INSPECTION OF AIRCRAFT COMPONENTS WITH THE AID OF PORTABLE DIGITAL SHEAROGRAPHY

D Findeis and J Gryzagoridis

Department of Mechanical Engineering University of Cape Town,

Private Bag, Rondebosch, 7701, South Africa email: dfindeis@eng.uct.ac.za, profg@engfac.uct.ac.za

website: www.meceng.uct.ac.za Abstract: Digital Shearography is an optical, non-contacting NDE inspection tool which can be used to detect material discontinuities and other defects within objects. The authors present a portable Digital Shearography Camera, which has just been completed and is currently undergoing evaluation. After presenting the preliminary results of laboratory based evaluation and testing of the Digital Shearography System, the authors report on the findings of the first on-site inspection, which was conducted at the regional airforce base in Cape Town. The results in the form of images exhibiting fringe patterns are presented, discussed and the presence or absence of flaws is highlighted. The performance and suitability of the Digital Shearography System for on-site application appears quite promising. Keywords: Digital Shearography, Non-Destructive Evaluation, Flaw detection, Laser applications Nomenclature: ∆φ = correlation phase ∂d/∂x = displacement rate S = magnitude of shear n = no of fringes 1. Introduction Shearography is a novel non-destructive evaluation method used to inspect manufactured components for defects and flaws. Shearography is tolerant to the hostile environment found in many industries and is a full field optical inspection technique which has inherently higher inspection rates than other non-destructive testing methods. Originally, shearography was developed as a strain-measuring device but was more readily accepted by industry as a flaw detection tool (Stanley, 1995). Material discontinuities within objects, which alter the strain distribution and hence weaken the structure, can be detected with this inspection method. As a result, this optical non-destructive testing technique has been used routinely in the aerospace and rubber tire industry and is endorsed by the American Federal Aviation Administration. Conventional methods of experimental stress analysis require laborious mounting of large numbers of strain gauges, or the application of contoured photo-elastic coatings, or the application of liquid brittle coating upon the surface of the material structure. In most cases, these methods either reinforce or alter the surface of the object. Shearography on the other hand is non-contacting, does not reinforce in any manner the structure being tested and yields full field surface strain information without the limitations associated with other methods (Stanley, 1995). In engineering components, defects arise either during manufacturing stages or due to adverse service conditions. When an object is stressed, the surface deforms in accordance to the applied stress. The presence of a defect weakens the structural integrity of an object, which in turn locally influences the object’s surface deformation when stressed. Shearography has been applied to a wide variety of objects and materials, including composites, and the typical types of defects which have been detected include cracks, voids, delaminations and debonds, as well as material corrosion (Gryzagoridis, 1995).

One major difference between shearography and other methods of non-destructive testing methods is the way in which discontinuities are revealed. Methods such as penetrants and magnetic particle testing reveal surface or subsurface discontinuities by enhanced visual means. Techniques such as ultrasound and radiography detect internal discontinuities by detecting heterogeneity in materials. Many of these detected discontinuities are unimportant; others again are. In contrast shearography looks for discontinuity induced strain anomalies which will only form if the discontinuity or flaw has an effect on the strain distribution within the object as a result of the weakening of the structure’s integrity (Findeis, 1996). 2. Theory of Operation Historically, shearography had received limited acceptance due to the need to use photographic film as the recording medium, which is a slow and costly procedure. The subsequent Fourier filtering process needed to be able to read the fringe pattern further delayed the output of the test results. During the late Eighties a new method, which eliminated the photographic recording and subsequent fringe readout process, was developed and dubbed digital shearography or electronic shearography (Hung, 1989). This new method replaced the photographic recording medium with a video camera linked to an image-processing module. Electronic shearography allowed non-destructive testing results to be generated digitally at video frame rates without the use of film or chemicals. Because shearography is based upon the principles of speckle pattern correlation interferometry (Jones, 1989), a monochromatic light source of sufficient coherent length, typically a laser, has to be used to illuminate the object which is to be tested. A video camera is then used to view the illuminated object through a shearing device. The net effect of this device is to split one point on the object into two at the image plane i.e. two sheared images. This is equivalent to bringing two separate points on the object to coincide in the image plane and allowing them to interfere with one another. The intensity based speckle pattern that is produced by the interference process of the two sheared images is resolved by the video camera and viewed on a personal computer (PC) after being digitised by a frame-grabber. The typical process of generating a shearography fringe pattern can be described as follows: The intensity based speckle pattern of the unstressed object is initially captured by the video camera, digitized and stored in a computer. The object is then stressed, causing a surface displacement. This causes a relative movement between lateral points on the object which are focused via the shearing optics onto one common point on the CCD image plane. Because the speckle distribution is a function of the phase and intensity of the laser reflection off the object’s surface, any deformation of the object’s surface will cause the speckle pattern produced at the video cameras image plane to change. By recording these subsequent speckle patterns, digitizing them and comparing them to the initially stored image, areas of pixel intensity correlation and de-correlation can be determined and plotted graphically. The result is an image, which consists of alternating black and white ‘zebra-like‘ fringes. Mathematically the fringes can be represented by the following equation :

S

xd

∂∂

=∆λπφ 4

The above equation indicates that the correlation fringes along which ∆φ is constant, represent lines of constant displacement rates. The spacing between adjacent fringes is a function of the displacement gradient according to the following equation:

Sn

xd

2λ

=∂∂

This implies that for a given object surface area, an increase in displacement gradient will produce a corresponding increase in number of fringes.

3. The Portable Shearography Camera Prototype The authors have been implementing Shearography and Electronic Speckle Pattern Interferometry for NDE purposes under laboratory conditions for a number of years, and have recently developed a portable Digital Shearography Camera. The brief for the prototype was to make the unit portable, versatile, robust but also lightweight, and to keep costs within limits. The Digital Shearography Camera, as depicted in Figure 1 is made up of the following items: A fully adjustable Shearography Head Unit (1) which is mounted on a sturdy professional tripod (2), an Intel PII based PC using the Microsoft NT operating system (3), keyboard (4), SVGA monitor (5), and diode laser controller and CCD camera power supply (6). Figure 2 is a schematic diagram of the interaction between the various components as well as a breakdown of the components which make up the Head Unit. Housed inside the Head Unit is a 50 mW infra-red diode laser which is fitted with a beam expander for object illumination purposes. A proprietary shearing device, which is made up of mirrors and beamsplitters, is used to achieve the desired image shear. The sheared images are focused onto a conventional CCD camera, which is fitted with a zoom lens and narrow band infrared filter. The camera zoom and focussing controls are mounted on the top of the Head Unit, as can be seen in Figure 1. The image shear magnitude and orientation can be adjusted via two vertical and horizontal control knobs, mounted on the side of the Head Unit, adjacent to the video data, video power and laser controller connectors.

Figure 1. The Digital Shearography Camera

Object

Image Processing Computer

Data

Shearography Head Unit

Camera Shearing Device

Laser

The video cable connects to an image digitiser which is housed in the PC and controlled by a custom written software package. Specific software also manages the image acquisition procedures and fringe pattern generation routines.A typical operating procedure of the Shearography camera can be described as follows:

Figure 2. Schematic of the Shearography Camera

The object to be inspected, if loose, should be placed on a sturdy work surface in view of the front side of the Head Unit. In order to set up the Head Unit, the software has to be initialised and a video document opened. This allows the video feed from the camera to be displayed on the PC monitor and gives the operator the opportunity to align the camera height, focus and image shear with respect to the test object via the relevant control knobs. At the same time, the laser illumination direction, which can be adjusted at the Head Unit, can be set to correctly illuminate the object. It must be remembered that the Shearography Camera utilises an infrared diode laser, which is invisible to the naked eye but not the camera. The next step in the process is to open a shearography document, which controls the image acquisition, processing and fringe generation sequences. To commence this sequence, an image of the unstressed object is captured and stored in the PC memory. The software routine then automatically captures the incoming video feed, compares it with the previously stored image, and displays the result in real-time on the PC monitor. Depending on the type and size of object being tested, one of the following forms of object stressing can be applied: - thermal stressing, - mechanical stressing, - vacuum or pressure stressing - vibration stressing. Whilst the operator stresses the object under inspection and views the resultant fringe patterns, the option exists to temporarily freeze the ‘live’ fringe patterns displayed on the computer monitor, either for closer scrutiny or storage. The inspection process can also be stopped at any point and either repeated or set up for the testing of another object.

Figure 3. Aluminium Honeycomb Panel

4. Laboratory Testing of Shearography Panel In order to evaluate the performance of the prototype, a series of controlled laboratory tests were conducted. The first inspection was conducted on an aluminium honeycomb panel, measuring 330x290 mm, as seen in Figure 3. It was constructed of two outer aluminium sheets with a 6 mm aluminium honeycomb mesh sandwiched between them. Two identical panels were tested. The first panel was defect free. The second panel had two artificially induced flaws. The first flaw was a cut (discontinuity) through the back aluminium sheet measuring approximately 50 mm as seen in Figure 4. The second flaw, depicted in Figure 5, was a 50mm diameter section where the honeycomb mesh had been crushed.

Figure 4. Cut through Aluminium Sheet Figure 5. Crushed Honeycomb

Figures 6 depict the fringe patterns obtained from the defect free panel with the shearing optics configured for shearing in the horizontal direction. The amount of image shear was approximately 30% of the width of the viewed image. Thermal heating via an infrared lamp was applied for approximately 5 seconds. As can be seen, this configuration generates uniform vertical fringes with no marked irregularities or fringe concentrations. This is due to the stressing of a uniform panel, which is not constrained at the boundaries.

Figure 6. Horizontal Shearing, Defect Free

After the defect free panel had been inspected, the “flawed” aluminium panel containing the two defects was inspected in the same manner. The panel was flipped around so that the sheet containing the defects was at the back and could not visibly be seen by the camera. An infrared lamp was again used to stress the object. Figure 7 depicts two circular fringes which are separated by a central vertical fringe. The close fringe spacing between the two circular fringes and the central vertical fringe indicate a localised maximum displacement gradient in the horizontal direction. The same fringe configuration was recorded when the object was inspected with the shearing set for the vertical direction, indicating a symmetrical defect. This type of fringe representation is characteristic of a bulging displacement; due to the weakened substructure; in response to the applied stress.

Figure 7. Crushed Honeycomb, Horizontal Shearing

Figure 8. Discontinuity, Horizontal

The result of the inspection of the discontinuity is shown in Figure 8. A defect free panel viewed with this inspection configuration should have generated symmetrical vertical fringes. Figure 8 however reveals that the discontinuity through the rear panel has a marked influence and significantly weakens and randomly alters the panels’ response to an applied stress. This is evidenced by the anomaly in fringe density when scanning the image from left to right along the dotted white line, as well as the fringe irregularity in the centre of the image, and a change in fringe orientation in the upper right region of Figure 8. 5. On Site Inspection Encouraged by the results obtained from the laboratory investigations, the authors proceeded to perform an on-site evaluation of the prototype at the local Air Force Base in Cape Town. The tests were conducted in an aircraft hanger with no special lighting or anti-vibration measures applied. One of the items tested was an aluminium aircraft landing gear tail fork, shown in Figure 9. The Air Force Base’s NDT division had rejected the tail fork due to the suspected presence of a welding crack in the

casting, assumed to have been created during the repair process of a flaw, which had originated in the casting process. The approximate wall thickness of the material in this region is 7 mm. The Shearography Head Unit was placed horizontally in front of the tail fork and focused on the region where the alleged crack had been detected. This region was then inspected using both vertical and horizontal shearing. Due to the material wall thickness, the thermal heating period using the infrared lamp was in the order of 10 seconds. Figures 10 and 11 below depict the results obtained from the inspection of the tail fork using horizontal as well as vertical shearing configurations. In both Figures the fringe spacing is uniform and corresponds with fringe patterns expected from a defect-free surface area. The widening of the fringe spacing towards the bottom of the image in Figure 10 is due to the presence of a strengthening rib. The change in orientation of the fringes on the left-hand side of Figure 11 is due to the curved edge of the casting as well as the presence of the strengthening rib at the bottom of Figure 11. The above results indicate that the flaw suspected to be a crack has no effect on the stress distribution within the tail fork. Figure 9. Aircraft Tail Fork

Figure 10. Tail Fork, Horizontal Shearing Figure 11. Tail Fork, Vertical Shearing

As a result of the above investigation it was decided to manufacture a crack in the tail fork casting and compare the results obtained from the manufactured flaw with those obtained from the suspected defect. To produce the flaw, a 1 mm slitting saw was used to machine a 75 mm long cut through the skin on the flat side of the tail fork. The cut was then welded closed again in such a way that the cooling process would produce a crack within the weld. This manufactured crack was then inspected with the shearing camera set up for vertical shearing as well as 45° shearing. The results of these tests are depicted in Figures 12 and 13. In both images the presence of the crack is clearly visible.

Figure 12. Man-made Flaw, Vertical Shearing

In Figure 12 the vertical shearing configuration reveals the crack location via the abrupt change in fringe direction. This is indicative of a sudden change in displacement gradient at the flaw. The above is supported

by the results depicted in Figure 13, where the change in fringe orientation and density also outlines the location of the crack. When comparing the results of the man-made crack investigation (Figures 12 & 13) with those of the suspected crack investigation (Figures 10 & 11), it becomes questionable whether the suspected crack did in fact exist. The crack was suspected when the personnel at the Air Force Base used Eddy current methods to inspect the tail fork, after the casting flaw had been welded closed. Eddy current is not only suited for crack detection, but also for material composition detection. If the aluminium used during the welding repair process was different from the aluminium used during the casting process, it is probable that the eddy current inspection procedure erroneously identified the dissimilar aluminium as a crack.

Figure 13. Man-made Flaw, 45° Shearing

6. Conclusion Both the laboratory and on-site inspection procedures have shown that the Shearography Camera is suitable as a non-destructive testing tool. Due to the small size and weight of the components making up the prototype, the system is truly portable. The proprietary shearing device is able to shear the viewed image in any desired orientation as well as magnitude. The custom developed software flawlessly controlled the image acquisition and processing routines. The inspection of the tail fork also revealed that the ability of shearography to record an object’s real response to an applied stress is a valuable asset and eliminates uncertainty. 7. References Findeis, D. and Gryzagoridis, J. (1996) Inspecting Glassfibre Reinforced Plastic Piping Using Portable ESPI and Shearography, Proceedings of the 14th World Conference on NDT (India) Vol 3, pp.1521-1524. Gryzagoridis, J., Findeis, D., Schneider,D. (1995) The Impact of Optical NDE Methods in Vessel Fracture Protection .Int. J. Pres. Ves. & Piping, pp.457-469. Hung, Y.Y. (1998) Apparatus and Method for Electronic Analysis of Test Object (US Patent 4887,899) Jones, R . and .Wykes, C. (1989) Holographic and Speckle Interferometry, 2nd Edition ( Cambridge University Press). Stanley, R. K. , Moore, P. O. , McIntire, P. (1995) Nondestructive Testing Handbook, 2nd Edition (American Society for Nondestructive Testing).