K. Elliott Cramer

Thermographic imaging of cracks in thin metal sheets

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.

Status of Thermal NDT of Space Shuttle Materials at NASA

Since the Space Shuttle Columbia accident, NASA has focused on improving advanced NDE techniques for the Reinforced Carbon-Carbon (RCC) panels that comprise the orbiters 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.

Line scan versus flash thermography: comparative study on reinforced carbon-carbon

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.

Fixed eigenvector analysis of thermographic NDE data

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.

The Application of Thermal Diffusivity Imaging to Sic-Fiber-Reinforced Silicon Nitride

Strong, tough, high temperature ceramic matrix composites are currently being developed for application in advanced heat engines. These new materials require new nondestructive inspection and material characterization techniques to insure the final integrity, as well as reduce the time require for development. One of the most promising of these new materials is SiC fiber-reinforced silicon nitride ceramic matrix composite (SiCf/ Si3N4). The high temperature thermal and mechanical performance of ceramic matrix composites is strongly dependent on the thermal diffusivity and interfacial bond strength of the material. Previous work has shown a interaction between the thermal diffusivity and the fracture toughness of SiCf/Si3N4. A thermal imaging technique has been developed to provide rapid large area measurements of the thermal diffusivity perpendicular to the fiber direction in these composites. Results are presented for a series of SiCf/Si3N4 (reaction bonded silicon nitride) composite samples heat-treated under various conditions. These results are compared with previous experimental and analytical work in this field.

The thermal photocopier: a new concept for thermal NDT

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.

Application of the thermal line scanner to quantify material loss due to corrosion

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.

Characterization of Effluents Given off by Wiring Insulation

When an insulated wire is heated, the insulation emits a variety of effluents. This paper discusses the basis of emissions of effluents from wiring insulation. Several species are emitted at relatively low temperatures, while others are emitted when the wire reaches higher temperatures. We isolate the emissions by relative molecular weight of the effluents and measure the effluent concentration both as a function of time (temperature held constant) and by wire temperature. We find that the Law of Mass Action describes and predicts the time‐dependence of the emission of a specific effluent caused by the heating. The binding energy is determined by performing an Arrhenius Plot on the temperature data. These dependencies are discussed and working equations are derived. Data collected from 20 gauge wire (MIL‐W‐22759/11‐20) is used to illustrate and confirm the validity of the theory.

Thermographic Detection of Corrosion in Aircraft Skin

The aging of aircraft and the integrity of lapjoints and doublers is a growing concern. With the increasing average age of the commercial aircraft fleet, there exists an increased need for the development of new NDE techniques for the detection of critical flaws in aircraft airframes. The current techniques being either too time consuming or unreliable are a primary reason for a requirement of major mandatory modification to the existing fleet. Improved NDE techniques offer the possibility for increased safety and reliability at reduced costs.

Processing infrared images of aircraft lapjoints

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.