[Welcome] [Introduction to SaNDI] [The project partners]
Fibre reinforced composites in the form
of sandwich construction have been widely applied in naval structures in the Nordic
countries since the mid-1970’s. More recently other European navies have also used
this form of construction for certain applications. Sandwich construction enables low
weight to be combined with high stiffness and strength. However, despite a quarter of a
century of use the material is relatively new, and certain gaps are still to be found in
our knowledge of these materials and structures. One such gap concerns methods for dealing
with damage and defects caused for example by accidental impacts or inadequate production
quality. Limitations in the ability of existing non-destructive inspection (NDI) methods
to detect defects and damage in sandwich structures were identified in a previous project.
The problem of deciding how to deal with defects and damage that have been detected is
faced by all navies operating vessels with this form of construction.
The objectives of this programme are:
WP1: Technical co-ordination, integration of results, compilation of manuals
WP1 will provide overall co-ordination of the technical work in the project to ensure
that the separate work packages are carried out in a consistent manner and in such a way
as to achieve the overall goals of the project. The objective is to establish inspection
and repair strategies based on consistent treatment of detectable defects, inspection
costs, initiation and growth times, repair methods/costs etc. as treated in the separate
Work Packages WP2, WP3 and WP4. In WP1 the results of the separate work packages will be
integrated to provide these inspection and repair strategies.
The main deliverables from the project will be manuals, incorporating these strategies,
that can be used by the respective participating nations to develop inspection and repair
handbooks for specific vessels or types of vessels. The manuals will be divided into
sections on production control and damage inspection. A central task of WP1 will be to
plan and compile these manuals.
As part of the technical co-ordination, WP1 will include an introductory study of the
types and sizes of manufacturing flaws and in-service damage that are encountered in
practice with sandwich structures in naval ships. The navies and shipyards involved in the
programme will be asked to provide information based on their experience.
WP2: Characterisation and criticality of flaws and damage
The objective of WP2 is to obtain an understanding and quantitative description of the
initiation and growth of defects and damage in FRP sandwich structures typical for naval
vessels, as a basis for deciding whether and when a given type and size of defect or
damage must be repaired. This activity is the very core of the project and will have high
priority.
It will be necessary to limit attention to 3-4 material combinations (sandwich panel
lay-ups) representing typical applications in naval vessels. Furthermore, the studies in
WP2 will have to be focused mainly on a selection of the most important types of
defects/damage identified in WP1.
Studies will be performed concerning growth and criticality of these cases of
defects/damage. This joint research effort between organisations in the participating
nations will involve studies of face sheet, core and interface related damage, focusing on
both damage growth and residual strength, to enable criticality to be determined in terms
of damage size and location. Finally, an attempt will be made to establish how large the
damage of a given kind and location may be without the need for repair.
In the longer term it may well be desirable to find ways of improving the damage tolerance
of naval sandwich structures so as to reduce the costs of inspection and repair. Where
possible within the project framework the needs and possibilities for such improvements
will be identified and subjected to preliminary investigation.
WP3: Production quality control, inspection and condition monitoring
WP3 consists of two distinct types of activity: studies of the capabilities and costs
of existing NDI methods and development/improvement of NDI and monitoring techniques.
The NDI methods are expected to include ultrasound, shearography, X-ray back-scatter and
X-radioscopy techniques as well as some acoustic/vibration methods In-service monitoring
based on technology involving optical fibres impregnated with Bragg gratings will also be
studied with a view to detecting onset and development of damage.
Capabilities and costs of existing NDI methods:
The first objective is to establish which types and sizes of defects can be detected by
each existing method i.e. the sensitivity of existing NDI methods. For some NDI methods
much of this has been done in earlier projects such as EUCLID RTP3.8. However, some
additional work is needed to review the latest state-of-the-art and to extend the
benchmarking results to methods not considered adequately in these projects. Further,
recent developments in image/signal processing techniques will be taken into account.
The second objective is to establish the speed and cost of scanning for each method, for
conditions when the ship is in dry dock and also in service (where this is practically
possible) as well as during production.
Development/improvement of NDI and monitoring techniques:
The objectives are to provide NDI methods with improved capability (either by improving the sensitivity of existing methods or by developing new methods) and to increase the speed and reduce the cost of scanning. The studies will focus especially on improvement of scanning and image/signal processing techniques. Development of in-service monitoring methods (in particular the use of optical fibres) to detect damage will also be attempted.
WP4: Repair technologies and evaluation of repairs
Ideally, to enable a fully integrated inspection and repair strategy to be developed,
it is desirable to establish methods for repairing each type of defect or damage, the cost
of performing a repair, and the properties of the repaired structure in terms of residual
strength and lifetime. However, it will not be possible within the project to perform
comprehensive studies of this type.
WP4 will nonetheless aim to perform a limited survey of available repair methods for
defects/damage of various types and sizes. Some limited studies of novel repair techniques
will also be performed.
Norway
Denmark
Finland
Sweden
UK
Editor: Philippe Noury (DNV)
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