Researchers at the Defence Science and Technology Organisation (DSTO), part of Australia’s Department of Defence, are helping to develop a system to measure the structural health of a vessel at sea, using a sophisticated network of fibre-optic sensors.
The researchers are working with colleagues at the US Naval Surface Warfare Centre, US Naval Academy and CRC for Advanced Composite Structures to develop the system, which will provide a means to quickly and non-destructively inspect large composite structures.
Composites are increasingly used in the construction of new air and maritime structures, due to their strength-to-weight ratio and corrosion resistance. However, excessive pounding from high seas and underwater explosions and impacts with submerged objects can cause sub-surface structural damage, requiring the structures to be withdrawn from service for inspection.
‘The current system, known as SIDER, was developed for the US Navy and relies on procedures applied when the vessel is docked. We are adapting the technique so it can be performed at sea, reducing platform downtime,’ said Dr Claire Davis, DSTO research scientist.
As part of the new system, sensor networks using advanced Bragg gratings in optical fibres, will replace SIDER excitation grids. The sensors will record structural responses to environmental excitation, caused by a vessel’s movement through the sea.
Using modal analysis, a technique used to determine the vibration characteristics (natural frequencies and mode shapes) of a structure, the researchers will then be able to determine any change in the composite structure’s stiffness, implying damage or a structural anomaly.
The sensory network’s optical fibres are lightweight, corrosion resistant and immune to electromagnetic interference. Consequently, the new system can be surface mounted or embedded within a vessel’s structure with minimal intrusion.
‘The number of sensors that can be written on a single fibre is limited only by the length of the gratings and wavelength bandwidth of the light source available for interrogation,’ said Dr Davis.
DSTO researchers in Melbourne aim to validate the new system soon. Successful validation could mean the system is fitted to an in-service naval composite structure.
The programme is funded by the US Office of Naval Research Global, via the Naval Cooperative Opportunities in Science and Technology Program.