Shape memory alloy active reinforcement method based on phase change impedance differential feedback and thermal-mechanical coupling

Abstract

The application discloses a shape memory alloy active reinforcement method based on phase change impedance differential feedback and heat-power coupling and belongs to the technical field of civil engineering and building structure reinforcement, and comprises the following steps: S1, initial tensile strain is applied when SMA is in a martensite phase, and absolute displacement is locked; S2, through pulse electric heating and four-wire impedance acquisition, first-order and second-order differentials of resistance are calculated in real time, reverse phase change nodes are identified, heating power is adaptively regulated and controlled, and higher recovery stress is obtained; S3, when the surface temperature of SMA is cooled to a target temperature window, a heat-curing structural adhesive is injected; S4, the residual heat is used to make the local glue liquid dynamic viscosity of the interface drop by at least 80%, and the interface penetration and in-situ non-isothermal curing of the structural adhesive are promoted; and S5, the locking is released. The shape memory alloy active reinforcement method based on phase change impedance differential feedback and heat-power coupling provided by the application improves the problems of insufficient excitation control and limited interface bonding effect in traditional reinforcement.

Description

Technical Field

[0001] This invention relates to the field of civil engineering and building structure reinforcement technology, and in particular to an active reinforcement method for shape memory alloys based on phase change impedance differential feedback and thermo-mechanical coupling. Background Technology

[0002] Shape memory alloys (SMAs) are pre-stretched and anchored to the surface of a structure, and then their reverse phase transformation (martensite to austenite transformation) is induced by external heating to generate active recovery stress. This is a cutting-edge technology in the field of structural strengthening. However, in practical engineering applications, existing SMA active strengthening methods face two long-standing technical barriers: I. Challenges in Phase Transformation Monitoring under Multi-Field Coupling: When SMA is heated under actual reinforced boundary conditions of "absolute displacement locking," its resistance change is simultaneously affected by the positive effect of intrinsic thermal resistance, the negative effect of phase transformation, and the piezoresistive effect caused by large stress abrupt changes. Existing technologies struggle to effectively extract the start and end points of the martensitic inverse phase transformation from high-noise composite impedance signals, potentially leading to localized overheating relaxation or underheating failure of the SMA when using constant power heating.

[0003] II. Limited Interfacial Anchoring Strength Due to Thermal-Chemical Mismatch: SMA materials typically require high temperatures to generate significant recovery stress, often exceeding the glass transition temperature (Tg) of conventional structural resins, posing a risk of thermal degradation. Existing anti-degradation processes often involve injecting adhesive after the SMA has completely cooled to room temperature. However, at room temperature, the adhesive viscosity is high, making it difficult to fully penetrate the micropores of the interface to be reinforced, resulting in insufficient effective contact area and consequently affecting the active prestress of the SMA.

[0004] In summary, there is an urgent need for an active reinforcement method that can accurately identify the reverse phase transformation process of SMA under displacement locking conditions, achieve adaptive control of the heating process, and take into account the subsequent interface injection and reliable stress transfer. Summary of the Invention

[0005] The purpose of this invention is to provide an active reinforcement method for shape memory alloys based on phase change impedance differential feedback and thermo-mechanical coupling, which aims to solve the problems in the prior art such as difficulty in identifying phase change signals under strong coupling of multiple physical fields, insufficient control of heating excitation, and limited interface anchoring performance under room temperature injection molding process.

[0006] To achieve the above objectives, this invention provides an active strengthening method for shape memory alloys based on phase change impedance differential feedback and thermo-mechanical coupling, comprising the following steps: S1. Pre-tensioning and displacement locking: at ambient temperature T envBelow the austenite transformation initiation temperature of shape memory alloy SMA A s Under the condition that the SMA is in the martensitic phase, an initial tensile strain is applied to the SMA. ξ 0, and lock the displacement of both ends of the SMA in the preset groove on the surface of the structure to be reinforced, thereby locking the displacement of both ends of the SMA; S2. Electrothermal Excitation and Phase Change Adaptive Control: Pulsed electric heating is applied to the displacement-locked SMA, and the resistance value is collected in real time. R (t) and calculate the first differential of the resistance with respect to time d. R / d t and second-order differential d 2 R / d t 2 When d is detected 2 R / d t 2 The appearance of negative peak mutation feature and d R / d t When the phase changes from positive to negative, the reverse phase transition is initiated and the heating power is reduced; when d R / d t When the value approaches zero and its absolute value is less than a set threshold within a continuous preset time period, the phase change is determined to be complete and the electric heating power supply is cut off. S3. Natural Cooling and Adhesive Injection: During the natural cooling phase after power is cut off, the surface temperature of the SMA is monitored in real time. T s ;when T s When the temperature drops to the target temperature window, thermosetting resin structural adhesive is injected into the preset tank; S4. Heat-curing adhesive penetration and in-situ curing: The residual heat released by the cooling of the SMA is used to reduce the local viscosity of the thermosetting resin structural adhesive in the interface area, so that the thermosetting resin structural adhesive penetrates into the surface pores or interface micro-defect areas of the structure to be reinforced, and cures during the subsequent cooling process to form an interface bonding layer for transmitting recovery stress. S5. Unlocking stress transfer: After the thermosetting resin structural adhesive has cured to the predetermined requirements, the displacement lock at both ends of the SMA is released, and the recovery stress generated by the phase change is transferred to the structure to be reinforced, thus completing the active prestress reinforcement.

[0007] Preferably, the initial tensile strain mentioned in step S1 ξ 0 represents 4% to 8% of the original length of the SMA; and throughout steps S2 to S4, the displacements at both ends of the SMA are kept locked, so that the total strain increment is zero.

[0008] Preferably, in step S1, the SMA surface is coated with a thermally conductive and insulating coating; the thermally conductive and insulating coating is a modified polyimide resin doped with aluminum nitride micropowder or a thermally conductive and insulating epoxy potting compound, with a thermal conductivity ≥1.5 W / (m·K) and a dielectric strength ≥15kV / mm after curing.

[0009] Preferably, in step S2, the Kelvin four-wire measurement method is used to obtain the voltage drop across the pure material in order to calculate the resistance value. R (t); and before calculating the first and second derivatives, the resistance time series is denoised and smoothed by a combination algorithm based on sliding time window mean filtering and first-order low-pass filtering.

[0010] Preferably, in step S2, constant current electric heating is applied to the SMA using pulse width modulation (PWM); during reverse phase change start-up, the PWM duty cycle is reduced to 30% to 40% of the initial full load.

[0011] Preferably, the continuous preset time period in step S2 is 10 seconds, and the set threshold is 10^ -5 Ω / s.

[0012] Preferably, the upper limit of the target temperature window in step S3 satisfies the following condition: T s ≤ T g -10℃, among which T g The glass transition temperature of the thermosetting resin structural adhesive; the lower limit of the target temperature window satisfies the following condition: T s ≥ T env +20℃ and T s ≥40℃.

[0013] Preferably, the thermal viscosity reduction effect in step S4 causes the local dynamic viscosity of the thermosetting resin structural adhesive to decrease by at least 80% compared to the initial dynamic viscosity at the ambient temperature of the construction site.

[0014] Therefore, the above-mentioned active strengthening method for shape memory alloys based on phase change impedance differential feedback and thermo-mechanical coupling has the following beneficial effects: (1) An intrinsic feedback excitation mechanism that penetrates multi-field coupling noise was constructed: the "negative abrupt change characteristic of the second derivative of SMA impedance under confined state" was used as one of the phase transformation control signals. This characteristic signal helps to reduce the impedance drift interference caused by thermal expansion and stress abrupt change, and effectively characterizes the initiation process of austenite inverse phase transformation, thereby reducing the risk of lattice damage and prestress relaxation caused by overheating, and realizing non-contact adaptive closed-loop fine excitation; (2) Synergistic enhancement of "thermal viscosity reduction-deep anchoring" is achieved by utilizing the non-steady-state high-temperature window period: Unlike the conventional process of injecting adhesive after cooling at room temperature, this invention provides a method that utilizes the non-steady-state high-temperature window period to achieve synergistic enhancement of "thermal viscosity reduction-deep anchoring": T g -10℃ is the upper limit, and 40℃ and T are the upper limits. env A specific cooling and adhesive injection window with a lower limit of +20℃ is used. Under the premise of ensuring that the resin adhesive does not undergo high-temperature glass transition degradation, the residual heat released by the cooling of SMA causes the high-viscosity structural adhesive to instantly undergo a violent "thermal viscosity reduction effect" (the dynamic viscosity decreases by orders of magnitude), thereby deeply penetrating and wedging into the microscopic capillary pores of the concrete surface; (3) Significantly improve the interfacial shear capacity: The thermal de-viscosity penetration during a specific window period, combined with the subsequent non-isothermal in-situ curing, creates a highly dense micro-mechanical interlocking interface between the SMA, the colloid and the original structure, thereby improving the effective shear force transmission performance of the interface.

[0015] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0016] Figure 1 This is an overall process flow diagram of the active reinforcement method according to an embodiment of the present invention; Figure 2 This is a diagram showing the evolution of the impedance and its first and second order differential characteristic curves during the heating and excitation process of the SMA reinforcement in an embodiment of the present invention. Detailed Implementation

[0017] The following detailed description of embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0018] This embodiment takes the active reinforcement of the tension zone on the bottom surface of a reinforced concrete T-beam that has been in service for 20 years as an example, but the application scope of the present invention is not limited to this, and it can also be applied to other types of structural reinforcement scenarios such as steel structures, bridges, and building walls.

[0019] I. System Composition and Preparatory Work The shape memory alloy (SMA) material used in this embodiment is a Ni-Ti-Nb based wide-hysteresis shape memory alloy cable with a diameter of 5mm. Its phase transformation characteristic parameters are: martensitic phase transformation end temperature. M f = -15 ℃, austenite transformation initiation temperature A s =45 ℃, the temperature at which the austenite transformation ends Af = 85℃. Ambient temperature at the construction site. T env = 20℃.

[0020] To prevent short circuits and ensure efficient heat transfer during electric heating, the surface of the SMA cable is pre-coated with a flexible thermally conductive insulating coating. In this embodiment, the coating is made by coating and curing a modified polyimide (PI) resin doped with aluminum nitride (AlN) thermally conductive micropowder (or commercially available thermally conductive insulating epoxy potting compound). After curing, the coating thickness is controlled between 0.15 mm and 0.25 mm, the thermal conductivity is not less than 1.5 W / (m·K), and the dielectric strength is not less than 15 kV / mm.

[0021] like Figure 1 As shown, the overall process flow of the active reinforcement method of the present invention includes five core stages: pre-stretching and displacement locking, electrothermal excitation and phase change adaptive control, natural cooling and glue injection, heat-induced viscosity reduction and in-situ curing, and unlocking stress transfer. Each stage is described in detail below.

[0022] Step 1: Pre-stretching and displacement locking A U-shaped shallow groove, 15mm wide and 20mm deep, is cut axially along the tension zone of the bottom surface of the T-beam as a pre-designed groove. (The remaining text appears to be incomplete and requires further context.) T env = 20℃ lower than the austenite transformation initiation temperature of the selected SMA A s Under the condition of 45 ℃ and SMA itself being in the martensitic phase, hydraulic tensioning jacks are used to apply pressure to the SMA cable. ξ 0 = 5% initial tensile strain (the initial tensile strain range can be 4% to 8%).

[0023] After tensioning, the tensioned SMA cable is placed in a U-shaped groove, and both ends are fixed to the bottom of the beam using specially designed ceramic-lined insulated wedge-shaped anchors. A high-precision through-hole force sensor is connected in series to achieve absolute displacement locking. Throughout the subsequent steps S2 to S4, the total strain increment of the SMA is kept zero.

[0024] Step 2: Electrothermal Excitation and Phase Change Adaptive Closed-Loop Control (I) Four-wire impedance acquisition The resistance value is calculated by obtaining the voltage drop of the pure material using the Kelvin four-wire measurement method. R (t). Specifically, a PWM constant current source (current) is output using an independent power supply circuit. I = 40A), and another pair of high-impedance voltage probes are directly connected to the inside of the anchors at both ends of the SMA cable to obtain the pure material voltage drop in real time at a sampling frequency of 50Hz. U(t), calculate the intrinsic resistance. R (t)= U (t) / I .

[0025] (II) Signal Denoising and Differential Calculation The controller has a built-in digital filtering module that uses a combination algorithm of mean filtering and first-order low-pass filtering based on a sliding time window. R The (t) sequence is smoothed in real time to filter out high-frequency electrical noise caused by power frequency and stress abrupt changes. Based on the processed data, the first-order derivative of the resistance with respect to time, d, is calculated in real time. R / d t and second-order differential d 2 R / d t 2 .

[0026] (III) Phase transition feature point identification and adaptive control like Figure 2 The figure shows the impedance and its first and second order differential characteristic curves during the heating and excitation process of the SMA reinforcement in this embodiment. The horizontal axis represents heating time / temperature, and the left vertical axis represents the real-time resistance value of the SMA. R The right vertical axis represents the first differential of the resistance, d. R / d t With the second-order differential d 2 R / d t 2 The signal amplitude. The figure contains three characteristic curves: the resistance R curve, the first-order differential of the resistance d... R / d t Curve, second-order differential of resistance d 2 R / d t 2 curve.

[0027] During the initial full-power heating phase, the SMA resistor R The resistance increases continuously with increasing temperature, and the first differential d... R / d t Keeping it positive, the second derivative d 2 R / d t 2 No obvious abrupt change. As the internal temperature of the SMA approaches... A s At approximately 45°C, the controller detects d. 2 R / d t 2 The curve shows a clear abrupt change in the negative peak, and d R / d tThe temperature changes from positive to negative at the zero point. Based on this, the system determines that the martensitic reverse phase transformation has started and automatically reduces the heating power—in this embodiment, the PWM duty cycle is reduced from the initial full load to 35% (the reduction range can be 30% to 40%), entering the isothermal phase transformation mode. When heating reaches the austenitic transformation end temperature... A f When, the first-order differential d R / d t It approaches zero again, and its absolute value remains below a set threshold for a continuous preset time period (in this embodiment, it is below 10^10 for 10 consecutive seconds). -5 When the pressure reaches Ω / s, the reverse phase transition process is considered to be completely finished, and the electric heating power supply is cut off. At this time, the recovery stress generated by the SMA is locked at 480MPa.

[0028] Step S3: Natural cooling and glue application After the power is cut off, the SMA enters the natural cooling phase, and its surface temperature is monitored in real time. T s The glass transition temperature of the modified epoxy resin structural adhesive selected in this embodiment is... T g =75℃. Calculated based on the target temperature window settings: Maximum: T s ≤ T g -10℃ = 65℃; Lower limit: T s ≥ T env +20℃ = 40℃ and T s ≥40℃; Using an infrared thermometer to monitor the surface temperature of the cable in real time T s .when T s When the temperature drops to 60℃ (within the 40℃~65℃ range), start the low-pressure grouting pump (pressure 0.2~0.3MPa) and inject thermosetting resin structural adhesive into the U-shaped groove.

[0029] Step 4: Heat-cooled adhesive penetration and in-situ non-isothermal curing At room temperature (20℃), the initial dynamic viscosity of the selected structural adhesive is approximately 8000 mPa·s. After contacting the SMA cable at 60℃, the adhesive absorbs the residual heat released by the cooling of the SMA, resulting in a strong "thermal viscosity reduction effect". The local dynamic viscosity drops sharply to approximately 800 mPa·s (the viscosity reduction reaches 90%, meeting the requirement of at least 80% reduction).

[0030] Under low pressure, the extremely low-viscosity, high-flow-rate adhesive deeply penetrates the micropores and microcracks of the concrete at the bottom of the U-shaped channel, reaching a depth of 3-5 mm. After injection, the structural adhesive undergoes in-situ non-isothermal cross-linking and curing within a wide temperature range of 60℃ to 20℃. This forms a highly dense micro-mechanical interlocking interface between the SMA, the adhesive, and the original structure.

[0031] Step 5: Unlock stress transfer After 72 hours of curing, once the thermosetting resin structural adhesive has reached the predetermined curing level, the mechanical anchors at both ends are slowly removed. At this point, the 480MPa axial tensile stress generated during the phase transformation of the SMA is uniformly and smoothly transferred to the bottom surface of the T-beam through the micro-interlocking high-strength structural adhesive layer in the form of interfacial shear stress, thus completing the active reinforcement.

[0032] Tests showed that, under the conditions of this embodiment, the effective interfacial shear strength achieved by the method described herein is more than 40% higher than that of the traditional room temperature injection molding process. The SMA recovery stress excitation process achieves non-contact adaptive closed-loop control, reducing the risk of lattice damage and prestress relaxation caused by overheating.

[0033] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0034] Therefore, the present invention employs the above-mentioned active reinforcement method for shape memory alloys based on phase change impedance differential feedback and thermo-mechanical coupling, which can accurately identify the reverse phase change node of SMA in a multi-physics coupling environment and realize adaptive closed-loop temperature control excitation; at the same time, it utilizes the residual heat of SMA cooling to achieve local thermal debonding and deep penetration curing of the structural adhesive interface, significantly improving the interface anchoring strength and reinforcement reliability, effectively solving the technical problems of uncontrollable excitation and insufficient interface bonding performance in traditional active reinforcement of SMA, and has good engineering applicability and promotion value.

[0035] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for active strengthening of shape memory alloys based on phase change impedance differential feedback and thermo-mechanical coupling, characterized in that, Includes the following steps: S1. Pre-tensioning and displacement locking: at ambient temperature T env Below the austenite transformation initiation temperature of shape memory alloy SMA A s Under the condition that the SMA is in the martensitic phase, an initial tensile strain is applied to the SMA. ξ 0, and lock the displacement of both ends of the SMA in the preset groove on the surface of the structure to be reinforced, thereby locking the displacement of both ends of the SMA; S2. Electrothermal Excitation and Phase Change Adaptive Control: Pulsed electric heating is applied to the displacement-locked SMA, and the resistance value is collected in real time. R (t) and calculate the first differential of the resistance with respect to time d. R / d t and second-order differential d 2 R / d t 2 When d is detected 2 R / d t 2 The appearance of negative peak mutation characteristics and d R / d t When the phase changes from positive to negative, the reverse phase transition is initiated and the heating power is reduced; when d R / d t When the value approaches zero and its absolute value is less than a set threshold within a continuous preset time period, the phase change is determined to be complete and the electric heating power supply is cut off. S3. Natural Cooling and Adhesive Injection: During the natural cooling phase after power is cut off, the surface temperature of the SMA is monitored in real time. T s ;when T s When the temperature drops to the target temperature window, thermosetting resin structural adhesive is injected into the preset tank; S4. Heat-curing adhesive penetration and in-situ curing: The residual heat released by the cooling of the SMA is used to reduce the local viscosity of the thermosetting resin structural adhesive in the interface area, so that the thermosetting resin structural adhesive penetrates into the surface pores or interface micro-defect areas of the structure to be reinforced, and cures during the subsequent cooling process to form an interface bonding layer for transmitting recovery stress. S5. Unlocking stress transfer: After the thermosetting resin structural adhesive has cured to the predetermined requirements, the displacement lock at both ends of the SMA is released, and the recovery stress generated by the phase change is transferred to the structure to be reinforced, thus completing the active prestress reinforcement.

2. The active strengthening method for shape memory alloys based on phase change impedance differential feedback and thermo-mechanical coupling according to claim 1, characterized in that, The initial tensile strain mentioned in step S1 ξ 0 represents 4% to 8% of the original length of the SMA; and throughout steps S2 to S4, the displacements at both ends of the SMA are kept locked, so that the total strain increment is zero.

3. The active strengthening method for shape memory alloys based on phase change impedance differential feedback and thermo-mechanical coupling according to claim 1, characterized in that, In step S1, a thermally conductive and insulating coating is applied to the surface of the SMA. The thermally conductive and insulating coating is a modified polyimide resin doped with aluminum nitride micropowder or a thermally conductive and insulating epoxy potting compound. After curing, the thermal conductivity is ≥1.5 W / (m·K) and the dielectric strength is ≥15 kV / mm.

4. The active strengthening method for shape memory alloys based on phase change impedance differential feedback and thermo-mechanical coupling according to claim 1, characterized in that, In step S2, the Kelvin four-wire measurement method is used to obtain the voltage drop of the pure material in order to calculate the resistance value. R (t); and before calculating the first and second derivatives, the resistance time series is denoised and smoothed by a combination algorithm based on sliding time window mean filtering and first-order low-pass filtering.

5. The active strengthening method for shape memory alloys based on phase change impedance differential feedback and thermo-mechanical coupling according to claim 1, characterized in that, In step S2, constant current electric heating is applied to the SMA using pulse width modulation (PWM); during reverse phase change start-up, the PWM duty cycle is reduced to 30% to 40% of the initial full load.

6. The active strengthening method for shape memory alloys based on phase change impedance differential feedback and thermo-mechanical coupling according to claim 1, characterized in that, In step S2, the continuous preset time period is 10 seconds, and the threshold value is set to 10^. -5 Ω / s.

7. The active strengthening method for shape memory alloys based on phase change impedance differential feedback and thermo-mechanical coupling according to claim 1, characterized in that, The upper limit of the target temperature window mentioned in step S3 meets the following condition: T s ≤ T g -10℃, among which T g The glass transition temperature of the thermosetting resin structural adhesive; the lower limit of the target temperature window satisfies the following condition: T s ≥ T env +20℃ and T s ≥40℃.

8. The active strengthening method for shape memory alloys based on phase change impedance differential feedback and thermo-mechanical coupling according to claim 1, characterized in that, The heat-induced viscosity reduction effect described in step S4 causes the local dynamic viscosity of the thermosetting resin structural adhesive to decrease by at least 80% compared to the initial dynamic viscosity at the ambient temperature of the construction site.

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