Optical fiber sensor for simultaneously monitoring stress and initial rust time of reinforcing steel bar

Abstract

The invention provides an optical fiber sensor for simultaneously monitoring the stress and the initial rust time of a reinforcing steel bar, and belongs to the technical field of structural health monitoring. The optical fiber sensor comprises a single-mode optical fiber core, a single-mode optical fiber cladding, a structural adhesive, a telescopic rubber sealing ring, an inner-layer stainless steel tube, an outer-layer stainless steel tube, stainless steel tube capillary pores, a porous stainless steel tube, a coreless optical fiber, a steel bar coating film, a single-mode optical fiber, a Bragg grating, a connecting key and a steel bar. According to the device, the deviation of the transmission spectrum of the sensor is monitored in real time, the stress and the initial rust time of the steel bar are monitored in real time at the same time, the stress and corrosion states of the steel bar are judged in time, maintenance measures are reasonably formulated, and the durability and the safety of a reinforced concrete structure are guaranteed. The sensor is small in size, light in weight, resistant to electromagnetic interference, resistant to corrosion, high in sensitivity and worthy of application and popularization.

Description

Technical Field

[0001] The invention relates to an optical fiber sensor for simultaneously monitoring steel bar stress and initial rust time, belonging to the technical field of structural health monitoring. Background Art

[0002] Steel bar corrosion is one of the main causes of durability failure of reinforced concrete structures. The alkaline environment of the concrete pore solution will form a passivation film on the surface of the steel bar. The corrosion of the passivation film caused by chloride ion erosion or concrete carbonization will cause steel bar corrosion, which will lead to cracking of the concrete protective layer and degradation of the bond between the steel bar and concrete, thereby reducing the bearing capacity of the structure. In addition, the stress state of the steel bar determines the safety performance of the structure. Therefore, the effective prediction of the initial rust time of steel bars in concrete structures in offshore environments and the monitoring of the stress state can not only reduce the structural maintenance cost but also ensure the safe operation of the structure. Therefore, it is necessary to develop a fiber optic sensor that can simultaneously monitor the stress and initial rust time of steel bars, so as to improve the safety and stability of reinforced concrete structures.

[0003] At present, the monitoring methods of steel corrosion are mainly divided into two categories: electrochemical methods and non-electrochemical methods. Electrochemical methods include half-cell potential method, linear polarization method, AC impedance method, electrochemical noise, etc. However, the measurement accuracy of half-cell potential is affected by the degree of wetness of concrete surface, whether the steel bar is coated, chloride ion content, concrete carbonization, etc.; the actual polarized steel bar area in polarization impedance test is uncertain and easy to cause measurement errors; AC impedance spectrum has the characteristics of long test cycle and complex impedance data. Non-electrochemical methods are mostly based on sound signals, electromagnetic signals, etc., including acoustic emission, ultrasonic detection, ground penetrating radar, magnetic flux leakage method, etc. However, these non-electrochemical methods are mostly used to monitor the late stage of steel corrosion, that is, concrete corrosion, rust expansion and cracking, steel bar corrosion cross-sectional area reduction, etc., and cannot be used for corrosion monitoring of some major and important engineering structures such as cross-sea bridges where the initial time of steel bar corrosion is set as durability life failure. Steel bar stress monitoring methods include differential resistance strain gauges and vibrating wire instruments. Differential resistance instruments are made using the principle that the deformation of the resistance wire is proportional to the resistance ratio. They have good sealing and high stability, but the resolution of the instrument is low. Vibrating string instruments are made using the principle that the vibration frequency of the steel string changes with the stress of the steel wire. They have high accuracy, large range, long-distance transmission, and small size, but poor waterproof performance and high production cost.

[0004] Due to a series of advantages such as small size, light weight, good corrosion resistance, anti-electromagnetic interference, high sensitivity, and the ability to be connected to a network to achieve real-time continuous monitoring, various fiber optic sensors have been proposed for monitoring in the fields of physics, chemistry, biology, and medicine. The Bragg grating (FBG) is packaged and then installed on the surface of the steel bar to monitor the strain of the steel bar, but it cannot monitor the initial rust time of the steel bar. Multimode optical fiber is sensitive to the refractive index of the external environment. If a layer of iron-carbon alloy film with a composition similar to that of steel bar is electroplated on the surface of the multimode optical fiber, when the iron-carbon alloy film rusts, the refractive index around the multimode optical fiber changes, causing the transmission spectrum to shift. By monitoring the transmission spectrum shift, the initial corrosion time of the iron-carbon film can be monitored.

[0005] Therefore, the present invention proposes a fiber optic sensor based on a Bragg grating-multimode fiber structure, which can be used to monitor the stress and initial rust time of steel bars at the same time. The sensor provides a new idea and method for stress and corrosion monitoring of steel components in the fields of engineering structures, petrochemicals, steel metallurgy, and aerospace. Summary of the invention

[0006] In order to solve the above technical problems, the present invention proposes an optical fiber sensor for simultaneously monitoring steel bar stress and initial rust time, the purpose of which is to achieve simultaneous monitoring of steel bar corrosion stress and initial rust time to ensure the safety and stability of the engineering structure.

[0007] The technical solution of the present invention:

[0008] An optical fiber sensor for simultaneously monitoring steel bar stress and initial rust time comprises a single-mode optical fiber core 1, a single-mode optical fiber cladding 2, a structural adhesive 3, a retractable rubber sealing ring 4, an inner stainless steel tube 5, an outer stainless steel tube 6, a retractable rubber sealing ring 7, stainless steel tube capillaries 8, a porous stainless steel tube 9, a coreless optical fiber 10, a steel bar coating 11, a retractable rubber sealing ring 12, a single-mode optical fiber 13, a Bragg grating 14, a connecting key 15, a connecting key 16, a connecting key 17, and a steel bar 18.

[0009] The single-mode optical fiber 13 includes a single-mode optical fiber core 1 and a single-mode optical fiber cladding 2. The Bragg grating 14 of the single-mode optical fiber 13 can be prepared by a phase mask method. The inner stainless steel tube 5 and the outer stainless steel tube 6 are connected and sealed with the single-mode optical fiber 13 using a structural adhesive 3. The inner stainless steel tube 5 and the outer stainless steel tube 6 are sealed by a retractable rubber seal 4. The inner stainless steel tube 5 is welded to the steel bar 18 through a connection key 15, and the outer stainless steel tube 6 is welded to the steel bar 18 through a connection key 16. The porous stainless steel tube 9 is sleeved on the outside of the coreless optical fiber 10. The porous stainless steel tube 9 has uniformly distributed stainless steel tube capillaries 8. The surface of the coreless optical fiber 10 is coated with a steel bar coating 11 by vacuum ion sputtering, and the porous stainless steel tube 9 is welded to the steel bar 18 through a connection key 17.

[0010] The outer stainless steel tube 6 and the porous stainless steel tube 9 are sealed by a retractable rubber sealing ring 7, and the porous stainless steel tube 9 and the single-mode optical fiber 13 are sealed by a retractable rubber sealing ring 12;

[0011] The diameter of the coreless optical fiber 10 is equal to the diameter of the single-mode optical fiber 13;

[0012] The single-mode optical fiber core 1, the single-mode optical fiber cladding 2, the single-mode optical fiber 13, the structural adhesive 3, the retractable rubber sealing ring 4, the inner stainless steel tube 5, the outer stainless steel tube 6, the Bragg grating 14, the connecting key 15, and the connecting key 16 constitute a steel bar stress sensing unit;

[0013] The porous stainless steel tube 9, the coreless optical fiber 10, the steel bar coating 11, the retractable rubber sealing ring 12 and the connecting key 17 constitute a steel bar initial rust time sensing unit. The porous stainless steel tube 9 protects the coreless optical fiber 10 while ensuring that the environment of the coreless optical fiber 10 is consistent with the external environment. The steel bar initial rust time sensing unit is welded and connected to the steel bar 18;

[0014] The inner stainless steel tube 5 and the outer stainless steel tube 6 are made of stainless steel, and their telescopic length range can be adjusted according to the size of the steel bar to be tested. After the length is determined, the steel bar stress sensing unit is sealed using structural adhesive 3 and a telescopic rubber sealing ring 4;

[0015] The distance between the connecting key 15 and the connecting key 16 is determined according to the length of the fiber Bragg grating sensor unit;

[0016] The retractable rubber sealing ring 4 can ensure that the sensor is in a sealed state, and does not limit the expansion and contraction of the inner stainless steel tube 5 and the outer stainless steel tube 6, and does not provide a constraint effect for the sensor;

[0017] The thickness of the steel bar coating 11 and the length and diameter of the coreless optical fiber 10 can be adjusted within a reasonable range according to the requirements for sensor sensitivity in actual engineering.

[0018] Working principle of the present invention:

[0019] The sensor is placed on the surface of the steel bar to be tested. The steel bar is strained by force, causing the Bragg wavelength of the transmission spectrum corresponding to the stress sensing unit fixed on the surface of the steel bar to be tested to shift. By establishing the relationship between the Bragg wavelength shift and the steel bar strain, the steel bar strain can be monitored.

[0020] When the strain of the environment in which the fiber Bragg grating sensor unit is located changes, the initial spacing L between the connection keys 15 and 16 of the fixed fiber Bragg grating sensor unit is fixed. 0Changes will be found, causing the telescopic stainless steel tube to expand and contract, with a telescopic distance of ΔL:

[0021] ΔL=L 0 ε s (1)

[0022] In the formula, ε s is the steel bar strain. The total length of the Bragg fiber grating stress sensing unit is L 1 , then:

[0023]

[0024] Measure strain ε f Then, the steel bar strain is calculated by formula (3):

[0025]

[0026] Then the true stress of the steel bar σ s for:

[0027] σ s =Eε s (4)

[0028] Where E is the elastic modulus of the steel bar to be tested.

[0029] In addition, when the steel bar corrodes, the steel bar coating on the surface of the coreless optical fiber corrodes, causing the refractive index around the coreless optical fiber to change, resulting in a shift in the characteristic wavelength corresponding to the sensor transmission spectrum. The shift in the characteristic wavelength and the amount of corrosion of the steel bar coating have the following relationship:

[0030] Δλ ncf =F(Δm) (5)

[0031] In the formula, Δλ ncf is the characteristic wavelength offset corresponding to the coreless optical fiber transmission spectrum, Δm is the mass loss of steel film corrosion. F is the functional relationship between the characteristic wavelength offset of the coreless optical fiber transmission spectrum and the mass loss of steel coating, which can be obtained through experiments. Thus, the initial rust time of steel bars can be monitored.

[0032] Beneficial effects of the present invention:

[0033] (1) The present invention can meet the sensitivity required by actual engineering by adjusting the thickness of the steel bar coating, the length and diameter of the coreless optical fiber.

[0034] (2) The present invention utilizes coreless optical fiber and Bragg grating optical fiber to realize simultaneous monitoring of steel bar stress and initial rust time.

[0035] (3) The present invention is small in size and light in weight. After the sensor is welded to the steel bar to be tested using a connecting key, the mechanical properties of the steel bar in the structure are not changed, and non-destructive monitoring can be achieved.

[0036] (4) The present invention uses a spectrometer to monitor the offset of the characteristic wavelength during the corrosion process of the steel bar to be tested in real time. The monitoring result is relatively accurate and can be used to determine the initial rust time of the steel bar to be tested, and then take maintenance measures.

[0037] (5) The present invention has a simple structure, is easy to use, and is low in price, thus having good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 The figure is a schematic diagram of the structure of the optical fiber sensor used for simultaneously monitoring the stress and initial rust time of steel bars according to the present invention.

[0039] Figure 2 AA cross-sectional view of the optical fiber sensor structure for simultaneously monitoring the stress and initial rust time of steel bars according to the present invention;

[0040] Figure 3 BB cross-sectional view of the optical fiber sensor structure for simultaneously monitoring the stress and initial rust time of steel bars of the present invention;

[0041] Figure 4 It is a schematic diagram of a steel bar stress sensing unit of an optical fiber sensor for simultaneously monitoring steel bar stress and initial rust time of the present invention;

[0042] Figure 5 A schematic diagram of a steel bar initial rust time sensing unit of an optical fiber sensor for simultaneously monitoring steel bar stress and initial rust time of the present invention;

[0043] Figure 6 It is a schematic diagram of the spectrum principle of the optical fiber sensor used for simultaneously monitoring the stress and initial rust time of steel bars of the present invention;

[0044] Figure 7 The figure is a schematic diagram of an engineering application of the optical fiber sensor of the present invention for simultaneously monitoring the stress and initial rust time of steel bars.

[0045] In the figure: 1 single-mode optical fiber core, 2 single-mode optical fiber cladding, 3 structural adhesive, 4 retractable rubber sealing ring, 5 inner stainless steel tube, 6 outer stainless steel tube, 7 retractable rubber sealing ring, 8 stainless steel tube capillaries, 9 porous stainless steel tube, 10 coreless optical fiber, 11 steel bar coating, 12 retractable rubber sealing ring, 13 single-mode optical fiber, 14 Bragg grating, 15 connecting key, 16 connecting key, 17 connecting key, 18 steel bar. DETAILED DESCRIPTION

[0046] The specific implementation of the optical fiber sensor for simultaneously monitoring the stress and initial rust time of steel bars will be described below in conjunction with the accompanying drawings.

[0047] As shown in the figure, the present invention provides an optical fiber sensor for simultaneously monitoring steel bar stress and initial rust time, including a single-mode optical fiber core 1, a single-mode optical fiber cladding 2, a structural adhesive 3, a retractable rubber sealing ring 4, an inner stainless steel tube 5, an outer stainless steel tube 6, a retractable rubber sealing ring 7, stainless steel tube capillaries 8, a porous stainless steel tube 9, a coreless optical fiber 10, a steel bar coating 11, a retractable rubber sealing ring 12, a single-mode optical fiber 13, a Bragg grating 14, a connecting key 15, a connecting key 16, a connecting key 17, and a steel bar 18.

[0048] The single-mode optical fiber 13 includes a single-mode optical fiber core 1 and a single-mode optical fiber cladding 2. The Bragg grating 14 of the single-mode optical fiber 13 can be prepared by a phase mask method. The inner stainless steel tube 5 and the outer stainless steel tube 6 are connected and sealed with the single-mode optical fiber 13 using a structural adhesive 3. The inner stainless steel tube 5 and the outer stainless steel tube 6 are sealed by a retractable rubber seal ring 4. The inner stainless steel tube 5 is welded to the steel bar 18 through a connection key 15, and the outer stainless steel tube 6 is welded to the steel bar 18 through a connection key 16. The porous stainless steel tube 9 is sleeved on the outside of the coreless optical fiber 10. The porous stainless steel tube 9 has uniformly distributed stainless steel tube capillaries 8. The surface of the coreless optical fiber 10 is coated with a steel bar coating 11 by vacuum ion sputtering, and the porous stainless steel tube 9 is welded to the steel bar 18 through a connection key 17.

[0049] The outer stainless steel tube 6 and the porous stainless steel tube 9 are sealed by a retractable rubber sealing ring 7; the porous stainless steel tube 9 and the single-mode optical fiber 13 are sealed by a retractable rubber sealing ring 12;

[0050] The diameter of the coreless optical fiber 10 is equal to the diameter of the single-mode optical fiber 13;

[0051] The single-mode optical fiber core 1, the single-mode optical fiber cladding 2, the single-mode optical fiber 13, the structural adhesive 3, the retractable rubber sealing ring 4, the inner stainless steel tube 5, the outer stainless steel tube 6, the Bragg grating 14, the connecting key 15, and the connecting key 16 constitute a steel bar stress sensing unit;

[0052] The porous stainless steel tube 9, the coreless optical fiber 10, the steel bar coating 11, the retractable rubber sealing ring 12 and the connecting key 17 constitute a steel bar initial rust time sensing unit. The porous stainless steel tube 9 protects the coreless optical fiber 10 while ensuring that the environment of the coreless optical fiber 10 is consistent with the external environment. The steel bar initial rust time sensing unit is located around the steel bar to be tested;

[0053] The inner stainless steel tube 5 and the outer stainless steel tube 6 are made of stainless steel, and their telescopic length range can be adjusted according to the size of the steel bar to be tested. After the length is determined, the steel bar stress sensing unit is sealed using structural adhesive 3 and a telescopic rubber sealing ring 4;

[0054] The distance between the connecting key 15 and the connecting key 16 is determined according to the length of the fiber Bragg grating sensor unit;

[0055] The retractable rubber sealing ring 4 can ensure that the sensor is in a sealed state, and does not limit the expansion and contraction of the inner stainless steel tube 5 and the outer stainless steel tube 6, and does not provide a constraint effect for the sensor;

[0056] The thickness of the steel bar coating 11 and the length and diameter of the coreless optical fiber 10 can be adjusted within a reasonable range according to the requirements for sensor sensitivity in actual engineering.

[0057] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An optical fiber sensor for simultaneously monitoring steel bar stress and initial rust time, characterized in that: The optical fiber sensor for monitoring the stress and initial rust time of steel bars comprises a single-mode optical fiber core (1), a single-mode optical fiber cladding (2), a structural adhesive (3), a retractable rubber sealing ring (4), an inner stainless steel tube (5), an outer stainless steel tube (6), a retractable rubber sealing ring (7), stainless steel tube capillaries (8), a porous stainless steel tube (9), a coreless optical fiber (10), a steel bar coating (11), a retractable rubber sealing ring (12), a single-mode optical fiber (13), a Bragg grating (14), a connecting key (15), a connecting key (16), a connecting key (17), and a steel bar (18); The single-mode optical fiber core (1), the single-mode optical fiber cladding (2), the structural adhesive (3), the retractable rubber sealing ring (4), the inner stainless steel tube (5), the outer stainless steel tube (6), the Bragg grating (14), the connecting key (15), and the connecting key (16) constitute a steel bar stress sensing unit; The stainless steel tube capillary pores (8), the porous stainless steel tube (9), the coreless optical fiber (10), the steel bar coating (11), the retractable rubber sealing ring (12) and the connecting key (17) constitute a steel bar initial rust time sensing unit; The single-mode optical fiber (13) comprises a single-mode optical fiber core (1) and a single-mode optical fiber cladding (2); a Bragg grating (14) of the single-mode optical fiber (13) is prepared by a phase mask method; an inner stainless steel tube (5) and an outer stainless steel tube (6) are connected and sealed with the single-mode optical fiber (13) by using a structural adhesive (3); and the inner stainless steel tube (5) and the outer stainless steel tube (6) are sealed by a retractable rubber sealing ring (4); The inner stainless steel pipe (5) is connected to the steel bar (18) via a connecting key (15), and the outer stainless steel pipe (6) is welded to the steel bar (18) via a connecting key (16); The outer stainless steel tube (6) and the porous stainless steel tube (9) are sealed by a retractable rubber sealing ring (7); The porous stainless steel tube (9) and the single-mode optical fiber (13) are sealed by a retractable rubber sealing ring (12); The diameter of the coreless optical fiber (10) is equal to the diameter of the single-mode optical fiber (13); The inner stainless steel tube (5) and the outer stainless steel tube (6) are made of stainless steel, and their telescopic length range can be adjusted according to the size of the steel bar to be tested. After the length is determined, the steel bar stress sensing unit is sealed using structural adhesive (3) and a telescopic rubber sealing ring (4); The retractable rubber sealing ring (4) ensures that the sensor is in a sealed state, and does not restrict the retraction of the inner stainless steel tube (5) and the outer stainless steel tube (6), and does not provide a constraint on the sensor; The porous stainless steel tube (9) protects the coreless optical fiber (10) while ensuring that the environment of the coreless optical fiber (10) is consistent with the external environment. The porous stainless steel tube (9) is connected to the steel bar (18) via a connecting key (17).

2. The optical fiber sensor for simultaneously monitoring steel bar stress and initial rust time according to claim 1, characterized in that: The distance between the connecting keys (15) and (16) is determined according to the length of the fiber Bragg grating sensor unit.

3. The optical fiber sensor for simultaneously monitoring steel bar stress and initial rust time according to claim 1, characterized in that: The thickness of the steel bar coating (11) and the length and diameter of the coreless optical fiber (10) are adjusted within a reasonable range according to the requirements for sensor sensitivity in actual engineering.

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