Technical report | Evaluation of a Distributed Fibre Optic Strain Sensing System for Full- Scale Fatigue Testing

Abstract 

This report details an experimental comparison between the performance of conventional electrical resistance foil strain gauges and a commercially available fibre optic distributed strain measurement system based on Rayleigh scattering. Results are presented which compare strain response, spatial resolution and noise levels between the two systems, firstly on coupon specimens containing fatigue-induced cracks and secondly on a full-scale fatigue test article consisting of a centre barrel of an ex-service F/A-18 under simulated operational loading spectrum.

Executive Summary

Current industry practice for the measurement of strain is the electrical resistance foil strain gauge. These sensors are time consuming to install and require three shielded wires per sensor, which can add considerable weight and complexity to the structure under test when high-density strain surveys are required. Electrical gauges are also prone to fatigue and require frequent calibration when installed on operational aircraft. Distributed fibre optic strain measurement systems present the opportunity to significantly reduce installation costs and complexity as well as addressing some of the durability and performance issues associated with electrical gauges.

This report details an experimental comparison between the performance of conventional electrical resistance foil strain gauges and a commercially available fibre optic distributed strain measurement system based on Rayleigh scattering. The results presented compare strain response, spatial resolution and noise levels between the two systems, firstly on coupon specimens containing fatigue-induced cracks and secondly on a full-scale fatigue test article consisting of a centre barrel of an ex-service F/A-18 subject to simulated operational spectrum loading.

In most areas the optical strain data compares well with measurements made using foil strain gauges however, there are some limitations to the system particularly when measuring strains in regions of high strain gradient. Despite these limitations, in many cases there is potential for Rayleigh scattering to provide detailed strain measurements  at a substantially reduced cost per sensing point compared to conventional electrical resistance foil strain gauges.