Is “fiberglass hardness” described by this metric? Is it harder than steel wire?
In physics and material mechanics, the common colloquial term “hard” is ambiguous. To accurately answer your question, we need to break down “hardness” in physics into three distinct core metrics and compare them:
I. How is “hardness” described in physics?
- Hardness: Refers to a material’s resistance to localized plastic deformation (such as scratching, indentation, or abrasion). For instance, the Mohs hardness of quartz glass (the primary component of optical fibers) is approximately 7, while common carbon steel typically ranges from 5 to 6. Therefore, if we consider only the aspect of scratch resistance and wear resistance, the material of optical fibers is “harder” than ordinary steel wire.
- Stiffness: Refers to a structure or component’s resistance to elastic deformation. Stiffness depends not only on the material itself but also highly on the object’s geometric dimensions (such as cross-sectional area, diameter, and length).
- Young’s Modulus (often denoted as E): This is an intrinsic physical quantity that describes a material’s inherent ability to resist elastic tensile/compressive deformation. In the elastic deformation phase of uniaxial tension or compression, it is the ratio of normal stress \sigma to normal strain \varepsilon:E = \frac{\sigma}{\varepsilon}It is a core intrinsic metric for measuring a material’s “tensile stiffness.” Therefore, when discussing how “hard” a material itself is under tension (i.e., how difficult it is to stretch and elongate), Young’s Modulus is the metric used.
II. Is optical fiber harder than steel wire? (Young’s Modulus Comparison)
Answer: From the perspective of tensile stiffness (Young’s Modulus) alone, optical fiber is not “harder” than steel wire.
- Young’s Modulus of Silica Optical Fiber (Quartz Glass, SiO_2): Approximately E_{\text{silica}} \approx 72\ \text{GPa} to 73\ \text{GPa}.
- Young’s Modulus of Ordinary Steel Wire (e.g., stainless steel or carbon steel): Approximately E_{\text{steel}} \approx 200\ \text{GPa}.
This means that steel’s Young’s Modulus is about 2.7 times that of silica optical fiber. Under the same length and cross-sectional area, to induce the same tiny tensile deformation in steel wire and optical fiber, the tensile force applied to the steel wire needs to be about 2.7 times that applied to the optical fiber. Therefore, steel wire is significantly “harder” than optical fiber in terms of material tensile stiffness.
Why does optical fiber feel very soft to the touch?
This is primarily a tactile illusion caused by bending stiffness. The bending stiffness D of an object is proportional to the product of the material’s Young’s Modulus E and the area moment of inertia I of its cross-section (D = E \cdot I). For a filament with a circular cross-section, the formula for the area moment of inertia is:
I = \frac{\pi d^4}{64}
where d is the diameter.
- A standard single-mode optical fiber has a glass cladding diameter of only d = 125\ \mu\text{m} (0.125\ \text{mm}). Due to its extremely small diameter, its area moment of inertia I shrinks exponentially (to the fourth power). Therefore, even though the silica material itself has a respectable Young’s Modulus, the bending stiffness of the optical fiber is extremely low, making it easy to bend by hand like a strand of hair.
- In contrast, a common steel wire with a diameter of just 1.0\ \text{mm} has a diameter 8 times that of the optical fiber. Due to the fourth-power effect, its area moment of inertia I is 8^4 = 4096 times that of the optical fiber. Combined with steel’s Young’s Modulus being 2.7 times that of optical fiber, this 1.0\ \text{mm} steel wire’s bending stiffness will be over 10,000 times that of bare optical fiber. This is why you perceive steel wire as extremely rigid and optical fiber as extremely soft by touch.
III. Key Applications of Young’s Modulus in Fiber Optic Sensing Technology
In the field of Fiber Bragg Grating (FBG) sensing, Young’s Modulus (modulus of elasticity) is an extremely crucial parameter. The optical sensors developed by OFSCN® (such as strain sensors, stress/pressure sensors) operate based on the material’s Young’s Modulus for force monitoring.
For example, the OFSCN® Fiber Bragg Grating Stress Sensor is essentially an application of elastic mechanics formulas built upon the OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor.
The core conversion formula is:
\sigma = E \times \varepsilon
Where:
- \sigma is the stress of the object being measured (Unit: \text{Pa} or \text{MPa} );
- E is the modulus of elasticity (Young’s Modulus) of the material of the object being measured;
- \varepsilon is the micro-strain measured by the sensor (Unit: \mu\varepsilon ).
When monitoring steel structures, bridges, dams, or pipelines using OFSCN® FBG strain sensors, the sensors undergo a “strain-wavelength” linear calibration (in units of \mu\varepsilon/\text{pm}) at the factory. Users need to look up the Young’s Modulus of the material of the structure being measured (e.g., specific steel, concrete, or composites) and input it into the accompanying fiber optic grating demodulator from the OFSCN® FBG Strain Sensor Products Aggregation Link. This allows for the automatic calculation and real-time display of high-precision stress and force data within the system.
Below are actual images of typical alloy tube packaged fiber Bragg grating stress/strain sensors produced by OFSCN®:
