Strength and Fatigue Properties of Optical Glass Fibers Containing Microindentation Flaws

Abstract

The inert strength and dynamic fatigue properties of fusedsilica 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: (I) 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 initiationcontrolled 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 evalnations. 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.

Inert strength of subthreshold and post-threshold Vickers indentations on fused silica optical fibres

“…Baikova, Pukh and Talalakin (1974) and Aslanova et al (1982) present data for indentation on HF-etched silica surfaces. Their data roughly agree with those of Dabbs and Lawn (1985) and Jakus et al (1988) but it is not clear what test environment they used.…”

“…This technique is particularly useful since the residual strength can be controlled over a wide range by varying the indentation load. Dabbs and Lawn (1985) observed that, while radial cracks form at the indentation comers at high loads, radial cracks are not formed below some threshold load. They found that the residual strength of subthreshold and post-threshold indents do not follow the same trends with the presence of radial cracks causing post-threshold indents to be discontinuously weaker than subthreshold indents.…”

“…Most work on indentations has concerned the post-threshold regime since most materials are substantially weaker than optical fibre. The only available subthreshold data for fused silica are due to Dabbs and Lawn (1985) and Jakus et al (1988) but their data only span a narrow range of loads (1 50-680 mN) which does not overlap the post-threshold region (about 10 N and above). The data therefore neither characterize subthreshold behaviour in detail nor provide a sensitive test of subsequently developed fracture mechanics models (Jakus et al 1988, Choi, Ritter and Jakus 1990, Lathabai, Rodel, Dabbs and Lawn 1991.…”

<title>Indentation experiments on silica optical fibers</title>

“…in high strength specimens failure is initiation controlled; in low strength specimens failure results from crack "pop-in" followed by subcritical crack growth (propagation controlled failure). 5 We observed that the results for subthreshold flaws measured in two point and four point bending do not form one continuous line. The results from the four point apparatus are calculated assuming small deflections of the fiber.…”

Mechanical properties of optical glass fibers damaged by nanoindentation and water ageing

“…According to Dabbs and Lawn [6] values for n of about 20 and lower corresponds to damages made at sub threshold loads. Then, the calculated value for n indicates the indentations made in the fibers generate flaws into the glass surface in a post threshold regime.…”