Method for monitoring damage of steel structure based on carbon nanotube resin-based strain sensor

Abstract

The application discloses a kind of steel structure damage monitoring methods based on carbon nanotube resin base strain sensor, comprising the following steps: step 1: carbon nanotube resin base sensor and metal foil strain gauge are pasted on steel structure, carbon nanotube and metal foil strain gauge are pasted in adjacent position, to ensure that the part of steel structure monitored is same;Step 2: utilize LCR digital bridge and strain box to monitor carbon nanotube resin base sensor and strain gauge, and record data;Step 3: process the data collected, draw resistance change rate-strain curve;The slope of curve is compared with the sensitivity of carbon nanotube sensor measured by pasting undamaged steel, and a conclusion is drawn.The application realizes the health monitoring of steel structure with independently developed carbon nanotube resin base strain sensor, and promotes the further development of structure monitoring field.

Description

Technical Field

[0001] This invention belongs to the field of steel structure damage monitoring, specifically relating to a steel structure damage monitoring method based on a carbon nanotube resin-based strain sensor. Background Technology

[0002] Steel structures have a significant drawback: they are susceptible to corrosion. When exposed to the atmosphere, a thin liquid film forms on the steel surface, leading to corrosion during alternating wet and dry periods. Atmospheric corrosion of steel not only causes enormous economic losses to countries worldwide but also deteriorates various mechanical properties such as strength, modulus of elasticity, and elongation, reducing the ductility and seismic resistance of steel structures and increasing the probability of structural safety accidents. Due to environmental erosion, human activities, and inadequate maintenance, steel structures inevitably suffer cumulative damage and fatigue failure during their long-term service life. Therefore, real-time monitoring and diagnosis of steel structure performance, timely detection of structural damage, prediction of potential disasters, and assessment of safety have become essential requirements for the development of civil engineering.

[0003] Currently, structural health monitoring methods are mainly divided into two categories: offline monitoring and online monitoring. Offline monitoring primarily utilizes traditional non-destructive testing techniques, such as ultrasonic C-scanning and acoustic emission techniques. These methods involve complex equipment, low levels of intelligence, and poor timeliness, hindering the development of modern industry. Online monitoring methods involve attaching or embedding appropriate sensors, such as resistance strain gauges, FBG fiber optic grating sensors, and PZT piezoelectric sensors, onto or within the surface or interior of the structure to monitor stress and strain information in real time. These parameters are then fed back to a computer in a timely manner, enabling the prediction and avoidance of damage. These sensors have some limitations. For example, FBG sensors are generally insensitive to cracks propagating parallel to the fiber orientation; some piezoelectric materials in PZT sensors are susceptible to moisture and require moisture-proof measures; their output signals are poor, necessitating the use of charge amplifiers or high input impedance circuits. In contrast, this patent utilizes a self-developed carbon nanotube resin-based strain sensor monitoring method that is simple, highly sensitive, and suitable for steel structure damage monitoring.

[0004] This patent utilizes a self-developed carbon nanotube resin-based strain sensor to monitor steel structures. The sensor can monitor structural damage without direct contact, clearly identifying damage within a range of five times the sensor's side length, and can also differentiate the magnitude of structural damage. Furthermore, the sensor can identify the elastic, yield, and strengthening stages of the steel. This not only ensures structural safety but also further promotes the development of structural health monitoring systems. Summary of the Invention

[0005] The purpose of this invention is to address the lack of application of strain sensors in the field of steel structure damage monitoring, and to provide a steel structure damage monitoring method based on a carbon nanotube resin-based strain sensor. This method uses an independently developed carbon nanotube resin-based strain sensor to achieve health monitoring of steel structures, thereby promoting the further development of the structural monitoring field.

[0006] The technical solution adopted in this invention is: a method for monitoring steel structure damage based on a carbon nanotube resin-based strain sensor, comprising the following steps:

[0007] Step 1: Attach the carbon nanotube resin-based sensor and the metal foil strain gauge to the steel structure. Place the carbon nanotube and the metal foil strain gauge in adjacent positions to ensure that the monitored parts of the steel structure are the same.

[0008] Step 2: Use an LCR digital bridge (model: VC4090A) and a strain gauge (model: TST3826F-H) to monitor the carbon nanotube resin-based sensor and strain gauge, and record the data;

[0009] Step 3: Process the collected data and plot the resistance change rate-strain curve; compare the curve slope with the sensitivity of the carbon nanotube sensor measured on the undamaged steel. If the difference between the curve slope and the sensor sensitivity is greater than 10% of the sensor sensitivity, it can be determined that the steel structure is damaged near the sensor. If the difference between the curve slope and the sensor sensitivity is less than 10% of the sensor sensitivity, it is considered that the steel structure is not yet damaged.

[0010] Preferably, the carbon nanotube resin-based sensor described in step 1 is randomly attached to the surface of the steel structure. The carbon nanotube resin-based sensor responds to damage within a range of 5 times the side length of the carbon nanotube resin-based sensor and to damage of different sizes through non-direct contact.

[0011] Preferably, the carbon nanotube resin-based sensor comprises epoxy resin, curing agent, carbon nanotubes, and dispersant, with the following mass fractions for each component:

[0012] Carbon nanotubes mass fraction 3.5 wt.%

[0013] Dispersant mass fraction: 0.7 wt.%

[0014] Epoxy resin mass fraction: 73.69 wt.%

[0015] The curing agent has a mass fraction of 22.11 wt.%.

[0016] The specific parameters are shown in Table 1 and Table 2:

[0017] Table 1 Sensor Raw Materials

[0018]

[0019] Table 2 Basic Properties of GE7118A / GE7114B Epoxy Resins

[0020]

[0021] The preparation process includes:

[0022] 1) Weigh the carbon nanotube powder and dispersant according to a carbon nanotube to dispersant mass ratio of 5:1. First, pour the dispersant into the container of the sand mill, and slowly add the carbon nanotube powder in multiple batches. The zirconium bead milling speed is 2300 rpm, and the machine is cooled with circulating water throughout the process. The total dispersion time is controlled at 2.5 hours. The carbon nanotubes are uniformly dispersed without damaging their integrity and conductivity to prepare a carbon nanotube dispersion.

[0023] 2) Mix the prepared carbon nanotube dispersion with epoxy resin in a predetermined ratio (Table 1), ultrasonically disperse for 30 min, and magnetically stir at 400 rpm for 30 min.

[0024] 3) After adding the curing agent (see Table 1 for specific proportions), stir at 400 rpm for 10 min, remove air bubbles from the composite material by vacuuming, and let stand for 10 min.

[0025] 4) When the viscosity is appropriate, the uniformly mixed carbon nanotube resin-based composite material is screen-printed onto the cut polyimide film (the size of the polyimide film can be determined according to actual needs, but should not be less than 20mm×5mm).

[0026] 5) Place in a vacuum drying oven and cure at 100℃ for 5 hours;

[0027] 6) After the composite material has fully cured, use conductive silver paste (any type of conductive silver paste can be used) to attach copper sheets (0.05 mm thick) as electrodes at both ends of the sensor. Place them in a vacuum drying oven at 70°C for 3 hours (below the glass transition temperature of epoxy resin, 75°C) to improve the conductivity and adhesion of the conductive paste. Finally, weld wires onto the copper electrodes to complete the fabrication of the carbon nanotube sensor.

[0028] The prepared sensor is used in a similar manner to the metal foil strain gauge. No power supply is required during use. When measuring the resistance of the carbon nanotube resin-based sensor, an LCR digital bridge (model: VC4090A) instrument is required.

[0029] This invention discloses a carbon nanotube resin-based sensor for monitoring steel damage. By analyzing changes in sensor sensitivity, it can identify the elastic, yielding, and strengthening stages of steel, as well as damage within a range of 5 times the sensor's side length, and can also differentiate the magnitude of structural damage. This is because stress concentration exists near the damage, altering the strain state near the sensor. The spacing between adjacent carbon nanotubes changes significantly, and the tunneling effect causes the resistance of the carbon nanotube sensor to increase more rapidly, resulting in an improved sensitivity coefficient. This patent employs a resistance-based damage sensing method, specifically the piezoresistive effect. The piezoresistive effect is the phenomenon where the resistivity of a conductive material changes when a specimen is subjected to force. By measuring the resistance of a sensor made from conductive material, information about structural deformation under stress can be obtained. The effect of strain on material resistivity is called piezoresistive strength. When a structure is subjected to stress, the strain state at structural damage (such as cracks, holes, etc.) changes, thus affecting the sensor's resistivity. Therefore, processing and analyzing the monitored data can determine whether the structure is damaged. The following further explains the relationship between sensor resistance change and structural deformation using composite material piezoresistive theory:

[0030] Studies have shown that the piezoresistive effect of carbon nanotube / polymer composites is mainly attributed to the following three working mechanisms: (1) the tunneling resistance between adjacent carbon nanotubes changes due to the change in spacing; (2) the change in the conductive network of carbon nanotubes, i.e., the loss or degradation of the contact between carbon nanotubes; and (3) the piezoresistive property of the individual carbon nanotubes themselves due to deformation under strain. However, under small strain (<1%), the piezoresistive property of the carbon nanotubes themselves and the degradation of the carbon nanotube contact can be ignored because the resistance change is very small. In carbon nanotube-based nanocomposites, when the distance between tubes is no greater than 1.8 nm, the tunneling resistance dominates the conduction mechanism (the three positional relationships between adjacent carbon nanotubes are as follows). Figure 1 Furthermore, this effect gradually weakens as the concentration of MWCNTs increases.

[0031] In conductive composite materials, the total resistance is a function of the resistance through each conductive particle and the polymer matrix. Assuming the matrix resistance is constant, the resistance perpendicular to the current direction can be neglected. The conductive network composed of randomly dispersed carbon nanotubes within the matrix can be modeled as N parallel paths composed of carbon nanotubes, each path consisting of a series of M carbon nanotubes (e.g., ...). Figure 2 The total resistance of composite materials can be expressed as (“Resistivities of Conductive Composites”, Ruschau et al., pp. 953-959, Journal of Applied Physics, 1992).

[0032]

[0033] Where R is the total resistance of the composite material, R m R is the contact resistance between adjacent particles. i Let M be the intrinsic resistance of the conductive particle, M be the number of particles forming a single conductive path, and N be the number of conductive paths.

[0034] The piezoresistive properties of strain sensors based on conductive filler polymer nanocomposites are attributed to the tunneling effect, a dominant mechanism for DC conduction in such structures: when the distance between adjacent conductive particles is less than 2 nm, tunneling current flows through the particle gap. According to Simmons' tunneling theory, the tunneling current J at low voltage is given by the following equation ("Generalized Formula for the Electric Tunnel Effect between Similar Electrodes Separated by a Thin Insulating Film", Simmons, p. 1793, Journal of Applied Physics, 1963):

[0035]

[0036] Where m is the electron mass, e is the electron charge, h is Planck's constant, V is the applied voltage, d is the closest distance between the conducting particles, and E c Let be the potential barrier height between adjacent particles (typically 0.5 eV to 2.5 eV for epoxy resin). Since electron tunneling depends on the barrier height, it can be assumed that all tunneling occurs within a small area. Assume a 2 R is the effective cross-sectional area corresponding to the tunneling effect. m It can be represented as follows:

[0037]

[0038]

[0039] Since the conductivity of the polymer matrix is ​​much lower than that of the filler, the intrinsic resistance R of the conductive particles can be ignored. i R i ≈0. Substituting equations (3) and (4) into equation (1), we can obtain the total resistance R of the nanocomposite material:

[0040]

[0041] Under axial tensile load, the resistance changes due to interparticle separation; more specifically, the resistance increases with the increase in interparticle distance. Assuming the interparticle distance is distributed proportionally from d0 (initial particle distance) to d, the interparticle distance d under external force can be expressed as ("Highly sensitive and stretchable piezoresistive strain sensor based on conductive polystyrene-butadiene-styrene) / few layergraphene composite fiber", Wang et al., pp. 291-299, Composites Part a-Applied Science and Manufacturing, 2018):

[0042] d=d0(1+kε) (6)

[0043] Where k is a constant and ε is the strain elongation. Considering that resistivity increases more rapidly for larger strains (plastic region), and assuming that the number of conduction paths changes at a faster rate, it can be expressed as follows ("Fluctuation-Induced Tunneling Conduction in Carbon-Polyvinylchloride Composites", Sheng et al., pp. 1197-1200, Physical Review Letters, 1978):

[0044]

[0045] Where N0 and N(ε) represent the number of conductive paths in the initial state and under deformation, respectively. A, B, C, and D are fitting parameters. Therefore, the rate of change of resistance can be expressed by a combination of equations (4) to (7):

[0046]

[0047] When ε is very close to zero, the exponential term evolves toward the value 1, therefore equation (8) describes a linear relationship, with the slope k representing the sensitivity coefficient GF. Based on the above considerations, equation (8) can be reorganized into the following format:

[0048]

[0049] τ=γd0(GF) (10)

[0050] Since exp(τε) is close to 1, equation (9) can be approximated as:

[0051]

[0052] The beneficial effects of this invention are:

[0053] 1) Convenient data collection

[0054] This technology addresses the problems of complex equipment, low intelligence, and poor timeliness in traditional monitoring methods. The carbon nanotube resin-based sensor only requires a resistance meter to acquire data, making it simple and quick to operate.

[0055] 2) Performance optimization

[0056] The carbon nanotube resin-based sensor of this invention has a strain sensitivity of approximately 3.37, which is about 1.69 times that of a conventional metal foil strain gauge. This provides a novel strain sensor with high sensitivity and high stability for the engineering field.

[0057] 3) Ensure structural safety

[0058] Applying carbon nanotube sensors to steel damage identification can improve the safety of steel structures and reduce maintenance costs, further promoting the development of structural health monitoring systems. Attached Figure Description

[0059] Figure 1 This diagram illustrates three possible positional relationships between adjacent carbon nanotubes.

[0060] Figure 2 This is a schematic diagram of the conductive path of a conductive composite material.

[0061] Figure 3 This is a drawing of a damaged steel tensile specimen. D: Hole diameter; L: Sensor distance.

[0062] Figure 4 This is a monitoring chart of the entire stress-strain process of steel.

[0063] Figure 5 Sensor resistance response curves at different monitoring distances for steel with the same degree of damage;

[0064] Figure 6 The sensor resistance response curves are shown for steel with different degrees of damage at the same monitoring distance. Detailed Implementation

[0065] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0066] To achieve steel damage monitoring based on carbon nanotube resin-based sensors, holes of different diameters and locations were drilled in the steel to simulate different degrees of damage. The piezoresistive response of the carbon nanotube sensor to different types of steel damage was investigated to realize steel structure damage monitoring.

[0067] Example 1: Carbon nanotube resin-based sensors and ordinary strain gauges were attached to the same positions on both sides of Q235 steel, and a tensile test was conducted. The test results are as follows: Figure 4 As shown, the sensitivity coefficient of the carbon nanotube sensor is 3.37 when the steel is in the elastic stage. When the steel is stretched to the yield stage, the sensitivity coefficient is 3.76. When the steel is stretched to the strengthening stage, the sensitivity coefficient is 13.68. The results indicate that the elastic, yield, and strengthening stages of steel can be clearly identified by the change in the slope of the piezoresistive curve of the carbon nanotube sensor.

[0068] Example 2: A Q235 steel plate with a length of 450mm, a thickness of 3mm, and a width of 45mm was used. Holes (2.5mm, 5mm, and 7.5mm in diameter) were then symmetrically drilled at distances of 0mm, 50mm, and 100mm from the center line of the steel plate. Subsequently, carbon nanotube resin-based sensors were attached to the same positions on both sides of the intact and damaged steel plates. Figure 3 A tensile test was performed on the steel specimen at a strain rate of 0.00025 s⁻¹. The change in DC resistance across the sensor was measured using a bridge tester.

[0069] Test results are as follows Figure 5 As shown, with the increase of hole diameter, i.e. damage degree, the sensitivity coefficient of the sensor increased by 19.6%, 51.0% and 65.9% respectively compared with the undamaged steel plate (damage degree of 10%, 20% and 30%, sensor distance of 100mm).

[0070] Example 3: A Q235 steel plate with a length of 450mm, a thickness of 3mm, and a width of 45mm was used. Holes (2.5mm, 5mm, and 7.5mm in diameter) were then symmetrically drilled at distances of 0mm, 50mm, and 100mm from the center line of the steel plate. Carbon nanotube resin-based sensors were then attached to the same positions on both sides of the intact and damaged steel plates. Figure 3 A tensile test was performed on the steel specimen at a strain rate of 0.00025 s⁻¹. The change in DC resistance across the sensor was measured using a bridge tester.

[0071] Test results are as follows Figure 6 As shown, with the increase of sensor distance, the sensitivity coefficient of the damaged steel plate (damage degree of 10%, sensor distance of 0mm, 50mm and 100mm) increased by 112.8%, 81.6% and 19.6% respectively compared with the undamaged steel plate. The increase in sensitivity coefficient caused by damage became less and less obvious, and the CNT sensor had more and more difficulty in identifying the damage of the hole. The effective monitoring range was 100mm.

[0072] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations of these embodiments within the scope of the principles and technical concept of the present invention still fall within the protection scope of the present invention.

Claims

1. A method for monitoring damage of a steel structure based on a carbon nanotube resin-based strain sensor, characterized by, Includes the following steps: Step 1: Attach the carbon nanotube resin-based sensor and the metal foil strain gauge to the steel structure. The carbon nanotube resin-based sensor and the metal foil strain gauge should be attached to adjacent positions to ensure that the monitored parts of the steel structure are the same. Step 2: Monitor and record data using an LCR digital bridge and a strain gauge on the carbon nanotube resin-based sensor and strain gauge; Step 3: Process the collected data and plot the resistance change rate-strain curve; compare the curve slope with the sensitivity of the carbon nanotube sensor measured on the undamaged steel. If the difference between the curve slope and the sensor sensitivity is greater than 10% of the sensor sensitivity, it can be determined that the steel structure is damaged near the sensor. If the difference between the curve slope and the sensor sensitivity is less than 10% of the sensor sensitivity, it is considered that the steel structure is not yet damaged.

2. The method of claim 1, wherein the carbon nanotube resin-based strain sensor is attached to the steel structure. The carbon nanotube resin-based sensor described in step 1 is randomly attached to the surface of the steel structure. The carbon nanotube resin-based sensor responds to damage within a range of 5 times the side length of the carbon nanotube resin-based sensor and to damage of different sizes through non-direct contact.

3. The method for monitoring steel structure damage based on a carbon nanotube resin-based strain sensor according to claim 1, characterized in that, The carbon nanotube resin-based sensor is made of epoxy resin, curing agent, carbon nanotubes, and dispersant, with the following mass fractions for each component: Carbon nanotubes mass fraction 3.5 wt.% Dispersant mass fraction 0.7 wt.% Epoxy resin mass fraction: 73.69 wt.% The curing agent has a mass fraction of 22.11 wt.%.

4. The method for monitoring steel structure damage based on a carbon nanotube resin-based strain sensor according to claim 3, characterized in that, The fabrication process of the carbon nanotube resin-based sensor includes: 1) Weigh carbon nanotube powder and dispersant according to a carbon nanotube to dispersant mass ratio of 5:1; first pour the dispersant into the sand mill container, then slowly add the carbon nanotube powder in multiple batches. The zirconium bead milling speed is 2300 rpm, and the machine is cooled with circulating water throughout the process. The total dispersion time is controlled at 2.5 h. The carbon nanotubes are uniformly dispersed without damaging their integrity and conductivity to prepare a carbon nanotube dispersion. 2) Mix the prepared carbon nanotube dispersion with epoxy resin, ultrasonically disperse for 30 min, and magnetically stir at 400 rpm for 30 min; 3) After adding the curing agent, stir at 400 rpm for 10 minutes, remove air bubbles from the composite material by vacuuming, and let stand for 10 minutes; 4) Once the viscosity is suitable, the uniformly mixed carbon nanotube resin-based composite material is screen-printed onto the cut polyimide film. 5) Place in a vacuum drying oven and cure at 100℃ for 5 hours; 6) After the composite material has fully cured, copper sheets are attached to both ends of the sensor as electrodes using conductive silver paste. The sensor is then placed in a vacuum drying oven at 70°C for 3 hours to improve the conductivity and adhesion of the conductive paste. Finally, wires are welded onto the copper electrodes to complete the fabrication of the carbon nanotube sensor.

5. The method for monitoring steel structure damage based on a carbon nanotube resin-based strain sensor according to claim 4, characterized in that, The screen printing deposition area in step 4) is 20mm × 5mm, and the size of the polyimide film shall not be less than 20mm × 5mm.

6. The method for monitoring steel structure damage based on a carbon nanotube resin-based strain sensor according to claim 4, characterized in that, In step 6), the thickness of the conductive silver paste used to bond the copper sheet is 0.05 mm, and the wires welded to the copper electrode are copper core wires with an outer diameter of 1.8 mm.

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