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INTRODUCTIONLaser ultrasonics is rapidly becoming a well-known industrialtechnique for inspecting composites that provides largebenefits over conventional systems in the inspection ofcomplex contoured carbon-fiber reinforced polymer (CFRP)components (Osterkamp and Kaiser, 2008). The advantagesof laser ultrasound over conventional systems for theinspection of complex shaped CFRP are multi-fold. Systemsare typically able to rapidly scan the laser beam across thesurface of the part with no knowledge of the part curvature,no special tooling or fixtures, and no contour following.Teaching procedures are quick and simple and there isminimal recurring setup time. Typical laser ultrasonic systemsuse a robot manipulator to move a scan head around acomplex shape from a standoff distance of approximately 1.8 m (6 ft). A fast, two-axis galvanometer scanner is used toscan the laser beams across the surface, thereby covering largeareas of complex shaped parts in a rapid fashion. These typesof system characteristics make laser ultrasound extremelyversatile for rapidly testing a large variety of part types. Inaddition, the variety of system configurations availableprovides the flexibility needed for all types of factoryenvironments and requirements.Laser generation of ultrasound in CRFP materials occurs

due to the absorption of the laser wavelength in the top 10 to100 µm of the surface layer. This surface layer is generally anorganic matrix such as an epoxy resin, bismaleimide orthermoplastic, although other surfaces such as paint topcoatscan also provide excellent generation. The size of the laserspot hitting the target is approximately 5 mm (0.2 in.) indiameter. This provides an absorption volume that efficientlydirects the bulk of the ultrasonic energy in a longitudinal

mode normal to the material surface regardless of the laserangle of incidence. A short pulse laser, often CO2 (10.6 µm)or Nd:YAG (1.064 µm), is used as the generation laser. Laserenergies are kept below the damage threshold, and thegeneration mechanism is therefore a thermo-elastic process inwhich the material returns to its prior state and the laserleaves no lasting effect. The rapid expansion of the materialdue to heating causes a stress wave in the material, which isthe ultrasound.In general, ultrasound generated by a laser is similar to that

from conventional ultrasonic devices with the exception thatlaser-generated ultrasound typically contains a much broaderfrequency spectrum. To some degree, both frequency andmode content are dependent on the generation laser spot size.Although the laser generates several types of spatial modes, asthe spot size increases, a higher percentage of the energy isshifted to the longitudinal mode (a more piston-like source)and less is available for other modes such as shear or lambmodes. In addition, a higher percentage of the total energytends to shift to the lower frequency domain as the spot sizeis increased. This is analogous to transducer diametersincreasing as their center frequency decreases.For detection, a second laser is directed to the same point

on the material under test. The detection pulse (typically anNd:YAG at 1.064 µm) is a long pulse laser of 50 to 100 µs inlength. This pulse must illuminate the surface of the materiallong enough for the ultrasound to reverberate several times inthe material thickness. The ultrasonic vibrations on thesurface modulate the phase and frequency of the lightscattered from the surface and collected by the system optics.This light is sent to a discriminating device where these phaseand frequency modulations are converted to amplitude

The Advantages of Laser Ultrasonics for theInspection of Complex Composite Partsby Kenneth R. Yawn, Mark A. Osterkamp, David Kaiser and Chase Barina

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From NDT Technician, Vol. 13, No. 1, pp: 4–6.Copyright © 2014 The American Society for Nondestructive Testing, Inc.

modulations and in turn are converted to electrical signals. Duringthe scanning process all data acquisition, system timing and laserlight intensity are automatically controlled by the system. There areno user parameters to input or adjust. Optional user defined filterparameters and techniques are available in the post-processing anddata analysis phase to optimize the detection of anomalies.For production components, material integrity is of the utmost

importance. Nondestructive testing (NDT) is by definitionnondestructive. To that end, the laser pulse energies and spot sizesused in laser ultrasonics are designed to keep the energy densitybelow the damage threshold of the CFRP matrix. Differentmaterials may have different damage thresholds and can reactdifferently to the laser wavelengths. Material testing to determinethe characteristic damage threshold is needed prior to productionuse. For a number of years, laser ultrasonics has been used on thefactory floor at a military aircraft manufacturing facility in FortWorth, Texas and more recently at a military and commercialaircraft manufacturing facility in Winnipeg, Canada to inspectproduction composites parts for military aircraft (Drake et al.,1998; Yawn et al., 1999). The technology has been approved andimplemented on an array of programs such as the F-16, F-22, F-2and F-35. To date, the systems installed in the Fort Worth facilityhave tested in excess of 40 000 flyaway production parts.

Carbon-fiber Reinforced Polymer InspectionDifficultiesAs the use of composites in the aerospace industry continues toexpand, more complex and difficult to inspect components arebecoming common. Part diversity can range from small bracketsand clips, floor panels, stringer sections and complex inlet ducts upto large wing and fuselage skins. In commercial aircraft, very largebarrel structures for the fuselage and large flat floor panels arecommonplace. The amount of square footage of CFRP that mustbe tested daily to meet production rates can be enormous.Throughput of this magnitude and of such a wide variety of

components often requires companies to purchase and maintain anumber of different machine configurations for ultrasonicinspection alone. Often, these systems are dedicated to inspectingonly one type of part configuration such as floor panels and stillmay require extensive setup time when parts are changed in thesystem. Laser ultrasound has the potential to alleviate many of thosedifficulties. Often, inspections of large complex contoured partswith tight radii, ply drops and drilled holes can be accomplished onthe same machine as a large batch of small clips and brackets. Setuptime and machine configuration changes between parts are simpleor virtually nonexistent. The next section will describe two commonproduction testing problems and their laser ultrasonic solutions.

Two Inspection Problems and Their SolutionsTwo specific examples of difficult inspection scenarios will now bedetailed. In the first example, consider a set of small bracketsapproximately 152.4 mm (6 in.) in length, each of a slightlydifferent configuration and size. These brackets typically have twoor three sides oriented at 90° to each other with the corners havingapproximately 6.4 mm (0.25 in.) radii. Production rates mayrequire on the order of 2000 of these various brackets to be testedeach month. Inspection of large numbers of small, complex shapedbrackets such as these create two main difficulties: part transfer andsetup for each bracket would be extremely time consuming, and thesubsequent inspection of that number of brackets in a month wouldswamp any system. On the other hand, the inspection of individualbrackets with a laser ultrasonic system is easily accomplished withlittle setup, and the inspection of a three-sided bracket can becompleted from a single inspection view of the part. In addition, a laser ultrasonic system could easily be designed with automatedingress and egress to the inspection cell of a rack loaded with manybrackets at one time, all tested from a single inspection view. Figure 1shows an amplitude C-scan of a two-sided bracket scanned from asingle view. The angle of incidence of the laser beams to the bracketfaces is approximately 45°. Figure 2 shows a B-scan cut across the

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Figure 1. High-resolution amplitude C-scan of areas around drilledholes and porosity on a two-sided angle bracket (angle of incidenceof laser beam to surface is approximately 45°).

1 2 3 4Minor porosity

90° radius

Figure 2. High-resolution B-scan of areas around drilled holes andporosity on a two-sided angle bracket (angle of incidence of laserbeam to surface is approximately 45°).

Top surface of part

Back wall

1 2 3 4

(four) drilled holes in the two-sided bracket. The image clearlyshows the four holes by the absence of a back wall signal. (Thefront wall signal inside the holes is the result of the generation laserinteracting with the floor behind the bracket.) The front surface ofthe part together with the back wall is noted in the figure.The three-sided bracket of this material (shown in Figure 3) can

also be tested from a single view with an angle of incidence of over65°. In principle, this would allow a single, simply designed rack tohold many different styles of brackets at one angle convenient forthe system to scan with minimal adjustment to the scan head angleor position. The rack could be designed to move in and out of theinspection cell in an automated fashion. The only manual task atthat point would be placing the brackets on the rack and removingthem after inspection.

The second example is of a hat stringer section. In commercialaircraft production these types of stringer section can be upwards of10 m (32.8 ft) in length and of varying widths and curvatures.These types of stringers can present many difficulties forconventional systems. Part loading and unloading into fixtures,system setup and calibration can all be time consuming anddifficult to get correct, producing results that are frequentlydependent upon the skill of the system operator.Figure 4 shows an example of a commercial aircraft stringer

section with challenging tight radii. Figure 4a shows a photo of the

part taken by the system camera located on the scan head. This is aview from the actual scanning position. From this view the systemis able to scan the flange, inside radius, web, outer radius and top ofthe hat without repositioning. The amplitude C-scan image fromsuch a scan appears in Figure 4b. Only one additional view from asimilar orientation on the other side would be required to test theentire part. In laser ultrasound, software controls the entireultrasound generation and detection process. By removingadjustable parameters, the data quality is consistent from operatorto operator.

CONCLUSION

In over 15 years of daily production inspection of complexaerospace composite structures at the Fort Worth and Winnipegfacilities, laser ultrasound has shown itself to be an extremelyversatile, adaptable and cost-effective NDT technique. This articlehas shown two examples of component types that, in a highproduction rate environment, are very difficult to inspect quicklyand easily with conventional NDT. Herein has been demonstratedhow laser ultrasound can be used as a simpler, faster and moreefficient solution to the inspection of these challenging componentswhile providing results that are less dependent on the skill of theoperator running the system. Laser ultrasound, while still a newtechnology to many in the aerospace community, is rapidly provingitself to be a capable and versatile NDT tool.

AUTHORSKenneth R. Yawn: PaR Systems, Inc., LaserUT Technology Center, FortWorth, Texas.

Mark A. Osterkamp: PaR Systems, Inc., LaserUT Technology Center, FortWorth, Texas.

David Kaiser: PaR Systems, Inc., LaserUT Technology Center, FortWorth, Texas.

Chase Barina: PaR Systems, Inc., LaserUT Technology Center, FortWorth, Texas.

REFERENCESDrake, T.E., Jr., K.R. Yawn, S.Y. Chuang, M.A. Osterkamp, P. Acres, M.Thomas, D. Kaiser, C. Marquardt, B. Filkins, P. Lorraine, K. Martin and J.Miller, “Affordable NDE of Aerospace Composites with Laser Ultrasonics,”Review of Progress in Quantitative Nondestructive Evaluation, Vol. 17, 1998,p. 587.

Osterkamp, M. and D. Kaiser, “Application of Laser Ultrasonics for theNon-destructive Inspection of Complex Composite Aerospace Structures,”1st International Symposium on Laser Ultrasonics: Science, Technology andApplications, Montreal, Canada, 16–18 July 2008.

Yawn, K.R., T.E. Drake, Jr., M.A. Osterkamp, S.Y. Chuang, P. Acres, M.Thomas, D. Kaiser, C. Marquardt, B. Filkins, P. Lorraine, K. Martin and J.Miller, “Large-scale Laser Ultrasonic Facility for Aerospace Applications,”Review of Progress in Quantitative Nondestructive Evaluation, Vol. 18, 1999,p. 387.

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FYI | The Advantages of Laser Ultrasonics

Figure 3. Photographic image of a three-sided angle bracket (angleof incidence of laser beam to surface is approximately 65°).

Figure 4. Laser ultrasonic scan of commercial aircraft stringer:(a) part photo; (b) amplitude C-scan.

(a)

(b)