If a piece of the outer colored jacket flakes off, how long can the inner glass last?
The “colored skin” of an optical fiber typically consists of primary/secondary polymer coatings (such as polyacrylate, polyimide, etc., with an outer diameter usually around 250\ \text{µm}), while the interior comprises a cladding (diameter 125\ \text{µm}) and a core (e.g., 9\ \mu\text{m} for single-mode) made of high-purity silicon dioxide (\text{SiO}_2) glass.
The longevity of exposed silica glass depends on its stress state, environmental humidity, and mechanical contact conditions. The specific failure mechanisms and lifetime assessments are as follows:
I. Failure Mechanism of Coating Loss on Glass Lifespan
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Drastic Increase in Sensitivity to Mechanical Damage
Silica glass possesses extremely high theoretical tensile strength, but its surface is highly susceptible to microscopic cracks (Griffith cracks). The function of the coating is to protect the glass surface from external abrasion. Once the coating is lost, even slight friction from dust or touch can create micro-cracks on the bare fiber surface, drastically reducing its mechanical strength by several orders of magnitude. -
Moisture and Stress Corrosion (Static Fatigue Failure)
In environments with ambient moisture, water molecules ( \text{H}_2\text{O} ) can chemically react with the siloxane bonds on the glass surface:\text{Si—O—Si} + \text{H}_2\text{O} \longrightarrow 2\text{Si—OH}If the exposed area is simultaneously under tensile or bending stress, the chemical hydrolysis reaction at the crack tip accelerates (subcritical crack growth). As the crack gradually extends to a critical size, the glass will undergo sudden brittle fracture.
II. Expected Lifespan of Exposed Glass Under Different Conditions
- Scenario A: Bent state, under tension, or in an environment with slight vibration
- Expected Lifespan: Seconds to days.
- Explanation: Tensile stress from minor bending, combined with ambient humidity, can rapidly induce stress corrosion fracture; or it may break directly with slight external contact.
- Scenario B: Completely stationary, no tensile stress, dry, and contact-free environment
- Expected Lifespan: Months to years.
- Explanation: Under ideal conditions with no tension, no bending, and no disturbance, silica glass is chemically stable, and its optical transmission performance will not degrade immediately in the short term.
- Scenario C: Humid, acidic/alkaline, or alternating high/low temperature environment
- Expected Lifespan: Hours to weeks.
- Explanation: Water vapor and chemical corrosion accelerate the chemical degradation and micro-erosion of the bare fiber surface.
III. Engineering Measures and Protection Recommendations
After localized loss of the fiber coating, the exposed fiber segment is in an extremely unreliable, sub-healthy state. Engineering practices recommend the following measures:
- Temporary Protection: Avoid touching, bending, or pulling the exposed area; keep the fiber segment straight.
- Recoating / Mechanical Reinforcement:
- Use a fiber recoating machine to re-cure the polyacrylate adhesive layer;
- Or use a fiber splice protection sleeve with reinforcing steel wire for localized heat-shrink protection;
- In harsh industrial or sensing applications, methods like seamless stainless steel metal sleeving are typically employed for comprehensive mechanical and environmental protection of the fiber.
- Resplicing: If the exposed area already has visible scratches or damage, it is recommended to cut off the damaged section and proceed with stripping, cleaving, and resplicing.