Hybrid Optical Fiber Sensors for Smart Materials and Structures

There has been a rapid growth in the use of advanced composite materials in a variety of load-bearing structures, for example in aviation for structures such as rotor blades, aircraft fuselage and wing structures. Composite materials embedded with fiber-optic sensors (FOS) have been recognized as one of the prominent enabling technologies for smart materials and structures. The rapid increase in the interest in composite materials embedded with FOS has been driven by numerous applications, such as intelligent composite manufacturing/processing, and safety-related areas in aircrafts. Research has been focused recently on using several optical sensor types working together to form so called “hybrid optical fiber sensors” in order to overcome the limitations of the individual sensor technologies. The main aim of the research described in this thesis is to investigate a hybrid sensing scheme that utilizes polarimetric sensors and FBG sensors working in a complimentary fashion to measure multiple physical parameters in a composite material, with a particular focus on measuring the complex indirect parameters thermal expansion and vibration. The research described in this thesis investigates the performance of a hybrid sensing scheme based on polarimetric sensors and FBG sensors after embedding in a composite material. It is shown that the influence of thermal expansion within a composite material on embedded polarimetric sensors is the main source of errors for embedded fiber sensor strain measurements and that for practical strain sensing applications buffer coated PM-PCF are more suitable for embedding in composite. Further, using a buffer stripped PM-PCF polarimetric sensor, a measurement scheme to measure a composite material\u27s thermal elongation induced strain is proposed. A novel hybrid sensor for simultaneous measurement of strain, temperature and thermal strain is demonstrated by integrating polarimetric sensors based on acrylate coated high bi-refringent polarization maintaining photonic crystal fiber (HB-PM-PCF), and a coating stripped HB-PM-PCF sensor together with an FBG sensor. Flexible demodulation modules that can be embedded or surface attached is a challenge for composite materials containing fiber-optic sensors. In this thesis an interrogation method that allows intensity domain operation of hybrid sensor is demonstrated. Further focusing towards the miniaturization of the hybrid sensor interrogator, a miniaturized flexible interrogator for the demonstrated hybrid sensing scheme embedded in a composite material is also designed. Low frequency vibration measurements are performed for glass fibre-reinforced composite material samples with two different strain-sensitive polarimetric sensor types embedded. It is shown that the strain sensitivity of polarimetric sensors limits the vibration measurements to a certain range of vibration amplitudes. A polarimetric sensor based buffer stripped HB-PM-PCF is demonstrated for monitoring the different stages of the curing process for a Mageneto-Rheological composite material. By providing information about multiple parameters such as strain, temperature, thermal strain, vibration amplitude and vibration frequency the proposed and demonstrated hybrid sensing approach has a high potential to change the paradigm for smart material design in the future

Optical Fiber Sensors for Smart Composite Materials and Structures

Composite material structures are widely used in the aerospace, marine, aviation, transport, sport/leisure and civil engineering industries. Such composite material structures are frequently subjected to external perturbations and varying environmental conditions, which may cause the structures to suffer from fatique damage and/or failure, and thus require real-time structural health monitoring (SHM). Of necessity, the diagnosis process and condition monitoring of composite structures are usually carried out during their working life. The goal of such diagnosis is to detect, identify, locate, and assess the defects that my affect the safety or performance of a structure

Overview of Fiber Optic Sensor Technologies for Strain/Temperature Sensing Applications in Composite Materials

This paper provides an overview of the different types of fiber optic sensors (FOS) that can be used with composite materials and also their compatibility with and suitability for embedding inside a composite material. An overview of the different types of FOS used for strain/temperature sensing in composite materials is presented. Recent trends, and future challenges for FOS technology for condition monitoring in smart composite materials are also discussed. This comprehensive review provides essential information for the smart materials industry in selecting of appropriate types of FOS in accordance with end-user requirements

A Photonic Crystal Fiber Based Polarimetric Sensor for Cure Monitoring of Magneto-Rheological Smart Composite Material

A buffer stripped high birefringent photonic crystal fibre based polarimetric sensor is developed for monitoring the curing process of magnetorheological elastomer (MRE) smart composite material. Using the developed sensor, different phases of the MRE curing process are clearly visible from the phase shift variation of the embedded polarisation maintaining photonic crystal fibre (PM-PCF) sensor. During the curing process, the buffer stripped PM-PCF exhibits a stress/strain induced phase shift variation from 0 to 1.98 rad. This is a significantly large phase change, which can be used to very clearly identify the different stages in the curing process. For comparison, a fibre Bragg grating sensor is also used for monitoring the internal strain during the curing process and its response does not allow one to reliably distinguish all the curing stages. The present investigation offers a simple non-destructive method to monitor the curing process of MRE smart composite material

A Miniaturized Flexible Surface Attachable Interrogator for Hybrid Optical Fiber Sensing

In this paper, we propose a miniaturized flexible interrogator for polarimetric and fiber Bragg grating (FBG) sensors based hybrid sensing scheme embedded in composite materials. The flexible interrogation module comprises of an Arrayed Waveguide (AWG) for FBG demodulation and an Electro Optic Mach- Zehnder Interferometric (EO-MZI) intensity modulator for polarimetric sensor demodulation. A customized design of AWG and EO- MZI are presented, and the designed performance resulted in a wide channel spacing of 5 nm and low cross talk of -34 dB between adjacent channels for the AWG, and a low Vπ voltage of ±1.5 Volt for the MZI-EO. The method for fabrication of the flexible interrogator device is also presented, based on post processing of platinum coated polyimide substrate. The film type interrogation module is compact, with a size of 3.4 cm x 0.1 cm x 0.01 cm. The surface attachable flexible hybrid sensor interrogator is proposed to integrate with photo detector arrays and wireless communication technology to enhance the competency of hybrid sensing scheme in smart sensing composite parts in motion for applications in aircraft, wind rotor blades, and so forth

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

Whispering Gallery Mode Micro Resonators for Multi-Parameter Applications

A novel fiber optic sensing configuration for simultaneously measuring ammonia vapor (NH3) concentration and relative humidity (RH) in air is proposed and experimentally demonstrated. The system comprised two silica whispering gallery mode (WGM) microsphere resonators coated with different polymer layers. One of the microspheres was dip-coated with sol gel silica polymer and another with a 0.5 % wt./vol. agarose hydrogel. WGMs in both microspheres were excited simultaneously by evanescent coupling using a single adiabatic fiber taper. The optical properties of both coating layers change due to their exposure to ammonia and water molecules in the surrounding atmosphere, resulting in the spectral shifts of the WGM resonances relevant to each of the microspheres. By measuring the relevant WGMs\u27 spectral shifts, the NH3 concentration in air and the RH can be determined simultaneously. The experimentally demonstrated sensitivity of the proposed sensor array to ammonia was estimated as 19.07 pm/ppm (NH3 molecules in air) and its sensitivity to relative humidity as 1.07 pm/% RH. Detailed studies of the coatings\u27 cross-sensitivity and temperature dependence are also presented. The proposed sensor array is compact, highly sensitive and potentially low cost

Anisotropic magnetism in PrCrSb<SUB>3</SUB>: bulk properties of single crystals

We report anisotropic dc magnetic susceptibility &#967;(T), isothermal magnetization M(H), electrical resistivity &#961;(T), and heat capacity C(T) measurements on a single crystalline sample of PrCrSb3. The crystal exhibits ferromagnetic ordering below TC=112 K due to ordering of Cr spins. The ferromagnetic phase is highly anisotropic. The anisotropy becomes even more evident at low temperatures at which the rare-earth ion gets magnetically ordered. Pr moments antiferromagnetically order at TN=12 K. The magnetic interplay between these two species (Cr3+ and Pr3+ ions) gives PrCrSb3 a complex magnetic phase diagram

Anisotropic magnetism in PrCrSb<SUB>3</SUB> and NdCrSb<SUB>3</SUB>

In this work we compare resistivity, susceptibility, magnetization and heat capacity of single crystals of PrCrSb3 and NdCrSb3. Both of them exhibit ferromagnetic ordering of Cr moments at 112 K and 100 K, respectively. Pr and Nd moments order at low temperatures (TPrN=12K and TNdC=10K) leading to moment compensation effects, even though bulk ferromagnetism of Cr moments persists down to the lowest temperature. The transport studies on PrCrSb3 show existence of gap in the spin-wave spectrum which seems to be absent in NdCrSb3. Moreover, the observed anisotropy in bulk properties and the ferromagnetic ordering of Cr moments are found to decrease with decrease in the size of the rare earth ion

Anisotropic magnetism in NdCrSb<SUB>3</SUB>

We report anisotropic dc magnetic susceptibility &#967;(T), isothermal magnetization M(H), electrical resistivity &#961;(T) and heat capacity C(T) measurements on a high quality single crystalline sample of NdCrSb3. The crystal exhibits ferromagnetic ordering below TC=100 K due to ordering of Cr spins. The ferromagnetic phase is highly anisotropic. The anisotropy becomes even more evident at low temperatures where the rare earth ion (Nd3+) gets magnetically ordered. Nd moments order ferromagnetically below 10 K. The magnetic interplay between these two species (Cr3+ and Nd3+ ions) gives NdCrSb3 a complex magnetic phase diagram