Technical Presentations at the October 2003 Meeting
2.1 ‘Rules for, and Applications of, Marine Composites’, John Howson (Lloyds Register)
Resumé: Special
Service Craft rules have been developed for craft made of composite
materials and software based on these rules is available for design
purposes. For the rules, a
class notation is used and
service area of operations is defined.
Materials must be approved and a list of approved composites can be
found on the Lloyds Register website (www.lr.org).
Inspection of composite
craft involves tap testing to check for delamination, thickness
measurement and thermography to determine integrity of structure. CT
2.2
‘Inspection of GRP and composites for marine applications’,
Gary Penney (TWI) & ‘State-of-the-art techniques for the
ultrasonic inspection of large area (marine) composite structures’,
Richard Freemantle (NDT Solutions)
‘Inspection
of GRP and composites for marine applications’, Gary Penney (TWI)
The Non-Destructive Testing (NDT) of composites is a
very complex subject. For many years this area has not been given due
consideration, as has been the case for metallics. In essence there is no
one tool that satisfies the numerous inspection challenges provided by
composites. The variables in terms of material type, manufacturing process
and component design mean that often more than one NDT discipline must be
considered to provide an adequate inspection program.
Build damage, accidental damage and environmental
degradation are all mechanisms that can manifest themselves as defects in
a component. The are many defect forms of which delamination, fibre
misalignment, inclusions, porosity and matrix cracking are but a few.
Similarly, mechanical impedance, ultrasonics, D-site, thermography and
shearography are but a few of the NDT methodologies and disciplines that
may be employed to detect or characterise the defects. In essence there is
no panacea for the total inspection requirements, when composite materials
are utilised. Serious consideration of the many methodologies available
should be given before choosing a single or multi-discipline approach to a
particular application. (Gary
Penney: gary.penney@twi.co.uk)
‘State-of-the-art techniques for the ultrasonic
inspection of large area (marine) composite structures’,
Richard Freemantle (NDT Solutions)
There has been an increase in use of new composite
materials (glass fibre and carbon fibre) in many industries including the
aerospace industry for civilian aircraft, the power industry for wind
turbine energy and the marine industry for high performance mast and hull
structures. This is
driving a need for new NDT techniques that can rapidly scan large
structures and provide quantitative data on the structure integrity.
An example of this is in wind turbine energy where a guaranteed
product lifetime of 20 years is requiring NDT to be used at all levels in
the supply chain, from the inspection of raw materials through to
in-service monitoring.
In many of the applications there are common
requirements for the ultrasonic inspection of composites for porosity,
delaminations, foreign body contamination and fibre wrinkling. The traditional method of ultrasonic inspection
requires the use of a single point probe which is manually applied to the
structure. Although these
techniques are well established they are time consuming
and require a skilled operator. NDT
Solutions have developed a new NDT imaging technology based on medical
ultrasonic arrays housed within a rubber coupled wheel sensor.
This allows ultrasound beams to be electronically controlled and
moved rapidly over the structure to form an image.
The wheel can be manually applied or scanned over the structure
with an automated scanning system.
Examples of the application of the technology include real time imaging and reporting of impact damage, porosity sizing and adhesive bond characterisation. Future work in the development of this technology for composites includes bringing NDT and stress analysis techniques closer together to allow not only detection of defects but an analysis of the criticality of the defect on the performance of the structure. (Richard Freemantle: r.j.freemantle@ndtsolutions.com)
4.1
‘The Degradation of Polymer Composites in Marine Environments’,
Bill Broughton (NPL)
Polymer Matrix Composites are used because they have the properties of being light and strong, they can be tailored to suit the shape and application required, they have good environmental resistance, good fire resistance and good fatigue resistance.
Typical applications for PMCs are:
- Chemical (pipes, tanks, pressure vessels),
- Marine (boat hulls/decks/masts/propellers, buoys, submersibles)
- Offshore (platforms, high pressure + drill pipes, pump risers)
- Electrical/Power (insulators, pylons, switch gear, wind turbines)
- Automotive (car chassis/bumper/leaf springs/drive shafts)
- Aerospace (wings, fuselage, landing gear, helicopter blades)
Conclusions: Prolonged
or even short-term exposure to marine environments can cause irreversible
chemical and physical property changes in PMCs.
Degradation
rate is dependent upon chemical concentration, temperature and stress
levels. Such degradation
leads to loss of structural integrity and reduction of loading bearing
properties, giving shorter service life.
The rate of degradation is
often predictable, using reliable computer modelling methods.
4.2 ‘Composites used in Minesweepers’, Jody Dazel-Job (QinetiQ, Rosyth)
‘Modelling the properties of ageing marine laminates’, by G Kotsikos,
J Dalzel-Job, K Morris and J Mawella.
The effect of water diffusion
on the durability of glass reinforced isophthalic polyester and vinylester
marine laminates has been investigated through accelerated conditioning
tests by immersion in seawater at elevated temperatures. A gradual
reduction in the static and fatigue properties with increased immersion
time has been observed. For laminates conditioned at 40°C, water
diffusion can be modelled by Fick’s law. For 60°C conditioning the
Langmuir model seems applicable. Mechanical property degradation models,
based on the water diffusion coefficients determined from water uptake
measurements, are presented and can be used for predicting the
material’s long-term mechanical strength. Observations of damage
development during fatigue testing of composites affected by a seawater
environment have shown that the onset of delamination invariably takes
place after a decrease of stiffness of 20-30%. The modulus variation tends
to be governed by an empirical power law function relating strain
amplitude and number of cycles. This relationship is demonstrated for two
laminate materials currently used in the UK minehunter fleet. The Nuclear
Magnetic Resonance (NMR) technique, employed to image the diffused water
in the composite has shown that the water has penetrated deeper than
predicted by the Fickean process, by following a preferential path along
the fibre reinforcement network (wicking).