Temperature dependence of strength and fatigue of fused silica fiber in the range 77 to 473 K M. John Matthewson,* Charles R. Kurkjian, Christopher D. Haines, Navin Venugopal Photonic Component Reliability Group, Department of Ceramic & Materials Engineering, Rutgers University, Piscataway, NJ 08844 ABSTRACT The strength of optical fiber at low temperature is an important parameter since it approximates the inert strength, i. e. the starting strength of the material before degradation by fatigue. Published data suggest that the fatigue may abruptly slow below some temperature. However, published data are limited to strength vs temperature or fatigue in liquid nitrogen. We report strength and fatigue data for both bare (stripped) and metal coated fused silica optical fiber at temperatures down to 77 K. While fatigue slows as the temperature is reduced (i.e. the stress corrosion parameter increases with falling temperature) fatigue is still measurable at 77 K. This is the case even for hermetic metal coated fiber with extremely low water activity at the glass surface. The results confirm that fused silica exhibits "intrinsic" fatigue, i.e. fatigue in the absence of moisture. Keywords: Optical fiber, strength, fatigue, stress corrosion, temperature, intrinsic fatigue. 1. INTRODUCTION When the strength of fused silica optical fiber is measured under ambient conditions, moisture causes fatigue so that the measured strength is the initial, inert or starting strength of the material (i.e. the strength that would be measured in the absence of moisture) minus the loss in strength due to subcritical stress corrosion cracking during testing. The inert strength is important since it is a key parameter in models for mechanical reliability, plus it is needed to properly assess the measured strength — a weak specimen may be weak either because it contained a large flaw or because it fatigued rapidly. The most convenient way to estimate the inert strength is to measure the strength in liquid nitrogen, at which temperature the fatigue process (a chemical reaction between environmental moisture and strained bonds in the region of stress amplifying surface defects) is greatly slowed. However, in early work on fused silica fiber strength, Proctor et measured the strength of bare, hand-drawn fibers in tension and found that the strength in boiling helium (4.2 K) is higher than in boiling nitrogen (77 K) indicating that perhaps fatigue is not stopped at 77 K.2'3 Assuming that the fatigue process is a thermally activated chemical reaction, one expects the strength to roughly follow Arrhenius behavior.4 Fig. 1 shows an Arrhenius plot of the data of Proctor et al.1 (triangles). Apart from the point with the arrow which corresponds to a temperature of 4.2 K, the data roughly form a straight line suggesting that fatigue still proceeds at low temperature. Duncan et al.5 measured the strength of silicone coated optical fibers in two-point bending. Their data are also shown in Fig. 1 (squares). Their data also form a straight line on the Arrhenius plot, except that the measurement in boiling nitrogen is much lower than projected from the higher temperature data. This suggests a break in the behavior around 125 K (lIT =8 x iO K1); below this temperature fatigue is very much slower. However, this result may be an artifact of the silicone coating. At sufficiently low temperature the coating will embrittle and so could cause premature failure of the glass. Fortunati and Matthewson6 measured the strength of bare (i.e. acid stripped) fibers in two-point bending. Their experiments had poor temperature control so that measurements could not be averaged at a particular temperature. Their results (individual tests, circles in Fig. 1) show considerable scatter but do not show any evidence of a break in the linear behavior down to 77 K. Nonlinear behavior of strength on the Arrhemus plot has been found for a borosilicate glass,5 a soda-lime silica glass7 and E-glass.8 * mjohnmfracture.rutgers.edu; phone 1-732-445-5933; fax 1-732-445-3258; http://www.rci.rutgers.eduImjohnmImypage; Department of Ceramic & Materials Engineering, Rutgers University, 607 Taylor Road, Piscataway, NJ, USA 08854-8065 Reliability of Optical Fiber Components, Devices, Systems, and Networks, Hans G. Limberger, M. John Matthewson, Editors, Proceedings of SPIE Vol. 4940 (2003) © 2003 SPIE · 0277-786X/03/$15.00 74