Strength and Fatigue Properties of Optical Glass Fibers Containing Microindentation Flaws

Abstract

The inert strength and dynamic fatigue properties of fused‐silica optical fibers are studied using subthreshold indentation flaws, i.e., flaws without radial cracks. These subthreshold properties differ from those obtained in comparative tests on silica rods containing postthreshold indentation flaws in three major respects: (1) the inert strengths are significantly higher than predicted by extrapolation of the postthreshold data; (2) the slopes of the dynamic fatigue plots are likewise greater, indicating a greater susceptibility of the subthreshold flaws to chemical kinetic effects; and (3) the scatter in strengths is wider. These trends reflect the change in mechanical response reported for optical fibers with “natural” flaw populations in going from ordinary to ultra‐high‐strength regions. Direct observations of the indentation sites up to the point of failure indicate that the property differences can be interpreted in terms of a transition from propagation‐controlled to initiation‐controlled fracture instabilities at reduced contact loads. The subthreshold instability condition is modeled qualitatively as a two‐step, deformation‐fracture process, with strong emphasis on the importance of residual stress fields in parametric evaluations. The relevance of the results to the practical issue of fiber reliability, most notably in connection with the potential dangers of using macroscopic crack velocity data to predict long‐lifetime characteristics, is addressed.