1,627 research outputs found
Encapsulation of FBG sensor into the PDMS and its effect on spectral and temperature characteristics
Fiber Bragg Grating (FBG) is the most distributed type of fiber-optic sensors. FBGs are primarily sensitive to the effects of temperature and deformation. By employing different transformation techniques, it is possible to use FBG to monitor any physical quantity. To use them as parts of sensor applications, it is essential to encapsulate FBGs to achieve their maximum protection against external effects and damage. Another reason to encapsulate is increasing of sensitivity to the measured quantity. Polydimethylsiloxane (PDMS) encapsulation appears to be an interesting alternative due to convenient temperature and flexibility of the elastomer. This article describes an experimental proposal of FBG PDMS encapsulation process, also providing an analysis of the FBG spectral characteristics and temperature sensitivity, both influenced by high temperature and the process of polydimethylsiloxane curing itself. As for the PDMS type, Sylgard 184 was employed. Encapsulation consisted of several steps: allocation of FBG to PDMS in its liquid state, curing PDMS at the temperature of 80°C ± 5 %, and a 50-minute relaxation necessary to stabilize a Bragg wavelength. A broadband light source and an optical spectrum analyzer were both used to monitor the parameters during the processes of curing and relaxation. Presented results imply that such a method of encapsulation does not have any influence on the structure or functionality of the FBG. At the same time, a fourfold increase of temperature sensitivity was monitored when compared to a bare FBG
Review of Fiber Optic Sensors for Structural Fire Engineering
Reliable and accurate measurements of temperature and strain in structures subjected to fire can be difficult to obtain using traditional sensing technologies based on electrical signals. Fiber optic sensors, which are based on light signals, solve many of the problems of monitoring structures in high temperature environments; however, they present their own challenges. This paper, which is intended for structural engineers new to fiber optic sensors, reviews various fiber optic sensors that have been used to make measurements in structure fires, including the sensing principles, fabrication, key characteristics, and recently-reported applications. Three categories of fiber optic sensors are reviewed: Grating-based sensors, interferometer sensors, and distributed sensors
The viscosity of silica fibres
The viscosity of an optical fibre over 1000 to 1150 {\deg}C is studied by
inscribing an optical fibre Bragg grating that can withstand temperatures up to
1200 {\deg}C and monitoring fibre elongation under load through the Bragg
wavelength shift. This optical interrogation offers high accuracy and
reliability compared to direct measurements of elongation, particularly at
lower temperatures, thus avoiding significant experimental error. An excellent
Arrhenius fit is obtained from which an activation energy for viscous flow of
Ea = 450 kJ/mol is extracted; addition of an additional temperature dependent
pre-exponential does not change this value. This value is less than that
idealised by some literature but consistent with other literature. The log plot
of viscosity is overall found to be consistent with that reported in the
literature for silica measurements on rod and beams, but substantially higher
to past work reported for optical fibres. The discrepancy from an idealised
activation energy Ea ~ 700 kJ/mol may be explained by noting the higher fictive
temperature of the fibre. On the other hand, past optical fibre results
obtained by beam bending with much lower values leave questions regarding the
method of viscosity measurement and the time taken for structural
equilibration. We note that because regenerated gratings already involve
post-annealing to stabilise their operation at higher temperature, the
structures are much more relaxed compared to normal fibres. This work
highlights the need to stabilize components for operation in harsh environments
before their application, despite some mechanical compromise. Given the
increasing expectation of all-optical waveguide technologies operating above
1000 {\deg}C, the need to study the behaviour of glass over the long term
brings added significance to the basic understanding of glass in this regime.Comment: Submitted to Acta Material
Study on fibre optic sensors embedded into metallic structures by selective laser melting
Additive Manufacturing, which builds components layer by layer, opens up exciting
possibilities to integrate sophisticated internal features and functionalities such as
fibre optic sensors directly into the heart of a metal component. This can create truly
smart structures for deployment in harsh environments. The innovative and
multidisciplinary study conducted in this thesis demonstrates the feasibility to
integrate fibre optics sensors with thin, protective nickel coatings (outer diameter
~350 μm) into stainless steel (SS 316) coupons by Selective Laser Melting
technology (SLM).
Different concepts for fibre embedment by SLM are investigated. The concepts
differ in which way the fibre is positioned and how the powder is deposited and
solidified by the laser in respect to the optical fibre. Only one approach is found
suitable to reliably and repeatable encapsulate fibres whilst preserving their structural
integrity and optical properties. In that approach SS 316 components are
manufactured using SLM, incorporating U-shaped grooves with dimensions suitable
to hold nickel coated optical fibres. Coated optical fibres containing Fibre Bragg
Gratings (FBG) for strain and temperature sensing are placed in the groove. Melting
subsequent powder layers on top of the FBGs fuses the fibre’s metallic jacket to the
steel and completes the integration of the fibre sensor into the steel structure.
Cross-sectional microscopy analysis of the fabricated components, together with
analysis of fibre optic sensors’ behaviour during fabrication, indicates proper stress
and strain transfer between coated fibre and added SS 316 material. During the SLM
process embedded Fibre Bragg grating (FBG) sensors provide in-situ temperature
measurements and potentially allow measuring the build-up of residual stresses.
Subsequently, FBG sensors embedded into SS 316 structures using our approach
follow elastic and plastic deformations of the SS 316 component, with a resolution of
better than 3 pm*μɛ-1. Temperature measurements are also conducted with a
precision of 3 pm*K-1. Such embedded fibre sensors can also be used to high
temperatures of up to ~400 °C.
However, at elevated temperatures issues arise from the significantly larger thermal
expansion coefficient of SS 316, leading to delamination of the more rapidly
expanding metal from the glass. Rapid thermal expansion of the metal also leads to
high axial stresses within the glass exceeding the fibres tensile strength and
ultimately leading to structural damage of the optical fibre
Experimental Study and Analysis of a Polymer Fiber Bragg Grating Embedded in a Composite Material
The characteristics of polymer fiber Bragg gratings (FBGs) embedded in composite materials are studied in this paper and are compared with characteristics of their silica counterparts. A polymer FBG of 10 mm length which exhibits a peak reflected wavelength circa 1530 nm is fabricated and characterized for this purpose. A silica FBG with a peak reflected wavelength circa 1553 nm is also embedded in the composite material for a comparison study. The fabricated composite material sample with embedded sensors is subjected to temperature and strain changes and the corresponding effects on the embedded polymer and silica FBGs are studied. The measured temperature sensitivity of the embedded polymer FBG was close to that of the same polymer FBG in free space, while the silica FBG shows elevated temperature sensitivity after embedding.With an increase in temperature, spectral broadening was observed for the embedded polymer FBG due to the stress induced by the thermal expansion of the composite material. From the observed wavelength shift and spectral bandwidth change of the polymer FBG, temperature and thermal expansion effects in the composite material can be measured simultaneously
Femtosecond laser micro-machined optical fiber based embeddable strain and temperature sensors for structural monitoring
Structural monitoring technology is becoming increasingly important for managing all types of structures. Embedding sensors while constructing new structures or repairing the old ones allows for continual monitoring of structural health thus giving an estimate of remaining utility. Along with being embeddable, miniaturized sensors that are easy to handle are highly sought after in the industry where in-situ monitoring is required in a harsh environment (corrosive atmosphere, high temperatures, high pressure etc.).
This dissertation demonstrates the use of femtosecond laser-fabricated Fabry-Perot interferometer (FPI) based optical fiber sensors for embedded applications like structural health monitoring. Two types of Fabry-Perot interferometer sensors, extrinsic FPI and intrinsic FPI, have been designed, developed and demonstrated for strain and temperature monitoring applications. The absence of any movable parts make these sensors easy-to-handle and easy to embed inside a material. These sensors were fabricated using a laboratory integrated femtosecond (fs) laser micromachining system. For the extrinsic Fabry-Perot interferometer (EFPI) design, the fs-laser was used to ablate and remove the material off the fiber end face while for intrinsic Fabry-Perot interferometer (IFPI) design, the laser power was focused inside the fiber on the fiber core to create two microstructures. The scope of the work presented in this dissertation extends to device design, laser based sensor fabrication, sensor performance evaluation and demonstration.
Feasibility of using these sensors for embeddable applications was investigated. A new type of material called Bismaleimide (BMI) was used for demonstrating the embeddability of the sensors. Experimental results of strain and temperature testing are presented and discussed. The EFPI sensor has low temperature sensitivity of 0.59 pm/⁰C and a high strain sensitivity of 1.5 pm/µε. The IFPI sensor has the same strain sensitivity as EFPI but is 25 times more sensitive to the temperature. These sensors were tested up to 850 ⁰C in non-embedded condition and they produced a linear response. A hybrid approach combining the EFPI and IFPI sensors was demonstrated for simultaneous measurement of strain and temperature --Abstract, page iii
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Structurally Embedded Electrical Systems Using Ultrasonic Consolidation (UC)
Current research has demonstrated the use of Ultrasonic Consolidation (UC) to embed
several USB-based sensors into aluminum, and is working toward embedding suites of
sensors, heaters and other devices, connected via USB hubs, which can be monitored and
controlled using an embedded USB capable processor. Additionally, the research has
shown that electronics can be embedded at room temperature, but with some inter-layer
delamination between the ultrasonically bonded aluminum layers. Embedding sensors
and electronics at 300o
F to overcome the delamination issues resulted in optimal
bonding, and the sensors used thus far have functioned normally. Future investigation
will explore other UC parameter combinations to ascertain the quality of embedding at
lower temperatures.Mechanical Engineerin
Development of a fiber optic high temperature strain sensor
From 1 Apr. 1991 to 31 Aug. 1992, the Georgia Tech Research Institute conducted a research program to develop a high temperature fiber optic strain sensor as part of a measurement program for the space shuttle booster rocket motor. The major objectives of this program were divided into four tasks. Under Task 1, the literature on high-temperature fiber optic strain sensors was reviewed. Task 2 addressed the design and fabrication of the strain sensor. Tests and calibration were conducted under Task 3, and Task 4 was to generate recommendations for a follow-on study of a distributed strain sensor. Task 4 was submitted to NASA as a separate proposal
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