The use of composite materials continues to increase in the aerospace community due to the potential benefits of reduced weight, increased strength, and manufacturability. The ability to characterize damage in carbon fiber reinforced polymer composite components is required to enable damage progression models capable of yielding accurate remaining life predictions. As these composite structures become larger and more complex, nondestructive evaluation (NDE) techniques capable of quantifying and fully characterizing the material state are needed. This paper will present an overview of current NDE research activities for quantitative characterization of aerospace composites as well as a discussion of future directions in NDE research.
Automated Fiber Placement (AFP) systems have been developed to help take advantage of the tailorability of composite structures in aerospace applications. AFP systems allow the repeatable placement of uncured, spool fed, preimpregnated carbon fiber tape (tows) onto substrates in desired thicknesses and orientations. This automated process can incur defects, such as overlapping tow lines, which can severely undermine the structural integrity of the part. Current defect detection and abatement methods are very labor intensive, and still mostly rely on human manual inspection. Proposed is a thermographic in situ inspection technique which monitors tow placement with an on board thermal camera using the preheated substrate as a through transmission heat source. An investigation of the concept is conducted, and preliminary laboratory results are presented. Also included will be a brief overview of other emerging technologies that tackle the same issue.
The application of the quadrupole method for simulating thermal responses of delaminations in carbon fiber reinforced epoxy composites materials is presented. The method solves for the flux at the interface containing the delamination. From the interface flux, the temperature at the surface is calculated. While the results presented are for single sided measurements, with ash heating, expansion of the technique to arbitrary temporal flux heating or through transmission measurements is simple. The quadrupole method is shown to have two distinct advantages relative to finite element or finite difference techniques. First, it is straight forward to incorporate arbitrary shaped delaminations into the simulation. Second, the quadrupole method enables calculation of the thermal response at only the times of interest. This, combined with a significant reduction in the number of degrees of freedom for the same simulation quality, results in a reduction of the computation time by at least an order of magnitude. Therefore, it is a more viable technique for model based inversion of thermographic data. Results for simulations of delaminations in composites are presented and compared to measurements and finite element method results.
KEYWORDS: Principal component analysis, Composites, Data modeling, Inspection, Thermal modeling, Thermography, Resistance, Data acquisition, Nondestructive evaluation, Data processing
Principal Component Analysis (PCA) has been shown effective for reducing thermographic NDE data. While a reliable technique for enhancing the visibility of defects in thermal data, PCA can be computationally intense and time consuming when applied to the large data sets typical in thermography. Additionally, PCA can experience problems when very large defects are present (defects that dominate the field-of-view), since the calculation of the eigenvectors is now governed by the presence of the defect, not the "good" material. To increase the processing speed and to minimize the negative effects of large defects, an alternative method of PCA is being pursued where a fixed set of eigenvectors, generated from an analytic model of the thermal response of the material under examination, is used to process the thermal data from composite materials. This method has been applied for characterization of flaws.
Principal Component Analysis (PCA) has been shown effective for reducing thermographic NDE data. This paper
will discuss an alternative method of analysis that has been developed where a predetermined set of eigenvectors is
used to process the thermal data from both reinforced carbon-carbon (RCC) and graphite-epoxy honeycomb
materials. These eigenvectors can be generated either from an analytic model of the thermal response of the
material system under examination, or from a large set of experimental data. This paper provides the details of the
analytic model, an overview of the PCA process, as well as a quantitative signal-to-noise comparison of the results
of performing both conventional PCA and fixed eigenvector analysis on thermographic data from two specimens,
one Reinforced Carbon-Carbon with flat bottom holes and the second a sandwich construction with graphite-epoxy
face sheets and aluminum honeycomb core.
Since the Space Shuttle Columbia accident, NASA has focused on improving advanced NDE techniques for the Reinforced Carbon-Carbon (RCC) panels that comprise the orbiter's wing leading edge and nose cap. Various nondestructive inspection techniques have been used in the examination of the RCC, but thermography has emerged as an effective inspection alternative to more traditional methods. Thermography is a non-contact inspection method as compared to ultrasonic techniques which typically require the use of a coupling medium between the transducer and material. Like radiographic techniques, thermography can inspect large areas, but has the advantage of minimal safety concerns and the ability for single-sided measurements. Details of the analysis technique that has been developed to allow in situ inspection of a majority of shuttle RCC components is discussed. Additionally, validation testing, performed to quantify the performance of the system, will be discussed. Finally, the results of applying this technology to the Space Shuttle Discovery after its return from the STS-114 mission in July 2005 are discussed.
Thermographic nondestructive inspection techniques have been shown to provide quantitative, large area damage detection capabilities for the ground inspection of the reinforced carbon-carbon (RCC) used for the wing leading edge of the Shuttle orbiter. The method is non-contacting and able to inspect large areas in a relatively short inspection time. Thermal nondestructive evaluation (NDE) inspections have been shown to be applicable for several applications to the Shuttle in preparation for return to flight, including for inspection of RCC panels during impact testing, and for between-flight orbiter inspections. The focus of this work is to expand the capabilities of the thermal NDE methodology to enable inspection by an astronaut during orbital conditions. The significant limitations of available resources, such as weight and power, and the impact of these limitations on the inspection technique are discussed, as well as the resultant impact on data analysis and processing algorithms. Of particular interest is the impact to the inspection technique resulting from the use of solar energy as a heat source, the effect on the measurements due to working in the vacuum of space, and the effect of changes in boundary conditions, such as radiation losses seen by the material, on the response of the RCC. The resultant effects on detectability limits are discussed.
KEYWORDS: Signal to noise ratio, Scanners, Thermography, Inspection, Principal component analysis, Lamps, Quartz, Infrared cameras, Cameras, Imaging systems
Thermographic inspection techniques fundamentally vary by method of heat deposition. Some systems use a short burst of energy from a flash lamp while others control the motion of a quartz lamp over the material. Both techniques have had a history of successful inspections on aircraft and boiler tubes, for example. Historically, the system used for inspections was determined by the thermographic equipment available to the researcher. This paper will compare the flash and line scan thermographic systems on Reinforced Carbon-Carbon. Reinforced Carbon-Carbon (RCC) is a brittle composite material that is found on the Space Shuttle’s nose section, wing leading edges, and chin panel. It is used to protect the orbiter’s aluminum frame from superheated air during flight. In the time since the Columbia accident, impact tests on RCC panels have been ongoing. Flash thermography has been successfully used to scan the impact site for delaminations. While the system has proven effective, it is not without limitations. A single scan yields only that section of material that is in the field of view of the infrared camera. Additionally, delaminations deep within the material may not be resolved as well as with quartz heating. A comparative study was conducted using a RCC panel with flat-bottom holes varying in diameter and depth. The panel was scanned with the Thermal Line Scanner, the Thermal Photocopier, and the Echotherm from Thermal Wave Imaging. Signal to noise ratios were calculated for the defects and used to compare the three systems. This paper will discuss the details of the study and show the results obtained from each of the three systems.
The thermal line scanner has proven to be a successful method of rapidly scanning large areas of aircraft fuselage for delaminations and metal pipes for corrosion. The limitation of this technique is with the finite depth by which flaws can be located due to the fixed distance that the thermal camera follows the moving line source. To identify deeper flaws within a material, the thermal imager and line source must have a greater separation distance so that the heat has more time to propagate through the material. Ultimately, one would want to identify flaws at any depth requiring continual scans with greater separation between the line source and imager. The Thermal Photocopier is a hybrid of the thermal line scanner. It utilizes a moving line source and a stationary infrared camera. Any one image captured by the computer shows the sample in gradient cooling due to the moving heat source. An algorithm has been developed that reconstructs full-field images of the material at specific cool down times. These frames represent various depths into the sample as the heat propagates through the thickness of the material. Therefore, an object can be analyzed from the front to the back surface for flaws using this modified thermal detection system. This system has been tested on aluminum and composite materials of varying thickness yielding results consistent with thermographic images obtained with flash and quartz lamps.
Peltier cooling devices are used on the Hubble Space telescope for temperature control of various detector packages. A typical construction of these devices involves sandwiching an array of Bismuth-Tellurium (Bi2Te3) posts between two ceramic plates. When a DC current is applied to the device heat is moved from one side of the device to the other, depending on the polarity of the current. Because these devices can change temperature very rapidly, there is the potential for damage due to thermal expansion and contraction of the constituents. A failure in the bonding of the Bi2Te3 to the ceramic sheet can lead to reduced efficiency or failure of the device. NASA Langley Research Center has developed a nondestructive thermal imaging technique to determine the integrity of the Bi2Te3 posts through the ceramic surface of the peltier device. By driving the peltier device with a time varying DC current, a corresponding temperature rise and fall can be observed on the surface of the device using a commercial infrared camera. Lock-in thermography can then be used to construct both phase and amplitude images of the front surface temperature. It has been found that failure of Bi2Te3 posts results in a measurable change in both the amplitude and phase. This paper will describe an inspection method that has been developed and show results of the inspection of the extremely small Bi2Te3 posts whose dimensions are 0.81mm by 0.81mm and approximately 1.45mm tall.
Wall thinning due to corrosion in utility boiler waterwall tubing is a significant operational concern for boiler operators. Historically, conventional ultrasonics has been used for inspection of these tubes. Unfortunately, ultrasonic inspection is very manpower intense and slow. Therefore, thickness measurements are typically taken over a relatively small percentage of the total boiler wall and statistical analysis is used to determine the overall condition of the boiler tubing. Other inspection techniques, such as electromagnetic acoustic transducer (EMAT), have recently been evaluated, however they provide only a qualitative evaluation - identifying areas or spots where corrosion has significantly reduced the wall thickness. NASA Langley Research Center, in cooperation with ThermTech Services, has developed a thermal NDE technique designed to quantitatively measure the wall thickness and thus determine the amount of material thinning present in steel boiler tubing. The technique involves the movement of a thermal line source across the outer surface of the tubing followed by an infrared imager at a fixed distance behind the line source. Quantitative images of the material loss due to corrosion are reconstructed from measurements of the induced surface temperature variations. This paper will present a discussion of the development of the thermal imaging system as well as the techniques used to reconstruct images of flaws. The application of the thermal line source coupled with the analysis technique represents a significant improvement in the inspection speed and accuracy for large structures such as boiler waterwalls.
The thermal line scan technique has been shown to be an effective technique for rapid inspection of aerospace specimens. Past efforts have focused on thermal measurements far behind the line source where the heat flow normal to the surface is negligible. This paper focuses on measurements closer to the line source to enable the measurement of the thermal diffusivity in the surface normal direction. This measurement also enables an independent characterization of the thermal diffusivity in the direction of motion of the thermal line source. An analytical solution is given for a line source moving with constant velocity across an anisotropic plane. A nonlinear least squares fitting routine is used to reduce the temporal response of a specimen to images of the thermal diffusivity in both the directions normal to the surface and parallel to the motion of the line source. Measurements are presented on specimens with known variations in effective diffusivity. Measurements on these specimens allow a comparison of this technique to more conventional techniques for diffusivity measurement.
Localized wall thinning due to corrosion in utility boiler water-wall tubing is a significant inspection concern for boiler operators. Historically, conventional ultrasonics has been used for inspection of these tubes. This technique has proven to be very manpower and time intensive. This has resulted in a 'spot check' approach to inspections, documenting thickness measurements over a relatively small percentage of the total boiler wall area. NASA Langley Research Center has developed a thermal NDE technique designed to image and quantitatively characterize the amount of material thinning present in steel tubing. The technique involves the movement of a thermal line source across the outer surface of the tubing followed by an infrared imager at a fixed distance behind the line source. Quantitative images of the material loss due to corrosion are reconstructed from measurements of the induced surface temperature variations. This paper will present a discussion of the development of the thermal imaging system as well as the techniques used to reconstruct images of flaws. The application of the thermal line source coupled with the analysis technique represents a significant improvement in the inspection speed for large structures such as boiler water-walls. A theoretical basis for the technique will be presented which explains the quantitative nature of the technique. Further, a dynamic calibration system will be presented for the technique that allows the extraction of thickness information from the temperature data. Additionally, the results of applying this technology to actual water-wall tubing samples and in situ inspections will be presented.
A scanned thermal line source is a rapid and efficient technique for detection of corrosion in aircraft components. Reconstruction of the back surface profile from the data obtained with this technique requires a nonlinear mapping. Neural networks are an effective method for performing nonlinear mappings of one parameter space to another. This paper discusses the application of neural networks to the reconstruction of back surface profiles from the data obtained from a thermal line scan. The neural network is found to be a very effective method of reconstructing arbitrary surface profiles. The network is trained on simulations of the thermal line scan technique. The trained network is then applied to both simulated and experimentally obtained data. The reconstructed profiles are in good agreement with independent characterizations of the profiles. Limitations of the reconstruction technique are illustrated by presenting results for several different configurations.
Recent advances in thermal imaging technology have spawned a number of new thermal NDE techniques that provide quantitative information about flaws in aircraft structures. Thermography has a number of advantages as an inspection technique for aircraft. It is a totally noncontacting, nondestructive, imaging technology capable of inspecting a large area in a matter of a few seconds. The development of fast, inexpensive image processors has aided in the attractiveness of thermography as an NDE technique. These image processors have increase the signal to noise ratio of thermography and facilitated significant advances in post- processing. The resulting digital images enable archival records for comparison with later inspections, thus providing a means of monitoring the evolution of damage in a particular structure.
Wall thinning in utility boiler waterwall tubing is a significant inspection concern for boiler operators. Historically, conventional ultrasonics has been used for inspection of these tubes. This technique has proved to be very labor intensive and slow. This has resulted in a `spot check' approach to inspections, making thickness measurements over a relatively small percentage of the total boiler wall area. NASA Langley Research Center has developed a thermal NDE technique designed to image and quantitatively characterize the amount of material thinning present in steel tubing. The technique involves the movement of a thermal line source across the outer surface of the tubing followed by an infrared imager at a fixed distance behind the line source. Quantitative images of the material loss due to corrosion are reconstructed from measurements of the induced surface temperature variations. This paper will present a discussion of the development of the thermal imaging system as well as the techniques used to reconstruct images of flaws. The application of the thermal line source, coupled with this analysis technique, represents a significant improvement in the inspection speed for large structures such as boiler waterwalls while still providing high-resolution thickness measurements. A theoretical basis for the technique will be presented thus demonstrating the quantitative nature of the technique. Further, results of laboratory experiments on flat panel specimens with fabricated material loss regions will be presented to demonstrate the capabilities of the technique. Additionally, the results of applying this technology to actual waterwall tubing samples will be presented.
Recent advances in thermal imaging technology have spawned a number of new thermal nondestructive evaluation (NDE) techniques that provide quantitative information about flaws in aircraft structures. Thermography has a number of advantages as an inspection technique for aircraft. It is a totally noncontacting, nondestructive, imaging technology capable of inspecting a large area in a mater of a few seconds. The development of fast, inexpensive image processors have aided in the attractiveness of thermography as an NDE technique. These image processors have increased the signal to noise ratio of thermography and facilitated significant advances in post-processing. The resulting digital images enable archival records for comparison with later inspections thus providing a means of monitoring the evolution of damage in a particular structure. NASA Langley Research Center has developed a thermal NDE technique designed to image and quantitatively characterize the thickness of thin aluminum sheets. The technique involves the movement of a thermal line source across the outer surface of a sample followed by an IR imager at a fixed distance behind the line source. Images of the material loss due to corrosion are reconstructed from measurements of the induced surface temperature variations. This paper will present a discussion of the development of the thermal imaging system as well as the techniques used to reconstruct images of flaws. The application of the thermal line source coupled with the analysis technique represents a significant improvement in the quantification of flaws over conventional thermal imaging. Results of laboratory experiments on specimens with fabricated material loss region swill be presented to demonstrate the capabilities of the technique. An integral part of the development of this technology is the use of analytic and computational modeling to optimize the technique and reduce the data. The experimental results will be compared with simulations to demonstrate the utility of such an approach.
The thermographic inspection of materials and structures typically involves the application of a heat flux to the surface and measuring the subsequent surface temperature profiles. The nature of typical heat flux sources requires the incident flux has the shape of either a short pulse or a step function. This pulse shape for the flux typically will not maximize the contrast between a response from a flaw in the structure and the unflawed regions of the structure. Optimal shaping of the pulse is experimentally difficult, if not impossible. However, its consideration serves as a useful tool for developing post-processing techniques for the data. The concept is to design filters for processing the data in a manner that emulates shaping the input flux. Convolving the measured thermal response with this optimized filter effectively maps the measured response to the response for an optimally shaped input heat flux. A method for generating this filter is presented. Applying the filter to the thermal response of the structure increases the contrast between flawed and unflawed regions. Results with experimental data illustrate the advantages of the technique over conventional techniques.
Recent advances in thermal imaging technology have spawned a number of new thermal NDE techniques that provide quantitative information about flaws in aircraft structures. Thermography has a number of advantages as an inspection technique. It is a totally noncontacting, nondestructive, imaging technology capable of inspecting a large area in a matter of a few seconds. The development of fast, inexpensive image processors have aided in the attractiveness of thermography as an NDE technique. These image processors have increased the signal to noise ratio of thermography and facilitated significant advances in post- processing. The resulting digital images enable archival records for comparison with later inspections thus providing a means of monitoring the evolution of damage in a particular structure. The National Aeronautics and Space Administrations's Langley Research Center has developed a thermal NDE technique designed to image a number of potential flaws in aircraft structures. The technique involves injecting a small, spatially controlled heat flux into the outer surface of an aircraft. Images of fatigue cracking, bond integrity and material loss due to corrosion are generated from measurements of the induced surface temperature variations. This paper presents a discussion of the development of the thermal imaging system as well as the techniques used to analyze the resulting thermal images. Spatial tailoring of the heat coupled with the analysis techniques represent a significant improvement in the detectability of flaws over conventional thermal imaging. Results of laboratory experiments on fabricated crack, disbond and material loss samples are presented to demonstrate the capabilities of the technique. An integral part of the development of this technology is the use of analytic and computational modeling. The experimental results are compared with these models to demonstrate the utility of such an approach.
Aircraft structural integrity is a major concern for airlines and airframe manufacturers. To remain economically competitive, airlines are looking at ways to retire older aircraft, not when some fixed number of flight hours or cycles has been reached, but when true structural need dictates. This philosophy is known as `retirement for cause.' The need to extend the life of commercial aircraft has increased the desire to develop nondestructive evaluation (NDE) techniques capable of detecting critical flaws such as disbonding and corrosion. These subsurface flaws are of major concern in bonded lap joints. Disbonding in such a joint can provide an avenue for moisture to enter the structure leading to corrosion. Significant material loss due to corrosion can substantially reduce the structural strength, load bearing capacity and ultimately reduce the life of the structure. The National Aeronautics and Space Administration's Langley Research Center has developed a thermal NDE system designed for application to disbonding and corrosion detection in aircraft skins. By injecting a small amount of heat into the front surface of an aircraft skin, and recording the time history of the resulting surface temperature variations using an infrared camera, quantitative images of both bond integrity and material loss due to corrosion can be produced. This paper presents a discussion of the development of the thermal imaging system as well as the techniques used to analyze the resulting thermal images. The analysis techniques presented represent a significant improvement in the information available over conventional thermal imaging due to the inclusion of data from both the heating and cooling portion of the thermal cycle. Results of laboratory experiments on fabricated disbond and material loss samples are presented to determine the limitations of the system. Additionally, the results of actual aircraft inspections are shown, which help to establish the field applicability for this technique. A recent application of this technology to aircraft repairs using boron/epoxy patches is shown illustrating the flexibility of the technology.
The aging of the commercial transport fleet increases the possibility of a reduction or loss of structural integrity through corrosion. Thermal imaging is a nondestructive evaluation (NDE) technique that is non-contacting and can rapidly inspect large areas. In this work, thermal NDE is used for characterization of corrosion in aircraft skin. Thermal images from an infrared camera are low in contrast and raw images give only qualitative results. The technique presented will use the time evolution of the thermal images to produce qualitative and quantitative information of the corrosion sample being imaged. This paper is going to show the results from fabricated material loss samples, electro chemical corroded samples and aircraft panels with corrosion. A quantitative comparison of results for the different samples will be shown.
A thermal technique is presented for imaging subsurface damage and computing the depth of damaged areas for low diffusivity materials. The measurement technique presented uses uniform heating with quartz lamps over a large area. The surface temperature of the sample is collected using a scanning IR radiometer and a real time image processor during the cooling of the sample after heating. Flaw depths are computed by performing a numeric approximation to the surface Laplacian on each temperature image in the time series. The depth of the damage is then calculated from the time required for the amplitude of the surface Laplacian to reach a minimum in the region over the damage. Experimental results from the application of the technique to low diffusivity materials with surface and subsurface defects at various depths are presented showing the technique's ability to give quantitative depth of damage information. Additionally, the effects of variations in defect size on the time for flux minimum, and thus on the calculated depth, is also investigated. Finally, finite element simulations are compared with experimental results.
Techniques for processing IR images of aging aircraft lapjoint data are discussed. Attention is given to a technique for detecting disbonds in aircraft lapjoints which clearly delineates the disbonded region from the bonded regions. The technique is weak on unpainted aircraft skin surfaces, but can be overridden by using a self-adhering contact sheet. Neural network analysis on raw temperature data has been shown to be an effective tool for visualization of images. Numerical simulation results show the above processing technique to be an effective tool in delineating the disbonds.
The presence of cracks significantly decreases the structural integrity of thin metal sheets used in aerospace applications. Thermographic detection of surface temperature variations due to these cracks is possible after external heating. An approximate line source of heat is used to produce an inplane flow of heat in the sheet. A crack in the sheet perturbs the inplane flow of heat and can be seen in an image of the surface temperature of the sheet. An effective technique for locating these perturbations is presented which reduces the surface temperature image to an image of variations in the inplane heat flow. This technique is shown to greatly increase the detectability of the cracks. This thermographic method has advantages over other techniques in that it is able to remotely inspect a large area in a short period of time. The effectiveness of this technique depends on the shape, position and orientation of the heat source with respect to the cracks as well as the extent to which the crack perturbs the surface heat flow. The relationship between these parameters and the variation in the heat flow is determined both by experimental and computational techniques. Experimental data is presented for through-the-thickness, subsurface and surface EDM notches. Data for through-the-thickness fatigue cracks are also presented.
Post-processing of infrared thermal image thta is a technique which finds many uses in a laboratory devoted to
non-destructhe evaluation (NDE) of materials. Among these are determination ofmaterial pmperty values and
detection/location of delaminations. Exanples are shown in which thermal diffusivity is measured for technique verification,
as a verification of the tensor nature of diffusivity measurements and as a proxy for porosity in a test sample of a material
under developmenL Another example is given in which the coefficient of thennal expansion is determined through the
phenomenon of thermoelasticity. A final example is given in which post-processing extrts the thermal signature of a
delamination from an image dominated by an unwanted feature. Following these examples of materials evaluation using
post-processing, a set of procedures common to the data analysis in the examples is extracted. Generic requirements are
given so that each procedure can operate consistently within the entire process to produce appropriate values of the material
characteristics sought.
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