Mechanical Degradation
Chemical and mechanical processes act together to cause slow crack growth in materials held under tensile stresses below their ultimate yield strength. This phenomenon of stress corrosion cracking represents a primary failure mode for silica optical fibers exposed to humidity. It occurs when chemical bonds at a crack tip are weakened by chemical reactions, allowing the crack to propagate.
Corrosive Environment
Moisture is the main environmental agent that drives this process in glass materials. The stress corrosion cracking rate is highly sensitive to water concentration and ambient temperature, which accelerate the chemical attack on the silica network. This dependency means that fibers installed in warm, damp underground conduits are at higher risk of failure than those in dry environments.
Protecting the glass with hermetic coatings helps to exclude these corrosive agents from the surface.
Failure Mechanism
Stress concentration at surface flaws accelerates the hydrolysis of silicon-oxygen bonds. During stress corrosion cracking, the reaction rate at the tip of a micro-crack exceeds the rate at the flat surface, leading to crack sharpening and eventual rapid fracture. This behavior is modeled using power-law relationships that relate time-to-failure to applied mechanical load.
Prevention Strategy
Mitigation requires applying protective hermetic coatings or polymer buffers during fiber draw. These coatings slow down the stress corrosion cracking process by preventing moisture from reaching the glass surface or by absorbing tension. In demanding subsea installations, carbon coatings are applied to ensure a prolonged service life.
