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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: • • • • 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: • • • • • • • • • • • • • • • • 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. 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