strength element 819 includes four sensing strands 850a-d that are concentrically disposed within the strength element 819 around and spaced-apart from the central axis. As illustrated in FIG. 8, the sensing strands 850a-d are placed at or very near an interface 822 between the high tensile strength core 820 and the galvanic layer 821. The four sensing strands 850a-d are evenly spaced about the central axis, e.g., are radially spaced apart by about 90°.

[0057] As illustrated in FIG. 9, three sensing strands 950a-c are placed within the galvanic layer 921, e.g., near the outer surface of the strength element 919, and are radially spaced apart by about 120°. As noted above, the galvanic layer 921 may comprise insulative glass fiber strands in a polymer binding matrix. When the sensing strands are conductive sensing wires, one potential advantage of placing the conductive sensing wires in the galvanic layer is that the conductive wires will be electrically isolated and more easily interrogated, e.g., as compared to being placed in a carbon fiber matrix. Further, placement near the outer surface may be advantageous for early detection of fractures that occur (e.g., are initiated) on the outer surface. However, the sensing strands 950a-c should not be placed so close to the outer surface that they are exposed and subject to being damaged or otherwise compromised, e.g., due to direct contact with the conductive layer (see FIG. 1).

[0058] FIG. 10 illustrates an embodiment of a strength member 1018 having a single strength element 1019, wherein four sensing strands 1050a-d are embedded in the high tensile strength core 1020. FIG. 11 illustrates an embodiment of a strength element 1119 wherein five sensing elements 1150a-e are placed at the interface 1122 between the inner core 1120 and the galvanic layer 1121. The five sensing strands 1150a-e are substantially equidistant from a central axis of the strength element and are substantially equally spaced apart, i.e. , radially spaced apart by about 72°. FIG. 11 illustrates that the strength element may include any number of sensing strands, including five or more sensing strands. FIG. 12 illustrates an embodiment of a strength element 1219 wherein three sensing strands 1250a-c in a first grouping are disposed at a first distance from a central axis of the strength element, and three sensing strands 1250d-f in a second grouping are disposed at a second distance from the central axis, where the second distance is different than the first distance. FIG. 13 illustrates an embodiment of a strength element 1319 wherein a sensing strand 1350a is disposed along a central axis of the strength element 1319 and is surrounded by a grouping of four sensing strands 1350b-e that are spaced apart from the central axis.

[0059] FIG. 14 illustrates an embodiment of a strength element 1419 that includes a high tensile strength core 1420 that is surrounded by a galvanic layer 1421 formed from a plastic, e.g., a high-performance plastic as is described above with respect to FIG. 6. As illustrated in FIG. 14, four sensing strands 1450a-d are disposed around the central axis and between the core 1420 and the plastic galvanic layer 1421. It will be appreciated that the arrangement of the sensing strands within the strength element illustrated in FIGS. 8 - 14 are merely examples of possible arrangements, and the present disclosure is not limited to the illustrated arrangements. Further, it will be appreciated that these strength elements may be implemented individually, i.e., as a single element strength member as illustrated in FIG. 1 or may be bundled to form a strength member as illustrated in FIG. 2.

[0060] As noted above, in one configuration of this embodiment, the sensing strands are selected to provide a pathway (e.g., a continuous pathway) for the transmission of a signal from the first end of the sensing strand to the second end of the sensing strand, e.g., from the first end of the strength element to the second end of the strength element. In one example, the sensing strand comprises an electrically conductive wire that provides a continuous electrical pathway along the length of the wire. The conductive wire comprises an electrically conductive material, such as a conductive metal, a conductive carbon material, a conductive inorganic oxide compound (e.g., a conductive ceramic or glass), a conductive inorganic non-oxide compound (e.g., a nitride or carbide compound) or a conductive organic compound, such as a conductive polymer. The conductive wire may be fabricated entirely from a single conductive material, may be fabricated from two or more conductive materials, or may be fabricated from a conductive material and one or more non-conductive materials. For example, the conductive wire may comprise a conductive material core and an outer layer of a non-conductive material such as a shellac or similar substance. In another example, the conductive wire may comprise an inner core of a non-conductive material and an outer layer of a conductive material. In another embodiment, the conductive wire may comprise one material phase (e.g., fibers or particulates) that is embedded in another material phase (e.g., a matrix phase).

[0061] Examples of electrically conductive metals that are particularly useful in the conductive wire include, but are not limited to, copper, nickel, aluminum, gold, silver, and the like, including alloys thereof, or other multi-component metallic materials such as steel. The metals may be in a work-hardened state or in an annealed state, for example. In one configuration, the conductive sensing wire is fabricated from copper and may advantageously be formed from annealed copper. In one example, the conductive wire comprises a steel matrix embedded with a high conductivity phase such as gold or copper. In another example, the conductive wire comprises a steel core surrounded by a layer (e.g., a coating) of a high conductivity material such as gold or copper.

[0062] According to this example, the conductive wires are configured such that a voltage may be passed through the wires to ascertain the integrity of the conductive wires. That is, a sensing device that is configured to measure an electrical property (e.g., electrical resistance) may be placed in operative contact with the conductive wires to determine if the continuity of the wires has been disrupted, e.g., if the measured resistivity has changed as compared to a known value, e.g., the original resistivity of the conductive wires.

[0063] One property of the conductive sensing wire that may be of importance is the resistivity (p) of the conductive wire. Because the overhead electrical cables are that are fabricated and installed in overhead electrical lines have a great length as noted above, the conductive sensing wire should have a relatively low electrical resistivity (i.e. , a relatively high conductivity) to enable a voltage to be passed from one end of the strength element to the other without requiring substantial electrical current, which can cause heating of the sensing wire. In one characterization, the resistivity of the conductive wire is not greater than about 3x1 O ⁶ W-rn, such as not greater than about 1x1 O ⁶ W-rn, such as not greater than about 8x1 O ⁷ W-rn, such as not greater than about 4x1 O ⁷ W-rn. In certain embodiments, the resistivity of the conductive wire is not greater than about 2x10 ⁷ Q-m, such as not greater than about 1x1 O ⁷ W-rn, not greater than about 5x1 O ⁸ W-rrn, or even not greater than about 3x1 O ⁸ Q-m. [0064] In another characterization, the conductive sensing wire has a cross-sectional area (e.g., a diameter or thickness) that is sufficiently large to enable a voltage/current to be passed through the wire to measure a voltage drop or current increase without significant resistance heating of the conductive sensing wire. For example, the conductive sensing wire may have a substantially circular cross-section and a diameter of at least about 0.2 mm, such as at least about 0.4 mm. In another example, the conductive wire has a cross-sectional area that is sufficiently small that the wire does not occupy an appreciable volume of the strength element and thereby does not appreciably degrade the properties (e.g., the tensile properties) of the strength element. For example, the conductive wire may have a substantially circular cross-section and a diameter of not greater than about 1.0 mm, such as not greater than about 0.8 mm, such as not greater than about 0.6 mm.

[0065] For some applications, it may be beneficial to coat the conductive material of the wire with a substantially non-conductive material to isolate (e.g., electrically isolate) the conductive material from the surrounding matrix. For example, some metallic materials may adversely react with the surrounding strength element components (e.g., with carbon reinforcing fibers), which could lead to a tensile strength degradation in the strength element. Further, direct contact between the conductive sensing wire and the slightly conductive carbon fiber may hinder the ability to obtain a reliable electrical reading along the length of the conductive sensing wire. The non-conductive coating may be fabricated from a polymer, e.g., a thermoset or a thermoplastic polymer. Examples include, but are not limited to, epoxy, polyvinyl, polyurethane, polyamide, polyesters, polyester-polyimides, polyamide-polyimides, and polyimides. Particularly useful polymers may include a high dielectric strength polymer such as PEEK (polyether ether ketone). Other useful polymers may include thermoplastics such as PVC (Polyvinyl Chloride), PE (Polyethylene), ECTFE (Ethylene Chlorotrifluoroethylene), PVDF (Polyvinylidene Difluoride) and Nylon (e.g., polyamides). Other useful thermoset polymers may include XLPE (cross-linked polyethylene), CPE (chlorinated polyethylene) and EPR (ethylene propylene). Another example of a non-conductive coating is a shellac coating. In another configuration, the non-conductive coating is a polymer that is selected from the group consisting of polyurethane, polyamide, polyester, polyester and polyimide. In one characterization, the coating has a thickness of at least about 5 pm.

[0066] In this configuration, the strength member may be interrogated by making electrical contact with both ends of each of the conductive wires and measuring the resistance of the conductive wires, e.g., measuring the voltage drop across the conductive wires. An anomaly (e.g., a deviation from a base value) in the resistance of one or more of the conductive wires will be indicative of a break in one or more of the conductive wires, and therefore indicative of a break in the strength member, e.g., in one or more of the strength elements. It is generally desirable that the conductive wires be selected such that the measurement does not require the use of a high voltage to reliably make the resistance measurement.

[0067] In the resistance method, the resistance is R=L/(oA), where L is the length of the wire, A is the cross-section area of the wire, and o is the electrical conductivity of the wire material. Using an Ohmmeter or similar device, the resistance may be measured along the wire. If the wire is broken, it is equivalent to reducing area A to zero and the wire is no longer conductive. Therefore, if the resistance increases dramatically, the strength element may be presumed to be damaged. The broken wire may not cause resistance to increase to infinity because the wire may still be electrically connected by carbon fiber, a weak conductor, in the matrix.

[0068] A current measurement may also be used. Ohm's law states that l=V/R, where I is current, V is voltage, and R is the resistance, and may also be written as l=oV/L A. If a constant voltage is applied at the ends of the wires, a current is generated, and the magnitude of the current may be measured. If the wire is damaged, its area A decreases so that current decreases, and in turn, a significant current reduction will be detected. For thin wires or low conductive materials, or if the length of the wire is very long, a high voltage may need to be applied to generate large current for an accurate measurement.

[0069] A voltage measurement may also be used. As noted above, Ohm’s law may be written in the form V=IL/oA. If a constant current is applied through the wire, a voltage may be measured at the ends of the wire. If the wire is damaged, its area A decreases so that voltage increases. Thus, significant voltage increase may indicate damage to the strength element.

[0070] In another embodiment, a technique such as ETDR (electrical time-domain reflectometry) method may be utilized to interrogate the strength element. Here, two or more conductive wires are embedded in the strength element and the distance between the two wires is relatively constant along the length of the strength element. When a ETDR sensing device is connected to the two wires and one of the two wires is broken, a pulse will be reflected to the ETDR, and the location of this reflection can be determined by ETDR so that the location of the damage may be identified. Specifically, ETDR measures reflections along a conductor. In order to measure those reflections, the ETDR transmits an incident signal into the conductor and detects reflections. If the conductor is of a uniform impedance and is properly terminated, there will be no reflections and the remaining incident signal will be absorbed at the far-end of the strength element by the termination. If there are impedance variations, then some of the incident signal will be reflected back to the source. The impedance of the discontinuity can be determined from the amplitude of the reflected signal, and the distance to the reflecting impedance can also be determined from the time that a pulse takes to return. One example of an ETDR technique is disclosed in US Patent No. 7,421 ,910 by Chen et al. which is incorporated herein by reference in its entirety.

[0071] In an alternative technique, a single conductive wire embedded in the strength element may be utilized, where the single conductive wire is electrically coupled to the outer conductor layer of the electrical cable (e.g., the stranded aluminum) to complete the circuit.

[0072] The foregoing electrical measurements may be measured by operatively contacting one or more conductors (e.g., wires or electrodes) to each end of the conductive wires, applying a voltage (e.g., a direct current voltage) and measuring the voltage drop across the wires, such as by using a multimeter. In one configuration, the strength member is a single element strength member having a plurality of conductive wires, and an electrode such as a single plate electrode is placed against the end of the strength member to simultaneously contact each of the conductive wires. In this regard, it may be advantageous to form smooth end surfaces on the strength element, e.g., by polishing the end surfaces, to facilitate electrical contact between ends of the conductive wires and the plate electrodes. Because the conductive wires are less brittle than the surrounding composite material, it may be possible to physically separate the wires from the strength element, e.g., by crushing the end of the strength element, leaving the conductive wires intact.

[0073] In one implementation, the interrogation technique includes placing a conductive structure on one end (e.g., a first end) of the strength member, where the conductive structure is configured to create a conductive pathway (e.g., a short circuit) among the conductive wires, e.g., among each of the conductive wires. For example, the conductive structure may comprise a planar body having a conductive metallic surface on at least one side of the planar structure. By placing the conductive surface against the end of the strength member, the end of each conductive wire will be in electrical communication with every other conductive wire. On the opposite end (e.g., a second end), electrical contact is made with each of the conductive wires individually or in pairs. In this manner, a property (e.g., electrical resistance) may be measured across any two conductive wires to determine if there is an anomaly across the two wires, e.g., if there is an unexpected increase in the measured resistance versus the expected resistance. This is an indication of possible damage to at least one of the two conductive wires. If confirmation is desired, or if it is desired to know if one or both of the conductive wires is compromised, the remaining pair combinations can be interrogated in a similar manner.

[0074] FIG. 15 illustrates a schematic cross-section of a strength element 1519 having four spaced apart conductive sensing wires, e.g., spaced apart by about 90° and substantially equidistant from a central axis of the strength element 1519. The four conductive sensing wires 1550a/1550b/1550c/1550d are disposed in a galvanic layer 1521 comprising glass fibers in a binding matrix. In one implementation of an interrogation technique, schematically illustrated in FIG. 16, a conductive plate 1560 is operatively contacted with (e.g., pressed against) one end of the strength element 1519 to contact an end of each of the conductive sensing wires. For example, the plate 1560 may be fabricated from copper. As illustrated in FIG. 16, the conductive plate 1560 is secured to the end of the strength element 1519 by a clamp structure 1560 that maintains the plate 1560 against the end of the strength element 1519, e.g., that slightly compresses the conductive plate 1560 against the end. On the opposite end of the strength element 1519, four conductive leads 1552a/1552b/1552d/1552c are operatively (e.g., electrically) connected to the four conductive sensing wires 1550a/1550b/1550d/1550c (FIG. 15). For example, the four conductive leads 1552a/1552b/1552d/1552c may be connected to the wires 1550a/1550b/1550d/1550c by welding, with a conductive adhesive, or by other means known to those skilled in the art.

[0075] As a result, the resistivity measured across any two conductive wires, i.e. , made using the conductive leads, will be indicative of the resistivity over a length equal to two times the length of the strength element 1519. If an anomaly is detected across, for example, opposite conductive wires 1550a and 1550c using leads 1552a and 1552c, this would be an indication of a fracture within the strength element 1519. If it is desired to know if one or both conductive wires 1550a and 1550c are fractured, or to confirm the result, other combinations of conductive wires may be interrogated. In this example, if an electrical measurement is made across conductive wires 1550a and 1550d, and no anomaly is detected, this would be an indication that the anomaly is in conductive wire 1550c. A further test across wire 1550b and wire 1550c, or wire 1550d and wire 1550c, would confirm this result.

[0076] The interrogation of the strength member including the conductive sensing wires may be carried out at any point in the life cycle of the strength member and/or of the electrical cable. The interrogation method may be implemented immediately after manufacture of the strength element, e.g., as the strength element is disposed on a spool. It may also be implemented after a stress-test (e.g., a bend test). It may be implemented after the fabrication of an end product (e.g., after stranding the strength element(s) with a conductor), and may be implemented after installation, e.g., installation of the overhead electrical cable, to ensure the structural integrity of the installed electrical cable.

[0077] As noted above, in a second configuration of this embodiment, the elongate sensing strands are selected to enable interrogation from outside of the strength element, e.g., by relative movement of an interrogation device along the length of the strength element(s), e.g., along a bare strength element or along an electrical cable including the strength element. In one example, the sensing strand comprises a radiopaque material, e.g., a material that is opaque to a selected incident radiation. In one configuration the radiopaque material is opaque to radio waves and/or x-ray radiation such that an x-ray emission and detection device can be used to detect a break or other anomaly in the radiopaque strand. The selected materials should have a relatively high radiopacity, particularly as compared to the other components of the overhead electrical cable such as aluminum, carbon fibers, glass fibers, and epoxy resins. Examples of such radiopaque materials that are useful for forming the radiopaque wires include, but are not limited to, lead-containing fibers, such as lead-containing glass fibers, and fibers coated in a radiopaque substance such as barium sulfate. The radiopaque wire may be fabricated entirely from a single radiopaque material, may be fabricated from two or more radiopaque materials, or may be fabricated from a radiopaque material and one or more non-radiopaque materials. For example, the radiopaque wire may comprise a radiopaque material core and an outer layer of a non-radiopaque material. In another example, the radiopaque wire may comprise an inner core of a non- radiopaque material and an outer layer of a radiopaque material. In another embodiment, the radiopaque wire may comprise one material phase (e.g., fibers or particulates) that is embedded in another material phase (e.g., a matrix phase).

[0078] The radiopaque wires may have a cross-sectional area (e.g., a diameter or thickness) that is sufficiently large to enable the reliable detection of a break in the wire using an x-ray device. For example, the radiopaque wire may have a substantially circular cross-section and a diameter of at least about 0.2 mm, such as at least about 0.4 mm. In another example, the radiopaque wire has a cross-sectional area that is sufficiently small that the wire does not occupy an appreciable volume of the strength element and thereby does not appreciably degrade the properties (e.g., the tensile properties) of the strength element. For example, the radiopaque wire may have a substantially circular cross- section and a diameter of not greater than about 2 mm, or not greater than about 1 mm.

[0079] As with the sensing strand configuration implementing conductive sensing wires, the interrogation of the radiopaque strands may be carried out at any point in the life cycle of the strength member and/or of the electrical cable. The interrogation method (e.g., x-ray interrogation) may be implemented immediately after manufacture of the strength element, e.g., as the formed strength element is disposed on a spool. It may also be implemented after or during a stress-test (e.g., a bend test). It may be implemented after the fabrication of an end product (e.g., after stranding the strength element(s) with a conductor), and may be implemented after installation, e.g., installation of the overhead electrical cable, to ensure the structural integrity of the installed electrical cable. In one implementation, the strength member is interrogated as the strength member is moving during a manufacturing process or quality control process. That is, as the strength member is moving, a stationary interrogation device can be placed in the pathway of the moving strength member to interrogate the strength member as it passes by/through the device. Thus, the interrogation device may be placed immediately after formation of the strength element and before the strength element is wrapped on a spool to determine if the strength element was damaged during manufacture. The interrogation device may be placed immediately after the roller in a roller bend test to determine if the strength element was damaged by passing over the roller. The interrogation device may be placed immediately after the stranding operation and before the spool upon which the electrical cable is wrapped to determine if the strength element was damaged during the stranding operation. The interrogation device may be placed immediately after the bull wheels during installation of the electrical line to determine if the strength element was damaged by the bull wheels during the installation. If it is desired to perform an interrogation after installation of the electrical line, it may be necessary to pass the inspection device along the electrical line, such as by mounting the device on a radio- controlled drone.

[0080] One advantage of the configuration implementing radiopaque strands is that the location of the fracture in the strength member can be readily identified, thereby enabling a rapid repair or other curative action to be made. For example, after installation of the electrical cable in the support towers (e.g., pylons), an x-ray interrogation device may be passed over the electrical cable, such as by using remote controlled drone, to rapidly interrogate the length of the electrical cable.

[0081] The strength elements described above may be fabricated by means known to those of skill in the art, including the methods described in the above-referenced U.S. patents. In one particular embodiment, the strength element is formed by pultrusion process whereby tows of continuous reinforcing fibers (e.g., carbon and glass fibers) are pulled through a binding matrix material (e.g., through an epoxy resin bath), which is subsequently cured to bind the fibers and form a fiber-reinforced composite. Sensing strands may be provided in continuous lengths (e.g., of many thousands of meters) on spools in a manner similar to fiber tows (e.g., carbon fiber tows and glass fiber tows).

[0082] Thus, one embodiment of the present disclosure is directed to a method for the manufacture of an elongate fiber-reinforced composite strength element that is configured for use in a strength member. The method may include the steps of forming an elongate fiber-reinforced composite having a longitudinal central axis and comprising a binding matrix and a plurality of reinforcing fibers disposed in the binding matrix. At least a first sensing strand is embedded into the fiber-reinforced composite, e.g., during the formation of the composite, such that the sensing strand extends from a first end of the composite to the second end of the composite. For example, the sensing strands may be integrated into a pultrusion process wherein the reinforcing fiber tows are impregnated with a resin which is cured form the binding matrix. Sensing strands may be provided on spools (bobbins) in a manner similar to the reinforcing fiber tows, and therefore can be pulled through a pultrusion system in a similar manner.

[0083] As is noted above, the sensing strands may be oriented linearly (e.g., co-linear with the central longitudinal axis of the strength element), or may be wrapped (e.g., helically wound) relative to the central longitudinal axis of the strength element.

[0084] After the formation (e.g., by pultrusion) of an adequate length of the fiber- reinforced composite, the composite is then cut proximate to the first end of the composite to form a first cut end surface, e.g., that is substantially flat. When the sensing strand is configured to be interrogated by passing a signal from one end of the strength element to the other (e.g., using conductive wires) the cut end may be polished to form an elongate fiber-reinforced composite strength element having a polished first end surface, the polished first end surface comprising fiber-reinforced composite and a first end of the conductive wires.

[0085] As is discussed above, the strength element may include one or a plurality (e.g., four or more) of the sensing strands, and the plurality of sensing strands may be configured (e.g., embedded within) the fiber-reinforced composite to enhance the probability of the sensing strands to become disrupted due to a fracture or other defect in the surrounding composite material. Thus, the foregoing method may include embedding a plurality of sensing strands, e.g., two or more sensing strands, as is described above.

[0086] In one embodiment, the manufacturer of the strength element may desire to “stress-test” the strength element before the strength element is shipped, e.g., before the strength element is shipped to a stranding facility for stranding of a strength member formed from the strength element with a conductor to form an electrical cable. Such a stress-test may include unwinding the strength element from one spool and winding it onto another spool, where the strength element is threaded through one or more wheels (e.g., small sheaves or rollers) between the two spools. Proper selection of wheel size and wheel placement causes a known stress to be applied to the strength element to confirm that the strength element does not fracture under that known stress, e.g., to confirm that there are no significant manufacturing defects in the strength element. Thus, according to one embodiment, the system and method for interrogation of the strength element is applied to the strength element after the stress test to determine if the strength element passed the stress test, e.g., to determine if the strength element fractured during the stress test. For example, the system and method can be applied after the stress-test when the strength element is wrapped on the second spool. In the case of radiopaque sensing strands, the strength element may be interrogated immediately after passing over the wheels and before being wrapped around the spool. If it is determined that the strength element passed the stress-test using the interrogation system and method, the spooled strength element may be shipped for stranding.

[0087] In one implementation, particularly where the sensing strands include conductive wires, the cutting step may be carried out in a manner that has a high probability of forming an orthogonal end surface as is discussed above. For example, the cutting step may include cutting the fiber-reinforced composite with a mechanically actuated (e.g., powered) cutting edge. A gritted cutting edge, one that cuts through a material primarily due to the presence of abrasive grit (i.e. , a high hardness particulate material) on the cutting edge, is generally preferred over a cutting edge that includes cutting teeth. Although the use of fine cutting teeth is not precluded for cutting the fiber- reinforced composite, it is believed that cutting teeth will leave a rougher surface that abrasive grit, resulting in a more difficult (e.g., more time consuming) polishing step after the cutting step. In one characterization, the gritted cutting edge comprises abrasive grit having a size that is super fine or coarser, e.g., a size of at least about 30 pm (about 600 grit), such as at least about 40 pm (about 360 grit) or even at least about 68 pm (about 220 grit). It will be appreciated that the selection of cutting grit may be made by considering the speed (e.g., rotational velocity) of the cutting edge and the desire for a rapid cut and polish, i.e. , a coarser grit may cut faster but may necessitate a longer subsequent polish time.

[0088] To form a substantially orthogonal end surface as described in certain characterizations above, the cutting step may include securing (e.g., mechanically securing) the first end of the fiber-reinforced composite such that the first end is substantially orthogonally disposed relative to the cutting edge (e.g., to the cutting blade) during the cutting step. For example, the fiber-reinforced composite may be mechanically clamped so that the composite is orthogonally disposed to the cutting edge and is not able to move off-axis in any appreciable manner during the cutting step.

[0089] After cutting, the end surface may be polished, if necessary, to provide a smooth end surface. For example, the polishing step may include polishing the cut end with a polishing surface. For example, the polishing surface may include abrasive grit having an ultrafine size, for example a grit size of not greater than about 162 pm (100 grit), such as not greater than about 100 pm (150 grit), or even not greater than about 25 pm (800 grit).

[0090] The method of forming the end surface of the strength element is described above as including mechanically cutting and polishing the end of the fiber-reinforced composite, e.g., using a cutting edge and a polishing surface. Flowever, it is contemplated that a strength element having the desired end surface may be formed by other means. For example, a water jet or a laser may be used to obtain an end surface having the desirable properties for effective electrical contact for certain interrogation systems and methods disclosed herein. [0091] In another embodiment, a system is disclosed for the interrogation of a strength element that includes an inner core of reinforcing carbon fibers and a galvanic (e.g., insulating) layer surrounding the carbon fiber core. According to this embodiment, a relatively high voltage is passed through the carbon fiber core. Because the carbon fiber core has a moderate resistivity, a voltage drop can be measured across the carbon fiber core. In one characterization, a voltage of at least about 1 kV, such as at least about 2 kV or even at least about 3 kV may be applied across the carbon core. However, the applied voltage should typically not exceed about 10 kV, such as not greater than about 8 kV. In one example, the applied voltage is at least about 4 kV and is not greater than about 6 kV. The voltage may be applied constantly or as a voltage pulse. Once the voltage is applied, an anomaly in the strength element may be detected by detecting a voltage drop, an increase in resistance, or similar methods.

[0092] This embodiment may be implemented with or without sensing strands being disposed in or around the strength element. In one implementation, the strength element includes ferromagnetic wires (e.g., steel or nickel wires) extending through the strength element, and the interrogation method includes the detection of eddy currents that arise where the ferromagnetic wires have been damaged. Other useful materials may include iron wires, cobalt wires, iron-laden ceramic fibers, and the like.

[0093] While various embodiments of a system, method and tools for the interrogation of a strength element have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present disclosure.