SIMoNET Seminar Report
Report on SIMoNET seminar
held at UCL (London) on April 2nd 2003
Bill
Dover opened the meeting welcoming those attending and gave a brief outline of
the SIMoNET programme. Brief descriptions of each of the papers presented
follows:
1.
Non-destructive evaluation of Marine Composites, Dr Janice Barton, School of Engineering Sciences, University of Southampton
Dr. Barton described the non-destructive evaluation techniques in use at Southampton and gave a list of current projects in this area. The presentation then moved on to describe the analysis of a joint in an RNLI lifeboat. The joint was of foam cored sandwich construction composite material and modelled the hull and a bulkhead in an RNLI lifeboat. The joint was hygrothermally aged in a specially designed chamber and tested periodically using thermoelastic stress analysis (TSA). The results showed marked differences between the aged and unaged specimens as a result of degradation of the sandwich skin. Dr Barton also showed an example of how TSA could be used to quantify sub-surface damage in composite materials. This used a coupon of material with simulated damage and the results showed that the thermoelastic signal was equal to that for a material with a reduction in cross sectional area equivalent to the damaged specimen.
Dr
Barton then moved on to discuss the activities of SIDAnet. These are summarised
in an article included in the May 2003 Simonet Newsletter. The next meeting
of SIDAnet would be on the 21st
May 2003 at Warton Aerodrome. She suggested that anyone interested should visit
www.sidanet.org for more details.
2.
Composite Monitoring, West
Mill Bridge, Dr.Lee Canning, Mouchel
Dr.
Canning described the monitoring of the composite Westmill Bridge in Oxfordshire,
which formed part of project ASSET, a four year EU funded programme which
started in 1998. Members of ASSET include the Oxfordshire County Council,
Skanska, Mouchel, KTH and IETCC. The last two being a Swedish University and
test house. He described the new single carriageway bridge with a span of 10m .It
had on-line monitoring equipment installed, including electrical resistance strain
gauges, dial gauges and optical fibre Bragg sensors developed by City and
Cranfield Universities. The main sensors were located on the main beams at mid
span to measure any sagging or hogging. There were also sensors on bonded
joints. Sensor protection during manufacture and installation were key items.
An on-site data logger has been installed. Load test results from a
28 tonne lorry traversing the bridge were shown and it was encouraging that the result from the optical fibre gauges matched
those from strain gauges. Real time access to the bridge structural data is
planned, as well as a web camera view of the bridge in operation.
3.
Acoustic Emission monitoring of Advanced Composites, Chris Rowland, Pancom
Chris
Rowland described acoustic emission testing and its principles, mainly used for
GRP tanks and vessels and more recently CFRP type structures. Examples of testing of aero-engine parts, satellite
structures, FI racing cars and hip joints were shown. Glue line unzipping was a
recognised problem and examples of this were also shown.. F1 car components
were proof tested for each race. The basis of the 'felicity ratio' (FR) was
described; results less than unity indicating permanent damage. Sites of failure
and damage were shown linked to values of the FR. Difficulties with composite structures
using AE were high attenuation, a noisy material and the need for a history to
determine levels of significant damage. Overall AE was a proven technique,
which was now being transferred to CFRP type structures with some success.
4.
ISO 19902, Structural
Integrity Management, David Galbraith, Galbraith Consulting.
David
Galbraith described the background to the ISO standard development, and related
offshore standards such as API. Sixty eight ISO standards in the oil and gas
field had been published to date. He outlined the development of the structural
standards in the ISO 19900 series, noting that 19901-3 will be the world's
first offshore standard for topsides. ISO 19902 is concerned with fixed
offshore structures and expected to reach the DIS stage this June. This
standard includes a strategy for structural integrity, which was briefly
described. In terms of inspection the ISO standard includes a 'default'
inspection programme, as well as an alternative system for structural integrity
management (SIM). He showed the flow chart for SIM, which includes steps such
as data collection and updating, evaluation, development of inspection strategy
and an inspection programme. Inspection types include baseline, periodic, unscheduled
and special. The periodic type includes four levels, ranging from a basic level
I (mainly above water visual) to level IV (underwater NDE). The draft standard
also includes a section on personnel qualifications, not only for those
involved in the actual inspections but also for those planning the inspections
and undertaking related structural engineering. The draft standard is available
on a web site : www.galbraithconsulting.co.uk/iso.
5.
PHOTOGRAMMETRY – “another
way of looking at it”, Rory Stanbridge, Marketing Director – Photarc Surveys Ltd.
Rory Stanbridge noted that for many years,
photogrammetry has been the accepted method of data collection for topographic
mapping through the guise of aerial survey.
Although a
Prussian architect pioneered the technique in the 1860’s, it is only in recent
years that increased awareness of the technique has enabled it to establish
itself in the minds of engineers. Even today however, many professionals are
blissfully unaware of the potential use of photogrammetry for measurement
solutions.
Herein
lies a problem. If potential users do not know about it, how can we persuade
them to use it? The majority of academic institutions are either unwilling or
unable to teach anything but the most basic aspects of photogrammetry. The site
of this conference today being one of the few institutions actively teaching
the science. For many years it has been perceived as expensive, difficult to
understand, inflexible, etc. However, things are changing.
The
accepted use of the technique in aerial survey has been extended now to
architecture, engineering, forensic works and may other applications. The
advent of digital photography has also meant that the photogrammetry systems
have become more compact and therefore more portable. Companies like UK Perspectives
and Get Mapping have published aerial photography on the Internet, making it
more widely available and accessible for everybody.
Although
the technique is most commonly associated with the use of stereoscopic
photography, multi image software has now enabled measurements to be taken to
sub millimetre accuracy by non-photogrammetrists. Cheap desktop software
packages for the production of wire frames have also become widely available
bringing photogrammetry to a whole new audience.
He
explained that photogrammetry should not be looked upon as anything other than
an extra component in the toolbox of those people requiring good and accurate
data, preferably 3D to make the most advantage of its capabilities.
6.
Monitoring Structures with
close range photogrammetry, Stuart Robson, UCL
Stuart
Robson described the use of close range photogrammetry for measuring dynamic
changes in both engineering and medical structures. The process involves taking
a network of images, with a set of calibrated digital cameras. From this it is
possible to compute the required 3D spatial information, using at least two
lines of sight. With two cameras the accuracy was about 1mm. Examples of data
processing were provided, including the measurement of images, optimum location
of cameras etc. Measures of deformation could be made from two sets of images at different stages. . He explained that
calibration of the cameras was an important part of the process, using a
special calibrated wall at UCL. He provided examples of images of cracked
tubular columns, soil samples in a centrifuge, an abdominal aortic aneurysm.
Using 25 frames per second imaging the wing motion of an experimental micro-flight
vehicle had also been studied.
7.
Risk from Breaking Waves,
Prof. Rod Rainey, Atkins Process
Prof.
Rod Rainey explained that breaking waves are one of the main hazards to
floating bodies. For small vessels, like fishing boats, the risk is capsize –
the loss of the trawler Helland Hansen,
for example, is well-established as being the result of capsize in a breaking
wave. For large vessels like tankers and bulk carriers, the risk is of impact
damage to hull plating. Bow damage is reported not infrequently - a
well-documented case is the tanker Wilstar,
where the bow damage was very severe. A more recent case is the oil production
ship Schiehallion, where the bow
plating was ruptured over a small area well above the water line, by a pressure
which must have been about 75 tonnes/m2. Even
more recently, the tanker Prestige
appears to have been damaged by a breaking wave, which led to her subsequent
loss and the pollution of the Spanish coastline last winter.
To predict the likelihood of
such damage it is necessary to know the probability of encountering a wavefront
which is sufficiently steep. According to the ordinary linear theory of
irregular waves, the water surface elevation is described for example by the
Pierson-Moskowitz spectrum:
where wm is the mean frequency (1.408 times the peak frequency
wp in this case), and s is the average steepness based on it
(i.e. significant wave height ÷ length of wave of frequency wm), taken as 0.05 by
definition of the P-M spectrum. In deep water the transfer-function between
water surface elevation and water surface slope is iω2/g so the spectrum of water surface slope is:
Since this spectrum behaves
as ω-1 as ω → ∞, its integral is
infinite, and so the significant water slope is infinite too. Thus no useful
predictions of wavefront steepness can be made with conventional sea spectra,
all of which share this property.
Nor is it possible to make predictions
with fully-non-linear computer simulations of the sea surface, since they are
far too computationally-intensive to generate the required statistical data
over millions of waves.
Prof. Rainey described an 'alternative'
approach to the problem, based on a different definition of the 'water
surface'. He claimed that this enabled the breaking wave to be predicted, which
was an important breakthrough. In discussion to the paper Prof. Rainey explained
that further work needed to be undertaken with this model but in principle it
would allow through simulation an estimate to be made of the expected frequency
of damaging breaking waves, which is important for offshore operations.
8.
SIM in a Marine Environment,
David Sanderson, MMI Engineering
David
Saunderson outlined the background to flooded member detection and integrity assessment
of jacket type structures using this technique. He then described the development
of an on-line device for detection of flooded members, in a project about to start
with funding from NESTA, HSE and BP. The device consists of a passive detection
unit that is inside tubular members of jackets. When it is activated by becoming
wet a signal is emitted which can be detected externally. A preliminary feasibility
study has shown that signals can be transmitted acoustically to a receiver.
Small scale trails have been undertaken on a model of the Thistle jacket which demonstrated
that acoustic transmission was feasible. MMI will be providing the technical
guidance with UMIST developing the laboratory model and prototypes. In due course
field testing of the device would be undertaken and a suitable site still had
to be located. One of the objectives of the programme was to select the optimum
modes and frequency for effective transmission through the structure,
recognising that losses do occur at nodes. The power requirements also have to
be established, together with a suitable power source to provide long term energy.
In discussion it was noted that the device is primarily for fitting to new
structures, and individual devices would need to be coded in some way so that
their position could be identified if a signal was received.
9.
'Memory Track' - complete stress monitoring system, Dr.
F.Brennan, UCL
Dr.
Brennan explained that AMEC SPIE Rail (UK) with University College London and
FIOSTEC Limited had brought Stress Memory Technology to the Rail Industry
promising to optimise rail maintenance and maximise safety. Continuous Welded
Rail (CWR) needs to be installed under tension to protect it from compressive
buckling stress as the rail temperature rises in summer. By pretensioning rails
the thermal expansion induced compressive stresses are significantly reduced so
that the track ballast, sleepers and fastening components, can safely contain
these. Conversely, the pretension level must be optimised so that it does not
contribute to rail breaks under excessive tensile stress when rails contract in
low winter temperatures.
Dr.
Brennan said that the Stress Free Temperature (SFT) or Neutral Temperature is
the rail temperature at which the rail is the same length as it would be in an
unrestrained state and at which, therefore there is no thermal force present.
The rail is pulled by hydraulic rail tensors to achieve a SFT of 27ºC (in the
UK). Track buckles and rail breaks are
both potentially dangerous and monitoring of rail stress is crucial to the
safety of the railway. Network Rail approval has been given to trial "Memory
Track" for monitoring of SFT on the UK rail network from May 2003.
Future
production units are likely to have the following features:
·
·
Provide a report of tensioning operation
·
·
After installation, monitor SFT and temperature every hour
·
·
Automatic hot weather patrolling and site monitoring
·
·
Lifetime of ten years (including power and data storage)
·
·
Wirelessly communicate stored data to passing vehicles
10.
New Regulations for
Structural Safety, Brian Neale, HSE
Brian
Neale outlined new regulations for structural safety, as part of the
'Workplace, Health & Safety Welfare Regulations, which came into force
September 2002. . These concern the terms 'stability' and 'solidity' and as yet
no guidance has been made available. The new regulations follow from the 1989 EC
Workplace Directive, concerned with the safety of people in buildings. The regulations
apply to all parts of the structure, with an emphasis on land based buildings, except
where regulations already exist. He described the main requirements for an effective
management of structural safety, including a risk based approach, monitoring and
a life care plan. Brian Neale said that an information sheet was currently
being prepared, including definitions of solidity and stability, on which he
was seeking further input.
11. Properties of Guided Waves in Composite Plates, and Implications for
NDE, Peter Cawley, Dept. of Mechanical Engineering, Imperial College, London.
Professor
Cawley explained that guided waves are potentially very useful for the rapid
NDE of plate structures. In the
aerospace industry in particular, they have been proposed for the structural
health monitoring of carbon fibre skin panels, in either an active or a passive
configuration. In an active configuration a guided wave is deliberately
generated and then later received after it has travelled through the structure;
in a passive configuration an acoustic emission sensor is used to listen to
sound created by the structure itself.
The successful development of these ideas depends on a good knowledge of
the properties of the guided waves, yet it has been found that the properties
of guided waves in such materials are very much more complicated than those in
a simple metal skin. The complications
include steering of the directions of the phase and group velocities by the
anisotropy of the plate, and attenuation because of damping of the matrix
material and scattering by the fibres.
The authors (Mike Lowe, Peter Cawley, Marc Deschamps, Guillaume Neau
from Imperial College and LMP, Université de Bordeaux) have studied these
phenomena both analytically and experimentally, and have presented some of that
work at previous QNDE meetings. Professor Cawley presented an overview of the findings and identified the key
implications for practical developments of structural health monitoring
techniques.
12.
Closure
Bill Dover closed the meeting , thanking those for presenting and those who had attended. Another seminar would be held in the autumn - and suggested that a watch is kept on the SIMoNET web site for details.
J.V.Sharp,
23.7.03