The intensity of the infrared radiation emitted by objects is mainly a function of their temperature. In infrared thermography, this feature is used for multiple purposes: as a health indicator in medical applications, as a sign of malfunction in mechanical and electrical maintenance or as an indicator of heat loss in buildings. This paper presents a review of infrared thermography especially focused on two applications: temperature measurement and non-destructive testing, two of the main fields where infrared thermography-based sensors are used. A general introduction to infrared thermography and the common procedures for temperature measurement and non-destructive testing are presented. Furthermore, developments in these fields and recent advances are reviewed.

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Infrared thermography for temperature measurement and non-destructive testing.

A review provided about non-destructive testing (NDT) methods for the evaluation of composites. The review considers the capabilities of most common methods in composite NDT applications such as Visual Testing (VT or VI), Ultrasonic Testing (UT), Thermography, Radiographic Testing (RT), Electromagnetic Testing (ET), Acoustic Emission (AE), and Shearography Testing with respect to advantages and disadvantages of these methods. Then, methods categorized based on their intrinsic characteristics and their applications.

https://cyberleninka.org/article/n/593997.pdf

A review of non-destructive testing methods of composite materials

Active thermography has gained broad acceptance as a non- destructive evaluation method for numerous in-service and manufactur- ing applications in the aerospace industry. However, because of the dif- fusive 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 compo- nents in the image sequence and significantly reduces temporal noise. Time-derivative images derived from the reconstructed data allow detec- tion of subsurface defects at earlier times in the sequence than conven- tional 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. © 2003 Society of Photo-Optical In-

Reconstruction and enhancement of active thermographic image sequences

Review of progress, QNDE

Active infrared thermography is a fast and accurate non-destructive evaluation technique that is of particular relevance to the aerospace industry for the inspection of aircraft and helicopters’ primary and secondary structures, aero-engine parts, spacecraft components and its subsystems. This review provides an exhaustive summary of most recent active thermographic methods used for aerospace applications according to their physical principle and thermal excitation sources. Besides traditional optically stimulated thermography, which uses external optical radiation such as flashes, heaters and laser systems, novel hybrid thermographic techniques are also investigated. These include ultrasonic stimulated thermography, which uses ultrasonic waves and the local damage resonance effect to enhance the reliability and sensitivity to micro-cracks, eddy current stimulated thermography, which uses cost-effective eddy current excitation to generate induction heating, and microwave thermography, which uses electromagnetic radiation at the microwave frequency bands to provide rapid detection of cracks and delamination. All these techniques are here analysed and numerous examples are provided for different damage scenarios and aerospace components in order to identify the strength and limitations of each thermographic technique. Moreover, alternative strategies to current external thermal excitation sources, here named as material-based thermography methods, are examined in this paper. These novel thermographic techniques rely on thermoresistive internal heating and offer a fast, low power, accurate and reliable assessment of damage in aerospace composites.

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Recent Advances in Active Infrared Thermography for Non-Destructive Testing of Aerospace Components.

The use of pulsed thermography as an NDE solution for manufacturing and in-service applications has increased dramatically in the past five years, enabled by advances in IR camera and computer technology. However, the basic approaches to analysis and processing of pulsed thermographic data have remained largely unchanged. These methods include image averaging, subtraction, division, slope calculation and contrast methods (e.g. peak contrast and peak slope time mapping). We have developed an alternative approach to analysis of pulsed thermographic data, based on developing a parametric equation for the time history of each pixel. The resulting synthetic image provides increased spatial and temporal resolution, and significantly extends the range of defect depths and sample configurations to which pulsed thermography can be applied. In addition, our approach reduces the amount of data that must be manipulated and stored, so that an entire array of image sequences from a large structure can be processed simultaneously.

Advances in pulsed thermography

A method for compiling thermographic data including obtaining data indicative of a monotonically changing characteristic of a specimen, sequencing the data or a surrogate of the data into a plurality of groups, categorizing, within each group, the frequency distribution of an attribute of the data or an attribute of said surrogate data, and compiling, from one or more groups, a collection of two or more of the frequency distributions.

System for generating thermographic images using thermographic signal reconstruction

Structural adhesive bonding is an enabling technology for the implementation of composite assemblies in automotive applications. Therefore, the quality and reliability of the composite bond must be assured. An advanced thermal non-destructive test (NDT) method, pulsed thermography, was evaluated for its capability to assess joint quality in an adhesively bonded composite pickup truck box. Pulsed thermography, used under in-plant, pre-production conditions as would exist during process start-up and optimization trials, was shown effective in determining both the quality of the structural adhesive bonds and the quality of the composite itself. With one exception, NDT showed that bonding was performed correctly, i.e. the bond was continuous and properly placed. The exception was a ‘starved’ bond-line that we believe exists due to poor fixturing at that location. Pulsed thermography illustrated the effects of environmental and mechanical exposure on the bonded joints. Finally, the NDT method was able to show mechanical damage to the composite itself, identifying impact damage not visible to the unaided eye.

Non-destructive testing of structural composites and adhesively bonded composite joints: pulsed thermography

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.

Steven M. Shepard

Papers

Reconstruction and enhancement of active thermographic image sequences

Advances in pulsed thermography

System for generating thermographic images using thermographic signal reconstruction

Non-destructive testing of structural composites and adhesively bonded composite joints: pulsed thermography

Enhancement and reconstruction of thermographic NDT data