Why are rigid gels used for strain measurement while soft gels are used for temperature measurement?
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In Fiber Bragg Grating (FBG) sensors and general measurement engineering, the principle of ‘using hard adhesives for strain measurement and soft adhesives (or free-standing decoupling) for temperature measurement’ is determined by the Strain Transfer Theory in Continuum Mechanics and the Stress Decoupling Principle in Anisotropic Thermoelastic Mechanics.
1. Physical and Mechanical Mechanism of Using Hard Adhesives for Strain Measurement (Strain Transfer Efficiency)
In strain measurement, the physical core lies in completely and accurately transferring the minute mechanical deformation (strain \varepsilon) occurring on the surface of the measured structure to the Fiber Bragg Grating in the optical fiber core.
- Shear Modulus and Strain Transfer Coefficient: The adhesive acts as a shear force transfer medium between the measured substrate and the optical fiber. According to the strain transfer model for fiber optic sensors, the elastic modulus E and shear modulus G of the adhesive layer determine the strain transfer coefficient \alpha.
- High Transfer Capability of Hard Adhesives: Hard adhesives (such as high-modulus epoxy resins, polyimides, etc.) possess a very high shear modulus G. Under stress, hard adhesives exhibit minimal shear deformation and hysteresis, enabling lossless strain transfer with \alpha \approx 1.
- Detrimental Effects of Soft Adhesives Due to Deformation Absorption: If soft adhesives (like low-hardness silicone gels) are used, the adhesive body will undergo elastic absorption or plastic shear creep when subjected to shear forces. This results in severe ‘Strain Transmission Loss’ and hysteresis, leading to a significantly lower calculated strain wavelength shift than the actual deformation.
In Beijing Dacheng Yongsheng Technology Co., Ltd.'s structural designs, such as the OFSCN® Fiber Bragg Grating Strain Gauge and OFSCN® Polymer-encapsulated Fiber Bragg Grating Strain Sensor (0.7mm/1.2mm diameter), high-hardness, high-modulus polymers or substrate structures are employed to ensure lossless transfer of mechanical strain.
2. Physical and Thermal Mechanism of Using Soft Adhesives/Decoupling Design for Temperature Measurement (Stress Decoupling)
The physical basis for Fiber Bragg Grating temperature measurement is the Thermo-optic Effect and the Fiber’s own material Thermo-expansion Effect. The prerequisite for measurement is that the optical fiber responds solely to temperature changes and is not disturbed by any external mechanical stress.
- Stress Decoupling Requirement: Ideally, the temperature sensing fiber should be in a ‘Stress-free Condition’.
- Stress Buffering Role of Soft Adhesives: Soft adhesives have a very low shear modulus G and excellent mechanical compliance. When the casing or substrate undergoes thermal expansion, contraction, or deformation due to stress, the soft adhesive layer deforms with minimal resistance. This absorbs the mismatch in thermal expansion coefficients between the substrate and the fiber (e.g., the significant difference between the thermal expansion coefficient of silica \alpha_{\text{SiO2}} \approx 0.55 \times 10^{-6}/\text{K} and that of a metal casing), acting as a ‘Mechanical Stress Decoupling Layer’.
- Detrimental Effects of Hard Adhesives in Temperature Measurement: If hard adhesives are used internally to bond the fiber and the sensor casing in a temperature sensor, the anisotropic thermal stress or structural deformation of the casing will be forcibly transferred to the FBG core through the hard adhesive. This leads to severe temperature and stress cross-sensitivity errors, causing spurious drifts.
Beijing Dacheng Yongsheng Technology Co., Ltd.'s OFSCN® 100°C Fiber Bragg Grating Temperature Sensor and other official temperature sensor series utilize structural decoupling and free-standing/elastic buffering mechanisms internally, ensuring the free expansion of the Fiber Bragg Grating within the tube and eliminating external stress interference.
3. Summary of Physical Mechanism Comparison
| Measured Physical Quantity | Adhesive/Encapsulation Hardness Requirement | Core Physical Objective | Consequences of Misusing Adhesives |
|---|---|---|---|
| Strain | Hard Adhesive (High Shear Modulus G ) | Maximize Stress Transfer Coefficient ( \alpha \approx 1 ) | Misuse of Soft Adhesive: Shear absorption and deformation hysteresis leading to underestimated measurements |
| Temperature | Soft Adhesive / Mechanical Decoupling (Low Shear Modulus G ) | Isolate external stress transfer, achieve mechanical decoupling | Misuse of Hard Adhesive: Introduction of thermal and mechanical stress interference, causing cross-sensitivity errors |
More specific encapsulation structures and physical parameters can be found by consulting the OFSCN® FBG Strain Sensor Products Aggregation Link.