The stresses in a microstructured optical fiber are determined in tension and in bending. A model for the failure process in porous structures with applications to weak microstructured optical fibers with large flaws is proposed. The model assumes that a continuous crack front splits on contact with a hole and then propagates such that the direction of the front in contact with the boundary of the hole is normal to it. Three-dimensional finite perturbation analysis of Bower and Ortiz1 is extended to model the interaction of cracks with a periodic array of holes. Crack propagation in the cross-section is examined using the modified finite perturbation method. Test cases are presented for the stress intensity factor distribution in elliptical cracks, and periodic perturbations in straight cracks. During crack front interaction, the front can split upon contact with the hole or an array of holes, and then travel around them.
The inert strength of carbon-coated optical fibers (hermetic
fibers) has been observed to be less than that of standard polymer
coated optical fibers. A scaled version of the of the
carbon-coated optical fiber was developed in the laboratory and
used in an experimental investigation of the mechanical properties
of this system. The microstructure and phase similarity between
the scaled system and the optical fiber was established using
Raman spectroscopy. The mechanical properties, residual stress in
the film and the fracture toughness of the scaled system were
determined using nano-indentation. A fracture mechanics model was
developed to explain the mechanism of this strength reduction. The
model, based on the cracking of thin films in residual tension,
will be used to predict growth of flaws from the carbon film and
penetrating into the substrate. The model can be applied to all
brittle coatings where delamination of the coating is not
observed. Conditions under which cracks in the carbon film
propagate into the substrate were investigated using a recently
developed superposition scheme. Possible methods of crack arrest
will be discussed.
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