Patent Description:
This application claims the benefit of co-pending, commonly assigned <CIT>.

The present disclosure relates to damage detection systems and methods for detecting damage in fastened structures and, in particular, to a non-destructive and non-invasive systems for detecting damage in fastened joints that maintains the integrity of a structure without unreasonable disassembly and inspection cycles.

Two or more structures have, in some instances, been coupled together using a fastener at a joint (e.g., a rivet, or the like). Generally, the most common places for damage to initiate in structures are in the fastened joints. Holes drilled for the fasteners create stress concentrations, and minute defects created in the drilling process create ideal locations for cracks to nucleate. Corrosion can be accelerated in the areas of the joint due to the presence of dissimilar contacting materials, breaks in protective coatings at drilled holes, and moisture trapped between layers. These factors can be further exasperated by the fact that fastened joints are some of the most difficult areas for inspection. Damage is often hidden under fastener heads or between layers, and visual inspection is therefore not viable.

Traditional inspection methods, such as Eddy currents or ultrasonic detection, are typically hindered due to the complex geometry of fasteners and multiple stacked materials. Sensors or transducers have been placed along the structure surface, within the drilled hole, or under a fastener (e.g., between the fastener and structure). However, by placement of the sensor or transducer in such areas, the joint between the fastener and structure is modified and can lead to additional failure of the components.

Thus, a need exists for a damage detection system for fastened joints that maintains the integrity of the structure without unreasonable disassembly, without inspection cycles, and without modification of the fastener installation. These and other needs are addressed by the damage detection systems of the present disclosure. <CIT> discloses a threaded fastener incorporating a removable ultrasonic transducer for obtaining preload measurements as well as other measurements for quality control inspection or for monitoring purposes. The transducer may be removed for repair or replacement purposes. <CIT> discloses a threaded bolt having an opening in either its head or its opposite end with an ultrasonic transducer fixedly secured therein for use in obtaining preload measurements as well as other measurements for quality control inspection or for monitoring purposes. <CIT> discloses a load indicating device and a method of using a load indicating device for monitoring the deformation in and imparting torque to a load bearing member are disclosed and claimed. An ultrasonic transducer, grown on one surface of a load bearing member, such as a fastener, is used to determine the length, stress or other tensile load dependent characteristic of the member using ultrasonic techniques. <CIT> discloses a method of measuring the load in a member subjected to longitudinal stress, a load measuring device and a fastener tightening device using the method of measuring, a load indicating member and a load indicating fastener for use in conjunction with the method of measuring, a method of making the load indicating fastener, a method of tightening the load indicating fastener and a transducer for instrumenting a load bearing member. A thin piezoelectric sensor consisting of a piezoelectric film sandwiched between two thin electrodes is permanently mechanically and acoustically coupled to the upper surface of a member and is used to determine the length, tensile load, stress, or other tensile load dependent characteristic of the member by ultrasonic techniques. <CIT> discloses an apparatus and method of mounting a stress transducer to a fastener includes securing the transducer to a weld insert, and welding the insert to the fastener. <CIT> discloses an improved acoustic transducer, transducerized fastener and method of manufacture are presented wherein a piezoelectric crystal and a contact/retaining button assembly are permanently attached to a fastener. The contact/retaining button assembly has an insulator board sandwiched between upper and lower electrodes, with at least one plated thru-hole for electrical contact between the electrodes. <CIT> discloses a system and method to determine stress within the load bearing members of an aircraft, machine or structure in order to improve their design, safety and efficiency as well as enhancing their operation. Load or stress is calculated from signals generated in fasteners fitted with piezoelectric crystals. Rather than indirect stress indication through approximations from accelerometers, optical fibers, position sensors, strain gages and the like, the system and method calculates load or stress from sensors installed directly into load bearing elements. The system and method can perform stress indicating function in machines and structures such as, but not limited to, aircrafts, buildings, bridges, power generating stations, ships and engines. <CIT> discloses that the health of a structural joint clamped by fasteners is monitored by directly measuring the preload of the fasteners. The preload is measured by a transducer on the fastener and electronically transmitted to a monitoring station where the preload values may be analyzed to assess the health of the joint. <CIT> discloses a computer implemented method for analyzing the health of a structure by measuring the value of preload on each of a plurality of fasteners installed on the structure and correlating the measured values of preload with a set of specifications. A report is generated for each of the fasteners representing the results of the correlations. Groups of the fastener reports are used to infer the health of the structure.

In accordance with embodiments of the present disclosure, an exemplary transducer assembly for damage or flaw detection in a fastened structure, which is defined in the independent apparatus claim <NUM>, is provided. The transducer assembly includes a fastener and an electromechanical unit. The fastener includes a cavity disposed at one end of the fastener. The electromechanical unit includes a piezoelectric element and a substrate. The piezoelectric element is disposed outside of the cavity of the fastener and has a bottom surface being joined to a top surface of a top section of the substrate disposed outside of the cavity. A bottom section of the substrate, which protrudes or extends from a bottom surface of the top section of the substrate, is at least partially inserted into and mechanically coupled within the cavity of the fastener.

The substrate may aid in orienting and aligning the piezoelectric element relative to the fastener when the electromechanical unit is inserted into the cavity of the fastener.

The electromechanical unit can be configured to be driven or actuated by an electrical stimuli, and configured to output a response signal that can be indicative of or used to determine whether at least a portion of the fastened structure (e.g., structural and/or non-structural components) or the fastener is damaged. A data acquisition device can be electrically coupled to the electromechanical unit. The data acquisition device can receive the response signal output from the piezoelectric element and digitizes the response signal. In one embodiment, the data acquisition device can be physically packaged together with the electromechanical unit (e.g., in a shared housing). In one embodiment, the cavity of the fastener can be non-circular. In such an embodiment, the substrate can be shaped to substantially correspond to the shape of the cavity.

In one embodiment, the fastener can be a threaded fastener including a shaft, a head at a first end of the shaft, a threaded portion at a second end of the shaft opposite the first end, and the cavity of the fastener is in the shaft accessible from the second end. The threaded fastener can include a nut having a threaded body cooperatively engageable with the threaded portion of the threaded fastener. The threaded portion of the threaded fastener can include a terminal end at the second end that includes a smooth lateral surface extending circumferentially about the terminal end and substantially surrounding the cavity.

In one embodiment, the fastener can be a threaded fastener including a shaft, a head at a first end of the shaft, a threaded portion at a second end of the shaft opposite the first end, and the cavity of the fastener in the head accessible from the first end. The threaded fastener can include a nut having a threaded body cooperatively engageable with the threaded portion of the threaded fastener.

The fastener may include a distal end and a proximal end. The cavity may be formed at the distal end of the fastener and may extend towards the proximal end of the fastener. The piezoelectric element may be disposed outside of the cavity and adjacent to the distal end of the fastener.

A detection system for monitoring damage or flaws in a fastened structure may include a data acquisition device, and the transducer assembly.

In accordance with embodiments of the present disclosure, an exemplary method of damage detection of a fastened structure, which is defined in the independent method claim <NUM>, is provided. The fastened structure includes a fastener joining two or more structural components. The fastener includes a cavity. The method includes inserting a substrate of an electromechanical unit at least partially into the cavity of the fastener post-installation of the fastener relative to the two or more structural components. The electromechanical unit includes a piezoelectric element coupled to the substrate. The piezoelectric element is disposed outside of the cavity of the fastener. A top surface of a top section of the substrate to which a bottom surface of the piezoelectric element is joined is disposed outside of the cavity. A bottom section of the substrate extends or protrudes from a bottom surface of the top section of the substrate and is at least partially inserted into and mechanically coupled within the cavity of the fastener. The method also includes mechanically coupling the substrate to the fastener. The method also includes exciting the electromechanical unit by an electrical stimuli to mechanically stimulate the fastened structure. The method also includes measuring an output signal associated with excitation of the electromechanical unit at a data acquisition device to determine whether at least a portion of the fastened structure or the fastener is damaged.

The substrate may mechanically couple the piezoelectric element to the fastener. The method can include wirelessly transmitting a digitized output of the output signal from the electromechanical unit. Power for the electrical stimuli of the electromechanical unit can be harvested at an energy harvesting device by at least one of radio frequency energy, inductive energy, or mechanical energy.

Other objects and features will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed as an illustration only and not as a definition of the limits of the invention.

To assist those of skill in the art in making and using the disclosed damage detection system, reference is made to the accompanying figures, wherein:.

Exemplary embodiments of the damage detection system (e.g., electromechanical unit) disclosed herein include a sensor or transducer assembly incorporated into a fastener to determine whether the fastener and/or the surrounding structures are damaged. Rather than placing the transducer(s) on one of the skin layers of the structure or under a fastener, the transducer is mounted inside of the fastener itself. Neither the fastener itself nor the fastener installation is modified to accommodate the detection system. In accordance with exemplary embodiments, the detection system can be mounted inside of the fastener post-installation. As an example, for bolted joints, the detection system can be fit inside of the fastener head. As a further example, for a riveted joint, the detection system can be mounted within the recess (e.g., hex recess) located at the bottom of the pin or shaft. In some embodiments, the detection system can be formed or machined to substantially match the geometry of the opening in the fastener, and the detection system can be maintained coupled to the fastener via, e.g., a friction fit, epoxy, combinations thereof, or the like. The detection system provides mechanical and acoustic coupling with the surrounding structure once inserted or bonded therein. A sensor (e.g., a piezoelectric wafer), can be crystal bonded to the machined part and can serve as an ultrasonic transducer to send and/or receive wave energy (e.g., vibrations) during ultrasonic wave propagation across the structure and fastener.

The combination of the sensor and the machined part (e.g., substrate) can form an electromechanical unit or transducer that can be incorporated into a fastener to transform the fastener into an instrumented fastener or transducer. In some embodiments, the electromechanical unit can be formed by the sensor without the machined part (e.g., substrate). Different ultrasonic inspection methods can be used with embodiments of the detection system, including bulk wave inspection, guided wave inspection, modal analysis, acoustic emission, impedance monitoring, combinations thereof, or the like (e.g., external or remote propagation sources). In some embodiments, propagation of guided waves, shear waves, bulk waves, Rayleigh waves, impedance response, frequency/modal response, combinations thereof, or the like, can be used to excite the disclosed transducers. In some embodiments, the inspection can encompass only the area directly surrounding a single fastener (and the fastener itself). In some embodiments, the inspection can encompass several fasteners in a row at the same joint. In some embodiments, the inspection can encompass the area between two rows of fasteners. In some embodiments, combinations of transducers can be used to serve as actuators and sensors, and beamforming can be achieved to scan large areas for damage detection. By virtue of the case that the fasteners themselves are being excited, the detection system can be particularly sensitive to changes directly surrounding the fasteners in areas that are hidden and traditionally difficult to inspect. Further, ultrasonic energy is able to penetrate and propagate through multiple layers in contact with the fastener, and through air gaps within the fastener to reach the detection system. In some embodiments, the detection system can be permanently attached to digitizing elements in a wired or wireless network for on-demand or real-time health and usage monitoring (e.g., a health and usage monitoring system (HUMS) or structural health monitoring (SHM) system), or can be directly queried through a connector or wirelessly for ad-hoc inspections.

Turning to <FIG>, a perspective view of fasteners 100a-d usable with the exemplary damage detection system is provided. In some embodiments, the fasteners 100a-d can be, e.g., HI-LOK™ rivets, or the like. Each fastener 100a-d includes a pin 102a-d configured to be passed through a hole in a structure, and a collar 104a-d (e.g., a nut) configured to be threadingly coupled to the pin 102a-d to fasten two or more surfaces between the pin 102a-d and collar 104a-d. As will be discussed in greater detail below, during assembly, a wrenching element of each collar 104a-d can be torqued off after reaching a predetermined torque or torque range, thereby leaving the bottom section of the collar 104a-d permanently coupled to the pin 102a-d and the wrenching element decoupled from the collar 104a-d.

While static SHM methods (e.g., guided wave, eddy current, fiber optics, or the like) have generally shown good results for simple structures, no method to date has been successful for damage detection in complex joints. An example of this type of joint is provided in <FIG>, and can include three pieces of dissimilar metal structures or skins <NUM>, <NUM>, <NUM> in a stacked configuration joined by fasteners 100c (e.g., HI-LOK™ rivets). Traditional damage detection methods (such as guided wave) can find damage on just the metal skin to which the sensor is affixed. Damage in hidden layers is therefore missed using traditional damage detection methods. Guided wave sensors can use piezoelectric wafers that excite ultrasonic waves in elastic solids; however, these ultrasonic waves are unable to traverse other layers in a stacked joined structure because, while the ultrasonic wave (e.g., vibrations) may propagate through tight fasteners in the same plane, the ultrasonic waves are unable to turn tight "corners.

<FIG> show diagrammatic views of the steps involved in installing a fastener <NUM> to couple two structures <NUM>, <NUM> relative to each other. The fastener <NUM> includes a pin <NUM> and collar <NUM>. The collar <NUM> (e.g., a nut) includes a main body section <NUM> having a threaded interior and an element <NUM> configured to be torqued off to decouple the element <NUM> relative to the main body section <NUM> upon reaching a predetermined torque or torque range during installation of the collar <NUM>. The pin <NUM> includes a central body portion <NUM> (e.g., a shaft) having a threaded portion or section <NUM> on one end (e.g., a distal end, a second end, or the like) and a tapered cap <NUM> (e.g., a head) on the opposing end (e.g., a proximal end, a first end, or the like), where the tapered cap <NUM> typically has a diameter that is greater than the diameter of the central body portion <NUM>. The cap <NUM> includes a substantially flat or planar surface extending circumferentially around the body portion <NUM> and configured to be placed against a substantially flat surface of a first structural sheet or layer (e.g., structure <NUM>). The main body section <NUM> of the collar <NUM> includes a substantially flat or planar surface forming a bottom end of the collar <NUM> and configured to be placed against a substantially flat surface of a second structural sheet or layer (e.g., structure <NUM>). The endpoint of the pin <NUM> (e.g., at the distal end) includes a hole or cavity <NUM> (e.g., a drive recess or cavity) for insertion of a tool (e.g., a hex-shaped opening for a hex key). The endpoint of the pin <NUM> at the distal end defines a terminal end having a smooth lateral surface extending circumferentially about the terminal end (e.g., surrounding the hole <NUM>). Although illustrated as a hex-shaped opening, it should be understood that the hole <NUM> can be, e.g., circular, non-circular, square, star, oval, angled, X-shaped (e.g., Phillips head), or the like. Although referred to as a hole, it should be understood that any type of recess or cavity of a fastener can be used to incorporate an electromechanical unit, as discussed below. The pin <NUM> can include an opening at the distal end defining the entrance to the hole <NUM>, and the hole <NUM> can define a recessed volume within the pin <NUM> that extends only a partial distance into the pin <NUM> (see, e.g., <FIG>).

The fastener <NUM> can be installed by pressing or hammering the threaded pin <NUM> into an interference-machined hole <NUM> extending through the structures <NUM>, <NUM>. The collar <NUM> (e.g., nut) is threaded onto the pin <NUM>, such that the threaded interior of the collar <NUM> engages the threaded section <NUM> of the pin. The collar <NUM> is tightened using a ratchet until the wrenching element <NUM> breaks off of the main body section <NUM> of the collar <NUM> at a preset torque level. A hex-key can be inserted into the hole <NUM> during installation of the collar <NUM> on to the threaded pin <NUM> to prevent rotation of the pin <NUM> as the collar is threaded onto the pin <NUM>. After installation, the pin <NUM> and the main body section <NUM> of the collar <NUM> remain coupled together to maintain coupling of the structures <NUM>, <NUM> to each other. As will be discussed in greater detail below, embodiments of the electromechanical unit (e.g., one or more sensors) can be mounted inside the volume defined by the hole <NUM> in the pin <NUM> post-installation such that the fastener can be transformed into an instrumented fastener or transducer. As the electromechanical unit can be mounted inside the volume defined by the hole <NUM>, the process of transforming the fastener into an instrumented fastener or transducer does not require any modification of the fastener <NUM> itself or the fastener <NUM> installation process. For example, a special washer is not needed for installation and the detection system is not placed between the collar <NUM> and the structure <NUM>, <NUM>. The proposed position of the electromechanical unit within the hole <NUM> of the pin <NUM> advantageously negates any impact on fastener integrity and does not necessity any additional certification tests.

<FIG> shows different, non-limiting examples of pin cap <NUM> configurations for fastener <NUM>. For example, the cap 124a can include a protruding shear configuration, the cap 124b can include a protruding tension configuration, the cap 124c can include a <NUM>° flush shear configuration, the cap 124d can include a <NUM>° flush crown shear configuration, the cap 124e can include a <NUM>° flush shear configuration, the cap 124f can include a <NUM>° flush crown shear configuration, the cap <NUM> can include a <NUM>° flush MS20426 shear configuration, and the cap <NUM> can include a <NUM>° flush MS24694 tension configuration.

<FIG> shows different, non-limiting examples of collar <NUM> configurations for fastener <NUM>. For example, the collar 130a defines a standard basic collar, the collar 130b defines a standard self-aligning collar, and the collar 130c defines a standard self-sealing collar.

<FIG> are detailed side cross-sectional and top views of fastener <NUM> configured to receive embodiments of the electromechanical unit. The hole <NUM> at the end of the pin <NUM> can define a depth <NUM> measured from the distal end of the pin <NUM> in a direction towards the proximal end of the pin <NUM> along a central or longitudinal axis <NUM> of the central body portion <NUM> and an overall inner diameter or inner width <NUM> measured perpendicularly relative to the center or longitudinal axis <NUM>. In some embodiments, based on the tool used to machine the hole <NUM> in the pin <NUM>, the bottom of the hole <NUM> can include a dome-shaped tapered section <NUM> extending deeper than the depth <NUM> along the center or longitudinal axis <NUM> towards the proximal end of the pin <NUM>. The element <NUM> of the collar <NUM> can include a hole <NUM> formed therein and extending through the element <NUM> to align with the hole <NUM> in the pin <NUM> when the collar <NUM> is engaged with the pin <NUM>. The inner diameter <NUM> of the hole <NUM> can be dimensioned greater than the inner diameter or inner width <NUM> opening/hole <NUM> to permit insertion of a hex key through the hole <NUM> and into hole <NUM>. Thus, during installation, a hex key is inserted into the holes <NUM> and <NUM> to maintain the position of the pin <NUM> (e.g., to prevent the pin <NUM> from turning or shifting as the collar <NUM> is being threaded onto the pin <NUM>), and a wrench is used to thread the collar <NUM> onto the pin <NUM> until the predetermined torque is reached at which point the element <NUM> breaks off from the collar <NUM> to decouple the element <NUM> from the main body section <NUM>.

<FIG> are microscope images of the hole <NUM> of the pin <NUM>. The hole <NUM> can be formed during fabrication of the pin <NUM> by drilling a circular hole of approximately the correct diameter (e.g., similar to inner diameter or inner width <NUM>), and a stamping operation can be used to create flat sidewalls within the hole <NUM> about the center axis (e.g., to create a hexagonally-shaped interior volume) for the hex key. The result of this process, however, is that metal shavings from the sidewalls can be driven to the bottom of the hole <NUM> (e.g., the tapered section <NUM>), creating a "false" bottom that is not solid. In some embodiments, prior to installation of the electromechanical unit within the hole <NUM>, epoxy can be introduced into the bottom of the hole <NUM> to bond the metal shavings and/or fill any air pockets beneath the shavings to reduce the effect of the shavings and/or any air pockets on passage of ultrasonic energy through the pin <NUM>. In some embodiments, the electromechanical unit can be introduced into the hole <NUM> without placing epoxy at the bottom of the hole <NUM>, with the substrate of the electromechanical unit packed sufficiently into the hole <NUM> to prevent or reduce any effect on passage of ultrasonic energy (e.g., a friction or press fit within the hole <NUM>).

<FIG> are diagrammatic, top and cross-sectional views of an exemplary electromechanical unit <NUM> (hereinafter "unit <NUM>") (e.g., a structural health monitoring system, a damage detection system, or the like), which can be incorporated into and coupled with the fastener <NUM>. Although fastener <NUM> is shown, it should be understood that the electromechanical unit <NUM> can be incorporated into any type of recess or cavity in a fastener (e.g., a recess or cavity in the head of a Phillips screw). For example, a Phillips head screw can be used with a nut to assemble structures, and the unit <NUM> can be incorporated into the cavity of the screw head. In some embodiments, beamforming sensing and actuating elements can be incorporated into existing fasteners <NUM>. In some embodiments, non-traditional excitation sources and sensing elements can be used. The unit <NUM> can have a "nail-type" configuration. First, the fastener <NUM> can be used to couple structures 142a-c together, and the element <NUM> of the fastener <NUM> can be detached upon reaching the predetermined torque. In some embodiments, the installation process of the fastener <NUM> can include placement of a washer <NUM> between the collar <NUM> and the top surface of the structure 142a. Use of the washer <NUM> is optional and is not necessitated for installation of the unit <NUM>.

The unit <NUM> can include a sensor element or electromechanical device <NUM> coupled to a substrate <NUM>. In some embodiments, the sensor element <NUM> can be a piezoelectric element. The piezoelectric element can be in any form, e.g., circular, annular, hexagonal, square, rectangular, or the like. The width or diameter of the sensor element <NUM> can be dimensioned greater than the inner width or inner diameter <NUM> of the hole <NUM>. The piezoelectric element can defines substantially flat or plate-like configuration having uniform top and bottom surfaces. In some embodiments, the piezoelectric element can define nonuniform top and/or bottom surfaces. For example, the piezoelectric element can include a hole in the middle of the piezoelectric element for coupling of an electrode to the bottom surface, and a "bullseye" electrode pattern on the top surface. In some embodiments, the piezoelectric element can have a diameter of approximately <NUM> inches (<NUM>) to approximately <NUM> inches (<NUM>) or can have a diameter of approximately <NUM> inches (<NUM>), and a thickness of approximately <NUM> inches (<NUM>) to approximately <NUM> inches (<NUM>) or a thickness of approximately <NUM> inches (<NUM>).

The substrate <NUM> can be fabricated from, e.g., metal, ceramic, plastic, or the like. The substrate <NUM> can define a substantially T-shaped side profile, and can include a horizontal or top section <NUM> (e.g., a cap) and a vertical or bottom section <NUM> (e.g., a shaft or an extension). The bottom section <NUM> protrudes or extends from a bottom surface of the top section <NUM> in a substantially perpendicular manner. The top surface and the bottom surface of the top section <NUM> can be substantially flat and uniform (except for the bottom section <NUM> extending therefrom). In some embodiments, the top section <NUM> can define, e.g., a circular, hexagonal, square, rectangular, or the like, configuration when viewed from the top.

For example, as shown in <FIG>, the top section <NUM> can define a hexagonal configuration when viewed from the top, while the sensor element <NUM> can define a circular configuration when viewed from the top. In some embodiments, the configuration of the top section <NUM> of the substrate <NUM> and the sensor element <NUM> can correspond, e.g., the top section <NUM> and the sensor element <NUM> can each have a circular <NUM>, a hexagonal, square, rectangular, or the like, configuration. The width or diameter of the top section <NUM> can be dimensioned greater than the width or diameter of the sensor element <NUM> to ensure that the entire bottom surface of the sensor element <NUM> is bonded to the top surface of the top section <NUM>. In some embodiments, the width or diameter of the top section <NUM> can be dimensioned smaller than the width or diameter of the collar of the fastener <NUM> to prevent extension of the unit <NUM> beyond the edges of the fastener <NUM>. In some embodiments, the width or diameter of the top section <NUM> can be dimensioned substantially equal to or greater than the width or diameter of the collar of the fastener <NUM>.

The bottom section <NUM> (e.g., extension) can define a length substantially corresponding with the depth <NUM> of the hole <NUM> in the pin <NUM>. The length of the bottom section <NUM> as measured from the bottom surface of the top section <NUM> to a bottom or end <NUM> of the bottom section <NUM> can be selected based on the type of fastener <NUM> being used. Thus, when the bottom section <NUM> is inserted into the hole <NUM>, the bottom surface of the top section <NUM> can be positioned adjacent to or abutting the distal end of the pin <NUM>. In some embodiments, the bottom section <NUM> can define a substantially cylindrical shape with a circular cross-section. In some embodiments, the bottom section <NUM> can define a hexagonal cylinder with a hexagonal cross-section corresponding with the hexagonal hole <NUM>. In some embodiments, the bottom or end <NUM> of the bottom section <NUM> can be substantially flat (as shown in <FIG>). In some embodiments, the bottom or end <NUM> of the bottom section <NUM> can be machined to match the dome-shaped cavity or tapered section <NUM> at the bottom of the hole <NUM>.

In some embodiments, the bottom section of the substrate <NUM> can have a length of approximately <NUM> inches (<NUM>) to approximately <NUM> inches (<NUM>) or can have a length of approximately <NUM> inches (<NUM>). For embodiments in which the bottom section <NUM> is formed as a hexagonal cylinder, the bottom section <NUM> can have flat sides, each of which can be dimensioned to be approximately <NUM> inches (<NUM>) to approximately <NUM> inches (<NUM>) or approximately <NUM> inches (<NUM>). For embodiments in which the top section <NUM> (e.g., cap) has a hex-shape with flat sides/edges, each of the sides/edges can be dimensioned to have a width of approximately <NUM> inches (<NUM>) to approximately <NUM> inches (<NUM>) or have a width of approximately <NUM> inches (<NUM>) and to have a thickness of approximately <NUM> inches (<NUM>) to approximately <NUM> inches (<NUM>) or have a thickness of approximately <NUM> inches (<NUM>). However, it should be understood that the dimensions of the sensor element <NUM> and substrate <NUM> can be selected based on the type of fastener used.

The dimensions of the bottom section <NUM> can be selected such that the bottom section <NUM> can be press fit into the hole <NUM>, such that friction between the bottom section <NUM> and the sidewalls of the hole <NUM> can maintain coupling between the unit <NUM> and the fastener <NUM> without the use of epoxy or other substance. The friction fit can also maintain tight contact between the unit <NUM> and the fastener <NUM> to prevent air gaps that may affect ultrasonic signal travel and the readings of the ultrasonic signal during testing of the joint. In some embodiments, epoxy can be introduced into the hole <NUM> to couple the unit <NUM> to the fastener <NUM>. The epoxy can ensure that the unit <NUM> remains in place, and reduces air gaps between the hole <NUM> and the unit <NUM>. For example, the epoxy can be introduced into the hole <NUM> sufficiently to remove all air gaps within the hole <NUM> after insertion of the unit <NUM>. In some embodiments, an embodiment of the unit <NUM> can be capable of wirelessly transmitting data from the sensor element <NUM>. For example, the sensor element <NUM> can include a radiofrequency transmitter and/or receiver <NUM> to facilitate wireless data transfer between the sensor element <NUM> and a receiving unit external to the sensor element <NUM> and the fastener <NUM>. In such embodiments, only the sensor element <NUM> and the substrate <NUM> can be incorporated into the fastener <NUM>. Alternatively or in addition, in some embodiments, a wire can be coupled to the sensor element <NUM> to transmit data from the sensor element <NUM> to a receiving unit external to the fastener <NUM>. Coupling of the sensor element <NUM> with the substrate <NUM> ensures the orientation or level position of the sensor element <NUM> relative to the fastener <NUM>, by providing a substantially level surface against which the sensor element <NUM> is mounted. The sensor element <NUM> can be oriented to be disposed at or substantially at the midpoint of the fastener <NUM>.

<FIG> are diagrammatic, top and cross-sectional views, respectively, of an exemplary electromechanical unit <NUM> (hereinafter "unit <NUM>") (e.g., a structural health monitoring system, a damage detection system, or the like) incorporated into and coupled with an embodiment of the fastener <NUM>. The unit <NUM> can be in a "plate-type" configuration. The unit <NUM> can include a sensor element or electromechanical device <NUM> coupled to a substrate <NUM>. The sensor element <NUM> and substrate <NUM> can be substantially similar to the sensor element <NUM> and substrate <NUM>, except for the distinctions noted herein.

In some embodiments, the sensor element <NUM> can be a piezoelectric element. The piezoelectric element can be in any form, e.g., circular, hexagonal, square, rectangular, or the like. The width or diameter of the sensor element <NUM> can be dimensioned smaller than the inner width or inner diameter <NUM> of the hole <NUM>, such that the unit <NUM> can be placed within the hole <NUM>. The piezoelectric element can define a substantially flat or plate-like configuration having uniform top and bottom surfaces. In some embodiments, the piezoelectric element can have a diameter of approximately <NUM> inches (<NUM>) to approximately <NUM> inches (<NUM>) or can have a diameter of approximately <NUM> inches (<NUM>) and can have a thickness of approximately <NUM> inches (<NUM>) to approximately <NUM> inches (<NUM>) or can have a thickness of approximately <NUM> inches (<NUM>). In some embodiments, an embodiment of the unit <NUM> can be capable of wirelessly transmitting data from the sensor element <NUM>. For example, the sensor element <NUM> can include a radiofrequency transmitter and/or receiver <NUM> to facilitate wireless data transfer between the sensor element <NUM> and a receiving unit external to the sensor element <NUM> and the fastener <NUM>.

The substrate <NUM> can similarly define a substantially flat or plate-like configuration having uniform top and bottom surfaces. The width or diameter of the substrate <NUM> can be dimensioned slightly greater than the width or diameter of the sensor element <NUM> to ensure that the bottom surface of the sensor element <NUM> is bonded to the top surface of the substrate <NUM>. The width or diameter of the substrate <NUM> can be dimensioned smaller than the inner width or inner diameter <NUM> of the hole <NUM>, such that the unit <NUM> (e.g., the entire unit) can be placed and contained within the hole <NUM>. In some embodiments, the dimensions of the substrate <NUM> can be selected such that a friction fit is used to maintain the unit <NUM> within the hole <NUM>. In some embodiments, epoxy can be introduced into the hole <NUM> to ensure the position of the unit <NUM> is maintained. Thus, rather than filling the entire hole <NUM> with epoxy (as performed in <FIG>), epoxy is only introduced into the bottom of the hole <NUM> to sufficiently couple the bottom of the unit <NUM> within the hole <NUM>. In some embodiments, the bottom surface of the substrate <NUM> can be machined to correspond with the tapered section <NUM> at the bottom of the hole <NUM>. In some embodiments, the substrate <NUM> can define a hex-shape, with flat sides/edges dimensioned to have a width of approximately <NUM> inches (<NUM>) to approximately <NUM> inches (<NUM>) or a width of approximately <NUM> inches (<NUM>) and to have a thickness of approximately <NUM> inches (<NUM>) to approximately <NUM> inches (<NUM>) or a thickness of approximately <NUM> inches (<NUM>). While <FIG> illustrate an embodiment of the unit <NUM> including the substrate <NUM>, exemplary embodiments of the unit <NUM> can be devoid of a substrate such that the sensor <NUM> without the substrate <NUM> is inserted into the cavity.

In some embodiments, a substrate having a hex-shaped top section or cap can be used. In some embodiments, a substrate having a cylindrical top section or cap can be used. In some embodiments, a substrate having a hex-shaped bottom section or extension can be used. In some embodiments, a substrate having a cylindrical bottom section or extension can be used. In some embodiments, a flat bottom section of the substrate extension can be used. In some embodiments, a dome-shaped bottom section of the substrate can be used to substantially fill the volume at the bottom of the pin hole. In some embodiments, sensor elements having an overall width or diameter of approximately <NUM> inches (<NUM>) or less can be bonded onto a small delay line substrate. In some embodiments, sensor elements having an overall width or diameter of approximately <NUM> inches (<NUM>) or less can be bonded onto a cap on the outside of the collar head. In some embodiments, the sensor element can be cast to have a substantially complementary geometry to the pin hole, thereby fitting the sensor element into the, e.g., hex cavity, without the use of a delay line substrate.

In some embodiments, a hex-shaped bottom section of the substrate can provide improved mechanical and ultrasonic coupling to the fastener based on the tighter fit of the substrate within the hole. In some embodiments, a cylindrical bottom section of the substrate can be used for instances in which the dimension of the pin hole does not allow a more precisely machined part. In some embodiments, the material used for the substrate can match the material of the pin and/or collar of the fastener. For example, <NUM> steel substrates can be used to match the acoustic impedance of a fastener formed of <NUM> steel for maximum ultrasonic energy propagation. In some embodiments, a <NUM> copper substrate can be used to reduce the overall stiffness for exciting modes. In such embodiments, the acoustic impedance of the material of the substrate would be at a value between the acoustic impedance of the piezoelectric material and the steel material of the fastener. Thus, in some embodiments, the material of the substrate can be selected to have an acoustic impedance between the acoustic impedance of the piezoelectric material and the material of the pin and/or collar of the fastener.

As will be discussed in greater detail below, the electromechanical units can be used to detect cracks, corrosion or damage hidden in-between layers of fastened joints. Although the term electromechanical unit is used herein to refer to the combination of the piezoelectric element and substrate, in some embodiments, only the piezoelectric element can be used as the electromechanical unit. The piezoelectric wafer element is bonded to a piece of metal (e.g., a substrate), and then bonded at least partially within the fastener. The substrate can assist in aligning the piezoelectric element relative to the fastener, and can act as an amplifier for the input/output signals. A sinusoidal voltage is directed or applied into the piezoelectric wafer element to turn the entire fastener into an inspection device, serving as either an ultrasonic sensor or receiver. Because an electrical stimulus is applied to the piezoelectric element, the assembly is referred to herein as the electromechanical unit. The combination of the electromechanical unit with the fastener can be referred to herein as the transducer assembly.

Electrically stimulating the piezoelectric element in turn mechanically stimulates the metal substrate which, in turn, mechanically stimulates the fastener which then in turn mechanically stimulates the fastened structure. The measured or received output from the piezoelectric element can be an analog voltage that can be digitized to determine the state of the structure. The voltage signal can be digitized by an analog-to-digital converted (e.g., of a data acquisition device) and sent to a processor to pass through an algorithm to infer if the structure/fastener is damaged. The damage detection system can include the transducer assembly, the data acquisition device, memory, a processor, communication chip (wired or wireless) and an antenna. The damage detection system can be locally attached to a single sensor (e.g., within an inch or <NUM>) or more remotely attached to multiple transducer assemblies (e.g., each several inches or centimeters away).

<FIG> show perspective and top views of a fastener having a collar <NUM> (e.g., nut) and pin <NUM> and damage detection units <NUM>, <NUM>. Particularly, <FIG> shows the collar <NUM> and the pin <NUM>, <FIG> shows an embodiment of the unit <NUM> having the sensor element <NUM> and the substrate <NUM>, and <FIG> shows an embodiment of the unit <NUM> coupled to a wire <NUM>. The wire <NUM> can be used to obtain data from the sensor element <NUM> for embodiments of the unit <NUM> that does not wirelessly transmit the data. A dime <NUM> is provided for sizing reference.

In one non-limiting example, for feasibility testing experiments, the pin <NUM> selected can be a HI-LOK™ HL-<NUM> pin and the collar <NUM> selected can be a HI-LOK™ HL-<NUM> collar. In the feasibility testing experiment, the pin <NUM> was stainless steel with a small protruding shear cap, and the collar <NUM> was self-leveling aluminum. The pin <NUM> includes a ¼ inch (<NUM>) hole (e.g., hole <NUM>), and the resulting hex was approximately <NUM> inches (<NUM>) wide by approximately <NUM> inches (<NUM>) deep. Several HL-<NUM> rivets were used during the feasibility testing experiments, and a molding compound was used to cast impressions of the hex-key volume to measure tolerances across a range of parts. The opening (nominally <NUM>/<NUM> inches or <NUM>) was found to be well-toleranced across a dozen parts, approximately ±<NUM> inches or <NUM>. The depth of that cavity was also found to be well-toleranced across a dozen parts, approximately ±<NUM> inches or <NUM>. Therefore, an embodiment of the substrate <NUM> can generally be machined to fit tightly or snugly in stock HI-LOK™ rivets without customizations to the rivet nor rivet-specific or individual SHM parts.

<FIG> is a block diagram of an exemplary system <NUM> for structural health monitoring (e.g., a damage detection system for fastened structures and/or joints). The system <NUM> includes one or more fasteners <NUM> and one or more electromechanical units <NUM>. Each electromechanical unit <NUM> can be at least partially inserted into and mechanically coupled to a cavity or recess of a respective fastener <NUM>. Each electromechanical unit <NUM> includes a substrate <NUM> and a piezoelectric element <NUM> mounted and coupled to the substrate <NUM>. The system <NUM> includes one or more data acquisition devices <NUM> for collection, processing and transmission of signals and data receive from the electromechanical units <NUM> during or after excitation. The data collected and processed by the data acquisition devices <NUM> can be wired and/or wireless transmitted to a computing system <NUM>. In some embodiments, the data acquisition device <NUM> can include an analog-to-digital converter.

In one embodiment, a data acquisition device <NUM> can be incorporated into each one of the electromechanical units <NUM> and/or electrically coupled to the electromechanical units <NUM> (e.g., one data acquisition device to one electromechanical unit) such that the output of the piezoelectric element in the electromechanical unit can be received as an input to the data acquisition device. In some embodiments, the data acquisition device can be mechanically coupled to or incorporated with the electromechanical unit <NUM>. In one embodiment, a data acquisition device <NUM> can be electrically coupled to several of the electromechanical units <NUM> (e.g., one data acquisition device to many electromechanical units), and the outputs of the electromechanical units can be received as inputs by the data acquisition device <NUM>. In one embodiment, data acquisition devices <NUM> can be incorporated into two or more electromechanical units <NUM>, and signals from electromechanical units <NUM> can be transmitted in a wired and/or wireless manner to the respective data acquisition devices <NUM>. In one embodiment, the acquisition device <NUM> can be mechanically coupled to the structure surrounding the electromechanical units <NUM>, and signals output from the electromechanical units <NUM> can be received as inputs by the data acquisition device <NUM>. After digitizing and processing the data from the electromechanical units <NUM>, the data can be transmitted to the computing system <NUM> for further analysis.

The system <NUM> can include an excitation source <NUM>, one or more energy harvesting devices <NUM> (e.g., having power harvesting circuitry), signal and power conditioning circuitry <NUM>, a processing device <NUM>, such as a microcontroller or microprocessor, a communication chip <NUM> (e.g., a radio frequency (RF) transceiver), an antenna <NUM>, a power source <NUM>, a memory <NUM>, and a storage device <NUM>. Although illustrated as external and separate from the data acquisition device <NUM>, in some embodiments, components of the system <NUM> can be incorporated into a shared housing with the data acquisition device <NUM>. The excitation source <NUM> can actuate the piezoelectric element <NUM> of the electromechanical unit <NUM> via an electrical and/or mechanical stimuli. Signals output from piezoelectric element <NUM> of the electromechanical unit <NUM> in response to the excitation can be digitized by the analog-to-digital converter of the data acquisition device <NUM> and subsequently processed by the processing device <NUM>. The signal and power conditioning circuitry <NUM> can receive as input the signal from the piezoelectric element <NUM>, and can condition the signal before it is input to the data acquisition device <NUM>. In some embodiments, the signal from the piezoelectric element <NUM> can be input to the data acquisition device without first being processed by the circuitry <NUM>. The processing device <NUM> can collect and process the signals into data representative of damage to structure associated with the fasteners <NUM> and/or the structure assembly to which the fasteners <NUM> are secured. For example, damage can be detected of structural and non-structural elements of the structure assembly, such as adhesives between structural layers. The processing device <NUM> can be programmed and/or configured to operate the analog-to-digital converter of the data acquisition device <NUM> to convert and digitize the signals into a format capable of being further analyzed by the processing device <NUM> and/or for transmission to the computing system <NUM>.

The communication chip <NUM> can be configured to transmit (e.g., via a transmitter of an RF transceiver) and/or receive (e.g., via a receiver of an RF transceiver) wireless transmissions via an antenna <NUM>. For example, the communication chip <NUM> can be configured to transmit data, directly or indirectly, to one or more external devices (e.g., computing system <NUM>) and/or to receive data, directly or indirectly, from one or more external devices (e.g., computing system <NUM>). The communication chip <NUM> can be configured to transmit and/or receive messages having a specified frequency and/or according to a specified sequence and/or packet arrangement. As one example, the communication chip <NUM> can be a BlueTooth® transceiver configured to conform to a BlueTooth® wireless standard for transmitting and/or receiving short-wavelength radio transmissions typically in the frequency range of approximately <NUM> gigahertz (GHz) to approximately <NUM>. As another example, the communication chip <NUM> can be a Wi-Fi transceiver (e.g., as defined IEEE <NUM> standards), which may operate in an identical or similar frequency range as BlueTooth®, but with higher power transmissions. As another example, the communication chip can transmit data according to a proprietary communication and messaging protocol. Some other types of the communication chip <NUM> that can be implemented include RF transceivers configured to transmit and/or receive transmissions according to the Zigbee® communication protocol, and/or any other suitable communication protocol.

The storage device <NUM> can include any suitable, non-transitory computer-readable storage medium, e.g., read-only memory (ROM), erasable programmable ROM (EPROM), electrically-erasable programmable ROM (EEPROM), flash memory, and the like. In exemplary embodiments, operations for controlling the excitation source <NUM>, the energy harvesting device <NUM>, the processing device <NUM>, the communication chip <NUM>, the power source <NUM>, the memory <NUM>, the storage device <NUM>, and/or the data acquisition device <NUM> can be embodied as computer-readable/executable program code stored on the non-transitory computer-readable storage device <NUM> and implemented using any suitable, high or low level computing language and/or platform, such as, e.g., Java, C, C++, C#, assembly code, machine readable language, and the like.

The memory <NUM> can include any suitable non-transitory computer-readable storage medium (e.g., random access memory (RAM), such as, e.g., static RAM (SRAM), dynamic RAM (DRAM), and the like). In some embodiments, the data/information and/or executable code for implementing an operation of the system <NUM> can be retrieved from the storage device <NUM> and copied to memory <NUM> during and/or upon implementation of the processes described herein. Once the data/information has be used, updated, modified, replaced, and the like, the data/information may be copied from memory <NUM> to the storage device <NUM>.

The processing device <NUM> can include any suitable single- or multiple-core microprocessor of any suitable architecture that is capable of implementing and/or executing operations of the system <NUM>. For example, the processing device <NUM> can be programmed and/or configured to execute to excite one or more electromechanical units <NUM>, receive signals output from the electromechanical units <NUM> (e.g., via the communication chip <NUM>), and transmit digitized data to an external device (e.g., computing system <NUM>). The processing device <NUM> can retrieve information/data from, and store information/data to, the storage device <NUM> and/or memory <NUM>. For example, excitation signal values, received signal values, infrastructure damage values, baseline values, and/or any other suitable information/data for implementing the system <NUM> or that may be used by the system <NUM> may be stored on the storage device <NUM> and/or a memory <NUM>.

In exemplary embodiments, the processing device <NUM> can be programmed to execute the system <NUM> to receive and process information/data from the excitation source <NUM>, electromechanical unit <NUM>, communication chip <NUM>, data acquisition device <NUM>, and/or memory <NUM> and/or can be programmed to output information/data to the communication chip <NUM>, the storage device <NUM>, and/or the memory <NUM> based on the execution of the system <NUM>. The power source <NUM> can be implemented as a battery or capacitive elements configured to store an electric charge. In some embodiments, the power source <NUM> can be a rechargeable power source, such as a battery or one or more capacitive elements configured to be recharged via a connection to an external power supply and/or to be recharged by the energy harvesting device <NUM>. As one example, the rechargeable power source can be recharged using solar energy (e.g., by incorporating photovoltaic or solar cells as the energy harvesting device <NUM>), through physical movement (e.g., by incorporating a piezo-electric elements as the energy harvesting device), through energy received from radiofrequency transmissions (e.g., by incorporating an inductive charging circuit as the energy harvesting device <NUM>) and/or through any other suitable energy harvesting techniques using any suitable energy harvesting devices. In some embodiments, the energy harvesting device <NUM> can be a device for harvesting radio frequency energy, inductive energy, mechanical energy, combinations thereof, or the like. The energy harvesting device <NUM> can include energy or power harvesting circuitry.

As shown in <FIG>, after cleaning the substrate <NUM>, <NUM>, piezoelectric sensor elements <NUM>, <NUM> of units <NUM>, <NUM> were bonded to their respective substrates <NUM>, <NUM> to form electromechanical units using electrically conductive epoxy film preforms. The assembly was cured under vacuum at approximately <NUM> for approximately <NUM> minutes. Conductive epoxy was used because the substrates <NUM>, <NUM> themselves were to be part of the ground path. Signal carrying wires <NUM> were attached to the exposed plated surfaces of the piezoelectric sensor elements <NUM>, <NUM> with conductive epoxy cured at approximately <NUM> for approximately <NUM> minutes. The wire joints were then reinforced with epoxy, cured at room temperature for approximately <NUM> hours.

Transducers (e.g., an embodiment of the unit <NUM>) were bonded into the holes of the pins <NUM> using a low viscosity epoxy cured at approximately <NUM> for approximately <NUM> minutes. During installation, epoxy was injected into the floor of the recesses with <NUM> gauge needles while observing through a microscope to ensure substantially all air bubbles escaped from the recess irregularities. Hex-plate type transducers (e.g., unit <NUM> of <FIG>) were submerged in epoxy and nail-type transducers (e.g., unit <NUM> of <FIG>) were bonded at the tip and sides of the shank (e.g., the bottom section <NUM>), as well as the on underside of the head (e.g., the top section <NUM>). Metallic substrates (and the pins, by extension) were used as ground electrodes. Therefore, electrical contact between the substrates <NUM>, <NUM> and the pins <NUM> was critical during and after bonding. Piezoelectric capacitance was monitored during assembly with one probe on the signal wire <NUM> and one probe on the pin <NUM> being assembled. When good contact was verified, parts were fixtured and cured.

<FIG> show unassembled and assembled views of a test bed <NUM> for experimental testing of embodiments of the units <NUM>, <NUM>. To compare performance of the four transducer combinations (two configurations in two substrate materials), a test bed <NUM> was constructed from three layers <NUM>, <NUM>, <NUM> of approximately. <NUM> inch (<NUM>) thick <NUM> aluminum. The bottom and middle layers <NUM>, <NUM> were approximately <NUM> inch (<NUM>) squares. The top layer <NUM> included four, approximately <NUM> inch (<NUM>) squares, one at each corner of the test bed <NUM> assembly. Fastener mounting holes <NUM> were drilled and reamed through all three layers <NUM>, <NUM>, <NUM> according to HI-LOK™ manufacturer specifications, and four pins <NUM> were installed per standard process and instrumented post-installation.

As shown in <FIG>, the test bed <NUM> was instrumented with two types of units, e.g., embodiments of the units <NUM>, <NUM>. Two conventional flex circuit mounted piezoelectric sensor elements <NUM> were bonded to the exposed surface of the middle layer <NUM> with the piezoelectric sensor element <NUM> centered on the top of the plate, and on the bottom layer <NUM> with the piezoelectric sensor element <NUM> centered on the bottom of the plate. The centered piezoelectric sensor element <NUM> was equidistant to the four instrumented pins <NUM> to facilitate a direct comparison of signal strength relative to the instrumented pins <NUM>. The transducer data was collected in the following manner. One transducer was selected as the actuator, and the other four transducers (including the two standard flex circuits) were used as sensors. The actuator was driven using <NUM> sine waves under a Hanning window generated by an Agilent 33220A function generator and display via an oscilloscope <NUM> (see, e.g., <FIG>). The actuator was tested using a frequency of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. The actuator and sensors were measured using a TDS 3014B oscilloscope. Data averaged <NUM> times.

The initial feasibility testing successfully demonstrated the ability to convey ultrasonic energy into multiple fastened layers by exciting HI-LOK™ rivets and sensing the propagated signal. All configurations of embodiments of the units <NUM>, <NUM> tested provided positive results. The steel hex-nail configuration provided the strongest results. In some embodiments, the steel hex-nail configuration (e.g., an embodiment of the unit <NUM>) can be implemented for purposes of reduced fabrication complexity and improved field durability.

As shown in <FIG>, it should be understood that different methods excitation of the transducers can be used. In one embodiment, a signal source generator <NUM> (e.g., energy harvesting, power transmission, wave generation, exciter, or the like) external to the transducers can electrically or mechanically actuate or stimulate one or more of the transducers, and signals from the transducers can be received by an external data acquisition device <NUM>. In such embodiments, the signal source generator <NUM> and/or the data acquisition device <NUM> can be communicatively coupled to the transducers via wired or wireless means. In some embodiments, the signal source generator <NUM> can be incorporated into one or more of the transducers. As discussed above, in some embodiments, the data acquisition device <NUM> can be incorporated into one or more of the transducers (see, e.g., <FIG>).

In some embodiments, one or more transducers can be selected as the actuator, exciter or sending transducer(s) to propagate waves to the remaining transducers, and one or more adjacent and/or remote transducers can be used as sensors or receivers to receive the generated signal (e.g., pitch catch or phased array configuration). In some embodiments, one or more transducers can be selected as the actuator, exciter or sending transducer(s), and the same transducer(s) can be used as a sensor or receiver (e.g., pulse echo configuration). In some embodiments, the signal source generator <NUM> can be incorporated into one or more of the data acquisition devices <NUM>.

<FIG> show perspective and detailed views of a test setup <NUM> for experimentation of an embodiment of the unit <NUM>. The test setup <NUM> was designed to more fully characterize the behavior of the selected sensor assembly configuration. Specifically, test setup <NUM> assisted in measuring wave speed, attenuation rate (i.e., detection range), and effect of damage near a rivet. A larger plate <NUM> was assembled that more closely resembled the final test design, with a <NUM> inch x <NUM> inch (<NUM> x <NUM>) bottom plate (not visible) with ¼ inch (<NUM>) holes at an approximately <NUM> inch (<NUM>) pitch, assembled with two strips <NUM> of <NUM> inch (<NUM>) wide aluminum stacked at either end. Each plate was approximately <NUM>/<NUM> inches (<NUM>) thick. Three HI-LOK™ rivets <NUM> were installed, two adjacent ones in the middle of one end, and one directly opposite the pair, forming an L-shaped configuration In the middle sheet of aluminum, approximately <NUM> inch x <NUM> inch (<NUM> x <NUM>) channels were machined leading away from the rivet hole to simulate a crack.

Steel shims <NUM> were chemically etched to the same dimensions of the channels with a wider grip area at the end, and were pressed into the channels using an arbor press as seen in <FIG>. The intention was to collect a baseline signal with the shim <NUM> intimately pressed into the channel, and then use the grip end to pull out the shim forcefully to simulate crack growth. As seen in <FIG>, one shim <NUM> was installed behind the rivet at the "elbow" of the L shape, and a second shim <NUM> was placed in an empty hole on the opposite side of the adjacent rivet. Data was collected using each of the three rivets as a <NUM> actuator one at a time, while collecting at the other two positions as sensors. A baseline signal was collected, data was again collected after each shim <NUM> was removed, and a new baseline was collected the following day for comparison with the prior damage case. It should be noted that while the first pressed in shim <NUM> was held in tightly and required much force to remove, the adjacent shim <NUM> not behind a rivet readily slid out of the channel. The second shim <NUM> was therefore not expected to produce a changed signal because it likely was not compressed enough to be ultrasonically coupled to the plate.

<FIG> show perspective, detailed and front views of a test setup <NUM> for experimentation of units <NUM>. After it was determined that the ultrasonic waves radiating from the rivet positions could propagate across a <NUM> inch (<NUM>) plate, and that the signals changed sufficiently due to detected simulated damage, a more elaborate proof-of-concept test setup <NUM> was conducted. In the test setup <NUM>, a plate <NUM><NUM> inches (<NUM>) wide by <NUM> inches (<NUM>) long with a <NUM>/<NUM> inch (<NUM>) aluminum base plate was used, with two <NUM> inch (<NUM>) wide strips <NUM> of <NUM>/<NUM>" (<NUM>) doubler aluminum plates at either end. Two rows of HL-<NUM> rivets <NUM> were installed with a <NUM> inch (<NUM>) pitch, as seen in <FIG>. Three channels were milled with pressed-in shims <NUM> (similar to the prior test) and integrated into the plate for alternating rivets (see, e.g., <FIG>). The rivets were instrumented with hex-nail sensor assemblies (e.g., an embodiment of the unit <NUM>) identical to those used in the feasibility testing experiments, using a gantry to strain relieve the cables as seen in <FIG>.

Four <NUM>-channel networked oscilloscopes <NUM> were used to simultaneously collect data from the <NUM> instrumented rivets (i.e., rivets that include an embodiment of the unit <NUM> coupled thereto), triggered by the <NUM> <NUM> cycles sinusoidal burst excitation signal sent by the arbitrary function to the actuating rivet. Data was stored remotely by a laptop. For each damage case, each instrumented rivet was manually configured to be an actuator (by switching BNC connections) one at a time while the rest of the transducers would be sensing (for the fielded unit this can be achieved using a simple multiplexing circuit). Shims <NUM> were removed one at a time, left to right. It is noted that two of the shims <NUM> broke without moving and, therefore, no signal change was expected at such locations. Three holes <NUM> were drilled sequentially using a <NUM>/<NUM> inch (<NUM>) precision bit in the center of the middle layer <NUM> behind the rivet and between those configured with channels <NUM> and shims as seen in <FIG> and the diagrammatic view of <FIG>, for a total of four damage cases (e.g., one shim and three holes). Particularly, in <FIG>, reference to "Shim" locations refers to the channels <NUM> provided with shims, and reference to "Drill" locations refers to the drilled holes <NUM> in the middle layer <NUM>.

The experimental objective for the testing discussed above was to determine the feasibility of exciting and receiving guided ultrasonic waves through the use of an existing rivet structure. The following abbreviations are using in the analysis graphs shown in <FIG>. "Cen" refers to a center piezoelectric sensor element mounted at the top (see, e.g., <FIG>); "Bot" refers to a center piezoelectric sensor element mounted a the bottom (see, e.g., <FIG>); "CuT" refers to a copper substrate having a nail-type configuration (see, e.g., <FIG>); "StT" refers to a steel substrate having a nail-type configuration; "StH" refers to a steel substrate having a hex-type and plate-type configuration (see, e.g., <FIG>); "Act" refers to actuator; and "Sens" refers to sensor.

As an initial approach, the theoretical Lamb modes were calculated for the top plate layer (thickness of <NUM> inches or <NUM>) to find that the A0 and S0 modes are the only propagating Lamb modes until about <NUM>. Generally, guided wave modes induced with a piezoelectric disc have the greatest transfer of energy into the structure below <NUM>. Because of this, a Hanning windowed sinusoid with center frequency of [<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>] kHz was selected as an input signal. To determine if induced signals from the transducers are similar to Lamb waves, the first arrival wave packet was isolated and time of flight was used to calculate group velocity. The calculated group velocity was then compared to theoretical A0 and S0 mode group velocities.

<FIG> plot the theoretical A0 and S0 Lamb mode group velocity along with the calculated group velocity of the first arrival wave packet for the central two transducers as actuators. <FIG> show the received guided wave signal at <NUM> for the various actuator and sensor combinations. In each plot, the received signal envelope is shown along with the estimated first arrival wave packet amplitude indicated by a black point. The location in time of this peak is used along with the known propagation distance to calculate the group velocity of wave packet. <FIG> show the normalized amplitude of the received first arrival packet for the various sensor actuator combinations versus frequency.

From the feasibility portion of testing, it was concluded that the waves were propagating in each layer of the assembly, and the steel "nail-type" configuration (e.g., unit <NUM> of <FIG>) provided the optimal results. The goal of the subsequent round of testing was to establish the ability of waves generated and received by the hex-nails (e.g., the substrate <NUM> of <FIG>) to propagate <NUM> inches (<NUM>) or more, and to establish the ability of the sensors to detect changes caused by introduction of representative flaws. <FIG> show the time traces for waves propagated between the furthest actuator with the damaged rivet sensing (<FIG>) and vice-versa (<FIG>). Overlaid are four traces representing the initial baseline signal after the first shim was removed, the signal after the second shim was removed, and a second baseline collected the following day. <FIG> are subtracted differences between each of the four data sets. As can be seen in <FIG>, while there is nearly no difference once the second shim was removed (expected since there was no retention force) nor in the second baseline, there was a significant change in the third wave packet following the first shim being removed. It is believed that the third wave packet is most affected because the first two arrivals are the A0 and S0 modes and the third wave is a reflection that has passed through the damaged area before passing through the sensing rivet.

The final experiment discussed above sought to demonstrate the full proof of concept for damage detection using the proposed hex-nail sensors. The first three damage cases attempted were removal of shims pressed into narrow channels as seen in <FIG>. In the prior characterization experiment, the shim removal worked well for a channel behind a rivet, but easily slid out of a channel where there was no rivet. The opposite issue was experienced in the final experiment, where the shims were too snugly held in the channels when assembled with all eight rivets on the same plate. The first and third shim simply snapped while attempted to pull them out, without any apparent movement in the channel. This can be seen in <FIG>, where virtually no damage metric was recorded for the shims near rivets <NUM> and <NUM> (see, e.g., <FIG>).

For the middle case of rivet <NUM>, it can be seen in <FIG> that a significantly pronounced damage metric was calculated for rivets <NUM>-<NUM>. However, in <FIG>, it can be seen that rivet <NUM> still had roughly twice the metric of the adjacent sensors. In order to detect further damage cases, <NUM>/<NUM> inch (<NUM>) holes were drilled behind rivets <NUM>, <NUM> and <NUM> (see, e.g., <FIG>). As seen in <FIG>, the correct rivet was properly identified in each case with the maximum damage metric (all on the same scale). For the first and third holes drilled, only the rivet near the damage showed a high damage metric, with the others being well below the threshold value. For the middle case near rivet <NUM>, relatively strong damage indications were also calculated for both adjacent sensors (rivets <NUM> and <NUM> were about <NUM>% of the rivet <NUM> value). These results clearly demonstrate the ability to reliably detect subtle hidden damage in riveted assemblies when the exemplary damage detection units are incorporated into the rivets.

The experimentation and results demonstrated the ability to use pre-existing HI-LOK™ rivets with exemplary damage detection units as guided wave actuators and sensors to detect hidden damage in complex fastened joints. Multiple configurations and materials were explored, and the steel "nail-type" configuration (e.g., unit <NUM> of <FIG>) provided the most effective results. During experimentation, a hex shaft similar to the hex key used to hold the rivet pin during standard installation was used as a delay line to ultrasonically couple to the rivet, and a transducer was bonded to a larger hex-shaped head positioned outside of the hex recess. The resulting waves were close in velocity to the theoretical A0 Lamb wave modes. Central transducers placed on the middle and bottom plates demonstrated that the waves being generated by the nails were propagating in all layers. A few tests were also conducted sending waves from one pin-type transducer to another which resulted in high transmitted wave energy received between transducers spaced <NUM> inches (<NUM>) apart.

After feasibility had been established for exciting each layer using the "nail-type" transducer design (e.g., unit <NUM> of <FIG>), the subsequent experiment was set up to determine the range of wave propagation in a layered medium, and to determine the feasibility to detect damage in the form of a representative crack. The same rivets and transducers were used as in the previous experiment, but the rivets were positioned <NUM> inches (<NUM>) apart from each other. To represent a crack, a narrow (<NUM> inch (<NUM>) wide by <NUM> inch (<NUM>) deep) channel was machined between the rivet hole and the back edge of the plate on the middle layer, and a similarly dimensioned steel shim was pressed into that gap. After a baseline was collected, the shim was forcefully removed from the assembly representing crack growth. Both the transducer adjacent to the damage and the transducer <NUM> inches (<NUM>) away from the damage was able to detect the removed shim, thereby further emphasizing the ability to detect damage with the exemplary systems.

The subsequent experiment was similarly dimensioned, with two rows of eight rivets spaced by <NUM> inches (<NUM>). Channels were machined into three of the rivet holes to sequentially simulate multiple cracks while using the sensor array to beamform the resulting signal. Unfortunately, during testing the shims broke before they were able to be removed in the channel, resulting in no signal change. Therefore, a <NUM>/<NUM> inch (<NUM>) bit was used to drill small holes in the middle of the central <NUM>/<NUM> inch (<NUM>) aluminum plate near the rivet holes without shims. The results from this experiment showed that the exemplary systems are effective in detecting and localizing damage. The rivet mounted sensors are particularly sensitive to changes occurring around them, amounting to boundary condition changes for the pins. Thus, during use, a threshold value can be set after installation of the rivet(s) and subsequent testing can provide signals that reliably detect changes caused by damage, regardless of whether the damage is fatigue cracks and/or corrosion.

Additional testing was performed to demonstrate crack detection capability in a complex multi-layer fastened joint. <FIG> is a diagrammatic, cross-sectional view of a test specimen <NUM> including an exemplary unit <NUM>. Eight variations of the multi-part test specimen <NUM> (e.g., a jointed test specimen (JTS)) were subjected to tensile fatigue. The specimens <NUM> were fundamentally similar, each including four plates, two inner plates <NUM>, <NUM>, and two outer plates <NUM>, <NUM>. The two inner plates <NUM>, <NUM> each include a hole near one end for passage of the pin <NUM>. The outer plates <NUM>, <NUM> each have two holes for passage of both pins <NUM>. The two inner plates <NUM>, <NUM> are aligned vertically with the hole-ends near one another. A single specimen <NUM> was assembled from four plates by sandwiching the inner plates <NUM>, <NUM> between two outer plates <NUM>, <NUM>, passing threaded HI-LITE™ pins <NUM> (e.g., HST12 pins) through all three layers, and securing the pins <NUM> with threaded collars <NUM> (e.g., HST1087 collars) (see, e.g., <FIG>).

In some variations of the specimen <NUM>, a washer <NUM> (e.g., NAS1149-D0432J washer) was inserted between the collar <NUM> and the inner plate <NUM> and/or a carbon fiber spacer <NUM>. Fiberglass grip tabs <NUM> were included on the ends of the inner plates <NUM>, <NUM> opposing the fastener assembly. All interfaces of the specimen <NUM> were coated with AMS <NUM> sealant. Crack starter notches were cut into the inner diameter of one or more holes in each specimen to facilitate quick crack initiation. The eight different specimen configurations were formed by varying starter notch type and location, plate material, plate thickness, and the presence or absence of a carbon fiber spacer.

Five stages of testing were performed to provide the desired results. First, a test specimen similar to a 1XX-series specimen was fabricated and instrumented with two piezoelectric-based fastener SHM sensors. The specimen was used to characterize the influence of sealant on pitch-catch performance, as well as the effect of looser hole dimensions (as compared to testing using a press fit between the electromechanical unit and pin).

One of each specimen type (plus extras) were assembled without instrumentation and tested in tensile fatigue until failure. Such testing included two required "baseline" specimens. The testing was used to verify occurrence of failure under reasonable loads and cycle counts, and permitted validation of all test setups before testing instrumented samples. If crack initiation took too long at this stage, adjustments were made prior to further testing.

At least three of each specimen type were assembled, instrumented with exemplary electromechanical units, and tested under tensile fatigue. Testing was regularly paused for data collection from the SHM fastener sensors. The test was stopped when confident that a crack had grown. The testing refined and calibrated damage detection algorithms to accommodate a crack/no-crack criteria, using a binary decision tree pattern recognition algorithm based on guided wave features. Subsequently, the specimens were tested until sensors detected crack initiation, at which point testing was ceased and specimens were further analyzed.

<FIG> show top and perspective views of components for assembly of the test specimen <NUM> (see, e.g., diagrammatic representation of <FIG>). The inner plates <NUM>, <NUM> include a hole <NUM> near one end and the outer plates <NUM>, <NUM> each include two holes <NUM>. The specimen <NUM> was assembled by sandwiching the inner plates <NUM>, <NUM> between two outer plates <NUM>, <NUM> and passing HI-LITE™ pins (HST12-<NUM>-<NUM>) through all layers and securing them with HI-LITE™ collars (HST1087-<NUM>). Washers <NUM> (NAS <NUM>-D0463J) and solder lugs <NUM> (Keystone Electronics <NUM>) were added between the collar <NUM> and outer plates <NUM>, <NUM>. The washers <NUM> were used to fill up the grip length on the HI-LITE™ pins <NUM> left from the variations of the specimens <NUM>. Three washers <NUM> were used on all HI-LITE™ pins <NUM> in the absence of a carbon fiber spacer <NUM>. With the presence of a carbon fiber spacer <NUM>, one washer <NUM> was used to keep overall pin <NUM> length and presence of washers <NUM> constant. Solder lugs <NUM> were inserted into the stack up to provide a surface for more reliable electrical connection. All interfaces of the specimen <NUM> were coated with sealant (AMS <NUM>). Eight different specimen <NUM> configurations were used, with variations in starter notch type and location, plate material, plate thickness, and presence or absence of a carbon fiber spacer. Upon receipt of the specimen <NUM> components, all parts were measured to ensure that all tolerance specifications were met.

Prior to assembly, all fastening and transducer components were cleaned through a Branson <NUM> Ultrasonic Cleaner. All plates were cleaned with a rag and acetone. Tooling was designed to ensure proper alignment of all components of the test specimen <NUM> during assembly. As shown in <FIG> and <FIG>, a base plate <NUM> and two aligning rails <NUM> were used to ensure proper alignment of all components during assembly. Prior to assembly, one rail was fixed down such that all components referenced the same fixed wall. The sealant was mixed and cured. Sealant was painted using an acid paint brush on the upper surfaces of all components to ensure that all faying surfaces were covered. The test specimen <NUM> was built from the bottom up as seen in <FIG>, including units <NUM>. The shank of the HI-LITE™ pin was painted and then threaded through the outer plate. As each component was laid down, all faying surfaces were thinly coated with sealant. Care was taken to ensure that the hex recess of the HI-LITE™ pins remained sealant-free to ensure reliable electrical connection to the transducers.

After the entire stack up was assembled, the second aligning rail <NUM> was fixed to restrain movement and rotation of the specimen <NUM> in the Y-direction. The cut-out in the base plate <NUM> and HI-LITE™ pin prevented movement in the X-direction. Tightening the collar constrained movement of the components in the Z-direction, driving out excess sealant from the pin hole and ensured minimal alignment variation across all specimens <NUM>. Collars were tightened using a Lang ROW-<NUM><NUM>/<NUM> inch (<NUM>) ratchet box wrench and Hex Plus keys. This allowed for a greater contact area between the hex recess and the hex key, which yielded a higher success rate compared to standard hex keys. After AMS <NUM> sealant was fully cured, G10 grip tabs were bonding on using epoxy or adhesive. A thin layer of epoxy or adhesive was painted on using camel-hair paint brushes and left to cure under load for <NUM> hours at room temperature. Specimens <NUM> were engraved with their respective part numbers, indicating the type of specimen <NUM> as well as location of the notched hole. The specimens <NUM> were engraved with a carbide-tipped high strength electric engraver, and were engraved in the grip tab region such that the engraving would not change the stress conditions at the notch. <FIG> show top, detailed, front and side views of the assembled test specimen <NUM>, including the engraving <NUM> in <FIG>.

As shown in <FIG>, the specimens <NUM> were instrumented with electromechanical units <NUM> (e.g., transducers) in a similar fashion as described above. The electromechanical units <NUM> were substantially similar in structure and function to the unit <NUM> of <FIG>, except for the distinctions noted herein. The transducer configuration interfaced with the walls and floor of the <NUM> inch (<NUM>) wide by <NUM> inch (<NUM>) deep hexagonal drive recesses or holes of the HST12-<NUM>-<NUM> pin. The transducers or sensor elements <NUM> were circular piezoceramic elements measuring <NUM> inches (<NUM>) in diameter and were cut from gold plated wafer stock. The sensor elements <NUM> were bonded to a substrate <NUM> using conductive epoxy at <NUM> for <NUM> minutes. The substrate <NUM> material was chosen as Ti-6AL-4V per AMS4967 to match the pin (HST12-<NUM>-<NUM>) material.

Transducers were bonded into the hex recess of the HI-LITE™ pins using a low viscosity epoxy and cured at room temperature for <NUM> hours. During bonding, care was taken to ensure that no air bubbles formed that would cause irregularities. While observing through a microscope, epoxy was injected into the floor of the recesses with a <NUM> gauge needle. The substrates <NUM> were inserted into the hex recess of the pin and allowed to cure under load to ensure good contact. Wires <NUM> were then bonded to the exposed plated surfaces of the piezoelectric elements with conductive epoxy and cured at <NUM> for <NUM> minutes. A ground wire <NUM> was soldered to the solder lug <NUM>. After the desired electrical contact was verified by measuring resistance and capacitance, the wire joints were secured with Loctite <NUM> and cured at room temperature for five minutes. The wires were subsequently strain relieved.

Testing in a loadcell having a maximum load capability of <NUM>,<NUM> lbf (<NUM> kN) was performed. The specimen loading parameters assumed a stress ratio (R) of <NUM> and cycled below <NUM>. Three uninstrumented specimens underwent tensile loading. Specimen <NUM>-<NUM> experienced <NUM>,<NUM> cycles with a maximum load of <NUM> lbf (<NUM> kN). Subsequently, the load was increased to a maximum load of <NUM>,<NUM> lbf (<NUM> kN) and an additional <NUM>,<NUM> cycles were run. The specimen did not fail. <FIG> shows a graph of the load history for the <NUM>-<NUM> specimen. Specimen <NUM>-<NUM> was run for <NUM>,<NUM> cycles with a maximum load of <NUM>,<NUM> lbf (<NUM> kN). An additional <NUM>,<NUM> cycles were run at the same maximum load before load was increased. <NUM>,<NUM> cycles at a maximum load of <NUM>,<NUM> lbf (<NUM> kN) resulted in failure. Specimen <NUM>-<NUM> experienced <NUM>,<NUM> cycles and <NUM>,<NUM> cycles with no failure. Specimen <NUM> was run for <NUM>,<NUM> cycles at <NUM>,<NUM> lbf (<NUM> kN) without failure, followed by another <NUM>,<NUM> cycles at <NUM>,<NUM> lbf (<NUM> kN) without failure. Adjustments were made to loading parameters to achieve failure under reasonable loading and cycle counts.

A concern with testing at high cycle counts was that it would be challenging to pick practical points for sensor recording over such a large range without risking big steps in crack size, and only <NUM>,<NUM> cycles could be performed each day. Consistency of boundary conditions for specimens that fail over <NUM>,<NUM> cycles was a concern, and required multiple days of testing with un-gripping/re-gripping after each day. Both of the specimens that had experienced <NUM>,<NUM> to <NUM>,<NUM>,<NUM> cycles were corner notch electrical discharge matching (EDM) specimens (XX3), and it was determined that it would take too many cycles for the corner notch specimens to develop natural cracks to be practical for the test matrix and budget.

After such determination, an uninstrumented <NUM> specimen (e.g., without a electromechanical unit) was tested at <NUM>,<NUM> lbf (<NUM> kN) for <NUM>,<NUM> cycles with no signs of failure. However, when the load was increased to <NUM>,<NUM> lbf (<NUM> kN), a catastrophic failure occurred within <NUM>,<NUM> cycles. A second uninstrumented <NUM> specimen was subsequently tested at <NUM>,<NUM> lbf (<NUM> kN) and stopped at <NUM>,<NUM> cycles such that it could be confirmed that a natural fatigue crack was growing and, once disassembled, cracks of approximately <NUM>/<NUM> inch (<NUM>) were observed. Based on these results, it was determined that <NUM>,<NUM> lbf (<NUM> kN) was a good load level for successful and practical fatigue tests. Subsequently, the same load level of <NUM>,<NUM> lbf (<NUM> kN) was tested for an uninstrumented <NUM> specimen which failed at <NUM>,<NUM>. A second <NUM> specimen was stopped at <NUM>,<NUM> cycles to observe if there was a crack, and it was discovered that the specimen had a <NUM> inch (<NUM>) crack. An uninstrumented <NUM> specimen was tested at the same <NUM>,<NUM> lbf (<NUM> kN) load level, and completely failed at <NUM>,<NUM> cycles. Therefore, another <NUM> specimen was tested and stopped at <NUM>,<NUM> cycles to look for cracks and a <NUM> inch (<NUM>) crack was found. Finally, two <NUM> specimens were tested at <NUM>,<NUM> lbf (<NUM> kN) as well. <FIG> shows Table <NUM> with uninstrumented test results for multiple specimens, including the maximum loads, minimum loads, cycles and crack formation information.

Next, specimens were instrumented with electromechanical units and tested to calibrate a damage metric. <FIG> shows Table <NUM> with instrumented calibration test results for <NUM>, <NUM> and <NUM> specimens. Since the <NUM>, <NUM> and <NUM> specimens were mostly the same (identical parts assembled other than a single EDM notch for <NUM> versus dual notches for <NUM> and <NUM> specimens, and an additional carbon fiber spacer added onto the <NUM> stack-up), it was decided to proceed with the same test procedure and load level for these three sets of specimens, and it was assumed that the specimens would have the same or similar damage metrics. Two <NUM> specimens were tested to failure at <NUM>,<NUM> lbf (<NUM> kN) while collecting data from the piezoelectric transducer rivet sensor. Two <NUM> specimens were tested, and the tests were stopped after <NUM>,<NUM> and <NUM>,<NUM> cycles, respectively, to observe the fatigue crack lengths at those metric levels for calibration purposes. Similarly, two <NUM> specimens were tested to <NUM>,<NUM> and <NUM>,<NUM> cycles and stopped to observe the cracks at those metric levels.

After the calibration data was collected, a metric was determined that could be used to stop the test as a small fatigue crack was growing in the specimen. The metric that was used compared a baseline piezoelectric sensor signal to a signal at a specific test cycle number using a matched filter applied to a specific window in time corresponding to the first guided wave arrival (e.g., a correlation statistic). <FIG> shows three time series for <NUM>, <NUM>,<NUM>, and <NUM>,<NUM> cycles along with the first arrival region highlighted with a dashed box. A change in the sensor response from <NUM> cycles to <NUM>,<NUM> cycles can be seem, which is attribute to "settling" of the specimen under load and likely dominated by changes to the sealant. After approximately <NUM>,<NUM> cycles, the change in the first arrival region stabilizes and any additional change is assumed to be due to crack growth surrounding the sensor. To capture the change in signal due to crack growth, a matched filter represented as Equation <NUM> was used: <MAT> where h[n] is the base signal for the windowed time series signal at <NUM>,<NUM> cycles, and x[n] is the test signal at a cycle count greater than <NUM>,<NUM>. The matched filter is a linear convolution between the base and test signal. <FIG> shows an example windowed time series for a base signal at <NUM>,<NUM> cycles and a test signal at <NUM>,<NUM> cycles at a time lag of <NUM> and <NUM>. <FIG> is a graph of the matched filter output for all possible time lags. For these signals, maximum correlation occurs at a time lag of <NUM> where the two signals nearly directly overlap. For other time lags, such as <NUM>, the correlation is near zero since the signals do not overlap. The metric used to relate crack growth to signal change can be represented as Equation <NUM>: <MAT> where T represents the metric, and y[n] represents the value from Equation <NUM>. As the crack grows, the correlation of the base signal and test signal generally decrease, resulting in an increase of the crack metric. The -<NUM> value in the metric equation serves as a normalization factor.

<FIG> shows Table <NUM> of instrumented damage matric test results for <NUM>, <NUM> and <NUM> specimens. <FIG> is a graph of the matched filter versus cycles for the specimens in Tables <NUM> and <NUM>. As seen in <FIG>, when plotted against cycles, the metric never truly holds at zero before increasing when a crack forms. Rather, there is always some small amount of change from the start of testing, which is likely due to fretting (observed on all dissected specimens) and potentially plastic zone formation. However, uniformly through all the tests, after the metric reaches a certain level a crack is definitely present, and is well correlated to crack size across multiple specimens, as seen in <FIG>.

For the first three "blind" tests (one specimen each from <NUM>, <NUM> and <NUM>), a conservative metric goal of about <NUM> was selected, such goal expected to yield a measurable crack based on the data from the calibration tests. After each block of <NUM> cycles, ultrasonic data was collected while holding at the minimum load, and the data was plotted in real-time for the Matched Filter metric. The test was stopped if the metric was at (or near) the <NUM> metric. Since the metric was only processed every <NUM> cycles, the test was stopped in some cases based on a value close to <NUM> with a large enough slope that seemed inevitable that the <NUM> would be exceeded before the next loading block was completed. After dissection, cracks of about <NUM> mil were present for each specimen. For the specimens with multiple cracks, the metric appeared to correspond to the longest crack growing from the same hole, rather than the sum of the crack lengths across the hole. Subsequently, an extra <NUM> specimen test was conducted with a more aggressive metric goal of about <NUM>, which yielded an approximately <NUM> mil crack. A summary of the blind test results for the <NUM>, <NUM> and <NUM> specimens is provided in Table <NUM> of <FIG>.

Following testing on the similarly constructed specimens (<NUM>, <NUM> and <NUM>), testing commenced for the <NUM> specimens. Similar to the prior specimens, an instrumented <NUM> was cycled through a larger range of metric values to verify the metric behavior (the same metric approach was used for these specimens). The test was stopped at <NUM>,<NUM> cycles to measure the crack that had developed (about <NUM> inches or <NUM>). Based on the first test and the assumed crack growth rate of about <NUM> mil per <NUM>,<NUM> cycles post-initiation, it appeared that a damage metric of about <NUM> (with the same logic for stopping based on slope) would again provide for a crack length of about <NUM> mil. The final results for the calibration testing and three "blind" specimens is provided in Table <NUM> of <FIG>, and is graphically represented in <FIG>. The approximately <NUM> metric provided similar sized cracks as seen in the previous specimen. However, the correlation between crack length and metric value was not as strong for these specimens, albeit with half the sample size. It is believed that the minimized correlation is due to the differences between how cracks initiate and grow in aluminum versus titanium, which was also visually apparent when looking at the cracks under a microscope.

One test not mentioned in the above matrix is a <NUM> specimen that failed prematurely as a completely titanium fracture (a part without an EDM notch that was not expected to crack and did not crack for any of the other tests). Once disassembled, a <NUM> inch (<NUM>) crack was observed on one side of the aluminum (fretting surface) growing out of the EDM notch, and no crack was observed on the back side of the same specimen (this ended up being consistent with other tests for crack initiation). The specimen was sectioned using a carbide-toothed rotary saw in order to better understand the crack evolution as it grew from the EDM notch tip. Cuts were made perpendicular to the direction of crack growth in <NUM> inch (<NUM>) to <NUM> inch (<NUM>) increments, starting at the far surface of the plate. <FIG> show the crack <NUM>, sectioned faces <NUM>, and crack depth <NUM>. A flat, smooth cross-section was revealed at every step with each sectioning cut creeping closer to the crack leading edge. After each cut, the part was examined under magnification using dye penetrant. From crack depth measurements taken at each section (see Table <NUM> of <FIG>), the two-dimensional representation of the crack leading edge was constructed and is shown in <FIG>. A single through notch <NUM> was formed in the specimen, and <FIG> shows the crack <NUM>. <FIG> show photographs of the crack in the <NUM> specimen.

Each specimen tested underwent a disassembly procedure after fatigue testing to permit inspection of each component. The head of one HI-LOK™ pin was clamped in a smooth-jawed machinists' vise without overtightening. The corresponding collar was gripped with locking pliers. The locking force was adjusted to grip just firmly enough to turn the collar without crushing it. The collar was removed, with the unscrewing action pulling the T-pin and piezoelectric sensor element from the pin's hex recess. The assembly was moved to an arbor press, and the pin was pressed out with an undersized drift pin. The individual sealant joints were broken by hand to separate the four plates. The plates were wiped with acetone-soaked cotton rags to clean off sealant.

<FIG> are photographs of the cracks that were discovered through the blind testing process. A red line was placed where it was believed that the crack stopped through visual inspection with a microscope. No red mark was placed on specimen 101F as it was not completely clear where the crack stopped. However, it is believed that there is a <NUM>-<NUM> mil fine crack in specimen 101F.

Fatigue testing was performed using the set-up <NUM> shown in <FIG>, including a test specimen <NUM> clamped in the test frame. The load frame used in the test series is an Instron model <NUM> servo-hydraulic uniaxial tensile tester equipped with fatigue rated mechanical wedge style grips (cat # <NUM>-<NUM>). The grips are rated to +/-<NUM>,<NUM> lbf (<NUM> kN) and have two-inch (<NUM>) wide serrated jaw faces. The tester's load cell can measure up to <NUM>,<NUM> lbf (<NUM> kN). Load cells and stroke measurement equipment were calibrated annually by an independent calibration lab that is ISO <NUM> accredited. Preventative maintenance was performed annually as well.

Claim 1:
A transducer assembly for damage or flaw detection in a fastened structure, the transducer assembly comprising:
a fastener (<NUM>) including a cavity (<NUM>) disposed at one end of the fastener (<NUM>); and
an electromechanical unit (<NUM>) including a piezoelectric element (<NUM>) and a substrate (<NUM>) having a T shaped profile, the piezoelectric element (<NUM>) is disposed outside of the cavity (<NUM>) of the fastener (<NUM>) and has a bottom surface being joined to a top surface of a top section (<NUM>) of the substrate (<NUM>) disposed outside of the cavity (<NUM>), and a bottom section (<NUM>) of the substrate (<NUM>), which protrudes or extends from a bottom surface of the top section of the substrate (<NUM>), is at least partially inserted into and mechanically coupled within the cavity (<NUM>) of the fastener (<NUM>).