Effect of embedded optical fiber sensors on transverse crack spacing of smart composite structures

In this study the effect of the presence of embedded optical fiber sensors on the transverse cracking of cross-ply laminates was investigated. The transverse crack spacing of cross-ply laminates with embedded optical fiber sensors was predicted using modified shear-lag analysis considering the presence of optical fibers and compared with experimental results. The effect of the orientation and quantity of optical fibers was evaluated and the effect of the coating of optical fiber was also investigated. Specimens were made with transparent glass/epoxy prepreg because the transverse crack and other damages such as delamination, splitting and bleeding of laser can be examined directly and visually. It has been found that the transverse crack spacing was not affected significantly by the embedding of optical fibers at low volume fraction of optical fibers. However, the cracks of specimens with embedded optical fibers which were initiated at a slightly lower stress level showed smaller spacing at the same stress level than those of specimens without embedded optical fibers. The theoretical crack spacing evaluated from the shear lag analysis showed good agreements with experimental results.     

The mechanical characteristics of smart composite structures with embedded optical fiber sensors

In this study, the mechanical characteristics of composite laminates with embedded optical fiber sensors were evaluated to investigate the effect of embedded optical fiber on the mechanical properties of composite laminates under the static tensile and the low cycle fatigue load. Testing specimens were fabricated with glass fiber/epoxy composites with embedded optical fiber sensors to observe initiation and growth of damage in the specimens and laser signal behavior transmitted through the optical fiber visually and directly. By using this transparency of glass fiber/epoxy composites, the damage of sensors and associated laser signal behavior was observed. Under the static load, the embedded optical fibers do not have significant effect on the stiffness and the strength, while the embedded optical fibers show significant effect on the fatigue life of composite specimens. Especially, the embedded optical fiber sensors show the very low resistance to the fatigue load.     

Surface-bonded and embedded optical fibers as ultrasonic sensors

The effectiveness of surface-bonded and embedded optical fibers for the detection of ultrasonic Lamb waves in 2-3-mm-thick steel, carbon-fiber-reinforced plastic (CFRP) and glass-reinforced plastic (GRP) plates are compared. A novel integrating ultrasonic sensor was achieved using the signal arm of an actively stabilized 633-nm homodyne Mach-Zehnder fiber interferometer which was either bonded directly to the plate surface or spliced to single-mode fibers embedded within a composite plate during manufacture. An embedded fiber is shown to be about 20 times more sensitive to Lamb wave motions than a surface-bonded fiber. However, the latter may be more practical.     

Signals of an intermodal fiber interferometer induced by laser frequency modulation

Signals of an intermodal fiber interferometer induced by laser optical frequency modulation are studied. Dependences of signal amplitudes and spectra on the laser frequency deviation are examined theoretically and experimentally for various optical fibers. It is established that the sensitivity of the intermodal fiber interferometer to the laser frequency variation essentially depends on the fiber refractive index profile. The minimal sensitivity corresponds to graded-index (α ≈ 2) multimode optical fibers. Step-index optical fibers (α = ∞) are more sensitive to the laser frequency variation by more than a factor of 100.     

Research on the interaction of three contact fibers in the fiber suspensions

Based on the slender-body theory, the governing equations for the mechanical interaction of one settling fiber with two fixed fibers in fiber suspensions were derived and solved numerically. In order to validate the model and method used in the paper, the interactions of two contact fibers were simulated and the results are in good accordance with the experiments. The results of three interacting fibers show that the initial contact point and the initial orientation angle of settling fiber have a significant effect, however, the angle between two fixed fibers has an insignificant effect, and the initial angular velocity of settling fiber has no effect on the interaction process of fibers. The settling fiber aspect ratio is not an independent parameter involving in the interaction between the fibers. The interaction duration increases as the fiber aspect ratio increases, however, the effect caused by reducing the diameter is more significant than that caused by increasing the length. The interaction duration is directly proportional to the solvent viscosity and verse directly to the fiber specific weight remarkably. Finally, a synthetic parameter A which contains the quantities affecting the interaction duration is derived to uniquely describe the total duration of interaction of the fibers.     

Analysis of optical fibers by the effective-index method

The effective-index method for determining waveguide dispersion is derived from the scalar wave equation and applied to optical fibers of arbitrary cross-sectional shapes. In the simplest use of the method, the optical fiber is replaced by an equivalent slab waveguide with an index profile derived from the geometrical shape of the fiber. Results from analyzing circular, elliptical, and cusp-shaped fibers are used to illustrate the general features of the method. A procedure is also given for improving the accuracy of the method applied to a class of single-mode fiber.     

Chemically etched fiber tips for near-field optical microscopy: a process for smoother tips

An improved method for producing fiber tips for scanning near-field optical microscopy is presented. The improvement consists of chemically etching quartz optical fibers through their acrylate jacket. This new method is compared with the previous one in which bare fibers were etched. With the new process the meniscus formed by the acid along the fiber does not move during etching, leading to a much smoother surface of the tip cone. Subsequent metallization is thus improved, resulting in better coverage of the tip with an aluminum opaque layer. Our results show that leakage can be avoided along the cone, and light transmission through the tip is spatially limited to an optical aperture of a 100-nm dimension.     

光纤缠绕式应变传感器

研究了一种可用于机敏复合材料与结构状态监测的本征型强度调制光纤应变传感器,它由两根以上多模光纤相互缠绕绞合形成。分析了该传感器的应变传感原理,得到其既能测量拉应变、又能测量压应变的结论。传感器对应变的响应具有良好的线性和重复性,灵敏度高,无迟滞现象。对植入碳纤维/环氧复合材料内的光纤缠绕型应变传感器的实验结果与理论分析一致,表明该传感器是适合于机敏复合材料与结构状态监测的较为理想的光纤传感器。     

碳纤维制造过程中径向差异表征及演变机理

为优化PAN基碳纤维结构,采用AES表征PAN纤维在低温碳化与高温碳化后C,N,O沿纤维径向的分布,并用以阐明预氧化碳化过程径向差异的形成机理.结果表明:预氧丝径向结构不均匀,由外向内氧化程度降低;预氧时物理阻隔与化学阻隔导致径向形成氧浓度梯度,热物理传递与化学反应放热导致径向形成温度梯度.低温碳化时,热物理传递与化学反应放热形成温度梯度加剧了预氧时的径向差异;纤维分3部分,最外层氧含量低,由氧化程度高的预氧皮层外部强烈脱氮脱氧形成,最内层由氧化程度低的预氧芯层转化而成;中间是过渡层,一部分由氧化程度较高的预氧皮层内部少量脱氧脱氮而成,氧含量高,而后过渡到预氧程度低的低含氧量芯部.高碳丝径向组分差异变小,纤维分两部分,外层厚度仅为纤维直径的10%,是碳含量逐渐降低的过渡性皮层,其余部分为组成均一的芯层.     

Remote fluorescence imaging of dynamic concentration profiles with micrometer resolution using a coherent optical fiber bundle

Dynamic concentration profiles within the diffusion layer of an electrode were imaged in situ using fluorescence detection through a multichannel imaging fiber. In this work, a coherent optical fiber bundle is positioned orthogonal to the surface of an electrode and is used to report spatial and temporal micrometric changes in the fluorescence intensity of an initial fluorescent species. The fluorescence signal is directly related to the local concentration of a redox fluorescent reagent, which is electrochemically modulated by the electrode. Fluorescence images are collected through the optical fiber bundle during the oxidation of tris(2,2'-bipyridine)ruthenium(II) to ruthenium(III) at a diffusion-limited rate and allow the concentration profiles of Ru(II) reagent to be monitored in situ as a function of time. Tris(2,2'-bipyridine)ruthenium(II) is excited at 485 nm and emits fluorescence at 605 nm, whereas the Ru(III) oxidation state is not fluorescent. Our experiments emphasize the influence of two parameters on the micrometer spatial resolution: the numerical aperture of optical fibers within the bundle and the Ru(II) bulk concentration. The extent of the volume probed by each individual fiber of the bundle is discussed qualitatively in terms of a primary inner-filter effect and refractive index gradient. Experimentally measured fluorescence intensity profiles were found to be in very good agreement with concentration profiles predicted upon considering planar diffusion and thus validate the concept of this new application of imaging fibers. The originality of this remote approach is to provide a global view of the entire diffusion layer at a given time through one single image and to allow the time expansion of the diffusion layer to be followed quantitatively in real time.     

Finite-element modal analysis of large-core multimode optical fibers

Plastic optical fibers that are a typical large-core multimode optical fiber support a great number of modes compared with conventional silica-glass multimode optical fibers. So far the WKB method hasbeen used for most of the modal analyses of these fibers because of a great number of guided modes. We describe the accurate eigenmodal analysis of large-core multimode optical fibers with the finite-element method (FEM) and compute the propagation constants of all LP modes. In addition, the FEM has a strong advantage for arbitrary core profiles whereas the WKB method is not suitable fornonmonotonic profiles. To demonstrate the advantage, we apply the FEM to the fiber having sinusoidal fluctuations.     

Fiber Bragg grating flow sensors powered by in-fiber light

This paper presents an active fiber Bragg grating temperature and flow sensor based on self-heated optical hot wire anemometry. The grating sensors are directly powered by optical energy carried by optical fibers. In-fiber diode laser light at 910 nm was leaked out from the fiber and absorbed by the surrounding metallic coating to raise the temperature and change the background refractive index distribution of the gratings. When the diode laser is turned off, the grating is used as a temperature sensor. When the diode laser is turned on, the resonance wavelength and spectral width change of the self-heated grating sensor is used to measure the gas flow velocity. The grating flow sensors have been experimentally evaluated for different grating length and input laser power. The grating flow sensors have demonstrated a 0.35- m/s sensitivity for nitrogen flow at atmosphere pressure.     

Hydrogenated amorphous carbon films used for carbon-sealed double-coated optical fibers

The carbon-sealed double-coated optical fiber is constructed from double-coated optical fiber sealed with a hydrogenated amorphous carbon film fabricated by plasma enhanced chemical vapor deposition. The pure methane (CH₄) and hydrogen (H₂) were selected as the precursor gases, and eight kinds of carbon film thicknesses by different deposited times were prepared with fixed CH₄/H₂ ratio. The radio-frequency power, substrate temperature, and working pressure were 200 W, 298 K, and 17 Pa, respectively. Characteristics of the carbon-sealed double-coated optical fibers prepared with different carbon film thicknesses were investigated. Experimental results indicated that when the carbon-film thickness was 113 nm, the carbon-sealed double-coated optical fibers had the highest mechanical strength, high resistance to moisture penetration, and good ability to withstand thermal loading. Additionally, external mechanical damage protection, abrasion resistance, bending flexibility, adhesion, stripping force, and bending insensitivity meet the fiber coating requirements. The carbon-sealed double-coated optical fiber is an optical fiber for optical transmission, and their properties are better than or equal to those of conventional double-coated optical fiber. Moreover, the carbon-sealed double-coated optical fiber is superior to nickel-sealed double-coated optical fiber in terms of water repellency and bending flexibility.     

Prediction and verification of an iridescent synthetic fiber

An optical model that predicts the reflection of light by a synthetic fiber of arbitrary cross-sectional shape is described. The model uses a Monte Carlo simulation of an exact ray trace of light for incident rays directed at a selected angle to the fiber axis. The model revealed an optical effect in round fibers that led to the prediction of a new mechanism for iridescence (change of color with angle of illumination or view) in a fabric by means of round, concentric, sheath-core fibers, with core size 相似文献   

Integration of micrometer-sized polymer elements at the end of optical fibers by free-radical photopolymerization

A simple method of manufacturing micrometer-sized polymer elements at the extremity of both single-mode and multimode optical fibers is reported. The procedure consists of depositing a drop of a liquid photopolymerizable formulation on a cleaved fiber and using the light that emerges from the fiber to induce the polymerization process. After exposure and rinsing a polymer tip is firmly attached to the fiber as an extension of the fiber core. It is shown that the tip geometry can be adjusted by the variation of basic parameters such as the geometry of the deposited drop and the conditions of drop illumination. When this process is applied to a multimode fiber three-dimensional molds of the fiber's linearly polarized modes can be obtained. The process of polymer-tip formation was simulated by a numerical calculation that consisted of an iterative beam-propagation method in a medium whose refractive index is time varying. It is shown that this process is based on the gradual growth, just above the fiber core, of an optical waveguide in the liquid formulation. Experimental data concerning two potential uses of the tipped fibers are presented.     

Expanding the realm of fiber optic confocal sensing for probing position, displacement, and velocity

We describe a fiber optic confocal sensor (FOCOS) system that uses an optical fiber and a lens to accurately detect the position of an object at, or close to, the image plane of the fiber tip. The fiber characteristics (diameter and numerical aperture) and optics (lens F/# and magnification) define the span and precision of the sensor and may be chosen to fit a desired application of position and displacement sensing. Multiple measurement points (i.e., fiber-tip images) may be achieved by use of multiple wavelengths in the fiber, so that each wavelength images the fiber at a different plane due to the chromatic dispersion of the optics. Further multiplexing may be achieved by adding fibers on the optical axis. A FOCOS with multiplexed fibers and wavelengths may also be used for velocity measurements.     

Precision fiber-optic sensor based on an optoelectronic recirculation system

It is proposed that an optoelectronic recirculation system can be used as an instrument for measuring the length of optical fibers and as a temperature sensor. The measured quantity is identified from the recirculation frequency, which is recorded with comparative ease and high accuracy. The optoelectronic recirculation system can be used to monitor the quality of optical fiber connections and other optical elements, and also to estimate the refractive index profile of multimode graded-index optical fibers. Translated from Izmeritel’naya Tekhnika, No. 7, pp. 32–35, July, 1999.     

氢与光纤的物化反应对井下永久性测量的影响

用于井下永久性测量的光纤传感器,由于在高温高压等恶劣环境下会与井下物质发生各种反应从而引起光纤吸收损耗的增加.为了掌握井下永久性测量用光纤吸收损耗的增加情况,本文针对影响光纤吸收损耗的重要因素-氢与光纤的物理及化学反应进行了研究,分析了其影响机理,讨论了其对光纤测量性能及使用寿命的影响,仿真计算了氢与光纤发生的各种反应所导致的光纤吸收损耗的增加情况,得到了化学反应引起的吸收损耗增加是影响永久测量性能的主要因素的结论,并由此给出了有效减少氢对光纤井下永久测量影响的预防措施.     

Numerical analysis of thermoplastic composites laser welding using ray tracing method

Laser technology is a good alternative for continuous joining of thermoplastics composites structures. Presence of continuous fibers at a high fiber volume fraction (superior to 30%) does not allow using traditional development as for pure thermoplastic materials, due to the presence of fiber clusters or polymer rich areas. Those heterogeneities induce macroscopic light scattering through the structure, reducing the resulting energy level absorbed at the welding interface. The study proposed here takes into account the real microstructure of the composite in order to evaluate changes in local energy diffusion directly linked with local fiber arrangements. The objective of this work is to develop an affordable numerical simulation of the laser welding process modeled with adapted physics mechanism and taking into account the microstructure heterogeneity of the considered materials regarding optical and thermal properties. To model the optical path of the laser beam through the composite fibrous structure, a simulation tool based on geometrical optic is developed. Weldability is considered on composites with different thicknesses, showing the non linear relationship between welding energy and substrate thickness.     

Influence of fiber optic probe geometry on the applicability of inverse models of tissue reflectance spectroscopy: computational models and experimental measurements

Accurate recovery of tissue optical properties from in vivo spectral measurements is crucial for improving the clinical utility of optical spectroscopic techniques. The performance of inversion algorithms can be optimized for the specific fiber optic probe illumination-collection geometry. A diffusion-theory-based inversion method has been developed for the extraction of tissue optical properties from the shape of normalized tissue diffusion reflectance spectra, specifically tuned for a fiber probe that comprises seven hexagonally close-packed fibers. The central fiber of the probe goes to the spectrometer as the detecting fiber, and the surrounding six outer fibers are connected to the white-light source as illumination fibers. The accuracy of the diffusion-based inversion algorithm has been systematically assessed against Monte Carlo (MC) simulation as a function of probe geometry and tissue optical property combinations. By use of this algorithm, the spectral absorption and scattering coefficients of normal and cancerous tissue are efficiently retrieved. Although there are significant differences between the diffusion approximation and the MC simulation at short source-detector (SD) separations, we show that with our algorithm the tissue optical properties are well retrieved within the SD separation of 0.5-3 mm that is compatible with endoscopic specifications. The presented inversion method is computationally efficient for eventual real-time in vivo tissue diagnostics application.