Dynamic fatigue experiments on optical fibers
- PDF / 547,786 Bytes
- 5 Pages / 597 x 774 pts Page_size
- 35 Downloads / 261 Views
I.
INTRODUCTION
THE important
developments and applications of optical waveguides that occurred during the last decade have given a strong impetus to research on the mechanical properties of optical fibers. Particular interest has been focused on the study of the mechanical strength of these materials and the control of fatigue and fracture in them. In a fiber submitted to a long period of stress, fatigue, i.e., degradation of mechanical strength with time, can lead to eventual fracture of the fiber. Fatigue in an optical fiber is a result of the slow growth of microdefects. This growth is produced by chemical attack from ions present in the surrounding medium. Such reagents are apparently inert if stress is absent, but when tensile stress is present, they can cause rupture of the material. It is widely accepted that the principal cause for the growth of microdefects is the presence in the surrounding medium of moisture. Even low levels of humidity have considerable effects on the mechanical strength of optical fibers. A technical requirement for optical fibers is that they should have a minimum life of 25 years. I11 Proof testing is often done to establish a minimum strength for the optical fibers. The behavior of these fibers is studied using the so-called universal fatigue curve, t2] The maximum stress allowed in the operation of optical fibers is calculated from this curve. The main difficulty with this procedure is the need to extrapolate the results obtained in a period of months to 25 years. In addition, reports in the literature t3,4] seem to show that the usual linear extrapolation indicates an apparent allowable stress that is too large. In other words, the usual predictions have been impugned as neither accurate nor safe. In the present study, fibers that had not been proof tested were used to measure strength in order to obtain fatigue results for low stresses and short fracture times. Results are represented in a universal fatigue curve, corMAX ANTONIO RAMOS LUCAS, Research Assistant, and RICARDO ENRIQUE MEDRANO, Professor, are with the Instituto de Fisica, UNICAMP, C.P. 6165, 13081, Campinas, SP, Brazil. PETER P. GILLIS, Professor, is with the Department of Materials Science and Engineering, University of Kentucky, Lexington, KY 40506-0046. Manuscript submitted December 22, 1989. METALLURGICAL TRANSACTIONS A
recting all the values for differences in inert strengths using the Evans and Wiederhorn tS} method. II.
THEORY
Degradation of the mechanical strength of glass due to the presence of water has long been known by glass blowers, but only in the last decade have models been developed to relate the change of strength with time. Evans and Wiederhorn [51 have used a relationship between the crack growth rate and the stress intensity factor, Ki V = AK~
[1]
Here, V is the crack growth velocity and A and n are constants related to the specimen geometry and material. The stress intensity factor is related to the applied tensile stress, or, and the flaw size, a, by Ki = YorX/-aa
[2]
Here, Y is a ge
Data Loading...
