Yulong Li

Temperature Sensing of Metal-Coated Fiber Bragg Grating

Optical fiber Bragg grating (FBG) sensors have attracted considerable interest due to their special wavelength-encoded nature, small size, lightness, and immunity to electric-magnetic interference. Before being embedded into metal structures, FBG sensors must be protected and metal coating is one of the most efficient protective methods. The fluctuation of temperature induces thermal stresses between metal coating and the optical fiber due to their different thermal expansion coefficients. The thermal stresses affect the temperature sensitivity of FBG sensors. This paper presents a new method for analyzing the effects of thermal stresses on the Ni-Cu coated FBG, Cu-Ni coated FBG, and one-layer metal-coated FBG. Specifically, copper was chosen as the conductive layer for deposition on FBG and different thickness of nickel coating was then deposited on the top of the copper as the protective coating by electroless plating and electroplating. The sample with different thickness of metal coating was thermally tested from 35 °C to 95 °C. The electroplating Ni-Cu coated FBG was embedded into #45 steel by the brazing method and tested from 35 °C to 95 °C as well. The obtained data verified the soundness of the presented model.

A plating method for metal coating of fiber Bragg grating

We present a method for metal coating optical fiber and in-fiber Bragg grating. The technology process which is based on electroless plating and electroplating method is described in detail. The fiber is firstly coated with a thin copper or nickel plate with electroless plating method. Then, a thicker nickel plate is coated on the surface of the conductive layer. Under the optimum conditions, the surfaces of chemical plating and electroplating coatings are all smooth and compact. There is no visible defect found in the cross-section. Using this two-step metallization method, the in-fiber Bragg grating can be well protected and its thermal sensitivity can be enhanced. After the metallization process, the fiber sensor is successfully embedded in the 42CrMo steel by brazing method. Thus a smart metal structure is achieved. The embedding results show that the plating method for metallization protection of in-fiber Bragg grating is effective.

Highly Sensitive Ni-Cu Duplex Metal Coated Fiber Bragg Grating Temperature Sensor

Fiber Bragg grating (FBG) sensors have great potential for measuring temperature or strain and even detecting fractures in metal materials. The use of metal coatings is one of the most efficient methods for protecting FBG sensors. This article describes a new type of etchless duplex metal coating especially developed for FBG for sensing purposes. Copper was chosen as the conductive layer for deposition on FBG by electroless plating and nickel was then coated on top of the copper by electro deposition. Thermal shock testing confirmed that the copper coating had excellent surface adhesion to the FBG. SEM images revealed that the duplex coating has smooth boundaries. To compare the temperature sensing performance of Ni-Cu-coated FBG with the Ni-coated FBG, two samples were thermally tested from 30 to 100 °C in this study. The results indicate that Ni-Cu-coated FBG sensor not only has the better temperature repeatability but also has the higher temperature sensitivity.

Influence of alloy elements on microstructure and mechanical properties of Al/steel dissimilar joint by laser welding/brazing

The problem in dissimilar material joining (e.g., Al/steel) is the degradation of joint mechanical properties by formation of the hard and brittle interfacial intermetallic compounds. In the present study, in order to improve the joint mechanical properties, alloy elements Si and Zn are added in the form of Al-Si and Zn-Al filler metals, respectively. The effects of alloy elements on the joints are investigated in terms of interfacial microstructure and mechanical properties. The results have shown that element Si is able to suppress the growth of interfacial reaction layer, which leads to the improvement in the fracture load, while element Zn is capable of reducing the brittleness of reaction layer, and it consequently enhances the fracture load.

Laser welding/brazing of 5182 aluminium alloy to ZEK100 magnesium alloy using a nickel interlayer

ABSTRACT Aluminium alloy and magnesium alloy were successfully joined by using laser welding/brazing technology via a nickel interlayer. Microstructure and mechanical properties of the dissimilar Al/Mg joints with and without a nickel interlayer were comparatively investigated. No joints were achieved without a nickel interlayer; after welding, specimens were separated without applying any force. By inserting a nickel interlayer, sound metallurgical bonding were obtained at the interfaces. Hence, the joint strength reached 410 N with the failure at Mg/Ni interface. The influence of Ni interlayer on the weld defect, microstructure and joint strength was studied, and the joint formation mechanism was also discussed.

An Electroplating Method for Surface Mounting Optical Fiber Sensors on the Metal Substrate

An electroplating method for surface mounting fiber Bragg gratings (FBGs) on metal structures is presented. A process to electrolytically embed fiber sensors on the metal surface is elaborated. In addition, the mechanical property of the joint is investigated and the interface of the joint is studied. The temperature sensing characteristics as well as the failure temperature of the surface mounted FBGs are systematically studied. Results of the sensing tests show that the central wavelength keeps linear relationship with the temperature change and the temperature sensitivity is 30.7 pm/°C. The surface mounted FBG shows a similar response sensitivity to the transient temperature change to that of bare FBG. Furthermore, the failure temperature of the surface mounted FBG is tested, which is ~900 °C.

Formation and Toughening Mechanisms of Dispersions in Interfacial Intermetallics of Dissimilar Laser Al/Steel Joints

The key to improve the strength of Al/steel dissimilar joint is to toughen the hard and brittle interfacial intermetallics. Introducing soft and tough dispersions into the interfacial intermetallics would be a promising method. In the present study, Zn-Al alloy (Zn-22Al) was used as the filler metal to generate dispersions in the interfacial intermetallics matrix of dissimilar laser Al/steel joint. The results have shown that soft and tough dispersions, i.e., FeZn10, Al-rich amorphous phase, and Zn solid solution, were successfully introduced into the hard and brittle Fe2Al5−xZnx matrix. The formation of these dispersions was resulted from diffusion of elements Fe and Al and infiltration of liquid Zn along the grain boundaries of Fe2Al5−xZnx matrix and the subsequent chemical reaction and solidification. With this kind of interfacial microstructure, the joint exhibited a stable peak fracture load of 1200xa0N even though the interfacial layer thickness increased from 20 to 45xa0μm. This was because the interfacial layer was toughened by the soft and tough dispersions. Using this example, introduction of soft and tough dispersions into hard and brittle intermetallics matrix to toughen interfacial layer and then strengthening of dissimilar joints will be highlighted.

Theoretical and experimental study of macrobending losses in coated single-mode fibers

Abstract. A macrobending loss formula for single-mode fibers, which is shown to accurately predict the bend loss of a single-mode fiber with complex or real refractive index coating, is presented. Theoretical simulations and experimental measurements of bend loss in a fiber with nickel coating obtained by chemical plating and acrylate buffer coating are also presented. Agreements between simulations and measurements support the validity of the theoretical formula. Results show that the large imaginary part of the refractive index of the coating can intensify the bend loss oscillation behaviors, which implies that the bent fiber with metal coating can be utilized in fiber-optic devices.