Enrique Cuellar

Calculation of static fatigue lifetime of optical fiber

Two approaches are compared for estimating static fatigue lifetime at low cumulative failure probability. Both approaches use equations based on linear elastic fracture mechanics, but one uses the distribution of fiber strength to estimate the lifetime at low failure probability, and the second uses the distribution of time to failure. The two approaches give the same result, within experimental error, even though different sets of data are used in the calculation. Both approaches have advantages. Use of the distribution of time to failure is simpler experimentally because it does not require measurement of inert strength. However, this approach is limited to cases in which the strength distribution is unimodal. Use of the strength distribution is more generally applicable because it does not require that the strength be unimodal. Experimental data are presented for fiber tested in two-point bending at 80°C and 60% relative humidity. Lifetime predictions for a bending application are made using both approaches of extrapolation to low failure probability. The uncertainty in the calculations is estimated and the results compared. Two techniques are used to estimate the uncertainty in the lifetime calculation, the propagation of error technique and Monte Carlo simulation. A sensitivity analysis is presented that shows the sensitivity of the lifetime calculation to parameters such as fiber diameter, bend radius, and strength of the fiber.

Stress-free aging of optical fibers in liquid water and humid environments: part 1

The strength degradation of three commercially available telecommunications fibers was evaluated after stress-free aging in 85°C liquid water, 85°C at 94% r.h., 85°C at 85% r.h., and 85°C at 60% r.h. Interim results show that the three fibers show different aging resistances, depending on the environment. Strength loss of up to 50% after 45 days in 85°C water was observed. Atomic Force Microscopy was used to image the cladding surface, and estimate the strength determining flaw size. The data show qualitative correlation between strength loss and flaw size. While the mechanisms underlying stress-free crack growth are not well understood, strength degradation must be predicted, nonetheless, in order to ensure acceptable reliability for fiber-based systems in the outside plant.

Static fatigue lifetime of optical fibers in bending

Abstract An experimental program aimed at defining the effects of applied stress, temperature, humidity, and buffer coating on the static fatigue behavior of optical fibers in bending configurations is in progress. Data are presented below which demonstrate that the static fatigue behavior of fiber is strongly dependent on the polymeric buffer coating. Furthermore, the effect of humidity is readily evident by the comparison of times to failure at 30% RH and in water immersion. The ultimate objective of this research is to determine an allowable bend radius for fiber optic cable which is based on measurements of both static fatigue and strength in bending and which will assure reliable performance of the fiber over the design lifetime.

Stress-free aging of optical fiber in water and humid environments: part 2

Three commercially available telecommunications fibers were aged under zero mechanical stress in 85 degree(s)C and 30 degree(s)C water and in two hot, humid environments, 85 degree(s)C at 85% r.h. and 85 degree(s)C at 60% r.h. The strength of the fiber, as well as the stability of its protective polymeric coating, have been monitored periodically for aging times of up to 1 year in these environments. The fiber strength and coating integrity must be maintained in order for fibers to be handled safely during routine termination procedures in the outside plant.

Effect of buffer coating on static fatigue of optical fibers in bending

An important concern in achieving long service lifetime of optical fibers, particularly in adverse environments, is their susceptibility to mechanical failure by static fatigue. This phenomenon of delayed fracture is believed to be caused by the growth over time of microscopic surface flaws into larger critical flaws. The median time to failure of an optical fiber is a strong function of the applied stress, temperature, and chemical environment, specifically the presence of water.1

Design Requirements For Optical Fiber In Bending

An experimental methodology is presented for determining the allowable design stress on an optical fiber in service. The design stress must allow adequate static fatigue lifetime at high reliability. The experimental methodology and data analysis presented here have been used to successfully model static fatigue behavior in a wide range of environments in tests of up to 44 months duration.

Static Fatigue Of Optical Fibers In Bending II: Effect Of Humidity And Proof Stress On Fatigue Lifetimes

The goal of this work is to determine an allowable bending strain (bend radius) for optical fiber cable which is to be held in bending for long periods of time. The particular interest is in military and aerospace applications. In these applications it is desirable to use space efficiently and to route cable with as tight bending radii as possible. However, reliability cannot be compromised. In determining the lifetime of fiber it has been shown that the fiber must be tested under conditions that resemble those of actual use as closely as possible. For this reason static fatigue measurements are made in bending, and not in tension. In order to correlate static fatigue data with strength we have found that the strength must also be measured in bending. Most importantly for making reliable predictions the measurements must include long term tests; extrapolations from short term data are not reliable.

Determination of safe design stress

Mechanical stress on optical fiber must be low enough to insure that the fiber does not fracture due to static fatigue. In this paper sample data are presented to illustrate the calculation of design stress. The methodology is based on the concepts of linear elastic fracture mechanics, although a strictly empirical treatment of the data would yield essentially the same result. The calculation involves extrapolation of a plot of time to failure versus applied stress. It also requires extrapolation to low failure probability, as well as corrections for the amount of fiber under stress. The temperature and humidity are taken to be constant over the service life, and equal to the worst case field conditions. The calculations require both strength and fatigue data. The example illustrates the relative magnitude of the various terms in the equation for static fatigue lifetime, such as the term for applied stress, the term for failure probability, and the term for amount of fiber under stress. It also shows the sensitivity of the lifetime prediction to uncertainties in the various parameters.

Accelerated static fatigue behavior of optical glass fibers

Optical glass fibers can exhibit an accelerated static fatigue behavior at long times under moderate stresses. Similarly, optical fibers can exhibit a pronounced strength degradation due to zero-stress aging. The onset of significant strength loss due to zero-stress aging in water occurs at about the same time as the static fatigue transition. The two-point bending static fatigue and zero-stress aging behaviors of two commercial telecommunications fibers were measured at 85 degree(s)C in water immersion and in 60% and 85% r.h. In tests extending out to 680 days, neither fiber has shown evidence of a transition occurring in relative humidity. Similarly, both fibers show modest changes in their two-point bending strength after aging for 1.5 years in the same humidity environments. A model was developed which accounts for the simultaneous effects of zero-stress aging and stress corrosion on crack growth. This model can be used to predict the occurrence of the static fatigue transition, and was applied to both the 85 degree(s)C water immersion and the 85 degree(s)C, 85% r.h. data.

Effect of color removal on optical fiber reliability

Several fiber contacting devices used in the fiber optic industry rely on the ability to transmit light through the protective polymeric coating surrounding the glass fiber. These include fiber identifiers, local injection and detection systems, and optical taps. Optimal operation of these devices may require removal of the inks used to color code the fiber. This paper addresses the question of whether the solvents typically used for color removal affect the strength of the fiber. Since strength is strongly correlated to fiber reliability, operations which degrade fiber strength are of great concern. Test results show no measurable strength degradation after color removal, regardless of stripping technique. The long term reliability of fibers in the field is therefore unlikely to be affected adversely by removal of the ink layer from fiber coatings.