NDT Detection and Quantification of Induced Defects on Composite Helicopter Rotor Blade and UAV Wing Sections

Transcription

1 NDT Detection and Quantification of Induced Defects on Composite Helicopter Rotor Blade and UAV Wing Sections Dirk Findeis*, Jasson Gryzagoridis, Vincent Musonda Dept of Mechanical Engineering, University of Cape Town, 14 University Avenue, Rondebosch, 7700, South Africa. ABSTRACT Digital Shearography and Infrared Thermography (IRT) techniques were employed to test non-destructively samples from aircraft structures of composite material nature. Background information on the techniques is presented and it is noted that much of the inspection work reviewed in the literature has focused on qualitative evaluation of the defects rather than quantitative. There is however, need to quantify the defects if the threshold rejection criterion of whether the component inspected is fit for service has to be established. In this paper an attempt to quantify induced defects on a helicopter main rotor blade and Unmanned Aerospace Vehicle (UAV) composite material is presented. The fringe patterns exhibited by Digital Shearography were used to quantify the defects by relating the number of fringes created to the depth of the defect or flaw. Qualitative evaluation of defects with IRT was achieved through a hot spot temperature indication above the flaw on the surface of the material. The results of the work indicate that the Shearographic technique proved to be more sensitive than the IRT technique. It should be mentioned that there is no set standard procedure tailored for testing of composites. Each composite material tested is more likely to respond differently to defect detection and this depends generally on the component geometry and a suitable selection of the loading system to suit a particular test. The experimental procedure that is reported in this paper can be used as a basis for designing a testing or calibration procedure for defects detection on any particular composite material component or structure. Keywords: NDT, speckle techniques, digital Shearography, thermography, defect detection, composites 1. INTRODUCTION Nondestructive testing and evaluation (NDT/E) is a very broad, interdisciplinary field that is applied to structural components, parts and materials in order to ensure that they function in a reliable and cost effective fashion. These techniques are widely used in the manufacturing, power, petrochemical and aircraft industries, both as a tool to certify the integrity of manufactured components and for routine maintenance inspections. There are many different materials such as steel, aluminium alloy, titanium, plastics and composite materials used in the manufacture of for example aircrafts. In addition, the manufacturing process requires that components either be glued, welded, riveted or fastened together. As a result, there is no single NDT method suitable for the routine inspection of all components and joints of the finished product. The conventional NDT/E techniques used for inspection purposes include liquid penetrants, magnetic particle inspection, eddy current, ultrasonics, radiography, infrared thermography, visual and optical interference inspection techniques to mention but a few. Changes in the selection of materials used in the aircraft industry pose new challenges to the NDT industry. 25% of the materials used in the manufacture of the Airbus A380 are composites. The Boeing 787 is even more ambitious and it is estimated that up to 50% of the materials used, including the manufacture of the fuselage will be of a

2 composite nature. Similarly the new Gripen fighter jet uses approx 20% by weight carbon fibre and epoxy composite laminates in the structure. Some of the conventional NDT methods such as x-ray inspection, ultrasound and eddy current were designed for the needs of metal structures and cannot meet the challenges posed by modern composite structures. Other NDT techniques are reliant on probe contact with the inspection area, which can end up as a time consuming process if large aircraft areas are to be inspected. This approach is unsuitable, as it is desirable that the NDT inspection and maintenance turnaround time within the commercial aviation industry be as fast as possible. 2. THEORY Two techniques which have attracted interest within the academic and commercial applications community are Infrared Thermography (IRT) and Digital Shearography. Both techniques are non-contacting, non intrusive, area based inspection techniques and thus are faster than conventional contact inspection methods. IRT is a NDE method used for the localization and characterization of thermally resistive defects. The method relies on the fact that heat energy flows through a solid, the rate of which is governed by the thermal diffusivity of the material. The temperature distribution on the viewed surface can be defined by equation 1 below: 2 1 T T = 0 α t Where: α is the thermal diffusivity (m 2 s -1 ) T is the temperature, and t is the time. For uniform heating and in the case of a homogeneous material, the surface temperature distribution would be uniform. Where an anomaly is encountered within the solid, the heat flow rate or pattern is altered 1. These changes in heat flow cause localized temperature differences or thermal patterns in the material, which are transferred to the surface of the material or object under inspection. The heating of the material can be done with quartz lamps or hot air from a dryer and applied either continuously or in a pulsed manner using flash lamps. The type of heating of the object and thus transient energy flow can be either in transmission or reflection mode. The technique makes use of an infrared camera which transforms the energy radiated from the object surface in the infrared band of the electromagnetic spectrum into an electronic video signal which is processed into a visible image of the object being viewed. In the image each radiation energy level is characterized by either a colour, or a grey scale level 2. In other words, images presented in the video feed from the camera are a record of the temperature variations of the surface and can range either from white for warm regions to black for cooler areas on a black and white image or in colours ranging from reds to blues using a pseudo colour system. These images can then be interrogated to detect localized areas of differing temperatures, which can be related to the presence of either surface or subsurface material or structural variations. In order to capture and process these images, infrared cameras are often connected to and driven by a computer. Figure 1. 3 below, shows a typical setup of an IRT system. It has been indicated that IRT is one of the many other NDT techniques which can be used for the detection of flaws in composites and is potentially useful in the aerospace industry 1,4,5,6. The range of defects detectable include, but are not limited to: delamination, debonding and fracture in composite laminate parts, delamination, debonding or object damage due to foreign objects as well as liquid contamination and corrosion in composites. (1)

3 Figure 1. Infrared thermography testing setup Digital Shearography is another technique which has produced promising results when used to inspect composites for defects. The technique was originally developed for strain measurement as it is an optical interference technique used to reveal an object s surface displacement gradient in response to an applied stress. There are many ways to stress the object during the inspection process, the most common being thermal, pressure or vacuum stressing. Each object to be tested possesses unique structural or material characteristics and there thus is no standardized inspection procedure or object loading method suitable for all types of composites to be inspected. As the technique is so sensitive to the resultant displacements, the magnitude of the applied stress is very small, making it a truly non destructive inspection technique. Because defects in objects usually induce strain concentrations due to structural weakening, the presence of a surface or subsurface defect is easily revealed as strain anomalies or concentrations within the displacement gradient pattern. Moreover, a rigid-body motion is Laser not associated with a strain, thus shearography is desensitized to such Mirror displacements. This is an important advantage of shearography, and improves Mirror2 the suitability of the technique for in-situ applications. Camera Mirror The shearography technique is based on the recording of speckle images, created when two light waves interfere with one another 7. In order for this to occur, the light waves have to be monochromatic, which is why single mode lasers are employed. The expanded laser beam is used to illuminate the object. A video camera is used to view the object through a shearing device. The shearing device is often a proprietary design, but a conventional Michelson Interferometer, as indicated in Figure 2 alongside can be used to illustrate the process. The laser light reflected off the object is split into two by the beamsplitter and directed onto mirrors M1 and M2. By tilting mirror M1 either horizontally or vertically, the reflected images can be misaligned, Computer + Digitiser Video Monitor Partial Mirror Object Beam Expander Figure 2. Typical shearography set-up

4 or sheared with respect to each other when recombined at the beamsplitter surface before being focused onto the CCD. The recombined lightwaves interfere with each other and produce a speckle pattern which can be captured and digitized using a PC. When an object is stressed, either mechanically or thermally, the object surface deflects. This causes the laser beam path length used to illuminate the object to change. The associated change in phase of the laser light also causes the speckle interference pattern to change. By capturing the speckle interference pattern of the unstressed object and comparing it with the speckle interference pattern of the stressed object, it is possible to locate regions of correlation and decorrelation between the two images. This produces a familiar zebra-like fringe pattern. For Shearography this can be represented mathematically by equation 2 below 8. 4π d φ = S λ x (2) where: φ = correlation phase, d/ x = rate of displacement, S = magnitude of shear, λ = wavelength of the laser light, Equation 2 above 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 Equation 3, d x nλ = 2S (3) where: n = no of fringes. This implies that for a given object surface area, an increase in displacement gradient will produce a corresponding increase in number of fringes. 3. RESULTS In an attempt to quantify the position of a defect to the number of fringes produced using the Digital Shearography inspection technique, a section of a helicopter rotor blade was used and 9 induced defects of varying depth were generated on one side of the blade surface. The blade section was then exposed to three tests, each with a different heating period, and the fringe patterns recorded on the front defect free side of the blade. The second object inspected was a section of an Unmanned Aerospace Vehicle (UAV) wing section, in which a barely visible impact damage (BVID) area was created on the inside skin of the wing section. The test piece was then exposed to both Digital Shearography as well as IRT inspections, in order to establish which of the two techniques was more sensitive to detecting the defect. One of the artificial defects in the Helicopter rotor blade sample was also subjected to combined IRT and Digital Shearography inspection. The two samples can be seen in figure 2 below.

5 Figure 2. The Helicopter rotor blade section with 9 defects The UAV wing section with non visible impact damage All thermography inspections were conducted using an off the shelf IRISYS system, which is based on the IRISYS proprietary 256 pixel infrared array detector. Figure 3 below shows the typical laboratory layout, with the helicopter blade section in the foreground and the computer in the background behind it. The shearing device and camera are positioned next to the standalone IRISYS IRT system, and the laser can be seen behind the two inspection units. A mirror and beam expander for laser steering purposes completes the system. Figure 3. The laboratory inspection setup. The helicopter rotor blade was inspected using an infrared lamp, placed sufficiently far away to evenly heat the whole of the blade section. Three different heating durations were chosen, 1s, 2s and 3s. Each heating inspection was repeated 5 times in order to obtain a reasonable average of the fringe orders obtained for each defect. Figure 4 below is a typical image of the fringe patterns obtained from the 2 second inspection sequence. The defect annotation indicates how far from the inspection surface side of the blade the defect was made. The characteristic double bulls eye fringe pattern indicating the presence and location of the defect can be clearly seen. It is also evident that as the defects become shallower across the differing depth

6 defects, the number of fringes recorded reduces accordingly. This is further highlighted in the graphical representation of the results in figure 5 below. (a) 2 mm depth (b) 13 mm depth (c) 16 mm depth (d) 30 mm depth (e) 43 mm depth (f) 57 mm depth Figure 4. Shearography results from 2s heating of the helicopter rotor blade (g) 64 mm depth

7 Digital shearography inspection graphs:helicopter blade(far side heating) Stress for 1 sec. Stress for 3 sec. Expon. (Stress for 2 sec.) Stress for 2 sec. Expon. (Stress for 3 sec.) Expon. (Stress for 1 sec.) Number of fringes Depth of defect(mm) Figure 5. Graph of Shearographic fringe density results vs. depth of defect. Figure 6. IRT result after heating Equivalent Shearography result

8 The results of the UAV inspection are listed in figures 6 and 7 below. Both IRT and Digital Shearography were applied simultaneously whilst the wing section was heated from behind the composite skin section for 5 seconds. In figure 6, the thermographic image clearly reveals the presence of a heated section within the UAV wing structure. This is also visible within the Shearography results as a double bulls eye. However, between the two major bulls eyes, there also is a small double bulls eye which denotes the presence of the BVID defect. Care had to be taken when evaluating this result to not confuse the major double bulls eye with a defect as it only denotes an overall object displacement gradient in response to the overall heating of the UAV section. This detail cannot be extracted from the information provided by the IRT system. Figure 7. IRT result of the cooled UAV Shearography result of cooled UAV Figure 7 denotes the results obtained after the UAV was allowed to cool down significantly. In this result, the thermal map was non-descriptive with no identification of a defect present. The Shearography result however clearly still reveals the presence of the defect in the UAV. The loss in fringe density, also indicated that the thermal energy transferred into the UAV had all but dissipated. Figure 8. IRT of helicopter blade defect Shearography result of same defect

9 The next set of results are those obtained from the combined IRT and Digital Shearography inspection of one of the defects in the helicopter rotor blade, as depicted in figures 8 and 9. In figure 8 above which was taken immediately after heating was applied from behind the blade, the defect is clearly visible in both the IRT result as well as the Shearography result. In figure 9 which was taken after thermal dissipation had taken place, the defect under investigation as well as surrounding defects is still clearly visible. The IRT result however does not reveal the presence of the defects detected by Shearography. Figure 9. IRT of helicopter blade defect after cooling Shearography result of same defect 4. CONCLUSION From the above results it is evident that both IRT and Digital Shearography can be used to inspect objects for defects. The results further indicate that Digital Shearography has the ability to indicate the severity of a defect in terms of the fringe density in response to the applied test. This is evidenced in the graph in figure 5, where there is a clear decrease in the number of fringes recorded as the depth of the defect decreases, even at different stressing magnitudes. This phenomena can be exploited in reverse and be used to interpret the defect severity in relation to the fringe density and thus quantify the defect present. There is however no clear protocol for this approach, as each type of defect will present a different set of fringe patterns which in turn would require a new set of benchmark results in order to quantify any results obtained. The comparative study between IRT and Digital Shearography clearly reveals that the results obtained from Digital Shearography are superior to those obtained from the IRT system. The sensitivity of Digital Shearography far exceeds that of IRT especially in the low thermal stressing range as evidenced in the results in figures 7 and 9, where the IRT system was not able to detect any significant temperature gradient, even though a displacement gradient due to thermal diffusivity variations was present and recorded with Digital Shearography. At higher thermal gradient inspection levels however, IRT appears to be more promising, as seen in the detecting of the defect in figure 8. One must however remember that the IRISYS system is an entry level camera system with a resolution of 0.5K. A higher specified system with a better thermal resolution would very likely produce better results.

10 5. REFERENCES [1] Khan, A. U. Md., 1999, Non-destructive Testing Applications in Commercial Aircraft Maintenance, Proc. 7th European Conference on Non-destructive Testing, Copenhagen, June, Vol 4(6). [2] Meola, C. and Carlomagno, G. M., 2004, Recent Advances in the use of Infrared Thermography, Measurement Science and Technology. 15, pp [3] Cecchini, A., 2005, Damage Detection and Identification in Sandwich Composites using Neural Networks, MSc. Thesis, University of Puerto Rico. [4] Wong, B.S., Tu, C.G., Bai,W., Tan, P.H., Low, B.S., and Tan,K.S.,(n.d) Thermographic Evaluation of Defects in Composite Materials, [5] Gryzagoridis, J., and. Findeis, D., 2005, Simultaneous Shearographic and Thermographic NDT of Aerospace Materials, [6] Thomas, R.L., and Favro L.D, 1998, Thermal Wave Imaging of Fluid Intrusion in Composites, [7] D Findeis, J Gryzagoridis, M Matlali, 2005, Phase Stepping Shearography and Electronic Speckle Pattern Interferometry, Proceedings 3 rd US-Japan Symposium on Advancing Capabilities and Applications in NDE, Maui, June. [8] Venkatraman, B, Jayakumar T., Kalyanasundaram, P., and Raj, B., 2000, NDE Methodologies for Examination of Tail Rotor Blades of Helicopters, Proc.15 th World Conference on Non-Destructive Testing, Roma October.