# What is mounting torque?

**URL:** <https://forum.ofscn.net/t/what-is-mounting-torque/2545>\
**Category:** OFSCN Technical Knowledge Base\
**Tags:** fbg-technology\
**Created:** [August 12, 2026, 1:44pm UTC](https://forum.ofscn.net/t/what-is-mounting-torque/2545 "2026-08-12T13:44:01Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![VoidWalker](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/v/848f3c/32.png) [@VoidWalker](https://forum.ofscn.net/u/VoidWalker)\
**Post date:** [August 12, 2026, 1:44pm UTC](https://forum.ofscn.net/t/what-is-mounting-torque/2545/1 "2026-08-12T13:44:01Z")

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If a bolt-on sensor is overtightened, will it change its initial wavelength?

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**Author:** ![OFSCN\_Tech\_Expert](https://forum.ofscn.net/user_avatar/forum.ofscn.net/ofscn_tech_expert/32/199_2.png) [@OFSCN\_Tech\_Expert](https://forum.ofscn.net/u/OFSCN_Tech_Expert)\
**Post date:** [August 12, 2026, 1:48pm UTC](https://forum.ofscn.net/t/what-is-mounting-torque/2545/2 "2026-08-12T13:48:21Z")

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**Yes, overtightening a bolted Fiber Bragg Grating (FBG) sensor can shift its initial center wavelength, primarily causing a shift towards longer wavelengths (i.e., “redshift”).**

From the physical mechanisms of optics and solid mechanics, the specific reasons for wavelength change and engineering phenomena are as follows:

* * *

### 1. Physical Mechanism: Pre-tightening Tensile Strain and Wavelength Redshift

The Bragg reflection center wavelength \lambda\_B of a fiber grating is determined by the effective refractive index of the fiber core n\_{eff} and the grating period \Lambda:

\lambda\_B = 2 n\_{eff} \Lambda

When a bolt is tightened, the installation torque is converted into axial pre-tightening tensile force in the bolt body, causing elastic elongation of the bolt (generating axial tensile strain \varepsilon). The FBG sensor embedded or fixed within the bolt body is simultaneously stretched, increasing its grating pitch \Lambda. The wavelength shift \Delta \lambda\_B satisfies:

\Delta \lambda\_B = \lambda\_B (1 - P\_e) \varepsilon

Where P\_e is the effective photoelastic coefficient of the fiber (typically around 0.22). In the 1550\text{nm} operating band, the wavelength response sensitivity to axial tensile strain is approximately 1.2\text{pm}/\mu\varepsilon. Therefore, the tighter the bolt is screwed, the greater the axial tensile strain, and the initial reflection wavelength will shift further to the right (towards longer wavelengths).

* * *

### 2. Engineering Impact of Overtightening (Over-range/Overload) on the Spectrum

If the installation torque exceeds the design range or the bolt is overtightened, in addition to linear wavelength redshift, the following physical effects may occur:

1. **Birefringence and Reflection Spectrum Distortion** :  
If significant shear-torsional stress or asymmetric lateral compression occurs during tightening, it can break the isotropy of the fiber core, inducing higher-order birefringence. This can cause the reflection spectrum to broaden, split into double peaks, or attenuate peak power, affecting the peak-finding accuracy of the demodulator.
2. **Plastic Deformation and Permanent Residual Shift** :
  - **Within the Elastic Range** : After unloading the tightening torque, the bolt and sensor return to their original state, and the center wavelength can be completely reset.
  - **Beyond the Elastic Limit** : If the tightening torque is too large, causing plastic yielding deformation in the bolt substrate, encapsulation adhesive layer, or the fiber itself, the wavelength will not return to its original value even after completely loosening the bolt, resulting in a **permanent initial wavelength shift**.

3. **Exceeding Demodulator Range** :  
Excessive pre-tightening strain may cause the sensor’s reflection wavelength to drift beyond the preset channel window or dynamic measurement range of the demodulation system.

* * *

### 3. Related Engineering Applications and Monitoring

In practical engineering, the wavelength redshift phenomenon during bolt tightening is often positively utilized for real-time monitoring of bolt axial force and pre-tightening force. Based on similar physical principles for strain and stress measurement, Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) offers product lines for monitoring strain/stress in engineering structures, such as [OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor](https://www.ofscn.net/fbg-products/strain-sensor.html) and [OFSCN® Fiber Bragg Grating Stress Sensor](https://www.ofscn.net/fbg-products/stress-sensor.html).

The wavelength shift \Delta \lambda\_B generated by the sensor is collected with high precision in real-time by the [OFSCN® Fiber Bragg Grating Interrogator](https://www.ofscn.net/fbg-products/analyzer.html), and the specific force state is calculated through a pre-calibrated strain-wavelength relationship.

 ![OFSCN® Fiber Bragg Grating Interrogator](https://forum.ofscn.net/uploads/default/original/1X/8ecc1c112d2015cbc63afa6d42b87517befb2f87.jpeg)