NON-DESTRUCTIVE CHARACTERIZATION OF PLY ORIENTATION AND PLY TYPEOF CARBON FIBER REINFORCED LAMINATED COMPOSITES

Sarah Stair1, David A. Jack1,1, and John Fitch2

1Department of Mechanical Engineering, Baylor University. Waco, TX 767982Birkeland Current. Waco, TX 76798

Abstract

As the fuel efficiency of vehicles is being increased over the next decade due to both consumer de-mand and regulations, several automobile manufacturers are incorporating fiber-reinforced com-posites into their product line due to their high strength to weight ratio. While the fiber-reinforcedcomponents are lighter than the traditional aluminum and steel components, the processing andrepair of these parts is more complicated. The need for non-destructive testing methods to char-acterize the as-processed ply configuration compared to the as-designed ply configuration willbecome essential in next generation vehicle quality control and maintenance. Traditionally, ul-trasonic techniques have been focused on locating defects and delaminations within parts. Thecurrent study incorporates a novel ultrasonic C-scanning technique to determine the ply type, ori-entation and thickness of each lamina in a carbon fiber reinforced laminated composite. Initialtesting was performed inside of an immersion tank, but the system has since been reconfiguredto a non-immersion system which can be placed on the top surface of the carbon fiber reinforcedlaminate for scanning. This paper presents the results for a variety of parts along with the accuracyassociated with this non-destructive testing method.

Introduction

The automotive, aerospace, and athletic industries are incorporating fiber-reinforced compositesinto their product lines due to their high strength to weight ratio, but the manufacturing and mainte-nance of such parts pose increased engineering difficulties. Thus, non-destructive testing methodsare necessary for characterizing and inspecting these parts as there is typically an increase in partto part variability when using fiber-reinforced composites. As the use of composites continues toincrease, the non-destructive testing industry is expected to grow as well [1] with the developmentof new and inventive techniques [2].

Composite materials are significantly more complex than most metals. A technician’s abil-ity to make an informed decision regarding the structural stability of a composite part requiresbackground knowledge and training focused on this type of material. Goglia, who has worked inthe aerospace industry for over 40 years, has voiced concern about the repair and maintenanceof composite parts. As an example, consider an object falls onto a composite component of acar. Unlike metal materials, a composite material could appear healthy while significant damage,such as delaminations, may be lurking underneath the surface. Without background knowledgeof composite materials, a typical person would likely overlook such possibilities of damage, andthe part could experience catastrophic failure while in use [3]. In an effort to provide educationregarding inspection of such parts companies, such as GE, have developed training facilities fornon-destructive analysis of composite parts [4].

Several non-destructive testing techniques have been developed in recent years including op-tical, thermal, and acoustic methods, as well as techniques which include a combination of thesemethods. A key focus of non-destructive testing is damage identification. Amaro, et.al. [5] createda model to predict a composite’s response to a low velocity impact and compared their predictionswith measured results. After a comparison of the predicted and measured responses of the com-posite, they found their model to be reasonably accurate. Members of this group also performeda study in which they compared four non-destructive testing techniques to determine which was

1Author to who correspondence should be addressed: david jack@baylor.edu

most accurate at locating impact damage within a composite part. The techniques included: elec-tronic speckle pattern interferometry (ESPI), shearography, ultrasound, and X-radiography. All fourtechniques accurately located the damage within the laminate plane, but the ultrasound techniquewas the only method which accurately located the damage through the thickness of the part (in thevertical plane) [6]. Additional studies related to impact damage identification via non-destructivetechniques have also been performed by Bendada et.al. [7], Zwink et.al. [8], Gryzagoridis andFindeis [9] and Ben et.al. [10].

As previously mentioned, some studies have incorporated multiple non-destructive techniqueswhile analyzing parts. For example, Bendada et.al. [7] studied impact damage using thermaltechniques, optical techniques and a combination of the two techniques. Based on the results,they found the combination of the two techniques supplied more information about the part thanwhen either of the two methods were used individually. Cuadra et.al. [11] incorporated acousticemission, digital image correlation and infrared thermography to inspect damage within a fiber-glass composite exposed to both tensile and fatigue loading. Using the combination of thesenon-destructive methods, they were able to monitor several part properties including strains andresidual stiffness.

In the present study carbon fiber-reinforced laminated composites are inspected using an ul-trasonic C-scanning technique, and the resulting data is analyzed using a patent pending plydetection algorithm (see PCT/US13/33187) [12, 13]. The system’s ability to capture a carbonfiber tow (or a small bundle of individual carbon fibers) is what makes this an innovative non-destructive testing technique. The ply orientation and ply type of each lamina in the bulk laminateas determined from the C-scan data and ply detection algorithm provide information regarding theas-manufactured part at the individual ply level. This information can then be used in combinationwith the constitutive properties of the carbon fibers and the resin matrix to determine the failureenvelope associated with the as-manufactured part as compared with the as-designed part sincethe two rarely match [14].

Ultrasound BasicsUltrasound systems provide a relatively easy, low cost, portable means of performing non-destructive testing of parts. Figure 1(A) represents damage detection via ultrasonic analysiswhile Figure 1(B) and Figure 1(C) represent ultrasonic wave transmission options. The through-transmission option uses two ultrasonic transducers: a pulser and a receiver. While Figure 1(B)shows the pulser on one side of the part and the receiver on the opposite side of the part, thereare additional configurations available. For example, if only one side of a part is accessible forscanning purposes, the two transducers can be placed on the same side of part in a differentconfiguration. The pulse-echo transmission option seen in Figure 1(C) uses the same transduceras both the pulser and the receiver and is transmission option used in the present study. In thepulse-echo mode, the ultrasound wave is initiated by a computer controlled pulsar, and then thetransducer listens for and captures the reflected wave [12].

(A) (B) (C)

Figure 1: (A) ULTRASONIC DETECTION OF A DEFECT WITHIN A PART, (B) DIAGRAM OFTHROUGH-TRANSMISSION, AND (C) DIAGRAM OF PULSE-ECHO TRANSMISSION. [14]

There are several types of transducers available on the market with the flat front transducer

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being the most common (see Figure 2(A)). While this transducer is ideal for locating defects withina part, the resolution is dependent on the diameter of the transducer. For example, in Figure 2the black ovals and S-shape represent a carbon fiber weave while the yellow surrounding materialrepresents the resin matrix. The flat front transducer is unable to capture the weave pattern, butthe spherically focused transducer, as seen in Figure 2(B), is able to capture the detail of theweave [14]. The spherically focused transducer focuses the ultrasonic waves to a point a givenfocal distance away from the transducer face which allows the system to observe such detail as acarbon fiber tow and is the option chosen for the study presented in this paper.

(A) (B)

Figure 2: A COMPARISON OF (A) A FLAT FRONT TRANSDUCER AND (B) A SPHERICALLYFOCUSED TRANSDUCER. [14]

A wave traveling through an elastic solid can be described by Navier’s equations as [15]

∂2u

∂x2− 1

c2i

∂2u

∂t2= 0 (1)

where the Laplacian with respect to space x of the displacement u in one dimension is related toacceleration of the displacement scaled by the speed of sound ci within the ith acoustic medium.The incident wave’s speed is dependent upon the properties of the medium through which it istravels and is usually a function of the medium’s stiffness and density. When the wave approachesa change in medium (such as a carbon fiber-resin matrix interface), a portion of the wave isreflected back to the transducer while the remaining portion of the wave is refracted deeper intothe part as depicted in Figure 3. The sum of the intensities of the reflected and refracted waveswill always be less than or equal to the intensity of the initial wave as seen in Equation (2).

IIncident ≥ IReflection + IRefraction (2)

In a carbon fiber-reinforced laminated composite a reflection and a refraction wave is formedeach time the acoustic wave approaches a carbon fiber-resin matrix interface. This phenomenamay occur in between laminae or within a single lamina when a woven ply is used. Thus, afiber-reinforced laminated composite which consists of many plies will have several reflection andrefraction waves existing at the same time making the returned acoustic signal difficult to fullycomprehend.

An additional aspect affecting wave travel through the composite part is the acoustical attenu-ation of the signal. Attenuation becomes an important factor when determining the usable scan

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Figure 3: A DIAGRAM OF THE INCIDENT WAVE APPROACHING A CHANGE IN MEDIUM ATWHICH POINT IT SPLITS INTO A REFLECTION WAVE AND A REFRACTION WAVE [14]

depth before the signal intensity is damped below the detection threshold. Based on studies per-formed in-house, the higher the transducer frequency, the thinner the part that should be scanned.Similarly, the lower the frequency, the thicker the part that could be scanned. Deschamps andHosten [16] describe the prediction of signal attenuation through a viscoelastic medium similar tothe epoxy matrix used in this study. A similar study focused on the signal attenuation within acarbon fiber composite was performed by Lonne et.al. [17], and the results generated in this studycorrespond to the exponential decay form suggested by Schmerr [15] as seen in Equation (3)∣∣∣∣p1p2

∣∣∣∣ = exp (α∆x) (3)

where the ratio of the pressures p1p2

within a material separated by a distance ∆x = x2 − x1 canbe obtained once the attenuation constant α is known. The attenuation constant α is a functionof both the material’s stiffness and the frequency associated with the incident wave as discussedby Lonne, et.al. [17]. Based on experience with our system, we have a significant amount signalattenuation with the 15 MHz transducer which allows us to determine the ply orientations for a20-30 ply composite depending on the thickness of each lamina and the fiber volume fraction [12].

Considering this information, let us focus our discussion on a 16 layer composite stack com-prised of woven carbon fiber plies. The speed of sound through the resin matrix and theresin/carbon fiber tow regions were determined from in-house measurements, and the result isfound by solving Equation (1) with a custom Matlab program assuming the signal attenuation pro-vided in Equation (3). Figure 4 presents the single, focused A-scan solution for a randomly chosenlocation on the laminate. During the initial 3.5 µs, there is no returned signal as the wave travelsthrough the fluidic medium. Once the ultrasound wave touches the top surface of the laminate, aportion of the signal is reflected back to the transducer while the remaining portion of the signalis refracted deeper into the part. Once the reflected wave reaches the transducer, the transducerbegins observing the reflected acoustic signal based on the reflections and refractions inside ofthe laminate. If the material were homogeneous, such as a neat resin bar or an aluminum plate,the initial increase in pressure would occur only for the length of the initial pulse and no additionalsignal would be observed. For the carbon fiber-reinforced laminated composite, while the reflected

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Figure 4: RAW THEORETICAL A-SCAN RETURN SIGNAL FROM THE 16 LAYER LAMINATEFOR A 15 MHz SPHERICALLY FOCUSED TRANSDUCER.

wave travels back to the transducer, the refracted wave travels deeper into the laminate. When therefracted wave approaches another resin matrix-carbon fiber interface a second set of reflectionand refraction waves are formed. This second reflection wave travels back to the transducer andthe second refracted wave travels deeper into the part. The formation of additional reflection andrefraction wave pairs continues throughout the thickness of the laminate. Ultimately, these waveseither are reflected back to the transducer or attenuate inside of the viscoelastic medium belowthe minimum detection threshold.

The A-scan signal depicted in Figure 4 can be shifted to align with the front surface of thecomposite (see Figure 5). The range of time on this axis has been decreased compared to thatseen in Figure 4 in an effort to focus on the signal’s transition between lamina within the bulklaminate. Locating the individual laminae within this figure is difficult because the transducer mustcomplete a full cycle before exiting the signal transmitting mode. Therefore, the power going intothe system is not a discrete wave front, but instead it has a time duration associated with it. Thiscould potentially pose a problem as the wave length must be smaller than the smallest verticalfeature within the laminate, but also, the receiver must be fast enough to fully resolve the entiretransmitted signal.

Figure 5: SHIFTED A-SCAN RETURN SIGNAL CORRESPONDING TO THE FRONT SURFACEOF THE LAMINATE FOR A 15 MHz SPHERICALLY FOCUSED TRANSDUCER [12]

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Another important item to consider is how to use the returned acoustic signal to characterizea change in the part’s microstructure. Figure 6 presents multiple methods for capturing the signalintensity for post-processing. The first method is to use the system’s gate to capture points oftime that are of specific interest (time and depth are synonymous in this context) and integrate thesignal intensity over the chosen time interval. Other methods include selection of the maximumsignal intensity, the initial signal intensity and an integral average intensity value for a chosen rangeof data (also referred to as gate width). The integral average can be calculated as

Figure 6: Methods to define the gate width and selection of proper points for signal to be used incomputations.

Iavg =1

∆t

∫ ti+∆t

ti

I(t)dt (4)

where I(t) is the signal intensity, ti (often termed the z-depth) is the starting time for the integrationaveraging, and ∆t is duration of time for signal averaging (typically called the z-gate). While theintensity I(t) is only defined at discrete points, a moving spline can be formed allowing the integralto be determined at any point in time ti and not solely for those points at which the signal is stored.The continuous form of I(t) allows for a smoother, higher resolution intensity average. Providedthe gate width Deltat is small, the four method of analyzing the signal intensity will provide similarresults. Based on the analysis of C-scan data for several different parts, the time integrationmethod produces the highest resolution images but is slightly more computationally expensivethan the other methods discussed.

Experimental Configuration

the experimental setup consists of a US Ultratek PCIUT3100 ultrasonic pulser/receiver attachedto an x-y translation table built with parts purchased from Velmex and allows for a scan resolutionof 1/800th of a millimeter. The PCIUT3100 transducer operates at 100 MHz and is capable ofwithstanding 40 to 300 Volts. In order to obtain usable data from the ultrasonic transducer, theremust be a fluidic medium located between the transducer face and the front surface of the part.The initial experimental configuration (as depicted in Figure 7) included an immersion tank filledwith water. Distilled water is the best option for filling the immersion tank as tap water can behighly oxygenated depending on the faucet. The highly oxygenated water leads to the formationof air bubbles on the part surface and affects the scan data as seen in Figure 8.

The PCI board is connected to the computer and sends an electric pulse to the transducerbased on the user supplied inputs. The transducer converts the electric pulse to an acoustic pulsewhich travels through the water and the part. The power supplied to the system needs to be

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Ultrasonic Transducer Increasing Motor #1

Increasing Motor #2

Figure 7: EXPERIMENTAL SYSTEM WITH COMPOSITE SAMPLE IN IMMERSION TANK

Figure 8: HIGHLY OXYGENATED WATER FORMS AIR BUBBLES ON THE SURFACE OF THEPART AND THE TRANSDUCER.

large enough for the ultrasound waves to travel through the thickness of the part, but at the sametime, it should not saturate the top surface of the part. After working with various input powervalues, we have determined a user-defined power of 250 Volts adequately satisfies each of theseconsiderations for the carbon fiber laminated composites we have been analyzing [12,14].

An in-house LabVIEW program controls both the translation table and the ultrasonic transducerwith many of the test parameters being user-defined allowing for increased experiment tailorabilitydepending on the material or part being scanned. User-defined test parameters include but arenot limited to the number of samples in each the x and y directions (also referred to in this studyas the x1 and x2 directions, respectively), the number of steps between each sample, pulse width,pulse voltage, gain, and buffer length. Each of these variables contribute to the scan quality andcan be optimized for a given part.

C-Scans of Laminated CompositesThe data obtained from the ultrasonic C-scan is analyzed using a custom Matlab program whichgenerates images of the ply microstructure for each lamina within the composite laminate. Figure9 represents the transition regions between the first and second and fourth and fifth plys, respec-tively, of a 16 layer carbon fiber-reinforced laminated composite manufactured with woven fabric.In each of these images provided in Figure 9, the ply orientation has been designated with redarrows for ease of viewing. These images provide evidence that viewing the internal ply type andorientation is possible via non-destructive testing techniques.

A portion of the ply detection algorithm’s accuracy may be attributed to the high resolution

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(A) (B)

Figure 9: C-SCAN OF TRANSITION BETWEEN (A) THE FIRST AND SECOND PLYS AND (B)THE FOURTH AND FIFTH PLYS OF A 16 LAYER LAMINATE.

of the C-scan data. The system used in this study has an x1 − x2 plane corresponding to thelamina plane with the x3 axis corresponding to the thickness of the laminate. The typical carbonfiber tow (or small bundle of individual carbon fibers) is approximately one millimeter wide in thex1 − x2 plane with the lamina thickness being on the order of 1/10th of a millimeter measuredwith respect to the x3 axis. As discussed in an earlier section, the use of a flat front, pulse-echotransducer would not generate data at a high enough resolution to distinguish between the carbonfiber tows in a woven fabric since the transducer itself would need to be less than a millimeter indiameter. On the other hand the spherically focused transducer, which was used in this study,is capable of identifying the weave pattern as shown in Figures 9(A) and 9(B). The focal pointfor the 15 MHz transducer purchased from General Electric and used in this particular scan islocated approximately 38 millimeters away from the transducer face. The part itself is less thana millimeter thick, and the focal spread through the thickness of the part is less than 1/10th of amillimeter. In the x1 − x2 plane a resolution of 1/800th of a millimeter can be obtained with theVelmex x-y translation table, and the resolution in the x3 direction is larger as it can be representedby the material’s speed of sound divided by the transducer frequency. Since we used a 15 MHztransducer for this scan, the resolution in the x3 direction is approximately 0.2 millimeters. Whilethe resolution in the direction is close to the thickness of interest, the scan still provided reasonableresults.

While analyzing multiple parts, potential issues associated with the laminate manufacturingprocess arose. The final surface of a manufactured laminate is rarely 100% smooth. There areusually small surface features where the resin matrix was unable to fill a gap between the tows ofa woven fabric. There are methods to fill in these features, and the method presented in Figure 10involves applying a thin layer of polyurethane to the top surface on the laminate in ana attempt tofill these small surface voids. Unfortunately, once fully dried the polyurethane accentuates thesefeatures rather than diminishing their appearance. The part shown in Figure 10 was manufacturedusing a vacuum assisted resin transfer molding (VARTM) method. Similar surface phenomenawere also observed on carbon fiber laminates manufactured with prepreg materials.

While the surface features are evident on the C-scan results for both the laminate manufacturedwith the VARTM technique and the prepreg technique, the ply type and orientation of each layercan still be obtained from the C-scan data as seen in Figures 11 and 12. The orientation ofeach layer has been marked with red arrows for ease of viewing. As evident from theses images,useful information regarding the internal ply type and orientation can still be gleaned from partswhich have these types of surface features, but it is important to if there is an upper bound to theallowable amount of surface features and if so, what that limit is. For example, would 10% of thetotal surface area be acceptable, but 20% of the surface area be too much? This is a topic that isbeing considered for future research as the results would likely assist in improving the accuracy of

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Figure 10: POLYURETHANE IS APPLIED TO THE LAMINATE IN A THIN, EVENLY DIS-TRIBUTED LAYER AND ALLOWED TO DRY. AFTER THE POLYURETHANE HAS DRIED COM-PLETELY, IT HAS ACCENTUATED THE SURFACE FEATURES RATHER THAN DIMINISHINGTHEIR APPEARANCE [14]

(A) (B) (C) (D)

Figure 11: ULTRASONIC C-SCANNING RESULTS FOR THE (A) FIRST, (B) SECOND, (C) THIRDAND (D) SIXTH LAYERS OF A CARBON FIBER REINFORCED LAMINATED COMPOSITE MAN-UFACTURED WITH THE VARTM METHOD.

the ply detection algorithm.As seen in the C-scan results presented thus far, the current ultrasonic C-scanning system is

capable of determining the ply type and orientation associated with laminae located within thelaminate. Table 1 provides a comparison of the actual ply orientations and the ply orientations asdetermined using the C-scan data and the ply detection algorithm. The far left column representsthe layer of the ply within the stacking sequence, the next column represents the actual ply orien-tations and the third column represents the orientations calculated using the patent pending plydetection algorithm [13]. Looking at the results in this table, we see that the calculated orientationfor layer 1 is incorrect. While this is disappointing, the first layer can be seen with human eye, andthe user can manually correct or double check this calculated ply orientation. The result for layer2 is more interesting. The actual ply orientation was 30 degrees, and the algorithm calculated anorientation of 30.2 degrees. Similar results are seen for layers 3 through 6. Layer 7’s calculatedorientation appears to be 180 degrees off from the actual ply orientation, however it is noted in thetable’s title that all plys within this laminate are unidirectional carbon fiber fabric. Thus, an orienta-tion of 7 degrees is equal to an orientation of 180 degrees. The same principle applies for layers 8through 14. Thus, to assist in the comparison of the actual ply orientations with the calculated plyorientations, a fourth column has been added to Table 1. This fourth column accounts for the 180

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Figure 12: ULTRASONIC C-SCANNING RESULTS FOR THE (A) FIRST, (B) SECOND AND (C)THIRD LAYERS OF THE CARBON FIBER REINFORCED LAMINATED COMPOSITE MANUFAC-TURED WITH PREPREG MATERIALS.

degree difference between the actual and calculated ply orientations in layers 7 through 14, andthe calculated ply orientations for these layers have been rewritten after accounting for this 180degree difference.

Table 1: COMPARISON OF ACTUAL PLY ORIENTATIONS AND PLY ORIENTATIONS AS DETER-MINED FROM THE PLY DETECTION ALGORITHM. NOTE: ALL PLYS WITHIN THIS LAMINATEARE UNIDIRECTIONAL CARBON FIBER FABRIC.

Layer Actual Algorithm Interpreted Results1 30 −5.4 −5.42 30 30.2 30.23 56 56.3 56.34 83 83.4 83.45 117 117.4 117.46 146 146.0 146.07 7 187.1 7.18 145 −6.4 173.69 145 −34.8 145.210 123 −57.4 122.611 97 −82.8 97.212 67 −112.9 67.113 29 −150.7 29.314 26 −184.5 −4.5

From Table 1, the calculated ply orientations for layers 1, 8 and 14 are incorrect. Layers 1 and14 are the outer surface layers of the laminate and can be visually inspected after the part hasbeen manufactured. Considering this information, the ply detection algorithm correctly determinedthe ply orientations for 11 out of the 12 interior laminae which cannot be visually inspected post-manufacture. While these initial results are quite promising, research is still being performed toimprove the ply detection algorithm’s accuracy and will be the topic of an upcoming journal article.

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Future WorkRecently, we have been developing a non-immersion ultrasonic C-scanning system which will beable to scan parts that are either too large to fit in the immersion tank or that the customer prefersnot to have submerged. A fluidic medium is still maintained between the transducer face and thepart throughout the entire scan, and a variety of fluids are being analyzed to determine which isoptimal for the new setup.

Another interesting application we have been investigating with our system is the detection andanalysis of bondlines between dissimilar materials. For example, we have scanned carbon fiber-reinforced laminated composites bonded to aluminum plates as well as carbon fiber-reinforcedcomposites adhered to a honeycomb core. We are able to locate and see these bonds with ourMatlab program but are still in the early stages of analyzing these results.

ConclusionFiber-reinforced laminated composites are complex materials with the ply type and orientation ofeach layer impacting the performance of the as-manufactured part. This study focused on thedevelopment of an ultrasonic C-scanning technique and ply detection algorithm to determine theply type and orientation of each lamina within a manufactured carbon fiber-reinforced laminate.Several laminates were immersed and scanned with the ultrasound system, and the resulting datawas analyzed with a custom Matlab progam. The ply detection algorithm successfully determinedthe ply orientation of 11 out of 12 interior laminae, and further research is being performed toimprove the algorithm’s accuracy. Potential issues regarding the part surface quality have beenidentified and further investigations will be performed to better understand the effects of thesefeatures on the C-scan data. The focus of recent research has been on the development of a non-immersion scanning system and the identification of bondlines between two different materials.The results of this study are promising and have proven this to be a viable technology.

AcknowledgmentsThe authors would like to express their gratitude to L-3 Communications in Waco, Texas for theirfinancial support of the composites program at Baylor University and their support of this project.The authors would also like to thank the SPE ACCE Conference and Michigan Economic De-velopment Corporation for their support of this project in the form of an SPE ACCE GraduateScholarship. The authors are also grateful to Mr. David Moore for allowing us to scan a variety ofhis composite parts and for the efforts of Theresa Vo in constructing the C-scan system.

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