Search

Optical fibre strain and shape measurement sensors were deployed on a rotor blade during a full-speed helicopter ground run, with real-time data wirelessly streamed from rotor hub-mounted sensor interrogators. In part 2 of a 2-part paper series, two-dimensional direct fibre-optic shape sensing (DFOSS), using fibre segment interferometry-based interrogation is investigated. The concept of blade shape change visualisation over one rotation period using rotation displacement surfaces is introduced and the usefulness of DFOSS data to gain additional insights by determining operational modal frequencies independently for both horizontal and vertical vibration directions of the blade is demonstrated.

Optical fibre strain and shape measurement sensors were deployed on a 5-m long rotor blade during a full-speed (rotation rate 6.6 Hz) helicopter ground run, with real-time data wirelessly streamed from rotor hub-mounted sensor interrogators. In Part 1 of a 2-part paper series, the strain sensing capabilities of the two optical fibre-based sensing techniques, optical fibre Bragg grating (FBG) and fibre segment interferometry (FSI), are compared, while Part 2 (Kissinger et al 2022 Smart Mater. Struct. accepted) specifically investigates the blade shape measurement based on the FSI approach. In part 1, the rotor hub-mounted instrumentation is described, and data on the dynamics of the blade obtained from a sequence of controlled pilot inputs are analysed both in the time and spectral domains. It is shown that this can provide insights into the aeroelastic properties of the blade. Noise standard deviations of 0.2 n ϵ / Hz and 30 n ϵ / Hz for the FSI and FBG-based sensing approaches, respectively, were observed over a strain range of 3500 µϵ.

The performance of two complementary optical strain measurement techniques, speckle shearing interferometry (shearography) and fibre Bragg grating (FBG) sensors, is compared with that of resistance foil strain gauges (RFSGs) and with theoretical predictions. The test object used for the surface strain measurements was a hydrostatically loaded ABS pipe. A multi-component shearography instrument, capable of full surface strain measurement, was used to determine the displacement gradient components, from which the surface strain components were calculated. Six surface mounted wavelength division multiplexed FBG sensors were used to measure the axial and the hoop strains. RFSGs located on the surface of the pipe, adjacent to the FBGs, were used for comparison. Reasonable agreement between theory and the axial and hoop strains determined by the different techniques was found. Issues associated with deploying and comparing the techniques are discussed.

We report the development of a fibre Bragg grating (FBG) interrogation system that is capable of detecting independently and simultaneously the two orthogonally polarized signals reflected from a polarization-maintaining (PM) FBG. This relaxes the constraints on the bandwidth of the FBG spectrum, allowing the use of a shorter PM FBG (larger bandwidth) for sensing applications where a higher spatial resolution is required. It imposes no limitation on the measurable strain range and strain profile. The interrogation system is capable of decoding the complex spectral responses of a PM FBG subjected to a transverse load.