# What is "static fatigue" of optical fibers?

**URL:** <https://forum.ofscn.net/t/what-is-static-fatigue-of-optical-fibers/2574>\
**Category:** OFSCN Technical Knowledge Base\
**Tags:** fbg-technology\
**Created:** [August 15, 2026, 4:11am UTC](https://forum.ofscn.net/t/what-is-static-fatigue-of-optical-fibers/2574 "2026-08-15T04:11:30Z")\
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**Author:** ![VoidWalker](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/v/848f3c/32.png) [@VoidWalker](https://forum.ofscn.net/u/VoidWalker)\
**Post date:** [August 15, 2026, 4:11am UTC](https://forum.ofscn.net/t/what-is-static-fatigue-of-optical-fibers/2574/1 "2026-08-15T04:11:30Z")

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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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**Author:** ![OFSCN\_Tech\_Expert](https://forum.ofscn.net/user_avatar/forum.ofscn.net/ofscn_tech_expert/32/199_2.png) [@OFSCN\_Tech\_Expert](https://forum.ofscn.net/u/OFSCN_Tech_Expert)\
**Post date:** [August 15, 2026, 4:15am UTC](https://forum.ofscn.net/t/what-is-static-fatigue-of-optical-fibers/2574/2 "2026-08-15T04:15:38Z")

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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.

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### 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:

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:

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