Download Smart Materials in the Marine Environment A State of the Art Review

Smart Materials in the Marine Environment
A State of the Art Review
Professor Ajit Shenoi (University of Southampton)
Adrian Waddams (British Marine Federation)
Ashutosh Sinha (Shipbuilders and Shiprepairers Association)
Version 2: April 2009
SMART.mat
SMART.mat is one of the nine network groups operating within the Materials
Knowledge Transfer Network (KTN), one of the Knowledge Transfer Networks
funded by the Technology Strategy Board.
The Materials KTN is a partnership between business, research and technology
organisations, designers and universities to provide UK manufacturing and
services with additional resources to compete in global markets. The Materials
KTN incorporates groups related to metals, plastics, packaging, ceramics,
technical textiles, design, composite materials, smart materials and materials for
transport.
For more information on the Materials KTN, visit www.materialsktn.net or contact
Dr Robert Quarshie, Operations Director, [email protected]
Key SMART.mat Contacts
Jackie Butterfield, Operations Director
Tel: 01302 380906
[email protected]
Professor Ajit Shenoi
University of Southampton
[email protected]
Adrian Waddams
British Marine Federation
[email protected]
Ashutosh Sinha
Shipbuilders and Shiprepairers Association
[email protected]
Contents
1.
2.
3.
4.
5.
Introduction
Snapshot of the Marine Industry
2.1
Ships and Boats
2.2
Underwater Vehicles
2.3
Offshore Structures
Opportunities for Smart Technology in Recreational Marine Craft.
3.1
Hull and Structure
3.2
Propulsion and Power Transmission
3.3
Internal Combustion Engines
3.4
Electric and Hybrid Propulsion Systems
3.5
Transmission, Propulsion Systems and Steering Gear
3.6
Dynamic Trim, Stability and Drag Reduction Systems
3.7
Sailing Craft
3.8
Domestic and Hotel
3.9
Payload and Containment
3.10 Navigation and Communications
Drivers
Smart Materials – Functionality, Examples and Potential Applications
Appendix A – Literature Review on Smart Materials and Smart Structures
A1
A2
A3
A4
A5
A6
Introduction
Features of Smart Materials
A2.1 Piezoelectric Materials
A2.2 Shape Memory Alloys
A2.3 Controllable Fluids
A2.4 Controllable Elastomers
Control Systems for Smart Structures
Marine Applications
A4.1 Noise, Vibration and Damping Control
A4.2 Structural Health Monitoring
A4.3 Self Healing and Repair
A4.4 Shape Control
A4.5 Piezoceramic Actuators
A4.6 Electrorheological Actuators
A4.7 Shape Memory Actuators
Summary and Conclusions
References
1. Introduction
The introduction of smart materials, surfaces and structures into products adds
enhanced functionality, which is the key to securing competitive advantage across
a wide range of market sectors. Uptake of smart technology is gathering pace but
many opportunities for exploitation remain.
The purpose of this report is to inform government and opinion formers about the
status of technology in the marine sector and the potential for advances using
Smart materials and systems.
The marine sector, dealing with ships, boats, submersibles, offshore structures for
power (renewable energy) generation and oil extraction/exploration, etc., has
typical product characteristics as below:
• Bespoke and of unique design
• Made without recourse to prototypes
• Low profit margins
• Cyclic nature of industry
• Low concept-to-delivery times
• Fabrication usually labour intensive
• Conservative industry and highly risk averse
2. Snapshot of the Marine Industry
The ‘products’ in the marine industry can broadly be categorised under three
headings, namely Above Water Vehicles (Ships and Boats), Under Water Vehicles
and Fixed Marine Structures. Each of these is examined in turn.
2.1 Ships and Boats
Ships and boats can be divided into two broad categories, namely transport
vessels (including cargo, container, oil tankers, passenger/cruise ships etc.) and
non-transport vessels (such as fishing craft, tugs, rescue craft, warships,
recreational craft etc.). An overview of the wide range of craft is shown in Figure 1.
Figure 1: Ship types
(from The Maritime Engineering Reference Book by A F Molland, Elsevier, 2008)
A further classification can be made with respect to the manner in which the inertial
loads are supported by: (a) buoyancy derived from the displaced water, which is
the case for all displacement craft as shown in Figures 2a-f; (b) lift `derived from
dynamic/forward motion of the vessel, such as planning craft or hydrofoils as
shown in Figure 2g-h; or (c) lift wholly or partially derived from injected air cushions,
as shown in Figure 2i-j.
Figure 2a: Container ship
Figure 2b: Oil Tanker
Figure 2c: Cruise ships
Figure 2d: Warship
Figure 2e: Lifeboat
Figure 2f: Tug
Figure 2h: Hydrofoil craft
Figure 2j: Surface effect ship
Figure 2g: Planing craft
Figure 2i: Hovercraft
2.2 Underwater vehicles
These could be either manned or unmanned, with functions varying with the nature
of service. Manned submersibles could be used for naval purposes (Figure 2.2a)
or for tourism (Figure 2.2b). Some of the latter could be remotely operated (Figure
2.2c) and others autonomous (Figure 2.2d).
Figure 2.2a: Naval submarine
Figure 2.2b: Submersible for tourism
Figure 2.2d: Autonomous underwater vehicle
Figure 2.2c Remotely
Operated Underwater
Vehicle
2.3 Offshore structures
These could be used for renewable energy based power generation (e.g. wind
turbine assemblies) such as shown in Figure 2.3a or for offshore oil
exploration/extraction, a range of which are shown in Figure 2.3b.
Figure 2.3a: Offshore wind farm
Figure 7b: Range of offshore structure for oil extraction/exploration
3. 3. Summary - Smart Materials and Structures for Marine Applications
Many Smart Materials are still at the research and development stage and depend
on potential applications to justify further investment in their commercial
development. They may provide solutions to existing problems or new technical
possibilities that would otherwise not be feasible.
Some smart material
applications already exist in marine which is generally regarded as conservative in
adopting new materials and technologies. Other sectors such as the automotive,
aerospace, railway and construction industries are more adventurous and have
more experience of smart materials that could be adapted for marine applications.
To encourage the more promising applications consideration should be given to:
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Main drivers for new material applications
Marine applications that could work in both the short and longer terms
Applications from other sectors where the added stimulus of marine interest
could make further development more commercially rewarding
Priority applications for investment, and if so from whom
3.1 Main drivers for new material applications
In the marine industry, as with most others, design and manufacturing processes
and product performance are driven by many, often conflicting requirements.
These include:
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•
•
•
•
•
•
•
•
•
•
•
•
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Performance
Cost
Reliability
Durability
Safety
Sustainability
Amenity - comfort and convenience
Efficiency
Operator feedback and ease of control
Reduced load on structures and systems
Reduced wear and fatigue on components and systems
Increased service life/reduced maintenance/MTBF
Reduced whole life/operating cost/end of life recycle/dismantling
Advance warning of imminent system breakdown or failure
Environmental protection pollution
Noise and gaseous emissions
Improvements in meeting these requirements are what drive new product
development, and the changes may often be small leading to gradual evolution
rather than step changes in design which might present unacceptable risk because
of unknown longer term consequences, e.g. lighter structures that might be
stronger, but less durable than existing designs. Where new regulations and
legislation create a requirement for change this can accelerate the pace and
incentive to do so.
3.2 Marine Applications
By increasing awareness about smart materials and their potential benefits the aim
of this review is to encourage their wider application in the marine industry
covering a broad range of potential applications including large commercial ships,
offshore structures and recreational craft. The following types of smart materials
with some examples of existing uses and suggested applications are covered
within this review; see Appendix B for a fuller explanation and literature review.
Smart materials considered here can be summarised as:
•
•
•
•
•
Piezoelectric
Piezoceramic
Shape memory alloys
Controllable fluids - Eletrorhelogical and Magnetorheoloical
Controllable elastomers – similar to controllable fluids in responding to
electrical and magnetic responses
The range of applications so far identified includes:
•
•
Noise, vibration and damping control using variable rate materials in engine
and machinery mountings.
Sensors for systems monitoring and service information, including fibre optic
cabling.
These can be used for structural health monitoring using optical Fibre Bragg
Grating (FBG) sensors in composite structures such as hulls, rudders, masts and
booms in lifeboats and racing yachts, both for data collection during trials and inservice monitoring.
Self healing and repairing materials and structures, particularly composites, allow
barely visible damage in structures to self heal and prevent further propagation of
faults through the structure that could lead to more serious defects and possible
catastrophic failure. As structures are optimised and become lighter there may be
less redundant structure to rely on when damaged, and early warning and repair is
required when damage occurs, but regular inspections are not feasible on
submerged structures in regular use, such as large yacht hulls.
Self healing coatings (e.g. surface coatings and paints) to protect metal from
corrosion such as in offshore structures are already in use in aircraft where
corrosion of aluminium structures, especially around riveted joints is a major safety
and maintenance cost item.
Shape control of structures using self morphing and shape memory alloys are
another opportunity. Naval architects have always strived to reduce the resistance
of ships and boats as they are propelled through water, both to increase speed and
reduce the power and fuel needed. The ability to vary and optimise shapes when
underway according to speed and conditions by morphing is attractive and could
be applied to rudders, keels, propellers, hull trim wedges, and other hydrodynamic
parts that are critical to performance.
Where a change in surface shape is
required at different speeds this could be achieved by self morphing automatically
in response to inputs such as speed and pressure of water flow over the surface
that then responds by changing shape. Propeller pitch can be varied by moving
blades relative to the hub, but the blade shape remains fixed and efficiency can be
compromised. As a possible alternative for controllable or variable pitch propellers,
self morphing could alter blade shape as well.
Actuators that are Smart could also be function to vary hydrodynamic
characteristics, eg by mechanically moving the propeller blades relative to the hub
to alter pitch in response to pressure on the blade surface.
3.3 Applications from other sectors
Smart “self healing” surface coatings are already used on aircraft to reduce
corrosion of aluminium structures, and on other structures where the application of
surface coatings is costly due to scale and accessibility of the structure. Self
healing could extend the effective life of the coating, and of the structure,
potentially reducing its whole life cost.
Piezoelectric coatings have been tried on aircraft wings as a de-icing solution, so
that when energised by electrical current to create a vibration any ice is loosened
and can fall off from the treated area. Similar techniques might be investigated as
an antifouling solution for marine craft and structures, in a similar way to ultrasound
that has been tried as a solution without the need for toxic or other environmentally
unfriendly coating solutions.
3.4 Priority applications
Design requirements that tend to become first priorities are those most affected by
regulation, legislation and sustainability such as environment, emissions and
carbon reduction. Legislation and regulation affects commercial marine activities
in particular, but increased regulatory pressure on the recreational sector from the
RCD and the need for fuel efficiency and “greener” solutions are encouraging
alternative approaches to boat design, including the development of hybrid
propulsion, lower resistance hulls, keels and appendages, higher efficiency sailing
rigs, more effective anti-fouling paints and surface coatings, waste water
management and disposal, gas and noise emissions from engines, ease of repair
and maintenance, and safer operation, navigation and control.
Product innovation and market leadership must be maintained for companies to
compete in global markets, and product differentiation especially for high value
luxury products is often key to sales growth particularly where a perceived high
technology edge is important. In the luxury yacht sector market leading features in
the accommodation and equipment are as important as the structure and
propulsion system. This offers scope for self cleaning glass and surfaces, smart
control of heating, ventilating and air conditioning systems, and novelties such as
ambient lighting, self adjusting window blinds and so on.
Given the above and the known range of materials under development surface
coatings including anti-fouling paint and drag reduction improvements would
appear high on the priority list for further investigation and development using
smart materials.
Appendix A provides a detailed background to Recreational Marine Craft and some
opportunities for applying technical solutions based on smart materials and
systems.
Appendix B is a literature review on smart materials and smart structures with
more detailed technical review of the materials and some possible marine
applications.
Appendix A: Opportunities for Smart Technology in Recreational Marine
Craft
The following overview of the main systems and equipment used in recreational
marine craft is intended to illustrate the wide range and therefore the very
significant scope for the application of smart materials and systems. Many of the
design principles, functional requirements and characteristics are similar to larger
craft and ships. Recreational craft by definition include all vessels from the
smallest canoe or dinghy to speedboats and motor cruisers and the similarly
diverse range of sailing craft of overall length up to 24 metres. Other craft included
in these considerations are large motor and sailing yachts up to 70 or 80 metres in
length, so called superyachts.
A.1 Hull and structure
Hulls, decks, superstructure and bulkheads are the essential primary structural
components that define a vessel’s shape, buoyancy, stability, water tightness and
its ability to withstand the loads and impacts it may encounter when used in its
expected operating environment. The shape, materials and construction methods
chosen directly affect the safety, performance, efficiency and initial cost of building
a vessel, its service life and the cost of maintaining it during that life. Any
component or system that becomes part of the hull and structure that influences
these characteristics must be designed to withstand these loads. Thus all hatches,
windows and gazing and other openings in the hull, deck or superstructure, such
that if they failed they would create structural weakness or openings exposed to
the sea and weather, must meet relevant design standards.
Recreational craft up to 24 metres overall length sold in any European Union
member state must comply with the requirements of the Recreational Craft
Directive (RCD). Larger superyachts must meet even stricter codes and standards,
such as the MCA Large Yacht Code. Similarly, working craft such as fishing and
small commercial vessels must comply with relevant regulations and meet
applicable standards. Other national and international standards for small craft
exist, such as those of the American Boat and Yacht Council in the USA. The
requirement to meet European and national requirements is usually satisfied by
compliance with international standards for particular elements and systems of the
vessel such as ISO standards. These cover all aspects of the vessel where there
is a need to satisfy essential safety requirements, such as scantlings that take
account of the strength of the materials and construction used for the hull and
structure, and in other areas such as propulsion, electrical and fuel systems.
Hull and structural materials used may include any or a combination of all of the
following:
• Fibre reinforced plastics using glass and other fibres (GRP and FRP)
• Aluminium Alloy
• Steel
• Wood
As materials for ship and boatbuilding these appear in reverse chronological order
with wood now much less used whereas it was for centuries the only material
available before ferrous metal, first iron and then steel, became the main
shipbuilding materials. Small craft were still mainly built of wood until glass
reinforced plastic was developed after WWII and from the mid 1960s this rapidly
took over from wood and remains the material most used. More advanced
composite materials using a variety of resins and fibre reinforcements are now
gaining ground as alternatives to the original polyester resin and glass fibre mat
used in GRP. One attraction of GRP over other materials was and remains its
ability to be moulded to the required shape so that mass production became
possible turning out large numbers of identical boats from a single mould, and it
requires less skill than traditional boatbuilding in wood.
GRP is also strong, watertight, long lasting and light. Steel remains popular for
work boats and inland waterway craft where durability, ease of construction and
simple hull shapes make it attractive. Marine grade aluminium alloy is also popular
in many applications such as in larger sail and motor yachts or even small utility
boats where it offers ease of manufacture, durability and light weight.
Surface finishes and coatings may be applied to all these construction materials to
protect them and provide attractive appearance. In the case of wood it is essential
to preserve the material and protect it from water penetration by sealing with paint,
varnish or resin. Some wood such as teak may be left untreated on decks and
coamings, but all wood that is immersed must be coated. Wood remains popular
for interior bulkheads, doors, and joiner work and a variety of finishes is available.
Steel must be corrosion protected and can be coated using compatible primers,
surface fillers and paint as a finishing system to ensure long lasting protection,
cosmetic appearance and long intervals between costly repainting. Aluminium
alloys need not be painted in all instances as the marine grades are self-protecting
forming a layer of surface oxidation that resists further corrosion attack. Small
aluminium dinghies and skiffs are often sold as “low maintenance” with a bare
metal finish making the craft attractive for users not keen on applying regular polish
or paint.
GRP and other fibre reinforced plastic (FRP) composite materials may not require
any surface finish when new. Many early GRP boats were advertised as
“maintenance free” protected by a hard, shiny outer resin gel coat surface that is
very durable and weather resistant. This will retain its appearance for many years
with regular cleaning, polishing and wax protection, but if left uncared for it will fade
and eventually need painting. Underwater surfaces of GRP are particularly
vulnerable to a form of degradation called osmosis that results in many older GRP
boats suffering blisters and a pox like appearance that requires treatment and
refinishing by cutting back the surface, filling and re-finishing usually with epoxy
resin systems. Osmosis is now better understood and largely avoidable through
care during manufacture and with suitable choices of gel coat and laminating
resins. Painting still becomes desirable when GRP surfaces loose their shine or
suffer from accumulated abrasion and scratching.
To attach or secure vessels when towed, moored or anchored hull and deck fittings,
such as bollards, cleats, fairleads and safety rails need to be attached to the hull
and deck and able to withstand the loads applied without failure or detachment.
Craft such as tenders and lifeboats may need to be lifted using suitably attached
lifting points.
All ships and small craft left in seawater and some inland waters require an
antifouling paint to be applied to the immersed parts of the hull. The purpose is to
discourage marine growth and organisms from adhering to the surface and
reducing surface smoothness resulting in considerable additional drag that can
render craft unusable in extreme cases.
Sailing and power craft loose
performance and the latter also suffer significantly increased fuel consumption.
The chemistry of antifouling paints is complex and many of the products by their
nature are toxic and unpleasant encouraging a constant search for more effective
and environmentally friendly solutions, including other types of surface coatings
and finishes and ultrasonic systems to discourage organisms from attaching
themselves.
A.2 Propulsion and power transmission
The choice of propulsion for recreational craft includes internal combustion engines
and electric motors connected to a wide variety of systems for generating thrust.
Gas turbine engines are not covered here as they are employed mainly in larger
craft and ships. There is also growing interest in hybrid systems that combine an
internal combustion engine with a generator and electric motor offering some
environmental benefits. Fuel cells also offer an alternative power supply for
electric motors as primary or auxiliary propulsion, increasing their range according
to the fuel capacity for the fuel cell when used as an alternative or supplement to
batteries.
Sailing craft with or without auxiliary power also offer great scope for smart
materials, and at the high performance level sailing craft are now matching the
speed of powered craft and their range and endurance is of course unaffected by
fuel capacity. Examples of possible applications include smart materials in hull
structures and surface coatings, and in masts and rigging where embedded
sensors can measure strain, and feedback data when limits are reached allowing
loads to be reduced automatically through sail trim controls.
A.3 Internal combustion engines
Internal combustion engines can be generalised as either diesel (compression
ignition) or petrol (spark ignition). These can be four stroke or two stroke types,
although all the diesels used in recreational craft are now four stroke. Two stroke
petrol engines used in recreational craft sold in Europe and the USA are now
limited to some outboard motors and personal watercraft (jetski) where these can
comply with stringent emission requirements using advanced direct injection
technology, otherwise petrol inboard engines and outboard motors are four stroke.
The Recreational Craft Directive and many national, regional and inland authorities
around the world now require new engines in boats to meet noise and exhaust
emission regulations. This is much the same as for engines used in land transport.
Marine engines in recreational craft are mostly derived from automotive and
industrial engines. The technology in these applications usually meets or exceeds
marine regulations allowing successful adaption for marine use. Because of the
higher production volumes in these sectors and the R&D investment and technical
knowledge available from them, they are able to benefit marine applications where
the production volumes are smaller.
However, marine engines have particular needs and differ from land transport
engines in areas such as their exhaust, cooling, fuel, lubrication, electrical, air and
control systems. Operating at sea and remote from outside assistance they must
also be even more reliable by ensuring that their systems are accessible for
maintenance and able to function in harsh conditions, and generally at higher
continuous power outputs for longer periods. Engine systems include the following
main groups
• Exhaust systems (wet and dry)
• Engine cooling systems
• Engine instrumentation and controls
• Electronic management systems for performance and emissions control
• Fuel, water, air and ventilation systems (convection and fan assisted)
• Engine electrical and starting systems
• Fuel (including filtration and polishing)
• Hydraulic tanks and system
• Compressed air tanks and systems
A.4 Electric and hybrid propulsion systems
Electric motors offer silent and clean running, ideal on inland waters, and were first
popular a century or more ago when internal combustion engines were less
reliable and powerful. Usually DC motors running on rechargeable batteries they
offer simple installation, but low endurance between recharging which requires
access to shore side facilities. Again mirroring automotive developments these are
a minority choice, but with improving motor efficiency, battery performance and
energy capacity they can be attractive in certain applications, such as inland
sheltered waters with shoreside charging facilities.
The hybrid solution using an internal combustion engine and an electric motor
offers the benefits of electric power with the independence of the internal
combustion engine. This drives a generator that eliminates the need for access to
a recharging supply on shore. Different hybrid arrangements are possible so that
the IC engine/generator can also be coupled to the propeller drive and thus in
combination with the electric motor provide higher total power when it is needed or
until stored energy in the batteries is exhausted.
Fuel cells as the electrical energy source remain less developed as an option for
powering boats, but offer great scope if their current limitations of low power
density and cost can be overcome. Battery and fuel cell technology remain the
limiting factors in providing the required power density for electric propulsion,
whereas electric motors can produce very high power and torque from compact
units. Larger vessels already use diesel electric propulsion where their size and
operating requirements allow installation of several engine/generator sets that can
be run as required to provide variable power steps to electric motors directly
driving propellers or propulsors. This allows the engine(s) in use to operate at the
most efficient load for both fuel efficiency and emissions, and has benefits in more
flexible location for the engine room and by avoiding the need for long shaft runs.
Cruise ships and ferries use such systems. In general the components and
systems found in electric and hybrid systems include:
• Motors ac/dc
• Controllers
• Batteries and super-capacitors
• Chargers and inverters
• Fuel cells
A.5 Transmission, propulsion systems and steering gear
Factors affecting the choice of power transmission and propulsion system include
space and accommodation layout, the power to be transmitted, the type of craft
and its expected use. All propulsion systems generate reactive thrust by pushing
water away from the desired direction of travel, generally using screw propellers,
waterjet pump units and paddle wheels (the latter now being obsolete). For
completeness air screws moving air above the water are also used for boat
propulsion, but only for limited special applications such as swamp boats and are
not therefore considered further here. In all cases steering gear comprising a
rudder, bearings and associated mechanical, electrical and hydraulic actuators and
controls or some other means of directing thrust for the steering function is
required.
For safety and control the propulsion system can usually be disconnected from the
engine whilst it is running, to provide neutral as well as forward and reverse thrust
using a gearbox or where no reverse is required a clutch, or both. Systems may
be designed for individual craft or supplied as packages or modules to assist
installation. The simplest system is a propeller shaft coupled inline to the engine
and gearbox on the same axis with the shaft emerging through the bottom of the
hull or keel. Depending on hull shape and propeller size the shaft may be angled
downwards for clearance and the propeller may be fixed directly where the shaft
emerges from the hull or further aft with the extended shaft being supported just
ahead of the propeller by a bearing bracket or strut attached to the hull. This
conventional system remains popular and is rugged and reliable, but has
performance and efficiency limitations on high speed craft, including loss of thrust if
the shaft is angled down.
The simplest self contained system is the outboard motor that is clamped or bolted
to the stern and once connected to fuel, electrical and steering systems as
required it is ready for use. The smallest units weighing about 15 kg are easily
carried and are truly portable power units, whereas larger engines with some
weighing more than 300kg and developing up to 350 hp remain attached to the
boat only to be removed for major servicing or replacement. With one or two
special exceptions that are diesel powered outboard motors are increasingly four
stroke petrol engines or direct injection two strokes that can meet exhaust
emission regulations. Small battery powered electric outboard motors are also
available for slow speed applications in sheltered areas. Power transmission from
the outboard power unit is by a drive shaft coupled to the engine crankshaft,
directly or through gears, both being mounted vertically and contained in a faired
aluminium alloy casing forming the drive leg with a streamlined underwater
propeller hub. This contains bevel gears that turn the drive through 90 degrees
and provide the forward, neutral and reverse function. The axis of thrust is
therefore in the horizontal direction of travel and thus most efficient, with the facility
to adjust the angle of the outboard motor and the line of thrust by tilting the
complete motor to create a trim effect. The tilt mechanism also allows the
propeller to be lifted from the water for beaching, clearing fouling or replacement.
Steering is by swivelling the motor about its vertical axis, so no rudder is required.
Various mechanisms exist for controlling the steering and trim systems, from
simple mechanical types to electro-hydraulic devices.
So called inboard outboard or stern drive units were developed in the late 1950s
from outboard motors to provide the benefits of the compact installation, ease of
control and performance of the outboard motor with higher powered inboard
engines. An inboard engine is mounted inside the stern and coupled to the
transom-mounted outboard drive leg by a shaft passing through the transom.
Stern drives have been refined and developed to handle much larger power and
torque than outboard motors thus enabling more efficient diesel engines to be used
up to several hundred hp and therefore suitable for use in boats up to about 14
metres length often in twin installations. Other refinements carried from outboard
motors include underwater exhaust outlets through the propeller hub. Many stern
drives now also use contra rotating twin propellers to increase the propulsive
efficiency and thrust and to eliminate the torque reaction that can cause lateral trim
and steering effects affecting boat handling and safety.
Pod drives have developed quite recently for recreational craft and again use a
streamlined underwater bevel gear box to provide a horizontal propeller thrust line
and steering by rotation of the underwater unit. Pod drives differ from outboards
and stern drives by being mounted under the hull rather than on the transom, with
the vertical drive shaft and exhaust passing through the hull and the tail of the
inboard engine close coupled to the drive unit. The whole assembly is mounted on
a robust double hull seal to ensure a long lasting and secure watertight installation.
Propellers are either rear or forward facing from the gear hub. Underwater exhaust
through the propeller hub is again used to reduce noise and bury fumes to clear
the boat. Contra-rotating propellers are generally used and the pod units when
mounted in pairs may be steered independently using electronic control to provide
vectored thrust for precise manoeuvring and positioning of the craft, and this can
be augmented by interfacing with an onboard Global Positioning System (GPS) for
holding station. Many large craft with conventional stern gear require bow and
stern thrusters for close manoeuvring in harbour. The vectored thrust facility of
pod units, and also some stern drives, eliminates the need for fitting separate
thrusters. Pod units up to 700 hp each are now available, but provide equivalent
performance to 900 hp engines driving conventional stern gear due to the
propulsive efficiency of the horizontal propeller shaft line, reduced appendage drag
and contra-rotating propellers.
Water jet propulsion units were originally created for small motorboats in the 1950s
and have been successfully developed for commercial and military craft and fast
ferries, and are almost universally used in fast personal watercraft. Generally
using the axial or mixed flow pump
principle water jet units have inlets flush mounted under the hull close to the stern
and the water flow is directed through the transom so that on a fast vessel it is
above the water surface when underway at planing speed. Steering and forward,
neutral and reverse thrust are controlled by guide vanes and a clamshell bucket
lowered to redirect waterflow. This eliminates the need for a gearbox or clutch in
many installations, although they are fitted where the facility to reverse water flow
in the unit casing assists clearing debris or inlet blockages.
Waterjets offer shallow draft and freedom from damage with no exposed sterngear
and they are safe to animals and people in the water, hence their use for personal
watercraft. In some high speed applications they are more efficient than propeller
systems. They are also quieter and less prone to vibration than propellers and
thus attractive for commercial passenger vessels, and can absorb very high power.
Although their useful thrust is generated in a relatively narrow operating speed
range and careful matching to boat and engine is required, they do not put undue
load on the engine as propellers can if the craft is overloaded or suffering from hull
fouling. Thus engine reliability, and ease of operation are positive attributes. As
with pod drives water jets in pairs or larger multiples can be electronically linked
and controlled, and often with a bow thruster, for vectored thrust and précis
manouvering using a single joystick control.
• Gearboxes, clutches and couplings
• Propellers, shafts and stern gear
• Stern drives and pod propulsion units
• Engine mountings
• Water jet units
• Thrusters
• Controls systems mechanical, hydraulic and electronic, plus interfacing for
manoeuvring and positioning
• Steering gear, rudders and steering controls
A.6 Dynamic trim, stability and drag reduction systems
Planing craft that overcome their bow and stern waves to lift and ride over the
water at high speed are sensitive to trim and heal. The resistance of the craft
increases rapidly as planing speed is approached, and if trim by the stern is too
great whether a function of load or hull design, the planing condition may not be
reached even with full power applied. Flaps or tabs extending at a small
downward angle from the base of the transom can be employed to generate lift to
the hull and reduce the trim by the stern sufficient for the craft to pass through the
transition speed to planing. This can assist making the transition from the static full
displacement condition to planing.
Some of the propulsion systems described above have a built-in adjustable trim
device to allow the trim of the craft to be adjusted using thrust. Other uses for trim
tabs are to alter hull trim according to sea state, for example to lower the bow to
reduce slamming in waves, and on smooth water to allow the bow to ride high and
reduce the wetted surface of the hull and resistance to gain speed or reduce fuel
consumption. Trim tabs are fitted in pairs or multiples symmetrically across the
transom and can be controlled independently port and starboard to induce heal to
counteract any uncomfortable list when the craft is side on to the wind. Some
deep-vee hulled craft are particularly susceptible to heal in cross winds and trim
tabs can reduce this.
An alternative to trim tabs are flow interceptors. These are metal or composite
blades that can slide down from the lower edge of the transom a few millimetres
into the water flow to divert it and create a lift component in the same way as a tab
does. Flow interceptors are simple devices for which reduced drag is claimed
compared to trim tabs. Both tabs and interceptors require electromechanical or
hydraulic actuators and there is scope for electronic control of these devices to be
interfaced with the controls for engine, steering and other parameters to optimise
the operation of the craft.
Roll and pitch damping on ships can be controlled and reduced using stabilisers to
improve comfort and sea keeping. As recreational craft become larger there is a
growing market for stabilisers which is already established in the larger motor
yacht sector. Retractable and fixed fin stabilisers external to the hull act on water
flow and require continuous dynamic control as the craft responds to inputs and
encounters waves. Gyroscopic stabilisers within the vessel act to resist movement
across the axis of the spinning flywheel, and again these require dynamic control
to constantly resist and manage uncomfortable motions. These devices are heavy
by their nature and technical solutions include using higher speed gyroscopes to
reduce weight for the same performance. They also require a constant power
input and so consume energy, making low friction bearings essential. The
spinning flywheel of high speed gyroscopes must be safely contained and operate
in a near vacuum for least friction. Gyroscopic stabilisers are now available in
sizes for smaller motor yachts and are particularly attractive for stabilising sport
fishing boats whilst at rest when they would otherwise roll uncomfortably whilst
fishing.
• Trim tabs and flow interceptors
• Stabilisers - gyro and fin
A.7 Sailing Craft
Sailing craft are as complex in their propulsion systems as powered craft, and most
seagoing sailing craft will also have an auxiliary engine. Apart from the sails the
main components are the mast(s) and boom(s) depending on the craft, and the
fixed or standing rigging which supports the mast(s) plus the running rigging which
is used to hoist, set and trim the sails. To these are added the attachments on
deck for securing the standing rigging, the blocks, clutches, fairleads, cleats and
winches for controlling the running rigging, and where fitted the reefing and furling
gear for stowing the sails and controlling the amount of sail set according to the
wind conditions. All the systems can be manually operated although on larger craft
these are normally powered by electric or hydraulic motors. Standing rigging can
be of rope using natural or synthetic fibres, but is more usually multi-stranded
stainless steel wire or solid rod, and in some cases low stretch synthetic fibre rope.
Running rigging needs to be more flexible to run around winches and blocks and
for comfort and ease of handling, and is available in ropes wound from a variety of
mainly synthetic fibres.
Masts and booms are still made in wood for traditional craft, but mainstream sailing
craft use aluminium alloy extrusions painted or anodised for corrosion protection
and good aesthetics. Carbon fibre composite masts are also used in very high
specification cruising yachts and more so in racing yachts. Carbon fibre masts and
booms are lighter and this improves performance by reducing the amount of keel
weight required.
Sail technology is very much a science aiming to achieve aerofoil shapes for
maximum lift for propulsion with minimum drag and resistance. Sail materials
range from simple Terylene or Dacron woven fabrics to laminated lightweight
composite plastic films that hold their shape plus offer ease of handling and low top
weight. Laser cutting, laminating and other computer controlled manufacturing
processes are highly advanced for the top level of racing and record breaking
sailing craft.
A.8 Domestic and hotel
Boats with accommodation for overnight and longer occupation have an increasing
range of domestic and hotel facilities that on larger craft can provide the same
levels of comfort and amenity as a luxury home or hotel. These comforts and
amenities are either achieved using land based domestic equipment, such as ac
electrical appliances for refrigeration, freezers and cooking, or special purpose
marine equipment. The accommodation on board can include sleeping cabins for
owner, guests and crew, saloon and entertaining areas, bathrooms, shower, and
wc facilities, and galley areas for cooking. Internal structure, fittings, furnishings
and the necessary plumbing and electrical services must be designed to suit the
marine environment, be compact and light in weight, and remain accessible for
servicing and maintenance. The accommodation spaces on board are often small
and close together so privacy and comfort are factors often requiring great
attention to sound and heat insulation, especially where near sleeping
accommodation, sanitary facilities and galley areas. Fabrics and furnishings are
matters of taste and style, and generally they will need to meet or exceed domestic
requirements for fire safety.
Heating, Ventilation and air conditioning requirements will depend on size of craft
and the climate it operates in. These all require power and that will need to come
from on-board sources, usually directly from generators or batteries charged
through the main engine on smaller craft. Heating systems are usually diesel oil
fired, or in some cases LPG, although electrical power can be used when sufficient
generator capacity is available, and for air conditioning and ventilation systems.
Lights can also be complex to suit the accommodation, and many large yachts
have deck lights and underwater lights to create attractive effects at night as well
as interior lighting throughout the accommodation.
All the above requires substantial electrical power, and electrical systems on board
will include both ac and dc systems. AC power is usually drawn from generators or
shore power when available in harbour. Ac power can be provided through
inverters from a dc supply, although power is limited and used mainly for powering
small domestic appliances, computers and onboard TV and entertainment systems.
Additional electrical power can be obtained from solar and wind/water driven
generators and from energy recovery from the propeller shaft freewheeling when
sailing. Batteries will be required to store electricity and these require safe storage
in separate ventilated compartments with access for maintenance and replacement.
The domestic services above have fuel requirements for heating and cooking and
the necessary storage tanks and fluid management systems including pumps,
valves, filtration and waste containment. Some are derived from land based
equipment, although marine specific equipment is preferred and in many cases
mandatory to meet relevant standards, particularly where fire or flooding would be
a consequential risk of any system failure. The need for self sufficiency generally
increases with size of vessel or the need for long range, and desalination systems
for making fresh water are among the systems required for this. Environmental
laws in many countries forbid discharges of grey waste water and black sewage
water near coasts or in harbours, so waste treatment, storage tanks and discharge
valves are also required, even on quite small yachts. It is a requirement of the
RCD that all craft with a wc able to discharge directly overboard has provision for
fitting a holding tank.
• Fresh/potable water
• Desalination systems/water makers
• Grey water
• Black water
• LPG Gas tanks and systems for HVAC/cooking
• Oil for HVAC/cooking
A.9 Payload and containment
Although not cargo carrying even modest cruising yachts carry dinghies, tenders
and life rafts and sometimes a shore access ramp or passerelle for passengers
and crew. All these must be securely attached when at sea and accessible when
required, often at short notice in an emergency. Larger yachts carry several boats
such as a jetski, fast tender for taking guests ashore and perhaps even a small car
or motorcycle. These require davits for lifting into the water or onto the dockside,
and stowage on deck or increasingly in a “garage” or hold. Helicopters may also
be carried and able to land and take off from helipads. Food, water, fuel (e.g.
highly flammable petrol for the jetskis) and other supplies must also be carried and
safely stored and suitable provision made for this.
A.10 Navigation and communications
This is to be developed with larger ships, as many systems are common if not to
the same scale.
All on-board navigation and communication systems including:
Logs, depth sounders, radar, sonar, GPS, Satellite Communications, VHF/DSC,
autopilot,
Appendix B: Literature Review on Smart Materials and Smart Structures
B.1 Introduction
Smart Materials can be described as materials that sense their environment and
respond to changes in temperature, moisture, pH, electric or magnetic field. They
can be used directly to make smart systems or structures or embedded in
structures, and to improve inherent properties to meet high value-added
performance needs. Similarly, smart structures might be described as those that
respond to environmental stimuli and self-actuate without external human
intervention. A smart structure has been called an active structure (Wada et al. ,
1990), an intelligent structure (Crawley, 1992), and an adaptive structure when
smart material is adapted to a conventional structure. This involves some vital
factors including static structural material, integral components such as sensors,
processors and actuators that sense a force from the environment and adjust the
shape, making the structure more rigid or more flexible depending upon the force
(Clark et al. 1998).
These integral components can be easily understood as biological organs. A
human body is a good example, which consists of skeleton, muscles, sensory
nerves and the brain. Skeletons can be considered as static structures. Muscles
could be actuators attached to bones that actuate parts of the skeleton. Sensory
nerves could be sensors which can sense the body’s environment.
When sensory nerves gather and transfer information through optic nerves to the
brain, the brain evaluates information and decides how to act and feedback to
actuators, and then actuators can act to respond. As illustrated in Figure B.1, when
someone experiences darkness, the retina of their eyes sense the amount of light
and transfer chemical information to their brain, and then the brain evaluates
information and decides how much the pupils should be dilated to see objects in
reduced light. Finally, an iris can be actuated by the signal of a brain.
Figure B.1: Feedback system of eye
In brighter light, the process will be reversed and the pupils will become smaller to
reduce light entering the eye..
The human body is thereby able to smartly interact with environmental stimuli such
as light, temperature, pressure and so on.
Like a human body, smart structures can be controlled by smart materials which
can change their shape or properties under power such as from electrical voltages
or magnetic fields. Smart materials such as piezoelectric, electrorheological,
magnetorheological materials and shape memory alloy are introduced into
structures by means of embedding or bonding to structural surfaces and used as
actuators, sensors and feedback controllers.
The development of smart materials has provided additional functionality and
changed design methodologies for structures that were previously passive. As a
result, smart structures could not only control structural responses in response to
stimuli, but adapt and give continuous feedback.
The purpose of using smart structures is to make the structure more adaptive than
conventional structures. One of important factors is to minimize the life cycle cost.
Inappropriate initial design may cause catastrophic disaster to buildings in an
instant. For large civil engineering structures such as bridges and buildings, the life
cycle cost will be increased. Smart suspension of building structures can respond
to earth tremors instead of trying to resist forces. Once sensors detect structural
changes, actuators activate dampers to minimize vibration forces, for example in
buildings to reduce damage during an earthquake.
Smart materials have been used where low maintenance cost is required for
example, when cracks occur in a structure, smart structures with an embedded
healing function can detect and repair damage without any intervention whereas in
a conventional structure damage would be first observed and then repaired. Smart
structures would have the potential to save the significant repair cost of dealing
with such damage.
B.2 Features of Smart Materials
Developments of new technologies for manufacturing materials and adaptive
structures have made possible various smart structures. Recently developed smart
materials, such as piezoelectric materials, shape memory alloys, controllable fluids
and elastomers have created new opportunities for structural control. Smart
materials that can perform the function of an actuator allow smart structures to
adapt to their changing environment. Such materials can change their stiffness,
shape, viscosity or other properties in response to variations in electrical field,
magnetic field, or temperature. Four distinctive groups smart materials are
discussed below.
B.2.1 Piezoelectric materials
Piezoelectric materials generate an electric field along their surface when subject
to mechanical stress. Conversely, Piezoelectric materials show change shape
when an electrical field is applied, either as compression or expansion depending
on the electrical polarity.
PZT (lead zirconate titanate) and PVDF (polyvinyldene fluoride) have been used
either as an actuator or sensor. PVDF exhibits piezoelectricity several times
greater than quartz and unlike ceramics, where the crystal structure of the material
creates the piezoelectric effect, in polymers the intertwined long-chain molecules
attract and repel each other when an electric field is applied. Most of the
piezoelectric actuators and sensors either surface bonded or embedded in the
adaptive structure system are based on either its extension or shear mechanism.
Piezoelectric materials respond quickly to small deformations. Piezoceramics have
a high structural stiffness, which can afford them a strong voltage dependent
actuation force. Since high voltages correspond precisely to only tiny changes in
the width of crystal, piezoceramics are used in common rail diesel fuel injection
systems (Randall et al., 2005). Moreover, there is intensive effort on developing
high performance piezo actuators which could be applied in various applications
such as suppressing vibrations of marine or aerospace structures, actuating
control surfaces of small airplanes, morphing wing sections, and so on.
There are several disadvantages with this sensitive type of material because the
brittle nature of ceramic makes them vulnerable to accidental breakage during
handling and bonding procedures. Also their small range of mechanical movement
may not suit large scale structures.
B.2.2 Shape memory alloys
Shape memory alloys (SMAs) are alloys that remember their geometry. SMAs
produce a significant shear strain, induced by temperature change and are
normally made in the form of wire or foil. Their length can be reduced when heated
and restored to original geometry by cooling. This is due to a temperaturedependent phase transformation from a low-symmetry to a highly symmetric
crystallographic structure which is known as Martensite and Austenite respectively.
Martensite is more changeable at the lower temperature phase present in shape
memory alloys, whereas austenite is stronger at the higher temperature phase.
The nickel titanium alloy (e.g. Nitinol, as for Nickel Titanium Naval Ordnance
Laboratory) exhibits strain up to about 8% and will recover its original shape when
it is heated above its activation temperature (Dittrich, 1998).
Various shapes of SMA can be used for different purposes, SMA wires can be
embedded or attached to host structures as hybrid composites which have a
combination of two or more reinforcement fibres or material. Due to the novelty of
changing its elastic modulus, SMA could be a vibration absorber (Rustighi et al.,
2005), a shape controller (Oh et al., 2001) and a device for self repairing (Peairs et
al., 2004). Among the major challenges of using SMA is the considerable
dependence of the control schemes on external temperatures.
Despite their advantages such as ductility and generating high strain (e.g. 100%
for a Shape Memory Polymer), many drawbacks also exist such as slow response,
low efficiency, unpredictable movement, only suitable for small-scale applications
(Prendergast & McHugh, 2004) and continuous high output heat source required
so energy consuming.
B.2.3 Controllable fluids
Controllable fluids can change their viscosity or rheological behaviour by external
stimuli such as electrical and magnetic fields. There are two commercially available
types of these fluid known as electrorheological fluid (ERF) and
magnetorheological fluid (MRF). ERF is a kind of smart material with variable
viscosity depending on electric field intensity. ERF consist of a low viscosity
insulating liquid, mixed with nonconductive particles usually in the range of 1-10μm
diameter. When the electric field intensity reaches a certain value, ERF will change
from liquid to a solid, as shown in Figure B.2. The change is almost instantaneous,
within a few milliseconds or so, and is also instantaneously reversible and on
switching of the electric field the liquid state is resumed.
Yield stresses in shear for modern materials are of the order of 10kPa for static
loading, and 5kPa for dynamic loading (Block & Kelly, 1988). However, they
require high voltages. The increase in electric field across the ERF from 0 to 2kV is
proportional to an increase of 25-50% in equivalent viscous damping (Phani &
Venkatraman, 2003).
Due to these novel properties, ER materials have been used for haptic vehicle
instrument control (Weinberg et al., 2005). In addition, they require a very high
dielectric constant to generate a large electomechanical force. ERF are vulnerable
to contamination, therefore requiring high structural integrity and sealing within the
smart systems in which they are used (Madden et al. , 2004).
Figure A.2: Electrorheological Fluid (a) Non-activated (b) Activated (Weinberg et al.,
2005)
Magnetorheogical fluid (MRF) belongs to the group of controllable fluids. MRF
produces a strain under the change of a magnetic field by micron sized
magnetisable particles dispersed in a carrier medium. Their viscosity changes with
the external magnetic field, whereas ERF experiences increased yield stress in the
presence of an electric field. MRF has applications where controlled vibration and
impact energy dissipations are required. In recent years its potential capability has
been increasingly recognised. MRF are being developed for use in the dampers of
car suspension (Carlson & Jolly, 2000), damping washing machine vibration,
prosthetic limbs, exercise equipment and surface polishing of machine parts.
MRF have been used for semi-active control devices with properties that can be
controlled to optimally reduce the response of large scale systems such as
buildings and bridges (Ribakov & Gluck, 2002). They can also overcome
bandwidth limitation (Housner et al., 1997). The rheological and magnetic
properties of several commercial MR dampers are presented by Jolly et al. (1999).
A schematic of the full-sized, MR seismic damper is shown in Figure B.3. The
damper utilizes a particularly simple geometry where the outer cylindrical housing
is part of the magnetic circuit. The fluid orifice is the entire annular space between
the piston outside diameter and the inside of the tubular damper housing.
Movement of the piston causes fluid to flow through this entire annular region. The
damper is double ended, with the piston supported by a rod at each end.
Figure B.3: Schematic of MR Seismic Damper (Jolly et al. , 1999)
There are several differences between ER and MR fluids as described in Table B.1.
The increase of external magnetic field strength causes the increase of MRF
viscosity. The advantages of MRF are that they are less affected by contamination.
Therefore, they can be used in salt laden marine environments. MRF may be
operated directly from low-voltage power supplies. Operating voltages for MRF
span from 2 to 25V while ERF requires high electrical voltages from 2KV to 5KV.
Due to the strong dipole interaction of the magnetic particles of the filler, MR fluid
has relatively high yield stress while ER fluid has low yield stresses, of the order of
only several kPa. MRF are less affected by contaminants and their temperature
ranges from -50 to +150C, as can be experienced during manufacture and use.
Conversely, ERF are quite sensitive to pollutants, and especially water, which can
have a profound influence on the strength of the material. Due to the suspended
iron particles, problems with both ERF and MRF are clumping, deposition,
environmental contamination and sealing problems, have so far prevented their
wide application.
Table B.1: Comparison of properties of typical MR and ER fluids (Carlson & Jolly,
2000; Housner et al., 1997)
Properties
Max Yield Stress τy
Operating Temperature
Range
Stability
Response Time
Density
Power Supply (typical)
MR Fluids
50-100 kPa
-50 - 150ºC
ER Fluids
2-5 kPa
10 - 90 ºC
Not affected by most Cannot
impurities
impurities
ms
Ms
3
3 – 4 g/cm
1 – 2 g/cm3
2 – 25V
2 – kV
1 -2 A
1 – 10mA
(2 – 50 W)
(2 – 50 W)
tolerate
B.2.4 Controllable Elastomers
The principle of controllable elastomers is similar to controllable fluids.
Controllable elastomers comprise natural or synthetic rubber and polarisable
particles dispersed within resulting in an elastomer that can undergo changes in
properties while subjected to an electrical or magnetic field. The applications
include variable stiffness mounts, such as engine mounts, made from the material.
Electroactive elastomers have exhibited up to 380% strain when they are highly
prestrained in area expansion at 5-6kV (Pei et al. , 2004) as well as having a high
elastic energy density. Because of excellent space saving, the material is used for
multi-degrees of freedom robot arms (Pei et al., 2004). However, the strain
depends on the elastic modulus of the polymer and the strain-electric field
relationship of electro-rheological material is nonlinear at large strains.
Magnetorhelogical Elastomer (MRE) is composed of tiny ferrous particles in chains
and an elastomer such as synthetic and natural rubbers. When liquid state rubber
combined with ferrous particles is exposed to a steady magnetic field, the ferrous
particles will form chain-like structures arranged along the magnetic field during the
curing process. Due to the characteristics of solid-state material, it operates in the
pre-yield region while MR Fluid (MRF) operates in the post-yield region. MRE is a
durable material which can be operated in severe temperatures (-50◦C to 150◦C)
with low voltage supplies. Their variable properties enable them to be used as an
adaptive structure for vibration minimization in a variety of applications. From a
number of experimental investigations, it is well known that adding a small amount
of iron particles to the elastomer makes its property much stiffer. In particular, the
incorporation of filler particles is known to increase the stiffness of the material,
change the strain history dependence of the stiffness and alter time-dependent
aspects of material behaviour such as hysteresis and stress relaxation. When
liquid state rubber combined with ferrous particles is exposed to a steady magnetic
field, the ferrous particles will form chain-like structures arranged along the
magnetic field as shown in Figure A.4. The difference between MRE and MRF is
that MRF is liquid and used in viscosity controllable devices whereas MRE is solid
and used for stiffness controllable devices (Zhou & Wang, 2005). The most
Figure B.4: Aligned MRE particles under the magnetic field (left) and randomly
dispersed MRE particles without magnetic field (right) (Stepanov et al., 2007)
noteworthy is that the particle chains within the elastomer composite are intended
to always operate in the pre-yield regime while MR fluids typically operate in a
post-yield continuous shear or flow regime (Jolly et al., 1996). MRE can be used in
applications where stiffness or resonance frequency change is needed (Jolly et al.,
1999). Toyota central R&D laboratory has developed an engine mount (Shiga et al.,
1992) and Ford motor company has developed an automotive bushing (Watson,
1997) with MRE to improve passenger comfort. Such applications demonstrate
that there is potential for the use of MR materials in marine structures as well and
therefore the subject is worthy of study by naval architects and marine engineers.
There are reports of MRE from experimental and numerical modelling. The
research to enhance MRE performance was found in the literature from (Blom &
Kari, 2005; Davis, 1998; Deng et al., 2006; Ginder et al., 2000; Jolly et al., 1999;
Lokander & Stenberg, 2003; Stepanov et al., 2007; Varga et al., 2006; Yalcintas &
Dai, 2004). Jolly et al. (1999) showed that the maximum percentage of change in
modulus was observed in the vicinity of 1-2% strain. Davis (1998) predicted the
optimal particle volume fraction for the largest MR effect was about 27% by
numerical value and verified the theory by experiment. Lokander & Stenberg
(2003) tested the shear property of MRE by using two different types of iron
particles to find out the substantial effect of MRE.
It was shown that MRE with large irregular particles have a large MR effect
although the particles are not aligned within the material and the rheological
properties of the matrix material do not influence the MR effect. Blom & Kari (2005)
showed that the maximum stiffness changes are up to 115% in the audible
frequency ranges. Varga et al. (2006) reported that the most significant effect was
found if the applied field is parallel to the particle alignment and to the mechanical
stress. Stepanov et al. (2007) investigated viscoelastic behaviour of highly elastic
magnetic elastomers by three different experimental techniques of elongation,
static and dynamic shears. It is shown that when the initial elastic modulus is small,
up to a 100 fold increase of the modulus has been observed at small 1-4%
deformations and the loss factor of aligned MRE is tunable with magnetic flux
density depending on frequency range.
The MRE tuned vibration absorber (TVA) device can modify mechanical systems’
responses to external disturbances through energizing an internal electromagnet
so that unexpected vibration loads can be dissipated through activation of the MRE
material via a closed loop control system (Deng et al. , 2006). The MRE TVA
system is fail-safe because the system will continue to function as a passive
isolator in the event of a power system failure. Deng et al. (2006) applied MREs to
an adaptive tuned vibration absorber (ATVA). It is shown that the MRE TVA
absorbs the vibrating force from an oscillating rigid mass and the relative frequency
change is up to 147% with a 60dB absorption of frequency response.
Ginder et al. (2000) showed a MRE application for an automotive bushing. The
MRE TVA devices can be developed with minimal engineering design changes to
meet other potential applications such as suspension elements for high speed
boats to enhance ride quality and handling over rough sea condition, and sensitive
vibration and noise isolation for improved resistance to shock and impact damage.
The effort to increase the performance of MRE has been revealed by using
different material selection, combination of materials or various kinds of
experimental scheme. The optimal particle volume fraction for the largest fractional
change of about 27% to 30% predicted by numerical means Davis (1998) and
proved with experiment (Choi et al., 2009; Demchuk & Kuz’min, 2002). Optimum
iron particle volume fraction has been predicted (Davis, 1998) and verified by
experiment (Zhou, 2003). They have commonly reported that maximum modulus
change arises at 27 % volume of iron particles. Kallio (2005) has measured
elastic/shear modulus and damping ratio/loss factor values with and without
applied magnetic field.
Varga et al. (2006) have found that the most significant effect was found when the
applied field is parallel to the particle alignment in conjunction with the direction of
mechanical stress.
These magnetic field sensitive materials with tunable elastic properties may find
many applications in elastomer bearings and vibration absorbers. Zhou & Wang
(2005) carried out a feasibility study showing that MRE can be used as a tuned
core material in a sandwich beam which showed resonant vibration at a particular
frequency.
Deng et al. (2006) presented the relative frequency change as 47% and vibration
absorption capacity is up to 60% when shear modulus changes up to 100%,
depending on the strength of the magnetic field. Recently Kallio (2005) showed
that MRE can change damping ratio depending on magnetic strength and the
maximum value of damping ratio occurs at 27 % volume of iron powder.
The strong strain-amplitude dependence of the MR effect suggests that MR rubber
materials are most suitable for low amplitude applications, such as sound and
vibration insulation (Lokander, 2004). From measurements at frequencies within
the audible frequency range, it can be seen that sound and vibration isolation is a
promising application for MR rubber materials.
For the material properties, it has been reported that MRE with large irregular iron
particles have a large MR effect although the particles are not aligned within the
material (Lokander & Stenberg, 2003). However the rheological properties of the
matrix material do not influence the MR effect (Lokander & Stenberg, 2003). It has
been also reported that cyclic loading conditions with different strain amplitudes
manifest a dependence of the viscoelastic storage modulus with the maximum
stiffness change being up to more than twice in the audible frequency range (Blom
& Kari, 2005). The maximum percentage of change in modulus is being observed
in the vicinity of 1-2% strain (Jolly et al., 1999; Stepanov et al., 2007). Therefore, it
is used as a complementary material for parts of structures and suitable for various
applications, such as vibration reduction in adaptive structures.
Thus, MRE has been used in applications where stiffness or resonance changes
are needed (Jolly et al., 1999). For example, the MRE tuned vibration absorber
(TVA) device can modify a mechanical systems response to external disturbances
through activating an internal electromagnetic field. Consequently, unexpected
vibration or impact loads can be dissipated through activation of the MRE material
via a closed loop control system.
The MRE TVA system can be developed with minimal engineering design changes
to meet other potential applications such as automotive suspension bushings to
enhance ride quality (Ginder et al., 2000) and marine equipment handling over
rough sea condition, sensitive electronic shock isolation for improved resistance to
shock and impact damage. Deng et al. (2006) carried out the dynamic test for
MREs as an adaptive tuned vibration absorber. It was shown that the MRE TVA
achieves a frequency change up to 147% with 60dB absorption of frequency
response. Furthermore, MRE TVA is fail-safe because the system will continue to
manage the device as a passive isolator in the event of a power system failure.
B.3 Control Systems for Smart Structures
Control systems are compared in Table B.2. An active control system is a
mechanical system with the ability to alter its configuration, form or properties in
response to changes in the environment rapidly. An active control system uses a
technique of controlling the outcome of the system by employing control actuators
into the system, whose output depends on the response of the system, whereas
passive control systems can change the outcome of the system without actuators.
However, an active control system has many requirements. First, efficiency is
required. For example, if a structure is too stiff to change the shape, an active
system might not be useful. Second, target structures must have structural integrity
to carry the design loads. The process of actuation should not affect adversely the
strength of the structure. Finally, unlike passive structures, active structures require
constant motion and therefore consistent external power should be provided. From
vibration and sound control to shape control and structural health monitoring the
range of applications of active control systems is wide. Due to the consistent power
requirement of actuating control systems and high initial cost, active vibration
control systems are less common than passive systems (Liu et al., 2005). However,
active control systems can contribute to cost savings over the whole life cycle.
Semi-active control systems are a class of active control systems for which the
external energy requirements are orders of magnitude smaller than for typical
active control systems. Semi-active control devices are often considered as
controllable passive devices (Housner et al., 1997). Therefore it combines the
advantages of active systems and passive systems such as energy efficiency and
low whole life cost. Similar to active control systems, semi-active systems also
need external power sources, but do not need constant power. Energy
requirements are orders of magnitude smaller than typical active control systems.
Therefore semi-active control systems are applied to building and bridge
suspension control against seismic forces which are instant rather than constant
(Ribakov & Gluck, 2002).
Table B.2: Comparison of active, semi-active and passive systems
Active System
Energy
Requirement
Control
Mechanism
Control
Bandwidth
Design
Methodology
Big
Semi-Active
System
Small
Passive System
None
Add and dissipate Bounded
input Impart Energy
energy
and output
Wide, Variable
Narrow, Variable Narrow, Fixed
Embedding and Combined
Cut out, Shaped,
bonding another between
active Formed
into
actuator
and
passive components for
systems
structures, Tuned
mass,
viscoelastic
material, Damper
Initial Cost / Life High / Low
Medium / Low
Low / High
Cycle Cost
Applications
Active vibration Tuned vibration Vibration
and
and noise control absorber
noise isolation
Actuator (Shape Semi-active joint
Self Healing
controller)
Structural Health
Monitoring
A passive control system does not need an external power source. For damping
vibration the control aim of passive systems is to impart energy developed in
response to the motion of the structure (Housner et al., 1997). Therefore, its
control bandwidth is narrow and fixed at an initial designed value. The term
passive control is closely related to isolation. As a result, passive control system
can be made by means of being cut out, shaped and formed into components for
structures, using viscoelastic material in the structure, using vibration or impact
dampers, or using tuned mass dampers. The difference between passive and
active control systems is that passive control does not make any real-time changes
in the system. Even though initial cost of passive systems is relatively low, life
cycle cost including maintenance and redesign could be high. Passive vibration
isolation (Dong et al., 2006), tuned mass damping (Setareh et al., 2006) and self
healing systems are examples of passive control systems.
Comparing all systems, power sources should be provided in active and semiactive systems for any real time control whereas passive systems do not require
power sources. The control mechanism of active systems is adding and dissipating
energy while semi-active systems use bounded input and output energy.
However passive systems just transfer energy from one system to another. The
active and semi-active systems have variable control bandwidth whereas passive
systems have fixed control bandwidth. Even though the initial cost of the active
system is high, life cycle cost is low whereas the initial cost of passive systems is
relatively low but life cycle cost is high.
Adaptive control is generally used to control systems whose parameters are
unknown or uncertain (Housner et al., 1997). If the amount of uncertainty is too
large to be compensated by a fixed controller, or the structural parameters may
have unpredictable variations so that gain scheduling may be inadequate, then in
these cases an adaptive controller should be considered. An adaptive passive
control is described by Bernhard et al. (1992) as a fourth classification of vibration
control.
Adaptive passive vibration control combines a tunable passive device with a tuning
strategy such that optimal performance is guaranteed (Franchek et al., 1996).
For adaptive passive control systems, the control does not require to switch rapidly.
The method for changing damping or stiffness operates in a on and off manner and
is well suited to vibration isolation in rotating machines (Liu et al., 2005) and as an
adaptive tuned vibration absorber. It can be tuned to suppress the modal
contribution to the vibration of a troublesome natural frequency of the host
structure over a wide band of frequencies. Alternatively, the TVA can be tuned to
suppress the vibration at a troublesome forcing frequency (Bonello et al., 2005).
Payload variation or component aging causes parametric uncertainties, component
failure leads to structural uncertainties, and external noises are typical
environmental uncertainties.
Typical adaptive control applications include temperature control, automotive
systems control, and ship steering control (Tao, 2004).
B.4 Marine Applications
Ranging from automotive systems, optical systems, machine tools for space
systems, medical systems and infrastructures, smart-structures technology is a
highly interdisciplinary field. Many applications have been explored by using
different kinds of smart materials such as piezoelectric, magnetorheological (MR),
electrorheological (ER) material and shape memory alloys (SMAs). Possible
marine applications are discussed herein.
B.4.1 Noise, Vibration and Damping Control
Attempts at reduction of noise and vibration in structures has always been an
important issue in mechanics ranging from small devices to large structures. Active
control systems can change output value of an object directly by adapting force
actuators whereas the only effect of a passive system is isolating or as a damping
patch on vibrating sources. Active control systems have been applied to a wide
variety of objects such as machines, structures, biological systems, etc. Active
controls for vibration and noise have been investigated by using piezoelectric
materials, MR, ER materials and SMAs.
The advance of processing technology in piezoelectric materials has resulted in
their successful applications in specialist applications in structures. In active control
systems, a greater number of smart structures incorporating piezoelectric materials
applications have been used for a variety of sophisticated mechanical tasks as
feedback systems controlled by a computer such as in the active control of noise
and vibration. Patch type Piezoelectric materials have been broadly used as
sensors and actuators by using anti-oscillations to reduce radiated noise (Hyde &
Agnes, 2000; Kalinke et al., 2001; Lee, 2000; Monner & Wierach, 2005).
Recent achievements have been made in the design and application of SMAs as
high-damping elements, utilizing pseudoelastic hysteresis, transient damping
effects in the two-phase and damping capacity of the martensitic phase
(Humbeeck & Kustov, 2005). SMA wires have been applied to active damping of
flexural vibrations of a cantilever beam (Baz et al., 1990; Choi & Cheong, 1996).
Due to the slow response to temperature, the tuned vibration absorber (TVA) has
become an established vibration control device which can be used to suppress a
troublesome resonance or to attenuate the vibration of a structure at a particular
forcing frequency (Rustighi et al., 2005).
Noise control excited by low frequency sources could be easy to achieve with low
energy input. However active vibration control has many difficulties when applied
to large systems requiring high amounts of energy. Active vibration control systems
should be accompanied by auxiliary actuators of which scale is proportional to the
size of the structure like ship structures.
For example, in the case of large scale structures such as buildings and bridges,
semi-active systems have been used to control vibration rather than active control
systems. A selective controllable base isolation system with MR dampers has been
developed to improve the seismic behaviour of multi-storey buildings (Ribakov &
Gluck, 2002) and to reduce pier drifts (Sahasrabudhe & Nagarajaiah, 2005)
respectively.
Zhou & Wang (2005) studied Magnetorheological Elastomer (MRE) as a foam core
to control shear stiffness of sandwich structures. MRE core has been introduced as
a kind of shunted system using very little magnetic energy.
B.4.2 Structural health monitoring
Light weight ships such as yachts, fishing boats and even naval vessels are
increasingly using composite materials to take advantages of their excellent
specific strength, stiffness and low electromagnetic signature. However, they have
the risk that damage can lead to catastrophic structural failure. Composite
structures tend to fail by internal damage of a distributive or interactive nature,
which have a number of failure modes that differ from steel structures. Agarwal &
Broutman (1990) showed that the internal material failure can appear in many
forms, i.e.
• Breaking of the fibres
• Micro-cracking of the matrix
• Separation of fibres from each other in a laminated composite (Debonding)
• Separation of laminates from each other in a laminated composite (Delamination)
Structural Health Monitoring (SHM) systems have the ability to detect material
failure and interpret adverse ‘changes’ in a structure able to improve reliability and
reduce life-cycle costs (Kessler et al., 2002). The expected benefits of the SHM
system in an FRP structure is the reduction of inspection cost required for large
marine structures and improving their structural reliability.
As the size of composite ships increases an efficient monitoring method for defects
in composite submerged structures will be required to estimate their life cycle more
effectively.
Even though there are many methods for non-destructive testing for small
laboratory specimens such as X-radiographic detection and hydro-ultrasonics,
these are not appropriate for in-service inspection of full size ship’s structures.
Techniques based on static displacement response and static strain measurement
which involve the measurement of actual displacement and strains are not easily
performed on large scale structures (Tseng & Naidu, 2002). Moreover, damage
detection in composites is more difficult than in metallic structures owing to the
anisotropy of the material (Kessler et al., 2002).
Thus fibre optic sensors have been used as structure monitoring sensors with
significant advantages. The applicability of using fibre optic sensors has been
examined (Kageya et al., 1998). Stress concentrations or residual strains resulted
from impact damage can be detected by fibre optic sensors. Suresh
et al. (2005) developed the Fiber Bragg Grating (FBG) based shear force sensor.
Baldwin et al. (2002) carried out monitoring on a UK Trimaran Research Vessel by
using Digital Spatial Wavelength Domain Multiplexing (DSWDM) which can
determine strain level as well as monitor frequency response of the ship to wave
slamming events. Herszberg et al. (2005) used FBG sensors and piezoelectric
strain sensors for structural health monitoring for composite T-joints.
The optical fibre sensor has potential advantages as a structure monitoring sensor
such as for dynamic measurement and is less affected by temperature.
However, it has difficulty in detecting displacement changes at a frequency of less
than 1Hz (Kageya et al., 1998). In other words, high resolution data can be
acquired but it cannot travel over long distances such as in the hull structures of
large vessels.
Ultra sonic wave techniques which have been used to interrogate structural
integrity could be suitable for long-range inspection. Diamanti et al. (2005)
demonstrated the potential of low-frequency ultra sonic waves being used for the
inspection of monolithic and sandwich composite beams. Yang & Qiao (2005)
carried out damage detection of composite structures by using the pulse-echo
method. Kessler et al. (2002) performed quasi-isotropic graphite/epoxy testing
using piezoelectric actuators and sensors.
Unlike waves on a planar surface, frequency dispersion occurs for surface waves
propagating along a curved surface (¨Uberall, 1973). The wave propagation in
curved composite structures has been investigated with analytical methods. Liu &
Qu (1998) developed an analytical approach for the transient wave propagation in
annular structures. Yang & Qiao (2005) defined the dispersion effect of laminated
composite structures by using numerical and experimental means.
Due to the significant difference between the acoustic impedance of each
laminated skin, the dispersion effects at low frequencies and multi-curved shells
should be considered simultaneously in composite ship structures.
B.4.3 Self healing and repairing
Self healing structures react to damage producing some action to restore it to its
undamaged condition (Peairs et al. , 2004) followed by air curing of the released
chemicals in the cracks, leading to restoring the mechanical properties of
undamaged composite (Li et al. , 1998). Once the glass fibres break, the chemical
agent is released into the cracks of the polymer matrix and the composite can be
healed. Significant crack arrest and life extension resulted when the in-situ healing
rate is faster than the crack growth rate (Brown et al., 2005). Li et al. (1998) carried
out a feasibility study on self healing which maintained the material’s water
tightness (sealing) and mechanical properties (healing) in structures.
Shape memory alloys (SMAs) have been used as actuator in a self repairing joint.
Layered SMA/PVDF films in flexible satellites were demonstrated in a self healing
system with a SMA-driven bolted joint that senses change and tightens itself (Hyde
& Agnes, 2000), which can reduce the maintenance cost of critical bolted joints.
Self healing can make the material safer, more reliable, longer lasting and require
less maintenance cost especially in ship structures subjected to unexpected
loading conditions such as hydraulic slamming or underwater explosions. It is also
useful to make up the healing agent in quantities for small crack sizes initially.
However, agents for self healing materials could be only a one shot solution rather
than as a permanent measure to repair damage resulting from continual loads
such as wave forces. In the case of SMA actuators for self repairing joints, a high
amount of power is required as well as time for heating and cooling to achieve
each material state (i.e. martensite and austenite).
B.4.4 Shape control
Smart structures featuring active shape control give the potential to widen the
optimum design operating parameters and significantly increase efficiency (Monner
& Wierach, 2005). The fundamental structural task of a wing is transferring the
loads between aerodynamic pressure and inertial forces. Adaptive wings can
change the entire shape of the structure by using active control only requiring an
electric current for movement (Hyde & Agnes, 2000). B¨ uter & Breitbach (2000)
introduced directly controlled helicopter blades by smart adaptive elements such
as piezoelectric or active fibres. Gern et al. (2005) analysed the smart wing of an
unmanned combat aircraft. Moreover, many experiments were carried out
including various kinds of space engineering, aerodynamic surfaces including
aircraft wings, twist-active rotor blades and missile fins adaptable for high speed
aerodynamics. Shape control actuators can unify various controls into a single
control and transmit information effectively.
For marine applications, precise control is particularly difficult especially for low
speed submersible vehicles. These low speed vehicles need manoeuvring
capabilities such as hovering, small-radius turning, diving, surfacing and precision
station keeping. To achieve these capabilities there are numerous investigations
into low speed controllers based on bio-robotics. The goal of bio-robotics research
is bridging the gap between nature and engineering by improving the performance
of the latter as compared with the former (Bandyopadhyay, 2005). Therefore,
shape control of structures could find possible marine applications by using
piezoelectric materials for precision adjustments of structures that require more
biological and less mechanical input in applications such as submergible vehicles.
Controllable pitch propellers are a good example of an application for low speed
submergible vehicles which could operate under range of flow conditions (Madden
et al., 2004). These vehicles could benefit from the added efficiency and
manoeuvrability, and the potentially low noise signature afforded by variable shape
propellers (Bandyopadhyay, 2002).
However, it is difficult to adapt smart materials to complex geometrical shapes.
An actuator required to vary the shape of a stiff structure such as a propeller blade
would require a high power electrical input.. Also for long term durability and
reliability protection from seawater would be essential.
High speed small and light submersible vehicles need precise control while
travelling to the target. Based on recent developments such as a missile fin
(Barrett, 1994), adaptive blade twist (B¨ uter & Breitbach, 2000), double plate
structure for smart wing (Oh et al. , 2001), analytical modelling and basic design for
smart fin (Rabinovitch & Vinson, 2003), smart fins will be used with precise
tolerances for high speed submersible vehicle using small adjustments of a high
stiffness fin to actuate twist and bend and to save space required for servomotors,
force transmission devices and dynamic control system. Thus, a submersible
smart fin could be positioned on any part of the outside structure of a vehicle and
could maximize the manoeuvring capability. Consideraton should however be
made of the massive density of deep water and the nonlinear vortexes occurring
from fin tips.
B.4.5 Piezoceramic actuators
The increase in performance and flexibility of actuators has resulted in a demand
for actuators in shape control systems. Shape control is determined by the
required range of displacement and responsiveness of the system. A lot of
theoretical and experimental research has been carried out on piezoceramic fibre
composite (PFC) actuators. These are more conformable, durable and responsive
than regular monolithic devices (Lloyd, 2004), and PFCs represent the best
application of piezoelectric materials. Many examples of applications are aircraft
wings to improve aerodynamic lift and reduce drag, rotor blades to increase hub lift
and reduce vibration due to imbalance, and reflector surfaces to improve antenna
performance (Marc, 1998). A Macro Fibre Composite actuator has been developed
by NASA Langley Research Center (LaRC-MFCTM), which has been used for
commercial purposes.
MFC strain performance exceeds 150% (Wilkie et al., 2000). Since piezoelectric
materials do not need electrical windings, they can be fixed to a structure and do
not require electrical or mechanical contacts. Sherrit (2005) has investigated these
materials for high temperature actuators. However, piezoelectric fibre composite
technology has some processing difficulties during actuator manufacture and is
limited by the high actuator voltage requirements (Yoshikawa et al., 1999).
B.4.6 Electrorheological actuators
Electrorheological materials suit applications in actuators, which can produce
different resistive torque values or shear stress depending on the applied voltage
and mode. Using these novel features of ER materials applications include a feed
back control knob made of ER Fluid (Weinberg et al., 2005) and a small working
robot with a rolled spring for each of its six legs made with Electro Elastomer (Pei
et al., 2004).
These actuators exhibit up to 380% strain in area expansion at 5 to 6kV and have
many advantages and operate effectively in lightweight, compact and powerful
interfaces (Sherrit, 2005). Due to the low elastic modulus ER materials cannot
develop sufficient power to actuate very stiff structures.
B.4.7 Shape Memory Alloy (SMA) Actuators
SMAs would be suitable when a large displacement and slow response are
required. SMAs have low bandwidth and are well suited mainly for slow actuation
requirements. However, SMAs have poor fatigue properties. SMA elements may
survive for a shorter time than steel components when subject to the same loading
conditions such as twisting, bending and compression. Moreover, the slow
response of SMA actuators are incompatible with high speed controllers.
Representative types of actuators including piezoceramic (PZT), electroelastomer
and shape memory alloys (SMAs) are compared in Table B.3.
Table B.3: Characteristics of smart materials as actuators (Dittrich, 1998; Hodgson
et al. 2004; Madden et al., 2004; Sodano, 2003; Wilkie et al. , 2000)
Actuation
Principle
Strain(%)
Efficiency(%)
Youngs Modulus
(Gpa)
Energy Density
(J/kg)
Tmax (ºC)
Relative Speed
PZT
Piezo-ceramic
VHB
Electro-elastomer
NiTiNol
Shape
Alloys
8
<5
0.2
90 (max)
63
380
90(max)
25 (typical)
0.003
0.013
3.4
30 (martensite)
90 (austenite)
15
-20 to 200
Fast
-10 to 90
Medium Fast
-100 to 200
Slow
Memory
Electroelastomer exhibits high strain (380%) whereas piezoceramic displays low
strain (0.2%). Efficiency is the ratio of work generated to input energy consumed,
which is almost the same (90%) between piezoceramic and electro-elastomer,
while SMAs exhibit low efficiency. Young’s modulus is the material stiffness
normalised by sample length and cross section area. It is important as it
determines the actuator’s passive ability to resist load changes and disturbances
(Prendergast &McHugh, 2004).
Young’s modulus between piezoceramic and electro-elastomer is significantly
different. Energy density is the ratio of energy generated in one actuator cycle per
unit volume of actuator. SMAs exhibit very high energy density (15 J/Kg), whereas
piezoceramic shows very low energy density (0.013 J/Kg).
There are various smart material applications in actuators such as feedback
control knobs using ER Fluid which can change their properties (Weinberg et al.,
2005) and adaptive torque-plate (Barrett, 1995) controlled directly by smart
adaptive elements. Marine applications may need considerable power to get the
required torque. Also consideration should be given to the different densities
between air and water, and vibration induced by vortexes. In the case of adaptive
blade twist (B¨ uter & Breitbach, 2000), the concept of using centrifugal force
seems similar to the propeller of the ship, so this could be applied to controllable
pitch propellers. However, it has several limitations such as the manufacturing
process and external factors affecting suitability.
B.5 Summary and Conclusion
Each smart material has its own features with respect to different purposes, and
even where smart materials have unique properties these may be suited to several
different applications.
For example, SMA could be used as vibration absorber (Humbeeck & Kustov,
2005), shape controller (Oh et al., 2001) or as self repairing (Peairs et al., 2004).
Piezoelectric materials can be sensors as well as actuators. Looking at future
applications of smart material, there are limitations with implementing piezoelectric
materials because their small range of mechanical movement may not suit large
scale structures. In the case of ceramic based materials, their brittle nature makes
them vulnerable to accidental breakage during handling and bonding procedures.
SMAs with their slow response, low efficiency, unpredictable movement and
continuous heat source makes them suitable only for small-scale applications. For
electro-rheological materials the strain depends on the elastic modulus of the
polymer and the strain-electric field relationship of material is nonlinear at large
strains. In addition, they require a very high dielectric constant to generate a large
electomechnical force. Also, they are vulnerable to contamination,requiring
protection within any smart structural system in which they are used. However,
magnetorheological materials are less affected by contamination making them
more attractive in salt laden marine environments. In addition, they can be
operated in extreme temperatures and from low-voltage power supplies. Due to the
strong dipole-dipole interaction of the magnetic particles of the filler,
magnetorheological materials are also more efficient than electrorheological
materials. Finally, they can overcome the bandwidth limitation.
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