Optical fibre sensors for structural health monitoring in aerospace airframes
Associate Professor Steven Hinckley
Edith Cowan University
A/Prof Steven Hinckley is a physicist working in the Photonics Research Laboratory and Centre for Communication and Electronics Research in Edith Cowan University (ECU). He joined ECU in 1993 having previously worked in CSIRO and Telstra Research Laboratories. Steve explained that he has a vision of an all-optical-fibre system for sensing and for control, including transmission of power over optical fibre. His focus in this presentation was on non-destructive structural health monitoring (SHM), particularly by the applications Fibre Bragg Gratings (FBG).
Steve started his presentation with an overview of SHM and current monitoring methods, particularly for aerospace airframes. Current aircraft life is dominated by very conservative limits imposed to avoid failure and consequent risk of loss of life. These conservative limits result in high costs for inspection and relatively short service lives for structures. Even so, human error in inspection and maintenance is estimated to be a factor in around 15% of accidents. The demands on inspection are increasing with the adoption of composite fibre reinforces structures, which are subject to manufacturing defects, fatigue (delamination), impact from dropped tools and bird strikes, and lightning strikes. The reinforcing fibres in composites are in themselves a kind of defect. The aim of SHM is increased safety at lower cost, and Steve referred to and to a current Federal Aviation Administration-Sandia SHM testing program on full-size B737 aircraft. This program is using a variety of current monitoring methods, but not methods based on optical fibres that are currently under development.
Common indicators of structural health are elongation (strain) and acoustic emission from crack growth events. These can all be monitored with optical fibres, notably with FBGs. An optical fibre consists of an optically transparent core with a transparent coating that has a slightly higher refractive index (RI). Light rays that would otherwise escape from the core undergo total internal reflection at the interface and so are confined to the fibre allowing transmission of light with very little loss. The use of optical fibres is not confined to the visible spectrum.
A FBG is an optical structure that interferes with this free transmission of light along the fibre so that light of a specific wavelength is reflected back from the FBG to the source end of the fibre. A FBG consists of a length of fibre in which the RI of the core has been changed (slightly) in a periodic pattern over short (several millimetres) length of the fibre. They are normally formed in situ by a high powered laser targeted at the desired location for the FBG. The periodic spacing of the ‘grid’ of tiny sections of fibre with changed RI determines the wavelength that will be reflected. Thus, if the section of fibre containing the FBG is stretched, the resultant change in spacing of the grid causes a change in the wavelength of the reflected light.
If the section of fibre containing the FBG is bonded to a structure, or embedded within a composite structure, any strain in the structure, including strain caused by the passage of acoustic waves, will change the reflected wavelength. In other words, by ‘interrogating’ the optical signal (measuring changes in the reflected wavelength) the FBG can be used as a strain gauge, and as a pressure wave transducer, monitored by the optical fibre in which it has been formed.
Moreover, an optical fibre can contain more than one FBG, each tuned to a different wavelength. Since light of non-reflected wavelengths can pass through an FBG unaffected, a single fibre can be multiplexed, with multiple signals from the various FBGs within the fibre sharing the same transmission medium. This multiplexing ability also allows transmission of power through the optical fibre at the same time as it is being used for monitoring. This is useful in situations where transmission of electrical currents would create a hazard. Light energy is passed through the fibre to a local photovoltaic (PV) receptor to generate an electrical current at the spot where it is required, for example, to power a transducer.
The additional actions required in achieving Steve’s vision of a practical all-optical SHM monitoring are interrogating the optical signal (the change in wavelength) and integrating the output into electronic circuits. Practical first steps involve integrating optical sensing into current electronic control systems so that the technology can be introduced progressively using existing control infrastructure. Steve’s current research at ECU includes extending laboratory-scale techniques to develop integrated planar FBGs devices that can be bonded to structural surfaces.
At the end of the presentation the audience was able to examine a number of different types of commercially available fibres with embedded FBGs that Steve uses in his research.
Photo L to R: Geoff Williams, A/Prof Steven Hinckley