CA2858866A1 - Method and system for detecting and locating damages in composite structures - Google Patents

Abstract

There is provided a method and system for detecting and locating damages occurring in large structures made of polymer matrix composite materials while the structures are subjected to loading. Carbon nanotubes are added to a resin to make the latter electrically conductive. The modified resin is incorporated with long fibers to make the composite structures, which are marked with grid points where electrically conductive materials are deposited. The electrical resistances and potentials between the grid points for electrically non-conductive fibers and conductive fibers reinforced polymer composite structures are measured and used as a reference set. Since the occurrence of a damage changes the electric resistance and potential between contact points surrounding the damage, such a change serves as an indication of occurrence of the damage. The position of the damage in the structure is also determined. Damages can be detected and located in-situ while the composite structure is in operation.

Description

METHOD AND SYSTEM FOR DETECTING AND LOCATING
DAMAGES IN COMPOSITE STRUCTURES
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 USC
119(e) of United States Provisional Patent Application No.
61/570,959 filed on December 15, 2011, the contents of which are hereby incorporated by reference.
FIELD OF THE APPLICATION
[0001] The present application relates to a method and system for damage detection and location in composite structures subjected to loading.
BACKGROUND OF THE ART

[0002] Polymer matrix composites have found augmented use in many important engineering structures, such as aircrafts.
This is due to their light weight, high strength, high stiffness and good fatigue resistance. However, along with advantages, there are many issues. One of the issues is the ability to detect failure in the composite structures, particularly for in-situ processes (i.e., detection and location of damage during service). Many techniques have been proposed to address this problem, yet with limited success.

[0003] In the technique using ultrasonics, ultrasonic waves are sent through the thickness of the composite laminate. If the laminate is good, the time of travel of the ultrasonic wave is short. If there are defects in the laminate such as delaminations or matrix cracking, the time of travel of the ultrasonic wave is altered. Scanning over the surface area of the laminate is done and a map of images of the time of travel is displayed. This map of images is compared against a reference map of images taken over a good laminate. Comparison between the actual map of images and the reference map of images can indicate the area of defect.
pm Ultrasonics appears to be the most widely used technique. However, this technique poses several problems.
First, the scanning needs to be carried out in a laboratory (in the case of small samples) or in a shop (for larger samples). The fact that the technique cannot be used in situ puts a limitation to its usefulness.
[0005]
Moreover, there is a need for a transfer medium to keep the ultrasonic beam coherent. Normally, water is used as the transfer medium. This presents the problem of introduction of water and the mess that may be created due to wetness. Ultrasonic laser has been introduced recently.
This helps the coherence of the beam to some extent.
However, the need for scanning still limits the usefulness of the technique.
[0006] Another technique is referred to as acoustic emission. In acoustic emission, an artificial ear (high-frequency sensor) is attached to a composite structure.
When a crack occurs, this crack creates stress waves that propagate in the structure. When the wave hits the sensor, the sensor records and displays a signal. By reading the signal displayed from the sensor, one may tell whether a crack has been formed. By placing many sensors in a certain geometrical pattern on the surface of the structure, the crack may be located. This technique is however subjected to the problem of the extraneous noise coming from many sources, such as the reflection and reverberation from stiffeners, free edges, etc., which can interfere with the desired signal. Some time unloading can also produce signals due to the rubbing of the existing crack surfaces.

The interference of other signals confuses the information and makes the technique intractable.
[0007] According to another technique, X Rays are used.
In this technique, the sample is subjected to X Ray diffraction.
Observation of the X Ray photograph can differentiate the defect from the good material. This technique can only be applied in laboratory.
[0008]
Thermography operates on the principle that the heat emitted from the surface of a structure depends on the stress state of the material below the surface. If there are defects in the material below the surface, the temperature distribution on the surface (down to a fraction of a degree) will be non-uniform. By comparing the thermal image of a reference sample to that of the sample studied, one can determine whether there are defects in the sample.
Again this technique is applicable in laboratory and cannot be used in situ.
[0009]
Shearography is an optical technique that involves holography and speckle interferometry. An expanded laser beam is used to illuminate the region to be examined on the surface of the object. The surface can be used as is or a layer of paint or powder layer can be applied on top. Light scattered by the surface is recorded using a camera. The camera has a glass wedge or shearing interferometer to generate a double image of the object surface.
[0010] To detect the deformation of the surface, a reference interferogram is first recorded. Subsequently interferograms are recorded either statically of dynamically as the object is loaded. Either a single camera or multi-camera sensor can be used to obtain the displacement gradient components.
[0011] The disadvantages of this technique include the fact that a high precision location between the part and the camera is required, which may not be suitable for field applications. Also, it cannot be used for the measurement of the bulk strain in the material.
[0012] As another technique, optical fibers with gratings (such as Bragg gratings) are either embedded inside the composite structure or bonded on its surface. The deformation of the material in the structure is transmitted to the optical fiber. The strain in the optical fiber changes the spacing between the gratings. By sending light waves in the fibers, the reflection of the light wave from the gratings changes. By observing the changes in the reflected wave as compared to referenced signals, one can determine the level of the strain. The technique is interesting in that it can be used in situ. However the size of the optical fibers is fairly large (about 100 microns).
Embedding these fibers in the composite structures with fiber diameters in the order of 10 microns creates stress concentration and may induce damage. Bonding the fibers on the surface of the structure only allows it to detect deformation locally. The difference between the use of optical fibers and strain gages needs to be proven.
Besides, the fibers are fragile and the equipment is bulky.
[0013] Over the past few years, the electrical conductivity of carbon fibers has been used as an indication for the presence of damage. Since carbon fibers are electrically conductive along the fiber direction, by applying an electric current over two probes at two points along the direction of the fibers, the change in electric voltage can be taken as an indication of damage in the carbon fibers composites. The problem with this technique is that since the resin is not conductive, one cannot use the technique to detect resin cracks. The majority of damages at the relatively low loads is due to matrix cracking and delamination, rather than to fiber breakage.
As such, the usefulness of this technique is limited. Also, for composites made of fibers such as glass or Kevlar which are not electrically conductive, the method does not work.
[0014] A number of resistance-based sensors have been developed for the sensing of deformation and damage in composite structures. For instance, a resistance-based sensor consisting of wire contacts embedded at the edge of carbon fiber composite has been developed. The sensor was shown to be capable of detecting barely visible impact damage, and accurately providing the damage location [L Hou and S A Hayes "A resistance-based damage location sensor for carbon-fibre composites" Smart Materials and Structures. 11 (2002) 966-969].
[0015] However, the size of the panel is small (12 cm by cm) and only one damage site was created. There have also been developed sensing systems that consist of depositing lines of conductive ink containing carbon nanofibers and polymeric resin on the surface of the structure in the form of a grid pattern. Electrical wires are connected to the ends of these grid lines for measuring the changes in resistances between grid lines [Rice, Brian P. "Sensing system for monitoring the structural health of composite structures" United States Patent Application 20050284232 Al]. A thin film containing carbon nanotubes intended for detecting defects in structures has been developed [Lynch.
Jerome P., Huo. Tsung-chin, Kotov. Nicholas A., Kam. Nadine Wong Shi, Loh, Kenneth J. "Electrical Impedance Tomography of Nanoengineered Thin Films" United States Patent Application 20090121727 Al]. Smits et al proposed a carbon nanotube based sensor which consists of a number of conductors and carbon nanotubes arranged in an array [Smits.
Jan M., Kite. Marlen T., Moore. Thomas C., Wincheski.
Russell A., Ingram. Joanne L.Watkins, Anthony N., Williams.
Phillip A. "Carbon nanotube-based sensor and method for detection of crack growth in a structure" United States Patent 7278324 B2, Oct.9, 2007]. Hayes and Jones presented a composite material system where the fiber reinforcement comprises electrically conductive fibers [Hayes. Simon, Jones. Frank "Electrical damage detection system for a self-healing polymeric composite" United States Patent Application 20090294022 Al] .
[0016]
Recently, carbon nanotubes (CNTs) have been added into resin such as epoxy to make the matrix electrically conductive in composite structures. This conductivity is due to the formation of a network of the carbon nanotubes.
Upon the application of a mechanical load, the network is stretched. If the load is high enough to create cracks in the matrix material, the configuration of the network is affected and there is a change in the conductivity. Chou et al. added carbon nanotubes into epoxy matrix to make glass/epoxy composite samples and showed that cracking in tensile and impact samples corresponds with the increase in electrical resistance of coupons of about 4 inches by 6 inches or smaller [Limin Gao, Tsu-Wei Chou, Erik T.
Thostenson, and Magali Coulaud "In situ sensing of impact damage in epoxy/glass fiber composites using percolating nanotubes networks" Carbon, 2011, 49, 3382-3385; Limin Gao, Erik T. Thostenson, Zuoguang Zhang, and Tsu-Wei Chou "Sensing of Damage Mechanisms in Fiber-Reinforced Composites under Cyclic Loading using Carbon Nanotubes" Adv. Functional Materials, 2009, 19, 123-130.; E. Thostenson and T. Chou "Carbon nanotube networks: sensing of distributed strain and damage for life prediction and self-healing" Advanced Material, 2006, 18, 2837-2841; E. Thostenson and T. Chou "Real-time in situ sensing of damage evaluation in advanced fiber composites using carbon nanotubes networks"
Nanotechnology 2008, 19, 215713; Thostenson, Erik T, Chou, Tsu-wei "Method and system for detecting damage in aligned carbon nanotube fiber composites using networks" U.S. Patent No. 7,786,736-] .
[0017] S.V.Hoa et al. incorporated carbon nanotubes into glass fiber reinforced epoxy composite to detect damage using electrical resistance measurement and found that there is correspondence between the change in electrical resistance and damage accumulation during fatigue and tensile testing in the small coupon specimen [ Mohammadreza Nofar, Suong V.
Hoa and Martin Pugh "Self sensing glass/epoxy composites using carbon nanotubes" ICCM 17 Edinburgh, UK, 27-31 July 2009; M.Nofar, Suong V. Hoa and Martin Pugh "Failure detection and monitoring in polymer matrix composites subjected to static and dynamic loads using carbon nanotube networks" Composites Science and Technology, August 2009, Volume 69, Issue 10 ,Pages 1599-1606; H. Hena-Zamal and S.
V. Hoa "Fatigue damage behavior of glass/epoxy composite using carbon nanotubes as sensors" 26 th ICAF Symposium -Montreal, 1-3 June 2011;H. Hena-Zamal "Monitoring Fatigue Damage Behavior of Glass/Epoxy Composites Using Carbon Nanotubes as Sensors" Master thesis, Concordia University, April 2011].
[0018] Athanasios Baltopoulos et al. described the forward and inverse methods for detecting the location of cracks in small specimen with size of 4 inch by 4 inch glass fibers/epoxy/CNTs composite plate using Electric Resistance Tomography (ERT) technique. They mounted electrodes around the boundary of the specimen which fails the proposed technique to detect damage at the center of the plate.[Athanasios Baltopoulos, Nick Polydorides, Antonios Vavouliotis, Vassilis Kostopoulos, Laurent Pambaguian "sensing capabilities of multifunctional composite materials using carbon nanotubes"IAC-10.C2.9.2,61st International Astronautical Congress, Prague, CZ,2010].

[0019] Zhang et al. embedded carbon nanotubes in graphite fiber/carbon nanotube/epoxy laminates and showed that there is a good correspondence between crack propagation and increase in electrical resistance in the sample [Zhang W., Sakalkar V., Koratkar N. "In situ health monitoring and repair in composites using carbon nanotubes additives" Appl Phys Lett 2007; 91(31) :1-3] .
[0020] Proper et al. embedded carbon nanotubes in epoxy reinforced with Kevlar fibers. They then incorporated an electrical grid on the surface of the sample and applied electric potential across the grid. They showed that when the sample (4 inch by 6 inch) is damaged by mechanical impact, there is a correspondence between the change in voltage across the grid and the impact damage. In their arrangement, the distance between the grid points is fairly small (0.25 inch). [A. Proper, W. Zhang, S.
Bartolucci, A. Oberai and N. Koratkar "In-Situ Detection of Impact Damage in Composites Using Carbon Nanotube Sensor Networks"
Nanoscience and Nanotechnology Letters 1, 3-7, 2009].
[0021] Wardle et al. developed a system where carbon nanotubes are grown radially from fibers made of alumina.
Resins are then incorporated into the fibers to make laminates. Silver ink is then deposited to form electrode lines on the surface of the laminate. The distance between the electrode lines is about 0.118 inch. The resistances across the electrodes are measured, and it has been found that there is correspondence between the changes in resistance and damages created by impact loading. [Sunny Wicks, Derreck Barber, Ajay Raghavan, Christopher T. Dunn, Leo Daniel, Seth S. Kessler, Brian L. Wardle "Health monitoring of carbon nanotube (CNT) hybrid advanced composite for space applications" MIT; Ajay Raghavan, Seth S. Kessler Christopher T. Dunn, Derreck Barber, Sunny Wicks, Brian L. Wardle "Structural health monitoring using carbon nanotube (CNT) enhanced composites", Proceedings of the 7th International workshop on structural health monitoring (IWSHM07), Stanford University, September 9-11, 2009].
[0022] Boger et al. added carbon nanotubes and carbon black in the glass/epoxy composites and found that there is correspondence between the change in strain and the change in electrical resistance [Boger L., Wichmann M.H.G., Meyer, L.O., Schulte K., "Load and health monitoring in glass fibre reinforced composites with an electrically conductive nanocomposite epoxy matrix", Composites Science and Technology 2008; 68:1886-1894].
[0on] Shang-lin, Gao et al. deposited carbon nanotubes onto glass fiber surfaces and found that epoxy composites made using this fiber system may be used for in-situ sensing of strain and damage [Shang-lin Gao, Rong-Chuan Zhuang, Jie Zhang, Jian-Wen Liu, and Edith Mader "Glass Fibers with Carbon Nanotube Networks as Multifunctional Sensors" Adv.
Functional Materials. 2010, 20, 1885-1893].
poN The above works show very interesting and innovative attempts to monitor deformation and damage in composite materials. Many sensors have been developed and show promise. However it remains to be seen whether these sensors can win out over other existing sensors such as strain gages, fiber optics, etc. The incorporation of carbon nanotubes in the epoxy used for the fiber/epoxy composites is very interesting. However, the works in this area have been limited to composite coupons of relatively small sizes. Works have been made on coupons of about 4 inches by 6 inches or smaller. Cracks or damages occurring in these samples would certainly interrupt the flow of electrical current from one electrical probe to another.
Extending the technique from small coupons to larger structure runs into many challenging issues.

[0025] One issue is the spatial non-uniformity in the electrical conductivity over the surface of the structure.
Mixing the carbon nanotubes in the epoxy resin takes a lot of effort. Incorporating the modified epoxy with the long fibers requires the penetration of the nanoparticles in between the long fibers. Many of the works cited above use vacuum assisted resin transfer molding to make the laminates. In this process, only vacuum is used and there may not be sufficient pressure to compact the laminates well. For prepreg and autoclave curing, it is important that the uniform distribution of the carbon nanotubes in the whole of the structure to be obtained.
Otherwise, the technique may not work.
SUMMARY OF THE APPLICATION
[0026] It is therefore an aim of the present disclosure to provide a method and system to detect and locate damages in composite structures using carbon nanotubes.
[0027]
Therefore, in accordance with the present application, there is provided a damage detection and location method for a composite structure of the type incorporating carbon nanotubes, the method comprising receiving from a plurality of electrical contacts arranged on a surface of the composite structure measurements of an electrical property of the composite structure, calculating a change between the received measurements and reference values of the electrical property, and identifying a damage if the change is above a predetermined threshold.
[0028] Still further in accordance with the present application, identifying the damage comprises locating the damage by correlating the change to a location in the composite structure of selected ones of the plurality of electrical contacts having provided the measurements.

[0029] Still further in accordance with the present application, receiving measurements of the electrical property of the composite structure comprises receiving a plurality of the measurements from a first grid of electrically conductive points arranged on the surface of the composite structure, each one of the plurality of the measurements comprising an electrical resistance measured between a first one and a second one of the electrically conductive points.
[0030] Still further in accordance with the present application, the composite structure is an electrically non-conductive fiber reinforced composite structure and the range of the electrical resistance is between 103 ohm and 106 ohm.
[0031] Still further in accordance with the present application, calculating the change between the received measurements and the reference values comprises calculating for each one of the plurality of the measurements a difference between the measured electrical resistance and a reference electrical resistance between the first and the second electrically conductive point.
[0032] Still further in accordance with the present application, receiving measurements of the electrical property of the composite structure comprises receiving a plurality of the measurements from a second grid of electrically conductive lines arranged in a first and a second orientation on the surface of the composite structure, each one of the plurality of the measurements comprising a first electrical resistance measured between two consecutive ones of the electrically conductive lines arranged in the first orientation and a second electrical resistance measured between two consecutive ones of the electrically conductive lines arranged in the second orientation.

[0033] Still further in accordance with the present application, calculating the change between the received measurements and the reference values comprises, for each one of the plurality of the measurements, calculating a first difference between the first measured electrical resistance and a first reference electrical resistance between the two consecutive electrically conductive lines arranged in the first orientation, calculating a second difference between the second measured electrical resistance and a second reference electrical resistance between the two consecutive electrically conductive lines arranged in the second orientation, and computing an average of the first and the second difference.
[0034] Still further in accordance with the present application, receiving measurements of the electrical property of the composite structure comprises receiving a plurality of the measurements from a first grid of a first set of electrically conductive points and a second grid of a second set of electrically conductive points, the first and second grids arranged on the surface of the composite structure, the first set of electrically conductive points for applying a constant electric current to the composite structure and the second set of electrically conductive points for measuring an electrical potential.
[0on] Still further in accordance with the present application, receiving measurements of the electrical property of the composite structure comprises receiving the plurality of the measurements each comprising the electrical potential measured between a first one and a second one of the second set of electrically conductive points.
[0036] Still further in accordance with the present application, calculating the change between the received measurements and the reference values comprises calculating for each one of the plurality of measurements a difference between the measured electrical potential and a reference electrical potential between the first one and the second one of the second set of electrically conductive points.
[0037] In accordance with the present application, there is further provided a method for fabricating a composite structure of the type incorporating carbon nanotubes, the method comprising preparing a first amount of an epoxy resin, preparing a second amount of a curing agent to the epoxy resin, preparing a third amount of the carbon nanotubes, mixing the first amount of the epoxy resin with the second amount of the curing agent to produce the epoxy matrix, dispersing the third amount of the carbon nanotubes into the epoxy matrix to produce the modified epoxy matrix, and incorporating the modified epoxy matrix into long fibers.
[0038] Still further in accordance with the present application, incorporating the modified epoxy matrix into the long fibers comprises incorporating the modified epoxy matrix into one of electrically non-conductive long fibers and electrically conductive long fibers.
[0039] Still further in accordance with the present application, the carbon nanotubes are provided in the composite structure in a presence ensuring uniform distribution of the carbon nanotubes in the composite structure, electrical conductivity of the composite structure, and detectability of a damage in the composite structure.
[0040] In accordance with the present application, there is further provided a damage detection and location system for a composite structure of the type incorporating carbon nanotubes, the system comprising a plurality of electrical contacts arranged on a surface of the composite structure, a damage detection unit for detecting and locating a damage in the composite structure, the damage detection unit comprising an acquisition module for receiving from the plurality of electrical contacts measurements of an electrical property of the composite structure, a value comparator for calculating a change between the received measurements and reference values of the electrical property, and for identifying a damage from the change having a magnitude beyond a predetermined threshold, a damage locator for determining a location of the damage, and an output for providing data pertaining to the damage and the location thereof.
[0041] Still further in accordance with the present application, the damage locator is adapted to determine the location of the damage by correlating the change to a position on the composite structure of selected ones of the plurality of electrical contacts having provided the received measurements.
[0042] Still further in accordance with the present application, the plurality of electrical contacts is arranged on the surface of the composite as one of a first grid of electrically conductive points deposited on the surface of the composite structure and a second grid of electrically conductive lines arranged in a first and a second orientation on the surface of the composite structure.
[0043] Still further in accordance with the present application, the acquisition module receives a plurality of the measurements each comprising an electrical resistance measured between a first one and a second one of the electrically conductive points.
[0044] Still further in accordance with the present application, the value comparator calculates the change by calculating for each one of the plurality of the measurements a difference between the measured electrical resistance and a reference electrical resistance between the first and the second electrically conductive points.
[0045] Still further in accordance with the present application, the acquisition module receives a plurality of the measurements each comprising a first electrical resistance measured between two consecutive ones of the electrically conductive lines arranged in the first orientation and a second electrical resistance measured between two consecutive ones of the electrically conductive lines arranged in the second orientation.
[0046] Still further in accordance with the present application, the value comparator calculates the change by calculating, for each one of the plurality of the measurements, a first difference between the first measured electrical resistance and a first reference electrical resistance between the two consecutive electrically conductive lines arranged in the first orientation, a second difference between the second measured electrical resistance and a second reference electrical resistance between the two consecutive electrically conductive lines arranged in the second orientation, and an average of the first and the second difference.
[0047] Still further in accordance with the present application, the plurality of electrical contacts is arranged on the surface of the composite structure as a first grid of a first set of electrically conductive points and a second grid of a second set of electrically conductive points, the first and second grids deposited on the surface of the composite structure, the first set of electrically conductive points for applying a constant electric current to the composite structure and the second set of electrically conductive points for measuring an electrical potential.

[0048] Still further in accordance with the present application, the acquisition module receives a plurality of measurements each comprising the electrical potential measured between a first one and a second one of the second set of electrically conductive points.
[0049] Still further in accordance with the present application, the value comparator calculates the change by calculating for each one of the plurality of measurements a difference between the measured electrical potential and a reference electrical potential between the first one and the second one of the second set of electrically conductive points.
glom In accordance with the present application, there is further provided a computer readable medium having stored thereon program code executable by a processor for damage detection and location in a composite structure of the type incorporating carbon nanotubes, the program code executable for receiving from a plurality of electrical contacts arranged on a surface of the composite structure measurements of an electrical property of the composite structure, calculating a change between the received measurements and reference values of the electrical property, and identifying a damage if the change is above a predetermined threshold.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] Fig. la is a flowchart of a damage detection and location method for composite structures, in accordance with an embodiment of the present disclosure;
[0052] Fig. lb is a flowchart of the step of Fig. la of fabricating composite structures;

[0053] Fig. lc is a flowchart of the step of Fig. la of detecting, locating, and determining the severity of damages in composite structures;
[0054] Fig. id is a flowchart of the step of Fig. lc of monitoring damages of composite structures;
[0055] Fig. 2a is a graph of a distribution of electrical resistances between measurements points in a 22x13"
composite laminate containing 0.20 wt % CNTs, showing a non-uniform electrical resistance distribution;
[0056] Fig. 2b is a graph of a distribution of electrical resistances between measurements points in a 22x13"
composite laminate containing 0.25 wt% CNTs;
[0057] Fig. 2c is a graph of a distribution of electrical resistances between measurements points in a 22x13"
composite laminate containing 0.30 wt% CNTs;
[0058] Fig. 2d is a graph of a distribution of electrical resistances between measurements points in a 22x13"
composite laminate containing 0.40 wt% CNTs;
[0059] Fig. 2e is a graph of a distribution of electrical resistances between measurements points in a 22x13"
composite laminate containing 1 wt% CNTs;
[0060] Fig. 3a is a block diagram of a damage detection and location system for composite structures, in accordance with an embodiment of the present disclosure;
posq Fig. 3b is a flowchart of an algorithm implemented by the analysis module of Fig. 3a for detecting and locating damages in electrically non-conductive fibers reinforced polymer composite structures containing CNTs;
[0062] Fig. 3c is a flowchart of an algorithm implemented by the analysis module of Fig. 3a for detecting and locating damages in electrically conductive fibers reinforced polymer composite structures containing CNTs;
[0063] Fig. 4a is a schematic view of the system of Fig.
3a showing a matrix of measurement points;

[0064] Fig. 4b is a schematic view of an alternative arrangement of the system of Fig. 3a, with sets of two grid points on the surface of a carbon fibers/epoxy/CNTs composite structure;
[0065] Fig. 4c is a schematic view of an alternative arrangement of the system of Fig. 3a, showing a network of electrical lines;
[0066] Fig. 5a is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after hole of size 1 (1/16) inch is drilled;
V1067] Fig. 5b is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after two holes of sizes 1 and 2 (1/16 and 2/16) inch are drilled;
[0068] Fig. 5c is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after three holes of sizes 1, 2 and 3 (1/16, 2/16 and 3/16) inch are drilled;
g069] Fig. 5d a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt % CNTs composite laminate after holes of sizes 1, 2, 3 and 4 (1/16, 2/16, 3/16 and 4/16) inch are drilled;
[0070] Fig. 5e is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after five holes of sizes 1, 2, 3, 4 and 5 (1/16, 2/16, 3/16, 4/16 and 5/16) inch are drilled;
[0071] Fig. 5f is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after six holes of sizes 1, 2, 3, 4, and 6 (1/16, 2/16, 3/16, 4/16, 5/16 and 6/16) inch are drilled;
[0072] Fig. 6 is a graph showing a severity of damage due to various sizes of holes;

[0073] Fig. 7a is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after impact damage 1 (78J) is made;
[0074] Fig. 7b is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after impact damage 1 and 2 (78J
each) are made;
[0075] Fig. 7c is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after impact damage 1, 2 and 3 (78J
each) are made;
[0076] Fig. 7d is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after impact damage 1, 2, 3 and 4 (78J each) are made;
[0077] Fig. 7e is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after impact damage 1, 2, 3, 4 and 5 (78J each) are made;
glom Fig. 7f is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after impact damage 1, 2, 3, 4, 5 and 6 (78J each) are made;
[0079] Fig. 8a is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after barely visible impact damage 1 (1J) is made;
[0080] Fig. 8b is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after barely visible impact damages 1 and 2 (1J and 2J) are made;
voirq Fig. Sc is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%

CNTs composite laminate after barely visible impact damages 1, 2 and 3 (1J, 2J and 3J) are made;
[0082] Fig. 8d is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate 8 after barely visible impact damages 1, 2 ,3 and 4 (1J, 2J, 3J and 4J) are made;
[0083] Fig. 8e is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after barely visible impact damages 1, 2, 3, 4 and 5 (1J, 2J, 33, 43 and 5J) are made;
[008] Fig. 8f is a graph of an electrical resistance change distribution of a 22x13" glass fibers/epoxy/ 0.30 wt%
CNTs composite laminate after barely visible impact damages 1, 2, 3, 4, 5 and 6 (1J, 2J, 3J, 4J, 5J and 10J) are made;
[0085] Fig. 9 is a graph showing a severity of damage due to various applied impact energies;
[0086] Fig. 10 is a graph of an average electrical resistance change distribution of 22x13" glass fibers/epoxy/
0.30 wt% CNTs composite laminate after holes of sizes 1, 2, 3, 4, 5 and 6 (1/16, 2/16, 3/16, 4/16, 5/16 and 6/16) inch located at A, B, C, D, E and F respectively are drilled;
[0087] Fig. lla is a graph of an electric potential distribution of 22x13" carbon fibers/epoxy/ 0.3 wt % CNTs composite laminate;
[0088] Fig. lib is a graph of an absolute electric potential change distribution of 22x13" carbon fibers/epoxy/
0.30 wt% CNTs composite laminate after six holes of sizes 1, 2, 3, 4, 5 and 6 (1/16, 2/16, 3/16, 4/16, 5/16 and6/16) inch respectively are drilled;
[0089] Fig. 11c is a graph showing the effect of hole size on the change in electrical potential;
[0090] Fig. lid is a graph of an absolute electric potential change distribution of 22x13" carbon fibers/epoxy/
0.30 wt% CNTs composite laminate after impact damage 1, 2, 3, 4, 5 and 6 are made using 318 mg aluminum particles travelling at 700 m/sec (78J).; and [0091] Fig. lie is a graph of an absolute electric potential change distribution of 22x13" carbon fibers/epoxy/
0.30 wt% CNTs composite laminate after barely visible impact damages 1, 2 and 3 (1J, 2J and 3J) are made.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0092] Referring now to Fig. la, a method 100 for damage detection and location in composite structures will now be described. The composite structures are, for instance, of relatively large scale, as commonly found in the aerospace industry, the wind-turbine industry, the automotive industry, the naval ships, the civil structures (composite rebars), space structures, such as satellites, etc., to name a few of many instances using composite structures. The composite structures may consist of any polymer matrix composite materials, with any appropriate type of resin binding the fibers.
[0093] The method 100 illustratively comprises fabricating at step 102 composite structures using carbon nanotubes (CNTs), depositing at step 104 a grid of electrical contacts, such as electrically conductive adhesive points or lines, as will be discussed further below, on the surface of the composite structures manufactured at step 102, and at step 106 detecting, locating, and determining the severity of damages in the composite structures in real-time and in-situ.
[0094] Referring to Fig. lb, the step 102 of fabricating the composite structures comprises mixing at step 108 carbon nanotubes in a polymer resin, such as an epoxy resin, and incorporating at step 110 the modified epoxy resin with long electrically conductive or electrically non-conductive fibers to make the composite structures.
[0095] The step 108 may require preparing a predetermined amount of polymer resins such as epoxy resin, preparing a predetermined amount of curing agent for the epoxy resin, preparing a predetermined amount of electrically conductive nanoparticles such as carbon nanotubes, mixing the predetermined amount of epoxy resin with the predetermined amount of curing agent and obtaining an epoxy matrix which is a combination of epoxy resin and curing agent, dispersing the predetermined amount of carbon nanotubes into the epoxy matrix to make the epoxy matrix electrically conductive, and obtaining modified epoxy matrix which is a combination of epoxy resin, curing agent and carbon nanotubes.
[0096] The step 110 may require preparing a predetermined amount of long electrically non-conductive fibers such as Kevlar and glass fibers or long electrically conductive fibers such as carbon fibers, incorporating the modified epoxy matrix with the predetermined amount of long electrically nonconductive fibers such as glass fibers or long electrically conductive fibers such as carbon fibers, and obtaining smart structures made of fibers reinforced polymer matrix composite materials.
[0097] The presence of carbon nanotubes in epoxy resin provides electrical conductivity for the resin. According to an embodiment, mixing may be done at step 108 by using a three-roll calendering machine. Multiwall carbon nanotubes may be used. The amount of the carbon nanotubes should not be too large or too small. If the amount of the nanotubes is too small, it will not provide a good percolation network for electrical conductivity, or stability of the results.
If the amount of carbon nanotubes is too high, it creates high viscosity which renders subsequent incorporation of continuous fibers difficult. Also, too small an amount of nanotubes will tend to give spatial variability in the electrical conductivity. On the other hand, too large an amount of carbon nanotubes will reduce the sensitivity of the change in electrical resistance due to the occurrence of damage.
[0098] It is desirable for the incorporation at step 110 of epoxy containing CNTs into long fibers to take into account uniformity of distribution of carbon nanotubes. Non-uniform distribution of carbon nanotubes may not provide good results due to the short-circuit phenomenon. For a structure where there are regions of good conductivity and regions of poor conductivity, the electrical current will follow the path of good conductivity and avoid the path of poor conductivity. When there is a defect in the region of poor conductivity, the system cannot sense it due to the lack of current flow. As such, damage detection and location thereof, as effected at step 106 of the method 100, may not be efficient in the case of non-uniform CNT
distribution.
[0099] The incorporation at step 110 of the modified epoxy into the long fibers requires the penetration of the nanoparticles in between the long fibers. Some of the prior art works use vacuum-assisted resin transfer molding to make the laminates. In this process, only vacuum is used and there may not be sufficient pressure to compact the laminates well. For prepreg and autoclave curing, it is desirable to obtain the uniform distribution of the carbon nanotubes in the whole structure, as will be discussed further with reference to Fig. 2a to Fig. 2e.
[00100]
Referring to Fig. lc, the step 106 of detecting, locating, and determining the severity of damages in the composite structures illustratively comprises the step 112 of coupling the grid deposited at step 104 of Fig. la to a damage detection unit. For this purpose, electrical wires may be attached to the grid to make electrodes. The electrodes may in turn be connected to the damage detection unit. The damage detection unit, which will be discussed further below with reference to Fig. 4, may comprise an acquisition module for collecting and storing information regarding electrical properties of the composite structures and an analysis module for analyzing the collected information to detect, locate, and determine the severity of damages in the composite structures.
[00101] Once the grid is coupled to the damage detection unit at step 112, reference values of electrical properties of the composite structures may be established at step 114.
For this purpose and as will be described further below, the electrical properties, e.g. electrical resistance or potential, between pairs of contacts on the grid, e.g. grid points or grid lines, may be measured. In this manner, reference values for all measurements points of the composite structure containing carbon nanotubes may be obtained. The reference values are, for instance, obtained before the composite structure is used, or at a reset, for instance after an inspection or a use.
[00102] The damages of the composite structures may then be monitored in-situ and in real-time at step 116. In particular, and referring to Fig. id, the step 116 of monitoring the damages may comprise obtaining at step 118 present values during the use of the composite structure.
According to an embodiment, the present values are continuously measured or punctually measured, at given time intervals. A change or difference (A) between present values and reference values may then be calculated for each set of measurement points. If the difference is identified at step 120 as being above a given threshold or tolerance, this may be an indication of damage. It may be desired to continue monitoring the values of the set of measurement points, as well as sets of measurement points neighbour to the identified above-threshold set of measurement points, to confirm that there is indeed a damage. It is considered to set to zero the calculated difference of the identified above-threshold set of measurement points and neighbour sets of measurement points to confirm that there is a damage.
polim When damage is confirmed, the location of the damage relative to the composite structure is provided at step 122. This is done by correlating the identified above-threshold set of measurement points to its location or position on the composite structure. It is anticipated that multiple values of the measured electrical properties, e.g.
electrical resistance or electrical potential, may be above the threshold. In such case, the maximum values of the measured electrical properties may be used to identify the specific location of the damage.
[00104] The damage is then output at step 124. The damage may be output in the form of a simple notification, an identification of the location, and/or a quantification of the damage. The output may be provided as a plot or any other suitable presentation means. An example of output is provided below in an exemplary embodiment.
gmq Referring to Fig. 2a to Fig. 2e and as discussed above with reference to Fig. lb, in performing the step of incorporating long fibers into the modified epoxy resin, it is desirable to achieve uniform distribution of the carbon nanotubes in the whole structure. Having an adequate electrical conductivity further gives suitable results. If the conductivity is too high, the occurrence of a defect may be picked up as the change in electrical resistance between contact points in the vicinity of the crack, for larger cracks only. This was reflected in the cases of samples containing 0.3 wt% CNTs and 1 wt% CNTs where the changes in electrical resistance of 4.8% and 0.446% respectively were observed. Too small an amount of CNTs in the composite structure A (even more than the percolation threshold of CNTs for the case of small samples) will not guarantee uniform distribution of electrical resistance. If the distribution of electrical resistance is not uniform, it makes it difficult to perform a comparison in electrical resistance between new or present resistance values and reference resistance values, leading to imprecise damage detection and location.
[00106] In an experiment, the measured electrical resistances show that a twenty-two (22) inch by thirteen (13) inch composite laminate containing 0.10% CNTs does not behave as a conductive material and it is approximately an insulator. Fig. 2a to Fig. 2e show the distribution of the electrical resistance between grid points in a composite laminate (22 inch by 13 inch) made using different amounts of CNTs (from 0.20 wt% to 1.0 wt%). It can be seen that the distribution of electrical resistance is not uniform over the surface of the laminated composite plates having 0.2 wt% CNTs and 0.25 wt% CNTs. This indicates that there is a window of CNT percentages that provides damage detection and location monitoring with high sensitivity and uniform distribution of electrical resistance. In particular, the amount of CNTs ranging between 0.3 and 0.4 wt% CNTs, and more particularly about 0.30 wt% CNTs, provides good damage detection and location for glass fibers/epoxy composite structures. It should however be understood that the optimal amount of CNTs may vary depending on the particular fiber and/or resin used. The weight ratio depends on the specifications of the CNTs. For instance, when longer CNTs are used, the ratio of CNT may be smaller for a same conductivity than a higher ratio with smaller CNTs. Put in terms of resistance, the range of resistance for electrically non-conductive fibers reinforced composite structures between 103 ohm to 106 ohm provides good damage detection and location capability. These values are provided simply as an example and therefore non-exclusively, and other values are considered as well.
N0107]
Referring now to Fig. 3a, a system 200 for damage detection and location in a composite structure, generally identified as A, will now be described. As discussed above, the composite structure A is made of fibers (electrically conductive or electrically non-conductive) reinforced polymer matrix composite materials containing CNTs. The damage detection system 200, and method 100 described hereinabove, are of the type that can be used during operation of the composite structure (e.g., flight conditions of an aircraft). The damage detection system 200 has a damage detection unit 202, and within the composite structure A either a matrix of measurement points generally shown at 204, featuring measurement points labeled 1-40 in Fig. 4a, a matrix of measurement points 204', featuring two sets of grid points as shown in Fig. 4b, or a network of electrical lines 205 as shown in Fig. 4c.
[00108]
Referring to Fig. 4a in addition to Fig. 3a, each matrix point 1-40 can be made of conductive silver paste, silver paint, or similar materials. A thin film of grid points may also be used. Each point serves as an electrical contact point, and thus as a measurement point. It is desirable to precisely set the distance between the measurement points. Indeed, if the distance between the measurement points is too large, the change in the resistance between the two measurement points may not reflect the occurrence of damage that has occurred in the vicinity of the measurements points. If the distance between the measurement points is too small, a very large number of measurement points would be required for a structure of a certain size. Too many measurement points may render the technique impractical. The distance between measurement points 1-40 may thus be selected as a function of the composite structure A.
[00109] In the illustrated embodiment, for the matrix 204 of measurement points having a first dimension dl of thirteen (13) inches and a second dimension d2 of twenty-two (22) inches, the distance between the measurements points, e.g. the distance d3 between measurement points 4 and 5 or the distance d4 between measurement points 5 and 10, is set to three (3) inches (or 76.20mm). Such a spacing was found to be more sensitive to changes in electrical resistance between adjacent points, thus resulting in a more accurate detection and location of damages on the composite structure A. Electrical conductive wires 206 are further illustratively attached to the measurement points 1-40 for electrical measurements and to the damage detection unit 202 for data gathering. The damage detection unit 202 then applies a constant source voltage by electrodes of the wires 206 mounted on the surface of the composite structure A.
The electrical current is measured by the damage detection unit 202 to calculate electrical resistance.
[00110]
Referring to Fig. 4b in addition to Fig. 3a, another arrangement of grid points is generally illustrated at 204', for being used as part of the damage detection system 200. The arrangement 204' of Fig. 4b depicts a representation of a plurality of sets of two grid points.
Each point of the grid can be made of conductive silver paste or similar materials. Each point of a set of two grid points serves as an electrical contact point. The first set of grid points, e.g. points 11-401, which is geometrically similar to the grid points of Fig. 4a, is mounted on the composite structure A. However, in this case, the first set of grid points is used to apply a constant electrical current through the composite structure. The second set of grid points, e.g. points 1V-40V, is diagonally shifted (e.g., by 5 mm) with respect to the first set and is mounted to measure electric potential. The electrical conductive wires 206 are attached to the grid points to make electrodes. In particular, the wires 206 attached to first grid points 11 to 401 are used to apply a constant current through the composite structure. The wires 206 attached to the shifted grid points 1V to 40V are used to measure electric potentials. The electric potentials across the second grid points monitored by the damage detection unit 202 are used as input values. In the illustrated embodiment, for the matrix 204' with a first dimension dl' of thirteen (13) inches and a second dimension d2' of twenty-two (22) inches, the distance between the grid points, e.g. distance d3' between measurement points 41 and 51 or the distance d4' between measurement points SV and by, is set to three (3) inches.
prill] As schematically illustrated in Fig. 4c, a network 205 of electrical lines may be used as an alternative to matrix points 204 or 204'. The network 205 comprises a plurality of electrical contact lines in a first, e.g.
vertical, orientation (X lines) and a plurality of electrical contact lines in a second, e.g. horizontal, orientation (Y lines). The electrical lines are mounted as electrical contacts on the surface of the structure A, to measure the electrical resistance using a two-probe method (described further below). In this manner, it becomes possible to detect damages of large electrically non-conductive fibers such as glass-fiber and kevlar-fiber reinforced epoxy composite structures containing CNTs. The electrical conductive wires 206 are bonded to these grid lines to make electrodes for electrical measurements. The electrical conductive wires 206 are further attached to the damage detection unit 202 for data gathering. A constant voltage may be directly applied by the damage detection unit 202 via the mounted electrodes on the surface of the composite structure A and the electrical current is measured by the damage detection unit 202 to calculate the electrical resistance. In such a case, insulator material may be used at the junction of the vertical and horizontal lines, to create non-contacting nodes as in 208. In the illustrated embodiment, with the network 205 having a first dimension dl' of thirteen (13) inches and a second dimension d2' of twenty-two (22) inches, the electrical lines are spaced apart by a distance d3" of three (3) inches along each direction.
[00112]
Referring back to Fig. 3a, the damage detection unit 202 comprises an acquisition module 220 that will obtain the signals (e.g. electrical resistance or potential) between measurement points of the network 204 (or the network 204'), or from the lines of the network 205. The signals are obtained in a reference state (e.g., prior to a first use, at a reset, etc.), and during use as well. The reference values can be further stored for future comparison with a new set of values when damage occurs. The acquired measurements may then be sent to an analysis module 221, which may be used to analyze the received information for the purpose of detecting, locating, and determining the severity of damages in the composite structure A. For this purpose, the analysis module 221 illustratively comprises a value comparator 222 communicating with a database 224, a damage locator 226, and an output module 228.
[00113] The acquisition module 220 may comprise any data acquisition unit suitable for acquiring measurements of a given electrical property, e.g. electrical resistance or potential. The analysis module 221 may be implemented as program code stored in a memory (not shown) and executable by a processor (not shown) of a computer (not shown). The program code, when executed, illustratively causes real-time and in-situ determination of severities, detections, and locations of damages in composite structures as in A. The processor may be any device that can perform operations on data. Examples are a central processing unit (CPU) and a microprocessor. The memory accessible by the processor may received and store data. The memory may be a main memory, such as a high speed Random Access Memory (RAM), or an auxiliary storage unit, such as a hard disk or flash memory.
The memory may be any other type of memory, such as a Read-Only Memory (ROM), Erasable Programmable Read-Only Memory (EPROM), or optical storage media such as a videodisc and a compact disc.
[00114] One or more databases 224 may be integrated directly into the memory. The database 224 described herein may be provided as collections of data or information organized for rapid search and retrieval by a computer. The database 224 may be structured to facilitate storage, retrieval, modification, and deletion of data in conjunction with various data-processing operations. The database 224 may consist of a file or sets of files that can be broken down into records, each of which consists of one or more fields. Database information may be retrieved through queries using keywords and sorting commands, in order to rapidly search, rearrange, group, and select the field. The database 224 may be any organization of data on a data storage medium, such as one or more servers.
[00115] For the network 204 of measurement points (Fig. 4a), an electrical resistance measurement technique is used to detect both internal and surface damages of large non-conductive fibers, such as kevlar and glass fibers, reinforced epoxy composite structures containing CNTs. The electrical resistance measurement technique adopts the electrically conductive CNTs themselves as networks of in-situ sensors and hence requires inexpensive equipment to operate. As discussed above, a constant source voltage may be directly applied by the mounted electrodes on the surface of the composite material and the electrical current is measured and received at the acquisition module 220 to calculate the electrical resistance. In order to detect the severities and locations of damage in the large-scale composite structures, grid points may be examined based on the electrical resistance measurement.
[00116] The Electrical Resistance Change (ERC) between two measurement points of Fig. 4a is expressed using the following equation:
¨RR.
AR(%) = _____________________ x 100 (1) R1,14 [00117] Where is the electrical resistance between points i and j before damage, or reference resistance; and [00118] RF,1,3 is the electrical resistance between points i and j during use, also referred to present resistance.
[00119] Using the measurements of electrical current received from the acquisition module 220, the value comparator 222 calculates the present values of the electrical resistance. Using equation (1), the value comparator 222 then computes the ERC for the purpose of comparing present values to reference values retrieved from the database 224 of any appropriate type. As discussed above, the complete set of electrical resistances between grid point pairs before damage establishes the reference resistance values. These reference values are illustratively stored in the database 224 to enable the value comparator 222 to effect the above-mentioned comparison with the set of present values.
polm For refereeing purposes, the electrical resistances between adjacent pairs of measurement points are obtained using for instance a two-probe method. For the array of Fig. 4a, examples of electrical resistances between adjacent pairs of measurement points corresponding to the first cell are:
R1,2, R1,6, R2,7, R6,7, [00121] Where R stands for the electrical resistance between two measurement points represented by the subscript numbers.
A similar approach may be adopted for the grid lines of network 205 (Fig. 4c). Indeed, for the grid line arrangement 205 of Fig. 4c, the Average Electrical Resistance Change (AERC) for each box between a foursome of grid lines is expressed using the following equation:
',J+1+AYw AERC(96) - (2) [00122] Where and A
Y3,j+1 are the electrical resistance changes between two consecutive vertical (X lines) and horizontal electrical contact lines (Y lines), respectively, which are defined in the following equations:
X
(%) = __ y x 100 (3) 1,1,t+1 Y
AY11-0 (%) = ___ 1'4'1+1 14'1+1 x 100 (4) [00123] Where i is the number of the electrical contact line in a first orientation, e.g. vertical in Fig 4c;
[00124] j is the number of the electrical contact line in a second orientation, e.g., horizontal in Fig 4c;
polzq x1,1 is the initial electrical resistance measured between two consecutive vertical electrical contact lines before damage, or reference resistance;

[00126] XF,i,õ1 is the final electrical resistance measured between two consecutive vertical electrical contact lines during use, or present reference;
[00127] 171,4.1 is the initial electrical resistance measured between two consecutive horizontal electrical contact lines before damage, or reference resistance; and [00128] 17F,3,j,/ is the final electrical resistance measured between two consecutive horizontal electrical contact lines during use, or present reference.
[00129] The complete set of electrical resistances between measurement point pairs before damage establishes the reference resistance values, stored in the database 224, after the acquisition of measurements of the electrical current by the acquisition module 220 and computation of the electrical resistance by the value comparator 222. The reference of electrical resistance values can be stored for future comparison by the value comparator 222 with a present set of electrical resistance numbers when damage occurs. The comparison may be done by computing the AERC for each box between a foursome of grid lines using equations (2), (3), and (4).
[00130] According to the embodiment of the grid point arrangement shown in Fig. 4b, an electric potential measurement technique may be used to detect both the internal and surface damages of large conductive fibers such as carbon-fiber reinforced epoxy composite structures containing CNTs. For this purpose, the electric potentials between adjacent pairs of second grid points are measured using a four-probe method.
[00131] The electric potential measurement technique adopts the electrically conductive carbon fibers and CNTs themselves as networks of in-situ sensors. Since there are two electrically conductive networks made by carbon fibers and CNTs additives into the composite, two sets of grid points need to be mounted on the surface of carbon-fiber reinforced epoxy composite containing CNTs to eliminate the contact resistance of measuring electrodes to the structure.
As discussed above, a constant source current may be directly applied by the first grid points mounted to the surface of the composite laminate and the electric potential was measured across the second grid points, using the four-probe method. The Electric Potential Change percentage (EPC) between the shifted grid points (Fig. 4b) is expressed using the following equation:
V -V
AV(%) = _______________ x 100 ( 5) [00132] Where VI,iv,jv: is the initial electric potential before loading, or reference electric potential, between grid points iv and jv; and polm VF,Iv,jv: is the final electric potential after loading, or present electric potential, between grid points iv and jv.
polIq For the rectangular array with two grids shown in Fig. 4b, examples of electric potentials between adjacent pairs of shifted grid points corresponding to the first cell are: Vlv, 2v / Vlv, 6v/ V2v, 7v/ v6v,7\r/ \fly, 7v/ V2v, 6v/ where V stands for electric potential between two grid points represented by the subscripts. The subscript numbers, separated by a space represent the associated grid points. The electrical wires 206 from the first grid points can be connected to the data detection unit 202 for the collection and storing of the information. The complete set of electric potentials between pairs of shifted grid points before damage establishes the reference potential values. This reference of electric potential values can be stored for future comparison with a new set of electric potential numbers when damage occurs.

[00135] When the composite structure is in operation, the detection unit 202 is turned on and a scan rate of the values of the resistance or potential can be done at a given interval of time by the acquisition module 220. The present set of values of the electrical resistance or potential is compared against the reference values by the value comparator 222. The difference, e.g. (LIR) as in equation (1) for the network 104 of Fig. 4a or (L17) as in equation (5) for the network 204' of Fig. 4c, between the new values of the resistance or potential between each pair of measurement points and the reference values is determined by the value comparator 222. The value comparator 222 will then determine if the difference exceeds a threshold value. If the difference exceeds the threshold value, the damage detection unit 202 will identify the difference as an indication of damage.
[00136] The Average Electrical Resistance Change as in equation (2) may be used for the network 205 of grid lines (Fig. 4c) to determine the average change of resistance.
AX+1 and A 1113,3+1 are used to provide the change of resistance between each set of adjacent lines. Again, if the difference exceeds the threshold value, the damage detection unit 202 will identify the difference as an indication of damage.
[00137] The value comparator 222 may also use the difference in electrical resistance (or potential) to quantify the damages. A correlation is made between the measured difference and the magnitude of the damages.
[00138] The damage locator 226 will then indicate where the damage is in the composite structure A, by relating the identified difference to the location of the set of measurement points having provided the measurements, relative to the composite structure A, in the case of the network 204 of Fig. 4a or the network 204' of Fig. 4b. In the case of the network 205 of grid lines shown in Fig. 4c, the location of the damage will be obtained using the values of AXu and A in the AERC. For instance, the box section having the highest value of AERC as in equation (2) may indicate the location of damage.
[00139] In particular, in locating the damage in electrically non-conductive fibers reinforced polymer composite structures containing CNTs, the damage locator 226 may, upon identifying that the difference exceeds the threshold value, e.g. a tolerance of 0.0005, detect the maximum electrical resitance changes of rows and columns in the matrix of points as in 204 of Fig. 4a or the network 205 of grid lines of Fig. 4c. The damage locator 226 may then find the row and column corresponding to the maximum electrical resistance changes, thereby identifying the location of the damage.
[00140] In order to locate the damage in electrically conductive fibers reinforced polymer composite structures containing CNTs, the damage locator 226 may, upon identifying that the difference exceeds the threshold value, e.g. a tolerance of 0.0005, detect the maximum electrical potential changes of rows and columns in the matrix of points as in 204' of Fig. 4b. The damage locator 226 may then find the row and column corresponding to the maximum electrical potential changes, thereby identifying the location of the damage.
[00141] The output module 228 of the damage detection unit 202 may then produce an output with an indication of damage, a location of the damage, and/or a quantification of the damage. The output may be any appropriate signal, interface, alarm or report that provides such information.

[00142] Fig. 3b and Fig. 3c show examples of algorithms that may be implemented by the analysis module 221 of Fig.
3a to detect and locate damages in a composite structure in the manner described above. The algorithm of Fig. 3b may be used for electrically non-conductive fibers reinforced polymer composite structures containing CNTs while the algorithm of Fig. 3c may be used for electrically conductive fibers reinforced polymer composite structures containing CNTs.
[00143] The data detection unit 202 may be configured for real-time and in-situ determination of detection, location and severity of damages in composite structures, respectively. As detailed above, the data detection unit 202 can be used for both non-conductive fibers, such as glass fibers and kevlar fibers, and conductive fibers, such as carbon fibers reinforced epoxy composite structures, by monitoring electric resistance and electric potential, respectively as input values for real-time and in-situ determination of severities, detections and locations of damage in composite structures.
[00144] The damage detection method 100 and system 200 have the ability to detect and locate the damages in composite structures in situ.
Moreover, the damage detection method 100 and system 200 enable the location of the damage, without effects from extraneous sources.
[00145] In an embodiment, the spacing between the measurement points of the network 104 is three (3) inches, as discussed above. This is compared with the spacing of 0.25 inch in prior-art cases. The large spacing is preferred to avoid using an excessively large array or network. The larger spacing is possible when an optimal value of the CNT is used in the resin of the composite structure A, as this allows maximum sensitivity of the change in electrical resistance to the occurrence of damage.

Another reason is due to the use of the voltage-resistance measurement method, rather than current-voltage methods used in prior-art references.
[00146] Many examples are provided to demonstrate the application of the proposed technique for determination of detection, location, and severities of damages in electrically non-conductive and conductive fibers reinforced epoxy containing CNTs. The experimental results for glass fibers and carbon fibers reinforced epoxy composites containing CNTs are presented in the following:
[00147] Exemplary embodiment: Electrically non-conductive fibers reinforced epoxy containing CNTs composite structures using grid points [00148] As discussed above, when an electrically non-conductive fibers, e.g. glass-fibers, reinforced epoxy composite structure containing CNTs is in operation, the system 200 can be turned on and a scan of the values of the electrical resistance can be done at a certain interval of time using two-probe electrical resistance measurement. The new set of values of the electrical resistance is compared against the reference values. The difference between the new values of the electrical resistance between each pair of grid points and the reference values is determined. This can be recorded and displayed.
[00149] According to a simulation, damages have been made in a few composite plates and the changes in electrical resistance have been measured and recorded. Two types of damages have been created. A first type was done by drilling holes of different sizes at different locations in the plate. A second type was done by impacts caused by the collision with high velocity projectiles and drop weights.
Fig. 5a to Fig. 5f show the locations and values of the changes in electrical resistance due to the drilling of holes in 22 inch by 13 inch glass fibers reinforced epoxy composite structures containing 0.30 wt% CNTs. Fig. 6 shows the effect of hole size on the change in electrical resistance. In Fig. 6, the numbers below the curve represent the pairs of electrical contact points for the measurement of the electrical resistance. This pair of electrical contact points is closest to the hole. The numbers above the curve represent the change in resistance in percentage. A
clear correlation is observed between hole size and change in electrical resistance. Accordingly, the technique can also be used to indicate the severity of damages, in addition to the detection and location capabilities.
[00150] Fig. 7a to Fig. 7f show the locations and values of the changes in electrical resistance due to the collision with high velocity projectiles, e.g. a 318 mg aluminum particle travelling at 700 m/sec (78J). It can be seen that the damages can be detected and located distinctly. Fig. 8a to Fig. 8f show the locations and values of the changes in electrical resistance due to the collision with low velocity projectiles. The energy levels vary from 1 J to 10 J as produced by drop-weight impact tests. Some of the damages at the lower energy levels are barely visible to the naked eye.
These locations correspond to the location of the damages created and barely visible damages zone. Fig. 9 shows the correlation between the change in electrical resistance and the energy level. Again the severity of damages can be shown.
[00151] Simulations may also be performed with a kevlar fibers reinforced epoxy composite structure containing CNTs to reach similar conclusions as above with respect to the glass-fibers reinforced epoxy composite structure.
[00152] Exemplary embodiment: Electrically non-conductive fibers reinforced epoxy containing CNTs composite structures using grid lines [00153] As discussed above, a variation of the technique using grid points is to use lines of contacts. In one simulation, lines of conductive paints are drawn on the surface of a glass fibers reinforced epoxy composite structure containing 0.30 wt% CNTs. Fig. 10 shows the average changes in electrical resistance between the lines when holes of different sizes have been drilled in the composite laminate. In particular, holes of sizes 1 (1/16 inch), 2 (2/16 inch), 3 (3/16 inch), 4 (4/16 inch), 5 (5/16 inch), and 6 (6/16 inch) have been respectively drilled at locations A, B, C, D, E, and F. The results of Fig. 10 can be compared to those of Fig. 5f, where holes of the same sizes have been drilled in the composite laminate, although at different locations thereon. It can be seen that the grid points technique enables to achieve better damage location than the grid lines technique for glass fibers reinforced epoxy composite structure containing 0.30 wt % CNTs.
[00154] Exemplary embodiment:
Electrically conductive fibers reinforced epoxy containing CNTs composite structures using two sets of grid points [00155] As was the case for non-electrically conductive fibers reinforced epoxy composite structures, electrically conductive, e.g. carbon fibers reinforced epoxy composite structures containing CNTs may be tested using the system 200. In this case, the system 200 can be turned on and a scan rate of the values of the electrical potential can be done at a certain interval of time. The new set of values of the electrical potential is compared against the reference values and the absolute difference between the new values of the electrical potential between each pair of grid points and the reference values is determined.
[00156] Damages, e.g. drilling of holes of different sizes at different locations in the plates or impacts caused by the collision with high velocity projectiles, may be made to 22 inch by 13 inch carbon fibers reinforced epoxy composite laminates containing 0.30 wt% CNTs. Fig. ha indicates electrical potential distribution of the composite laminate.
The changes in electrical potential are then be measured and recorded.
[00157] Fig. llb shows the locations and values of the changes in electric potential due to the drilling of holes of sizes 1, 2, 3, 4, 5, and 6. Fig 11c shows the effect of the hole size on the change in electrical potential. As in the case of electrically non-conductive fibers reinforced epoxy composite structures containing CNTs, a correlation may be observed between the hole size and the change in electrical potential. Accordingly, the technique can also be used to indicate the severity of damages, in addition to the detection and location capabilities.
polsq Fig. lld shows the locations and values of the changes in electric potential due to the collision with high velocity projectiles for the carbon fibers reinforced epoxy CNTs composite plate. It can be seen that damages due to the collision with high velocity projectiles can be detected and located distinctly in electrically conductive fibers reinforced epoxy composite structures containing CNTs. Fig.
lie shows the locations and values of the changes in electric potential due to the collision with low velocity projectiles. This shows that some of the damages at lower energy levels (e.g. 1 to 3J) are barely visible to the naked eye.
[00159] The structure illustrated is provided for efficiency of teaching the present embodiment. It should be noted that the present invention can be carried out as a method, can be embodied in a system, or on a computer readable medium. The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.

Claims (24)

1. A damage detection and location method for a composite structure of the type incorporating carbon nanotubes, the method comprising:
receiving from a plurality of electrical contacts arranged on a surface of the composite structure measurements of an electrical property of the composite structure;
calculating a change between the received measurements and reference values of the electrical property; and identifying a damage if the change is above a predetermined threshold.

2. The method of claim 1, wherein identifying the damage comprises locating the damage by correlating the change to a location in the composite structure of selected ones of the plurality of electrical contacts having provided the measurements.

3. The method of claim 1, wherein receiving measurements of the electrical property of the composite structure comprises receiving a plurality of the measurements from a first grid of electrically conductive points arranged on the surface of the composite structure, each one of the plurality of the measurements comprising an electrical resistance measured between a first one and a second one of the electrically conductive points.

4. The method of claim 3, wherein the composite structure is an electrically non-conductive fiber reinforced composite structure and the range of the electrical resistance is between 103 ohm and 106 ohm.

5. The method of claim 3, wherein calculating the change between the received measurements and the reference values comprises calculating for each one of the plurality of the measurements a difference between the measured electrical resistance and a reference electrical resistance between the first and the second electrically conductive point.

6. The method of claim 1, wherein receiving measurements of the electrical property of the composite structure comprises receiving a plurality of the measurements from a second grid of electrically conductive lines arranged in a first and a second orientation on the surface of the composite structure, each one of the plurality of the measurements comprising a first electrical resistance measured between two consecutive ones of the electrically conductive lines arranged in the first orientation and a second electrical resistance measured between two consecutive ones of the electrically conductive lines arranged in the second orientation.

7. The method of claim 6, wherein calculating the change between the received measurements and the reference values comprises, for each one of the plurality of the measurements, calculating a first difference between the first measured electrical resistance and a first reference electrical resistance between the two consecutive electrically conductive lines arranged in the first orientation, calculating a second difference between the second measured electrical resistance and a second reference electrical resistance between the two consecutive electrically conductive lines arranged in the second orientation, and computing an average of the first and the second difference.

8. The method of claim 1, wherein receiving measurements of the electrical property of the composite structure comprises receiving a plurality of the measurements from a first grid of a first set of electrically conductive points and a second grid of a second set of electrically conductive points, the first and second grids arranged on the surface of the composite structure, the first set of electrically conductive points for applying a constant electric current to the composite structure and the second set of electrically conductive points for measuring an electrical potential.

9. The method of claim 8, wherein receiving measurements of the electrical property of the composite structure comprises receiving the plurality of the measurements each comprising the electrical potential measured between a first one and a second one of the second set of electrically conductive points.

10. The method of claim 9, wherein calculating the change between the received measurements and the reference values comprises calculating for each one of the plurality of measurements a difference between the measured electrical potential and a reference electrical potential between the first one and the second one of the second set of electrically conductive points.

11. A method for fabricating a composite structure of the type incorporating carbon nanotubes, the method comprising:
preparing a first amount of an epoxy resin;
preparing a second amount of a curing agent to the epoxy resin;
preparing a third amount of the carbon nanotubes;

mixing the first amount of the epoxy resin with the second amount of the curing agent to produce the epoxy matrix;
dispersing the third amount of the carbon nanotubes into the epoxy matrix to produce the modified epoxy matrix;
and incorporating the modified epoxy matrix into long fibers.

12. The method of claim 11, wherein incorporating the modified epoxy matrix into the long fibers comprises incorporating the modified epoxy matrix into one of electrically non-conductive long fibers and electrically conductive long fibers.

13. The method of claim 12, wherein the carbon nanotubes are provided in the composite structure in a presence ensuring uniform distribution of the carbon nanotubes in the composite structure, electrical conductivity of the composite structure, and detectability of a damage in the composite structure.

14. A damage detection and location system for a composite structure of the type incorporating carbon nanotubes, the system comprising:
a plurality of electrical contacts arranged on a surface of the composite structure;
a damage detection unit for detecting and locating a damage in the composite structure, the damage detection unit comprising:
an acquisition module for receiving from the plurality of electrical contacts measurements of an electrical property of the composite structure;

a value comparator for calculating a change between the received measurements and reference values of the electrical property, and for identifying a damage from the change having a magnitude beyond a predetermined threshold;
a damage locator for determining a location of the damage; and an output for providing data pertaining to the damage and the location thereof.

15. The system of claim 14, wherein the damage locator is adapted to determine the location of the damage by correlating the change to a position on the composite structure of selected ones of the plurality of electrical contacts having provided the received measurements.

16. The system of claim 14, wherein the plurality of electrical contacts is arranged on the surface of the composite as one of a first grid of electrically conductive points deposited on the surface of the composite structure and a second grid of electrically conductive lines arranged in a first and a second orientation on the surface of the composite structure.

17. The system of claim 16, wherein the acquisition module receives a plurality of the measurements each comprising an electrical resistance measured between a first one and a second one of the electrically conductive points.

18. The system of claim 17, wherein the value comparator calculates the change by calculating for each one of the plurality of the measurements a difference between the measured electrical resistance and a reference electrical resistance between the first and the second electrically conductive points.

19. The system of claim 16, wherein the acquisition module receives a plurality of the measurements each comprising a first electrical resistance measured between two consecutive ones of the electrically conductive lines arranged in the first orientation and a second electrical resistance measured between two consecutive ones of the electrically conductive lines arranged in the second orientation.

20. The system of claim 19, wherein the value comparator calculates the change by calculating, for each one of the plurality of the measurements, a first difference between the first measured electrical resistance and a first reference electrical resistance between the two consecutive electrically conductive lines arranged in the first orientation, a second difference between the second measured electrical resistance and a second reference electrical resistance between the two consecutive electrically conductive lines arranged in the second orientation, and an average of the first and the second difference.

21. The system of claim 14, wherein the plurality of electrical contacts is arranged on the surface of the composite structure as a first grid of a first set of electrically conductive points and a second grid of a second set of electrically conductive points, the first and second grids deposited on the surface of the composite structure, the first set of electrically conductive points for applying a constant electric current to the composite structure and the second set of electrically conductive points for measuring an electrical potential.

22. The system of claim 21, wherein the acquisition module receives a plurality of measurements each comprising the electrical potential measured between a first one and a second one of the second set of electrically conductive points.

23. The system of claim 22, wherein the value comparator calculates the change by calculating for each one of the plurality of measurements a difference between the measured electrical potential and a reference electrical potential between the first one and the second one of the second set of electrically conductive points.

24. A computer readable medium having stored thereon program code executable by a processor for damage detection and location in a composite structure of the type incorporating carbon nanotubes, the program code executable for:
receiving from a plurality of electrical contacts arranged on a surface of the composite structure measurements of an electrical property of the composite structure;
calculating a change between the received measurements and reference values of the electrical property; and identifying a damage if the change is above a predetermined threshold.