Introduction

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 [2]. 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]. Considerable attempts have contributed to studying the strain transfer mechanism of optical fiber sensors. The earliest research started from the 1990s, and the strain relationship between the concrete and sensing fiber was studied with a polymer-to-glass modulus ratio of 1/200 given [21]. Host material with optical fiber sensor embedded was then simplified to infinite elastic cylinder model, and plane-strain theory was adopted to explore the strain transfer mechanism [24]. However, the simplified model in the two theories couldn’t be used to accurately determine the strain transfer relationships for various host materials with non-elastic behavior. In 1998, a systematic strain transfer theory of a three-layered structure embedded with optical fiber sensor was established [3]. To analyze the effects of a local interfacial slippage on the strain transfer ratio, a two-layered mechanical model consisted of host material and optical fiber was discussed [13]. For a multi-layered structure with various packaging layers, the unified strain transfer formula was conducted [33]. The improved strain transfer deduction of a three (multi)-layered sensing model by the use of simplified geometrical and physical functions was proposed [15]. Strain transfer analysis was also extended to special cases for considering the viscoelastic material properties of the monitored structure [30].