Objective
A.BACKGROUND
The safety of many advanced machines and structures relies on the mechanical capability of their components to sustain strains and stresses. The overall resistance to such stresses and deformations is often based on a metallic framework, which may be seriously affected by inherent processes of increasing corrosion and/or by structural defects originating at the production stage or due to ageing. Key examples include the load-bearing fuselage of airliners, the rotating turbines in the engines and rotors of aeroplanes and helicopters, the metallic frameworks of civil buildings and bridges and so on. Thus, it is a fundamental safety requirement to study continuously the improvement of these materials in terms of resistance to corrosion and fatigue. An opportunity to reduce defects rate at the production stage is another key point in improved quality assurance. Non-destructive testing procedures are important because every sample should be checked individually; however, it is difficult to evaluate reliably the actual conditions of the specimen both at the production stage and during periodical maintenance. Many techniques have been proposed to achieve this goal. Among them, neutron imaging techniques, and in particular neutron radiography and tomography, offer peculiar properties that give them unique capabilities.
These neutron techniques are in principle similar to the analogous applications in the medical field, where X-ray generators are used as the radiation source. However, the particular physical process of neutron to matter interaction makes them well suited to the detection of hydrogen (always present as a component of corrosion product) or of discontinuities in a bulk metallic structure. At this time, the achievable spatial resolution of neutron systems is of the order of 40 æm when using conventional photographic films coupled to gadolinium sheets. However, these performances need to be improved to satisfy fully the requirements for an early detection of corrosion and defects in metallic samples, and digital acquisition should be exploited to process quickly and enhance the image.
Detailed work has been carried out since the 1960's to develop these testing methodologies and to exploit their powerful capabilities. In the United States, major studies on the corrosion of airplanes' honeycomb structures were carried out in California by J.P. Barton of the NRE in cooperation with the US Air Force research group. This work led to the production of an advanced automated neutron inspection system that is currently operative at the McClellan Air Force Base, Sacramento.
In Japan, scientific research has concentrated on the industrial applications. In particular, all the main mechanical industries have historically maintained important research and development departments, and neutron imaging techniques have been developed for the non-destructive testing in automobile production. Most manufacturers (i.e. Toyota, Mitsubishi and so on) own a neutron-based imaging facility that plays a relevant role in the testing procedure and quality control. These plants are used again for early detection of corrosion to locate the critical points in the structures when ageing studies are carried out. It allows engineers to redesign these critical areas, thereby increasing the reliability of their products and hence contributing to the well-known world-wide penetration of Japanese cars on the market.
The diffusion in Europe of neutron imaging techniques for non-destructive applications is fragmented. France is undoubtedly very advanced in this field, since this country has for a long time maintained strong activity in the area of applied nuclear physics. For example, France currently inspects pyrotechnic devices to be used in the ARIANE launchers; the integrity and the correct positioning of rubber-based O-rings inside the wall of the ARIANE booster could also have been tested by neutron radiography, but this idea was given up due to the lack of available and appropriate equipment. France and the UK produce pulsed neutron sources that are well suited for in situ applications.
Relevant academic work has been done in recent years in Germany and Italy to develop advanced neutron detectors based on electronic devices. In the proposing country, in particular, since the beginning of the nineties, an initially simple system based on a scintillator screen neutron detector viewed by a cooled TV camera has been tested and developed at the ENEA research reactor in Rome by the group led by the proponent of this action. Thus, having begun with simple radiographs, the system is now able to perform quantitative tomographic examinations of small objects. The system employed now offers a resolution in the order of 150 æm, while the final target is now a value lower than 25 æm. Italian manufacturers of advanced composite material for aeronautics have already declared their interest if such performances are achieved.
The desire to have a portable system, able to operate in situ without any link with the main laboratory is pushing forward extensive work on data processing algorithm. Thus, the development of specific code to be used on personal computers is actually underway, as PCs are much more suited to be carried together with a portable acquisition instrumentation.
Central and Eastern European countries, such as Hungary, Poland, the Czech Republic, Slovenia, and obviously the Russian Federation, also have a good tradition in this field, but they are now struggling with economic troubles and uncertainties. There is a significant knowledge in the field of neutron production, detection and application that is likely to be dispersed and lost.
It is worth noticing that the environmental risk of the equipment needed to carry out the neutron tests is negligible and the exposure to professionally involved personnel is lower than the exposure to other equivalent technicians (i.e. medical radiology technicians). Indeed, the neutron research reactors are characterized by a power that is more than 100 times lower than a small power plant.
The adoption of the proposed pulsed sources further reduces the environmental risk because they do not involve any nuclear materials and/or critical assembly. A radiation throughput occurs only when the power supply devices are switched on under the control of a trained operator. When measures are performed in the field, the safety of staff requires either that a no-entry area is delimited all around the source of about ten metres or that a physical shield is placed between the staff and the source. By following the directives on transportation (i.e. by using authorized vans), no risk can occur when the portable sources are carried to the site where the measures are performed, because no nuclear material is present.
B.OBJECTIVES
The main objective of the Action is the development of high-performance non-destructive testing facilities employing non-reactor neutron sources in order to study the corrosion and defects in metallic and bulk structure and the improvement of industrial processes.
The achievement of such a target will provide several useful results in many other fields of industrial and non-industrial inspections. In particular it will allow:
- The early detection of corrosion and defects in composite materials employed for technologically advanced industries (for example aerospace, aeronautics and nuclear applications) as well as for civil engineering (buildings, bridges) or for works of art that need restoration (statues, arms, paintings). In turn, this allows an improvement of the reliability of some critical components used in high-risk technologies in order to prevent those failures which could have catastrophic consequences in terms of losses of human lives, environment disasters and loss of expensive equipment.
- The study and improvement of new materials (composite, ceramics) that could benefit the above applications and quality control during their manufacture.
- The development of new lubricants and chemicals e.g. for cars and refrigerators, with great benefits for our environment.
It is worth citing main intermediate actions that will be needed to achieve the final goal and yet independently are of value to the scientific and industrial communities whilst also benefitting international cooperation. These points may be summarized as follows:
- The study of requirements for facilities capable of performing neutron imaging in situ, that is at the location of the object to be tested (this may be essential for civil and some cultural applications) and during its normal operation. It is anticipated that the ability to locate critical elements in complex components or structures will also be improved. The use of such testing techniques during the production process and during routine maintenance should significantly reduce their overall costs.
- The development of advanced transportable neutron sources and of adequate detection systems. These will be required to perform on-site examination of aeroplanes, helicopters, cultural heritage and civil building for maintenance. European industries are active in the fields of source and detector production and their penetration on the world market will greatly benefit the demonstration of large-scale applicability of neutron-based non-destructive imaging techniques.
- The study of novel software techniques for real-time image processing, enhancement and automated evaluation as well as for 3D computed tomography.
- The study of the feasibility of applications of such techniques in the life sciences and environmental protection (improved detection of pollutants in soil, production of moisture retaining materials).
- The qualification of existing research centres both in Western Europe and in CEC\NIS countries enabling a standardization of testing procedures among these countries.
- The sharing of valuable expertise and\or expensive instrumentation between research centres of Western Europe and CEC\NIS countries, to be achieved mainly by short-term visits of young researchers, preferably at the post-doctoral level, from CEC\NIS countries to Western Europe centres.
C.SCIENTIFIC PROGRAMME
Part 1 - Improvement of equipment and materials
The quality assurance of products has become in recent years one of the most important challenges which industries must meet in order to be competitive in the world-wide market.
This is particularly true for those technologically advanced products whose reliability must be very high to ensure the safety of consumers, such as those employed in transportation and construction. Thus nowadays it is vital for producers to employ advanced techniques for a continuous monitoring of the quality of production, possibly at an intermediate stage of the production process to reduce the cost of rejection of high value finished products. Furthermore, the lifetime of advanced materials has been extended, often requiring a deeper and more demanding scheduled maintenance during its lifetime. These procedures must detect any early indication of incipient failure or corrosion to avoid risk of failure during operation; at the same time, economic constraints dictate that the tests must be non-destructive. It is therefore important to develop both advanced materials more resistant to failure while, at the same time, improving non-destructive testing methodologies.
The target of the first part of the work programme is the improvement of the capability to detect small amounts of corrosion or microdamages within metallic structures or in composite materials and concrete, while the second part is devoted to the study using this new upgraded method for the improvement of the materials themselves, being much easier to characterize and to qualify the performances of candidates advanced materials. This important task could be achieved by the exploitation of the powerful techniques based on neutron radiography (i.e. a technique similar to the conventional X-ray radiography) which employs a neutron beam as the probe to investigate the properties of the examined object.
Furthermore, it is of fundamental importance to be able to perform such investigations in situ, that is, where the object is located or produced.
The following working plan is drawn up as a result of this main target:
(a) preliminary detailed analysis of the previously described applications and the characterization of the technical requirements for their achievement.
The activity will then run in parallel with working groups covering two broad areas. The Instruments area will be divided into working groups studying:
(b) an improvement of existing pulsed neutron sources. These sources are needed to set up either a portable or a fixed and small size facility, but are to date characterised by a reduced neutron throughput. As the main difficulties arise from the need to use a neutron fluence as low as possible, so as to reduce the safety constraints and to keep the overall weight of the equipment as low as possible, the use of sealed tube neutron generators or other small size accelerators is a solution which is worth studying;
(c)the development of well-matched novel neutron detectors. The better detectors used at present for neutron radiography purpose were designed for operating with high neutron fluxes supplied by the more intense research reactor sources. Now such fluxes cannot be used in an industrial environment for mandatory and security reasons. Furthermore, moving capability of the equipment requires the use of mobile neutron sources that involve a reduction of a factor of 100 in the available neutron fluxes.
Therefore, there is a need to develop advanced detectors characterised by an improved sensitivity to counterbalance this drawback. At the same time the detector must be characterised by an increased spatial resolution, that is, the capability to distinguish fine details in an image;
(d) the development of new processing software tools to exploit the possibility of enhancing the quality of acquired images by numerical processing, for both cases of dynamic radiography and neutron tomography;
(e)the determination, after the completion of these preliminary tasks, of the optimal neutron energy for the testing of different materials, as it is well known that image quality is by far superior if a monoenergetic beam is employed rather than a polycromatic source;
(f)the testing of the new detectors on several sources, including non-reactor sources. This job will be done employing reference samples to be provided by industrial producers as well as by Material Research Institutes. Comparison versus results achievable with other non-destructive techniques, with particular regard to X-ray based testings, will be performed at all the development stages.
The concurrent activity of WGs in the area of Materials will concern:
(a)the continuous updating of testing procedures and protocols when relevant improvements are achieved in the instrumentation, beginning with the facilities that are actually available;
(b)as a consequence of (a), more accurate characterization of studied materials from time to time; the results will be used by research centres and industrial producers to begin the study on small scale refinements of the production processes.
Part II - Application Programme
Taking into account the future availability of an industrial neutron radiography equipment able to operate on site, the institutes and private companies involved in the programme will start neutron radiography inspection of samples as soon as the new action begins. At this early step, the inspection will be done close to the radiography centres using reactor sources, but, following the progress of the action, these inspections will be performed using both non-reactor sources and new detectors. These studies will focus on those fields where new interest is growing in this research from both the academic and the industrial sides to exploit the applicability of the peculiarities of the neutron-to-matter interaction.
These fields can be summarized as follows:
-the search for corrosion on aluminium complex surfaces, on lubricated assemblies and on cooling devices, with particular interest directed at the aircraft's maintenance procedure. The interest on this activity has already been expressed by a consortium comprised of the main French aeronautic groups, the British-based worldwide leader flight company and an Italian co-producer of aircraft.
-the improvement of coolers and lubricants used in refrigerators and air conditioners. An interesting application is carried out at the Hungarian site based on the coupled study by neutron imaging and vibrational technique to detect incomplete circulation of oil. Recently, this study allowed a Hungarian producer that expressed great interest in the current action optimally to reshape the path of the oil in its devices, reducing the defective rate. The coupled study is typical of many in the field of non-destructive testings, because often a single technique cannot completely satisfy the inspection requirements. Interest was also expressed by a main European producer of refrigerators and air conditioners.
-the testing of composite and ceramics as well as civil engineering materials. Cracks, intrusions and defects can severely affect the reliability of these devices. The small dimensions of typical defects in these materials which are designed to operate under thermally and mechanically extreme conditions makes it very difficult to find an efficient testing procedure. However, boron-based fiber ceramics can be tested reliably by neutron imaging due to the high absorption coefficient of boron for neutrons. A feasibility study has already been successfully completed. As far as the composite materials are concerned, it must be considered that they are made by an inhomogeneous structure with a basic matrix reinforced by fibres and/or particles. The more frequent defects may be due to the presence of intrusions, bubbles, broken fibres and so on. It is well known that this kind of material is very difficult to test. Indeed, they are bad conductors, so that eddy current techniques are not useful while the X-ray techniques can perform badly due to the wide variation in elemental composition of the materials. More sophisticated techniques have been exploited (infra-red thermography, ultrasonic tomography etc.) but those materials containing particles and long fibers still give unsatisfactory results. The particular sensitivity of neutron techniques to light elements can solve the above problem as was shown in the technical bibliography.
Cooperation with industrial partners has already started, and a relevant German industry (Siemens AG) has declared officially its interest in the work to be carried out during the programme.
It is worth noting that a German federal institute for material research and testing will participate in the programme.
The present work programme has a very interesting impact on the European market of advanced productions. The main advantage is clearly the improvement of the quality of several European products in the mechanical area (cars, refrigerators) as well as the high-technology field (aircraft production, microelectronics, ceramics). A reduction of the cost of maintenance procedures will further help the penetration of European industries on the market.
Two different approaches are foreseen for the industries interested to exploit the advantages of these advanced techniques for material analysis: either they establish their own facility based on a pulsed source (being European producers in a leading position thanks to the completion of the programme) or they can ask an external service-provider to carry out the measurements. The latter possibility is particularly suited for those small-scale productions that cannot justify setting up a dedicated facility. The provision of these irradiation and measurements services could be an interesting possibility of funding for several specialized research centres currently in trouble, particularly in the EC and NIS countries.
D.ORGANIZATION END TIMETABLE
The activities of the already existing European Neutron Radiography Working Group (ENRWG), that has been developing and disseminating neutron techniques for the last five years, constitutes the basis of the Action. However, the enlargement of this informal group towards all the interested partners, with a particular concern towards other partners interested in the study of materials, will be a major concern.
A general meeting of the partners will be held at the very beginning of the Action. This meeting will specify the technical needs to be fulfilled for the applications foreseen. After this initial meeting, the activity of partners will continue in seven working groups in the two areas. Each group will be chaired by a chairperson nominated by the MC; the chairpersons will coordinate the activities of the groups and will report directly to the MC at scheduled meetings. The four working groups of the Instruments area will deal with the development of improved sources (WG Sources), the development of related detection equipment (WG Detectors), the production of novel software specially devoted to image processing, enhancement and visualization (WG Soffware) and the standardization of the irradiation procedures (WG Standardization). The WG Sources and Detectors will then merge and work to achieve the best matching of sources and detectors characteristics. The WG Software and Standardization will continue for another 6 months, after which they will merge into the previous WG and a full characterization of the achieved performances will be carried out.
On the Materials side, a WG Analysis will study the optimization of the testing procedures and protocols to be followed for better performances while working on main facilities already available through the COST countries. The activity of the WG Development will be based on a continuous exchange of knowledge and expertise with the WG analysis and is aimed at:
- reducing the corruptibility by corrosion of metals to be used by mechanical industries;
-increasing the homogeneity of ceramics, being in many cases the performances of these materials seriously affected even by a reduced fluctuation (lower than 2%) of the density even on a long range. These defects are as dangerous as the traditional cracks and intrusions but are much less detectable with conventional techniques;
- improving the capability of correctly driving lubricants into engines by optimizing their physical characteristics. New recipes could be exploited by studying the diffusion of components during the usual duty cycle. The easier visualization of the path of lubricants inside engines is the key point to achieve this result.
The WG Other Techniques will from time to time monitor the achieved performances with respect to other NDT techniques and will exploit every possible joined application of these non-nuclear techniques with the neutron imaging.
A general middle term meeting will be held to report on the status of the action and to plan the subsequent activities, that will be targeted exclusively to the applications. This part of the activity will be carried out through a different WG which deals with the study of the corrosion and the development of more resistant metallic structures, the development of new chemicals and lubricants, the testing of civil engineering structures and the development of advanced ceramics and composites respectively. On these topics, the refinements that have been already achieved on small-scale productions will move to routine activities, while new improvements are envisaged up to the third annual meeting. Finally, the last year will be employed to transfer more extensively the results to routine productions. A complete report of the results achieved will be made at the final report meeting after 54 months from the beginning of the Action.
Short-term scientific missions will be made when appropriate, the MC deciding on the allocation of funding.
E.ECONOMIC DIMENSION
The following COST countries have actively participated in the preparation of the Action or otherwise indicated their interest:
Italy, France, Germany, United Kingdom, Slovenia, Switzerland, Hungary, Czech Republic, Poland.
On the basis of national estimates provided by the scientists representative of these countries and taking into account the coordination costs to be covered by the European Commission over the COST budget, the overall cost of the activities to be carried out has been estimated at ECU 18,5 million at 1996 prices. This estimate is valid on the assumption that all countries mentioned above, but no other countries, will participate in the Action. The main problems that limited the setting up of a common project have been the lack of coordination and limited funding for such a task. This can be overcome by joining a COST action; indeed no centres have had funding to provide the coordination among researchers until now, while the COST framework is properly devoted to providing and funding coordination efforts for concerted action. In the opinion of the proposer, the above project fits at best in the scope of the COST cooperation; moreover, opening a new COST action is also a good way for encouraging new partners to join the group and then to complement its already existing expertise with the result of increased efficiency.
Furthermore, this framework seems perfectly suited to cooperation that encompasses several institutions from both the academic and the industrial field that, in some cases, wish to retain their independence concerning the funding of basic research expenses.
Part II - Additional Information
History of the proposal
The European Neutron Radiography Working Group was founded 5 years ago. It is devoted to the study and diffusion of the neutron imaging techniques as a powerful tool for industrial non-destructive testings. The group is now composed of many scientists and engineers from both the academic and the production world and there is major participation from Central and Eastern Europe countries. Within the group, different types of expertise are available in the various fields of non-destructive testing employing neutron sources. The sharing of such valuable expertise has taken place during periodic meetings held annually, but effective scientific and technical cooperation has tended to occur only on a person-to-person basis. This has reduced the effectiveness of such a group. In the preliminary part of the work the following institutions and scientists played a relevant role in the drafting of the proposals:
Proposer:Professor Franco Casali
Dipartimento di Fisica
Universita di Bologna
40126 BOLOGNA
ITALIA
Ph: +39-51-253274
Fax: +39-51-247244
France:Dr Guy Bayon
CEA/DRE/SRO
91191 GIF SUR YVETTE
Ph: +33-1-69083881
Dr Serge Cluzeau
SODERN
20, Av. Descartes
94451 LIMEIL DE BREVANNES
Ph: +33-1- 45957066
Hungary:Dr Marton Balasko
KFKI Atomic Energy Research Institute
AEKI
POB 49
H 1525 BUDAPEST
Ph: +36-1-1699499
Germany:Dr Christian Rausch
Fakult"t fur Physik E21
D-85747 GARCHING
Ph: +49-893-209 2185
Slovenia:Dr Joze Rant
J. Stefan Institute
POB 100
SLO-61111 LJUBLJANA
Ph: +386-61-1885357
Russian Federation:Dr Yuri Tarabrin
Kurchatov Institute
MOSCOW
Czech Republic:Dr Frantisek Peterka
ATG and Technical University of Praha
17100 PRAHA 7
Ph: +42 2 6880111
Great Britain:Dr Nigel Boulding
Oxford Instruments
Osney Mead
OXFORD OX2 ODX
Ph: +44-1865-269500
Preliminary work programme:
In the opinion of the drafting group, the technical annex is complete and could also be submitted to non-experts. However, it is worth explaining the basic principles of the neutron imaging and its physical basis (covered in more detail in [29]). Basic instruments for those techniques are a suitable neutron source and a detector for the evaluation of the neutron transmission factor through the object to be tested. Such a factor obviously depends on the thickness of the object but also on the absorption coefficient of its constituent elements. The cross-section for thermal neutrons of the natural elements is shown in the following figure; the most important feature is that no continuous function of the atomic or mass number can satisfactorily fit the cross-section. This is clear if it is considered that the neutron-to-matter interaction occurs only at the nuclear level, while the above numbers are related to the electronic shell that does not interact at all with neutrons.
As a neutron source, thermal neutron reactors have been widely used because they are able to produce high intensity neutron beams, so allowing the reduction of the time required for the completion of the analysis. These sources are available in the form of research reactors (having total power not exceeding few MW) or non-reactor facilities in several European countries of the EU (France, Italy, Germany, UK, Austria) as well as in other countries of the COST co-operation such as Hungary (a high flux reactor is located in Budapest), Slovenia (at the J. Stefan Institute near Ljubljana), and the Czech Republic or accelerator sources such as in Switzerland at the Paul Scherrer Institut.
The other element in the proposed system for the detection of corrosion is a neutron image detector. In the scientific and technical literature, several materials (i.e. gadolinium sheets, cerium activated glasses or zinc sulphide powder coupled to a neutron absorber to form a scintillating screen) have been proposed for this purpose and are now in use at the main research centres. More advanced detectors would be able to reveal with increased accuracy the early traces of corrosion thanks to an increased sensitivity to small and localized fluctuations of the neutron pattern emerging from an object. Many new solutions should be tested so as to determine which is the most suitable detector for each application. Studies should begin aiming to improve the efficiency of actual neutron detectors and their spatial resolution; at the same time more advanced detectors could be developed as well. A study on the applicability of improved electronic components such as CCD, Multi-Channel Plates (MCP) and so on will greatly help the optimization of improved neutron detectors. In any case, the adoption of innovative detectors will greatly improve the achievable results both in terms of the spatial resolution of the final image (the final goal is to detect small grains of corrosion down to 20 æm diameter while it is currently not better than 150 æm when acquiring digital images) and of the minimum detectable hydrogen concentration. This latter capability is not limited to the detection of hydrogen but involves the mapping of its distribution inside the material.
The technology and expertise for the development of advanced detectors have already been developed in European Western and Eastern countries: for example in Italy an interesting activity on scintillating fibres is the outcome of the long-term expertise in the craftsmanship of high quality glasses near Venice, while in the Russian Federation the production of radiation-resistant fibre optics to be used in neutron environment has been well established for many years. Indeed, Kurchatov Institute is the leading centre in the optical fibres radiation testing. It has vast experience in different fibre optic devices development for specific Nuclear Environment. At the same time, the robust cooperation between Japanese research centres and the Czech Technical University of Prague enables the latter to develop improved scintillator screens following the advanced techniques defined in Japan.
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Italy