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Over the last few decades, theaerospace industry has turnedincreasingly to carbon compositestructural materials to make aircraftlighter and more economical. Withthis increased use, it has becomeapparent that conventionalnondestructive testing methods areoften impractical or unsuitable fortests of these materials.

Many current composite designsuse concepts similar to the sheetand stringer methods found inconventional aluminum airframes.However, designers today may uselong unibundle laminate tape plies(stringers) to reinforce honeycombcomposite sandwich panels (sheet orskins). Where conventionalaluminum structures may have beenriveted or welded in the past, today’smodern aerospace compositestructures are adhesively bonded.

The ability to find discontinuities(including those introduced duringthe manufacturing process andthose that result from use) incomposite structures withconventional nondestructive testinghas proven to be challenging. As aresult, laser shearography hasbecome one of the primarymethods used for nondestructiveinspection of composite structures.

Using Laser Light to DetectSurface Deformation

Laser shearography is a large areaoptical inspection technique thatuses laser light to detect very slightsurface deformations that formwhen subsurface discontinuities arepresent and the test part issubjected to an appropriate changein strain. The noncontact, full fieldtechnique allows large areas to becovered quickly. The results are alsonear real time. Shearography usesthe interference of coherent,monochromatic laser light to detectsurface displacements as small as30 nm.

Surface deformations caused bythe presence of discontinuities maybe generated by subjecting the partto controlled stressing mechanismssuch as:· pressure,· heat,· mechanical force or· vibrationIn many cases, a reference image isrecorded first. An excitation(stressing) technique is then used togenerate a response in the materialwhile a second image is recorded.The second image is subtractedelectronically from the referenceimage. This subtraction orcorrelation results in an image thatshows only the differences betweenthe excited surface and theunstressed surface. The resultingsurface deformations are directlylinked to surface strains and withthe correct excitation method, theimages show subsurface weaknessesor indications that may bediscontinuities or structuralinformation in the test object. Inreality, the images update in real

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Laser Shearography of Aerospace Compositesby Terry R. Tamberg* and Matt Crompton†

TNT · July 2011 · 1Vol. 10, No. 3

* General Atomics Aeronautical Systems, Inc.;14200 Kirkham Way; Poway, CA 92064;(858) 312-3002, fax (858) 312-4247; e-mail<terry.tamberg@uav.com>.† Dantec Dynamics Inc., 750 Blue Point Rd.,Holtsville, NY 11742; (831) 621-3107, fax(631) 654-1293; e-mail<matt.crompton@dantecdynamics.com>.

From NDT Technician, Vol. 10, No. 3, pp: 1–4.Copyright © 2011 The American Society for Nondestructive Testing, Inc.

time. A technique called phase stepping allows the directionof surface movement to be determined and images withhigher resolution to be obtained.

In order to detect internal discontinuities, it is necessary tostress the test object with the appropriate type andmagnitude of excitation in a manner that causes the internaldiscontinuity to manifest itself as a deformation of thesurface.

Laser shearography is sometimes referred to as electronicspeckle pattern shearing interferometry (ESPSI). The termspeckle refers to the grainy appearance of the laser light thatreflects from the surface in a diffuse manner. When coherentand monochromatic laser light is projected through adiverging lens onto a surface, the illumination appears not asan even spread, but as a speckled, granular pattern. Minuteareas of high and low intensity light, these speckles aregenerated by the interference of the reflecting laser light onthe viewing optic. They form the basis of shearography andwithout them, measurement would not be possible (Fig. 1).

In comparison to other nondestructive testing techniques,laser shearography can offer the advantage of performancetesting. Indications are seen as the result of applied stressand can therefore be indicative of the materials or structuralstrength. Conventional NDT methods can showdiscontinuities that may or may not be related to actualstrength. A typical shearography result shows the gradient ofdeformation of the surface and not the absolutedeformation. The corresponding colors, typically white andblack or blue and red, come together to form a shearographyresult. Commonly referred to as a butterfly image, thispattern shows the positive and negative gradient of surfacedeformation above the discontinuity or anomaly as the pointis stressed (Fig. 2).

Detecting Discontinuities and Anomalies

Within size and depth limitations, laser shearography candetect many of the discontinuities that commonly occur incomposite structures. These include:· delaminations,· disbonds,

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Figure 1. Optical setup for laser shearography inspection.

LaserShear element

Livespeckleimage

Resulting shearographyimage

Camera

Deformation

= Differential displacement(between sheared points)

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· impact damage,· wrinkles,· kissing bonds,· separation of structural components,· crushed core and· porosityAdditionally, structural informationsuch as ply drops, overlaps, bulkheads,splices and ribs may also be shown.

Establishing System Capabilitywith Evaluation Standards

Test or evaluation standards areessential as points of reference whenconducting any nondestructiveinspection. In any inspection, thesestandards demonstrate the capabilitiesof the inspection system for detectinganomalies or discontinuities within thecomponents being investigated.Evaluation or reference standards areused to determine the sensitivity ofthe inspection technique and todemonstrate the ability of thetechnique to detect the maximum sizediscontinuity allowed per the part’sestablished acceptance criteria.

Nondestructive testing evaluationstandards are necessary for setting upinspection parameters. Theconfiguration of the nondestructiveinspection evaluation standard shouldbe representative of the testpart/assembly with respect tothicknesses, material types, ply layups,orientation, bonding medium, and

underlying structure. It is possible touse a replica of a part or sections ofactual production parts as evaluationstandards. It is also possible to useparts taken from service that may havediscontinuities caused by normalloading or overloading to representthe type of discontinuity beinginvestigated. The evaluation standardmay have built in discontinuitiesplaced in proper locations with respectto the area of inspection and the typeof anomaly being investigated. Thestandard should have the same surfacecharacteristics such as coating, paint orfinish as the part being tested.

Composite test panels can also befabricated representing structuressimilar to those being tested withengineered voids of specific sizesplaced in specific locations torepresent disbonds or delaminations.It is important to ensure that surfacesof the engineered voids do not bondto the structure during panelfabrication. Typically two circularpieces of peel ply or fluorocarbonresin tape are placed against eachother to form a small pillow. Theedges of the pillow are sealed prior toplacement in the test panel to preventthe entrance of resin or adhesive intothe space between the pillow plies.This technique ensures that a knownnonbonded surface exists in thecomposite test panel (Fig. 3).

Setup and Inspection

Inspection of aerospace compositestructures typically follows establishedprocedures that have been developedor approved by the original equipmentmanufacturer in compliance withgovernment or industry standards.1

The procedure should call out an areaof inspection for each shearogramthat is based on pixel count, types ofdiscontinuities to be found, maximumallowable discontinuity size andrejectable discontinuity size limits.

Depending on the lasershearography equipment used for theinspection, the procedure will define

the image scale and shear vector aswell as the stand-off distance requiredfor a portable system using a cameraor the grid pattern required forinspection with a vacuum hood typesystem. In the case of a portablecamera laser shearography systemusing thermal excitation, a quartz lampwould be used to load the surface byheating the part. Only a degree or twoof change in the surface temperatureis required to conduct a properevaluation for laser shearography(Fig. 4). The camera and inspectionsoftware are activated and, once thesurface is loaded, the real time imageupdate can be used to observe thematerial as it responds to the heat.Individual images may be captured orrecorded for future evaluation. Anautomatic procedure can also be setup with predefined timings for bothloading and image acquisition.

Evaluating Images

Live laser shearography images can beused for real time evaluation of thetest part during the initial inspection orthe images can be captured and storedfor evaluation at a later time. Imagescan be viewed in grayscale or color.Similar to other graphical inspectionmethods, the results of the inspectionshould be compared to a knownsample or test standard. Structuraldissimilarities and discontinuities canbe compared to a valley, hill or ridgestanding out in relief on atopographical map (Fig. 5).

Figure 2. Illustration of shearographyresult for detection of disbond inhoneycomb material.

Figure 3. Shearography result forevaluation standard showing multipleengineered disbonds on honeycombcore (left) and foam core ((right).

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Core

Disbonding

Afterloading

Deformation Slope

Shearographicresult

Skin

Advantages

Advantages of the laser shearographytechnique include the following:· noncontact – does not use or

require couplant,· portability for use in field,· provides full field inspection and

overcomes many problems ofrecording and interpreting pointmeasured data of large area,

· real time display of results,· uses multiple stressing techniques

applicable to wide range ofmaterials, structures and

discontinuity types,· quickly covers large areas with high

sensitivity and· personnel qualifications are

established2,3,4

Limitations

Limitations of the laser shearographytechnique include the following:· requires mains power for lasers and

vacuum pumps,· laser safety issues from site to site,· deep discontinuities and monolithic

structures may prevent detection ofdiscontinuities and

· thick composite structures and

metallic structures may not lendthemselves to laser shearographyinspection

Conclusion

Compared to other NDT techniquessuch as ultrasonic testing, X-ray, orbond testing, laser shearography offersthe advantage of a fast, full field andnoncontact inspection technique. Thedetection of discontinuities is donewhile stressing the sample andidentifying local changes in materialproperties such as stiffness andadhesion due to the excitation. Lasershearography can rapidly inspect areasup to 0.5 m2 (5.4 ft2 ) in one test,while being noncontact andnondestructive. The technique workswell on sandwich construction withhoneycomb or foam cores.

References

1. ASTM E 2581, Standard Practicefor Shearography of Polymer MatrixComposites, Sandwich Core Materialsand Filament-Wound Pressure Vesselsin Aerospace Applications. WestConshohocken, PA: AmericanSociety for Testing and Materials.(2007).

2. ASNT Recommended PracticeNo. SNT-TC-1A: PersonnelQualification and Certification inNondestructive Testing. Columbus,OH: American Society forNondestructive Testing (2006).

3. CEN EN 4179, Qualification andApproval of Personnel forNon-Destructive Testing. Brussels,Belgium: European Committee forStandardization (2006).

4. AIA NAS 410, National AerospaceStandard Certification andQualification of Nondestructive TestPersonnel. Arlington, VA:Aerospace Industries Associationof America (March 2008).

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Figure 4. Laser shearography setup for portable camera system.

Figure 5. Laser shearography images captured during inspection of carbon compositestructures: (a) section of wing structure and (b) section of radome.

(a) (b)

Impact damage

Ply overlapIndications

Ply overlap

Spar Repair patches

Stiffeners

Test object

Laser diodearray

Computer andelectronic controls

Resultingshearography

image

Heat lamp

Impact damage