What is "static fatigue" of optical fibers?

OFSCN Global Technical Center OFSCN Technical Knowledge Base
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Why do fiber optic cables suddenly break after a few years due to bending alone, even without external force?

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The physical phenomenon where an optical fiber suddenly breaks after several years without any external dynamic pulling, simply due to being kept in a bent state, is known as static fatigue in fiber optics and material mechanics.

Static fatigue is not merely mechanical wear; it is a stress corrosion process resulting from the combined action of sustained tensile stress and water molecules ( \text{H}_2\text{O} ) in the environment.

I. Physical and Chemical Mechanism of Static Fatigue

The static fatigue process involves the following four key physical and chemical stages:

  1. Presence of Microscopic Initial Cracks
    Quartz glass optical fibers (primarily composed of silicon dioxide \text{SiO}_2 ) inevitably have micron- or nano-sized micro-cracks (Micro-cracks) on their cladding surface after manufacturing and drawing.

  2. Bending-Induced Sustained Tensile Stress
    When a fiber is bent, the outer side of the curved arc experiences tension, while the inner side experiences compression. Sustained tensile stress \sigma is generated on the outer surface of the bend. The physical relationship between tensile stress and the bending radius R is:

    \sigma = E \cdot \frac{r}{R}

    where E is the Young’s modulus of quartz glass (approximately 72\ \text{GPa} ), r is the fiber cladding radius (standard single-mode fiber r = 62.5\ \mu\text{m} ), and R is the bending radius. The smaller the bending radius R, the greater the tensile stress \sigma on the outer surface.

  3. Water Chemical Bond Breaking and Subcritical Crack Growth
    In the stress concentration zone (crack tip), the silicon-oxygen bonds ( \text{Si-O-Si} ) in silicon dioxide molecules are stretched and in a high energy activated state. Water molecules ( \text{H}_2\text{O} ) in the environment infiltrate the crack tip and react with the strained silicon-oxygen bonds through hydrolysis:

    \text{Si-O-Si} + \text{H}_2\text{O} \to 2\text{Si-OH}

    The hydrolysis reaction forms silanol groups ( \text{Si-OH} ), breaking the covalent bond network of the glass. This chemo-mechanical coupling process causes micro-cracks to continuously propagate forward at an extremely slow rate along the stress direction.

  4. Critical Sudden Brittle Fracture
    As the crack depth slowly increases, the remaining effective load-bearing cross-sectional area continuously decreases. When the local stress intensity factor at the crack tip reaches the material’s critical fracture toughness ( K_{IC} ), the crack propagation speed instantaneously reaches the speed of sound, and the fiber undergoes explosive brittle fracture. This is why the fiber’s surface appears normal for several years, yet it