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Full-Field Advanced Non-Destructive Technique for Online Thermo-Mechanical Measurement on Aeronautical Structures

State of the Art - Background

Tags: Air

Non-destructive testing (NDT) techniques allow for the detection of flaws in materials and structures or the measurement of behaviour of such components under given stress. These techniques are widely used in various applications such as development, production and maintenance.

Optical full-field NDT techniques are gaining in interest since there is no contact and they allow for observing a complete image, which is a faster process than the single-point measurement with scanning.

The best-known and used optical NDT technique in aeronautics is thermography.

Thermography, or thermal imaging, is a type of infrared imaging. Thermographic cameras detect radiation in the infrared range of the electromagnetic spectrum at approximately between 0.9 and 14 µm. In NDT applications, like the assessment of materials, thermography can show differential thermal behaviours of defects located under their surface when the materials are illuminated by an infrared lamp which creates heat transfer through it.

The other range of optical NDT techniques is holographic techniques which allow full-field observation of micrometric deformations of objects undergoing stress. Different techniques exist, the best known being shearography, which is increasingly applied for flaw detection in aeronautical composites.


Both thermography and holography have advantages and disadvantages. The main drawback of holography is its sensitivity to on-site perturbation, mainly due to the use of visible lasers with a short wavelength.

The FANTOM project proposes the development of an advanced NDT technique which combines thermography and holography/shearography. The development is based on a holography/shearography sensor with a usual optical set-up, but instead of working with lasers at visible light wavelengths, it will work in the spectral range of thermographic cameras, say at long-wave infrared-light wavelengths. This has the advantage of greatly reducing the sensitivity to external perturbation, while increasing the measurement range of the technique.

Additionally, the fact that the imaging system of holography in infrared is a thermographic camera allows one to envisage simultaneous capture of thermal and holographic information. This will lead to a unique sensor which will allow a substantial gain in inspection time compared to the situation where two such separate sensors are used.

Description of Work

There are three main innovations.

The first is the study and development of different holographic techniques in the long-wave infrared. Critical segments such as the infrared sensor will be improved in order to match specific requirements of holographic techniques. The techniques envisaged are electronic speckle-pattern interferometry, digital holography and shearography.

The second is the study of the decoupling between the holographic and thermographic information which will be captured simultaneously. This is required since it is not known how the use of lasers (necessary for holography) affects the thermal signature of the object.

Thirdly, based on the laboratory studies, a prototype will be built with improved segments and, after being validated in known study cases, will be validated in Airbus facilities (or other potential end-users).

Expected Results

The expected results are:

- Specifying the new technique based on state-of-the-art and end-user requirements;

- Conceptual designs of the instrument, including selection of critical components;

- Development studies of different holographic techniques, of improved optical segments and thermographic image sensors and camera modules;

- Study of thermal and holographic signatures decoupling;

- Development and certification of representative samples for lab evaluation;

- Build the prototype and carry out the evaluation in the lab with certified samples;

- Validating the prototype in structural testing at an Airbus plant (or other end-users).

Principle of the ESPI out-of-plane option to be studied within FANTOM
Principle of the ESPI out-of-plane option to be studied within FANTOM