To realize the reliable and long-term strain detection, the durability of optical fiber sensors has attracted more and more attention. The packaging technique has been considered as an effective method, which can enhance the survival ratios of optical fiber sensors to resist the harsh construction and service environment in civil engineering. To monitor the internal strain of structures, the embedded installation is adopted. Due to the different material properties between host material and the protective layer, the monitored structure embedded with sensors can be regarded as a typical model containing inclusions. Interfacial characteristic between the sensor and host material exists obviously, and the contacted interface is prone to debonding failure induced by the large interfacial shear stress. To recognize the local interfacial debonding damage and extend the effective life cycle of the embedded sensor, strain transfer analysis of a general three-layered sensing model is conducted to investigate the failure mechanism. The perturbation of the embedded sensor on the local strain field of host material is discussed. Based on the theoretical analysis, the distribution of the interfacial shear stress along the sensing length is characterized and adopted for the diagnosis of local interfacial debonding, and the sensitive parameters influencing the interfacial shear stress are also investigated. The research in this paper explores the interfacial debonding failure mechanism of embedded sensors based on the strain transfer analysis and provides theoretical basis for enhancing the interfacial bonding properties and improving the durability of embedded optical fiber sensors.
The structural safety of civil infrastructures, ocean platforms and aerospace structures has received increasing attention, because the failure of those important structures usually leads to large abundant of casualties and economical loss. To characterize the structural performance, structural health monitoring (SHM) technology has been recognized as one of the most effective and intelligent measures [18,19,1,23,22,10,7]. By the use of smart sensors and components, the real-time, long-term and continuous information of the in-situ structures can be provided for the damage identification, disaster forecasting and warming, and safety and life-time assessment [35,17,20,16,26]. Among these smart sensing elements, optical fiber based sensors are the most popular in civil engineering for the unique advantages of high sensitivity and precision, corrosion resistance, anti-electromagnetic interference, good stability, geometrical shape-versatility, absolute measurement and convenient integration of network [25,34,32,12]. For the brittle material properties of silica fiber, bare optical fiber is weak to resist the shear or torsion force in structural construction and operation. Especially for the embedded case, the packaging technique is the most critical factor to guarantee the survival and enhance the durability of optical fiber based sensors. However, the existence of the protective layer introduces the intermedium between the sensing fiber and the monitored structure, which makes the strain measured by the sensor not entirely represent the actual strain of host material . The error between the measured strain and the actual strain is attributed to the strain loss in the transferring path. To eliminate the strain transfer error and improve the measurement accuracy of optical fiber based sensors, strain transfer theory has been developed to establish the quantitative relationship of strains between the host material and the optical fiber [13,33,9,28].