A k-joint crack adhesive SMA plate targeted prestress reinforcement device and method

Abstract

The present application belongs to the technical field of steel structure reinforcement, and specifically discloses a kind of K-shaped node crack pasting SMA plate targeted prestress reinforcement device, including the connecting plate being arranged on main pipe and branch pipe and the shape memory alloy plate being pasted on the connecting plate, wherein the connecting plate is two groups, and two groups of connecting plates are respectively welded on the main pipe and the branch pipe;Two ends of the shape memory alloy plate are connected with two groups of connecting plates respectively;By applying directional excitation to the shape memory alloy plate, it is prompted that the shape memory alloy plate generates targeted prestress perpendicular to the crack propagation direction through shape memory effect.The present application solves the problems of excessive materials, poor spatial geometric adaptability, difficult construction and inability to actively target and inhibit cracks in the prior art, has the advantages of lightweight, good spatial geometric adaptability, easy construction, active crack inhibition and targeted reinforcement, and is suitable for crack repair of K-shaped nodes in steel pipe truss structure.

Description

Technical Field

[0001] This invention belongs to the field of steel structure reinforcement technology, specifically relating to a targeted prestressed reinforcement device and method for bonding SMA plates with K-shaped node cracks. Background Technology

[0002] Steel pipe truss structures are widely used in engineering projects such as buildings, bridges, offshore platforms, and dams due to their light weight, strong load-bearing capacity, and excellent spanning ability. The K-shaped nodes in these steel pipe truss structures are mostly connected by direct welding of branch pipes to main pipes, and are key parts for structural force transmission.

[0003] However, K-shaped joints exhibit significant structural weaknesses: uneven stiffness distribution in the joint area, accompanied by obvious geometric abrupt changes, leads to prominent stress concentration. Furthermore, the welding process easily introduces welding defects, making K-shaped joints highly susceptible to crack initiation under static or fatigue loads, posing a serious threat to the structural safety and stability. In addition, considering that K-shaped joints are often located in high-altitude environments, and that the members at the joints are densely packed and have complex geometric shapes, reinforcement construction is quite difficult. Currently, the mainstream reinforcement methods for K-shaped joints are steel plate reinforcement and carbon fiber reinforced polymer (CFRP) reinforcement. Steel plate reinforcement suffers from problems such as high material consumption, poor adaptability to spatial geometry, high difficulty in high-altitude welding construction, and the potential for generating new welding residual stresses. CFRP reinforcement, on the other hand, has drawbacks such as high material consumption, the tendency for the adhesive layer to delaminate, and complex construction procedures.

[0004] In summary, both of the above reinforcement methods fall under the category of passive reinforcement. They can only provide post-incident reinforcement after cracks appear, and cannot actively and specifically improve the stress state of the crack area, making it difficult to inhibit the initiation and propagation of cracks from the root.

[0005] Therefore, there is an urgent need for a K-shaped node crack bonding reinforcement device that is lightweight, has good spatial geometric adaptability, is easy to construct, actively suppresses cracks, and provides targeted reinforcement. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, this invention provides a targeted prestressed reinforcement device for K-shaped node cracks using bonded SMA plates. This device has the advantages of being lightweight, having good spatial geometric adaptability, being easy to construct, actively suppressing cracks, and providing targeted reinforcement. It is suitable for repairing cracks in K-shaped nodes in steel pipe truss structures.

[0007] Another objective of this invention is to provide a method for targeted prestressed reinforcement of SMA plates with K-shaped node cracks.

[0008] The technical solution of the present invention to solve the above-mentioned technical problems is:

[0009] A targeted prestressing reinforcement device for K-shaped node cracks using bonded SMA plates includes connecting plates disposed on a main pipe and a branch pipe, and shape memory alloy plates disposed on the connecting plates. The connecting plates are in two sets, each set being perpendicularly welded to the main pipe and the branch pipe, respectively. Both ends of the shape memory alloy plate are bonded to the two sets of connecting plates. By applying directional excitation to the shape memory alloy plate, it generates targeted prestress perpendicular to the crack propagation direction through the shape memory effect.

[0010] Preferably, the shape memory alloy plate has a thickness of 3-5mm, a total length of ≥400mm, and a height of 40-60mm. After annealing, it undergoes pre-stretching and shaping with a strain of 6%-10%. The effective bonding length between the shape memory alloy plate and the connecting plate is ≥260mm.

[0011] Preferably, the shape memory alloy plates are in two sets, and both sets of shape memory alloy plates are adhered to the two sides of the connecting plate by an adhesive.

[0012] Preferably, the adhesive is a polymer interface adhesive or a steel bonding adhesive.

[0013] Preferably, after the shape memory alloy plate is attached to the connecting plate, before the polymer interface adhesive or steel adhesive is fully cured, magnets need to be placed on the outer surfaces of the two sets of shape memory alloy plates; the magnets provide temporary compressive stress to the shape memory alloy plates.

[0014] Preferably, after the shape memory alloy plate is attached to the connecting plate, it needs to be left to cure for 5-7 days in an environment with a temperature of 20-25°C and a humidity of less than or equal to 60%. Subsequently, directional excitation is applied to the shape memory alloy plate to induce it to generate targeted prestress perpendicular to the crack propagation direction through the shape memory effect.

[0015] Preferably, the connecting plate is a parallelogram.

[0016] Preferably, the connecting plate is made of Q355B grade steel plate, with a bottom edge length L of 160-220mm, a height H of 120-160mm, and a thickness T of 20mm.

[0017] Preferably, the distance between the welding positions of the connecting plate and the main pipe, and the connecting plate and the branch pipe, and the crack is greater than or equal to 75 mm.

[0018] A method for targeted prestressed reinforcement of bonded SMA plates with K-shaped node cracks includes the following steps:

[0019] Use a grinder to remove the coating and impurities from the crack tips and welded areas;

[0020] A laser calibrator was used to determine the welding plane, and the two sets of connecting plates were welded perpendicularly to the branch pipe and the main pipe, respectively, to ensure that the welding surface was perpendicular to the crack tip.

[0021] The shape memory alloy plate is glued to the connecting plate with adhesive, and the position of the shape memory alloy plate is fixed by magnets;

[0022] Once the predetermined time is reached, the shape memory alloy plate is thermally excited to induce the plate to generate targeted prestress perpendicular to the crack propagation direction through the shape memory effect.

[0023] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0024] 1. The K-shaped node crack bonding SMA plate targeted prestress reinforcement device of the present invention adopts the spatial reinforcement form of shape memory alloy (SMA) plate. By actively applying targeted prestress to the tip of the node crack, it effectively solves the three core problems existing in the existing reinforcement technology: limited reinforcement efficiency for crack tip, mostly passive repair (difficult to prevent crack from continuing to propagate), and construction limitations such as excessively large welding area of ​​reinforcement material or excessively high hole precision requirements in traditional methods.

[0025] 2. The SMA plate targeted prestressed reinforcement device for K-shaped node cracks of the present invention solves the problems of high material consumption, poor spatial geometric adaptability, difficult construction and inability to actively target and suppress cracks in the prior art. It has the advantages of lightweight, good spatial geometric adaptability, easy construction, active crack suppression and targeted reinforcement, and is suitable for crack repair of K-shaped nodes in steel pipe truss structures. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the targeted prestressed reinforcement device for bonding SMA plates with K-shaped node cracks according to the present invention.

[0027] Figure 2 This is the front view of the connecting plate.

[0028] Figure 3 This is a side view of the connecting plate.

[0029] Figure 4 This is the front view of the shape memory alloy plate.

[0030] Figure 5 This is a side view of a shape memory alloy plate.

[0031] Figure 6 This is a stress-strain diagram showing the stress-strain relationship of SMA plates stretched to fracture before and after annealing at an annealing temperature of 700℃.

[0032] Figure 7The stress-strain relationship diagram is shown for an SMA plate that has been stretched to 9% of its pre-deformation at an annealing temperature of 700℃ and then subjected to unloading of tensile force.

[0033] Figure 8 This is a stress-temperature relationship diagram for the SMA plate.

[0034] Figure 9 This is a schematic diagram of a fatigue loading test.

[0035] Figure 10 This is a schematic diagram of a fatigue loading test on a shape memory alloy plate.

[0036] Figure 11 shows the load-deformation relationship of the shape memory alloy plate in the initial stage of fatigue loading.

[0037] Figure 12 This is a load-deformation relationship diagram for shape memory alloy plates at different fatigue stages.

[0038] Figure 13 is a comparison of the peak deformation of shape memory alloy plates at different fatigue stages.

[0039] Figure 14 The image shows the SN curve of the excited shape memory alloy plate.

[0040] Figure 15 This is a schematic diagram showing the recovery stress loss of a shape memory alloy plate at different fatigue stages.

[0041] Figure 16 This is a schematic diagram of the annealing process for shape memory alloy plates.

[0042] In the diagram: 1-Main pipe; 2-Branch pipe; 3-Shape memory alloy plate; 4-Connecting plate. Detailed Implementation

[0043] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0044] See Figures 1-5 The SMA plate targeted prestressing reinforcement device for K-shaped node cracks of the present invention includes a connecting plate 4 disposed on the main pipe 1 and the branch pipe 2, and a shape memory alloy plate 3 disposed on the connecting plate 4. The connecting plate 4 consists of two sets, which are respectively vertically welded to the main pipe 1 and the branch pipe 2. Both ends of the shape memory alloy plate 3 are respectively bonded to the two sets of connecting plates 4. By applying directional excitation to the shape memory alloy plate 3, the shape memory alloy plate 3 generates targeted prestress perpendicular to the crack propagation direction through the shape memory effect.

[0045] In this embodiment, the connecting plate 4 is a parallelogram structure. For the method of using adhesive anchoring intermediate medium, the parallelogram structure of the connecting plate 4 can provide sufficient effective bonding area while minimizing its own weight, thereby avoiding bonding interface issues. The connecting plate 4 is made of Q355B grade steel plate, with a bottom edge length L of 160-220mm, a height H of 120-160mm, and a thickness T of 20mm. The shape memory alloy plate 3 has a thickness of 3-5mm, a total length greater than or equal to 400mm, and a height of 40-60mm. The shape memory alloy plate 3 undergoes pre-stretching and shaping with a strain of 6%-10% after annealing. The distance between the welding position of the connecting plate 4 and the main pipe 1 and the branch pipe 2 and the crack should be greater than or equal to 75mm. The effective bonding length between the shape memory alloy plate 3 and the connecting plate 4 is greater than or equal to 260mm.

[0046] See Figures 1-5 The shape memory alloy plates 3 are in two sets, symmetrically arranged on two sides of the connecting plate 4. Each set of shape memory alloy plates 3 is adhered to the two sides of the connecting plate 4 using a polymer interface adhesive or a steel bonding adhesive. The polymer interface adhesive resists the influence of a humid environment on the chemical bonding strength of the resin and does not require moisture-proofing after use; however, the steel bonding adhesive requires moisture-proofing measures after reinforcement. The polymer interface adhesive or steel bonding adhesive is prepared according to the specified ratio and then brushed onto the two sides of the connecting plate 4; the shape memory alloy... After the plate 3 is pasted onto the connecting plate 4, before the polymer interface adhesive or steel adhesive is fully cured, magnets need to be placed on the outer surfaces of the two sets of shape memory alloy plates 3; the magnets provide temporary compressive stress to the shape memory alloy plates 3 to fix their position; then, the plates are left to cure for 5-7 days in an environment with a temperature of 20-25℃ and a humidity of less than or equal to 60%; then, directional excitation is applied to the shape memory alloy plates 3 to induce them to generate targeted prestress perpendicular to the crack propagation direction through the shape memory effect.

[0047] See Figures 1-5 The present invention provides a method for targeted prestressed reinforcement of SMA plates with K-shaped node cracks, comprising the following steps:

[0048] A grinder is used to remove the coating and impurities from the crack tip and welding area to ensure a clean surface. A laser calibrator is used to determine the welding plane, and two sets of connecting plates 4 are vertically welded to the branch pipe 2 and the main pipe 1 respectively using a welding torch to ensure that the welding surface is perpendicular to the crack tip. The shape memory alloy plate 3 is attached to the connecting plate 4 with polymer interface adhesive or steel adhesive, and the position of the shape memory alloy plate 3 is fixed by a magnet. After the predetermined time is reached, the shape memory alloy plate 3 is thermally excited using a hot air gun to induce the shape memory alloy plate 3 to generate targeted prestress perpendicular to the crack propagation direction through the shape memory effect.

[0049] In this embodiment, the shape memory alloy plate 3 is welded to the intersection area between the branch pipe 2 and the main pipe 1 of the K-shaped node via a parallelogram-structured connecting plate 4, thereby ensuring that the positioning reference plane is perpendicular to the crack propagation direction and passes through the crack tip. The positioning reference plane is defined as follows: the interface between the parallelogram-structured connecting plate and the steel pipe is rectangular; if the line connecting the midpoints of the two short sides of the interface between the connecting plate welded to the main pipe 1 and the connecting plate welded to the branch pipe 2 is in the same plane, and this plane passes through the crack tip, then this plane is the positioning reference plane.

[0050] Finally, the SMA plate targeted prestressing reinforcement device for K-shaped node cracks of the present invention has the following advantages: (1) Targeted active reinforcement of crack tip: The SMA plate targeted prestressing reinforcement device of the present invention uses the compressive stress field generated by the shape memory alloy plate 3 to accurately act on the crack tip, minimizing the amount of reinforcement material used, thereby avoiding stress redistribution of the node; (2) Active prestress control: The shape memory alloy plate 3 is used to realize the active prestressing reinforcement of K-shaped node cracks; (3) Lightweight component minimally invasive construction: By welding the parallelogram structure connecting plate 4 to the round steel pipe (i.e., the main pipe 1 and the branch pipe 2), not only is the welding surface small, but also minimally invasive reinforcement can be achieved; (4) Convenient construction: The SMA plate targeted prestressing reinforcement method of the present invention does not require drilling holes in the shape memory alloy plate 3, and the construction process is simpler.

[0051] In the above process, the present invention employs a spatial reinforcement method. By precisely arranging the connecting plate 4 and the shape memory alloy plate 3 in three-dimensional space, the axial recovery stress of the shape memory alloy plate 3 is decomposed and projected into the vertical, transverse and torsional directions, forming a composite compressive stress field around the crack tip. Unlike traditional planar attached steel plates or CFRP, which can only disperse stress along the plate surface, the spatial reinforcement method adopted by the present invention can simultaneously suppress the mode I (opening) stress intensity factor of the crack, achieving true "targeted" reinforcement, thereby effectively preventing crack propagation.

[0052] In terms of construction technology, the spatial reinforcement method adopted in this invention only requires local welding at the two connecting plates 4, and then the shape memory alloy plate 3 is pasted onto the connecting plates 4. This makes the total weld length much shorter than that of the large-area plate pasting method, thereby greatly reducing the risk of heat input and residual stress redistribution. In addition, the lightweight shape memory alloy plate 3 and connecting plate 4 make construction at high altitudes or in confined spaces simpler and more efficient. Prestressing can be completed by quickly positioning and welding, replacing the plate, and applying thermal excitation, thereby greatly improving construction safety and repeatability.

[0053] More importantly, the spatial reinforcement method adopted in this invention has dynamic adjustability and re-excitation capability: the shape memory alloy plate 3 can be excited multiple times through temperature control to repeatedly obtain the design prestress, providing an active compensation means for subsequent structural health monitoring and maintenance; at the same time, the spatial reinforcement method adopted in this invention is not limited by the difficulty of traditional planar reinforcement bonding, and can achieve surface contact and precise bonding at complex K-shaped nodes, which significantly reduces the crack propagation rate and greatly extends the fatigue life.

[0054] Furthermore, it is necessary to determine the prestress loss of the shape memory alloy plate 3. To determine the prestress loss of the shape memory alloy plate 3, this embodiment uses a stress-bearing unit box to monitor its strain amplitude over a long period of time. Since strain measurement is based on the change in resistance of strain gauges caused by steel deformation to calculate the strain value, and temperature changes will change the thermal expansion coefficient of the steel, temperature is an important factor affecting strain readings. Therefore, temperature monitoring needs to be carried out simultaneously to assess the degree of influence of temperature changes on strain measurement. During the reinforcement construction, a temperature compensation module needs to be placed at the crack location. This temperature compensation module eliminates the false strain introduced by the thermal expansion characteristics of the steel by obtaining the initial strain value of the same type of unstressed steel under the same temperature environment, and finally obtains the strain change caused only by the actual load.

[0055] In this embodiment, the material of the shape memory alloy plate 3 is NiTi or NiTiNb.

[0056] Similar to traditional prestressed reinforcement methods, the prestress loss during SMA plate reinforcement in this embodiment is also a crucial and non-negligible indicator, as the magnitude of prestress loss directly impacts the reinforcement effect. Therefore, this embodiment sets different levels of fatigue load amplitudes and conducts a series of fatigue loading tests on SMA plates under prestressed conditions to obtain the fatigue loading life and prestress loss status of the SMA plates, thereby determining the key fatigue parameters for ensuring proper prestress maintenance during the actual reinforcement process.

[0057] Before fatigue loading is performed on the SMA plate, static loading is required to determine the mechanical property parameters of the SMA and obtain the maximum recovery stress of the SMA material at room temperature for subsequent fatigue tests. The NiTi-SMA plate used in the static loading test exhibits hyperelasticity at room temperature. To obtain the recovery stress of the NiTi-SMA plate, it is necessary to treat it with a suitable temperature to transform its hyperelastic properties into shape memory effect. This is the reason for heat treatment of the NiTi-SMA plate.

[0058] The composition of the NiTi-SMA plate material used in this experiment is shown in Table 1:

[0059] Table 1. Composition of NiTi-SMA plate material used in the experiment

[0060]

[0061] Ni accounts for the largest proportion of all components, reaching 55.740%, followed by Ti at 44.136%. In addition to these two main components, it also contains C, H, O, N, and some other small amounts of other metallic components.

[0062] like Figure 6 As shown, SMA plates were stretched to fracture before and after annealing at 700℃ to obtain their stress-strain relationship. Figure 6 It is evident that annealing increases the ductility of SMA sheets while slightly reducing their ultimate strength. Specifically, the ultimate strength of the SMA sheet before annealing was 913 MPa, which decreased to 855 MPa after annealing, a reduction of 6.4%. The elongation of the SMA sheet before annealing was only 7.5%, which increased to 43% after annealing, a 5.73-fold increase. Annealing induces a phase transformation plateau of approximately 338 MPa in the material, while the yield strength of 385 MPa becomes more pronounced. Therefore, annealing significantly improves the ductility of the material while slightly reducing its ultimate strength. This is mainly because annealing induces a phase transformation plateau, enhancing the material's plasticity. During the tensile process, the austenite in the SMA material transforms into martensite, producing a shape memory effect.

[0063] Figure 7 The stress-strain relationship diagram is shown for an annealed SMA plate stretched to 9% pre-deformation at an annealing temperature of 700℃ and then subjected to unloading of tensile force. Figure 8 This is a stress-temperature relationship diagram for the recovery of an SMA plate, where... Figure 7 and Figure 8 Specimens 1, 2, and 3 are the same shape memory alloy plates, and the purpose is to conduct three repeatability tests to increase the reliability of the results.

[0064] like Figure 7As shown, when an SMA sheet annealed at 700℃ is stretched to its optimal pre-deformation of 9%, and then the load is restored to its initial state, the material will produce approximately 6.8% residual strain. This residual strain is the source of the recovery stress. Figure 7 It can be clearly seen that the SMA material immediately enters the yield state after the phase transformation plateau ends. The yield strength corresponds to a 9% pre-deformation. This indicates that the SMA material needs to be stretched to the pre-deformation corresponding to the yield strength in order to obtain good recovery stress. This pre-deformation value is within the optimal pre-deformation range of the material and exceeds the phase transformation plateau of the SMA material.

[0065] like Figure 8 As shown, after the SMA plate was annealed at 700℃ and stretched to the optimal pre-deformation of 9%, both ends were fixed and subjected to thermal excitation. Temperature sensors were placed on the SMA plate to obtain the temperature change of the plate surface during the thermal excitation process. At the same time, the stress change of the SMA plate was read using an MTS tensile testing machine throughout the entire thermal excitation process. After fully recovering to room temperature, the final recovery stress of the SMA plate no longer decreased but remained stable. Therefore, it was determined to be the optimal recovery stress at room temperature. According to the recovery stress-temperature relationship, the optimal recovery stress of the SMA plate during the thermal excitation process is close to its yield strength. The highest thermal excitation temperature is about 155℃. After recovering to room temperature, the stress of the SMA plate can still be maintained above 300MPa. This stress value is close to the stress value corresponding to the material phase transformation plateau.

[0066] Before conducting fatigue loading tests on SMA plates, the SMA plates used in the tests must be annealed (700℃) and pre-stretched (9%). After applying the target pre-strain to the SMA plates, the load is restored to zero, i.e., the fatigue load is reduced to zero. Figure 7 The stress-strain final state shown does not require unloading the grips of the MTS (Mechanical Testing System), and the current position is set as the initial state. At the same time, the load and displacement balance of the MTS is zeroed.

[0067] Furthermore, the SMA plate was thermally excited using a hot air gun. During thermal excitation, it was ensured that every location in the SMA thermal excitation zone was heated uniformly. To maintain consistency, the thermal excitation time was set to 50 seconds each time, and the outlet temperature of the hot air gun was consistently set. Data showed that this thermal excitation method successfully brought the SMA plate to its maximum peak recovery stress (approximately its yield strength). After thermal excitation, the SMA plate was allowed to return to room temperature (≈24℃). The standard was that the recovery stress of the SMA plate no longer decreased and the recovery stress curve remained horizontal, thus obtaining the stable recovery stress of the SMA plate under room temperature conditions. The entire process lasted approximately 800 seconds. During this period, the load sensor of the MTS continuously read the recovery stress values ​​of the SMA plate. After completing the above process, a fatigue loading program was set to perform fatigue loading on the SMA plate with recovery stress at a frequency of 4Hz. Simultaneously, the fatigue was stopped and the plate returned to its initial position after a pre-set number of fatigue cycles. The remaining recovery stress was read using the load sensor of the MTS. This cycle was repeated until the SMA plate finally fractured.

[0068] The fatigue loading procedure is characterized by load control during loading to ensure the SMA plate reaches the preset fatigue load amplitude. When reading the remaining recovery stress, displacement control is used to ensure the SMA plate returns to its pre-stressed initial position before fatigue loading. If the remaining recovery stress is read via load control, the SMA plate will not reach its initial state before fatigue loading because some residual deformation remains when the load recovers to the initial state. This initial load position error can be significant, especially with large fatigue amplitudes, as confirmed by subsequent tests. Displacement control solves this problem, ensuring the SMA plate returns to its initial state. This fatigue loading scheme ensures the SMA plate is constantly under high pre-stress during fatigue testing, which is more consistent with actual engineering conditions. However, it places high demands on the fatigue loading testing machine, requiring it to maintain high hydraulic pressure for extended periods.

[0069] like Figure 9 As shown, the fatigue loading test was set with five stress amplitude levels (F). min =Fs,F s=5.06kN), where the peak value under fatigue load remains unchanged, corresponding to the initial prestressed state of the SMA plate, that is, the average recovery stress value of the SMA plate. The upper peak value of fatigue load increases stepwise from the lower peak value by 0.1-0.5 times the peak value under fatigue load. The five fatigue-loaded specimens are named specimens S1-S5 according to their respective fatigue stress amplitudes. The stress amplitudes ΔS of the five specimens are 34MPa, 67MPa, 101MPa, 134MPa, and 167MPa, respectively. According to the stress ratio calculation, the five specimens... The stress ratios R of the specimens were 0.1, 0.2, 0.3, 0.4, and 0.5, respectively. The specific peak and trough values ​​of the loads for the five specimens are detailed in Table 2. The NiTi-based SMA plate used for fatigue testing had dimensions of 200mm × 15mm × 1mm (length × width × thickness), with a thermal excitation zone length of 150mm. Previous material property tests yielded elastic moduli of 37.56 GPa and 60.51 GPa before and after annealing, respectively. The annealed SMA plate possessed a phase transformation plateau of 338 MPa and a yield strength of 385 MPa. To minimize errors and ensure uniformity of recovery stress across the test plates, the average recovery stress of the SMA specimens of this size was measured before fatigue testing (average recovery stress 337 MPa, close to the 338 MPa phase transformation plateau). This average recovery stress was used as the basis for determining the fatigue load amplitude. Because SMA materials are highly sensitive to temperature, the laboratory was kept at a stable room temperature (≈24℃) during the test.

[0070] Table 2 Characteristic values ​​of fatigue loads at various levels

[0071]

[0072] The fatigue life of the SMA plates under high stress conditions did not exceed 1 million cycles. The final fatigue lives of specimens S1-S5 were 742,229, 81,879, 21,239, 8,003, and 5,578 cycles, respectively. The fatigue failure of specimens S1, S2, and S3 exhibited typical three-stage failure characteristics: rapid increase in residual deformation and significant stiffness degradation in the early stage, stable evolution in the middle stage, and sudden fracture in the later stage. Figure 11 and Figure 12 The load-deformation evolution curves of each specimen at the initial stage of fatigue loading and at different fatigue stages are shown. In the initial stage of fatigue loading, the residual deformation of the load increases more significantly with the increase of stress amplitude. The load-deformation curves throughout the entire process show that the residual deformation of each specimen gradually increases with the increase of fatigue cycles. For specimens S4 and S5, due to the large fatigue amplitude, the peak stress has exceeded the yield strength of the material itself. The residual deformation continues to increase throughout the entire process, and the stiffness continues to decrease. Therefore, their fatigue life is low and does not exceed 10,000 cycles.

[0073] The peak deformation values ​​of the SMA plate at different loading stages were statistically analyzed to obtain the peak deformation-fatigue cycle relationship curves for each specimen, such as... Figure 13 As shown, the peak deformation of specimen S1 before final fatigue fracture was only 0.2 mm, and there was no significant increase throughout the process. Compared with the initial loading stage, the increase in peak deformation was only 0.08 mm. The change pattern of peak deformation of specimen S2 was similar to that of specimen S1. Its peak deformation at the initial loading stage was 0.5 mm, and the peak deformation before fatigue fracture was 0.9 mm, with an increase of 0.4 mm. The initial peak deformation of specimen S3 was 0.9 mm, and the peak deformation had increased to 1.7 mm before fatigue fracture. The increase in peak deformation was mainly concentrated in the early stage of fatigue loading (the first 100 fatigue loading cycles), and the increase in peak deformation had reached 0.8 mm. The peak deformation values ​​of specimens S4 and S5 increased, with the initial peak deformation exceeding 1 mm and increasing significantly throughout the process. Specifically, the peak deformation increased from 1.9 mm and 2.8 mm at the initial loading stage to 5.3 mm and 8.7 mm in the later stages, with increases of 3.4 mm and 5.9 mm, respectively. As the fatigue amplitude increases, the peak deformation increment of the specimen changes dramatically. The peak deformation increment is inversely proportional to the fatigue life of the specimen. The peak deformation increment of specimen S5 is hundreds of times larger than that of specimen S1, and its corresponding fatigue life also decreases significantly. The final fatigue loading life of each specimen is statistically analyzed to obtain the following results: Figure 14 shown SN The curve, using the fatigue loading stress amplitude as the standard, gives a formula for predicting the fatigue life of NiTi-SMA plates under prestressed conditions.

[0074] The residual recovery stress of SMA plates at different fatigue loading stages was statistically analyzed to obtain, as follows: Figure 15 The curves showing the relationship between different fatigue cycles and SMA recovery stress loss are derived from... Figure 15It can be seen that under fatigue load with stress amplitude ΔS=34MPa, the SMA recovery stress of specimen S1 hardly changed with the increase of fatigue cycles, and the recovery stress remained above 96% of the initial value throughout the fatigue process until the final fatigue fracture. Under fatigue load with stress amplitude ΔS=67MPa, the SMA recovery stress of specimen S2 dropped to below 80% of the initial value in the first 10 fatigue loading cycles, and then remained stable with a steady, small decrease, leaving 65% of the initial value of recovery stress before the final fracture. Under fatigue load with stress amplitude ΔS=101MPa, the SMA recovery stress of specimen S3 dropped even more in the first 10 fatigue loading cycles, leaving only 44% of the initial value, and then remained stable with a small decrease, leaving 22% of the initial value of recovery stress before the final fracture. Under larger stress amplitudes (ΔS=134MPa and 167MPa, respectively), specimens S4 and S5 showed significant plastic fatigue deformation of the SMA and a drastic loss of recovery stress, with no remaining recovery stress before fracture. Therefore, it can be seen that the greater the stress amplitude, the greater the recovery stress loss of the SMA material. The recovery stress loss value is faster in the early stage of fatigue loading and gradually stabilizes in the later stage.

[0075] Based on the fatigue performance evolution and recovery stress loss characteristics of the SMA plate, it can be seen that under high stress, plastic fatigue deformation is the main cause of recovery stress loss in the SMA plate; the greater the residual deformation of the specimen, the more severe the recovery stress loss. Among all specimens, only specimen S1 showed almost no decrease in recovery stress. Figure 6 and Figure 7 The comparison shows that the recovery stress σ of each specimen... min Basically consistent with the phase transition plateau, the yield stress of the material is used as the dividing point, and the maximum peak stress σ experienced by the specimen is... max When the stress is less than the yield stress of the material itself, the recovery stress generated by the specimen on the reinforced structure will not disappear completely. When the maximum peak stress σ... max When the stress exceeds the material's yield stress, the recovery stress will be lost rapidly.

[0076] The above are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above content. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A targeted prestressed reinforcement device for bonded SMA plates with K-shaped node cracks, characterized in that, The device includes connecting plates mounted on the main pipe and branch pipe, and shape memory alloy plates mounted on the connecting plates. The connecting plates are in two sets, and the two sets of connecting plates are respectively welded perpendicularly to the main pipe and the branch pipe. The two ends of the shape memory alloy plate are respectively bonded to the two sets of connecting plates. By applying directional excitation to the shape memory alloy plate, the shape memory alloy plate is prompted to generate targeted prestress perpendicular to the crack propagation direction through the shape memory effect.

2. The targeted prestressed reinforcement device for K-shaped node cracks in bonded SMA plates according to claim 1, characterized in that, The shape memory alloy plate has a thickness of 3-5mm, a total length of ≥400mm, and a height of 40-60mm. After annealing, it undergoes pre-stretching and shaping with a strain of 6%-10%. The effective bonding length between the shape memory alloy plate and the connecting plate is ≥260mm.

3. The targeted prestressed reinforcement device for K-shaped node cracks in bonded SMA plates according to claim 2, characterized in that, The shape memory alloy plates are in two sets, and both sets of shape memory alloy plates are glued to the two sides of the connecting plate with adhesive.

4. The targeted prestressed reinforcement device for K-shaped node cracks in bonded SMA plates according to claim 3, characterized in that, The adhesive is a polymer interface adhesive or a steel bonding adhesive.

5. The targeted prestressed reinforcement device for K-shaped node cracks in bonded SMA plates according to claim 4, characterized in that, After the shape memory alloy plate is attached to the connecting plate, magnets need to be placed on the outer surfaces of the two sets of shape memory alloy plates before the polymer interface adhesive or steel adhesive is fully cured; the magnets provide temporary compressive stress to the shape memory alloy plates.

6. The targeted prestressed reinforcement device for K-shaped node cracks in bonded SMA plates according to claim 5, characterized in that, After the shape memory alloy plate is attached to the connecting plate, it needs to be left to cure for 5-7 days in an environment with a temperature of 20-25℃ and a humidity of less than or equal to 60%. Then, directional excitation is applied to the shape memory alloy plate to induce it to generate targeted prestress perpendicular to the crack propagation direction through the shape memory effect.

7. The targeted prestressed reinforcement device for K-shaped node cracks in bonded SMA plates according to claim 1, characterized in that, The connecting plate is a parallelogram.

8. The targeted prestressed reinforcement device for K-shaped node cracks in bonded SMA plates according to claim 7, characterized in that, The connecting plate is made of Q355B grade steel plate, with a bottom edge length L of 160-220mm, a height H of 120-160mm, and a thickness T of 20mm.

9. The targeted prestressed reinforcement device for K-shaped node cracks in bonded SMA plates according to claim 1, characterized in that, The distance between the welding positions of the connecting plate and the main pipe, and the connecting plate and the branch pipe, and the crack is greater than or equal to 75 mm.

10. A method for targeted prestressing reinforcement of SMA plates using the K-shaped node crack targeted prestressing reinforcement device according to any one of claims 1-9, characterized in that, Includes the following steps: Use a grinder to remove the coating and impurities from the crack tips and welded areas; A laser calibrator was used to determine the welding plane, and the two sets of connecting plates were welded perpendicularly to the branch pipe and the main pipe, respectively, to ensure that the welding surface was perpendicular to the crack tip. The shape memory alloy plate is glued to the connecting plate with adhesive, and the position of the shape memory alloy plate is fixed by magnets; Once the predetermined time is reached, the shape memory alloy plate is thermally excited to induce the plate to generate targeted prestress perpendicular to the crack propagation direction through the shape memory effect.

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