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Embedding and characterization of fiber-optic and thin-film sensors in metallic structures
Abstract
It is often important to acquire information such as temperature and strain values from metallic tools and structures in situ. With embedded sensors, structures are capable of monitoring parameters at critical locations not accessible to ordinary sensors. To embed sensors in the functional structures, especially metallic structures, layered manufacturing is a methodology capable of rapidly and economically integrating sensors during the production of tooling or structural components. Embedding techniques for both fiber-optic sensors and thin-film sensors have been developed.
... In the same fashion the aerospace industry is embedding sensors on components of jet engines. Other industries that are taking advantage of the embedding technique are the automotive industry (components of motors), the oil industry (drilling equipment), the power industry (vessels and pipes) and the construction industry (structural components in buildings) [23]. ...
... It is important to maintain the integrity of the sensors to obtain functional metallic structures [23]. However, ultrasonic consolidation eliminates the need for thermal protection of sensors. ...
... A network of embedded fiber-optic sensors can allow a structure to monitor its integrity or health during manufacturing and service. Moreover, these sensors could replace many of the functions traditionally performed by human visual inspection and could provide real-time feedback in the event of structure failure [23]. ...
... However, key challenges remain that limit a direct embedding of such sensors to metal parts mainly owing a high process temperature induced thermal deformation during conventional manufacturing e.g, forging [11] and casting [12]. As a result many extant attempts have tried to monitor the physical and chemical status of metal components employing external sensors rather than on the inside [13][14][15]. ...
... Several researchers have attempted to embed and integrate commerically avaliable fibre-backed sensors into metal components using LPBF [15,[38][39][40]. Most of the work utilises the use of coating for a ceramic pre-coating for sensors to act as a thermal protective layer during the LPBF scanning process. ...
Metal additive manufacturing (AM) techniques like laser powder bed fusion can print highly complex geometries, ideally with internal sensors. Depending on the component shape these internal sensors, may only be accessible and monitored from the outer surface of the component. Hence, placing or printing of the sensors inside the component requires the sensor to communicate information from outside with remote sensing. Ideally, sensors would be added during printing; however, during LPBF very high temperatures (> 3347℃) are reached causing thermal damage to sensors. Two sets of results are presented. Firstly, four sensors were successfully designed and embedded using two types of novel sensors. An embedding methodology was developed and validated for strain monitoring in Ti-6Al-4V components. A powder protective layer was introduced to prevent damaging the sensors during the laser scanning process. An optimal 1 mm powder protective layer was determined using computational analysis and validated through three-point flexural bench testing. A 1 mm powder protective layer was effective for the strain gauges that were printed using direct ink write (DIW) with glass fibre (GF) reinforced phenolic backing and tripropylene glycol diacrylate (TPGDA) backing. Surface roughness affects the mechanical performance and durability of LPBF components. The surface topology requirements also vary on component application. The evolution mechanisms of surface roughness during LPBF are not well understood due to a lack of in situ characterisation methods. Therefore, the second set of experiment focused on defect dynamics are quantified using synchrotron X-ray imaging and ex situ optical imaging and explain the evolution mechanisms of side-skin and top-skin roughness during multi-layer LPBF of Ti-6Al-4V. Then a surface topology matrix was developed that accurately describes surface features. The results suggest that the proposed process can open new avenues for LPBF technology to realise metal components with a self-cognitive ability using integrated sensors and highlight the need for hybrid smart manufacturing to meet the demands of multiple sectors e.g., biomedical and aerospace.
... The coating process should not induce high temperature on the sensors as UVwritten gratings in optical fibers start to degrade at around 200 • C and are removed permanently at higher temperatures [1]. Low temperature processes such as electroplating, vacuum brazing, metal evaporation and ultrasonic consolidation have been utilized to deposit metallic coatings on bare FBG sensors [9][10][11][12]. ...
... Li et al [12] used a low temperature process of magnetron sputtering to coat FBGs with a titanium film ∼1 μm thick followed by an ∼2 μm thick Ni film over the titanium film. They then used electroplating to plate a protective Ni layer with a thickness of 0.5-1 mm. ...
This paper describes a combined fabrication method for creating a bi-material micro-scale coating on fiber Bragg grating (FBG) optical sensors using laser-assisted maskless microdeposition (LAMM) and electroless nickel plating. This bi-material coating alters the sensitivity of the sensor where it also acts as a protective layer. LAMM is used to coat bare FBGs with a 1–2 µm thick conductive silver layer followed by the electroless nickel plating process to increase layer thickness to a desired level ranging from 1 to 80 µm. To identify an optimum coating thickness and predict its effect on the sensor's sensitivity to force and temperature, an optomechanical model is developed in this study. According to the model if the thickness of the Ni layer is 30–50 µm, maximum temperature sensitivity is achieved. Our analytical and experimental results suggest that the temperature sensitivity of the coated FBG with 1 µm Ag and 33 µm Ni is almost doubled compared to a bare FBG with sensitivity of 0.011 ± 0.001 nm °C−1. In contrast, the force sensitivity is decreased; however, this sensitivity reduction is less than the values reported in the literature.
... In recent years, low-melting-point processes such as electroplating, vacuum brazing, sputtering, evaporation (CVD and PVD), ultrasonic consolidation, and electroless plating have been used for FBGs recoating [16][17][18][19][20]. In the present work, coating was made by ED. ...
Use of fiber Bragg gratings (FBGs) to monitor high temperature (HT) applications is of great interest to the research community. Standard commercial FBGs can operate up to 600 ∘ C. For applications beyond that value, specific processing of the FBGs must be adopted to allow the grating not to deteriorate. The most common technique used to process FBGs for HT applications is the regeneration procedure (RP), which typically extends their use up to 1000 ∘ C. RP involves a long-term annealing of the FBGs, to be done at a temperature ranging from 550 to 950 ∘ C. As at that temperature, the original coating of the FBGs would burn out, they shall stay uncoated, and their brittleness is a serious concern to deal with. Depositing a metal coating on the FBGs prior to process them for RP offers an effective solution to provide them with the necessary mechanical strengthening. In this paper, a procedure to provide the FBG with a bimetallic coating made by copper and nickel electrodeposition (ED) is proposed, discussing issues related to the coating morphology, adherence to the fiber, and effects on the grating spectral response. To define the processing parameters of the proposed procedure, production tests were performed on dummy samples which were used for destructive SEM–EDS analysis. As a critical step, the proposed procedure was shown to necessitate a heat treatment after the nickel ED, to remove the absorbed hydrogen. The spectral response of the FBG samples was monitored along the various steps of the proposed procedure and, as a final proof test for adherence stability of the bimetallic coating, along a heating/cooling cycle from room temperature to 1010 ∘ C. The results suggest that, given the emergence of Kirkendall voids at the copper–nickel interface, occurring at the highest temperatures (700–1010 ∘ C), the bimetallic layer could be employed as FBG coating up to 700 ∘ C.
The sensitivity required for optical fiber sensors such as fiber Bragg gratings (FBG) is highly dependent on applications, environment of use and the magnitude of physical parameters measured by the sensor. In addition, such sensitivity dependency is more complicated if FBGs are embedded in metallic structures using laser-assisted additive manufacturing. One of the methodologies for the improvement of the sensitivity of FBGs is to coat them with thin films. The coating not only protects the optical fiber sensors but also improves their sensitivity. In this paper, we propose a combined coating process for on-fiber thin film deposition using Laser Assisted Maskless Microdeposition (LAMM) followed by electroless Ni plating. An analytical/numerical model is also developed to study the effect of coating thickness on the sensor sensitivity. In addition, the role of laser parameters on the coating characteristics such as appearance quality and compositions is experimentally investigated.
This work proposes a new method for the fabrication of Multifunctional Metal Matrix Composite (MMC) structures featuring embedded printed electrical materials through Ultrasonic Additive Manufacturing (UAM). Printed electrical circuitries combining conductive and insulating materials were directly embedded within the interlaminar region of UAM aluminium matrices to realise previously unachievable multifunctional composites. A specific surface flattening process was developed to eliminate the risk of short circuiting between the metal matrices and printed conductors, and simultaneously reduce the total thickness of the printed circuitry. This acted to improve the integrity of the UAM MMC's and their resultant mechanical strength. The functionality of embedded printed circuitries was examined via four-point probe measurement. DualBeam Scanning Electron Microscopy (SEM) and Focused Ion Beam (FIB) milling were used to investigate the microstructures of conductive materials to characterize the effect of UAM embedding energy whilst peel testing was used to quantify mechanical strength of MMC structures in combination with optical microscopy. Through this process, fully functioning MMC structures featuring embedded insulating and conductive materials were realised whilst still maintaining high peel resistances of ca. 70 N and linear weld densities of ca. 90%.
The sensitivity required for optical fiber sensors such as fiber Bragg gratings (FBG) is highly dependent on applications, environment of use and the magnitude of physical parameters measured by the sensor. In addition, such sensitivity dependency is more complicated if FBGs are embedded in metallic structures using laser-assisted additive manufacturing. One of the methodologies for the improvement of the sensitivity of FBGs is to coat them with thin films. The coating not only protects the optical fiber sensors but also improves their sensitivity. In this paper, we propose a combined coating process for on-fiber thin film deposition using Laser Assisted Maskless Microdeposition (LAMM) followed by electroless Ni plating. An analytical/numerical model is also developed to study the effect of coating thickness on the sensor sensitivity. In addition, the role of laser parameters on the coating characteristics such as appearance quality and compositions is experimentally investigated.
Real time monitoring, diagnosis, and control of numerous manufacturing processes is of critical importance in reducing operation costs, improving product quality, and shortening response time. Current sensors used in manufacturing are normally unable to provide measurements with desired spatial and temporal resolution at critical locations in metal tooling structures that operate in hostile environments (e.g., elevated temperatures and severe strains). Microsensors are expected to offer tremendous benefits for real time sensing in manufacturing processes. Rapid tooling, a layered manufacturing process, could allow microsensors to be placed at any critical location in metal tooling structures. However, a viable approach is needed to effectively integrate microsensors into metal structures during the process. In this study, a novel batch production of metal embedded microsensor units was realized by transferring thin-film sensors from silicon wafers directly into nickel substrates through standard microfabrication and electroplating techniques. Ultrasonic metal welding (USMW) was studied to obtain optimized process parameters and then used to integrate nickel embedded thin-film thermocouple (TFTC) units into copper workpieces. The embedded TFTCs successfully survived the welding tests, validating that USMW is a viable method to integrate microsensors to metallic tool materials. Moreover, the embedded microsensors were also able to measure the transient temperature in situ at 50 μm directly beneath the welding interface during welding. The transient temperatures measured by the metal embedded TFTCs provide strong evidence that the heat generation is not critical for weld formation during USMW. Metal embedded microsensors yield great potential to improve fundamental understanding of numerous manufacturing processes by providing in situ sensing data with high spatial and temporal resolution at critical locations.
Fibre Bragg gratings (FBGs) of type I and IIA were fabricated in Ge-doped and B–Ge co-doped fibres using a 248 nm excimer laser and their performance characteristics were tested and compared with those of a chemical composition grating (CCG), written in a fluorine–germanium doped fibre, over a wide range of temperatures. Long-term testing (more than 600 h) involving a series of step-wise incremental temperature changes shows for the first time the potential of FBGs for high temperature measurement applications (up to and beyond 1100 °C), this depending on the type of FBG involved and the material and composition of the substrate fibre (the CCG was observed to be the most durable at very high temperatures). These gratings are likely to be useful for the simultaneous measurement of strain and temperature over these higher temperature ranges.
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The temperature dependence of the Bragg wavelength of fiber optic gratings was measured at temperatures from 4.2 to 350 K. The measured wavelength dependence at room temperature (293 K) corresponds to a refractive index dependence of dn/dT equals 9.1 X 10-6/ degree(s)C, which agrees with earlier results for fused silica. However, dn/dT decreases with lower temperatures, declining to 3.5 X 10-6/ degree(s)C at liquid nitrogen temperature (77 K), and to approximately zero at liquid helium temperature (4.2 K). The Bragg wavelength of a fiber grating embedded in a composite material displayed the same temperature dependence as a non-embedded grating.
Fiber Bragg gratings (FBGs) are simple intrinsic sensing elements which can be 'photo-imprinted' into fiber and represent one of the most exciting developments in the area of fiber optic sensing, probably since the inception of the all-fiber interferometer. These devices are based on the photosensitivity of optical fibers: Conventional tele- communications grade germanium-doped (Ge) optical fiber has been shown to exhibit a significant photosensitive response when illuminated with UV light in the region of 248 nm, a wavelength which corresponds to an absorption band, or 'color center,' in the glass associated with the Ge/SiO2 bonding. Absorption of the UV light in the glass breaks bonds creating changes in color centers which modify the absorption characteristics of the glass. This change in absorption results in a shift in the index of the glass at wavelengths removed from the absorption region through the Krammers-Kronig relationship. As the Ge dopant is usually confined only to the core (light guiding) region of the fiber, the effect is observed only in the core. The required UV light can be readily produced by various sources: KrF excimer lasers, dye lasers, frequency doubled Ar lasers and quadrupled Nd:YAG lasers. The optical power levels produced by these sources vary, and the most common source used is the KrF laser which can produce intense pulses at 10 to 50 Hz repetition rates. Changes in index are generally on the order of 10-3 or less, although recent work on fibers with enhanced photosensitive response (hydrogen- loaded fiber), index shifts of approximately 10-2 have been reported. The advent of the holographic side- exposure and phase-mask techniques for writing gratings has made the devices readily available for widespread usage in fiber optic systems. Although the primary application area for Bragg gratings appears to be in the fiber communications field, there has been strong interest in FBG-based sensing, and progress in this area has been rapid with many significant developments over the past 5 years.
Metal-matrix composite structural components' subsurface strain and temperature levels can be ascertained by means of embedded fiber-optic sensors, if these can be incorporated without degradation in the course of composite fabrication. Attention is presently given to a USAF program for the embedding of fiber-optic sensors in Ti-alloy matrix composites, whose severe conventional processing conditions are broken down into less severe steps. It is noted that the sensor fiber used should be of a composition optimized to minimize dopant migration, and should incorporate abusive processing-resistant coatings.
A technique for embedding one or more optical fibers in a cast metal part or structure while maintaining optical transmission through the fiber is presented. This technique provides nondestructive monitor of internal perturbations of the structure. Application of the method to embedded fiber optic sensors in metallic structures and to fiber-embedded metal feedthrough are reported and the performances of temperature and ultrasound fiber sensor embedded in a cast aluminum block are demonstrated.
A technique for embedding one or more optical fibers in a cast metal part or structure while maintaining optical transmission through the fiber is presented. This technique provides nondestructive monitor of internal perturbations of the structure. Application of the method to embedded fiber optic sensors in metallic structures and to fiber-embedded metal feedthrough are reported and the performances of temperature and ultrasound fiber sensor embedded in a cast aluminum block are demonstrated.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
Fiber optic smart structure technology offers the prospect of
enabling new systems that are capable of providing critical information
for manufacturing, nondestructive evaluation, health, damage and control
systems. These systems are synergistic with the expanding fiber optic
communication and optoelectronic markets and can be expected to have a
bright future
This article presents a new method for embedding optical fibers into a nickel alloy and gives the results for a long-term test with thermal cycling of two fiber-optic Bragg gratings embedded in nickel alloy. We embedded these Bragg gratings in a piece of Inconel 600 (a nickel alloy) using vacuum brazing. We then thermally cycled this piece between 500, 525, and 550°C for about six months while monitoring the reflected wavelengths of the gratings. We tested two other embedded gratings for 68 hours at 600°C. Some microscopic cross sections of the embedded fibers are presented. The results show that fiber sensors embedded in metal can operate reliably at very high temperature and in harsh environments. We hope that the results from the long-term, elevated temperature test will make it possible to apply the technology of fiber-optic sensing in new and demanding monitoring applications, especially at high temperatures in energy production.
We demonstrate the higher temperature sensitivity of a fiber Bragg grating (FBG) sensor when it is clad with a metal of a large thermal expansion coefficient. With lead (solder) cladding, the sensitivity of Bragg wavelength shift can be enhanced by about five (four) times. A theoretical model was adopted to show quite consistent results. It was found that thermal annealing was crucial for preparing high-quality fiber Bragg grating sensors with metal claddings.
Evaluation of a new method for metal embedding of optical fibres for high temperature sensing purposes”, Baltica V conference, Condition and life management for power plants
- S Sandlin
- L Heikinheimo
Sensing applications of fiber Fabry-Perot interferometers embedded in composites and in metals
- H F Taylor
- C E Lee