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Full-field Advanced Non Destructive Technique for On-line Thermo-Mechanical Measurements on Aeronautical Structures

Periodic Report Summary 2 - FANTOM (Full-field advanced non destructive technique for on-line thermo-mechanical measurements on aeronautical structures)

Project context and objectives:

The increasing use of composite materials in aeronautical parts induced an important stream of research in various domains like simulation of materials behavior when parts undergo life cycle or incidents (impacts, etc). Testing, and in particular non destructive testing (NDT), also needed a constant search of adapted techniques and methodologies in order to assess these new materials in a relevant way or to detect specific types of defects, like delaminations.

FANTOM (please see online) addresses testing during the development phase of aircrafts. This encompasses a variety of characterisation and test activities. In the case of small coupons, one is generally interested at materials characterisation (coefficient of thermal expansion, etc.) and going to larger parts like elements, sub-components or larger components (fuselage, wings, pylons) one is more interest in assessing structural viability under various mechanical loading cycles.

For all these reasons optical non-contact full-field technologies attracted high attention in the last decade because they avoid dismantling, yielding a better definition of the damaged area and they allow instant full-field capture of the desired information, the most popular ones being thermography and shearography/holography.

FANTOM aims at developing an innovative full-field NDT technique for the composite materials structures. It is based on holography incorporating a Long wave infrared (LWIR) laser. Working in such spectral range requires using thermographic imagers generally considered in full-field temperature measurements. The advantage of a LWIR holographic system is to extend the displacement/strain measurement range of the technique to larger values than obtained with their visible counterpart: a factor of typically 20 is envisaged in the project. Indeed the displacement measurement range of holography is proportional to the laser wavelength. This opens applications in structural testing where structures undergo large solicitation levels. In addition the system can be used in a more unstable environment than in laboratory conditions.

Besides these advantages, the FANTOM breakthrough resides in the fact that holographic and thermographic measurements can be performed simultaneously with a single sensor, allowing a minimum uncertainty between the same information obtained by two separate sensors. Indeed, the temperature background of the structure is inherently present in the holographic signal and it has to be properly decoupled from the other to allow correct interpretation of both information.

This concept aims at decreasing the time of inspection or testing since only one set-up need to be placed and aligned in the experiment. Also separate systems generate sets of data which need to be correlated one to another (deformation vs temperature), which induces an important post-processing activity to be able to interpret measurements. Therefore it is expected an important reduction of testing with such a single sensor giving at the same time two information. Placed in the context of European Union (EU) first call of the Seventh Framework Programme (FP7), FANTOM is strongly relevant to the objective AAT.2007.4.1.1 of decreasing aircraft development costs through advanced testing tools and methods to improve cost-efficiency and reduce testing time in laboratory and on-ground tests.

The first objective of the project is to study the holographic techniques in the LWIR range in the laboratory and in parallel develop new specific component such as a high resolution thermographic camera adapted to NDT requirements.

The second objective is to study the procedure for decoupling the holographic and thermal information from the signal captured by the system.

The third objective is to develop a transportable demonstrator that has to be first validated with representative samples developed in the project. The final system has then to be validated in true working conditions in structural testing facilities of club of end users partners.

Project results:

The main and major result of the FANTOM project is a mobile instrument capable of simultaneous acquisition of temperature variation and deformation on a full image and that was certified in a series of NDT applications. In order to achieve this with success, various research and developments activities were carried out by project participants during typically the first half of the project. These led to some intermediate results, some of them leading to new products that could be exploited independently to the main result. These intermediate results are listed hereafter:
- Development of LWIR holographic/speckle techniques.
- LWIR high resolution camera module with cooled detector.
- Optical systems for combining object and reference Carbon dioxide (CO2) laser beams into a thermographic camera. Two systems were developed and constitute separate results.
- Beam combiner made of Zinc selenide (ZnSe) plate with suitable coating.
- Modified needle with optics fiber and pinhole.
- Algorithm and procedure for decoupling thermal and deformation signal.
- Certified composite samples.

During the first period of the project, a series of concepts were elaborated and laboratory implementation were performed which showed that, among the various holography techniques studied, only Electronic speckle pattern interferometry (ESPI) was able to properly combine with thermography.

One of the more critical points was the incorporation of the reference beam into the thermographic camera, which led to a series of solutions, among which one was selected for further implementation.

A specific algorithm has been established and implemented for easy decoupling the holographic and thermal information contained in the images captured by the LWIR ESPI technique.

During the second project period, these concepts and laboratory studies were followed by the development and building of a transportable demonstrator prototype. After Computer-aided design (CAD), the different components were integrated and the prototype has been tested in laboratory conditions. Some experiments consisted in measurement of strain and comparison with measurement obtained by other methods. This allowed assessing the gain of FANTOM on the basis of its measurement range, the duration of measurement itself but also post-processing. We demonstrated that large solicitation levels provoking large deformations were measurable with FANTOM and filled the gap between high sensitive methods like holography/speckle in visible light and other methods like videogrammetry, stereo-correlation and fringe projection with lower sensitivities.

Another application was the detection of defects capability determined on certified composite samples developed in the project. The FANTOM combined technique was compared to separate thermography and shearography. The setup was then used in structural testing facilities made available by a project partner and which allowed us to assess the functionalities of the mobile prototype on-site.

A final task of the project was to assess the technique and instrument in industrial study cases provided by external end-user on a large composite part located in a hangar, with real working conditions. Non destructive tests have been performed and FANTOM allowed detecting defects in such structures with an improved probability: it was shown that some defects were observed only through the local temperature variation (as done with thermography) whereas others were observed only through the local deformation (as done by holography/shearography).

All the results obtained by FANTOM demonstrate its improved capability of detecting defects in inspection of composites as well as providing gain in time for measurement of materials and structures thermo-mechanical behavior assessment.

Potential impact:

We have to distinguish two main categories of results. On one hand we have main results which can lead to final products that will be or already are marketed. In this category category we have the following:
1) the advanced FANTOM NDT system for simultaneous temperature and strain measurement,
2) the new cooled high resolution thermographic LWIR camera, which can be incorporated in the former or used as stand-alone equipment.

In the second category, we have all other intermediate results which cannot lead to a commercial exploitation but rather are new expertise or specific component/software/procedure which have a limited or no impact in the aeronautics sector.

Concerning the advanced FANTOM NDT system, we foresee first its use for aircraft production and development phases. Indeed it was demonstrated that FANTOM gives advantages in term of measurement and post-processing time in NDT campaigns aiming at thermo-mechanical characterisation of materials. For this type of applications, FANTOM targets companies (small to large) as well as testing centers and research labs active in composite materials development and manufacturing. The main target sector is aeronautics but composites appear in an increasing number of applications and sectors. Let's cite marine, turbine blades, automotive and civil engineering. The latter has recently showed applications of repair or reinforcement of concrete structures with composites, which addresses new problems in adherence between these materials. The use of combined holography and thermography information will be justified more specifically in applications where NDT measurements are used to confirm predictions by finite element modeling. Second we foresee a huge potential in maintenance inspection, due to its higher probability of detecting defects than separate thermography and shearography. However this will require further developments towards better compactness and portability.

Concerning the new high performance cooled thermographic camera, it can also be used for NDT in production, development or inspection during maintenance, where a simpler thermography approach is sufficient. Thermography also has a broader market than NDT in aeronautics. The sectors it addresses are extremely varied, from marine, automotive, civil engineering to domestic. The new camera is the first one for exclusive commercial use in LWIR with such a high resolution and image homogeneity developed in Europe. It will constitute an alternative to equipment made available from the United States (US) and will guarantee European autonomy in the field. Aerial based thermographic imagery is increasingly gaining interest for surveillance of industrial plants, security, search of persons, energy audit of building, and so on. There is no doubt that the camera module will find a place in such a vast market as high level equipment.

Concerning socio-economic aspects, new NDT technique like FANTOM can be incorporated in various aspects of development and assessment of life cycle of composites which are increasingly incorporated in aircrafts. Therefore this constant search for the quality and reliability has undoubtedly an impact on maintaining current employment, even creating new high level position in materials engineers/technicians for developing or manufacturing composite parts.

Concerning wider societal aspects, any successful development and achievement in the field of NDT, which is the case with FANTOM, has direct impact on safety of persons onboard because it provides either a confirmation of diagnosis or deeper analysis of defects made by other conventional methods. Besides, the new thermography camera will be useful for any diagnosis in the field of energy audit which helps in reducing losses in building, which has a direct impact on reducing environmental human imprint.