During the past few years there has been an upsurge of interest in the use of composites in all applications, including maritime, civil engineering and offshore. This growing trend is clue to the exploitation of the unique properties that today’s advanced composites offer properties that are difficult to match with traditional materials such as steel, aluminium or wood. These include high strength-to-weight and modulus-to-weight ratios.
Coupled with this are the recent advances in fibre optic technology, brought about by massive growth in the telecommunications industry. The fusion of these two high technology industries has led, for the first time, to the possibility of truly ‘smart’ structures. Smart Fibres is pioneering such technology with the implementation of a fibre optic strain sensing system, which aims to revolutionise current structural load monitoring methodology, with major implications in accelerating design, development and production of structural health monitoring systems. Embedding smart fibre optic cables within a composite material allows the instantaneous monitoring of structural loads in real time. This will ultimately lead to ‘intelligent’ structures that can report unusual loads or fatigue load data for further analysis.
The fast-moving advances in the technology, which have taken the idea from research novelty to commercial reality, have only been possible thanks to the unique position that Smart Fibres is in. Its parent company, Carbospars, is one of the leading designers and manufacturers of carbon composite structures, specialising in masts and spars for the performance and luxury yacht industry. This link has enabled Smart Fibres to get involved in high-profile maritime projects, such as the DTI’s Maritime Applications of Smart Technology (MAST) programme.
The three-year, £l.l million government funded MAST project brought together the UK’s leading photonics and composites experts, including British Aerospace, Aston University, Pendennis Shipyards and Carbospars. The aim of the project was to examine the potential of embedding fibre optic strain sensors in a composite material to produce a truly smart structure. The dramatic increase in the use of composite materials within the maritime environment provided an excellent opportunity to test out the project’s prototype strain system and the project culminated in a fully operational optical ‘nervous’ system being embedded in a 38 m free-standing mast built from carbon-fibre reinforced polymer. The demonstration rig provided a platform to verify many of the techniques of optical fibre sensor installation and handling, through the processes of composite lay-up, curing, fitting-out, installation and final sea-trialing in a harsh maritime environment.
Optical Sensing Fibres
Optical sensing fibres are ideally suited for incorporation in composite materials at the lay-up stage. Although their diameter is typically ten times that of the structural carbon fibres, they are still sufficiently small in cross section, when embedded in the laminate, to be minimally intrusive to the structure. This was confirmed by an extensive programme of research and development work undertaken during the MAST project. Tests concluded that both strength knock-down and fatigue effects clue to sensor disbonding were minimal, and optical systems were sufficiently resilient to withstand composite fabrication, cure and prolonged fatigue conditions.
Carbon fibre and composite masts are now preferred for many racing and cruising yacht spars. The advantages of composite materials stem from their intrinsic properties, and structural designers have the freedom to vary the material’s structural properties by using a combination of different fibres and lay-up orientations to produce an application-specific material. A typical stayed carbon fibre reinforced polymer (CFRP) mast might have 68% of its carbon fibre aligned at 0° for compressive strength, with 22% at 45° for torsional strength and 10% at 90° for hoop strength. The carbon fibres are normally woven into fabric or tape, weighing between 100 g.m-2 and 600 gm-2, and are cut to shape, coated with resin and laid, by hand, onto male or female moulds to form spars and masts. A CFRP mast made in this way reduces the mass above the vessel’s centre of gravity and reduces pitching moments, resulting in a more stable boat. Loads in these structures are high - for instance the preload alone can exceed 150 tonnes.
The rigging arrangement is an intricate engineering structure that is set up to hold the mast straight throughout a range of load conditions. The preloading of the individual stays is a complex business, made even more complicated by the lack of real load data in each panel of the structure. The smart fibre system can deliver this preload data and provide an ongoing reference or zero point for future comparison. It can also provide load information to the onboard operators, who may be racing the yacht or simply cruising in arduous, heavy weather conditions.
Huge advances have been made in optical fibre technology since its initial development in the 1970s, most of these advances being the product of the current global telecommunications explosion. The applications are not limited to the telecommunication industry and fibre optics have found use in many areas of sensing. Optical fibres have a number of special qualities that give them important advantages over conventional electrical sensing methods. These include complete immunity to electrical and magnetic interference, small mass and size (an optical fibre is typically 0.25mm in diameter and weighs as little as 30 µg.m-1), very high information transmission rates (GHz and higher) and the ability to cascade many sensors along the same fibre cable to allow distributed sensing measurements.
How do Optical Strain Sensors Work?
The optical sensing technology used for this application employs a ‘Bragg grating’ strain sensor, technology that has only emerged over the past five years. These sensors are typically a few millimetres in length and are formed within the core of the optical fibre, making them invisible to an observer inspecting the surface of the fibre. They are imprinted in the fibre using two UV wavelength laser beams, which intersect the fibre at an angle and create a periodic interference pattern in the core of the fibre, similar to the production of a hologram. This exposed region of fibre is called a fibre Bragg grating.
These Bragg grating sensors measure strain by the selective reflection of light. When light is launched down the sensing cable, each sensor acts as a tiny mirror that only reflects one particular wavelength of light, all other wavelengths being transmitted through the sensor and continuing further down the fibre. As the sensor experiences an axial load and is stretched or compressed, the wavelength of light it reflects changes accordingly. By monitoring the wavelength of light reflected by each sensor, the equivalent strain can be measured. If the sensing region is bonded to a structure then it device can be used as an optical strain gauge. Unlike electrical strain gauges, the wavelength of each gauge is of an absolute nature and can be used as a permanent and accurate benchmark to monitor long-term loads or changes to the structure.
The Structural Load Monitoring System
The structural load monitoring system operates in a similar manner to our own nervous system. The location of each of the sensors, similar to human nerves, are carefully chosen to monitor crucial load bearing regions within the structure. The sensing fibres, analogous to the spinal cord, can either be bonded or embedded in the composite material, forming an integral part of the structure. The information received at each of the sensors is then transmitted back to a remote optoelectronic data processing unit (the ‘brain’) where it can be filtered, analysed and appropriate action taken.
For the MAST programme, the sensor network consisted of eight parallel fibres, each containing five sensors positioned at various locations along the mast and boom. The information on the status of each of the sensors was updated 500 times each second. From this abundance of available data, three levels of information feedback were identified as important to the yacht’s crew and design engineers - quick and simple real time data, longer-term structural health monitoring information and black-box recording.
The top level data retrieval provides a simple onboard visual display of the current status of the complete system, the strain condition of each sensor being represented by colour. In this way, an excessive load situation could be visually identified by a red onscreen sensor and also by an alarm or light, which would immediately alert the crew to the problem.
The second level of data retrieval provides a longer-term structural health monitoring system, aimed more towards structural design engineers. The data relayed here includes minimum, maximum and average strain measurements over a short time period, typically five seconds. Such data can be accrued over the lifetime of the structure and used to build up a picture of rig usage, so that data can be fed back into the rig design and manufacturing process. The accurate measurement of strain under all sailing conditions at key load bearing points in the rigging can allow fine tuning of the manufacturing safety margins of the structure, and inevitably will drive down the weight and cost of the structure. More accurate risk analysis for insurance underwriting purposes can also be accrued from the same long term records. This could impact favourably on the cost of ownership of such rigs, which are extremely high value items.
The final level of data acquisition is that defined as black box, where all the data from every sensor is stored in a continual loop for a specified time period. This data will prove exceedingly useful in the case of catastrophic failure of the structure, giving all important information as to the strain events that occurred immediately prior to the failure.
The potential applications for the standard electro-optical hardware and strain sensor arrays now available are not limited just to the maritime market, as every day high performance composite materials find new applications in the aerospace, civil engineering, transportation and offshore industries. For example, in the aerospace industry, the effects of flutter and vibration of an aircraft in flight could be monitored, or in the case of civil engineering, the system might prove useful to monitor buildings in earthquake zones or to examine the degradation of bonding in composite-reinforced structures. The flexibility that the system provides means that structural loading can be monitored not only in composites, but also in more traditional materials.
The combination of advanced composite manufacturing and pioneering optical technology has brought the idea of an intelligent structure from research curiosity to commercial reality. It heralds a new era in structural design engineering, with the technology being proven within the maritime environment but having wide ranging applications far beyond this.