Stress Concentration in a Connectorized Optical Fiber

For the most part, analyses of fiber fractures in connectors have been in the form of postmortem fractography. Typically in these works, characteristics of prefracture stress states have been inferred from fracture surfaces, and plausible qualitative explanations have been advanced about the likely structural mechanics and circumstances leading to fractures. The authors and their colleagues have undertaken a number of investigations of relevant structural mechanics. These have served the useful purpose of elucidating gross mechanisms, but the influence of the fine details of stress distributions have been missing. Considered here is a cylindrical-ferrule connector for which, typically, the ferrule is ceramic with an outside diameter of 2.5mm or 1.25mm. The fiber to be terminated is bonded into a small-bore axial hole (capillary) in the ferrule by an epoxy or similar adhesive. In addition, fiber insertion into the capillary is facilitated by ferrule designs that provide a conical entrance cavity leading to the capillary. A very high percentage of fiber failures, both in the laboratory and the field, occur at the transition region between the fiber and the capillary; so analysis is focused on that region. The stress distribution within an optical fiber adhesively bonded to a ceramic ferrule is determined by the finite element method for uniform remote tension acting on the fiber. An axisymmetric model is constructed to represent the fiber, epoxy, and geometry of the ferrule under this particular loading condition. The resulting stress distribution is determined within the fiber and the epoxy layer using the ANSYS finite element code. Analysis of the stress distribution reveals the presence of two stress concentrations located at the surface of the fiber as the fiber enters the ferrule. One stress concentration occurs as the fiber encounters the epoxy within the conical cavity. The second stress concentration occurs as the fiber enters the capillary. These stress concentrations when combined with surface damage (flaws) may lead to fiber breakage. Further analysis reveals that a smooth fillet transition between entrance and capillary could significantly reduce the stress concentration at the capillary entrance. Finally, a simulation of epoxy debonding within the entrance cone reveals an increase of stress concentration at the capillary entrance.