Until relatively recent times, most periods of technological development have been linked to changes in the use of materials (eg the stone, bronze and iron ages). In more recent years the driving force for technological change in many respects has shifted towards information technology. This is amply illustrated by the way the humble microprocessor has built intelligence into everyday domestic appliances. However, it is important to note that the IT age has not left engineered materials untouched, and that the fusion between designer materials and the power of information storage and processing has led to a new family of engineered materials and structures.
Most familiar engineering materials and structures until recently have been ‘dumb’. They have been preprocessed and/or designed to offer only a limited set of responses to external stimuli. Such responses are usually non-optimal for any single set of conditions, but ‘optimised’ to best fulfil the range of scenarios to which a material or structure may be exposed. For example, the wings of an aircraft should be optimised for take-off and landing, fast and slow cruise etc. However, despite the partial tailoring of these structures by the use of additional lift surface, which we see deployed as each passenger aircraft approaches an airport, such engineering components are not fully optimised for any single set of flight conditions.
Similarly, advanced composites such as glass and carbon fibre reinforced plastics, which are often thought to be the most flexible engineering materials since their properties (including strength and stiffness) can be tailored to suit the requirements of their end application, can only be tailored to a single combination of properties.
‘Dumb’ materials and structures contrast sharply with the natural world where animals and plants have the clear ability to adapt to their environment in real time. The field of biomimetics, which looks at the extraction of engineering design concepts from biological materials and structures, has much to teach us on the design of future manmade materials. The process of balance is a truly ‘smart’ or intelligent response, allowing, in engineering terms, a flexible structure to adapt its form in real time to minimise the effects of an external force, thus avoiding catastrophic collapse.
The natural world is full of similar properties including the ability of plants to adapt their shape in real time (for example, to allow leaf surfaces to follow the direction of sunlight), limping (essentially a real time change in the load path through the structure to avoid overload of a damaged region), reflex to heat and pain. The materials and structures involved in natural systems have the capability to sense their environment, process this data, and respond. They are truly ‘smart’ or intelligent, integrating information technology with structural engineering and actuation or locomotion.
Applications of Smart Materials
There are many possibilities for such materials and structures in the man made world. Engineering structures could operate at the very limit of their performance envelopes and to their structural limits without fear of exceeding either. These structures could also give maintenance engineers a full report on performance history, as well as the location of defects, whilst having the ability to counteract unwanted or potentially dangerous conditions such as excessive vibration, and effect self repair. The Office of Science and Technology Foresight Programme has stated that `Smart materials ... will have an increasing range of applications (and) the underlying sciences in this area ... must be maintained at a standard which helps achieve technological objectives', which means that smart materials and structures must solve engineering problems with hitherto unachievable efficiency, and provide an opportunity for new wealth creating products.
Smart Materials in Aerospace
Some materials and structures can be termed ‘sensual’ devices. These are structures that can sense their environment and generate data for use in health and usage monitoring systems (HUMS). To date the most well established application of HUMS are in the field of aerospace, in areas such as aircraft checking.
An airline such as British Airways requires over 1000 employees to service their 747s with extensive routine, ramp, intermediate and major checks to monitor the health and usage of the fleet. Routine checks involve literally dozens of tasks carried out under approximately 12 pages of densely typed check headings. Ramp checks increase in thoroughness every 10 days to 1 month, hanger checks occur every 3 months, ‘interchecks’ every 15 months, and major checks every 24000 flying hours. In addition to the manpower resources, hanger checks require the aircraft to be out of service for 24 hours, interchecks require 10 days and major checks 5 weeks. The overheads of such safety monitoring are enormous.
An aircraft constructed from a ‘sensual structure’ could self-monitor its performance to a level beyond that of current data recording, and provide ground crews with enhanced health and usage monitoring. This would minimise the overheads associated with HUMS and allow such aircraft to fly for more hours before human intervention is required.
Smart Materials in Civil Engineering Applications
However, ‘sensual structures’ need not be restricted to hi-tech applications such as aircraft. They could be used in the monitoring of civil engineering structures to assess durability. Monitoring of the current and long term behaviour of a bridge would lead to enhanced safety during its life since it would provide early warning of structural problems at a stage where minor repairs would enhance durability, and when used in conjunction with structural rehabilitation could be used to safety monitor the structure beyond its original design life. This would influence the life costs of such structures by reducing upfront construction costs (since smart structures would allow reduced safety factors in initial design), and by extending the safe life of the structure. ‘Sensual’ materials and structures also have a wide range of potential domestic applications, as in food packaging for monitoring safe storage and cooking.
The above examples address only ‘sensual’ structures. However, smart materials and structures offer the possibility of structures which not only sense but also adapt to their environment. Such adaptive materials and structures benefit from the sensual aspects highlighted earlier, but in addition have the capability to move, vibrate, and exhibit a multitude of other real time responses.
Potential applications of such adaptive materials and structures range from the ability to control the aeroelastic form of an aircraft wing, thus minimising drag and improving operational efficiency, to vibration control of lightweight structures such as satellites, and power pick-up pantographs on trains. The domestic environment is also a potential market for such materials and structures, with the possibility of touch sensitive materials for seating, domestic appliances, and other products. These concepts may seem ‘blue sky’, but some may be nearing commercial readiness as you read this.
Approaches vary from the use of mechatronics (essentially hybrid mechanical/electronic systems) to the development of truly smart materials, where sensing and actuation occurs at the atomic or molecular level. The mechatronic approach is familiar from systems already in existence such as ABS and active ride control in road vehicles, and such an approach has already been employed in the vibration control of high rise Japanese buildings. However, in truly smart structures the integration of sensing and actuation is generally greater than that in pure mechatronic systems, with the required function integrated within the structural material itself. Such structures have been compared to Frankenstein's monster since separate sensors and actuators are integrated (or bolted) together into a structural material, but without the materials themselves being smart. Examples include sensual structures containing optical fibre sensors for monitoring load history and damage accumulation in bridges, dams and aircraft and adaptive structures containing novel piezoceramic, electrostrictive, magnetostrictive and shape memory actuators, for real time vibration and shape control.
‘Mechatronic’ smart structures have demonstrated the capability of this technology, but raise the important issue of the complexity of the resulting system. These smart structures contain a multitude of different materials, and in the case of sensual structures will generate large amounts of data. This increase in complexity has been described by Hiroaki Yanagida as the ‘spaghetti syndrome’, and has led to the proposal for an alternative type of smart structure based on the concept of ken materials (the Chinese characters meaning wisdom, structure, monitoring, integration and benignity being pronounced ken in the Japanese language). Such structures would move functional integration into the constituent engineering materials themselves.
Few practical examples of ken materials exist at present, although a structural composite based on this concept has been developed in Japan. This is a carbon and glass fibre reinforced concrete which is able to monitor concrete structures using only the structural reinforcing fibres, thus reducing the complexity of the system.
At the Atomic Level
The ultimate integration is a level beyond ken materials where functionality occurs at the microstructural or atomic and molecular scale. This produces what is commonly known as a ‘smart material’. Few examples of true smart materials exist at present, although the function of such a material can be illustrated by the familiar photochromic glass. Such glasses have inbuilt sensing and response but have only one response to the one stimulus.
The development of true smart materials at the atomic scale is still some way off, although the enabling technologies are under development. These require novel aspects of nanotechnology (technologies associated with materials and processes at the nanometre scale, 10-9m) and the newly developing science of shape chemistry.
Worldwide, considerable effort is being deployed to develop smart materials and structures. The technological benefits of such systems have begun to be identified and, demonstrators are under construction for a wide range of applications from space and aerospace, to civil engineering and domestic products. In many of these applications, the cost benefit analyses of such systems have yet to be fully demonstrated.
The Office of Science and Technology’s Foresight Programme has recognised these systems as a strategic technology for the future, having considerable potential for wealth creation through the development of hitherto unknown products, and performance enhancement of existing products in a broad range of industrial sectors.
The concept of engineering materials and structures which respond to their environment, including their human owners, is a somewhat alien concept. It is therefore not only important that the technological and financial implications of these materials and structures are addressed, but also issues associated with public understanding and acceptance.
The core of Yanagida’s philosophy of ken materials is such a concept. This is ‘techno-democracy’ where the general public understand and ‘own’ the technology. Techno-democracy can come about only through education and exposure of the general public to these technologies. However, such general acceptance of smart materials and structures may in fact be more difficult than some of the technological hurdles associated with their development.