Relationship between mirror dimensions and failure stress for optical fibers
Standard glass optical fiber was intentionally damaged and strength tested in tension for a wide range of failure stress values. The mirror/mist boundary on each specimen was measured and compared to the measured failure stress. When the size of the fracture mirror is small compared to the fiber diameter, the well-known linear square root dependence of strength on mirror size was reaffirmed. However, when the mirror size approaches the fiber diameter, this relationship does not hold. The classical relationship overestimates the failure stress for large mirrors. It was determined that the circular shape of the fiber contributes to, but can not fully account for, departure from the classical strength/mirror relationship. A new empirical relationship was developed to enable predictive determinations of failure stress to levels as low as 20 to 25 kpsi (0.14 to 0.17 GPa).
Prediction of Fracture Energy and Flaw Size in Glasses from Measurements of Mirror Size
Fracture strengths (δ), fracture-initiating flaw sizes, and mirror radii (r), as outlined by either the mist or the hackle boundary, were measured for silicate and nonsilicate glasses (e.g. As2S3 and glassy carbon). For all glasses, δr1/2= constant. The average ratios of inner and outer mirror radii to flaw size were ∼10:1 and ∼13:1, respectively, for most of the glasses. Critical fracture energies calculated from either flaw or mirror size agreed very well with those obtained by double-cantilever-beam measurements.
Fractographic analysis of optical fibers
Abstract : The fracture surfaces of over 50 fibers tested at Hughes or ITT reasearch Laboratories were examined on the scanning electron microscope (SEM). The sources of failure in these fibers were determined to be either mechanically induced sharp cracks, cracks resulting from inclusions or foreign particles, bubbles or inclusions. It was suggested that many of these sources of failure can be eliminated by changing production procedures. For example, some larger inclusions can be eliminated by using synthetic, high grade silica, starting material. Another example for eliminating low strength defects is by using high quality silica tubing in the CVD process. The fracture surface demarcations known in glass as mirror, mist, hackle, and crack branching were identified and related to the stress, at failure. In all cases but one, fracture surface analysis agreed with measured stresses during testing within experimental accuracy. Thus, fractography can be used to determine the stress and source of unexpected service failures.