NDT.net - November 1998,
Vol. 3 No. 11
Table of Contents ECNDT '98
LASER-ULTRASONICS INSPECTION AND CHARACTERIZATION OF AERONAUTIC MATERIALS
J.-P. Monchalin, C. Néron, M. Choquet, D. Drolet, M. Viens
Industrial Materials Institute, National Research Council of Canada
75 de Mortagne Blvd, Boucherville, Qué, J4B 6Y4, Canada
Tel : (450) 641 5116, Fax : (450) 641 5106
P. Bouchard, R. Héon, C. Padioleau
27 de Lauzon, Boucherville, Qué, J4B 1E7, Canada
Corresponding Author Contact:
Email: Jean-Pierre.Monchalin@nrc.ca , URL: http://www.imi.nrc.ca/
Laser-ultrasonics, an advanced ultrasonic technique based on the generation and detection of ultrasound with lasers, is finding more and more applications in many industrial fields. In the aeronautic sector, the technique is now being used for inspecting parts and structures made of composite materials and is being developed for the detection of corrosion in aging aircraft and the characterization of thermal barriers coatings.
Aeronautics, because of the requirement of very high safety makes extensive use of nondestructive techniques to verify that the materials and parts used are free of critical flaws. The presence of large areas to be inspected, contoured shapes, the introduction of advanced materials and the aging of current aircraft require further development of these techniques and innovative solutions. Among those, laser-ultrasonics that uses lasers for the generation and detection of ultrasound at a distance presents several advantages for the inspection of aeronautic parts and materials.
In laser-ultrasonics, the source of ultrasound is located at the surface of the material and detection of ultrasonic motion is performed off the same surface, which eliminates the normal incidence requirement of conventional piezoelectric-based ultrasonics and allows easy inspection of complex shapes . To explain further this unique capability of laser-ultrasonics, we consider a simple curved part, sketched in cross section in Figure 1. As shown in Figure 1, when the two lasers, one for generation and the other one for detection, impinge at the same location on the surface of the curved part a train of ultrasonic echoes is generated and then detected. Generation produces essentially an ultrasonic wave propagating normal to the surface. When the two laser beams are scanned across the surface by using a rotating mirror, the ultrasonic echoes are still observed, even when the optical beams are far off the normal to the surface. Since there is no need to determine the shape of the inspected part to follow its contour and no need of complex fixturing, laser-ultrasonics has the capability to be much faster than any conventional mechanical scanning system.
We describe first below the system that has been developed, then present the results obtained on the inspection of composite parts, on the detection corrosion in airframes and on the measurement of the thickness of thermal barrier coatings.
Figure 1 : Laser-ultrasonic inspection of a curved part. The two arrows indicate the generation and detection laser beams, which are shown at two locations, determined by the optical scanner. The inserts indicate schematically the ultrasonic echoes, which can, for example, be observed on an oscilloscope.
DESCRIPTION OF THE SYSTEM DEVELOPED
The laser-ultrasonic system developed is made of two units, a detection unit and a generation unit, linked by optical fibers . The generation unit comprises a TEA CO2 laser for ultrasound generation, optics for coupling colinearly the generation and detection laser beams and large size optics for collecting the scattered light from the surface of the inspected part. In addition, a two-axis mirror scanner, the same size as the collection optics, is used for scanning across the part. The generation spot is typically 5 mm. Light from the CO2 laser penetrates below the surface of the material, resulting in distributed absorption. This is valid for bare composites where penetration occurs essentially in the epoxy, painted composite or metallic parts where penetration occurs in the paint layer and zirconia coatings. This causes a constrained ultrasonic source and consequently the emission of a longitudinal wave normal to the surface.
The generation unit is linked by optical fibers to the detection unit comprising the detection laser and the demodulating interferometer, which is a confocal Fabry-Perot . One fiber is used to transmit light from the detection laser, while the other one is used to bring scattered light to the interferometer. Both fibers are approximately 15 meters long and shielded. The detection laser is based on Nd-YAG technology, provides long pulses ( 50 µs long) at the repetition rate of 100 Hz, is highly stable in frequency and has high peak power ( 2 kW). The confocal Fabry-Perot is one meter long and has an optical bandwidth of 8 MHz. A PC computer controls the optical scanner and houses a sampling card for digital acquisition of the A-scans. Specially developed imaging software is then used to display C-scans and B-scans.
The system we have developed operates at a distance of 1.5 m between the surface and the optical scanner and can scan from a fixed system position an area of 1.8 m x 1.8 m or even larger depending upon the scattering properties of the surface.
Such a system is now commercially available. In more recent developments, a system which allows the scanning of much larger surfaces was assembled by UltraOptec for the US Air Force . By mounting the generation unit at the end of the arm of a gantry robot, parts of the size of a wing can be inspected. This system includes also a distance measuring device to determine at the same time during the scan the shape of the part. This system is associated to a 3-D imaging software which allows to visualize part shape and ultrasonic results at the same time (as shown in Figure 3 below).
INSPECTION OF COMPOSITE PARTS
The use of the system described above has been demonstrated on parts of more than one meter across and on a large variety of smaller parts with acute curvature [5-6]. These parts included laminates of various thicknesses (from a few plies to a thickness of more than one inch), made of various materials (graphite epoxy, kevlar epoxy, glass epoxy), with a bare surface or a painted surface. Specimens with slopes and corners and parts with U-shape, T-shape and sine wave cross sections were also tested. The technique was also demonstrated on various honeycomb structures. Defects such as porosity, disbonds, delaminations and impact damage can be detected.
As an example we show in Figure 2 the detection of the delaminations caused by an impact on flat specimen. The specimen was impacted at 4 locations with various amount of energy. Delaminations were produced at two locations as shown by laser ultrasonic inspection. As a second example, in a 3-D view, we show in Figure 3 the results of the inspection of a contoured specimen, which has a U-shape and has implanted flaws at various depths along two flat surfaces and along one corner. The various implanted flaws, including the ones implanted along the corner are all clearly located. A thickness increase when going through the corner is also observed.
Figure 2 : Laser-ultrasonic C-scan image of the damage produced by an impact on a graphite-epoxy laminate. Right: time-of-flight C-scan, left : B-scan though the severe impact.
Figure 3 : Laser ultrasonic time-of-flight C-scan of the corner part shown in 3-D.
INSPECTION OF A CF-18 PLANE IN A MAINTENANCE HANGAR
These tests were conducted at the maintenance facility for CF-18 fighters of Bombardier Inc./Canadair Defense System Division, located at Mirabel, Québec, within the airport area [5-6]. The test plane had already been serviced and inspected according to the prescribed maintenance procedure (using in particular manual ultrasonic inspection). It was in a flying condition and fully loaded with fuel. It had not been the object of any particular preparation for laser ultrasonic inspection.
Figure 4: Laser-ultrasonic C-scan image of the edge of the horizontal stabilizer of a CF-18.
Various parts of the aircraft fuselage, wing and stabilizer were inspected. We are presenting here, as example, only one result obtained on the horizontal stabilizer, which is a complex composite structure with an aluminum honeycomb core (see Figure 4 below). The laser-ultrasonic inspection reveals the complex inner structure of this part. As should have been expected, since the plane had gone through complete inspection before our tests, we did not find any important flaw over the whole airframe, which would have required a repair. Only a small defect was found, of a size well below the critical size requiring corrective action.
DETECTION OF CORROSION IN AGING AIRFRAMES
Corrosion is a serious problem as it may directly affect the airworthiness of an aircraft. In lap joint structures, corrosion causes thinning and pillowing of the lap joint skins which in turn lead to very high stresses and may cause degradation of structural integrity. Typical lap joint in an aircraft structure is formed by splicing together two aluminum skins and a stiffener, and fastening these with rivets. Some lap joints have adhesively bonded layers, while some have only a layer of sealant for corrosion protection. Different configurations of lap joints with tapers, doublers and triplers are found in the aerospace industry. The present study has been limited to a simple lap joint assembly. Before corrosion occurs between the skins of this lap joint, the bond between the two skins must degrade. As corrosion progresses, the skins may become totally disbonded, leading to the creation of an air gap. An ultrasonic pulse will be totally reflected by this air gap. The outer skin can then be considered as acoustically isolated at the location of corrosion (or disbond). The measurement of the outer skin thickness, and hence of material loss, can be made by monitoring the ultrasonic resonance frequencies of the isolated outer skin. The resonance frequencies are directly related to the skin thickness and provide a precision measurement of skin thickness.
Since corrosion occurring in a bonded lap joint may start with a disbond of the outer skin from the rest of the assembly, the first step should be to discriminate bonded areas from disbonded areas. The Industrial Materials Institute has recently developed an ultrasonic method to determine the quality of bonds within a multi-layered structure . The method is based on modeling the propagation of ultrasonic waves in the bonded structure and on introducing a very thin interface layer of proper stiffness to describe adhesion strength. Using this model, we have calculated the theoretical ultrasonic frequency response of this lap joint assembly and observe excellent correspondence between the location of the resonance peaks given by the model and those observed in the test data. The model provides us with a method to discriminate bonded areas from disbonded ones. In this case, for disbonded areas, there is a sharp resonance peak near 5 MHz, whereas for bonded areas, there is a valley at about the same frequency. We also observe that a peak near 7 MHz is always present, independent of the bond quality. This position of this peak can be shown to relate to the thickness of the outer skin of the lap joint.
The procedure to identify corrosion and image material loss in the lap joint under study is therefore composed of three steps . First, we make a mask (black and white image) using a narrow bandwidth centered at 5 MHz. For every acquisition point, we verify if the maximum value in the narrow bandwidth is above or below a given threshold determined by the noise level in the signal. Points for which the value is above the threshold are marked with a white pixel in the image mask, while points for which the value is below the threshold are marked with a black pixel. The result is a mask which blocks out the regions of the lap joint with intact bonds, i.e., with no corrosion. Second, we make an ultrasonic frequency image of the lap joint by color coding the resonance frequency value near 7 MHz. Third, both images, the mask and the frequency image, are superimposed resulting in a thickness image of the corroded area of the lap joint. Figure 5 shows the mask, the initial ultrasonic image and the final corrosion image. These results, obtained by laser-ultrasonics, are in good agreement with measurements done by an eddy-current system and an enhanced visual technique (D Sight(tm)) image analysis. At this time, however, we have not been able to correlate these results with a direct measurement of the skin thickness. The lap joint is presently being disassembled for skin thickness measurement using a precision X-ray method. The results of the comparison will be reported at a later date.
Figure 5: Corrosion identification and residual thickness mapping in a lap joint.
Fig.6 - Thickness variation of a zirconia coating specimen along a particular direction : (o) actual thickness measured destructively, the error bars represent the standard deviation at each position ; (+) propagation delay through the layer measured by laser-ultrasonics.
CHARACTERIZATION OF PLASMA-SPRAYED COATINGS
Another application of laser-ultrasonics is the characterization and in particular the thickness determination of thermal barrier coatings used on engine blades . Planar tests specimen were prepared by spraying zirconia on copper substrates. The condition of deposition was such that the coatings did not have uniform thickness. Figure 6 shows the results of the measurement of the propagation delay through the layer with laser-ultrasonics compared with destructive measurements obtained by microscopic observation after epoxy infiltration of the coating and sectioning. We observe a good correlation between the actual thickness and the laser-ultrasonic results, indicating in particular that the coating stiffness is approximately constant throughout the sample and that laser-ultrasonics can be used to accurately map the coating thickness. Being a non-contact technique, already used in severe environment , laser-ultrasonics has therefore the potential to be used in production for coating deposition control.
We have described several aeronautic applications of laser-ultrasonics, an advanced ultrasonic technique that uses lasers for the detection and generation of ultrasound. The technique has been in particular developed for inspecting by pulse-echo polymer-matrix composite materials. Its effectiveness was demonstrated on numerous parts of various shapes, thicknesses and compositions. The technique was also demonstrated to be appropriate for service inspection by inspecting a CF-18 airplane in a maintenance hangar. The technology we have developped is now commercially available and efforts are currently pursued to introduce it further into industry. The application to the detection of corrosion in metallic airframes and the measurement of corrosion thinning in lap joints is now actively pursued and present encouraging perspectives. A third application of significant importance, which also appears to offer encouraging perspectives, is the monitoring of the deposition of thermal barrier coatings on engine blades.
The CF-18 inspection tests were sponsored by the Department of National Defense (DND) of Canada under contract No. W8477-2-AC30/01-BB. The impacted specimen and the corrosion specimen were provided by the Institute for Aerospace Research (IAR) of NRC. The U-shape specimen was provided by the Defense Research Establishment Pacific of the National Defense of Canada. We acknowledge the contribution of J. P. Komorowski and R. W. Gould of IAR to the application to corrosion detection and the contribution of C. Moreau of IMI to the application to thermal barriers coatings.
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