Key Engineering Materials Vol. 558

Abstract: The emergence recently of a thermoelastic stress analysis (TSA) capability exploiting low-cost, compact and rugged microbolometer detector technology provides a significant opportunity topromote a broader use of this powerful non-contact full-field stress analysis technique. An area whereit has considerable and hitherto unexplored potential is in in-situ structural health monitoring (SHM).The present paper outlines the case for a nexus between SHMand TSA in this new form. It is proposedthat the approach should yield diagnostic and prognostic capabilities surpassing those of some existingSHM modalities. An F/A-18 centre-fuselage full-scale structural fatigue test is employed as a casestudy to illustrate the practical feasibility of the approach and to underscore some of its potential.Although the case study focuses on an aircraft structure, the concept has potential application to awide variety of different engineering assets across the aerospace, civil and maritime sectors.

Abstract: This paper reports on an experimental program of work which investigates the use of fibre Bragg gratings (FBGs) for the measurement of surface strains during full-scale structural testing. The test article was an ex-service F/A-18 wing attachment centre barrel (CB). The CB has three main structural bulkheads whose function is to carry wing loads into the fuselage. Two FBGs were surface mounted to each bulkhead on the starboard side with electrical resistance foil strain gauges bonded to symmetric locations on the port side of the structure for comparison. The FBGs were fabricated during the fibre draw process and were then packaged in a composite tape for secondary bonding to the structure under test using a standard aerospace qualified strain gauge adhesive. The centre barrel was subjected to a series of load surveys comprising 50%, 70% and 100% of the maximum spectrum load amplitude and low constant amplitude (8% of maximum spectrum load) cycling during thermoelastic stress analysis surveys prior to the initiation of variable amplitude accelerated fatigue spectrum loading until structural failure. The experimental data shows good agreement between the electrical-resistance foil and optical gauges when the loading is symmetric with additional detail provided by the optical gauges which had a higher sampling rate. These results confirm the potential of FBGs to provide detailed experimental strain data from across the structure without the associated complexity of wiring and installation often associated with electrical foil gauges.

Abstract: This paper summarises recent work conducted on the development of exposed core microstructured optical fibres for distributed corrosion sensing. Most recently, exposed-core fibres have been fabricated in silica glass, which is known to be reliable under a range of processing and service environments. We characterise the stability of these new silica fibres when exposed to some typical sensing and storage environments. We show the background loss to be the best achieved to date for exposed-core fibres, while the transmission properties are up to ~2 orders of magnitude better than for the previously reported exposed-core fibres produced in soft glass. This provides a more robust fibre platform for corrosion sensing conditions and opens up new opportunities for distributed optical fibre sensors requiring long-term application in harsh environments.

Abstract: In recent times there has been concerted pressure on military organisations worldwide to push for significant reductions in through-life support costs while maintaining and improving aircraft availability and safety. These goals, which on the surface may appear contradictory, can be achieved through the introduction of Structural Health Monitoring (SHM) systems. The paper describes the Australian Defence Science and Technology Organisation (DSTO) roadmap for SHM system development, validation, certification and implementation, in the context of technologies and systems that meet Australian Defence Force (ADF) future requirements. The current approach focuses on wide-area damage detection SHM systems using acousto-ultrasonic techniques.

Abstract: ncreased use of FRP composites for critical load bearing components and structures in recent years has raised the alarm for urgent need of a comprehensive health mentoring system to alert users about integrity and the health condition of advanced composite structures. A few decades of research and development work on structural health monitoring systems using Fibre Bragg Grating (FBG) sensors have come to an accelerated phase at the moment to address these demands in advanced composite industries. However, there are many unresolved problems with identification of damage status of composite structures using FBG spectra and many engineering challenges for implementation of such FBG based SHM system in real life situations. This paper details a research work that was conducted to address one of the critical problems of FBG network, the procedures for immediate rehabilitation of FBG sensor networks due to obsolete/broken sensors. In this study an artificial neural network (ANN) was developed and successfully deployed to virtually simulate the broken/obsolete sensors in a FBG sensor network. It has been found that the prediction of ANN network was within 0.1% error levels.

Abstract: In this paper, a data-driven approach to localizing structural damage subjected to ground motion is proposed by using the fractal dimension of the time-frequency features of structural dynamic responses. The time-frequency feature is defined as the real part of wavelet coefficient and the fractal dimension adopts the box-counting method. It is shown that the proposed fractal dimensions at each story of linear system are identical, while the fractal dimension at the stories with nonlinearity is different from those at the stories with linearity. Therefore, the nonlinear behavior of structural damage caused by strong ground motions can be detected and localized through comparing the fractal dimensions of structural responses at different stories. Shaking table test on a uniform 16-story 3-bay steel frame with added friction dampers modelling interstory nonlinear behavior was conducted. The experiment results validate the effectiveness of the proposed method to localize single and multi seismic damage of structures.

Abstract: Compressive sampling also called compressive sensing (CS) is a emerging information theory proposed recently. CS provides a new sampling theory to reduce data acquisition, which says that sparse or compressible signals can be exactly reconstructed from highly incomplete random sets of measurements. CS broke through the restrictions of the Shannon theorem on the sampling frequency, which can use fewer sampling resources, higher sampling rate and lower hardware and software complexity to obtain the measurements. Not only for data acquisition, CS also can be used to find the sparse solutions for linear algebraic equation problem. In this paper, the applications of CS for SHM are presented including acceleration data acquisition, lost data recovery for wireless sensor and moving loads distribution identification. The investigation results show that CS has good application potential in SHM.