Composite structures with embedded fiber optic sensors: A smart propellant tank for future spacecraft applications

A silicone rubber packaged distributed optical fiber sensing tape for strain-crack monitoring based on OFDR technique

We have developed a new distributed strain measurement system with a long gauge Fiber Bragg Grating (FBG) sensor based on Optical Frequency Domain Reflectometry (OFDR). This sensing system enables us to measure strain at an arbitrary position along the FBG sensor with a high spatial resolution below 1mm. However, the sensing range of this system is confined to 〜10m at present. To expand the measurement range, we have problems in several components of the measurement system, such as the signal processing capability, limitation of measurable range due to reflection properties of the FBG sensor, and the effect of length fluctuation in the arm length of the interferometer. This paper discusses these factors using a simulation model of based on coupled mode theory. The signal analysis time was shortened by confining the analyzing section. The simulated result showed that, the measurable range is approximately 1.5m, and the length fluctuation in the arm length of measurement interferometer has a significant effect on the measured result.

This article presents the structural assessment of an existing old steel railway bridge in Malaysia using fibre optic sensors. The steel bridge was built more than a century ago and the bridge structural drawings and other documentations were lost over the years. Keretapi Tanah Melayu Berhad (KTM), the Malaysian rail operator was upgrading the railway line to cater for heavier and faster locomotives. The bridge is subjected to a potential risk of structural health deterioration or fatigue due to corrosion and long service period in such a high humidity country. Full scale static and dynamic tests using locomotives were carried out to assess the structural integrity of the steel bridge. Two types of optical sensors were used. Discrete and distributed fibre optic sensors were mounted on critical steel panels to monitor the change in strain of these members under dynamic and static loads. The discrete sensor, Fibre Bragg Grating (FBG) strain sensors were able to measure the change in strain at 1000 Hz frequency, and were used during the dynamic test where a locomotive travelled through the bridge at maximum speed of 40 km/hour. Distributed fibre optic (DFOS) sensors were mounted on the bottom chord of the steel truss to measure the maximum tensile strain when the locomotive was placed at the centre of the bridge. The results from the two different fibre optic sensors (FBG and DFOS sensors) were then compared and the results showed good agreement to strain measurements. © International Conference on Smart Infrastructure and Construction 2019, ICSIC 2019: Driving Data-Informed Decision-Making.

We report on an approach for reducing the effects of temperature in a fiber optic distributed sensor. This technique employs a sensing fiber and a Brillouin optical time domain reflectometer (BOTDR). The BOTDR has been proposed for measuring both strain and optical loss distribution along optical fibers by accessing only one end of the fiber. The BOTDR analyzes changes in the Brillouin frequency shift caused by strain. This device can measure distributed strain with an accuracy of better than plus or minus 60 X 10-6 and a high spatial resolution of up to 1 m over a 10 km long fiber. However, temperature fluctuations have an adverse effect on the accuracy with which the Brillouin frequency shift can be measured because the shift changes with temperature as well as with strain. This has meant that both spatial and temporal fluctuations in temperature must be compensated for when a fiber optic distributed sensor is used for continuous strain measurements in massive civil structures. We describe a method for the simultaneous determination of distributed strain and temperature which separates strain and temperature in a fiber optic sensor.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Electric field-assisted embedding of fiber optic sensors in structural materials for structural health monitoring

Desiccation cracking can significantly affect the hydro-mechanical behaviour of soil in various engineering applications. Characterising the evolution of desiccation-induced shrinkage strain in soils is a key step in understanding the cracking mechanism. This study presents a novel framework for monitoring and early detecting soil cracking by utilising the distributed fibre optical sensing (DFOS) technique based on optical frequency domain reflectometry (OFDR). DFOS-OFDR was integrated into a set of experimental testing to simultaneously monitor water content, crack pattern and strain state. OFDR with 1 με strain measurement accuracy was employed to monitor the initiation and propagation of cracks. The results reveal a strong correlation between the spatiotemporal evolution of soil strain and crack, confirming the efficacy and reliability of the proposed framework. During the drying process, the compressive strain induced by soil shrinkage increases first and starts to decrease when cracking is initiated. Further, DFOS-OFDR demonstrated promising results to predict the position of cracks. Compared to traditional strain monitoring methods with discrete measurements, DFOS-OFDR offers a non-destructive, accurate yet efficient, high-resolution and continuous technique for monitoring and early detection of soil desiccation cracking.

Coherent optical sensing technology faces significant challenges in practical applications due to the extremely weak signal light, necessitating highly sensitive coherent receivers. A high-sensitivity, highly integrated, and low-noise coherent receiver module based on germanium-silicon photonic chips has been developed. A testing system was constructed to assess the performance of the coherent receiver module, revealing a reception sensitivity of -70dBm, with physical dimensions of 33×40×8.6 mm3 and an output noise voltage of ±4mV. Additionally, a distributed sensing system based on optical frequency domain reflectometry (OFDR) was established to verify the module’s application in OFDR systems, achieving a spatial resolution of 1mm.

Fibre optic (FO) sensors are becoming increasingly popular for different applications in structural monitoring. Among their excellent properties, a strong interest for this type of sensors are represented by the possibility of embedding FOs inside composite components. In this case, one of the factors that significantly influence the efficiency of the whole Structural Health Monitoring (SHM) system is the interfacial adhesion between FO sensors and the host material. The main objective of this work is to investigate the interfacial adhesion between embedded fibre optic sensors and epoxy matrix to find the best type of optical fibre to be used in epoxy matrices to produce smart composites. Four types of optical fibres with different diameters and coatings (i.e. polyimide, polyacrylate and ormoceramic) were used. Pull-out tests were carried out and different methods were used to obtain the composite/optical fibre interfacial properties. Finally, an optical microscopy and Scanning Electron Microscopy (SEM) analysis were performed to characterize the fibre/matrix interfaces. It was found that the optical fibre that presented the highest energy required for interface rupture and, consequently, less invasiveness to the host material was the ormoceramic fibre with the smallest diameter.

Measuring strain fields in FRP strengthened RC shear walls using a distributed fiber optic sensor

On the processing and evaluation of pultruded smart composites

The use of the pultrusion process for the manufacture of fibre reinforced polymer (FRP) composites with embedded fibre optic sensors is discussed. The specific application is the use of smart composite reinforcements for strain monitoring in innovative concrete bridges and structures. The Bragg grating and Fabry-Perot fibre optic sensors are embedded during the pultrusion of FRP rods and the process-induced residual strains are evaluated using these sensors. The behaviour of optic sensors during pultrusion is assessed, and the effect of the embeddment of optical fibres and their surface coatings on the mechanical properties of the composite material is investigated. To verify the operation of the optic sensors embedded in the smart pultruded rods, mechanical tests were conducted and the output of the fibre optic sensors was compared to that of an extensometer. These mechanical tests were performed at room temperature as well as under conditions of low and high temperature extremes. The reliability assessment of the fibre optic sensors further entailed the study of their fatigue and creep behaviour as well as their performance when the rods in which they are embedded are placed in a severe environment (e.g., alkaline solutions) that may simulate conditions encountered in concrete structures wherein the composite rods will be used as prestressing tendons or rebars.Key words: smart composite reinforcements, fibre optic sensors, pultrusion, residual strain, fatigue and creep behaviour, reliability assessment.

Fiber Bragg Grating sensors (FBG) have a great resistance to embedding processes. This property is very useful for monitoring parameters at inaccessible places. Embedded fiber optic sensors into composites have been studied for a long time, but embedding a fiber sensor into metallic structure is beginning to study. Recently, this has raised interest due to embedded FBG in the metallic structure provide capabilities for controlling parameters of the structural health status and also information about their own process of deterioration. The embedding process of the FBG sensors involves the fusion of structural metallic material. During this process, very high temperatures are achieved that could damage the Bragg grating or the silica fiber. To protect the sensor during the embedding process, a fiber coating is made with a metallic material with a high melting point. In this paper we study three different techniques for coating a FBG sensor: physical vapour deposition (PVD), electroless deposition and electroplating. This paper describes the experimental procedure for coating metallic fiber optic sensors and the optical characterization.

Metaheuristic inverse analysis on interfacial mechanics of distributed fiber optic sensors undergoing interfacial debonding

Fiber optic sensors (FOS) in long-term structural health monitoring (SHM) have drawn significant attention due to their pivotal role in detecting defects and measuring structural performance in diverse infrastructures. While using FOS, temperature variation due to environmental factors is still considered one of the major challenges to isolating sensing parameters. To address this issue, we reported a machine learning (ML)-augmented multi-parameter sensing system that enables simultaneous detection of strain and temperature effects based on one single fiber Bragg gratings (FBGs) sensor for SHM. The initial phase entailed designing, fabricating, and characterizing a novel FBG sensor in the laboratory, incorporating a set of four FBGs, each distinguished by distinct Bragg wavelengths. In the next phase, ML algorithms are employed to separate temperature effects from strain variations. As a proof of concept, mechanical loading tests are conducted on the sensor, exposing the FBG portion to various temperature conditions. In the final phase, data collected from a post-tensioned concrete bridge embedded with both strain and temperature FBG sensors are utilized, and the developed ML models are applied to observe real-environment outcomes. Despite the limited feature points of collected FBG spectrums, the developed ML models effectively address cross-sensitivity issues induced by temperature perturbations. The long-term benefit of using FOS is that it will enable a better understanding and utilization of aging infrastructure. This will potentially reduce embodied carbon of infrastructure in the future and assist in the global efforts to achieve Net-Zero.

Distributed fiber-optic sensing (DFOS) technologies have been used for decades to detect damage in infrastructure. One recent DFOS technology, Optical Frequency Domain Reflectometry (OFDR), has attracted attention from the structural engineering community because its high spatial resolution and refined accuracy could enable new monitoring possibilities and new insight regarding the behavior of reinforced concrete (RC) structures. The current research project explores the ability and potential of OFDR to measure distributed strain in RC structures through laboratory tests on an innovative beam–column connection, in which a partial slot joint was introduced between the beam and the column to control damage. In the test specimen, fiber-optic cables were embedded in both the steel reinforcement and concrete. The specimen was tested under quasi-static cyclic loading with increasing displacement demand at the structural laboratory of the Pacific Earthquake Engineering Research (PEER) Center of UC Berkeley. Different types of fiber-optic cables were embedded both in the concrete and the rebar. The influence of the cable coating and cable position are discussed. The DFOS results are compared with traditional measurements (DIC and LVDT). The high resolution of DFOS at small deformations provides new insights regarding the mechanical behavior of the slotted RC beam–column connection, including direct measurement of beam curvature, rebar deformation, and slot opening and closing. A major contribution of this work is the quantification of the performance and limitations of the DFOS system under large cyclic strains. Performance is quantified in terms of non-valid points (which occur in large strains when the DFOS analyzer does not return a strain value), maximum strain that can be reliably measured, crack width that causes cable rupture, and the effect of the cable coating on the measurements. Structural damage indices are also proposed based on the DFOS results. These damage indices correlate reasonably well with the maximum sustained drift, indicating the potential of using DFOS for RC structural damage assessment. The experimental data set is made publicly available.