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A new Fiber Bragg Grating sensor based circumferential strain sensor fabricated using 3D printing method
Sensors and Actuators A 295 (2019) 663–670
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A new Fiber Bragg Grating sensor based circumferential strain sensor fabricated using 3D printing method Yuyao Yang a,b , Chengyu Hong b,∗ , Zamir Ahmed Abro b,c , Lei Wang a , Zhang Yifan d a
College of Urban Rail Transportation, Shanghai University of Engineering Science, Shanghai, 201620, China College of Civil and Transportation Engineering, Shenzhen University, Shenzhen, 518060, China Department of Textile Engineering, BUITEMS, Airport Road, Quetta, 87300, Pakistan d Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong b c
a r t i c l e
i n f o
Article history: Received 17 March 2019 Received in revised form 14 June 2019 Accepted 24 June 2019 Available online 26 June 2019 Keywords: Circumferential strain sensor Fused deposition modeling Fiber Bragg grating Compression test
a b s t r a c t A new Fiber Bragg Grating (FBG) circumferential strain sensor was fabricated using fused deposition modeling (FDM) method and used to measure the circumferential strain of Rubber cylinders in a uniaxial compression test. Advantages of this FBG based sensor include small size, flexible, low cost, immunity to electromagnetic interference (EMI) and quick prototyping. Calibration test data indicate that the wavelength of FBG strain sensors is linearly proportional to the change of occurred elongation. Measurement sensitivity and resolution of the FBG sensor are 0.0218 nm/mm and 114 m, respectively. The maximum wavelength difference was 0.0124 nm with a maximum measurement error of 1.59%. The designed new FBG based sensor has shown excellent performance and can be used for both vertical pressure measurements and circumferential strain monitoring. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Poisson’s ratio is an important parameter to characterize the mechanical properties of materials [1]. It is physical definition refers to the absolute value of the rate of transverse strain to longitudinal strain of materials under axial tension (compression), which reflects the elastic constants of transverse deformation in practical measurement [2]. Design and fabrication of circumferential strain sensor have attracted significant attention due to the difficulty for monitoring the circumferential deformation [3,4]. The measurement of circumferential strain can usually be divided into contact method and non-contact method [5]. Non-contact measurement mainly includes an optical interferometry method and digital speckle correlation method (DSCM) [6]. The Contact method is the mainstream method of circumferential deformation measurement, which was represented by the mechanical process and electrical method [7]. A distributed optical fiber temperature sensor was used to measure the circumferential strain of the pipeline to determine the possible leakage location of pipe [8,9] and also
∗ Corresponding author. E-mail addresses: [email protected] (Y. Yang), [email protected] (C. Hong), [email protected] (Z. Ahmed Abro), wangle [email protected] (L. Wang), [email protected] (Z. Yifan). https://doi.org/10.1016/j.sna.2019.06.048 0924-4247/© 2019 Elsevier B.V. All rights reserved.
used the grating sensor to test the transverse-vertical deformation data and poison’s ratio of material [10,11] Fiber Bragg Grating (FBG) technology has been widely used in optical fiber sensing, optical fiber communication and other concerned fields [12]. Zhang et al. designed a shock-resistant FBG pressure sensor, which uses a circular flat diaphragm as a pressurebearing surface and uses deflection stretching FBG under consistent pressure to produce displacement. The sensor has been successfully used to measure the gas pressure change process after cavity explosion and explosion and achieved good test results [12]. Hu et al. applied the FBG sensor to a monitoring and warning system for expressway slopes by fabricating and protecting the FBG sensor on the inclinometer casing. The fiber Bragg grating–based monitoring system of soil nails and inclinometers forms a real-time sensing network, assisting engineers in adopting quick and essential measures to deal with potential landslide risk [13]. Fiber Bragg grating sensing is the well-developed technology in civil engineering applications because of its small size, light weight, high sensitivity and strong anti-electromagnetic interference ability [13]. Optical fiber sensors can be divided into functional sensors and non-functional sensors according to the suitability of the sensor signals modulations [14]. The FBG sensors are easy to be shared and broken in the application process, so it is necessary to pack an FBG sensor from external vibration and damage. There are two traditional packaging methods: a substrate, embedded [15]. The FBGs are encapsulated on
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Fig. 1. Principle of FBG sensor.
titanium alloy sheet by using a high temperature resistant organic adhesive. The linear expansion coefficient of the base material is more significant than that of FBG, which can improve its temperature sensitivity coefficient and substrate type is easy to operate but with low precision [16]. Zhou Guopeng et al. put forward a new type of polymer packaging technology. Bare FBG is encapsulated in two kinds of polymer substrates by special materials. Hot-melt materials are encapsulated on FBG, and FBG is encapsulated by thermal shrinkage characteristics of hot-melt materials. The temperature sensitivity coefficient of bare FBG can be increased 6 times by the new packaging technology, and the good strain characteristics can be maintained [18]. FBG sensor was put in the capillary steel tube and filled with modified acrylate in the middle and then; put it into the oven to bake and solidify [17]. Yu Xiujuan et al. put forward a copper chip packaging process. FBG was encapsulated with twocomponent M-Bond 610 glue in a copper chip with fine grooves by using copper with larger linear expansion coefficient as packaging material. Copper chip packaging process does not change the strain sensitivity coefficient of FBG, but the temperature sensitivity coefficient is increased by 2.78 times [20]. Fused deposition modeling (FDM) process to encapsulate the FBG sensor in Polylactic Acid (PLA) material can effectively solve the shortcomings of high loss and complex encapsulation process [18,19]. The measurement of circumferential strain is often applied in geotechnical engineering such as triaxial compression test and pipeline monitoring [20]. Considering the rapid development and application of pipeline transportation in various industries; the safety problems of oil and gas pipelines cannot be ignored because the damage of the pipeline interface, aging of the pipe wall and external causes will cause accidents. Therefore, it is essential to monitor the pipeline to avoid an accident. The conventional methods of pipeline detection include leak detection cable method, an airborne infrared method, and multi-fiber probe telemetry method [21,22]. These methods can improve the safety of pipelines, but there are still limitations of real-time monitoring of pipelines. The Real-time tracking of pipes can be realized by using an optical fiber sensor, which is a great significance to the study of pipeline deformation as well [23]. A triaxle compression test is an essential experiment for measuring soil parameters [24]. The shear strength index, internal friction angle and cohesive force of soil are determined by increasing the axial pressure until the specimen is sheared [25]. The new FBG circumferential strain sensor is applied to a triaxial compression test, which can effectively monitor the change of circumferential soil displacement.
In this study, an FBG sensor for measuring circumferential strain is designed and fabricated by FDM technology. The FBG sensor was used to measure circumferential strain of a rubber cylinder in uniaxial compression tests. Measured wavelength change of FBG sensor presents a linear relationship against displacement change during loading and unloading process. The test results indicate that the new FBG circumferential strain sensor could effectively monitor the circumferential displacement and vertical pressure of a cylinder.
2. Design and fabrication of FBG circumferential strain sensor 2.1. Sensing principle of FBG strain sensor Optical fibers are usually cylindrical. It can confine the electromagnetic wave energy in the form of light in its interface by using the law of total reflection and guide the light wave along the axis of the optical fiber. Fiber Bragg grating (FBG) sensor is a transmitting optical filter. The reflection spectra are about 200–300 pm. The FBG grating sensor has a smaller period. FBG sensors are widely used in the field of health monitoring in civil engineering in recent years [25]. At present, application of FBG sensor is based on the measurement of the central wavelength of FBG. FBG sensor senses the change of grating period according to the shift of reflected wavelength to measure physical quantities. Fig. 1 shows a schematic diagram of the FBG sensor when refractive index of the core region changes. The FBG sensor produces small periodic modulation and the perturbation of the periodical refractive index affecting the narrow spectrum. The wavelength of the sensor depends on the refractive index of the core and the grating period. This relationship can be written as: ˇ = 2n
(1)
where ˇ is wavelength, n is the refractive index of the core, is a period of the grating. When the stress of FBG changes, the grating period and the refractive index of the core will change, and the wavelength of reflected light will change as follow:
ε=
ˇ ˇ (1 − Pe )
(2)
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where: ε is a strain, ˇ is wavelength variation, Pe is Effective optical coefficient of the fiber core, decided by the following formula: Pe =
n2 [P12 − v (P11 + P12)] 2
(3)
where:v is poison’s ratio of materials, P11 ; P12 is elasto-optical coefficient. Pe ≈0.22. 2.2. Design and fabrication of FBG circumferential strain sensor In this study, a new FBG circumferential strain sensor was designed by FDM technology. A schematic diagram of fabrication process of the new FBG circumferential strain sensor is shown in Fig. 2. FDM is a rapid fabrication method for various sensing components, which were grasped by heating Polylactic acid or polylactide (PLA) filament up to its melting point, and then by layer by layer deposition of this extruded PLA filament to build 3D sensing components. The raw material of these sensing components was PLA having the basic physical properties of the flexible material tensile modulus, tensile strength, and melting temperature range of PLA are around 2.7–16 GPa, 50 MPa, and 118–200 ◦ C, respectively. The fused deposition modeling process is necessary to protect the FBG sensor from external damage and also the smart PLA belt can be slipped easily and it can break the FBG sensor. Fig. 3(a) shows a design diagram of a new type of FBG sensor, including FBG sensor, PLA protective shell and strain induction belt. The FBG circumferential strain sensor can be applied in geotechnical engineering such as triaxial compression test and pipeline monitoring. The FBG sensor is attached to the strain sensing band and the FDM material at the same time. Fig. 3(b) shows the use of FDM technology to print PLA package shell. In the FDM process, the FBG sensor was embedded into PLA material with reserved space when 50% size was completed. Then the printing process was restarted so that FBG sensor and rubber bands was finally fabricated in PLA packaging shell. The selected infill density was 80%. The whole printing process lasted for 14 min. Finally, the heat-shrinkable casing was used
Fig. 2. Fabrication process of FBG strain sensor.
to reinforce the printing process. The wavelength of the sensor is 1543.001 nm. Fig. 3(c) shows a complete design of the FBG circumferential strain sensor for measurement and calibration. The new FBG circumferential strain sensor consists of a bare FBG sensor, Polyvinyl chloride (PVC), strain induction band, PLA protective shell and optical fiber cable. FBG sensor is used to measure the circumferential displacement. The sensing element is a fiber Bragg grating. The grating length is 5 mm and the total length of FBG is 600 mm. Length of the PVC protective tube is 7.5 mm from each end of the grating; it ensures that it is just outside the protective package, and the entire range is 300 mm. FBG sensor is connected with the optical fiber cable. The strain induction band is
Fig. 3. Design and fabrication of a new FBG circumferential strain sensor.
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Fig. 4. Force diagram of the FBG circumferential strain sensor placed on a cylinder.
Fig. 5. Relationship between wavelength changes against time at different displacement.
120 mm in length, 5 mm in width and 2 mm in thickness. The FBG sensor and strain induction band were printed in PLA material by FDM technology. The protective package printed with PLA material is 20 mm long, 6 mm wide and 3 mm thick. This short distance protection will not only constrain the FBG sensor circumferential displacement and but also effectively ensure the high sensitivity of an FBG sensor. The new FBG circumferential strain sensor is mounted on a cylinder to measure its circumferential strain effectively.
Fig. 4 shows the force diagram of the FBG circumferential strain sensor placed on a cylinder. Internal force p can be expressed as follow:
FR =
pb ·
0
FN =
d dϕ 2
=
pbd 2
pbd FR = 2 2
Fig. 6. Linear relationships between wavelengths change against FBG sensor displacement.
(4)
(5)
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Fig. 7. Relation between wavelengths difference against different displacement positions.
Fig. 8. Error of wavelength difference against different displacement values.
Fig. 9. Vertical compression test setup.
Therefore:
pe =
pd FN = A 2ı
(6)
According to the generalized Hooke’s law, hoop strain εpe can be shown as follow:
pe εpe = (7) Epe where: pe is hoop stress of cylinder, b is the width of cylinder, d is internal diameter of the cylinder, ıpe is wall thickness of cylinder, p is internal pressure in the cylinder, εpe is hoop strain of cylinder, Epe is the elastic modulus of cylinder. 3. Calibration test FBG sensors were encapsulated in 3D models for sensing matrix strain to protect the FBG sensor from the external environment and damage. Furthermore, the FBG circumferential strain sensor was calibrated under constant room temperature in an isolated laboratory.
Fig. 10. Shows a typical relationship of the wavelength of FBG circumferential strain sensor with elapsed time under the different vertical pressure values.
Calibration test of the FBG circumferential strain sensor was completed in a testing laboratory. Calibration test was realized by stretching of the FBG circumferential strain sensor with six different lengths on both sides, including 10 mm, 20 mm, 30 mm, 40 mm, 50 mm, and 60 mm. A total of 5 stretching cycles were repeated in the calibration test. Fig. 5 presents the relationships between wavelength changes and the elapsed time of the new FBG circumferential strain sensor under different displacement points. The wavelength of the sensor changes periodically in each cycle. Magnitudes of the wavelength peaks were close at the maximum and minimum displacements. The sensor wavelength varies from 1540.021 to 1540.388 nm, 1540.027 to 1540.823 nm and 1540.000 to 1541.362 nm which are raised by stretching FBG sensor about 20 mm, 40 mm and 60 mm, respectively. The larger the tensile displacement, the more significant the difference between the peak values of wavelength.
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Fig. 11. The relationship between wavelength differences against stress.
Fig. 12. The relationship between stress and strain.
Five groups of wavelength differences in Fig. 5 are averaged, with 1540 nm as the threshold value, and then added together to obtain the wavelength values under different displacements. Fig. 6 shows the wavelength change under the displacement 10 mm–60 mm. The maximum measured displacement of the FBG sensor was 60 mm. The corresponding relationship between center wavelength and displacement was y = 0.0218x+1540, so that measurement sensitivity of the sensor was 0.0218 nm/mm. The calibration results reveal that displacement increases linearly proportional to the change of FBG wavelength. Fig. 7 summarizes the wavelength difference of the FBG sensor against displacement in a calibration test. It can be seen the maximum wavelength difference are 0.0114 nm, 0.0124 nm and 0.0122 nm under the different displacement of 20 mm, 40 mm, 60 mm, respectively. The maximum wavelength difference is 0.124 nm under 40 mm displacement. Fig. 8 shows error values of the wavelength difference against different displacement values. The relative error value of the sensor was L/L, L is the difference between the actual wavelength difference and the mean value of the wavelength difference, L is the mean value of wavelength difference. The maximum errors of wavelength difference were 1.59%, 1.38% and 0.91% for the displacement of 20 mm, 40 mm and 60 mm, respectively. The maximum error was 1.59% at 20 mm displacement position.
4. Experimental methods and results 4.1. Experimental methods Uniaxial compression test of FBG circumferential strain sensor was carried out in a laboratory where room temperature was constant. Fig. 9 shows the experimental design of the uniaxial compression test in a laboratory. The uniaxial compression experiment consists of a test control end, an FBG data collection, FBG interrogator, a loading platform, a Polyurethane (PU) rod, and an FBG circumferential strain sensor. PU rod material was fixed with a new synthetic material between plastics and rubber. Diameter was 40 mm with a length of 80 mm. PU bar has the characteristics of high elasticity and low compression permanent deformation rate. The PU rod was located at the center of two loading plates where the FBG sensor was positioned. Uniaxial compression test was conducted by applying 7 different vertical pressures on the upper surface of PU rod including 100 kPa, 150 kPa, 200 kPa, 250 kPa, 300 kPa, 350 kPa, and 400 kPa. The frequency of data acquisition interrogator was 25 Hz. Total 10 loading cycles were achieved in the uniaxial compression test. 4.2. Results and discussion Fig. 10 shows a typical relationship of the wavelength of FBG circumferential strain sensor with elapsed time under the different vertical pressure values of 100 kPa, 150 kPa, 350 kPa and 400 kPa
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Fig. 13. Wavelength difference of sensor under different pressure.
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Fig. 13 shows the maximum wavelength difference of FBG circumferential strain sensor in calibration test. It can be seen the maximum wavelength difference are around 0.011 nm, 0.012 nm, 0.0056 nm, 0.017 nm, 0.0102 nm, 0.0054 nm and 0.0091 nm under the different vertical pressure of 100 kPa, 150 kPa, 200 kPa, 250 kPa, 300 kPa, 350 kPa and 400 kPa, respectively. The maximum wavelength difference was 1.017 nm under 250 kPa pressure. Fig. 14 shows the wavelength error values of the FBG sensor measured under different vertical pressures. The relative error of wavelength value under different vertical pressure is L/L. (L is the difference between the wavelength difference in each cycle and the mean value of wavelength difference. L is the mean value of the wavelength difference). To analyze the measurement error of the FBG sensor under the vertical pressure of 100 kPa to 400 kPa; the maximum error only accounts for 9.9%, 8.7%, 3.6%, 9.9%, 5.5%, 2.7%, 4.4% and the error was less than 10%. The maximum error was 9.9% at 250 kPa pressure. 5. Conclusions A new FBG circumferential strain sensor is designed and fabricated by using FDM technology and FBG sensor. The performance of the FBG sensor is verified by placing the sensor on the PU rod during the uniaxial compression test. The sensor could use to measure circumferential strain in pipeline deformation and triaxial compression test. The typical finding and conclusions are given below:
Fig. 14. Wavelength error of sensor under different pressure.
respectively, the wavelength of the FBG sensor shows cyclic change with time in uniaxial compression test. Variation ranges of all cycles are highly stable under the different pressures of 100 kPa, 150 kPa, 350 kPa and 400 kPa, respectively. The wavelength changes vary about 1540.099 and 1540.014 nm, 1539.982 and 1540.132 nm, 1540.088 and 1540.290 nm, 1540.083 and 1540.296 nm all cyclic changes. The wavelength ranges remain stable under the same loading period, which indicates that the FBG sensor shows reasonable measurement performance. By averaging the 10 groups of wavelength differences in Fig. 10, the wavelength values under different displacements are obtained. Fig. 11 describes the relationship of wavelength difference against pressure on FBG sensors under different vertical pressure ranging from 100 kPa to 400 kPa in the uniaxial compression test. It can be seen that change in stress is linearly proportional to the shift in strains. Slope ratio of FBG sensor was 3.7344, which indicate that the FBG sensor is sustainable for elastic deformation measurement. Fig. 12 presents the average values of all measured wavelength difference of FBG sensor against stress and strain under the vertical pressure values varying from 100 kPa to 400 kPa. The wavelength difference of the FBG circumferential strain sensor increases linearly with the change of vertical pressure. 1 kPa of vertical stress leads to a change in wavelength up to 0.0003 nm. Hence, the measurement sensitivity is 0.0003 nm/kPa. The maximum and minimum measurement strains were obtained from the FBG circumferential strain sensor was 0–171 in calibration test.
a Anew FBG circumferential strain sensor was developed by a combination of FBG sensor and FDM technology. FBG sensor was placed at 50% printing of object and after successful printing, FBG sensor was stable to sense strain deformation, which indicates that FDM technology has successfully packaged the FBG sensor in 3D printed porotype. b The calibration test showed that the displacement and wavelength change linearly with a correlation coefficient of 0.99, the measurement sensitivity of the FBG sensor was 0.02183 nm/mm and the resolution was 114 m. The wavelength differences of the sensor were within the range of 0.013 nm in the tension cycle test and the maximum error of wavelength was 2%. c The maximum pressure (400 kPa) was applied to the FBG sensor in a uniaxial compression test. Through seven groups of different vertical loads, the consistency of the peak and minimum values of each vertical force was higher under 10th cycles. The wavelength difference at the 95% sensors was less than 0.01 nm and the error of wavelength difference was less than 10%. It is verified that the new FBG displacement sensor has better performance with a sensitivity of 0.0003 nm/kPa. Vertical loading test validates the feasibility of the new FBG circumferential displacement sensor made by FDM technology. Declaration of Competing Interest There is no conflict of interest and this work was not harmful to society and animals. Acknowledgment The authors wish to thank the financial support from National Natural Science Foundation of China (NSFC) (Project No: 41602352). References [1] W. Gross, H. Kress, Simultaneous measurement of the Young’s modulus and the Poisson ratio of thin elastic layers, Soft Matter 13 (2017) 1048–1055.
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Biographies
Yang Yuyao was born in Jiuquan, Gansu province, China in 1994. She is now a Master student in the School of Urban Rail Transportation, Shanghai University of Engineering Science. Her research area is development and application of optical fiber sensing technologies for monitoring geotechnical structures
Chengyu Hong received the PhD degree from the Hong Kong Polytechnic University. He is currently an Associate Professor in College of Civil and Transportation Engineering, Shenzhen University. His research interests include design and fabrication of new FBG sensors using 3D printing technology, application of different optical fiber sensor for geotechnical monitoring. He has authored more than 30 SCI papers and referred more than 30 SCI journals.
Dr. Zamir Ahmed Abro was born in Pakistan in 1982; He secured his Master’s degree from Zhejiang SciTech University Hangzhou, China. He was serving as Assistant professor in Department of Textile engineering in BUITEMS, Quetta, Pakistan. He is currently finished Doctors degree in Department of Knitting and clothing of Donghua University Shanghai china. His research area is Development of smart garments based on FBG and Flex Sensor and design and fabrication of new FBG sensors using 3D printing technology.
Wang lei were born in 1985 in Xian, Shanxi Province, China. He received PhD degree in the Shanghai University in 2017. His research areas were related to the field of unsaturated soil mechanics, geotechnical engineering model test and numerical simulation, structural online detection technology. He is now working in Shanghai University of Engineering Science. He is now a reviewer for more than 10 SCI journals.
Zhang Yi-Fan was born in China, in 1983. She received her Ph.D. degree in 2013 from Institute of Textiles and Clothing, The Hong Kong Polytechnic University. After graduation, she joined in Knitting & Clothing Department, College of Textiles, and Donghua University as a lecturer. Her research interests include the development of smart textiles and wearable sensors based on fiber Bragg grating and MEMS sensing technologies.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A new Fiber Bragg Grating sensor based circumferential strain sensor fabricated using 3D printing method Yuyao Yang a,b , Chengyu Hong b,∗ , Zamir Ahmed Abro b,c , Lei Wang a , Zhang Yifan d a
College of Urban Rail Transportation, Shanghai University of Engineering Science, Shanghai, 201620, China College of Civil and Transportation Engineering, Shenzhen University, Shenzhen, 518060, China Department of Textile Engineering, BUITEMS, Airport Road, Quetta, 87300, Pakistan d Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong b c
a r t i c l e
i n f o
Article history: Received 17 March 2019 Received in revised form 14 June 2019 Accepted 24 June 2019 Available online 26 June 2019 Keywords: Circumferential strain sensor Fused deposition modeling Fiber Bragg grating Compression test
a b s t r a c t A new Fiber Bragg Grating (FBG) circumferential strain sensor was fabricated using fused deposition modeling (FDM) method and used to measure the circumferential strain of Rubber cylinders in a uniaxial compression test. Advantages of this FBG based sensor include small size, flexible, low cost, immunity to electromagnetic interference (EMI) and quick prototyping. Calibration test data indicate that the wavelength of FBG strain sensors is linearly proportional to the change of occurred elongation. Measurement sensitivity and resolution of the FBG sensor are 0.0218 nm/mm and 114 m, respectively. The maximum wavelength difference was 0.0124 nm with a maximum measurement error of 1.59%. The designed new FBG based sensor has shown excellent performance and can be used for both vertical pressure measurements and circumferential strain monitoring. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Poisson’s ratio is an important parameter to characterize the mechanical properties of materials [1]. It is physical definition refers to the absolute value of the rate of transverse strain to longitudinal strain of materials under axial tension (compression), which reflects the elastic constants of transverse deformation in practical measurement [2]. Design and fabrication of circumferential strain sensor have attracted significant attention due to the difficulty for monitoring the circumferential deformation [3,4]. The measurement of circumferential strain can usually be divided into contact method and non-contact method [5]. Non-contact measurement mainly includes an optical interferometry method and digital speckle correlation method (DSCM) [6]. The Contact method is the mainstream method of circumferential deformation measurement, which was represented by the mechanical process and electrical method [7]. A distributed optical fiber temperature sensor was used to measure the circumferential strain of the pipeline to determine the possible leakage location of pipe [8,9] and also
∗ Corresponding author. E-mail addresses: [email protected] (Y. Yang), [email protected] (C. Hong), [email protected] (Z. Ahmed Abro), wangle [email protected] (L. Wang), [email protected] (Z. Yifan). https://doi.org/10.1016/j.sna.2019.06.048 0924-4247/© 2019 Elsevier B.V. All rights reserved.
used the grating sensor to test the transverse-vertical deformation data and poison’s ratio of material [10,11] Fiber Bragg Grating (FBG) technology has been widely used in optical fiber sensing, optical fiber communication and other concerned fields [12]. Zhang et al. designed a shock-resistant FBG pressure sensor, which uses a circular flat diaphragm as a pressurebearing surface and uses deflection stretching FBG under consistent pressure to produce displacement. The sensor has been successfully used to measure the gas pressure change process after cavity explosion and explosion and achieved good test results [12]. Hu et al. applied the FBG sensor to a monitoring and warning system for expressway slopes by fabricating and protecting the FBG sensor on the inclinometer casing. The fiber Bragg grating–based monitoring system of soil nails and inclinometers forms a real-time sensing network, assisting engineers in adopting quick and essential measures to deal with potential landslide risk [13]. Fiber Bragg grating sensing is the well-developed technology in civil engineering applications because of its small size, light weight, high sensitivity and strong anti-electromagnetic interference ability [13]. Optical fiber sensors can be divided into functional sensors and non-functional sensors according to the suitability of the sensor signals modulations [14]. The FBG sensors are easy to be shared and broken in the application process, so it is necessary to pack an FBG sensor from external vibration and damage. There are two traditional packaging methods: a substrate, embedded [15]. The FBGs are encapsulated on
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Fig. 1. Principle of FBG sensor.
titanium alloy sheet by using a high temperature resistant organic adhesive. The linear expansion coefficient of the base material is more significant than that of FBG, which can improve its temperature sensitivity coefficient and substrate type is easy to operate but with low precision [16]. Zhou Guopeng et al. put forward a new type of polymer packaging technology. Bare FBG is encapsulated in two kinds of polymer substrates by special materials. Hot-melt materials are encapsulated on FBG, and FBG is encapsulated by thermal shrinkage characteristics of hot-melt materials. The temperature sensitivity coefficient of bare FBG can be increased 6 times by the new packaging technology, and the good strain characteristics can be maintained [18]. FBG sensor was put in the capillary steel tube and filled with modified acrylate in the middle and then; put it into the oven to bake and solidify [17]. Yu Xiujuan et al. put forward a copper chip packaging process. FBG was encapsulated with twocomponent M-Bond 610 glue in a copper chip with fine grooves by using copper with larger linear expansion coefficient as packaging material. Copper chip packaging process does not change the strain sensitivity coefficient of FBG, but the temperature sensitivity coefficient is increased by 2.78 times [20]. Fused deposition modeling (FDM) process to encapsulate the FBG sensor in Polylactic Acid (PLA) material can effectively solve the shortcomings of high loss and complex encapsulation process [18,19]. The measurement of circumferential strain is often applied in geotechnical engineering such as triaxial compression test and pipeline monitoring [20]. Considering the rapid development and application of pipeline transportation in various industries; the safety problems of oil and gas pipelines cannot be ignored because the damage of the pipeline interface, aging of the pipe wall and external causes will cause accidents. Therefore, it is essential to monitor the pipeline to avoid an accident. The conventional methods of pipeline detection include leak detection cable method, an airborne infrared method, and multi-fiber probe telemetry method [21,22]. These methods can improve the safety of pipelines, but there are still limitations of real-time monitoring of pipelines. The Real-time tracking of pipes can be realized by using an optical fiber sensor, which is a great significance to the study of pipeline deformation as well [23]. A triaxle compression test is an essential experiment for measuring soil parameters [24]. The shear strength index, internal friction angle and cohesive force of soil are determined by increasing the axial pressure until the specimen is sheared [25]. The new FBG circumferential strain sensor is applied to a triaxial compression test, which can effectively monitor the change of circumferential soil displacement.
In this study, an FBG sensor for measuring circumferential strain is designed and fabricated by FDM technology. The FBG sensor was used to measure circumferential strain of a rubber cylinder in uniaxial compression tests. Measured wavelength change of FBG sensor presents a linear relationship against displacement change during loading and unloading process. The test results indicate that the new FBG circumferential strain sensor could effectively monitor the circumferential displacement and vertical pressure of a cylinder.
2. Design and fabrication of FBG circumferential strain sensor 2.1. Sensing principle of FBG strain sensor Optical fibers are usually cylindrical. It can confine the electromagnetic wave energy in the form of light in its interface by using the law of total reflection and guide the light wave along the axis of the optical fiber. Fiber Bragg grating (FBG) sensor is a transmitting optical filter. The reflection spectra are about 200–300 pm. The FBG grating sensor has a smaller period. FBG sensors are widely used in the field of health monitoring in civil engineering in recent years [25]. At present, application of FBG sensor is based on the measurement of the central wavelength of FBG. FBG sensor senses the change of grating period according to the shift of reflected wavelength to measure physical quantities. Fig. 1 shows a schematic diagram of the FBG sensor when refractive index of the core region changes. The FBG sensor produces small periodic modulation and the perturbation of the periodical refractive index affecting the narrow spectrum. The wavelength of the sensor depends on the refractive index of the core and the grating period. This relationship can be written as: ˇ = 2n
(1)
where ˇ is wavelength, n is the refractive index of the core, is a period of the grating. When the stress of FBG changes, the grating period and the refractive index of the core will change, and the wavelength of reflected light will change as follow:
ε=
ˇ ˇ (1 − Pe )
(2)
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where: ε is a strain, ˇ is wavelength variation, Pe is Effective optical coefficient of the fiber core, decided by the following formula: Pe =
n2 [P12 − v (P11 + P12)] 2
(3)
where:v is poison’s ratio of materials, P11 ; P12 is elasto-optical coefficient. Pe ≈0.22. 2.2. Design and fabrication of FBG circumferential strain sensor In this study, a new FBG circumferential strain sensor was designed by FDM technology. A schematic diagram of fabrication process of the new FBG circumferential strain sensor is shown in Fig. 2. FDM is a rapid fabrication method for various sensing components, which were grasped by heating Polylactic acid or polylactide (PLA) filament up to its melting point, and then by layer by layer deposition of this extruded PLA filament to build 3D sensing components. The raw material of these sensing components was PLA having the basic physical properties of the flexible material tensile modulus, tensile strength, and melting temperature range of PLA are around 2.7–16 GPa, 50 MPa, and 118–200 ◦ C, respectively. The fused deposition modeling process is necessary to protect the FBG sensor from external damage and also the smart PLA belt can be slipped easily and it can break the FBG sensor. Fig. 3(a) shows a design diagram of a new type of FBG sensor, including FBG sensor, PLA protective shell and strain induction belt. The FBG circumferential strain sensor can be applied in geotechnical engineering such as triaxial compression test and pipeline monitoring. The FBG sensor is attached to the strain sensing band and the FDM material at the same time. Fig. 3(b) shows the use of FDM technology to print PLA package shell. In the FDM process, the FBG sensor was embedded into PLA material with reserved space when 50% size was completed. Then the printing process was restarted so that FBG sensor and rubber bands was finally fabricated in PLA packaging shell. The selected infill density was 80%. The whole printing process lasted for 14 min. Finally, the heat-shrinkable casing was used
Fig. 2. Fabrication process of FBG strain sensor.
to reinforce the printing process. The wavelength of the sensor is 1543.001 nm. Fig. 3(c) shows a complete design of the FBG circumferential strain sensor for measurement and calibration. The new FBG circumferential strain sensor consists of a bare FBG sensor, Polyvinyl chloride (PVC), strain induction band, PLA protective shell and optical fiber cable. FBG sensor is used to measure the circumferential displacement. The sensing element is a fiber Bragg grating. The grating length is 5 mm and the total length of FBG is 600 mm. Length of the PVC protective tube is 7.5 mm from each end of the grating; it ensures that it is just outside the protective package, and the entire range is 300 mm. FBG sensor is connected with the optical fiber cable. The strain induction band is
Fig. 3. Design and fabrication of a new FBG circumferential strain sensor.
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Fig. 4. Force diagram of the FBG circumferential strain sensor placed on a cylinder.
Fig. 5. Relationship between wavelength changes against time at different displacement.
120 mm in length, 5 mm in width and 2 mm in thickness. The FBG sensor and strain induction band were printed in PLA material by FDM technology. The protective package printed with PLA material is 20 mm long, 6 mm wide and 3 mm thick. This short distance protection will not only constrain the FBG sensor circumferential displacement and but also effectively ensure the high sensitivity of an FBG sensor. The new FBG circumferential strain sensor is mounted on a cylinder to measure its circumferential strain effectively.
Fig. 4 shows the force diagram of the FBG circumferential strain sensor placed on a cylinder. Internal force p can be expressed as follow:
FR =
pb ·
0
FN =
d dϕ 2
=
pbd 2
pbd FR = 2 2
Fig. 6. Linear relationships between wavelengths change against FBG sensor displacement.
(4)
(5)
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Fig. 7. Relation between wavelengths difference against different displacement positions.
Fig. 8. Error of wavelength difference against different displacement values.
Fig. 9. Vertical compression test setup.
Therefore:
pe =
pd FN = A 2ı
(6)
According to the generalized Hooke’s law, hoop strain εpe can be shown as follow:
pe εpe = (7) Epe where: pe is hoop stress of cylinder, b is the width of cylinder, d is internal diameter of the cylinder, ıpe is wall thickness of cylinder, p is internal pressure in the cylinder, εpe is hoop strain of cylinder, Epe is the elastic modulus of cylinder. 3. Calibration test FBG sensors were encapsulated in 3D models for sensing matrix strain to protect the FBG sensor from the external environment and damage. Furthermore, the FBG circumferential strain sensor was calibrated under constant room temperature in an isolated laboratory.
Fig. 10. Shows a typical relationship of the wavelength of FBG circumferential strain sensor with elapsed time under the different vertical pressure values.
Calibration test of the FBG circumferential strain sensor was completed in a testing laboratory. Calibration test was realized by stretching of the FBG circumferential strain sensor with six different lengths on both sides, including 10 mm, 20 mm, 30 mm, 40 mm, 50 mm, and 60 mm. A total of 5 stretching cycles were repeated in the calibration test. Fig. 5 presents the relationships between wavelength changes and the elapsed time of the new FBG circumferential strain sensor under different displacement points. The wavelength of the sensor changes periodically in each cycle. Magnitudes of the wavelength peaks were close at the maximum and minimum displacements. The sensor wavelength varies from 1540.021 to 1540.388 nm, 1540.027 to 1540.823 nm and 1540.000 to 1541.362 nm which are raised by stretching FBG sensor about 20 mm, 40 mm and 60 mm, respectively. The larger the tensile displacement, the more significant the difference between the peak values of wavelength.
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Fig. 11. The relationship between wavelength differences against stress.
Fig. 12. The relationship between stress and strain.
Five groups of wavelength differences in Fig. 5 are averaged, with 1540 nm as the threshold value, and then added together to obtain the wavelength values under different displacements. Fig. 6 shows the wavelength change under the displacement 10 mm–60 mm. The maximum measured displacement of the FBG sensor was 60 mm. The corresponding relationship between center wavelength and displacement was y = 0.0218x+1540, so that measurement sensitivity of the sensor was 0.0218 nm/mm. The calibration results reveal that displacement increases linearly proportional to the change of FBG wavelength. Fig. 7 summarizes the wavelength difference of the FBG sensor against displacement in a calibration test. It can be seen the maximum wavelength difference are 0.0114 nm, 0.0124 nm and 0.0122 nm under the different displacement of 20 mm, 40 mm, 60 mm, respectively. The maximum wavelength difference is 0.124 nm under 40 mm displacement. Fig. 8 shows error values of the wavelength difference against different displacement values. The relative error value of the sensor was L/L, L is the difference between the actual wavelength difference and the mean value of the wavelength difference, L is the mean value of wavelength difference. The maximum errors of wavelength difference were 1.59%, 1.38% and 0.91% for the displacement of 20 mm, 40 mm and 60 mm, respectively. The maximum error was 1.59% at 20 mm displacement position.
4. Experimental methods and results 4.1. Experimental methods Uniaxial compression test of FBG circumferential strain sensor was carried out in a laboratory where room temperature was constant. Fig. 9 shows the experimental design of the uniaxial compression test in a laboratory. The uniaxial compression experiment consists of a test control end, an FBG data collection, FBG interrogator, a loading platform, a Polyurethane (PU) rod, and an FBG circumferential strain sensor. PU rod material was fixed with a new synthetic material between plastics and rubber. Diameter was 40 mm with a length of 80 mm. PU bar has the characteristics of high elasticity and low compression permanent deformation rate. The PU rod was located at the center of two loading plates where the FBG sensor was positioned. Uniaxial compression test was conducted by applying 7 different vertical pressures on the upper surface of PU rod including 100 kPa, 150 kPa, 200 kPa, 250 kPa, 300 kPa, 350 kPa, and 400 kPa. The frequency of data acquisition interrogator was 25 Hz. Total 10 loading cycles were achieved in the uniaxial compression test. 4.2. Results and discussion Fig. 10 shows a typical relationship of the wavelength of FBG circumferential strain sensor with elapsed time under the different vertical pressure values of 100 kPa, 150 kPa, 350 kPa and 400 kPa
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Fig. 13. Wavelength difference of sensor under different pressure.
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Fig. 13 shows the maximum wavelength difference of FBG circumferential strain sensor in calibration test. It can be seen the maximum wavelength difference are around 0.011 nm, 0.012 nm, 0.0056 nm, 0.017 nm, 0.0102 nm, 0.0054 nm and 0.0091 nm under the different vertical pressure of 100 kPa, 150 kPa, 200 kPa, 250 kPa, 300 kPa, 350 kPa and 400 kPa, respectively. The maximum wavelength difference was 1.017 nm under 250 kPa pressure. Fig. 14 shows the wavelength error values of the FBG sensor measured under different vertical pressures. The relative error of wavelength value under different vertical pressure is L/L. (L is the difference between the wavelength difference in each cycle and the mean value of wavelength difference. L is the mean value of the wavelength difference). To analyze the measurement error of the FBG sensor under the vertical pressure of 100 kPa to 400 kPa; the maximum error only accounts for 9.9%, 8.7%, 3.6%, 9.9%, 5.5%, 2.7%, 4.4% and the error was less than 10%. The maximum error was 9.9% at 250 kPa pressure. 5. Conclusions A new FBG circumferential strain sensor is designed and fabricated by using FDM technology and FBG sensor. The performance of the FBG sensor is verified by placing the sensor on the PU rod during the uniaxial compression test. The sensor could use to measure circumferential strain in pipeline deformation and triaxial compression test. The typical finding and conclusions are given below:
Fig. 14. Wavelength error of sensor under different pressure.
respectively, the wavelength of the FBG sensor shows cyclic change with time in uniaxial compression test. Variation ranges of all cycles are highly stable under the different pressures of 100 kPa, 150 kPa, 350 kPa and 400 kPa, respectively. The wavelength changes vary about 1540.099 and 1540.014 nm, 1539.982 and 1540.132 nm, 1540.088 and 1540.290 nm, 1540.083 and 1540.296 nm all cyclic changes. The wavelength ranges remain stable under the same loading period, which indicates that the FBG sensor shows reasonable measurement performance. By averaging the 10 groups of wavelength differences in Fig. 10, the wavelength values under different displacements are obtained. Fig. 11 describes the relationship of wavelength difference against pressure on FBG sensors under different vertical pressure ranging from 100 kPa to 400 kPa in the uniaxial compression test. It can be seen that change in stress is linearly proportional to the shift in strains. Slope ratio of FBG sensor was 3.7344, which indicate that the FBG sensor is sustainable for elastic deformation measurement. Fig. 12 presents the average values of all measured wavelength difference of FBG sensor against stress and strain under the vertical pressure values varying from 100 kPa to 400 kPa. The wavelength difference of the FBG circumferential strain sensor increases linearly with the change of vertical pressure. 1 kPa of vertical stress leads to a change in wavelength up to 0.0003 nm. Hence, the measurement sensitivity is 0.0003 nm/kPa. The maximum and minimum measurement strains were obtained from the FBG circumferential strain sensor was 0–171 in calibration test.
a Anew FBG circumferential strain sensor was developed by a combination of FBG sensor and FDM technology. FBG sensor was placed at 50% printing of object and after successful printing, FBG sensor was stable to sense strain deformation, which indicates that FDM technology has successfully packaged the FBG sensor in 3D printed porotype. b The calibration test showed that the displacement and wavelength change linearly with a correlation coefficient of 0.99, the measurement sensitivity of the FBG sensor was 0.02183 nm/mm and the resolution was 114 m. The wavelength differences of the sensor were within the range of 0.013 nm in the tension cycle test and the maximum error of wavelength was 2%. c The maximum pressure (400 kPa) was applied to the FBG sensor in a uniaxial compression test. Through seven groups of different vertical loads, the consistency of the peak and minimum values of each vertical force was higher under 10th cycles. The wavelength difference at the 95% sensors was less than 0.01 nm and the error of wavelength difference was less than 10%. It is verified that the new FBG displacement sensor has better performance with a sensitivity of 0.0003 nm/kPa. Vertical loading test validates the feasibility of the new FBG circumferential displacement sensor made by FDM technology. Declaration of Competing Interest There is no conflict of interest and this work was not harmful to society and animals. Acknowledgment The authors wish to thank the financial support from National Natural Science Foundation of China (NSFC) (Project No: 41602352). References [1] W. Gross, H. Kress, Simultaneous measurement of the Young’s modulus and the Poisson ratio of thin elastic layers, Soft Matter 13 (2017) 1048–1055.
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Biographies
Yang Yuyao was born in Jiuquan, Gansu province, China in 1994. She is now a Master student in the School of Urban Rail Transportation, Shanghai University of Engineering Science. Her research area is development and application of optical fiber sensing technologies for monitoring geotechnical structures
Chengyu Hong received the PhD degree from the Hong Kong Polytechnic University. He is currently an Associate Professor in College of Civil and Transportation Engineering, Shenzhen University. His research interests include design and fabrication of new FBG sensors using 3D printing technology, application of different optical fiber sensor for geotechnical monitoring. He has authored more than 30 SCI papers and referred more than 30 SCI journals.
Dr. Zamir Ahmed Abro was born in Pakistan in 1982; He secured his Master’s degree from Zhejiang SciTech University Hangzhou, China. He was serving as Assistant professor in Department of Textile engineering in BUITEMS, Quetta, Pakistan. He is currently finished Doctors degree in Department of Knitting and clothing of Donghua University Shanghai china. His research area is Development of smart garments based on FBG and Flex Sensor and design and fabrication of new FBG sensors using 3D printing technology.
Wang lei were born in 1985 in Xian, Shanxi Province, China. He received PhD degree in the Shanghai University in 2017. His research areas were related to the field of unsaturated soil mechanics, geotechnical engineering model test and numerical simulation, structural online detection technology. He is now working in Shanghai University of Engineering Science. He is now a reviewer for more than 10 SCI journals.
Zhang Yi-Fan was born in China, in 1983. She received her Ph.D. degree in 2013 from Institute of Textiles and Clothing, The Hong Kong Polytechnic University. After graduation, she joined in Knitting & Clothing Department, College of Textiles, and Donghua University as a lecturer. Her research interests include the development of smart textiles and wearable sensors based on fiber Bragg grating and MEMS sensing technologies.