Tasdiq Ahmed

Reconstruction and enhancement of active thermographic image sequences

Active thermography has gained broad acceptance as a nondestructive evaluation method for numerous in-service and manufacturing applications in the aerospace industry. However, because of the diffusive nature of the process, it is subject to blurring and degradation of the signal as one attempts to image deeper subsurface features. Despite this constraint, active thermographic response is deterministic, to the extent that the postexcitation time evolution for a defect-free sample can be accurately predicted using a simple one-dimensional model. In the patented thermal signal reconstruction method, the time history of every pixel in the field of view is compared to such a model in the logarithmic domain, where deviations from ideal behavior are readily identifiable. The process separates temporal and spatial nonuniformity noise components in the image sequence and significantly reduces temporal noise. Time-derivative images derived from the reconstructed data allow detection of subsurface defects at earlier times in the sequence than conventional contrast images, significantly reducing undesirable blurring effects and facilitating detection of low-thermal-contrast features that may not be detectable in the original data sequence.

Parallel Thermal Wave Imaging Using a Vector Lock-In Video Technique

The appearance of the IR video camera has extended the wavelength range of the visible video camera to the thermal IR range (3–12 µm), thus providing a powerful tool to researchers in thermal wave imaging. However, imaging in the thermal IR range has its special handicaps, not shared by its visible counterpart. Most objects in conventional photography reflect rather than emit light of their own. As a result one often has the freedom to choose the intensity, direction and color of illumination to accentuate the aspects of the object to be photographed. In thermal IR imaging the situation is very different in that nearly all objects emit thermal radiation of their own, in addition to reflecting radiation of other objects. What is recorded in a thermograph is always a mixture of emitted and reflected radiation, some of which even comes from components of the camera itself, including lenses and their supporting structures. This problem is particularly severe in the 8–12 µm range, because it corresponds to the peak of blackbody radiation at room temperature. It is this same range of wavelength that is most relevent in non-destructive evaluation. In conventional scanned thermal wave imaging applications this problem is overcome by the use of a lock-in analyzer synchronized to the source of the thermal wave. Without the lock-in technique, the IR video camera is capable of observing only very slow thermal phenomena[1], despite the fact that the intrinsic band width of the camera is very broad. This limitation offsets the main advantage of the IR video camera, namely its high data-acquisition rate. In this paper we report on instrumentation development which combines the lock-in technique with the IR video camera. With this technique the information of each pixel of an image is handled in the manner of a lock-in analyzer, while the object is illuminated (i.e., heated) or stimulated (e.g., joule heating) with a signal which is synchronous with the reference signal of the lock-in detection. This way the unsynchronous background radiation is rejected and the signal-to-noise ratio is enhanced.

Experimental considerations in vibrothermography

Sonic, or thermosonic nondestructive testing, which is based on the vibrothermography method introduced in the late 1970’s, has attracted a great deal of recent interest as a means for detection of cracks that were previously considered to be undetectable using thermographic inspection methods. Excitation of a solid sample with bursts of high-energy (500 - 3000 Joule), low-frequency (10 - 50 kHz) acoustic energy has been demonstrated to be effective in generating transient localized heating at crack sites, making them detectable by an infrared camera. Despite the apparent simplicity of the scheme, there are a number of experimental considerations that can complicate, or in some cases even prevent, the implementation of vibrothermography-based inspection. Factors including acoustic horn location, horn-crack proximity, horn-sample coupling, and effective detection range all significantly affect the degree of excitation (or whether any excitation occurs at all) that occurs at a crack site for a given energy input. In cases where the experimental objective is precise measurement of crack length, the method used to visualize the data from the IR camera and its optic must also be taken into consideration.

Enhancement and reconstruction of thermographic NDT data

Visualization and analysis of pulsed thermographic data for NDT has generally been based on simple image averaging, subtraction or slope operations. Quantitative contrast methods, based on comparison to a defect free reference point or region, have also been used to a lesser extent. Despite their widespread use, all of these methods are highly susceptible to noise, nonlinearity of the IR camera response, and the presence of surface features on the sample. More importantly, the ability of any of these methods to significantly improve the ability to retrieve deep or weak subsurface features beyond the original unmodified image is limited. In a previous paper, we introduced the concept of Thermographic Signal Reconstruction (TSR) as a means of enhancing defect to background contrast while reducing the amount of data that must be stored by an order of magnitude. The TSR method increases the depth range over which pulsed thermography can be applied, and also reduces the amount of blurring due to lateral diffusion that is typical of thermographic imaging. In this paper we compare TSR with conventional thermographic approaches and consider the mechanisms for the resulting performance improvements.

Interfacial damage in EB-PVD thermal barrier coatings due to thermal cycling

Optical microscopy, acoustic microscopy, and thermal wave imaging techniques were used to characterize the interfacial damage in a thermal barrier coated single crystal superalloy subjected to thermal cycling. The thermal barrier coating (TBC) was applied to the superalloy using electron beam physical vapor deposition (EB-PVD) technique. The EB-PVD system seems to be stable at least up to the temperature of 1000°C for 300 cycles. Thermal expansion mismatch and oxidation between the bond coat and ceramic layer leads to separation at the interface which contributes to overall delamination and spallation. Experimental results suggest that buckling induced delamination is a possible mechanism for spallation.

Thermographic measurement of thermal barrier coating thickness

Flash thermography is widely used to inspect Thermal Barrier Coatings (TBC) during manufacturing and maintenance for defects such as delamination or contamination. However, attempts to use thermography to quantify TBC thickness have been less successful. In conventional thermographic NDT applications, the sample surface is opaque to an incident light pulse, and highly emissive in the infrared. The situation is more complex in TBCs, as the coatings are translucent to visible light and near-IR radiation (including the IR component of the flash). Furthermore, TBCs are translucent to the mid-IR wavelengths at which many IR cameras operate. Thus, in the absolute worst case, the flash pulse does not heat the coating, and the camera does not see the coating. Although the latter problem can be mitigated by judicious choice of camera wavelength, it must also be recognized that both the heating and cooling mechanisms in a flash-heated TBC are different from the usual thermography model, where transit time of a heat pulse from the sample surface to a layer interface is an indicator of coating thickness. The resulting time sequence is processed using the Thermographic Signal Reconstruction to generate thickness maps which are accurate to an accuracy of a few percent of the actual coating thickness.

Synchronous thermal wave IR video imaging for nondestructive evaluation

We describe a system for real-time processing of infrared video images in synchronism with the time-dependence of the target objects temperature. The system can either be used either with periodic or pulsed heating of the target. With periodic heating, the system operates as if it were a collection of lock-in amplifiers, one for each of the quarter of the million pixels of the image. With pulsed heating, it operates as if it consisted of a similar number of box-car averagers. In both cases, the signal-to-noise ratio and temperature sensitivity of the infrared camera are improved. The technique lends itself to a wide spectrum of NDE applications.

Thermal Wave Imaging of Aircraft Structures

In a previous report [1], we introduced the application of thermal wave imaging to adhesion disbonds and corrosion in aircraft. In the present paper, we describe the application of pulse-echo thermal wave imaging to NDT of aging aircraft. The technique uses high-power photographic flash lamps as a heat source and an IR video camera as a detector. The flash lamps launch pulses of heat into the skin of the aircraft and the IR camera images the returning thermal wave reflections from subsurface defects. The system also includes electronic hardware and software for carrying out the time-gated imaging and real time analysis of the defects. It also has the ability to image large areas in short times. The current inspection speed enables the imaging of over 90 feet of a 16″ strip of aircraft per hour. Here we present some examples of airframe defects, both for metal and composite structures.

Chronological evaluation of interfacial damage in TBC due to thermal cycling

A two layer electron beam-physical vapor deposited (EV-PVD) thermal barrier coating (TBC) on a single crystal superalloy (René N5) substrate was characterized prior to and after thermal cycling at 2, 18, 25, 44, 50, 75, 100, 110, 150, and 175 cycles in between 200 C-1177 C. Optical microscopy, scanning electron microscopy, and thermal wave imaging techniques were used to characterize the interfacial damage. Pt-Al was used as bond coat and 8 wt % YSZ was used as outer top layer. Interfacial cracking was observed even at two thermal cycles. Thermally grown oxide (TGO) layer increased with the number of thermal cycles. After numerous cycles over 100, interfacial separation was observed to be higher at the middle than at the edges of the sample. This observation is consistent with buckling induced delamination—a possible mechanism for spallation.

Quantification and automation of pulsed thermographic NDE

Until recently, thermographic methods for NDE have generally been quantitative, relying heavily on operator interpretation of image data. Although quantitative methods have been developed, they have generally required a priori knowledge of the sample physical properties, or identification of a defect free region within the field of view. Recent advances in pulsed thermography allow reference-free measurement of defect size, sample thickness and material properties without operator intervention or a priori knowledge of sample properties. An essential component of these advances are new signal processing methods based on both the spatial and temporal thermal response of the sample surface temperature to an instantaneous heat pulse. These methods provide a significant reduction in noise and blurring due to lateral diffusion of heat, and effectively increase the maximum penetration depth and spatial resolution beyond that of conventional thermography.