Fatigue Performance of Type I Fibre Bragg Grating Strain Sensors

Abstract

Although fibre Bragg gratings (FBGs) offer obvious potential for use in high-density, high-strain sensing applications, the adoption of this technology in the historically conservative aerospace industry has been slow. There are several contributing factors, one of which is variability in the reported performance of these sensors in harsh and fatigue prone environments. This paper reports on a comparative evaluation of the fatigue performance of FBG sensors written according to the same specifications using three different grating manufacturing processes: sensors written in stripped and re-coated fibres, sensors written during the fibre draw process and sensors written through fibre coating. Fatigue cycling of the fibres is provided by a customized electro-dynamically actuated loading assembly designed to provide high frequency and amplitude loading. Pre- and post-fatigue microscopic analysis and high-resolution transmission and reflection spectra scanning are conducted to investigate the fatigue performance of FBGs, the failure regions of fibres as well as any fatigue-related effects on the spectral profiles. It was found that because of the unique fabrication method, the sensors written through the fibre coating, also known as trans-jacket FBGs, show better fatigue performance than stripped and re-coated FBGs with greater control possible to tailor the optical reflection properties compared to gratings written in the draw tower. This emerging method for inscription of Type I gratings opens up the potential for mass production of higher reflectivity, apodised sensors with dense or complex array architectures which can be adopted as sensors for harsh environments such as in defence and aerospace industries.

Comparative strain survey of an aerospace structure using distributed fibre optic strain sensing technology

“…FOSs do not experience strain drift or fatigue damage under normal operating conditions, as specified in [1]. Previous testing by the authors showed that draw tower gratings, which are similar to AGF, survived an incremental constant amplitude loading sequence (Table 1) accumulating 3 million applied load cycles, with failure eventually occurring at a strain of 23,000 µƐ [2]. It was previously mentioned that FOSs provide significant reductions in weight and complexity of installation relative to FSGs.…”

“…A 3.25 m long AGF sensor with 2024 sensing points was selected to provide strain measurements every 1.6 mm with a notional strain measurement accuracy of ±1με. This type of FOS is mechanically robust with a 5% (50,000 𝜇𝜇𝜇𝜇) tensile failure strain [1] and exceptional fatigue resistance [2].…”

“…A high-strain fatigue resistant packaging and adhesive strategy described in [4] was used for the installation of the FOSs. The FOSs are adhered using LOCTITE EA9309.3NA two-part epoxy which has excellent peel strength and elongation properties, and can withstand harsh environments.…”

Trans-Jacket Fibre Bragg Gratings for In-Situ Health Monitoring of Defence Platforms in Harsh Environments

“…1). The dynamic range of this rig was designed to induce tensile strains in an optical fibre of up to 36,000 µε at a fixed 100 Hz cyclic loading frequency [20]. [20], with the FBG mounted between the moving part and the stable part (labelled).…”

Crack Detection Zones: Computation and Validation

“…Recently, many researchers have been using FBGs for crack detection [12][13][14][15] and SHM [16,17] because of their good linearity and resistance to harsh environments [18]. Even their fatigue performance is superior to the alternative electrical resistance SGs [19]. It must be noted that this is related to the sensor itself and not to a sensor that is glued to a structure.…”