International Workshop SMART MATERIALS, STRUCTURES & NDT in AEROSPACE Conference NDT in Canada 2011 2-4 November 2011, Montreal, Quebec, Canada Pulsed Thermography and Laser Shearography for Damage Growth Monitoring M. Genest 1, R. S. Rutledge 1, R. Kothari 2, K. Mourtazov 2, C. Turgeon 2 1 Institute for Aerospace Research, National Research Council Canada 1200 Montreal Rd, Building M-14, Ottawa, ON, Canada K1A 0R6 marc.genest@nrc-cnrc.gc.ca, robert.rutledge@nrc-cnrc.gc.ca 2 Bombardier Aerospace PO Box 6087, Station Centre-Ville Montreal, QC, Canada H3C 3G9 rushabh.kothari@aero.bombardier.com, khassan.mourtazov@aero.bombardier.com, cecil.turgeon@aero.bombardier.com ABSTRACT This paper compares the use of pulsed thermography and laser shearography for the inspection of a honeycomb sandwich composite structure during a Durability and Damage Tolerance Test (DADTT). Prior to the start of the DADTT, impact damages were introduced in the test article. The laser shearography and pulsed thermography were used to establish a baseline of the damage sizes. At several time during the DADTT, the shearography and thermography inspections were repeated to evaluate the areas for potential damage growth. The results showed that the shearography measurements predicted larger damage sizes than those obtained by pulsed thermography. This difference was due to the difference in the type of damage detected by each technique. Pulsed thermography mainly detects the delamination within the skin or disbond between the skin and the core, while laser shearography detects the presence of core damage, in addition to those detected by pulsed thermography. The variation in sizing between the inspection cycles were less pronounced in pulsed thermography results; having an average standard deviation of approximately 2 mm (0.08 inches) compared to approximately 4.83 mm (0.19 inches) for shearography. Keywords: Pulsed Thermography, Laser Shearography, Impact Damage, Damage Growth
INTRODUCTION A composite fuselage full-scale test was conducted as part of a composite research and development demonstration project between Bell Helicopters Textron Canada Limited (BHTCL), Bombardier Aerospace (BA), Composites Atlantic Limited (CAL) and the National Research Council Canada (NRC). As part of the project, a composite fuselage (barrel) was manufactured. The full-scale barrel test article geometry was designed and built to a similar size as an existing business aircraft fuselage. The specimen had a 2.5 m (8.5 feet) constant section barrel diameter and was approximately 4.8 m (16 feet) long. The structure was fabricated as a sandwich composite material with highly stiff carbon fiber reinforced plastic (CFRP) facesheets separated by honeycomb core. A full-scale test plan was developed to study the durability and damage tolerant (DADT) response of the structure [1]. As part of the DADT testing (DADTT), the barrel was impacted before fatigue testing. The impacts were produced by taking into consideration the energy level of different manufacturing and in-service damage types a typical fuselage structure endures during its lifetime. These types of damage include tool drop, hail impact, tire debris on runway, etc. The impact energies imparted on the specimen varied from 5 J to 80 J, with impactor head size varying from 12.7 mm (0.5 inch) to 89 mm (3.5 inch) in diameter. A schematic of the typical impact zones with multiple impact sites on the fuselage are shown in Figure 1. Fig. 1: Typical impact zones on the fuselage [1]. After the impact tests, non-destructive evaluation (NDE) of the sites was carried out using pulsed thermography, laser shearography and ultrasonic waves to evaluate damage areas and types. The composite fuselage/barrel test program was designed to conduct fatigue, as well as static testing on the same test article. The testing occurred in phases where each phase consists of fatigue cycling, limit load and ultimate load tests. NDE was carried out at different stages of testing to detect damage growth in the sandwich skin, as well as the core. This paper presents the pulsed thermography and vacuum laser shearography inspection results obtained during the first lifetime of DADTT cycling.
NON-DESTRUCTIVE EVALUATION BACKGROUND AND SETUP Since there were a high number of impact sites (38 in the sandwich area alone) and the test plan required inspections repeated 6 times during the test phase, fast and wide area techniques such as pulsed thermography (PT) and laser shearography (LS) were used to monitor potential damage growth during the DADTT. In addition to these, for reference purposes, a few damages were inspected by manual pulse-echo ultrasound. A typical inspection cycle during the DADTT is presented in Figure 2. The damage sites were monitored at the specified cycle (gray boxes). The damage growth in the sandwich skin and sandwich core was monitored during both the fatigue and static testing phase of the barrel. Depending on the location of the sites inspected a tripod or crane and slings were used to position and secure the PT or LS inspection hood during the image acquisition. Both pulsed thermography and laser shearography were carried out with minimal disruption to the DADTT. The total time per inspection cycle was less than a week. Pre-test NDI DADT 1000 Cycles NDI of all damage sites DADT 5 000 Cycles NDI of all damage sites DADT 10 000 Cycles NDI of all damage sites DADT 20 000 Cycles NDI of all damage sites Static test (Limit and Ultimate) NDI of all damage sites Result s Analysis Pulsed Thermography Fig. 2: DADT static and fatigue life of the fuselage barrel [1]. In pulsed thermography (PT), also known as flash thermography, energy is applied to the specimen using a brief duration excitation typically using high intensity flash lamps. The applied energy creates a thermal front that propagates from the specimen s surface through the specimen. During the cool-down process the surface temperature decreases uniformly for a samples without internal flaws. When the thermal front intersects an interface from a high to low conductivity layer, like in the case of delamination, disbond or porosity, the cooling rate is locally disrupted. This results in a small accumulation of heat above the flaw that is also manifested at the specimen s surface. This local increase in surface temperature is detected by an infrared (IR) camera allowing damage areas to be distinguished from pristine ones. In this work, the pulsed thermography inspections were carried out using a 240x320 pixels IR camera that has a thermal sensitivity of 0.020 C at 30 C and a spectral response in the long wave infrared, from 8.0 to 8.8µm. Two xenon flash lamps, each powered by a 2400J power supply,
were used as energy sources. Each thermogram contained 740 frames that were acquired at 30Hz. A photograph of a thermography setup used is shown in Figure 3. Pulsed thermography inspection hood Pulsed thermography computer and control system Composite barrel Fig. 3: Photograph of a pulsed thermography experimental setup on the composite barrel. Laser Shearography Laser shearography is a non-destructive evaluation technique that is suitable for rapid inspection of large areas. It is based on speckle pattern shearing interferometry and measures the out-of-plane deformation gradient of a structure under load to detect anomalies [ 2]. Figure 4 presents a schematic of a shearography setup. In a typical shearography system, diffuse light from an expanded laser beam or laser diodes is used to illuminate the surface of a specimen or structure of interest. A charge-coupled device (CCD) camera is used to monitor the light speckle inherent to diffuse laser illumination. A reference beam (P(x,y)) and object beam (P(x+ x,y)) are laterally sheared using a shearing interferometer, such as a Michelson interferometer, to create a sheared speckle image of the object. Under load, minute global and local out-of-plane displacements can occur due to variations in the specimen s stiffness and thermal properties inherent to the structure inspected or caused by the presence of flaws. These variations result in a new speckle distribution that is recorded by the CCD camera. By subtracting the original speckle distribution image (no load) from the new speckle distribution image (under load), a fringe (phase) image that is related to the out of plane gradient is obtained. The fringes are usually processed and unwrapped to provide the final output image (unwrapped phase map) that is easier to interpret. Examples of different images obtained by shearography are presented in Figure 5. Herein, the laser shearography inspections were carried out using a portable commercial off the shelf system (hood-type) that is equipped with both thermal and vacuum loading capability. Although two excitation modes were available, due to the barrel design that is mainly sandwich construction, the vacuum loading provided better results for the inspection of the impact damage sites and was used to carry the inspection discussed below. A photograph of a shearography setup used is shown in Figure 6.
Fig. 4: Schematic of laser shearography setup. a) b) c) Fig. 5: Example of shearography image a) no damage area; b) fringe image of a disbond area c) unwrapped phase map of a disbond/delamination. Shearography computer and control system Shearography inspection hood Composite barrel Fig. 6: Photograph of a shearography experimental setup used to inspect the composite barrel.
RESULTS AND DISCUSSIONS The initial sizings of the damage detected by pulsed thermography were carried out manually. However, due the number of impact sites and the repeated number of inspections it was found to be a time consuming exercise. Therefore, an algorithm was developed to automatically provide the width and length of the damage [ 3]. In addition to allowing faster processing, it was noticed that more consistent measurements were obtained compared to operator dependent manual measurements. An example of typical processed images obtained is shown in Figure 7. As the damage sizing was repeated for each inspection cycle, some variations in damage size were noticed as shown in Figure 8. Those variations made the damage sites appear inconsistent. It is believed that those variations are not the results of the damage growth, but rather a repeatability limitation of the combined inspection technique and algorithm used for the testing conditions. The average standard deviation for the 38 impact locations over the sandwich structure was 1.9 mm (0.074 inches) for the length and width. Considering the field of view of the IR camera each pixel in the IR image is 1 mm (~0.04 inches). Thus, the standard deviation corresponds to only 2 pixels. This demonstrates that pulsed thermography measurements using the automated flaw detection can be used to reliably measure damage size. The largest standard variation for an impact site was 3.8 mm (0.15 inches) (the equivalent of ~4 pixels). In order to reduce the standard variation between the measurements the spatial resolution of the IR camera used must be increased. This was not possible with the equipment available, which had a 240x320 pixels array. Nevertheless, the current pulsed thermography system used in this work is believed to be adequate to clearly detect growth larger than 6.3 mm (0.25 inches). length width Fig. 7: Damage map (left) and image indication blob corresponding to the damage map used for damage sizing (right).
Fig. 8: Typical variations in pulsed thermography damage width measurements over a life of the DADTT. A typical shearography image of an impact damage area and the corresponding line profile are shown in Figure 9. In the line profile image (Figure 9b), the extremities of the damage are indicated by the small circles. For each damage sites the line profile corresponding to the largest length of the damage was plotted and the inflection points corresponding to the damage extremities were selected manually. As in the case of pulsed thermography, there were some fluctuations in the damage size obtained. Typically these were less than 6.3 mm (0.25 inches). The average standard deviation between measurements was 4.6 mm (0.18 inches); the equivalent of 30 pixels. Based upon these numbers, vacuum shearography can reliably discriminate damage growth larger than 0.36 inches (twice the average standard deviation). It was noticed that 3 impact sites had a standard deviation significantly above 6.3 mm (0.25 inches): 10.9 mm (0.43 inches), 14 mm (0.55 inches), and 18.8 mm (0.74 inches), respectively. The larger discrepancies in the measurements of these three sites could not be explained with certainty. It was also noticed that, due to the optics used in the shearography system, the images obtained were slightly distorted. Therefore, the relative position of the damage in the image between two inspection cycles can affect the measurements. For example, during the inspection of a 38.1 mm (1.5 inches) wide sample, the distorted image shown in Figure 10 resulted in width measurements differing by 5.1 mm (0.2 inches) between the center of the image and the left and right extremities. Thus the location of the damage in the shearography image might have played a role in the sizing variations. After further investigation of the results, distortion was also noticed in the IR images but to a lesser degree. Therefore, it is suggested that location of the damage in the image might also explain in part the variations noticed in the pulsed thermography results.
250 200 150 100 50 0 0 200 400 600 800 1000 1200 1400 a) b) Fig. 9: An example of shearography image and profile. Fig. 10: Typical variations in shearography damage sites width measurements over a life of the DADTT. Fig. 11: Image showing distortion in the shearography measurement. A comparison of the measurements obtained by both techniques is presented in Figure 12. In this graph, the shearography measurements are larger damage than those obtained by pulsed thermography. This was expected based on a previous study [ 4] which showed that vacuum shearography indications relate to both core and skin damage, while pulsed thermography
indications relate only to skin damage. This graph also shows that variations in the PT and LS measurements are not correlated. Suggesting that the variations are likely due to the result of the inspection setup and conditions as mentioned previously. Manual pulsed echo ultrasonic measurements were carried out over a few damage locations, and showed no damage growth. Fig. 12: Image showing distortion in the shearography measurement. CONCLUSIONS Sizing of impact damage in a composite barrel was carried out during DADTT by pulsed thermography and laser shearography using vacuum loading. Although there were some variations in the damage size detected during the repeated inspection, it is believed that those were not the results of damage growth. Among the different factors responsible for the variations in the measurements, it is believed that the presence of distortion in the image caused by the camera s optics played a significant role. Thus, it is important that, whenever possible, the damage be as centered as possible in the field of view. This reduces the impact of distortion in subsequent measurements, which could be as much as 5.1 mm (0.2 inches) for the shearography system. The variations in the laser shearography were larger than those obtained with the pulsed thermography. In addition to be less affected by the optical distortion, the pulsed thermography measurements were automated and thus made the damage sizing more consistent. Damage sizing from a shearography image can be subjective and could in part explain the higher variations in the shearography measurements. It is recommended to identify a commonly accepted criterion to be considered as the edge of the defect and ideally develop an automated algorithm to obtain consistent results. The damage sizes obtained by vacuum shearography were consistently larger than those obtained by pulsed thermography. This is due to the different types of damage measured by the two techniques for the sandwich structure used herein: core damage versus skin damage. Although no damage growth occurred, it is believed that both vacuum shearography and pulsed thermography are suitable techniques to perform damage growth monitoring during DADTT and other fatigue tests.
ACKNOWLEDGEMENT The authors would like to thank everyone at Bombardier Aerospace, Bell Helicopter Textron Canada Ltd., Composite Atlantic Ltd. and the National Research Council Canada for their support in the composite barrel test program. REFERENCES 1. Korthari, R., Full-Scale Static and Fatigue Testing of Composite Fuselage Section, Proceedings of 26th ICAF Symposium, Montréal, 1 3 June, 2011. 2. Steinchen, W. and Yang L., Digital Shearography: Theory and Application of Digital Speckle Pattern Shearing Interferometry, SPIE The International Society for Optical Engineering, Bellingham, Washington, (2003). 3. Genest, M., Pulsed Thermography Image Processing for Damage Growth Monitoring, Review of Progress in Quantitative Non Destructive Evaluation 2011, Burlington, VT, July 17-22, 2011 (to be published) 4. Genest, M., Brothers, M., LeBlanc, R., and Fahr, A., Nde of Impact Damage in Sandwich Structures Using Ultrasonic, Thermography and Laser Shearography, Proceedings of Society for the Advancement of Material and Process Engineering (SAMPE) Conference 2010, Seattle, WA, 17-20 May, 2010.