Carbon nanotube acoustic lens for underwater high intensity acoustic delivery

Abstract

To provide an acoustic lens for focusing a compression wave generated by underwater plasma and used for evaluating a bond between two components.SOLUTION: A system and method for evaluating a bond using an acoustic lens is provided. The acoustic lens focuses an underwater plasma generated compression wave towards a bond in a structure being inspected. The acoustic lens can be formed of a plurality of cylindrical shaped structures, each of the cylindrical shaped structures can be formed of a material that has a stiffness and a strength to withstand damage from the underwater plasma and resultant compression wave, such as carbon nanotubes. The length and diameter dimensions of each of the cylindrical shaped structures can be configured to perform phase shift of the compression wave towards the structure being inspected.SELECTED DRAWING: Figure 1A

Description

[Technical Field]

[0001] The present disclosure is directed to an acoustic lens, and more particularly, to an acoustic lens for focusing compressional waves generated by an underwater plasma and used to assess coupling between two components. [Background technology]

[0002] Underwater plasma-generated compression waves can be used to inspect the quality of adhesive bonds within a structure. During inspection, plasma is generated within the liquid by supplying an electrical pulse to a pair of electrodes within the liquid. The resulting plasma generates a compression wave. The compression wave propagates through the liquid into the structure being inspected. The structure can be a composite structure, for example, containing a carbon fiber reinforced polymer (CFRP)-to-CFRP bond or a CFRP-to-metal bond. The compression wave mechanically generates a stress load on the adhesive bond. During exposure to this load, weak bonds fail, while strong bonds function. Therefore, structures with weak bonds can be identified and repaired or discarded.

[0003] Plasma-generated underwater compression waves propagate outward in all directions from their source, such that the strength of the compression wave is inversely proportional to the square of the distance from the source. In other words, the energy per unit area decreases as the compression wave moves away from the plasma source. Problems can arise if the compression wave's strength or energy per unit area applies an insufficient stress load to the adhesive bond. One solution is to increase the strength of the compression wave by increasing the electrical pulse. However, this requires a larger and more expensive power supply. It would be desirable to increase the strength of the compression wave in the structure under test without requiring a larger and more expensive power supply. Summary of the Invention [Means for solving the problem]

[0004] An acoustic lens is disclosed that includes an array of tubular structures, each tubular structure of the array having a length and a diameter, the long axis of each tubular structure of the array being oriented in the same direction, the array of tubular structures being formed from a plurality of carbon nanotubes, an incident side of the acoustic lens formed by a first end of the array of tubular structures having a concave shape, and the length and diameter of each tubular structure of the array being configured to phase shift a compressional wave.

[0005] A method for evaluating bonding is disclosed, the method including the steps of positioning an open portion of a first container against a bonded structure being inspected, drawing a vacuum between an outer surface of the first container and an inner surface of a second container surrounding the first container, whereby the vacuum seals the first container to the bonded structure, filling the first container with a liquid, where the liquid contacts the surface of the bonded structure being inspected at the open portion, initiating a spark discharge in the liquid to form a plasma that generates a compression wave in the liquid, using an acoustic lens to focus the compression wave to a focal point at or near the surface of the bonded structure to apply a force to the bond of the bonded structure, and inspecting the bond of the bonded structure.

[0006] A system for evaluating bonding is disclosed, the system including: a first container having one or more side walls and end walls, a liquid port connected to a supply source for filling the first container with liquid, and an open portion positioned relative to a bonded structure to be tested; a second container surrounding the open portion of the first container, the second container including a vacuum port connected to a vacuum system for drawing a vacuum in a space between an outer surface of the first container and an inner surface of the second container when the open portions of the first container and the second container are adjacent to the bonded structure to be tested; a pair of electrodes disposed within the first container, arranged to generate compression waves in the liquid in the first container that are directed toward the bonded structure to be tested; and an acoustic lens disposed within the first container between the pair of electrodes and the open portion, the acoustic lens comprising an array of tubular structures formed from carbon nanotubes, the length and diameter of each tubular structure configured to phase shift the compression waves.

[0007] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present teachings, as claimed.

[0008] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate aspects of the present teachings and, together with the description, serve to explain the principles of the present teachings. [Brief explanation of the drawings]

[0009] [Figure 1A] 1A and 1B schematically illustrate a side view of an acoustic lens, according to one implementation. [Figure 1B] 1A and 1B schematically illustrate an end view of an acoustic lens, according to one implementation. [Figure 2A] 1A and 1B schematically illustrate a cross-sectional view of a system for evaluating adhesive bonds of bonded structures, according to one implementation. [Figure 2B] 1A and 1B schematically illustrate an end view of a system for evaluating bonding of a bonded structure, according to one implementation. [Figure 2C]10A and 10B schematically illustrate a side view of a system for evaluating bonding of bonded structures having curved surfaces according to another implementation. [Figure 2D] FIG. 1 is a functional block diagram of a system for evaluating adhesive bonds in bonded structures, according to one implementation. [Figure 3] 1A and 1B schematically illustrate a cross-sectional view of a system for evaluating adhesive bonds of bonded structures according to another implementation. [Figure 4] 1 is a flowchart of a method for evaluating bonding of a bonding structure, according to one implementation. [Figure 5] 1A and 1B schematically illustrate a cross-sectional view of a system for assessing bonding of a bonding structure, according to one implementation. DETAILED DESCRIPTION OF THE INVENTION

[0010] It should be noted that some details of the drawings have been depicted in a simplified manner for ease of understanding, rather than to strictly maintain structural accuracy, detail, and scale.

[0011] Reference will now be made in detail to the present teachings, examples of which are illustrated in the accompanying drawings, wherein like reference numerals are used throughout to refer to like elements. In the following description, reference is made to the accompanying drawings which form a part hereof, and in which are shown, by way of illustration, specific examples embodying the present teachings. Accordingly, the following description is by way of example only.

[0012] The present disclosure is directed to an acoustic lens for focusing underwater plasma-generated compression waves. As used herein, the term "underwater" refers to immersion in any liquid suitable for forming a plasma and propagating the resulting compression wave toward a bond to a structure being inspected. The compression wave is also referred to herein as a stress wave or longitudinal wave. The disclosed acoustic lens, formed of a plurality of cylindrical structures, possesses rigidity and strength to withstand damage from the plasma and the resulting compression wave. The length and diameter dimensions of each of the plurality of cylindrical structures are configured to phase-shift the compression wave toward the structure being inspected.

[0013] FIG. 1A schematically illustrates a side view of an acoustic lens 100 according to the present teachings. An array of tubular structures 101a, 101b, 101c, through 101n forms the acoustic lens 100. Each tubular structure 101a, 101b, 101c, through 101n can be formed of a plurality of carbon nanotubes (CNTs), such as single-walled CNTs, multi-walled CNTs, or both. The CNTs provide the acoustic lens 100 with sufficient stiffness and strength to withstand deformation and damage from underwater plasma and compressional waves. The acoustic lens 100 can have a stiffness in terms of a Young's modulus of about 0.1 to about 5.0 TPa, about 1.0 to about 4.0 TPa, or about 1.5 to about 3.0 TPa. The acoustic lens 100 can further have a strength in terms of a tensile strength of about 50 to about 200 GPa, about 60 to about 100 GPa, or about 70 to about 90 GPa. An acoustic lens 100 formed from a plurality of CNTs can withstand immersion in liquid, heat and stress from underwater plasma, and stress from compression waves.

[0014] Each tubular structure 101a, 101b, 101c, through 101n can have a length and diameter that can cause a phase shift in the compression wave. For example, if the length of the tubular structure is a multiple of half the wavelength of the sound wave, e.g., the compression wave, constructive interference can occur, resulting in a resonance condition that leads to stronger amplitude of the sound wave. If the length of the tube is a multiple of the full wavelength, destructive interference can occur, resulting in a 180-degree phase shift. The length of the tubular structure can also cause a phase shift due to the time it takes the wave to travel from one end to the other and back. The diameter of the tubular structure can also affect the speed of the sound wave, e.g., the compression wave, within it. Generally, the speed of sound waves increases as the diameter decreases. A change in sound wave speed can further change the wavelength of a given frequency, leading to a phase shift. The diameter of the tubular structure can further affect the acoustic impedance of the tubular structure, which can affect how efficiently sound waves are transmitted or reflected at the boundaries of the tubular structure. This can also cause a phase shift in the sound wave. As a result, by varying the length and diameter of the tubular structures, the conditions under which the compressional waves propagate in the tubular structures can be adjusted, leading to changes in wavelength, resonance conditions, and interference patterns, thereby resulting in a phase shift of the compressional waves. For example, the tubular structures 101a, 101b, 101c through 101n can have diameters and lengths ranging from 0.2 to 10 mm, 1 to 8 mm, or 3 to 5 mm.

[0015] The side view of acoustic lens 100 in FIG. 1A further schematically illustrates acoustic lens 100 having a concave shape on input side 110, which is formed by the first ends of an array of tubular structures. As described in more detail below, underwater plasma-generated compression waves radiate 360 ​​degrees from a source. The concave shape on input side 110 of acoustic lens 100 helps capture as much of the compression wave as possible for focusing toward the structure under test. The top view of acoustic lens 100 in FIG. 1B schematically illustrates the array of tubular structures 101a, 101b, 101c, through 101n forming a circle. As described in more detail below, the circle can match the circle of one or more test vessels. For example, the acoustic lens can have a diameter ranging from 2 to 20 cm, 5 to 15 cm, or 9 to 11 cm.

[0016] 2A-2B show schematic diagrams of a system 200 for evaluating the coupling of a coupled structure using the acoustic lens 100, according to one implementation. As shown in the schematic cross-sectional view of FIG. 2A, the system 200 includes a first container 210 having an end wall 212, one or more side walls 214, and an open portion 230 opposite the end wall 212. The open portion 230 can be the entire wall (e.g., the end wall) opposite the end wall 212, or can be a portion of the wall opposite the end wall 212. The first container 210 further includes a liquid port 220 that can be used to fill the first container with liquid when connected to a liquid source. As described in more detail below, the first container 210 functions to contain a liquid, and the open portion 230 allows the liquid to contact the coupled structure under test. The coupled structure 290 shown in FIG. 2A can be, for example, a structure formed by a first component and a second component coupled to each other by bonding, as described in more detail below.

[0017] The system 200 also includes a second container 240 having one or more side walls 244, an end wall 242, an open portion 250, and a vacuum port 270 that can be connected to a vacuum system. The second container 240 surrounds the open portion 230 of the first container 210 and may completely enclose the first container 210 as shown in FIG. 2A , thereby allowing a vacuum to be drawn in a space 260 between the exterior surface of the first container 210 and the interior surface of the second container 240 when the system 200 is placed in contact with a bonded structure 290 to be tested. The system 200 further includes a pair of electrodes 280 disposed within the first container 210 and a power source 285 connected to the pair of electrodes 280.

[0018] The acoustic lens 100 is disposed within the first container 210 between the pair of electrodes 280 and the coupling structure 290 under test. Depending on the implementation, the acoustic lens 100 can match the diameter of the first container 210 to allow the acoustic lens to be placed in place by mounting it inside the first container 210. The acoustic lens 100 can also match the diameter of the first container 210 to capture and focus as many underwater plasma-generated compression waves as possible toward the coupling structure 290.

[0019] 2B shows an end view of the system 200, particularly the end including the open portion that contacts the bonded structure 290 under test. During testing, the bonded structure 290 is positioned over the open portion 230 of the first container 210 and the open portion 250 of the second container 240. The first container 210 is filled with liquid through the liquid port 220, and a vacuum is pulled in the space 260 via the vacuum port 270. To prevent leakage of the liquid within the first container 210 and to facilitate establishing the vacuum in the space 260, a seal 219 may optionally be included at the end of the sidewall 214 adjacent the open portion 230 of the first container 210. Similarly, a seal 249 may optionally be included at the end of the sidewall 244 adjacent the open portion 250 of the second container 240. While shown with a cylindrical shape, those skilled in the art will understand that other shapes may be used, particularly shapes that focus the compressional wave toward the structure under test.

[0020] The ends of the sidewalls 214 of the open portion 230 are shown as flat to allow for inspection of bonds within the bonded structure 290 having a flat surface. Structures under inspection that have curved or shaped surfaces can be inspected by conforming the ends of the sidewalls of the first container 210 and the second container 240 to have a shape or curvature that matches the shape or curvature of the surface of the structure under inspection. As shown in FIG. 2C , the bonded structure 296 can have a shaped or curved surface 297, and the ends of the sidewalls 244 of the second container 240 can have a shape or curvature that matches the shape or curvature of the surface 297 of the bonded structure 296. Similarly, the ends of the sidewalls 214 of the first container 210 can have a shape or curvature that matches the shape or curvature of the surface 297 of the bonded structure 296 under inspection. The sidewall edges, which match the shape or curvature of the surface of the structure being inspected, allow the first container 210 to be filled with liquid without leaking and allow a vacuum to be drawn on the space 260 between the first container 210 and the second container 240. This can optionally be achieved by seals 219 and 249 having a curvature and / or shape that matches the shape or curvature of the surface of the structure being inspected. Those skilled in the art will appreciate that a shaped or curved surface 297 can affect the propagation of the compression wave from the liquid in the first container 210 to the coupling structure 296, and that the pulse amplitude and / or pulse width of the compression wave may need to be adjusted using lens 100. The focus can also be adjusted as needed to account for the curved inspection surface.

[0021] As described above, the system 200 further includes a pair of electrodes 280 disposed within the first container 210 and a power source 285 connected to the pair of electrodes 280. As used herein, a pair of electrodes refers to an anode and a cathode separated by a gap. The pair of electrodes 280 may be, for example, a part of a spark plug, such as a spark plug for a combustion engine. The pair of electrodes 280 are positioned so that a compression wave generated in the liquid can propagate toward the acoustic lens 100 and the structure being inspected. For example, the pair of electrodes 280 may be positioned in front of the open portion 230 of the first container 210 so that the compression wave can propagate through the open portion 230 and into the structure being inspected. The power source 285 supplies an electric pulse to initiate a spark discharge at the electrodes 280. The electric pulse generates a compression wave having a high amplitude and a short pulse width, for example, a high-amplitude short pulse, for coupling inspection. The power supply 285 can supply approximately 40 kV to approximately 60 kV to the pair of electrodes 280 and can be, for example, one or more layers of capacitors. The one or more layers of capacitors can be variable capacitors including time delay capabilities. The power supply 285 can also supply voltage (e.g., and electrical pulses) via a transformer to generate an arc discharge across the pair of electrodes 280 at a desired voltage. Examples of transformers include, but are not limited to, an oscillator transformer and a flyback transformer. The power supply 285 can also be a Van de Graaff-type high-voltage electrostatic generator or other source of electrical pulses. Those skilled in the art will understand that the system 200 can include other components for supplying electrical pulses to the pair of electrodes, including, but not limited to, a waveform generator, a synchronization circuit, and a driver.

[0022] 2D is a functional block diagram of an exemplary system 200 for evaluating the bonding of bonded structures using underwater spark discharge. The system 200 is positioned adjacent to a bonded structure 290. A vacuum system 278 can draw a vacuum on a space 260 within a second container 240 through a vacuum port 270. The vacuum secures the system 200 against the surface of the bonded structure 290, and a seal 247 prevents loss of the vacuum from the space 260. A liquid source 228 can fill the first container 210 with liquid through a liquid port 220. The seal 247 can prevent liquid from leaking from the first container 210 to avoid voids / pockets within the first container 210 that could affect the propagation of compression waves into the bonded structure 290. A power source 285 can supply electrical pulses to the pair of electrodes 280, generating a spark discharge that generates a plasma between the pair of electrodes 280.

[0023] System 200 may further include an ultrasonic (UT) sensor 286, a waveform generator 287, a synchronization circuit 288, and a driver 289. UT sensor 286 can be used as a pulse-echo inspection mechanism, allowing for the capture of ultrasonic signals before or after the bond of the bonded structure is subjected to a compression wave generated by the underwater plasma. By subtracting the two signals, the extent of damage or failure to the bond of the bonded structure can be determined. UT sensor 286 can also be used to monitor the strength of the compression wave generated by the underwater plasma before it reflects off the surface and becomes a tension wave. When used as a radio frequency probe, UT sensor 286 can measure the amplitude and period of a single compression wave generated by the plasma.

[0024] Synchronization circuit 288 acts as a triggering mechanism to collect data from UT sensor 286 at a very high sampling rate just before and just after the plasma discharge, allowing data to be acquired only during testing of the bonded structure. Waveform generator 287 provides controlled electrical pulses to UT sensor 286 for high sampling rate pulse-echo detection of the compressional wave.

[0025] The spark discharge generates a compression wave that propagates from the liquid to the acoustic lens 100, which focuses the compression wave toward the coupling structure 290 to apply a force to the coupling. After applying the force to the coupling, the liquid source 228 can remove the liquid from the first container 210, and the vacuum system 278 can remove the vacuum from the second container 240 to release the system from the coupling structure 290.

[0026] FIG. 3 shows a cross-sectional view of a system 200 for evaluating bonding of bonded structures, according to another implementation, in which a first container and a second container share a common side wall and / or a common end wall. The system 300 includes a second container 240 having one or more side walls 244, an end wall 242, and an open portion 250 opposite the end wall 242. The second container 240 also includes a vacuum port 270 that can draw a vacuum within a space 260 when connected to a vacuum system. The system 200 also includes a first container 210 having an end wall 242, one or more side walls 214, 244, and an open portion 230 opposite the end wall 242. The first container 210 further includes a liquid port 220 that can fill the first container with liquid when connected to a liquid source. The first container 210 can optionally include a vent 223 for removing air, for example in the form of bubbles, when the first container 210 is filled with liquid.

[0027] 3, the second container 340 and the first container 310 share an end wall 342. The second container 340 also shares a portion of a side wall 344 with the first container 310. In other words, the end wall 342 serves as an end wall for both the first container 310 and the second container 340. Similarly, the side wall 344 serves as a side wall for the second container 340, and a portion of the side wall 344 also serves as a side wall for the first container 310.

[0028] During testing, the bonded structure is positioned above the open portion 330 of the first container 310 and the open portion 350 of the second container 340. The first container 310 is filled with liquid through the liquid port 320, and a vacuum is drawn in the space 360 ​​through the vacuum port 370. An optional seal at the end of the sidewall that contacts the bonded structure can prevent the liquid in the first container 310 from escaping and facilitate establishing the vacuum in the space 360. A pair of electrodes 380 (e.g., part of a spark plug) immersed in the liquid is supplied with an electric pulse from a power source 385. An electric arc formed between the pair of electrodes 380 generates a plasma in the liquid, which then generates a compression wave directed through the open portion 330 of the first container 310 toward the bonded structure. An acoustic lens 100 can be positioned between the pair of electrodes 380 and the open portion 330 where the structure to be tested is located. The acoustic lens 100 can be positioned anywhere between the pair of electrodes 380 and the open portion 330 where the structure to be inspected is located, but the placement of the acoustic lens 100 should focus the underwater plasma-generated compression waves toward a focal point at or near the surface of the structure being inspected and minimize interference from reflections of the compression waves from the side walls and / or end walls of the first container 310 and the second container 340.

[0029] FIG. 4 is a flowchart illustrating a method 400 for assessing the coupling of structures using an acoustic lens, according to one implementation.

[0030] Method 400 can optionally begin at 405 by selecting a predetermined force to apply to the bonds of the bonded structure. The predetermined force can be about 30% to about 70% of the force required to break the bond, about 40% to about 60% of the force required to break the bond, or about 45% to about 55% of the force required to break the bond. For example, the predetermined force can be about 50% of the force required to break the bond. The predetermined force can be selected from a lookup table constructed from lap shear and / or peel ply test results of the composite materials forming the bonds of the bonded structure. Once the predetermined force is selected, a nominal pulse energy and pulse width of the compression wave can be determined based on the materials comprising the bonds of the bonded structure. For example, if 50% of the force required to break the bond is 0.5 MPa, the compression wave can have an energy of about 1 MPa and a pulse width of about 0.1 to about 10 nanoseconds.

[0031] At 410 of method 400, a system for assessing bonding as disclosed herein can be positioned relative to a structure to be inspected. Referring to FIG. 5, which schematically illustrates the operation of a system as disclosed herein, system for assessing bonding 500 can be positioned relative to a structure 590 being inspected. Structure 590 can be, for example, a structure formed by a first component 592 and a second component 594 bonded to each other by a bond 593. First component 592 and second component 594 can be, for example, layers in a composite structure. As will be appreciated, there may be more bonded components, but for simplicity, only two components are shown. In some examples, first component 592 and second component 594 are made at least partially from CFRP. In another example, first component 592 is made at least partially from CFRP, and second component 594 is made at least partially from metal. Bond 593 can include a bonding material such as a resin, adhesive, or epoxy (e.g., boron epoxy or carbon epoxy).

[0032] System 500 is positioned relative to structure 590 such that the open portions of first container 510 and second container 540 are closed by structure 590. In other words, structure 590 closes the open portion of first container 510 to allow liquid to fill first container 510 and contact the surface of structure 590. Structure 590 also closes the open portion of second container 540 to allow a vacuum to be drawn on second container 540, for example, on space 560 between the inside of the sidewall of second container 540 and the outside of the sidewall of first container 510.

[0033] At 420 of the method 400, a vacuum is drawn within the second container 540. Referring to FIG. 5 , a vacuum is drawn within the volume 560 of the second container 540 via a vacuum port 570 connected to a vacuum system. The vacuum forms a watertight chamber within the first container 510 and holds the first container 510 and the second container 540 against the structure 590 to prevent the system 500 from moving during testing, for example, when a compression wave occurs. The vacuum also allows the system 500 to be positioned in various orientations, including where the system 500 can be held against the structure 590 by the vacuum, for example, where the structure 590 is against a vertical or inclined surface to be tested. A seal, sealant material, or gasket can be placed between the edge of the sidewall adjacent the open portion and the structure 590 to assist in drawing and holding the vacuum and / or to prevent liquid leakage. The vacuum further allows the system 500 to be positioned with the open portion of the first container 510 in various orientations, such as facing up as shown in Figures 2A, 2C, and 3, facing down as shown in Figure 5, or sideways at any angle.

[0034] At 430 of method 400, a first container is filled with liquid. Referring to FIG. 5, first container 510 is filled with liquid 525 through liquid port 520 connected to a liquid source. Liquid 525 can be water, e.g., deionized water, or oil, such as mineral oil or non-conductive oil. Liquid 525 serves as a medium for generating and propagating the compression wave generated by the underwater spark discharge. First container 510 can optionally be filled with liquid to a pressure of up to about 100 psi, e.g., about 5 to about 90 psi, or about 10 to about 75 psi. The pressure within first container 510 can increase the amplitude of the compression wave generated at 440 of method 400 below. System 500 can also optionally include an additional liquid port, e.g., second liquid port 521, to increase the rate at which container 510 is filled and / or to prevent contamination of the liquid source due to disruption of the liquid by the plasma. System 500 may also optionally include a vent 523 to allow for the removal of air, e.g., air bubbles, from container 510. Valves capable of withstanding the stress waves generated by system 500 may be used to close system 500 at vent 523, liquid ports 520, 521, and / or vacuum port 570. Examples of suitable valves include, but are not limited to, actuated valves and mechanical valves.

[0035] At 440 of the method 400, an electric pulse is transmitted to a pair of electrodes immersed in the liquid in the first vessel 510 to initiate a spark discharge. Referring to FIG. 5, a pair of electrodes 580 is immersed in the liquid 525 in the first vessel 510. In another implementation, the pair of electrodes can be modified to generate a different pulse width, an electric arc with different characteristics, a stress wave with different characteristics, or a combination thereof. For example, one or both electrodes of the pair can be replaced with a different electrode made of a different material and / or having a different size / shape. In addition, the electrode configuration, e.g., the gap length between the electrodes, can be varied to modify and / or control the plasma. For example, the gap length between the pair of electrodes and / or the liquid density can be selected or varied to control the spark discharge, which affects the parameters of the compressional wave, such as its pulse width, frequency, and power (dB).

[0036] The electrical pulses are supplied by a power source 585, such as a layer of capacitors, a voltage induction source, or a voltage switching source, to provide a voltage capable of forming an underwater plasma that generates a high-amplitude, short-wavelength compression wave for bonding testing. The high-amplitude, short-wavelength compression wave can have a pulse width of about 100 ns to about 300 ns and an energy of about 5 to about 40 joules. The high-amplitude, short-wavelength compression wave can be more comparable to the compression wave generated during LBI testing. The power source 585 can supply about 40 kV to about 60 kV to the pair of electrodes 580 to generate the underwater plasma and the resulting compression wave having a duration or pulse width of about 100 ns to about 300 ns and an energy of about 5 to about 40 joules.

[0037] Without wishing to be bound by any particular theory, it is believed that when delivered to a pair of electrodes, an electrical pulse generates a high-intensity electric field across the gap between the electrodes within the liquid. This results in ionization of the liquid molecules and the formation of a gaseous plasma. The high temperature and pressure generated by the plasma and acting on the liquid results in outwardly propagating acoustic waves, e.g., compression waves, of sufficient amplitude to change the density of the liquid.

[0038] The underwater plasma-generated compression wave propagates through the liquid 525 toward the acoustic lens 500. The acoustic lens 500 focuses the compression wave toward a focal point at or near the surface of the structure 590. For example, the focal point may be within the first component 592, the surface of the first component 592, or within the liquid 525 near the surface of the first component 592 of the structure 590. The focused compression wave propagates through the first component 592 and applies a force, optionally a predetermined force, to the coupling 593. The predetermined force applied by the focused compression wave can be controlled, for example, by the length of the gap between the electrodes, the size and width of the electrical pulse, and the size and shape of the first container and / or the liquid within the container. After the focused compression wave reflects from the surface of the first component 592, the surface of the second component 594, and / or the coupling 593, additional force can be applied to the coupling 593 from the reflected compression or tension wave.

[0039] At 440 of method 400, the bond can be inspected. For example, a non-destructive testing (NDI) system, including, but not limited to, an ultrasound imaging system or an ultrasound inspection system, can be used to inspect bond 593. The inspection detects misalignments and / or damage to the bond that occurs in response to compression waves and / or reflected compression waves reflected from the back surface of the structure. If the inspection reveals that bond 593 is broken or destroyed, the quality of the bond is determined to be poor (i.e., the bond does not pass the inspection). If the inspection reveals that bond 593 (e.g., the material forming the bond) is not broken or destroyed, the quality of the bond is determined to be good (i.e., the bond passes the inspection). Such inspection of the bond is comparable to laser bond inspection and inspection per ASTM D5528-13—Standard Test Method for Mode 1 Interlaminar Fracture Toughness of Unidirectional Fiber-reinforced Polymer matrix composites. Alternative or additional steps may also be performed.

[0040] The disclosed systems and methods can replace destructive tests such as peel ply and / or lap shear tests that use mechanically applied stress to pull the bond apart. The disclosed systems and methods can also replace laser bond tests, which use expensive and large lasers / power sources and also require sacrificial material for ablation to generate the compression wave.

[0041] Furthermore, this disclosure includes examples according to the following clauses:

[0042] Clause 1. An acoustic lens comprising an array of tubular structures, each tubular structure of the array having a length and a diameter, the long axis of each tubular structure of the array being oriented in the same direction, the array of tubular structures being formed from a plurality of carbon nanotubes, an incident side of the acoustic lens formed by a first end of the array of tubular structures having a concave shape, and the length and diameter of each tubular structure of the array being configured to phase shift a compressional wave.

[0043] Clause 2. The acoustic lens of clause 1, wherein the width and length of each tubular structure in the array phase shifts the compressional wave to focus the compressional wave at a focal point.

[0044] Clause 3. An acoustic lens according to clause 2, wherein the focal point is at, near or within the surface of the composite material being inspected.

[0045] Clause 4. An acoustic lens as described in clause 1, wherein the diameter and length of each tubular structure of the array phase shifts the compressional wave so as to collimate the compressional wave.

[0046] Clause 5. The acoustic lens of clause 1, wherein the array of tubular structures has a stiffness of 0.1 to 5.0 TPa.

[0047] Clause 6. The acoustic lens of clause 1, wherein the array of tubular structures has a tensile strength of 50 to 200 GPa.

[0048] Clause 7. The acoustic lens of clause 1, wherein the plurality of carbon nanotubes comprises multi-walled carbon nanotubes, single-walled carbon nanotubes, or a mixture thereof.

[0049] Clause 8. The acoustic lens of clause 1, wherein the cross-sectional shape of the acoustic lens is circular.

[0050] Clause 9. The acoustic lens of clause 1, wherein the diameter of the acoustic lens is from about 0.2 mm to about 10 mm.

[0051] Clause 10. A method for evaluating bonding, comprising the steps of: positioning an open portion of a first container against a bonded structure being inspected; drawing a vacuum between an outer surface of the first container and an inner surface of a second container surrounding the first container, whereby drawing the vacuum seals the first container to the bonded structure; filling the first container with a liquid, where the liquid contacts the surface of the bonded structure being inspected at the open portion; initiating a spark discharge in the liquid to form a plasma that generates compression waves in the liquid; using an acoustic lens to focus the compression waves to a focal point at or near the surface of the bonded structure to apply a force to the bond of the bonded structure; and inspecting the bond of the bonded structure.

[0052] Clause 11. The method of clause 10, wherein the acoustic lens is positioned between a surface of the coupling structure and a pair of electrodes that initiate the spark discharge.

[0053] Clause 12. The method of clause 10, wherein the acoustic lens comprises an array of tubular structures formed of carbon nanotubes, the long axes of each tubular structure in the array being oriented in the same direction, and the length and diameter of each tubular structure being configured to phase shift the compressional wave to a focal point.

[0054] Clause 13. The method of clause 10, wherein the bonding of the bonded structure comprises a metal-to-composite adhesive bond or a composite-to-composite adhesive bond.

[0055] Clause 14. The method of clause 10, wherein the step of drawing a vacuum between the exterior surface of the first container and the interior surface of the second container includes the steps of compressing a seal disposed between the first container and the bonding structure, and compressing a seal disposed between the second container and the bonding structure.

[0056] Clause 15. The method of clause 10, wherein the step of filling the first container with a liquid includes filling the first container with water or oil.

[0057] Clause 16. The method of clause 10, wherein filling the first container with liquid includes introducing a pressure of up to about 100 psi into the first container.

[0058] Clause 17. The method of clause 10, wherein the step of initiating a spark discharge within the liquid includes the step of supplying about 40 kV to about 60 kV to a pair of electrodes disposed within the first vessel.

[0059] Clause 18. The method of clause 10, wherein the step of initiating a spark discharge in the liquid to form a plasma generates a compression wave in the liquid having a pulse width of about 100 ns to about 300 ns.

[0060] Clause 19. The method of clause 10, wherein the force applied to the bond by the compression wave is about 30% to about 70% of the force required to break the bond.

[0061] Clause 20. A system for evaluating bonding, comprising: a first container having one or more side walls and end walls, a liquid port connected to a source for filling the first container with liquid, and an open portion positioned relative to a bonded structure to be tested; a second container surrounding the open portion of the first container, the second container having a vacuum port connected to a vacuum system for drawing a vacuum in a space between an outer surface of the first container and an inner surface of the second container when the open portions of the first container and the second container are adjacent to the bonded structure to be tested; a pair of electrodes disposed within the first container and arranged to generate compression waves in the liquid in the first container that are directed toward the bonded structure to be tested; and an acoustic lens disposed within the first container between the pair of electrodes and the open portion, the acoustic lens comprising an array of tubular structures formed from carbon nanotubes, the length and diameter of each tubular structure configured to phase shift the compression waves.

[0062] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the present disclosure are approximations, the numerical values ​​set forth in the specific examples are reported as precisely as possible. However, any numerical value inherently contains certain errors necessarily resulting from the standard deviation found in the respective testing measurements of the particular examples. Moreover, all ranges disclosed herein should be understood to encompass all subranges subsumed therein.

[0063] While the present teachings are illustrated with respect to one or more embodiments, changes and / or modifications can be made to the illustrated examples without departing from the spirit and scope of the appended claims. Additionally, while a particular form of the present teachings may be disclosed with respect to only one of several embodiments, such form may be combined with one or more other forms of other embodiments as desired and advantageous for a given function or particular functions. As used herein, the terms "a," "an," and "the" may refer to one or more elements or portions of an element. As used herein, the terms "first" and "second" may refer to two different elements or portions of an element. As used herein, the term "at least one of A and B," with respect to a list of items such as A and B, may mean A only, B only, or A and B. Those skilled in the art will recognize that these and other variations are possible. Furthermore, to the extent that the terms "including," "includes," "having," "has," "with," or variations thereof, are used in either the detailed description or the claims, such terms are intended to be inclusive terms similar to the term "comprising." Furthermore, in the discussion and claims of this specification, the term "about" indicates that the recited value may vary somewhat, provided that such variation does not render the process or structure non-compatible for the intended purpose described herein. Finally, "exemplary" indicates that the specification is used as an example, rather than implying that it is ideal.

[0064] It will be appreciated that the above-disclosed variations and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements may subsequently occur to those skilled in the art, which are also intended to be encompassed by the following claims. [Explanation of symbols]

[0065] 100, 500 acoustic lens 101a, 101b, 101c~101n Tubular structure 110 Incidence side 200, 300, and 500 systems 210, 310, 510 First container 212, 242, 342 End walls 214, 244, 344 side wall 219, 247, 249 seals 220, 320, 520, 521 fluid ports 223, 523 Vent 228 Liquid Source 230, 250, 330, 350 open part 240, 340, 540 Second container 260, 360, 560 space 270, 370, 570 vacuum ports 278 Vacuum System 280, 380, 580 1 pair of electrodes 285, 385, 585 power supply 286 Ultrasonic (UT) Sensor 287 Waveform Generator 288 Synchronous Circuit 289 Driver 290, 296 combined structure 297 Surface 590 Structure 525 Liquid 592 First Component 593 Combine 594 Second Component

Claims

1. 1. An acoustic lens comprising an array of tubular structures, each tubular structure of the array has a length and a diameter; the longitudinal axis of each tubular structure in the array is oriented in the same direction; the array of tubular structures is formed from a plurality of carbon nanotubes; an entrance side of the acoustic lens formed by a first end of the array of tubular structures has a concave shape; the length and the diameter of each tubular structure of the array are configured to phase shift a compression wave. Acoustic lens.

2. The acoustic lens of claim 1 , wherein the width and the length of each tubular structure of the array phase shifts the compressional wave to focus the compressional wave at a focal point.

3. The acoustic lens of claim 2 , wherein the focal point is at, near, or within a surface of a composite material being inspected.

4. The acoustic lens of claim 1 , wherein the diameter and the length of each tubular structure of the array phase shifts the compressional wave so as to collimate the compressional wave.

5. The acoustic lens of claim 1 , wherein the array of tubular structures comprises a stiffness of 0.1 to 5.0 TPa.

6. The acoustic lens of claim 1 , wherein the array of tubular structures comprises a tensile strength of 50 to 200 GPa.

7. The acoustic lens of claim 1 , wherein the plurality of carbon nanotubes comprises multi-walled carbon nanotubes, single-walled carbon nanotubes, or a mixture thereof.

8. The acoustic lens according to claim 1 , wherein the cross-sectional shape of the acoustic lens is circular.

9. The acoustic lens of claim 1 , wherein the acoustic lens has a diameter of about 0.2 mm to about 10 mm.

10. 1. A method for assessing binding, comprising: positioning an open portion of the first container against the bonded structure being inspected; drawing a vacuum between an exterior surface of the first container and an interior surface of a second container surrounding the first container, the drawing of the vacuum sealing the first container to the coupling structure; filling the first container with a liquid, the liquid contacting the surface of the bonded structure to be inspected at the open portion; initiating a spark discharge within the liquid to form a plasma that generates a compression wave within the liquid; using an acoustic lens to focus the compressional wave to a focal point at or near the surface of the coupling structure to apply a force to a bond of the coupling structure; inspecting the bond of the bonded structure; A method comprising:

11. The method of claim 10 , wherein the acoustic lens is disposed between the surface of the coupling structure and a pair of electrodes that initiate the spark discharge.

12. the acoustic lens comprises an array of tubular structures formed from carbon nanotubes; the longitudinal axis of each tubular structure in the array is oriented in the same direction; the length and diameter of each tubular structure is configured to phase shift the compression wave to a focal point; The method of claim 10.

13. The method of claim 10 , wherein the bond of the bonded structure comprises a metal-to-composite adhesive bond or a composite-to-composite adhesive bond.

14. 11. The method of claim 10, wherein drawing a vacuum between the exterior surface of the first container and the interior surface of the second container comprises compressing a seal disposed between the first container and the bonded structure and compressing a seal disposed between the second container and the bonded structure.

15. The method of claim 10 , wherein filling the first container with a liquid comprises filling the first container with water or oil.

16. 11. The method of claim 10, wherein filling the first container with the liquid comprises introducing a pressure of up to about 100 psi into the first container.

17. 11. The method of claim 10, wherein initiating the spark discharge within the liquid comprises supplying about 40 kV to about 60 kV to a pair of electrodes disposed within the first vessel.

18. 11. The method of claim 10, wherein initiating the spark discharge in the liquid to form the plasma generates a compression wave in the liquid having a pulse width of about 100 ns to about 300 ns.

19. 11. The method of claim 10, wherein the force applied to the bond by the compression wave is about 30% to about 70% of the force required to break the bond.

20. 1. A system for assessing binding, comprising: one or more side walls and end walls; a liquid port configured to connect to a source of liquid for filling the first container with liquid; an open portion configured to be positioned relative to a bonded structure to be inspected; a first container comprising: a second container surrounding the open portion of the first container, the second container including a vacuum port configured to connect to a vacuum system to draw a vacuum in a space between an outer surface of the first container and an inner surface of the second container when the open portion of the first container and the open portion of the second container are adjacent to the bonded structure to be inspected; a pair of electrodes disposed within the first container and arranged to generate a compression wave within the liquid within the first container that is directed toward the bonded structure under test; an acoustic lens disposed within the first enclosure between the pair of electrodes and the open portion, the acoustic lens comprises an array of tubular structures formed from carbon nanotubes; an acoustic lens, the length and diameter of each tubular structure being configured to phase shift the compressional wave; A system comprising:

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