Experiment on Bare Fiber Stress Fatigue Life Index

The optical fiber has been extensively used as a sensor in some special fields. Because the optical fiber exists crackles itself and also produce the micro-crack as well as local damage, the fatigue behavior of the optical fiber become the main problem we care about. This paper presents the experimental study on the fatigue performance of bare fiber under static, dynamic and cyclic load. The corresponding fatigue life index (Stress corrosion index) n can be obtained, which are 17.14, 11.83 and 5.88, respectively. The experiment data reveal that fatigue life index is related to loading mode and is treated as the indicator of crack propagation resistance of materials. Because dynamic and cyclic loads can induce additional damage, the fatigue life declines dramatically under the same stress levels. Introduction As the aerospace structure safety and reliability requirements are continuously improved, the online monitoring and diagnosis of the structure damage have attracted much attention [1]. The strain monitoring of the structure is the significant indicator of its damage degree. And optical fiber Bragg grating sensor, due to its improving properties such as small size, light weight, outstanding anti-jamming capability as well as characteristic of wavelength division multiplex and wavelength encoding, can be used to monitor the strain and the damage of the aviation and aerospace structures. The fiber is the sensory unit of the sensor. Through the fiber embedded in the composite material, the strain, degrees, cracks and the fatigue and damage information of the structure can be measured real-time [2], and then evaluation and prediction can be made on the fatigue failure of the material [2-6]. The optical fiber is also in the state of stress when used as a unit of the sensor. The fatigue performance of the bare fiber in various complex working conditions has become a topic of interest. The failure behavior of the optical fiber is mainly the result of subcritical crack growth due to its brittleness. Therefore the damage mechanism can be analyzed and revealed based on the linear fracture mechanism against the stress intensity in the crack tip area and the laws for the crack growth rate can also be obtained [7-13], which are valuable information for better application of the optical fiber. On the basis of fatigue theories of glass, this paper obtains fatigue indexes at three different loads from experiment to evaluate the failure behaviors of the optical fiber at different loads. Meanwhile, the static fatigue curve is fitted and obtained effectively based on the experimental data for the dynamic fatigue at the three loads. Experiment The optical fiber used in the experiments is produced by Wuhan ChangFei optical fiber and cable Co., LTD. The chemical composite of the external protective coating is the poly aluminum chloride (PAC). The bare optical fiber can be obtained by dissolving PAC from the surface of the optical fiber. The average diameter of the bare optical fiber is 107.62μm. Each end of the optical fiber is reinforced by the carbon fiber sheet. And the adhesive is solidified completely by being dried at an oven of 60° constantly for 24 hours. Specimens and experiment devices are shown in Figure. 1. 6th International Conference on Electronics, Mechanics, Culture and Medicine (EMCM 2015) © 2016. The authors Published by Atlantis Press 317 In this paper, the fatigue properties of the bare optical under static, dynamic and cyclic loads have been tested. All the samples are 50mm long. The static loading was applied by hanging a weight block at the end of the sample. The blocks weigh 0.85 b  , 0.8 b  , 0.75 b  , 0.7 b  , and 0.65 b  , respectively ( b  0.0005). The initial hanging time of every sample was recorded and the whole process filmed by computer and camera. Thus, the cracking-time of every sample could be obtained. The dynamic loading was generated by the Instron 5848 testing machine. The uniaxial tension was applied, and the tensile rates were 5mm/min, 0.5mm/min, 0.05mm/min, 0.005mm/min, and 0.0005mm/min, respectively. The cyclic loading was also generated by the Instron5848 testing machine. The uniaxial tension was chosen in the experiment. The maximum stresses were 0.9 b  , 0.85 b  , 0.8 b  , 0.75 b  , 0.7 b  , and 0.65 b  , respectively. The minimum tensile loading was 5N and the frequency 2Hz for all the cyclic loads. Figure. 1 A specimen and experiment device Results and Discussion Static Fatigue Test. The fatigue property of brittle materials is characterized by the subcritical crack growth V, which can be calculated by the stress intensity factor KI as: n I V AK  (1) where A is the constant and n the fatigue life index. 1/ 2 I a K Y a   (2) where Y is the crack geometry factor, a  the applied stress, and a the length of the crack. The value of the material fatigue life can be obtained by substituting the Eq. (2) into Eq. (1) to get the integration when a  is considered as a constant. The formulation of the fatigue life is written as: 2 2 [1 ( / ) ] n n n f c a a c t B            (3) Whereσc is the inherent intensity of the material, B is only related with the material and the working conditions, and can be written as: 2 2 2/[ ( 2) ] n IC B AY n K     (4) Because a  is smaller than c  and the n is generally large, the relationship between a  and c  can be obtained. It can be written as: 2 ( / ) 1 n a c     . The Eq. (3) can be simplified as follows: 2 n n f c a t B        (5) The logarithm has been taken on both sides of the Eq. (3). The formulation can be written as: 1/ 1/ og ( 2) / n a n f n n c Log n Log t n L B n n Log           (6)