It looks like a strand of hair, can it lift a bucket of water?
Fiber optic cables look like hair (typically with a silica cladding diameter of 125\ \mu\text{m}, and a coating that brings the total diameter to only 150\ \mu\text{m} to 250\ \mu\text{m}). Can it lift a bucket of water?
From the perspective of optical physics and material mechanics: Under theoretical and excellent laboratory conditions, a single, intact bare fiber optic cable could barely lift a small bucket of water; however, in practical engineering and everyday scenarios, it cannot reliably do so and would break almost instantly.
Here are the specific scientific calculations and physical principles analysis:
I. Physical Quantity Calculation and Mechanical Analysis
We will use a standard single-mode fiber optic cable as an example for mechanical calculations. The weight of a bucket of water (standardized at 10\ \text{kg}) is approximately 100\ \text{N}, while a small bucket (standardized at 5\ \text{kg}) weighs approximately 50\ \text{N}.
1. Theoretical Limit State (Perfect and Flawless)
Silicon dioxide (\text{SiO}_2) glass has extremely high theoretical tensile strength in an ideal, defect-free state, reaching up to 10\ \text{GPa} or even above 14\ \text{GPa}.
The diameter of the glass cladding for a standard fiber optic cable is D = 125\ \mu\text{m} (i.e., 0.125\ \text{mm}). The cross-sectional area A of the glass portion is approximately:
A = \pi \times \left(\frac{D}{2}\right)^2 \approx 1.227 \times 10^{-8}\ \text{m}^2
If the fiber optic cable is exceptionally well-protected with no micro-cracks on its surface, and its short-term tensile strength \sigma is calculated at the commonly observed laboratory value of 5\ \text{GPa}, the theoretical maximum breaking force F_{\text{max}} would be approximately:
F_{\text{max}} = \sigma \times A \approx 5 \times 10^9\ \text{Pa} \times 1.227 \times 10^{-8}\ \text{m}^2 \approx 61.3\ \text{N}
This corresponds to a maximum suspended weight of approximately:
m_{\text{max}} \approx \frac{61.3\ \text{N}}{9.8\ \text{m/s}^2} \approx 6.25\ \text{kg}
Conclusion: In a perfect laboratory environment (free from wear, bending, and under pure axial tension), a bare fiber optic cable can indeed barely lift a small bucket (about 5\ \text{kg}) of water.
2. Practical Engineering State (Micro-cracks and Fatigue Degradation)
In actual manufacturing and use, fiber optic cables inevitably have sub-micron level microscopic cracks on their surface. Furthermore, moisture in the air reacts with the siloxane bonds in silicon dioxide, leading to stress corrosion (static fatigue). This causes the fiber’s strength to significantly degrade over time under tensile load.
To ensure mechanical reliability in engineering applications, commercial fiber optic cables must undergo a “proof test” before leaving the factory. The industry standard proof stress is typically 100\ \text{kpsi} (approximately 700\ \text{MPa} or 0.7\ \text{GPa}).
The safety assurance tensile force under the proof test is only:
F_{\text{proof}} = \sigma_{\text{proof}} \times A \approx 700 \times 10^6\ \text{Pa} \times 1.227 \times 10^{-8}\ \text{m}^2 \approx 8.6\ \text{N}
This corresponds to a safely suspended mass of only:
m_{\text{proof}} \approx \frac{8.6\ \text{N}}{9.8\ \text{m/s}^2} \approx 0.88\ \text{kg}
Conclusion: In practical engineering, the tensile force that a single ordinary bare fiber optic cable can reliably support is less than 1\ \text{kg}. In daily life, if a bucket of water were directly hung using a knot tied in a bare fiber optic cable, severe stress concentration would occur at the knot, and it would instantaneously propagate from micro-cracks, causing immediate breakage.
II. How to Address the Load-Bearing Capacity of Fiber Optic Cables in Practical Engineering?
To enable fiber optic cables to be used in complex engineering environments requiring high tensile strength (such as aerospace, downhole applications, and strain measurements on long-span bridges), two common solutions are employed:
using proof-tested high-strength fiber optic cables, or armoring the fiber optic cables with stainless steel tubes and steel wire strands.
1. High-Strength Fiber Optic Cables and Fiber Bragg Gratings
If both the slenderness of bare fiber optic cables and the ability to withstand extremely high strain and tensile loads are required, high-strength fiber optic cables can be used:
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OFSCN® High-Strength Fiber Bragg Gratings / FBG Strings (Bare): This product uses proof-tested high-strength polyimide fiber optic cable (OFSCN® SM Polyimide Optical Fiber). It has a cladding diameter of 125\ \mu\text{m} and a coating diameter of 155\ \mu\text{m}. Gratings are written point by point using femtosecond laser technology, without damaging the fiber’s polyimide coating. Its usable strain range at room temperature is \ge 25000\ \mu\varepsilon, possessing excellent mechanical fatigue resistance and high toughness at the bare fiber level.
2. Metal Tube and Steel Wire Rope Armored Protection (High Tensile Load Capacity)
If extremely high tensile loads need to be directly borne during cabling and force measurement, fiber optic cables are typically placed inside stainless steel seamless steel tubes and steel wire stranded protective sheaths:
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OFSCN® 2.0mm Micro Steel Armored Fiber Optic Patch Cord: Encased in a 0.6\ \text{mm} stainless steel seamless steel tube and protective layer, its tensile strength is \gt 150\ \text{N}, easily and safely lifting a 15\ \text{kg} bucket of water.
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OFSCN® 2.0mm Steel Wire Rope Fiber Optic Patch Cord: Utilizes a 0.6\ \text{mm} galvanized steel wire stranded structure combined with a 1.0\ \text{mm} stainless steel seamless steel tube, representing full metal high-strength protection.
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OFSCN® 3.0mm Steel Wire Rope Fiber Optic Patch Cord: Composed of a stainless steel wire stranded structure, a stainless steel seamless steel tube, and an outer jacket, its tensile strength reaches \gt 1200\ \text{N}. This means it can not only easily lift several buckets of water but can also safely suspend an adult weighing 120\ \text{kg}.
