Optical fibres have proved to be an interesting means of measuring the stress on masts in storms. Design Engineering reports.
Optical fibres have revolutionised the telecommunications and data transmission industries and these areas are growing exponentially. In the world of instrumentation too, a revolution is also occurring which will render traditional mechanical and electronic methods obsolete.
Today, it is possible to incorporate fibre optic sensors into optical fibre networks to provide environmental strain and temperature information. The optical sensing arrays can be embedded into a composite material or simply bonded to the surface of the structure.
This embedded network of sensors can then monitor activity and warn if any excessive loading or damage occurs. The optical fibre transmits sensor information back to the central data processing unit where it can be filtered and analysed.
For its part, UK-based Smart Fibres offers such a sensing system for distributed strain and temperature monitoring applications. The use of fibre optic Bragg grating sensing technology allows sensors to be fabricated in a cable of diameter 0.25mm – a feature which makes them desirable in applications where non-intrusive load monitoring is required.
The complete smart fibre strain sensing system consists of a network of fibre optic strain and temperature sensors, data acquisition unit, display unit and customised software package. The system is aimed at users requiring distributed strain information from any structure. The flexibility of the system allows each of the components to be used independently, so that potentially any network of fibre grating strain sensors can be connected to the hardware unit for load analysis.
The system is a stand alone unit, which has data acquisition and preliminary data processing capability. The system allows multiple channel strain sensing data input, with an RS232 connection enabling data to be accessed from a remote location. The unit employs a scanning Fabry-Perot filter to monitor the loading applied to the sensors. An embedded PC, which initially acquires the raw data, has a black box data storage facility that can be triggered when loading exceeds a particular value or manually by the operator.
The advantages of the new system over older techniques are its low weight, integrity within the structure, and the fact that it is unaffected by electro-magnetic interference. What is more, it is expected to last for the complete lifetime of the structure.
The system also provides permanent load records which can be useful for determining maintenance schedules and working life which will give considerable financial benefits. It will also be used to provide design information feedback which will be a useful tool in the developing world of high-tech composites. It can be used for providing permanent records for insurance or other purposes.
The immediate proving ground for the system is in the marine market. The demand for increased performance and reduced cost means that more carbon fibre structures are being incorporated into yacht design, replacing conventional aluminium or wood. Even today the design of critical yacht components, such as masts, are over-engineered to cope with potential worst-case load situations, despite the usage of in-built load cell technologies. This places a size and cost burden and performance limitation on the yacht design.
In the past, strain gauge technologies have been used to experimentally measure applied loads at critical points on the mast. The information gained has been important in influencing mast design, but these techniques are not suitable for long term routine use and the data is limited to surface effects which do not accurately monitor the potentially more critical internal effects. Similar related problems apply to hull and keel design.
Mast failure, due to an underestimation of the environmental conditions and the subsequent overloading of the structure, or crew error, is therefore a not uncommon occurrence. The maritime environment of dampness, violent motions, salt water and electromagnetic equipment provides a harsh proving ground for both hardware and software systems. However, the development of generic embedded strain sensor technology for composite materials together with methodology for metallic, composite substructures and retrofit applications are seen as important enabling technologies for more advanced systems.
The use of smart fibre masts for the luxury and performance yacht industry will indicate any high-load events which the yacht has experienced over its lifetime and the black box facility will enable insurance companies to thoroughly analyse any incidents. Improved design feedback and in particular, analysis of long term load histories, will provide the basis for improved design and cost savings in the raw materials, particularly in the `superyacht’ masts where initial costs are high.
The smart fibre system will enable the rig and all its rigging elements to be correctly set up and give warning should an element of rigging be out of tune. The system will provide the crew with a comparison of the actual set-up compared to the required set-up. Load feedback from analysis of the data can also indicate to the structural design engineer whether the yacht is seeing or has seen high loads in areas of its structure, warranting examination of the relevant component and possible strengthening.
Alternatively, the load feedback could lead to a more rigid operating specification for example, reducing speeds further in certain wind and wave conditions. Using data obtained from the smart fibre system, computer graphics could be used to ensure the rig is set up in accordance with the design. This is comparable to tuning a radio by sound only rather than digital graphical and feedback tuning.
The employment of a complete fibre optic strain sensing system into the 38 m free-standing mast of the yacht Jacquelina has been the culmination of three years research and development work, under the DTI Link Photonics MAST project. The project was designed to develop promising laboratory work, examining the potential of embedded fibre optic sensors within a major carbon fibre structure. This project, with government funding of £1.1 million, involved Aston University, British Aerospace military aircraft, British Aerospace Sowerby Research Division and Smart Fibres parent company, Carbospars.
The first production load monitoring system has now been installed on a 42-foot luxury yacht signalling the start of a series of systems to be produced, not only for the maritime sector, but also for the aerospace industry, civil, power generation and offshore industries.
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