Distributed Fibre Optic Sensor-Based Continuous Strain Measurement along Semicircular Paths Using Strain Transformation Approach

Abstract

Distributed fibre optic sensors (DFOS) are popular for structural health monitoring applications in large engineering infrastructure because of their ability to provide spatial strain measurements continuously along their lengths. Curved paths, particularly semicircular paths, are quite common for optical fibre placement in large structures in addition to straight paths. Optical fibre sensors embedded in a curved path configuration typically measure a component of strain, which often cannot be validated using traditional approaches. Thus, for most applications, strain measured along curved paths is ignored as there is no proper validation tool to ensure the accuracy of the measured strains. To overcome this, an analytical strain transformation equation has been developed and is presented here. This equation transforms the horizontal and vertical strain components obtained along a curved semicircular path into a strain component, which acts tangentially as it travels along the curved fibre path. This approach is validated numerically and experimentally for a DFOS installed on a steel specimen with straight and curved paths. Under tensile and flexural loading scenarios, the horizontal and vertical strain components were obtained numerically using finite element analysis and experimentally using strain rosettes and then, substituted into the proposed strain transformation equation for deriving the transformed strain values. Subsequently, the derived strain values obtained from the proposed transformation equation were validated by comparing them with the experimentally measured DFOS strains in the curved region. Additionally, this study has also shown that a localised damage to the DFOS coating will not impact the functionality of the sensor at the remaining locations along its length. In summary, this paper presents a valid strain transformation equation, which can be used for transforming the numerical simulation results into the DFOS measurements along a semicircular path. This would allow for a larger scope of spatial strains measurements, which would otherwise be ignored in practice.

Smart monitoring of a full-scale composite hydrofoil manufactured using automated fibre placement under high cycle fatigue

“…The curved dips in the strain profile are due to the bend loops in the fibre optic sensor. The shape of these dips is not defined in the plots because the bend was not maintained perfectly semicircular, which would otherwise follow a defined curvature [60]. The peak strain value is comparable to the static test carried out at 25 000 cycles interval.…”

“…However, the optical sensor installation and curing condition of composites during manufacturing may impact optical signals at higher strains [59]. A practical continuous optical fibre path usually has semicircular curvature in between straight paths, where the strain values need to be transformed to desired orientation [60], which requires further study in an embedded sensor system. While the optical and MEMS accelerometers have small footprints, associated lightweight wireless data acquisition and transfer systems are critical for the practicality of these systems.…”

A Flexible Eddy Current TMR Sensor for Monitoring Internal Fatigue Crack

“…The sensors are arranged at the dangerous parts of the structure to sense the parameters representing the structural health status, and the maintenance plan of the structure is formulated through output collection, processing and structural health status judgment. At present, sensor monitoring technologies used for structural health monitoring mainly include strain sensor monitoring technology [1][2][3], optical fiber sensor monitoring technology [4,5], comparative vacuum sensor monitoring technology [6,7], ultrasonic guided wave monitoring technology [8][9][10], smart coating sensor monitoring technology [11][12][13], eddy current sensor monitoring technology [14][15][16], acoustic emission monitoring technology [17], etc. Both strain sensor monitoring technology and optical fiber sensor monitoring technology are used to monitor the real-time load of the structure and evaluate the remaining service life of the structure.…”

Airflow vibration of diaphragmatic breathing: model and demonstration using optical biosensor

“…SMF and FBG have characteristics that are superior to other fibers such as micro-sized [13], resistant to EM interference [14], easy to modify [15], and high precision [16]. The principle of strain in FOS usually uses a bending method with various patterns such as circular [17], straight [18], and sinusoidal [19]. However, sinusoidal patterns proved to be more effective than other forms in detecting changes in sensor parameters in the human body [20].…”