Contents lists available at ScienceDirect Engineering Fracture Mechanics journal homepage: www.elsevier.com/locate/engfracmech Equivalent initial flaw size model for fracture strength prediction of optical fibers with indentation flaws Yuxuan Cui, Yunxia Chen, Jingjing He ⁎ Beihang University, School of Reliability and Systems Engineering, No. 37, Xueyuan Road, Haidian District, Beijing 100191, China Science and Technology on Reliability and Environmental Engineering Laboratory, Beijing 100191, China ARTICLE INFO Keywords: Surface flaw Crack growth Residual strength ABSTRACT This work presents an equivalent initial flaw size (EIFS) model for indentation flaws to quantify the fracture strength of optical fibers. Three geometry parameters of the indentation flaws, namely, the total projection length, the average crack length and the total flaw depth are used to establish the EIFS model. Indentation flaws of different dimensions are made using a Berkovich indenter by adjusting the load force. A stereo microscopic imaging system is employed to acquire the geometry parameters of the indentation flaws. Tensile testing is then performed using 16 commercial optical fiber specimens. A groove fixture is employed to ensure the reliability of data acquisition. The experimental data are used for model development and performance validation. Furthermore, the proposed model is compared with existing models to demonstrate the effec- tiveness of the method. The proposed EIFS model gives more accurate prediction of fracture strength in current investigation. 1. Introduction Silica optical fibers are widely applied in the fields of telecommunication and structure health monitoring due to the outstanding performance of sensitivity, speed, and flexibility [1]. The mechanical integrity and the strength of optical fibers are critical to the reliability of the optical-fiber-based sensing systems [2,3]. Fracture is the most common failure mode of optical fibers under the combination of mechanical loads and harsh environments [4]. The mechanical strength of the optical fibers can reach up to 10GPa when they are in pristine state; however, surface flaws and impurities can reduce the strength to roughtly one order of magnitude [5,6]. Therefore, it is of great importance to quantify the influence of the surface flaws on the mechanical properties of optical fibers. Two types of flaws commonly exist in optical fibers. The first one is the contamination in glass interior and surface [7]. For example, the gritty particles originate from the impurities of ingredient or from the drawing environment conditions are typical contaminations [8]. The other type of flaws are introduced before the fiber is coated during the drawing process [9,10]. Although severe flaws could be eliminated by the screening procedure, those survived flaws can still pose a great risk for critical applications [6]. In this work the surface flaws are mainly studied to investigate their effect on the fracture strength of optical fibers. Artifical surface flaws have been reported to investigate the mechanical properties of optical fibers [11]. Mechanical abrasion with particles and indentation are two commonly used methods to create artificial flaws. The former one can best represent a natural flaw as the process is very similar to the naturally developed flaws [12]. However, it is difficult to control the flaw size and the residual strength of an optical fiber [13]. Indentation is an alternative method to create surrogate flaws and has been used for the analysis of fatigue https://doi.org/10.1016/j.engfracmech.2019.04.021 Received 5 December 2018; Received in revised form 15 April 2019; Accepted 16 April 2019 ⁎ Corresponding author at: Beihang University, School of Reliability and Systems Engineering, No. 37, Xueyuan Road, Haidian District, Beijing 100191, China. E-mail address: hejingjing@buaa.edu.cn (J. He). Engineering Fracture Mechanics 215 (2019) 36–48 0013-7944/ © 2019 Elsevier Ltd. All rights reserved. T