Table of Contents ECNDT '98
Developments and Applications of NDT Techniques for the Aeronautical Industry through EC funded Research ProjectsHervé TRÉTOUT*
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The current world-wide recession in the aeronautical industry forces many aircraft operators to postpone investments in new aircrafts and consequently stretch the lifetime of their existing fleet. Therefore, the European aeronautical industry nowadays sees itself faced with two main challenges: safe operation of an ageing aircraft fleet, and cost effective production and maintenance. For both areas, the aeronautical industry has the responsibility to provide the methods that enable the operators to offer safe and cost effective services to their passengers.
One of the approaches the industry has chosen to deal with these challenges, is the application of advanced materials. These materials offer the opportunity of reduced weight and more efficient production, lowering both operational and production costs. They require however another approach with respect to non-destructive testing (NDT): material behaviour and defect types differ largely from more conventional, metallic structures. At the same time stretched maintenance intervals and extended aircraft lifetime, have made quantitative detection of cracks and corrosion in metallic structures highly desirable. This calls for efficient NDT methods, that do not require long down times for in-service inspections, and are fast and reliable for production inspection. No single method can fulfil all requirements. In some cases the solution lies in the development of speedy full field methods that can inspect large areas. In other cases sophisticated signal processing for extraction of defect information from the measured signals yields the best result. In addition an approach using knowledge based diagnosis systems should offer increased inspection reliability.
A new approach lies in the tireless adaptation of increasing quality levels to these imperatives of cost and cycles reduction which can be facilitated, among other means, by the adoption of Unified Life-Cycle concepts that merge aspects of concurrent engineering and total quality management around the design. The actualisation of these concepts makes mandatory the integration of NDT at the various phases of a component life (see Figure 1).
Fig 1: Scheme of the United Life Cycle Engineering Environment (ULCE)
A large number of developers and end-users from the aeronautical industry have been involved in several consecutive projects funded by the European Union in the fields of Industrial and Materials Technologies. These research projects have been targeted towards solving today's ageing aircraft inspection issues, reducing production cost by improving inspection procedures and considering the in-service inspection problems expected to arise in the near future (see Figure 2). Following projects BE89-3209 (Ref.1) and BE91-5145 (Ref.2), we present an overview of the developments achieved and of the results obtained in the last completed project BE92-5879 (Ref.3) and of the work carried out in a current project BE95-1778 (Ref.4).The perpectives are based on a current proposal BE97-4057 (Ref.5).
Fig 2: N.D.T. / N.D.E. issues in production and maintenance
21 partners haver been involved in the two projects BE92-5879 (3) and BE95-1778 (4) as developer , end-user or vendor : Aerospatiale - FR (4), Agusta - IT (3) & (4), Alenia - It (4), BAM - DL (4), British Aerospace Defence Ltd. - GB (3) & (4), CASA - ES (4), CIRA - IT (4), CISE - IT (3), CSM Materialteknik AB - SE (3) & (4), DASA Airbus - DL (3), DASA LM - DL (3) & (4), University of Delft - NL (3) & (4), Eurocopter Deutschland - DL (4), Fokker Aircraft BV - NL (3), Helenic Aerospace Industry Ltd. - GR (3), Intrasoft S.A. - GR (3), NDT Expert - FR (4), Rohmann Gmbh - DL (3), University of Strathclyde - GB (3) & (4), University of Uppsala - SE (4).
One of the objective of the project was to improve the reliability and speed of in-field inspections of defects and damages in composite materials with lower detection level than currently possible and faster than with conventional ultrasonic methods. For this purpose application of optical non contact NDT methods was studied. A modelling effort using 2 finite element models was undertaken following a unique approach to calculate displacements of the surface of a composite material part, when submitted to thermal or mechanical (low pressure) loading. A specific vacuum loading system, controlled by computer, was designed; a flash-type and an infrared-type thermal loading systems, designed for infrared thermography, were adapted. A real time phase stepped shearing interferometer was developed it measures deformation of diffusely reflecting objects at video speed. It uses two CCD cameras and employs a four bucket method. The performance was evaluated by comparison with that of a temporal phase stepped shearing interferometer. For a similar sensitivity, the image quality of the new system did not equal that of conventional systems.
Optical interferometric techniques were investigated for in field detection of helicopter composite structures with the exploitation of natural vibration loads due to ground running conditions, in order to stress the structure and consequently to cause surface deformation. The most appropriate technique proved to be shearography, due to its lower sensitivity to environmental influences, compared with other optical interferometric methods (Ref. 6-8).
The second objective was to improve the ultrasonic inspection of composite materials.
The ultimate goal to develop an air coupled US inspection system is to improve upon and consequently provide alternative means of testing to existing water jet systems. This development included the most modern transducer technology: a composite transducer array was developed and its performance was additionally improved by a special matching layer to increase the efficiency of the sound path (Ref. 9). The status achieved now is a 8 channels prototype system with a receiver array and a large composite transmitting element. An Electronic Random Phase Array Transducer was investigated to improve defect characterisation.
New electronics and new software packages in the field of measurement data acquisition and evaluation caused all partners in the US task to analyse new solutions. A more stationary environment based on UNIX and VME-Bus architecture embedded in a computer network was investigated. Aspects of human interface to robot and software covered here the front end, while the selection of General Purpose (GP) software reduced the development costs and improved the availability of state of the art signal processing tools. This selection included software products like ORACLE for a NDT inspection database, HPVEE, PVWave, ONIX for data processing.
Signal analysis methods were carried out to improve ultrasonic testing of complex materials and structures by developing methods which use split spectrum processing and wavelet analysis, statistical pattern recognition, neural networks, frequency analysis. Finally a program system named CAMPUS was developed to determine porosity contents in CFRP and other material.
A major effort to model several structural configurations (layered bonded, T-stringer, double T structure) was done with the EFIT code from GHKassel.
Data compression and storage techniques were investigated for economic 3D ultrasonic data storage. Several common nonlossy and lossy methods were analysed and checked, depending on the factor of tolerable loss admitted, methods like JPEG and MPEG proved their power in trials.
The third objective was the development of new eddy current testing methods for improved corrosion depth measurement and fatigue crack sizing. For fatigue crack detection an original instrument was developed to investigate the feasibility of pulsed eddy current signals. Automatic detection was investigated using artificial intelligence. Results indicate the feasibility of this method for linear inner skin cracks. For corrosion detection a comprehensive experimental work was carried out to evaluate and optimise available standard probes, new probes designs and modified one's. Most of the probes were evaluated by its Point Spread Function (PSF) supported by the software developed.. High resolution was gained by new developed EC-probes. The use of multifrequency EC technique was investigated on parts with hidden corrosion. Multifrequency testing on aircraft structures seems to be applicable if the part inspected show only one 'noise signal' to be suppressed and if the easier one frequency method leads to no success. With the realization of pseudo-inverse filtering, data scanned with conventional and therefore everywhere available probes can be evaluated. Through the implementation of image processing processes the interpretation of test results is greatly facilitated and the resolution of the probes is improved considerably.
An investigattion of the critical factors and requirements was carried out for the development of a fieldable NDI-system to be operational with manual EC instruments but versatile enough to allow the use of other techniques. A semi-automatic miniscanner with two orthogonal axes has also been developed, it is aimed to be used for field inspections. Both scanners have been designed to be connected to the same computer environment to undertake their evaluation, they perform high quality inspections at a cost effective rate with several NDT techniques.
The main components of a plug-in-card for personal computers have been developed as well as a PC-based software evaluation system to provide C-scans and mapping of eddy-current test results. A universal scanner interface PC-card "ScanAlyser" has also been developed. An other new concept ESRIC has been developed, the instrument is fully digital and is controlled by a VME-bus system, it allows free programmable signals for evaluation, multisensor, multichannel and multifrequency testing (Ref.10).
Electrical and electromagnetic characterisations of probes were carried out. An equipment using magnetoresistive sensors was developed which allows the mapping of the EM field for a wide range of probes. Procedures were defined to set up the electrical and the electromagnetic characterisations of EC probes.Procedures were defined to set up the characterisation of eddy current instruments(Ref.11 & 12).
The currently running project will further develop and apply the NDT techniques that are most promising for the aeronautical industry and potentially for other industries. The developments are focused on 3 distinctive problem areas :
Fig 3: NDT WorkShop architecture building-up through brite euram projects
The first task comprises development and application studies of eddy current, laser ultrasonics and thermography. Much attention will be paid to development of new eddy current techniques, due to the vast amount of urgent problems that can potentially be solved with this technique such as hidden crack and quantitative corrosion detection. To this aim the multi-frequency approach, the introduction of C-scanning and the application of neural networks for signal analysis are very promising. The application of laser ultrasonics to aircraft corrosion problems is systematically investigated for the first time in Europe. For Infrared Thermography the combination of a new highly sensitive camera with pulsed excitation and the application of neural networks is expected to result in a fast corrosion detection system.
The main industrial objective of the second task is to suitably enhance the production NDT methods in order to perform large area coverage. This will be achieved by researching and developing potential non-contact inspection techniques. This will include the real time full field methods of thermography and shearography for faster production inspection by at least a factor of two of large metallic and composite components. Other developments include a real-time X-Ray technique combined with an expert system for the inspection of composite and metallic structures, air coupled ultrasound for large aerofoil composite configurations and laser generated ultrasonics for complex geometry applications. The development of a 32 element air-coupled ultrasonic array system, providing a minimum coverage of 100 mm at 2 mm resolution will be carried out.
The third task aims at the development of rapid single-sided access detection methods for quantification of defects in aeronautical composite parts, without removal or special surface preparation. Non-contact ultrasonics and thermography are most promising with regard to these industrial requirements.
The fourth task aims at specifying the software and hardware requirements for a NDT diagnosis tool that enables the combination of the inspection results from manifold NDT methods and CAD data with the idea of supporting and standardizing the diagnosis criteria. Some application examples will be developed. For the latter subject, technology transfers from the finalised RACE project TRAPPIST ("Transfer Processing and Interpretation of 3D-NDT-Data in a standard Environment")as well as the project RADICAD are investigated.
The complexity of the issues to be faced confirmed by the results of the previous projects and of a continously upgraded state of the art led to reconduct a wide co-operation, joining all the competencies of the majority of the previous partners. This has been achieved by setting up a new proposal "INDUCE".
To face the Unified Life-Cycle concept in all respects, it becomes necessary to put a greater effort on the integration of the nondestructive techniques at the level of design and manufacturing process, the INDUCE project has been conceived to make this objective feasible. In fact it's centred on :
The main expected aftermath of the program will be :
A particularity of these project is the important number of developments undertaken and achieved in several main NDT areas, Shearography, Infrared Thermography, X-rays, Ultrasounds, Laser Ultrasonics, Air coupled Ultrasounds and Eddy-Currents. This is typical with such large consortiums in which the contractors have many problems to solve. The achieved developments which are both problem and technology orientated can be classified in three mains categories as a function of the foreseen continuity :
These projects have been performed with the funding of the European Community. The co-ordinator would like to thank all the people who have contributed to the project BE92-5879 and in particular :
Hervé PERO, Michel ANDRIEU, Dominique OLLINGER (EC Scientific Officers), Vittorio PAGANI, Luigi MERLETTI (AGUSTA S.p.A), Kevin TAYLOR, John-Paul SMITH, Andrew COOK (BRITISH AEROSPACE Defence Ltd), Luciana RIZZI, Pierino DELVO (CISE S.p.A.), Wolfgang BISLE (DAIMLER BENZ AEROSPACEC AIRBUS), Friedrich RIEGLER, Ernst GRAUVOGL, Markus ALTMANN (DAIMLER BENZ AEROSPACEC AG), Peter SOMERS, Ad. MAAS, J.S. SPEJER, Theo MODDERMAN (FOKKER AIRCRAFT BV), Zaira MARIOLI-RIGA, Anna KARANIKA, Rubini MARINI, Th. SPATHOPOULOS, T. DRACOPOULOS (HELENIC AEROSPACE INDUSTRY Ltd), Yiannos AVRAMOPOULOS, George MATINOPOULOS, Sofia POUCAMISA, Dimitri LAGOS, Dimitri XANTHAKIS, L. ALEXANDROPOULOS (INTRASOFT S.A.), Martin JUNGER (ROHMANN Gmbh), Thomas GRYBACK (SAAB MILITARY AIRCRAFT), Magnus ENGSTR(tm)M, Bertil GRELSSON (CSM MATERIALTEKNIK AB), Prof. H. FRANKENA, Gon WEIJERS, A.J.P. van HAASREREN (TECHNISHE UNIVERSITEIT DELFT), Gordon HAYWARD, Stephen KELLY (UNIVERSITY OF STRATHCLYDE), Claude CORNET, Marc DESSENDRE, Jean-Yves MARIN, Alain LIOT, Nathalie RIVAULT, Claude SIDE, Fran‡oise THEVENOT, Hervé TRÉTOUT, Hubert VOILLAUME, Denis DAVID, Ren‚ DE MOL, Fabien CHIRON, Maurice RICARD (DASSAULT AVIATION), H.M. THOMAS (BAM), the University of Kassel (DE), L.O.A. - University Paris VI (FR), and others.