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.

This paper summarizes the basics of pulsed thermal nondestructive testing (TNDT) including theoretical solutions, data processing algorithms and practical implementation. Typical defects are discussed along with 1D analytical and multi-dimensional numerical solutions. Special emphasis is focused on defect characterization by the use of inverse solutions. A list of TNDT terms is provided. Applications of active TNDT, mainly in the aerospace industry, are discussed briefly, and some trends in the further development of this technique are described.

Review of pulsed thermal NDT: Physical principles, theory and data processing

Optically and non-optically excited thermography for composites: A review

Composite materials/structures are advancing in product efficiency, cost-effectiveness and the development of superior specific properties. There are increasing demands in their applications to loa...

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Non-destructive testing and evaluation of composite materials/structures: A state-of-the-art review

Eddy current pulsed thermography (ECPT) is an emerging nondestructive testing and evaluation (NDT&E) technique and has been applied for a wide range of conductive materials. In this paper, a single-channel blind source separation is proposed to process the ECPT image sequences. The proposed method enables: 1) automatically extract valuable spatial and time patterns according to the whole transient response behavior without any training knowledge, 2) automatically identify defect patterns and quantify the defects, and 3) to provide guidelines of choosing the optimal contrast functions that can improve the separation results. In this paper, both mathematical and physical models are discussed and linked. The basis of the selection of separated spatial and time patterns is also presented. In addition, an artificial slot and a thermal fatigue natural crack are applied to validate the proposed method.

Automatic Defect Identification of Eddy Current Pulsed Thermography Using Single Channel Blind Source Separation

Pulsed thermography (PT), being simple and fast, remains one of the most popular thermographic NDE techniques, though it has intrinsic limitations. An alternative method is suggested and named frequency modulated thermal wave imaging (FMTWI), where the heating waveform phase relations are adjusted over a bandwidth (B) in such a way, that a chirp (frequency modulated) signal of duration T, with much reduced peak power is produced. FMTWI, while retaining all characteristics of lock-in thermography, has the added advantage of overcoming the blind frequency problem. It can be processed by matched filtering (pulse compression) to improve sensitivity.

Frequency modulated thermal wave imaging

This paper studies the use of light-emitting diode (LED) as an excitation source for photothermal lock-in experiments, and investigates the energy absorption efficiency and signal-to-noise ratio of the established setup. Existing lock-in excitation sources usually use high power (in the range of kilowatt) halogen lamps. The present LED excitation source has an advantage of low power consumption making the setup much more energy efficient. Another advantage of LED in this paper is their low infrared (IR) content, in contrast to halogen lamps, the IR content gets reflected into the IR camera, and leads to an unpredicted change in phase image in lock-in thermography.

Characterization and Energy Absorption Efficiency Determination of LED as an Effective Photothermal Excitation Source in Lock-In Thermography

Fault detection of antenna arrays using infrared thermography

The electric field distribution (magnitude only) near a radiating source (antenna) can be easily determined using infrared thermography. A thin screen (made of carbon fiber reinforced polymers) is placed in front of a microwave source. The electromagnetic waves impinging on the screen are partially absorbed, resulting in temperature rise of the screen. This temperature rise is monitored by an infrared camera. The temperature distribution thus observed is mapped to the electric field strength (magnitude of electric field) of the electromagnetic waves. Points on the screen where the temperature rise is low correspond to weak electromagnetic fields whereas points with high temperature rise correspond to strong electromagnetic fields. In this paper electro-thermal modelling is done so as to obtain the temperature distribution over the screen, when an electromagnetic field is incident on it. This model can conversely be used for finding electromagnetic field distributions from IR thermal images.

Krishnendu Chatterjee

Papers

Frequency modulated thermal wave imaging

Characterization and Energy Absorption Efficiency Determination of LED as an Effective Photothermal Excitation Source in Lock-In Thermography

Fault detection of antenna arrays using infrared thermography

Modelling and Analysis of Power Distribution of Electromagnetic Waves on Plane Surfaces Using Lock-in IR Thermography

Bamboo-glue interface thermography for non-destructive testing.