If I pull it too hard accidentally, will it stretch or break?
If a fiber is pulled forcefully, it undergoes a very slight elastic elongation before breaking; however, when the tensile force exceeds its ultimate strength, it will undergo brittle fracture (break directly) rather than permanent plastic elongation (yielding) like a rubber band or plastic.
I. Physical Mechanism and Material Mechanical Properties
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Brittle Material Characteristics of Fused Silica Glass
The core of common communication and sensing fibers is primarily composed of high-purity silicon dioxide (silica glass \text{SiO}_2 ). Fused silica glass is a typical brittle material, whose mechanical behavior does not exhibit a plastic yielding phase like metals or polymer materials. -
Elastic Elongation Phase (Lengthening)
- When subjected to tensile force without exceeding its ultimate strength, the chemical bonds within the fiber are stretched, undergoing pure elastic deformation. At this point, the elongation \Delta L of the fiber is directly proportional to the applied tensile force (adhering to Hooke’s Law).
- The ultimate elastic strain of general silica fibers is typically around 1\% \sim 2\% (i.e., 10000\,\mu\varepsilon \sim 20000\,\mu\varepsilon ). For instance, a bare fiber 1 meter ( 1000\,\text{mm} ) in length can only elastically stretch by approximately 10\,\text{mm} \sim 20\,\text{mm} before breaking.
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Direct Brittle Fracture (Breaking)
- Once the strain induced by the tensile force exceeds the fracture limit of the glass material, micro-cracks within the fiber propagate instantaneously, causing the fiber to instantly and completely break.
- Upon removal of the external force, the broken segments return to their original shape (no plastic deformation is retained).
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Confusion Due to Jacketing and Cable Structure
If pulling a fiber optic jumper/cable with a plastic outer sheath (e.g., PVC, LSZH) or polymer coating layer (e.g., acrylate):- The outer plastic material possesses plasticity and may stretch and elongate;
- However, the fragile silica core inside typically brittle fractures first within the plastic before it elongates, leading to signal interruption.
II. Application in Fiber Optic Sensing (FBG Strain Perception)
Leveraging the physical characteristic of the fiber experiencing “elastic lengthening under force before breaking,” Fiber Bragg Gratings (FBGs) inscribed within the fiber can achieve precise measurements of strain and tensile force. When the fiber is stretched and elongates by \Delta L , the grating pitch changes, causing a shift in the reflected wavelength (Bragg wavelength \lambda_B ). This shift can then be used to infer the magnitude of the applied force.
To address the issues of fiber fragility and limited tensile strength, Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) has developed various protective packaging structures to enhance the fiber’s mechanical tensile strength and enable strain monitoring:
- OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor
This sensor uses a seamless elastic alloy tube for protective packaging of the Fiber Bragg Grating, significantly enhancing the fiber’s tensile and fracture resistance. It is suitable for monitoring minute deformations and high-frequency stress/strain.
- OFSCN® Ultra-Large Range Fiber Bragg Grating Strain Sensor
This sensor employs a composite structure of elastic alloy tube and polymer elastomer, with a default strain range of \ge 30000\,\mu\varepsilon . It can be customized to measure deformations from 3\% \sim 40\% and is specifically designed for applications requiring large tensile deformation and force monitoring.
