Under what circumstances are plastic-encased sensors preferable to steel-encased ones?
When designing the packaging for fiber optic sensors, particularly Fiber Bragg Grating (FBG) strain sensors, the choice between polymer encapsulation (commonly referred to as plastic tube encapsulation or polymer material encapsulation) and stainless steel tube/alloy tube encapsulation primarily depends on the physical environment, mechanical compatibility, and the measurement target.
Here are several typical scenarios where polymer (plastic) encapsulated sensors are more suitable and advantageous than metal steel tube encapsulated ones in engineering and academic applications:
1. Strain Measurement in Low Elastic Modulus Host Materials (Eliminating the “Reinforcement Effect”)
- Physical Principle:
Stainless steel tubes or metal alloy tubes possess a very high elastic modulus (typically between 190\text{ GPa} and 210\text{ GPa} ), making them extremely rigid. In contrast, the elastic modulus of polymer materials is significantly lower (often in the range of a few \text{GPa} or even less). - Application Scenario:
When the material being measured (substrate) is soft or has a low modulus, such as plastics, rubber, composite materials (like carbon fiber or glass fiber laminates), asphalt, soil, or biological tissues, a rigid metal steel tube encapsulation will cause a significant Reinforcement Effect. The sensor itself restricts local deformation, leading to a measured strain value that is much lower than the material’s true free strain.
In such cases, opting for a low-modulus polymer encapsulation achieves good Stiffness Matching with these soft or deformable substrates, ensuring that shear forces are realistically transmitted to the internal Fiber Bragg Grating and guaranteeing high measurement accuracy.
2. Applications Requiring Extremely Small Bending Radii and Complex Surface Mounting
- Mechanical Principle:
Once metal tube materials undergo large-angle bending, exceeding their yield limit results in irreversible permanent plastic deformation. This can even lead to the fracture of the internal fiber or significant bending loss. - Application Scenario:
For curved surfaces with complex geometries, mounting surfaces with small radii of curvature, or areas requiring follower bending, flexibility, and repeated large-range bending, polymer tubes exhibit excellent bending elasticity and flexibility. They can be bent to extremely small radii and return to their original shape without issues of kinking or yield hardening common to metal tubes, significantly improving mounting adaptability.
3. Measurement of Large Strain and Large Deformation
- Mechanical Principle and Application:
Metal materials are prone to plastic yielding under large deformations. However, polymers (like elastomers or high-performance engineering plastics) typically have a very wide elastic limit, making them suitable for structural components requiring measurement of large deformations and high elongation rates (e.g., bridge expansion joints, large deformation geotechnical structures, etc.). Polymer-encapsulated FBG sensors can support larger measurement ranges than traditional metal encapsulations (e.g., large strain ranges of \ge 3000\ \mu\varepsilon or even \ge 6000\ \mu\varepsilon ), and are less susceptible to fatigue during repeated large deformation cycles.
4. “Electromagnetically Neutral” Environments Such as Strong Electromagnetic Interference, High Voltage, or Nuclear Magnetic Resonance
- Electromagnetic Characteristics:
Metals are conductive and responsive to electromagnetic fields. In strong electromagnetic fields (e.g., near high-voltage substations, high-power microwave equipment, high-frequency radar systems, high-intensity Magnetic Resonance Imaging (MRI) scanners, etc.), metal encapsulation can distort magnetic fields, induce eddy currents, and even pose a risk of electrical discharge sparks. - Application Scenario:
When the protective steel tube is removed from the polymer outer layer, a pure polymer encapsulation (completely non-metallic structure) acts as an absolute insulator and non-magnetic material. It provides complete electromagnetic neutrality, completely eliminating safety hazards and signal interference in special physical, high-voltage, and strong magnetic field scenarios.
5. Structures Extremely Sensitive to Added Mass (Lightweight Requirements)
- Weight Density:
The density of polymer materials (typically between 1.0 and 1.5\text{ g/cm}^3 ) is significantly lower than that of stainless steel (approximately 7.9\text{ g/cm}^3 ). - Application Scenario:
On aircraft, unmanned aerial vehicles (UAVs), high-altitude balloons, or high-speed rotating lightweight structures (like wind turbine blades), any added mass can alter the structure’s dynamic characteristics or center of gravity. Polymer-encapsulated sensors are extremely lightweight, minimizing their self-weight load on the structure being measured.
Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) Related Product Solutions
Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) offers Fiber Bragg Grating strain sensor products that combine the advantages of polymer encapsulation with various housing options to meet different engineering needs:
1. OFSCN® Polymer-encapsulated Fiber Bragg Grating Strain Sensor (0.7mm/1.2mm diameter)
This sensor features a core encapsulated with polymer material. Its default product outer diameter is extremely small, \le 0.7\text{ mm} . At this base outer diameter, it is completely non-metallic and ideal for internal embedding or surface mounting on soft composite materials. If the installation environment is complex and requires waterproofing, moisture resistance, or protection against minor external mechanical abrasion, a seamless steel tube can be added around the polymer (increasing the outer diameter to \le 1.2\text{ mm} ). Its default strain range is \ge 3000\ \mu\varepsilon .
2. OFSCN® Polymer-encapsulated Fiber Bragg Grating Strain Sensor (1.5mm/2.3mm diameter)
This sensor also uses polymer material for core encapsulation. The default product outer diameter is \le 1.5\text{ mm} , and with the addition of an outer seamless steel tube protective layer, the outer diameter becomes \le 2.3\text{ mm} . In this larger diameter design, its normal temperature range can reach \ge 6000\ \mu\varepsilon , making it suitable for detection scenarios with larger strain variations.
For Comparison: All-Metal Tube Encapsulated Products
If your testing environment involves very high temperatures (e.g., outside the normal range of -20^\circ\text{C} to 55^\circ\text{C} ), significant friction from geotechnical materials or mechanical abrasion, and the test substrate is a steel structure or a metal substrate with a high elastic modulus, you should prioritize using strain sensors with full elastic alloy tube encapsulation:
OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor
This product utilizes an elastic alloy tube for encapsulation, with a default outer diameter of \le 1.1\text{ mm} . It offers excellent strain transfer performance from metal surfaces and abrasion resistance, a measurement range of \ge 6000\ \mu\varepsilon , and can be customized for operating temperatures up to 300^\circ\text{C} .
