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ABSTRACTLaser nondestructive testing (NDT) techniques based on interferomet-

ric imaging, primarily holography and shearography, have seen growingacceptance since the 1980s. With an increase in the use of composite mate-rials and sandwich structures, the need for high speed, large area testing forfracture critical, subsurface discontinuities, such as disbonds, delamina-tion, sheared cores or nonvisible damage in aircraft, missiles and marinecomposites, led to broad acceptance of laser based NDT techniques. LaserNDT techniques employing holography and shearography imaging inter-ferometers complement ultrasonic testing, thermography and other NDTtechniques as highly developed, mature and cost effective technology. Aswith all NDT methods, strengths and weakness must be completely under-stood, applications qualified through probability of detection verificationwith written procedures and rigorous training for operators and engineersalike. Once qualified for a particular application, holography and shearog-raphy systems can operate with extraordinary efficiency, reachingthroughputs of 2.3 to 111 m2 (25 to 1200 ft2) per hour, 2.5 to 120 times thetest rate for ultrasonic C-scans. As these technologies become more widelyknown, commercial applications in aerospace, electronics, marine compos-ites, high performance tires and medical devices have greatly increased. In2001, laser techniques reached a fundamental milestone with the inclusionof holography and shearography in SNT-TC-1A for Level III certification(American Society for Nondestructive Testing, 2001). This paper will pre-sent an overview of laser NDT technique applications in aerospace, elec-tronics and marine composites, where they serve as highly effective, fullyintegrated industrial process controls, improving manufacturing qualitywhile reducing costs.Keywords: holography, shearography, honeycomb, composites, abradableseals, helicopter blade, marine composites, thermal protection systems.

BACKGROUNDLaser interferometric imaging nondestructive testing (NDT)

techniques such as holography and shearography have seen dra-matic performance improvements in the last decade and wide ac-ceptance in industry as means of high speed, cost effective testingand manufacturing process control. These performance gains havebeen made possible by the development of the personal computer,high resolution charge coupled device (CCD) and digital videocameras, high performance solid state lasers and the developmentof phase stepping algorithms. System output images show qualita-tive pictures of structural features and surface and subsurface dis-continuities as well as quantitative data such as discontinuity size,area, depth, material deformation versus load change and materialproperties. Both holography and shearography have been imple-mented in important aerospace programs, providing cost effective,high speed discontinuity detection.

Holography images test part responses to changes in load as

well as part movement. Holography using continuous wave lasersand video frame rate data acquisition requires vibration isolationusually in the form of air supported isolation tables. Coupled withultrasonic vibration excitation of the test part, holographic systemsin production provide very high resolution images of disbonds insmall complex shaped components, such as turbine aircraft compo-nents and medical devices.

Shearographic NDT systems use a common path interferometerto image the first derivative of the out of plane deformation of thetest part surface in response to a change in load. This important dis-tinction is responsible for two key phenomena. First, shearographyis less sensitive to the image degrading effect of environmental vi-bration. Shearographic systems may be built as portable units orinto gantry systems, similar to ultrasonic C-scan systems, for scan-ning large structures. Second, the changes in the applied load re-quired to reveal subsurface discontinuities frequently induce grossdeformation or rotation of the test part. With holography, severalimportant test part stressing techniques, such as thermal and vacu-um stress, create gross part deformation. Discontinuity indicationsmay be completely obscured by these translation fringe lines.Shearography, on the other hand, is sensitive only to the deforma-tion derivatives and tends to show only the local deformation onthe target surface due to the presence of a surface or subsurface dis-continuity.

Shearography, in particular, offers unique and proven disconti-nuity detection capabilities in aerospace composites manufactur-ing. Shearographic images show changes in surface slope in re-sponse to a change in applied load. Shearographic whole field, realtime imaging of the out of plane deformation derivatives is sensi-tive to subsurface disbonds, delaminations, core damage, coresplice joint separations and surface damage. Secondary aircraftstructures have long used composite materials. The drive for bettervehicle performance, lower fuel consumption and maintainabilityare pushing the application of composites and sandwich designsfor primary structures as well. Faster and less expensive testingtools are necessary to reduce manufacturing costs and ensure con-sistent quality.

HOLOGRAPHIC NDT OF TURBINE AIRCRAFTCOMPONENTS

Holography has been used since the 1970s for tire testing. Beforethe development of electronic holography or electronic speckle in-terferometer cameras, film holography cameras were used in com-bination with vacuum stress. Early film holography cameras werealso used to solve a major production issue in the testing of abrad-able turbine engine components.

Since the 1970s, turbine aircraft engines have used abradableseals in the compressor stages of the engine to achieve high pres-sure ratios per stage, reducing the turbine power required to drive

Holographic and Shearographic NDTApplications in Aerospace Manufacturing

by John W. Newman*

Submitted May 2005

* Laser Technology, Inc., 1055 W. Germantown Pike, Norristown, PA19403; (610) 631-5043 X14; e-mail <jwnewman50@aol.com>.

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the compressor, reducing engine weight and increasing perfor-mance. The loss of this material can affect engine performance; test-ing of the bond line in production or engine overhaul is required.

Ultrasonic through transmission C-scan is capable of detectingdisbonds in parts where the shroud geometry is a straight or slight-ly conical cross section. However, in most engines, the design of thecompressor shrouds includes brazed stators, material thicknesschanges, flanges and other features that obscure or shadow theabradable seal material.

In 1982, a holographic NDT technique entered production atPratt & Whitney, combining time average holography with a lowfrequency ultrasonic vibration (Figure 1) applied to the compressorshroud (Figure 2). Holography provided excellent disbond detec-tion (Figure 3) with easily interpreted images essentially identical toultrasonic test results, but not affected by part geometry or materialthickness changes. Early systems used film holography with a onestep chemical process, invented by the author, which produced pro-duction quality holograms in approximately 10 s (Newman, 1984).The results were viewed on a video monitor. Electronic holographycurrently using megapixel CCD cameras has radically improvedsystem operation speed and reliability. Since 1982, holography hasbeen the testing standard for metallic abradable and plasmasprayed aircraft abradable seals.

Holographic testing has eliminated compressor seal bond fail-ures, yielded large financial savings to the manufacturers andgreatly enhanced customer satisfaction and cost savings. Similarapplications of holography with vibration part excitation of bonded

metallic materials on human orthopedic implants have shown dra-matic improvement in product quality. Highly automated systemsthat monitor all system parameters have been FDA approved. To-gether, these two applications of holography have yielded signifi-cant cost savings, increased manufacturing quality and provided ameans for engineering improvements to processes.

SHEAROGRAPHY The concept of using a common path interferometer to image

test part deformation derivatives to overcome the effects of envi-ronment vibration and loss of the discontinuity signal due to grosspart deformation, as seen with thermal stress holography, was firstintroduced by Butters and Leendertz (1971) and reduced to practiceby Nakadate and Saito (1985).

Shearographic cameras generally use a Michelson type inter-ferometer, as shown in Figure 4, with two essential modifications.First, one mirror may be precisely tilted to induce an offset, orsheared image, of the test part with respect to a second image ofthe part. The sheared amount is a vector with an angle and a dis-placement amount. The shear vector, among other factors, deter-mines the sensitivity of the interferometer to surface displacementderivatives.

The two laser speckle images of the test part, offset by the shearvector, interfere at every paired point over the surface in the field ofview. The single frequency laser light from the two sheared imagesof the part is focused onto the CCD camera array of photosensitivepixels. Light from pairs of points in each sheared image interfere.Each video frame, comprised of the complex addition of these twosheared images can be subtracted from a stored reference image.The absolute difference yields a fringe pattern observed on themonitor. The second mirror in the Michelson interferometer may bephase stepped using a piezoelectric device and the images com-bined to create a phase map. Further processing (Yang et al., 2004)

Figure 1 — Schematic of a holographic technique using low frequencyultrasonic part excitation to image braze bond discontinuities.

Figure 2 — Metallic abradable seal material bonded to a turbine enginecompressor shroud. This 12 stage also abradable seal material bonded tothe inner diameter of the stators, making conventional ultrasonictesting impractical.

Figure 3 — Holographic image of a turbine engine compressor showingdisbonds as black areas on the compressor seal material. Test time isapproximately 1 s per image or about 25 to 60 s per part, depending ondiameter.

Figure 4 — Schematic of a Michelson type shearographic interferometerobserving a flat metal plate with a 102 mm (4 in.) diameter machinedflat bottomed hole. A center load change on the plate provides a variabledeformation, observed on the computer monitor in real time.

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using any number of unwrapping algorithms may be used to gen-erate fringe free images of local surface deformation derivates.

In practice, each step in creating a shearogram is performedautomatically using image processing macros constructed bycombining each processing function in a sequence. Shearographysystem operators perform a test with a single keystroke. Portableshearography systems using voice recognition commands havealso been built, further freeing the operator from system function-al operations.

Quantitative Shearographic MeasurementsPrecise calibration of the shearogram image scale (pixels/inch)

and the shear vector allow further processing of shearography datato determine discontinuity indication dimensions, area and the de-formation of the material. The digital measurement of the deforma-tion derivative may be integrated to show the shape of the targetsurface deformation as well as the magnitude of the deformation atany location, as in Figure 5. Shearography can be used to measurethe deformation response of a structure to an applied load and as ameans of deriving material properties (Figure 6).

Typical shearographic camera systems include a built in laser il-lumination source, the shearing interferometer, image processingcomputer and remote controls. These systems may be used alonewith thermal or vibration stress or in test chambers with vacuumstress shearographic techniques.

SHEAROGRAPHIC NDT SYSTEMSShearographic NDT systems are either portable, for on vehicle

or structural testing, or fixed production systems using gantries toscan large panels or structures. As with all laser devices, exposureof the operator to laser emissions must be tightly controlled and incompliance with state and federal laws. Lasers interlocked to testcell doors or vacuum attachment features are an important designfeature.

Portable Shearographic NDT SystemsPortable shearographic systems generally are either tripod

mounted or attached in some manner to the test object. Portable systems use laser diodes and various means such as

vacuum changes, thermal flux or vibration to stress the object sur-face to detect subsurface discontinuities.

Shearographic techniques using portable systems are excellentfor engineered repairs in composite laminates. Figure 7 shows a re-pair to an aircraft laminate with far side, bonded stringers (diagonallinear features). The repair uses scarf plies built up thicker than the

Figure 5 — A phase map shearogram: (a) with horizontal shear vector,yielding a fringe pattern showing the first derivative of the out of planedeformation, ∂w/∂x; (b) using an unwrapping algorithm, the imageshows the positive (white) and negative (black) slope change — themetal plate with a 102 mm (4 in.) diameter flat bottomed hole wasdeformed by 7 µm (2.8 × 10-4 in.); (c) the integrated image showing theout of plane deformation; (d) integrated images of deformations derivedfrom shearographic data are free of errors due to gross objectdeformation or translation.

(a)

(b)

(c)

(d)

Figure 6 — A quantified measurement of the target deformation of7 µm (2.8 × 10-4 in.).

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original material, hence the signal from the stringers appears to dis-appear under the repair. Visible also are areas of porosity (circled inwhite). Test time is 15 s.

Portable shearography systems have seen extensive use in aero-space and marine composite testing since the introduction of thefirst systems in 1989. More than 170 composite boats and ships, in-cluding the Swedish Visby Naval Corvette ships measuring 73 m(240 ft), have used portable shearography systems. Portableshearography systems were also recently used to test the compositewind fairings covering the full 671 m (2200 ft) length of the BronxWhitestone Bridge.

Tripod mounted shearographic cameras are used frequentlywith thermal stress shearographic techniques. While thermographyis sensitive to changes in surface temperature (or the derivatives ofthe temperature change), thermal shearography images changes inthe thermal expansion of a structure. Damage, disbonds, foreignobject damage or delaminations produce local changes in the coeffi-cient of thermal expansion.

Thermal shearography is not generally affected by variations inemissivity or paint on the test part surface.

Fixed Production Shearographic SystemsFirst introduced on the US Air Force B-2 production program,

gantry mounted shearographic systems share many operationalfeatures with ultrasonic C-scan systems. These include:teach/learn part scan programming, electronic imaging of the en-tire part, image analysis, discontinuity measurement tools and au-tomated operation. However, shearographic systems operate atthroughputs typically in the range of 9.3 to 46.5 m2/h (100 to500 ft2/h) compared to a typical throughput of 0.9 m2/h (10 ft2/h)for ultrasonic C-scan systems. In addition, gantries are consider-

ably less expensive since precision part contour following is unnec-essary. Currently, dozens of these systems are in operation on aero-space manufacturing programs.

HELICOPTER BLADE TESTINGBoth metal and composite helicopter blades are easily tested in

production with either thermal or vacuum shearographic methods.The pressure reduction during test cycles is between ambient and2.1 kPa (0.3 lb/in.2) below ambient. Apache helicopter blade maybe scanned in less than 15 min (Figure 8).

PRODUCTION TESTING OF LAUNCH VEHICLE THERMALPROTECTION MATERIALS

Rocket launch vehicles require extensive thermal protection sys-tems to prevent damage from combustion flame and frictionalaerodynamic heating during flight in the atmosphere as well as toconserve cryogenic propellants. Spray on foam insulation usingpolyurethane materials is used on both the space shuttle externaltank and the Boeing Delta IV launch vehicle. For different reasons,reducing foam loss from these cryogenic fuel tanks is critical. Whilethe launch vehicle does not carry a fragile payload vehicle in thefoam debris path, loss of foam can affect the tank structure.

In 1998, Laser Technology, Inc., developed an acoustic shearog-raphy technique to image disbonds and voids in the spray on foaminsulation used on the aforementioned launch vehicle fuel tanks(Figure 9). Here, sonic frequency sound is used to excite the foammaterial. Disbonds and voids increase the local compliance of thefoam on the substrate, allowing the foam to resonate when excited.Shearography was qualified through extensive testing of 0.6 by 0.6 m(24 by 24 in.) foam/substrate panels with programmed discontinu-ities. Tanks were tested manually and probability of detection analy-sis performed on both laboratory test setups and finally the produc-tion systems. The written test procedures were tested with multipleoperators and the system shown to be robust, with highly repro-ducible results. In practice, shearography has been demonstrated tobe a highly effective testing technique for spray on foam insulation.The results have allowed multiple process refinements leading to

Figure 7 — Shearographic images: (a) engineered repair to a solidlaminate aircraft structure with far side bonded stringers (porosity iscircled); (b) thermal shearogram of an aircraft fairing showing layup ofthe composite material (thermal shearography is used for detection ofdisbonds and face sheet delamination).

(a)

(b)

Figure 8 — Helicopter blade testing: (a) discontinuity free honeycomband trailing edge area; (b) shearographic indications of disbonds in this610 mm (24 in.) section view. Discontinuties, such as crushed cores,disbonds and sheared cores, are easily detected and measured.

(a)

(b)

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improved foam application and reduced rework and costs. Testthroughput using acoustic shearography is extremely high. Theshearographic test system built for the launch vehicle operates at111 m2/h (1200 ft2/h).

AIRCRAFT COMPOSITE STRUCTURAL TESTINGSince 1987, shearography has seen extensive implementation

for aircraft composites testing. Shearography is capable of testing

complex shapes and large areas at a high throughput without cont-amination of bonding surfaces. Shearographic NDT is an excellentchoice for sandwich structures using foam or metal honeycombcores. Fracture analysis techniques can be used to determine themaximum allowable discontinuity size. The use of test panelswith programmed discontinuities and stepped face sheet thick-nesses will then allow determination of the minimum discontinu-ity detectable as a function of face sheet thickness. Once qualifiedfor an application, shearography can offer highly cost effectivetesting, even during ply buildup on the tool. Such techniques canyield zero discontinuity production by allowing testing, repair,retesting and application of more material during part manufac-ture. Shearographic NDT is used on many aircraft manufacturingprograms.

CONCLUSIONShearographic and holographic NDT techniques are mature and

cost effective production NDT techniques for many aerospace ap-plications. Shearography provides very rapid testing, allowing im-mediate feedback for process controls. Recent inclusion in SNT-TC-1A will help further the development of new applications andmethods.

REFERENCESAmerican Society for Nondestructive Testing, Recommended Practice No.

SNT-TC-1A, Columbus, Ohio, ASNT, 2001.Butters, J.N. and J.A. Leendertz, “Speckle Pattern and Holographic Tech-

niques in Engineering Metrology,” Optical Laser Technology, Vol. 3, 1971,pp. 26-30.

Nakadate, S. and H. Saito, “Fringe Scanning Speckle Pattern Interferome-try,” Journal of Applied Optics, Vol. 24, No. 14, 1985, pp. 2172-2180.

Newman, J.W., “Production Holographic Inspection of Turbine Aircraft En-gine Components,” Air Transport Association NDT Forum Proceedings,Washington, DC, Air Transport Association, 1984.

Yang, L.X. and Y.Y. Hung, “Digital Shearography for Nondestructive Evalu-ation and Application in Automotive and Aerospace Industries,” 16thWorld Conference on NDT, Montreal, Canada, 30 August-3 September2004.

Figure 9 — Acoustic shearogram showing triangular and squaredisbonds in 51 mm (2 in.) thick spray on foam insulation test panels.Test time for this 508 by 508 mm (20 by 20 in.) panel is 0.5 s.

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