The strength of silica optical fibers depends on the water activity at the glass surface. Polymer coatings are currently applied to optical fibers to protect them from mechanical damage during handling. They also act as a barrier diffusion of the surrounding humidity reaching the glass surface. Water is known to be one major factor of the propagation of cracks at fiber glass surface because it makes much easier the breaking of the Si-O bonds which build the vitreous network. The strength of the fiber is related to the water concentration at the glass surface. It is well known that flaws in glass subject to stress in humid condition grow subcritically. The crack velocity is related to the applied stress and also to the humidity rate . The kinetic of the reaction between silica and water changed at very low water concentration .
The strength of silica optical fiber is closely related to the activity of water at its surface. However, observations have shown that the polymeric coating is also a key factor contributing to the mechanical properties of the fiber. While the main role of the coating is to inhibit crack growth from the surface Griffith flaws, it also reduces the water concentration at the glass surface through diffusion processes.
Dynamic and static mechanical tests were implemented using a tensile test bench and a static fatigue test under uniform curve. The incidence of aging treatments at 65°C and 85°C was investigated on two standard silica optical fibers (with polyacrylate and fluorinated coatings). Fatigue under static tension was also investigated using a vertical static tensile bench.
Microscopic observations helped the understanding of the failure mechanism. It appears that the cyclic variations of the failure stress phenomenon, with respect to the aging time, are the result of the silicate gel which migrates towards the polymer coating.
Fluoride glass fibers are used for various passive and active applications. Current fibers are made from fluorozirconate glasses. Their intrinsic strength is smaller than that of silica fibers and their surface may be damaged by liquid water. The discrepancy between the reported values of the fiber strength reflects the major part of the extrinsic defects in the failure mechanism. The surface of the glass preform determines fiber strength to a large extent and numerous defects are induced during fiber processing. These defects are largely correlated to the chemical action of water.
Dynamic fatigue measurements have been carried out. They show that the average strength depends on fiber length even though when a proof test is made prior to measurement. Fiber strength is not constant for all fiber segments originating from the same preform. Atmospheric humidity also influences fiber strength and dried fiber show much larger failure stress. Aqueous solutions corrode surface fiber resulting in the significant decrease of the fiber strength. Structural relaxation may occur in fibers under stress inducing permanent bending and reduction of applied stress. Fibers hermetically packaged are more reliable and some of them have been used in outdoors conditions for years. While surface has to be protected from water in any case, controlling extrinsic defects should lead to the large improvement of the fluoride fiber strength.
The reliability and the expected lifetime of silica fibers are closely related to the chemical action of water molecules on silica. Strength and dynamic fatigue behaviour of silica optical fibers were measured in very dry atmosphere. Bare and coated fibers were compared to characterize the influence of the polymer. Dynamic mechanical tests were implemented using a two point bending set-up. The stress corrosion susceptibility parameter has been evaluated. At the higher stressing rate the slope of the dynamic fatigue curve was found to drop significantly. This behaviour is usually observed for specially damaged fibers tested at very high speed.
Strength and dynamic fatigue behavior of silica optical fibers has been measured in very dry atmosphere. Bare and coated fibers were compared to characterize the influence of the polymer. The stress corrosion susceptibility parameter has been evaluated. Subcritical crack growth effect was found to be different in coated and bare fibers while their strength was found to be close. The strength measurement have been done using a two-point bending set up.