STRENGTH AND DYNAMIC FATIGUE CHARACTERISTICS

OF AGED FIBER

Robert J. Castilone and Thomas A. Hanson

Corning Incorporated

Corning, NY 14831

Abstract

An experiment was performed to examine the strength and dynamic fatigue performance of polymer

coated optical fiber after various aging conditions including: 30-day hot-humid zero stress aging, shelf

life aging, and cyclic hot-humid aging. When compared to as-received fiber, the strength and stress

corrosion resistance (n

) increased with both 30-day hot-humid aging and shelf life aging. The strength

and fatigue values of the 2-year-old shelf life fiber showed a strong resemblance to the values from the

30-day hot-humid aging experiment. Testing was also performed at 50% R.H. and 100% R.H. to

determine how the test environment affects the mechanical properties of the fiber. Strength values at

100% R.H. were lower compared with values tested at the standard 50% R.H. This is consistent with

changes in the B parameter. For the as-received fiber, there was no significant change in n

between 50%

and 100% R.H., however the aged n

appeared to be slightly higher at the 100% R.H. condition compared

to the 50% R.H. environment. It was also determined that cycling humidity and temperature during aging

had no impact on strength or fatigue of the fibers tested.

Introduction

Zero stress aging of optical fiber has been and continues to be of interest in understanding the mechanical

behavior during the fiber’s lifetime. The aged fiber performance is commonly measured by performing

dynamic strength and fatigue measurements after exposure to a hot-humid environment for a period of

time. This condition is designed to simulate a lifetime of harsh aging in the field. The ability of a fiber

to withstand the environments in which it is exposed and maintain its pristine strength is of importance to

insure continued functionality and handleability in the field. Several studies have examined the effect of

aging on fiber strength. Aged glass surfaces have been examined using Atomic Force Microscopy

(AFM).

It was determined that strength degradation was directly related to pits which developed on the

glass surface. The roughness was attributed to dissolution of silica during the aging process. This

resulted in high stress concentrations in the pitted regions, thus resulting in lower strength values. It is

important to note that although strength reduction can occur due to zero stress aging, the strength levels

are not reduced to levels where long-term reliability is an issue. Recently, it has been suggested that

cyclic aging of fibers between wet and dry conditions may also lead to strength reduction.

It has been determined that water penetrates through typical polymer coatings rapidly, in as little as ten

minutes.

If moisture readily reaches the glass surface and leads to pitting, it would be expected that

strength degradation would consistently occur with standard accelerated aging tests. However, studies

have shown that strength degradation does not always occur after aging, and strength and dynamic

fatigue values can even increase after aging.

The secondary coating chemistry, which can act as a

moisture barrier, has been shown to play a role in the susceptibility of a fiber to strength reduction.

Clearly, a dependence between aging and coating type is present.

It is the purpose of this paper to investigate the strength and fatigue behavior of pristine fiber as it is

exposed to various environments and conditions. Zero stress aging at 85°C/85% R.H. for 30 days was

performed to understand how the strength and fatigue performs after hot-humid exposure. The test was

performed at both 50% R.H. and 100% R.H. environments to determine the influence of the test

environment on dynamic stress corrosion parameters (n

and B). Also performed was a time after draw

study to determine how the fiber’s mechanical properties vary with shelf life. Finally, the cyclic aging

study was performed to see how the fiber performs when altered between a hot-humid and a room

temperature-dry environment.

Experimental Procedure

All fibers tested were standard dual-polymer coated silica optical fibers. Samples were aged 30-days

using a standard temperature-humidity chamber. Fiber was aged on 6-inch diameter glass spools. The

tension on these spools is minimal and ‘zero stress’ aging is assumed. Cyclic aged samples were

alternated between an 85°C/85% R.H. environment and a 23°C/dry environment each day for 60 days.

Both the aged and the cyclic aged samples saw a total of 30-days in 85°C/85% R.H. for comparison

purposes. The dry environment was set to 0% R.H., however in actuality the humidity varied between 10

and 15%. Time after draw samples were tested at various shelf life intervals up to 3 years. The samples

were stored on standard shipping reels in a constant 23°C/50% R.H. environment.

Dynamic fatigue testing was performed on the samples using four strain rates: 0.025%/min., 0.25%/min.,

2.5%/min, and 25%/min. Samples were tested using the following two methods: a horizontal ganged

rotating-capstan tester, and a vertical stationary-capstan tester. Each apparatus contains sixteen separate

load capstans and corresponding load cells to minimize the time required to perform the tests. The

testing was performed in a controlled 23°C±2°/50%±5% R.H. environment. The 100% R.H.

environment was achieved using a water bath and cover over the horizontal test bench. Actual stressing

rates were recorded, and along with the breaking stress, were used to calculate fatigue parameters of each

fiber tested. Statistical Weibull strength information was obtained by examination of one or more of the

strain rates tested.