
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)13803-0
10.1016/j.heliyon.2024.e37772
e37772
Research Article
A tailless FBG smart bolt with good performance
Hou Dan
Zheng Min
Yang Zhimin
Li Run
Li Xiaojin nku_xjli@163.com
⁎
Guangdong Provincial Optical Fiber Sensing Engineering Technology Research and Development Center, T&S Communications Co., Ltd., Shenzhen, 518118, China
⁎ Corresponding author. nku_xjli@163.com
11 9 2024
30 9 2024
11 9 2024
10 18 e3777228 3 2024
4 9 2024
9 9 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
A novel type of tailless FBG smart bolt with good performance is developed. Owe to the innovative design of an integrated FBG sensor comprising a strain measured FBG and a temperature-compensated FBG, the tailless FBG smart bolt serves the purpose of torque monitoring or bolt looseness monitoring. We use a theoretical approach to establish the linear correlation between the applied torque and the central wavelength of the tailless FBG smart bolt. Additionally, a newly designed M24 bolt with a nominal strength has been integrated as a crucial component of the tailless FBG smart bolt. To enable the experimental study, the tailless FBG smart bolt is newly designed, fabricated and tested. The experimental results show a strong linear correlation between the applied torque and the central wavelength of the tailless FBG smart bolt, showing good agreement with the theory.

Keywords

Tailless FBG smart bolt
Integrated FBG sensor
Bolt looseness monitoring
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pmc1 Introduction

Bolted joints are crucial in engineering for creating strong structural connections. They are extensively used in large-scale structures with stringent safety requirements, including high-rise buildings, long-span bridges, and civil engineering structures [[1], [2], [3]]. When bolts become loose due to factors like high frequency vibration or high temperature, the consequences for these structures can be catastrophic, posing a significant risk to personal safety [4]. Thus, monitoring bolt looseness is of utmost importance.

Up to now, there are many effective methods to monitor the bolt looseness, including vision-based method [5,6], the piezoelectric method [7,8], ultrasonic method [9,10], electrical conductivity measurement method [11], wave interferometry method [12], entropy analysis method [13], polymer optical fiber method [14,15] and fused-silica optical fiber method [[16], [17], [18], [19]]. For instance, Huynh et al. applied the regional convolutional neural network (R-CNN) algorithm to detect loosened bolts [5]. For the same purpose, Ramana et al. proposed a novel approach to automatically detect loosened bolts based on the Viola-Jones algorithm and a support vector machine [6]. Nguyen et al. proposed a good method for bolt-looseness assessment based on the integration of the convolutional neural network and the frequency-domain impedance-based technique [7]. Shang et al. proposed a detection method utilizing phased array ultrasonic testing to assess the extent of washer damage and bolt looseness [9]. However, a limitation of these approaches is the difficulty in integrating sensors inside the bolt. In other words, a built-in direct measurement approach for bolt looseness is still lacking.

The optical fiber sensors, particularly the fiber Bragg grating (FBG) sensors, have gained significant attention in practical applications due to their small size, easy integration, electrical isolation, multiplexing, and immunity to electromagnetic interference [[20], [21], [22], [23]] Furthermore, the FBG (or FBG arrays) sensors, as an effective approach in monitoring bolts, showcase its good strain-sensitive property. For instance, Wang et al. discussed a complete distributed FBG force-measuring bolt and its installation [16]. Duan et al. developed a FBG-based smart bolt that resists tail fiber sloshing to monitor bolt looseness [4]. Ren et al. introduced a smart bolt that utilized embedded FBG sensors to simultaneously measure axial and shear forces [17]. Chen et al. proposed a novel approach to quantitatively monitor bolt looseness using a FBG enabled smart washer [18]. However, there are two notable drawbacks associated with the aforementioned FBG-based smart bolts. Firstly, the inclusion of a long tail fiber poses significant challenges during the installation of the bolt in large-scale structures. Secondly, the linearity of repeated tests falls short of expectations, as indicated by an unsatisfactory R-squared value of less than 0.990. These disadvantages considerably restrict the potential application of the aforementioned FBG-based smart bolts.

In this work, a tailless FBG smart bolt with good linearity (R-squared value ≥ 0.999) of repeated tests has reported. The innovative design of the integrated FBG sensor allows the bolt to simultaneously measure strain and temperature. For the experimental study, the tailless FBG smart bolt is newly designed, firstly fabricated and tested. Our experimental study has confirmed a significant linear relationship between the applied torque and the central wavelength of the tailless FBG smart bolt.

2 The principle of the tailless FBG smart bolt

The principle of the tailless FBG smart bolt is illustrated in Fig. 1 (a). The incident light enters the FBG (embedded in the tailless FBG smart bolt) and reflects a narrowband light named Bragg wavelength (λB), i.e., the reflected light. According to Bragg's law, λB can be expressed as followed [24]:(1) λB=2neff⋅Λ

where, neff is the effective refractive index of the fiber. Λ is the grating period of the FBG. If the FBG (embedded in the tailless FBG smart bolt) is subjected to the physical perturbations such as strain and temperature, the Bragg wavelength shift (Δλ) can be expressed by using the axial strain (ε) and the temperature change (ΔT) [25]:(2) ΔλλB=(1−Pe)⋅ε+(α+ξ)⋅ΔT

here, Pe is the photo-elastic constant of the fiber. For the fused silica, Pe=0.22. α represents the thermal expansion coefficient, while ξ represents the thermo-optic coefficient of the fiber. If the temperature remains constant, i.e., ΔT=0, Equation (2) can be written as:(3) ΔλλB=(1−Pe)⋅ε

Fig. 1 (a) The structure and principle of the tailless FBG smart bolt and (b) schematic of fastening bolt using a torque wrench.

Fig. 1

It is well known that in the case of fastening bolt using a torque wrench, as shown in Fig. 1(b), it is too difficult to directly measure the axial force in a bolt. However, one can be indirectly obtained the axial force (F) by the applied torque (Q) with the help of torque wrench. The expression can be written as [18]:(4) Q=k⋅d⋅F

here, k and d are the nut factor and the bolt diameter (in this experiment, k=0.13 and d=24 mm). For the FBG (or the FBG strain sensor) embedded in the tailless FBG smart bolt, according to Hooke's law, the axial strain (ε) can be obtained using the axial force (F) in bolt [24]:(5) ε=FE⋅A

where, E and A are the elasticity modulus and the cross-sectional area of the bolt (in this experiment, E=210 GPa). Since the cross-sectional area of the bolt is a circle, A can be written as A=π⋅(d2)2. Therefore, according to Equations (3), (4), (5), the relationship between the Bragg wavelength shift (Δλ) and the applied torque (Q) can be written as [18]:(6) Δλ=4⋅λB⋅(1−Pe)π⋅k⋅E⋅d3⋅Q

it can be observed that there is a linear relationship between the applied torque and the Bragg wavelength shift. Moreover, the proportional coefficient (i.e., 0.004 nm/N‧m) can be theoretically calculated based on those parameters mentioned above, when the Bragg wavelength (λB) is equal to 1536.686 nm.

3 The design of the tailless FBG smart bolt

3.1 The design of the integrated FBG sensor

Before design, the two-point FBG array (or string) written in fused-silica optical fiber has been fabricated by the UV laser based phase mask technology [26] and it enables simultaneous measurement of strain and temperature. The two-point FBG array fabrication consists of six steps:1) the high-quality SMF-28e optical fibers, coated with acrylate, were purchased from Corning Incorporated (a world leader in glass and ceramic products).

2) we conducted the high-pressure hydrogen loading process to enhance the photosensitivity of optical fibers.

3) mechanical stripping was required to remove the original acrylate coating. Notice that the stripping lengths of FBG1 and FBG2 are approximately 6 mm and 10 mm, respectively.

4) FBG writing. The schematic diagram of FBG writing, as shown in Fig. 2 (i.e., ref. [26]), illustrates a UV laser beam, generated by an excimer laser, passed through a beam shaping assembly consisting of a small aperture and two cylindrical lens. Then, it entered the phase mask vertically. After that, the UV laser beam was split into three primary components: +1 order, −1 order and 0 order diffractions. Then, +1 order and −1 order diffractions formed the interference fringes, which appeared near the stripped fiber, causing a periodic refractive index structure along the fiber core.Fig. 2 The two-dimensional drawings of two-point FBG array.

Fig. 2

5) FBG online check. The broadband light source emitted broad spectrum light which was then directed into the abovementioned FBG. In order to monitor the FBG spectrum in real-time, an optical spectrum analyzer was utilized, while the optical fiber was secured in place by two fiber clamps. During this step, it is important to verify the FBG parameters such as central wavelength, −3 dB value, side-mode suppression ratio (SMSR) value, and other relevant factors.

6) The annealing process was necessary to eliminate the excess hydrogen and improve the stability of the FBG.

Repeating the abovementioned six steps twice, the two-point FBG array can be obtained. In order to insert into the M24 bolt, the one side of the FBG2 needs to be cut off.

Before fabricating the integrated FBG sensor, the two-dimensional drawings of the two-point FBG array should be determined, as shown in Fig. 2. It can be observed that the distance between the FBG1 and the FBG2 is only 28 mm (3 mm + 20 mm + 5 mm, FBG is in the middle of the stripping length), which means that they are very close. In other words, the temperature distribution can be regarded as same between the locations of the FBG1 and the FBG2. Thus, it is reasonable to use the FBG2 as a temperature compensation sensor for the FBG1.

The integrated FBG sensor, a crucial component of the tailless FBG smart bolt, plays a significant role in enhancing the sensing performance. The design drawing of the integrated FBG sensor is presented in Fig. 3(a). It comprises the brake hoop, FBG strain sensor, spring leaf and fiber connector components. The brake hoop provides support for the FBG strain sensor and the spring leaf. The FBG strain sensor consists of a protective layer, polyimide (PI) film, limited blocks, thin stainless steel, fiber, jacket and protective tube. Detailed fabrication instructions for the FBG strain sensor can be found in our previous article [26]. The spring leaf applies pressure to securely fasten the FBG strain sensor using Loctite 406 adhesive in the tailless FBG smart bolt. Furthermore, the fiber connector components consist of the tailstock, spring, ceramic ferrule and ceramic sleeve. The ceramic sleeve guarantees precise coupling of the two ceramic ferrules.Fig. 3 The design drawing (a) and real pictures (b) of the integrated FBG sensor. In order to clearly exhibit the ceramic ferrule, the ceramic sleeve is removed in the real pictures.

Fig. 3

The real pictures of the integrated FBG sensor, offering a 360-degree horizontal view, can be seen in Fig. 3(b), the two-point FBG array embedded in the integrated FBG sensor includes FBG1 (i.e., the FBG used for strain measurement) and FBG2 (i.e., the FBG employed for temperature compensation). The FBG1 has a smaller central wavelength and is utilized to measure both strain and temperature in the integrated FBG sensor.

In order to determine the location of the FBG2, Fig. 4 exhibits the schematic diagram of the FBG2 located inside the ceramic ferrule. First, we put the FBG2 into the middle of the ceramic ferrule according to the mark point. Then, in order to paste the FBG2 and prevent it from occurring any movement, the 353ND epoxy resin adhesive (widely used for optical device assembly) is used at both ends of the ceramic ferrule. To obtain the better bonding effect, the 353ND epoxy resin adhesive needs to paste 2 mm–3 mm bare fiber, as shown in Fig. 4. In addition, due to packaging in the ceramic ferrule, the FBG2 is only affected by temperature and without strain effect. Thus, the FBG2 serves as a temperature compensated sensor for the FBG1.Fig. 4 Schematic diagram of the FBG2 located inside the ceramic ferrule.

Fig. 4

Based on previous experience, spectral observation is an effective method for evaluating the quality of a FBG sensor. Therefore, the first step is to measure the spectrum of the integrated FBG sensor, as depicted in Fig. 5. It is evident from the observation that the FBG1 has a central wavelength of 1538.225 nm, while the FBG2 has a central wavelength of 1549.250 nm. In addition, Furthermore, both FBGs exhibit clear Bragg peaks without any noticeable distortion near their central wavelengths. Consequently, we consider the fabricating quality of the integrated FBG sensor is satisfactory.Fig. 5 Two-point FBG array embedded in the integrated FBG sensor (a) and their spectra (b).

Fig. 5

The optical parameters of the integrated FBG sensor (with a size of approximately 7 × 6 × 40 mm3) are shown in Table 1. It can be observed that both FBG1 and FBG2 are of good quality, i.e., possessing a narrow bandwidth (≤0.65 nm), high SMSR value (≥15 dB) and good reflectivity (≥50 %).Table 1 Optical parameters of the integrated FBG sensor.

Table 1Parameters	Unit	Value	
Fiber type	–	Corning SMF-28e	
Gratings	–	FBG1&FBG2	
Wavelength1	nm	1538.225 (FBG1)	
Wavelength2	nm	1549.250 (FBG2)	
−3 dB Bandwidth	nm	≤0.65 (FBG1&FBG2)	
Absolute value of SMSR	dB	≥15 (FBG1&FBG2)	
Reflectivity	%	≥50 (FBG1&FBG2)	

3.2 The design of the bolt

The bolt is another key component of the tailless FBG smart bolt and plays an important role in the tailless FBG smart bolt. For this experiment, we have selected a M24 8.8 bolt with a nominal strength to accommodate the integrated FBG sensor. The newly design drawings of the bolt can be seen in Fig. 6, where it is evident that the bolt has a length of 85 mm and a diameter of 24 mm. To accommodate the integrated FBG sensor, a 40 mm deep irregular hole has been created using electric discharging machining. Additionally, four 6 mm deep M3 threaded holes have been machined using mechanical manufacturing technology to install the fiber adapter. Before use, it is important to clean the bolt with ultrasonic treatment to remove any oil stains.Fig. 6 The redesign drawings of the nominal strength of a M24 8.8 bolt. (a) Left view and (b) front view of the 2D view of the bolt; (c) The 3D view of the bolt.

Fig. 6

The material parameters of the M24 8.8 bolt are shown in Table 2. It can be observed that the M24 8.8 bolt has a proof load of 211.8 kN and a torque value of 682.4 N‧m.Table 2 Material parameters of the M24 8.8 bolt (according to ISO 898–1:2013).

Table 2Parameters	Unit	Value	
Bolt size	–	M24	
Pitch	mm	3.0	
Stress area	mm2	353	
Proof stress	N/mm2	600	
Proof load	kN	211.8	
Tensile stress	N/mm2	830	
Torque	N‧m	682.4	
Hardness	HRC	23–34	
Elongation	%	12.0	

3.3 The tailless FBG smart bolt assembly

After fabricating the integrated FBG sensor and the bolt, the tailless FBG smart bolt can be assembled manually using the pressure lever and the fiber adapter, as depicted in Fig. 7(a), Notice that the tailless FBG smart bolt has a length of approximately 90 mm and a diameter of 24 mm. The assembly process of the tailless FBG smart bolt consists of four steps. Firstly, apply Loctite 406 adhesive to the surface of the thin stainless steel of the FBG strain sensor. Secondly, slowly insert the integrated FBG sensor into the bolt. Thirdly, quickly insert the pressure lever into the bolt, which applies pressure to the spring leaf, tightening the FBG strain sensor with the assistance of the Loctite 406 adhesive. Finally, mount the fiber adapter onto the bolt using screw fixation. Once these four steps are completed, the tailless FBG smart bolt is fully fabricated. The top view and front view of the tailless FBG smart bolt can be seen in Fig. 7(b) and (c), respectively.Fig. 7 The assembly drawing (a) and real pictures (b)–(c) of the tailless FBG smart bolt.

Fig. 7

4 The performance of the tailless FBG smart bolt

To study the sensing performance of the tailless FBG smart bolt, we have developed a new experimental setup, as depicted in Fig. 8. The setup consists of a computer, a FBG wavelength demodulator a FBG wavelength demodulator from T&S Communication Co, Ltd. (model TS-WI) capable of achieving a wavelength resolution of 1 p.m., a test platform, a fixed block, a digital torque wrench, and the tailless FBG smart bolt and nut. The computer is responsible for recording and storing the experimental data. The FBG wavelength demodulator is utilized for demodulating the wavelength signals from the tailless FBG smart bolt. The tailless FBG smart bolt is securely fixed on the test platform using the fixed block. The digital torque wrench is utilized to apply a specific amount of torque to the tailless FBG smart bolt by acting on the nut. It is important to mention that the FBG wavelength demodulator and the tailless FBG smart bolt are connected through an optic fiber patch cord, which can be easily detached when necessary for testing purposes.Fig. 8 Schematic of the experimental setup for calibration test of the tailless FBG smart bolt.

Fig. 8

Firstly, we have conducted three loading processes to measure the central wavelength of the FBG2 as torque increased from 0 N‧m to 500 N‧m, and the measured results shown in Fig. 9. It is evident that the central wavelength of the FBG2 remains constant at 1549.25 nm as torque increases. This can be attributed to the consistent ambient temperature maintained by the air conditioning system during the tests. Thus, as a temperature sensor, the FBG2 demonstrates a stable central wavelength. Furthermore, Fig. 9(d) depicts good agreement between the measured results of the three loading processes and the theoretical data, indicating the exceptional temperature sensing performance of the tailless FBG smart bolt. By checking the experimental data, it can be found that the FBG2 has a maximum difference of 0.03 nm and the repeatability error (∼0.075 %) can be calculated using the methodology outlined in previous work [27].Fig. 9 The relationship between the central wavelength and the applied torque of the FBG2 in the tailless FBG smart bolt.

Fig. 9

In addition, the FBG2 demonstrates a strong linear correlation between wavelength and temperature across the entire range of temperatures measured, as shown in Fig. 10. The temperature sensitivity of the FBG2 is 0.020 nm/°C (i.e., 20 p.m./°C) and the degree of linearity (i.e., R-squared value) of the FBG2 is 0.999.Fig. 10 The central wavelength of the FBG2 was measured at temperatures ranging from −40 °C to 90 °C using a program-controlled high and low temperature/humidity test chamber.

Fig. 10

Afterwards, we conducted measurements on the central wavelength of the FBG1 with increasing torque from 0 N‧m to 500 N‧m in three loading processes. The results, displayed in Fig. 11, demonstrate a high level of linearity and repeatability for the FBG1. Specifically, the linearity and the R-squared value of three loading tests of the FBG1 are 0.999 and 0.004 nm/N‧m, respectively. In another word, the tailless FBG smart bolt performs exceptionally well in terms of linearity. Furthermore, Fig. 11(d) reveals that the measured results of three loading processes show good agreement with the theoretical data calculated using Equation (6), which confirms a good strain sensing capability of the tailless FBG smart bolt. By checking the experimental data, it can be found that the FBG1 has a maximum difference of 0.08 nm and the repeatability error (∼0.2 %) can be calculated according to previous work [27].Fig. 11 The relationship between the central wavelength and the applied torque of the FBG1 in the tailless FBG smart bolt.

Fig. 11

The FBG1 also demonstrates a strong linear correlation between wavelength and temperature across the entire range of temperatures measured, as shown in Fig. 12. The temperature sensitivity of the FBG1 is 0.023 nm/°C (i.e., 23 p.m./°C) and the degree of linearity (i.e., R-squared value) of the FBG1 is 0.997. The effect of temperature for the FBG1 can be removed by subtracting the changes of central wavelength of the FBG2. More precisely, the FBG1 can simultaneously measure both strain and temperature (i.e., Δλ1λB1=(1−Pe)⋅ε+k1⋅ΔT, λB1 = 1538.225 nm, Pe = 0.22, k1 = 23 p.m./°C) and the FBG2 can just measure temperature (i.e., Δλ2λB2=k2⋅ΔT, λB2 = 1549.250 nm, k2 = 20 p.m./°C). Thus, one can remove the effect of temperature by the subtraction of (Δλ1λB1−Δλ2λB2).Fig. 12 The central wavelength of the FBG1 was measured at temperatures ranging from −40 °C to 90 °C using a program-controlled high and low temperature/humidity test chamber.

Fig. 12

Although the tailless FBG smart bolt exhibits good strain sensing performance, it has its limitations. For instance, after creating a 40 mm deep irregular hole inside the bolt by using electric discharging machining, the whole structure of the bolt is destroyed, resulting in the decrease in mechanical strength of the bolt. In order to illustrate this, Fig. 13 exhibits the 3D finite element comparison results (with vs. without a hole) of the bolt under the same static tension. It can be observed that the strain value of the bolt near the head is about 1.748 × 10−3 with a 40 mm deep hole configuration. However, the strain value in the same position is about 1.470 × 10−3 without such a hole, as shown in Fig. 13(b). In other words, the bolt with a hole configuration is more susceptible to the same static tension, implicating that such a bolt has a lower value than the one (without a hole configuration) in mechanical strength.Fig. 13 The 3D finite comparison results of the bolt (section views) under the same static tension, (a) with and (b) without a 40 mm deep hole (with a size of approximately 7.2 × 6.2 × 40.2 mm3) configuration.

Fig. 13

5 Conclusion

In summary, this study has reported a tailless FBG smart bolt with good performance through the unique design of an integrated FBG sensor. The strain measured FBG and the temperature compensated FBG have been integrated in the integrated FBG sensor and their spectral characteristics have been studied. To evaluate the temperature and strain sensing capability of the tailless FBG smart bolt, a digital torque wrench-based experimental setup has been developed. The results demonstrate that the tailless FBG smart bolt exhibits good strain sensing performance, characterized by good linearity (R-squared value ≥ 0.999) and repeatability (repeatability error ≤0.2 %). Furthermore, the measured results of the tailless FBG smart bolt show good agreement with the theoretical data. Due to processing the above remarkable performance, the proposed tailless FBG smart bolt is well-suited for torque monitoring and bolt looseness monitoring in practical applications.

Funding

This work was supported by the Major Science and Technology Project of Shenzhen Municipality (Grant No. KJZD20230923114415031 ) and the 2023 Shenzhen High-Tech Zone Development Special Plan & Pingshan District Innovation Platform Project (Grant No. 91440300724721938J ).

Ethics declarations

The study complies with all ethics regulations.

Data availability

Data will be made available on request.

CRediT authorship contribution statement

Dan Hou: Writing – review & editing, Supervision, Investigation, Funding acquisition. Min Zheng: Software, Resources, Formal analysis. Zhimin Yang: Visualization, Formal analysis. Run Li: Visualization, Formal analysis. Xiaojin Li: Writing – original draft, Methodology, Investigation, Conceptualization.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Dan Hou reports financial support was provided by 10.13039/501100010877 Shenzhen Science and Technology Innovation Committee . If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was funded by the 10.13039/501100010877 Shenzhen Science and Technology Innovation Commission .
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