A k-joint crack targeted pre-stress reinforcement device and method for sma plate

Abstract

The application discloses a kind of K-shaped node crack SMA plate targeted prestress reinforcement device and method;Device includes connecting plate and shape memory alloy plate;The connecting plate is first connecting plate and second connecting plate;The first connecting plate is welded on main pipe, and the second connecting plate is welded on branch pipe;The first connecting plate and second connecting plate are located in the same plane, the plane is perpendicular to crack and the extension line of this plane passes through crack tip;Two ends of the shape memory alloy plate are respectively installed on the first connecting plate and the second connecting plate by fastening assembly;By directional excitation to shape memory alloy plate, to make the shape memory alloy plate generate targeted recovery stress perpendicular to crack propagation direction by shape memory effect.The SMA plate targeted prestress reinforcement device of the application has the advantages of lightweight, good spatial geometric adaptability, easy construction, active crack inhibition and targeted reinforcement, and is suitable for repairing cracks in K-shaped nodes of steel pipe truss structure.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of steel structure reinforcement, and particularly relates to a K-shaped node crack SMA plate targeted prestress reinforcement device and method. BACKGROUND

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

[0003] However, the K-shaped nodes have significant structural weakness: uneven stiffness distribution in the node area, accompanied by significant geometric discontinuity, leading to prominent stress concentration. In addition, welding defects are easily introduced during the welding process, making the K-shaped nodes prone to crack under static or fatigue loads, posing a serious threat to the safety and stability of the structure. Moreover, considering that the K-shaped nodes are often in high-altitude environments, and the bar members at the nodes are dense and have complex geometric shapes, the reinforcement construction is difficult. Currently, the mainstream reinforcement methods for K-shaped nodes are steel plate reinforcement and carbon fiber reinforced polymer (CFRP) reinforcement. However, steel plate reinforcement has the problems of large material consumption, poor spatial geometric adaptability, high difficulty in high-altitude welding construction, and easy generation of new welding residual stress. Carbon fiber reinforced polymer (CFRP) reinforcement has the problems of high material consumption, easy voiding of the glue layer, and complex construction process.

[0004] In summary, the above two reinforcement methods belong to passive reinforcement category, which can only be used for post-strengthening after the crack appears, and cannot actively and targetedly 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 reinforcement device that is lightweight, has good spatial geometric adaptability, is easy to construct, actively inhibits cracks, and targets reinforcement. SUMMARY

[0006] The present application provides a K-shaped node crack SMA plate targeted prestress reinforcement device to overcome the deficiencies of the prior art. The SMA plate targeted prestress reinforcement device has the advantages of lightweight, good spatial geometric adaptability, easy construction, active crack inhibition, and targeted reinforcement, and is suitable for repairing cracks in K-shaped nodes of steel pipe truss structures.

[0007] Another object of the present application is to provide a K-shaped node crack SMA plate targeted prestress reinforcement method.

[0008] The technical solution of the present application to solve the above technical problems is:

[0009] A targeted prestressing reinforcement device for SMA plates with K-shaped node cracks 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: a first connecting plate and a second connecting plate. The first connecting plate is welded to the main pipe, and the second connecting plate is welded to the branch pipe. The first and second connecting plates are located in the same plane, which is perpendicular to the crack, and the extension line of the plane passes through the crack tip. The two ends of the shape memory alloy plate are respectively mounted on the first and second connecting plates by fastening components. By applying directional excitation to the shape memory alloy plate, a targeted recovery stress perpendicular to the crack propagation direction is generated through the shape memory effect.

[0010] Preferably, the first connecting plate is a triangular structure, and the two bottom corners of the triangular structure are chamfered; the bottom edge length of the first connecting plate is 90-110mm, the height is 90-100mm, and the thickness is 20mm.

[0011] Preferably, the second connecting plate is also a triangular structure, with a base length of 80-100mm, a height of 90-100mm, and a thickness of 20mm.

[0012] Preferably, both the first connecting plate and the second connecting plate have through holes at their centers, and the diameter of the through holes is 14-16 mm.

[0013] Preferably, both the first connecting plate and the second connecting plate are made of Q355 steel; the distance between the welding position of the main pipe and the first connecting plate and the welding position of the branch pipe and the second connecting plate and the crack is greater than or equal to 75mm.

[0014] Preferably, the shape memory alloy plate is in two sets, and the two sets of shape memory alloy plates are respectively installed on the two sides of the first connecting plate and the second connecting plate; the thickness of each set of shape memory alloy plates is 3-5mm, the length is greater than or equal to 240mm, and the height is 40-60mm.

[0015] Preferably, the shape memory alloy plate has bolt holes with a diameter of 14-16 mm symmetrically formed along its length; the distance between the center of the bolt hole and both sides of the shape memory alloy plate along its length is less than or equal to 28 mm.

[0016] Preferably, the shape memory alloy plate needs to undergo annealing and pre-stretching processing.

[0017] Preferably, the fastening assembly includes a bolt, a nut, and a washer, wherein the bolt passes through a through hole in a first set of shape memory alloy plates, a first connecting plate or a second connecting plate, a second set of shape memory alloy plates, and a washer in sequence before being connected to the nut.

[0018] A targeted prestressing reinforcement method for SMA plates with K-shaped nodal cracks includes the following steps:

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

[0020] A steel plate of the same size as the shape memory alloy plate is mounted on a first connecting plate and a second connecting plate using fastening components. The first connecting plate is then welded to the main pipe, and the second connecting plate is welded to the branch pipe. The first connecting plate and the second connecting plate are located in the same plane, which is perpendicular to the crack, and the extension line of the plane passes through the crack tip.

[0021] After the first and second connecting plates have cooled down, the steel plates are removed, and the shape memory alloy plate is installed on the first and second connecting plates using fastening components. The shape memory alloy plate is then stimulated to generate targeted recovery stress through the shape memory effect. The targeted recovery stress is then converted into a targeted compressive stress field perpendicular to the crack propagation direction using fastening components. This targeted compressive stress field can actively suppress the stress intensity factor at the crack tip to prevent crack propagation.

[0022] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0023] 1. In the SMA plate targeted prestressed reinforcement device for K-shaped node cracks of the present invention, the shape memory alloy plate is thermally excited to induce the shape memory alloy plate to generate targeted recovery stress through the shape memory effect. The targeted recovery stress is converted into a targeted compressive stress field perpendicular to the crack propagation direction by fastening components. The targeted compressive stress field can actively suppress the stress intensity factor at the crack tip to prevent crack propagation, thereby solving the problems of existing reinforcement methods that cannot target and reinforce crack tips, have large residual stress, and are difficult to construct.

[0024] 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 the repair of cracks in K-shaped nodes in steel pipe truss structures. Attached Figure Description

[0025] Figure 1This is a schematic diagram of the structure of the SMA plate targeted prestressed reinforcement device for K-shaped node cracks according to the present invention.

[0026] Figure 2 This is the front view of the shape memory alloy plate.

[0027] Figure 3 This is the front view of the bolt.

[0028] Figure 4 This is the front view of the first connecting plate.

[0029] Figure 5 This is the front view of the second connecting plate.

[0030] 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℃.

[0031] Figure 7 The 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.

[0032] Figure 8 This is a stress-temperature relationship diagram for the SMA plate.

[0033] Figure 9 This is a schematic diagram of a fatigue loading test.

[0034] Figure 10 This is a schematic diagram of a fatigue loading test on a shape memory alloy plate.

[0035] Figure 11 This is a load-deformation relationship diagram for a shape memory alloy plate during the initial stage of fatigue loading.

[0036] Figure 12 This is a load-deformation relationship diagram for shape memory alloy plates at different fatigue stages.

[0037] Figure 13 A comparison of the peak deformation of shape memory alloy plates at different fatigue stages.

[0038] Figure 14 The image shows the SN curve of the excited shape memory alloy plate.

[0039] Figure 15 This is a schematic diagram showing the recovery stress loss of a shape memory alloy plate at different fatigue stages.

[0040] Figure 16 This is a schematic diagram of the annealing process for shape memory alloy plates.

[0041] In the diagram: 1 represents the main pipe, 2 represents the branch pipe, 3 represents the shape memory alloy plate, 4 represents the bolt, 5 represents the second connecting plate, and 6 represents the first connecting plate. Detailed Implementation

[0042] 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.

[0043] See Figures 1-5 The SMA plate targeted prestressing reinforcement device for K-shaped node cracks of the present invention includes connecting plates disposed on the main pipe 1 and the branch pipe 2, and shape memory alloy plates 3 disposed on the connecting plates. The connecting plates are in two sets: a first connecting plate 6 and a second connecting plate 5. The first connecting plate 6 is welded to the main pipe 1, and the second connecting plate 5 is welded to the branch pipe 2. The first connecting plate 6 and the second connecting plate 5 are located in the same plane, which is perpendicular to the crack, and the extension line of the plane passes through the crack tip. The two ends of the shape memory alloy plate 3 are respectively mounted on the first connecting plate 6 and the second connecting plate 5 by fastening components. By applying directional excitation to the shape memory alloy plate 3, the shape memory alloy plate 3 is prompted to generate targeted recovery stress perpendicular to the crack propagation direction through the shape memory effect.

[0044] In this embodiment, the first connecting plate 6 is a triangular structure with a chamfered bottom. The length B2 of the base of the first connecting plate 6 is 80-100mm, the height H2 is 90-100mm, and the thickness t2 is 20mm. The second connecting plate 5 is a triangular structure. The length B1 of the base of the second connecting plate 5 is 90-110mm, the height H1 is 90-100mm, and the thickness t1 is 20mm. Both the first connecting plate 6 and the second connecting plate 5 have through holes at their centers, and the diameter d1 of the through holes is 14-16mm.

[0045] In this embodiment, by setting the first connecting plate 6 and the second connecting plate 5 as a triangular structure, the welding area and residual stress can be reduced. In addition, the triangular welding element (i.e. the connecting plate) has the advantages of being lightweight and easy to construct compared with welding elements of other shapes. Furthermore, since the curvature of the welding surface of the first connecting plate 6 on the main pipe 1 is greater than that of the welding surface of the second connecting plate 5 on the branch pipe 2, the bottom of the first connecting plate 6 needs to be chamfered to ensure sufficient effective welding area.

[0046] See Figures 1-5 The first connecting plate 6 and the second connecting plate 5 are both made of Q355 steel; the welding position between the main pipe 1 and the first connecting plate 6 and the welding position between the branch pipe 2 and the second connecting plate 5 are both at a distance greater than or equal to 75mm from the crack.

[0047] See Figures 1-5 The shape memory alloy plate 3 consists of two sets, which are respectively installed on the two sides of the first connecting plate 6 and the second connecting plate 5; the thickness δ of each set of shape memory alloy plate 3 is 3-5mm, the length L1 is greater than or equal to 240mm, and the height H is 40-60mm.

[0048] In this embodiment, the shape memory alloy plate 3 is symmetrically provided with bolt holes of diameter d2 of 14-16mm along its length direction; the distance D between the center of the bolt holes and both sides of the shape memory alloy plate 3 along its length direction is less than or equal to 28mm; in order to achieve active crack repair, it is necessary to avoid the high temperature generated during welding from contacting the shape memory alloy plate 3, and it is also necessary to consider the high requirements of spatial anchoring on the position of the openings on the shape memory alloy plate 3; in this embodiment, a fastening assembly is used to install the shape memory alloy plate 3 on the first connecting plate 6 and the second connecting plate 5. Before welding, a steel plate of the same size as the shape memory alloy plate 3 is installed on the first connecting plate 6 and the second connecting plate 5 using the fastening assembly; then, a welding torch is used to weld the first connecting plate 6 to the main pipe 1 and the second connecting plate 5 to the branch pipe 2; after the temperature of the first connecting plate 6 and the second connecting plate 5 has cooled down, the steel plate is removed, and the shape memory alloy plate 3 is installed on the first connecting plate 6 and the second connecting plate 5 using the fastening assembly; in this way, the high temperature generated during welding can be avoided from stimulating the shape memory alloy plate 3.

[0049] See Figures 1-5 The shape memory alloy plate 3 undergoes annealing and pre-stretching processes. During pre-stretching, the distance between the clamping area of ​​the shape memory alloy plate 3 and the bolt hole is ≥10mm to ensure that the shape memory alloy plate 3 has sufficient residual deformation.

[0050] See Figures 1-5 The fastening assembly includes a bolt 4, a nut, and a washer. The bolt 4 passes through the bolt hole on the first set of shape memory alloy plates 3, the through hole on the first connecting plate 6 or the second connecting plate 5, the bolt hole on the second set of shape memory alloy plates 3, and the washer in sequence before connecting to the nut, thereby enabling the two sets of shape memory alloy plates 3 to be installed on the first connecting plate 6 and the second connecting plate 5.

[0051] See Figures 1-5 The method for targeted prestressed reinforcement of SMA plates with K-shaped node cracks according to the present invention includes the following steps:

[0052] Use a grinder to remove the coating and impurities from the crack tips and welded areas;

[0053] A steel plate of the same size as the shape memory alloy plate 3 is mounted on the first connecting plate 6 and the second connecting plate 5 using a fastening assembly. Next, a laser calibrator is used to determine the welding plane. Then, a welding torch is used to weld the first connecting plate 6 to the main pipe 1 and the second connecting plate 5 to the branch pipe 2, with the first connecting plate 6 and the second connecting plate 5 located in the same plane, perpendicular to the crack, and the extension line of this plane passing through the crack tip. After the first connecting plate 6 and the second connecting plate 5 have cooled, the steel plate is removed, and the shape memory alloy plate 3 is mounted on the first connecting plate 6 and the second connecting plate 5 using the fastening assembly. Finally, a hot air gun is used to excite the shape memory alloy plate 3, causing it to generate recovery stress through the shape memory effect. The fastening assembly converts the recovery stress into a targeted compressive stress field perpendicular to the crack propagation direction. This targeted compressive stress field actively suppresses the stress intensity factor at the crack tip, thus preventing crack propagation.

[0054] In the above process, the present invention employs a spatial reinforcement method. By precisely arranging the connecting plate and the shape memory alloy plate 3 in three-dimensional space, the axial recovery stress of the shape memory alloy is decomposed and projected onto 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 used in 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.

[0055] In terms of construction technology, the space reinforcement method only requires local welding at the two connecting plates. Before welding, a steel plate of the same size as the shape memory alloy plate 3 is installed on the first connecting plate 6 and the second connecting plate 5 using fastening components. Then, a laser calibrator is used to determine the welding plane. Subsequently, a welding torch is used to weld the first connecting plate 6 to the main pipe 1 and the second connecting plate 5 to the branch pipe 2, with the first connecting plate 6 and the second connecting plate 5 located in the same plane, which is perpendicular to the crack, and the extension line of the plane passes through the crack tip. After the first connecting plate 6 and the second connecting plate 5 have cooled down, the steel plate is removed, and the shape memory alloy plate 3 is installed on the first connecting plate 6 and the second connecting plate 5 using fastening components. This can greatly reduce the risk of heat input and residual stress redistribution. In addition, the lightweight shape memory alloy plate 3 and connecting plates 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.

[0056] More importantly, the spatial reinforcement method employed in this invention possesses dynamic adjustability and re-excitation capabilities: the shape memory alloy plate 3 can be repeatedly excited through temperature control to repeatedly obtain the designed prestress, providing an active compensation means for subsequent structural health monitoring and maintenance. Simultaneously, the spatial reinforcement method employed in this invention is not limited by the bonding difficulties of traditional planar reinforcement, and can achieve surface contact and precise bonding at complex K-shaped nodes, thereby significantly reducing the crack propagation rate and greatly extending fatigue life.

[0057] Furthermore, it is necessary to determine the prestress loss of the shape memory alloy plate 3. To determine this prestress loss, a waterproof lithium-ion battery AC complementary power receiving unit is used in this embodiment to monitor the strain amplitude of the shape memory alloy plate 3 over a long period. Since strain measurement is based on the change in resistance of the strain gauge caused by steel deformation to calculate the strain value, and temperature changes will alter the thermal expansion coefficient of the steel, temperature is a crucial 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 installed at the crack location. This module eliminates the false strain introduced by the thermal expansion characteristics of the steel by acquiring the initial strain value of the same type of unstressed steel under the same temperature environment, ultimately obtaining the strain change caused only by the actual load.

[0058] In this embodiment, the material of the shape memory alloy plate 3 is NiTi or NiTiNb.

[0059] 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.

[0060] 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.

[0061] The composition of the NiTi-SMA plate material used in this experiment is shown in Table 1:

[0062] Table 1. Composition of NiTi-SMA plate material used in the experiment

[0063]

[0064] 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.

[0065] 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.

[0066] 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.

[0067] like Figure 7 As 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.

[0068] likeFigure 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] Table 2 Characteristic values ​​of fatigue loads at various levels

[0074]

[0075] 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.

[0076] 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 S-N 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.

[0077] The residual recovery stress of SMA plates at different fatigue loading stages was statistically analyzed to obtain, as follows: Figure 15The curves showing the relationship between different fatigue cycles and SMA recovery stress loss are derived from... Figure 15 It 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.

[0078] 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.

[0079] 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 K-joint crack targeted prestress reinforcement device for SMA plate, characterized in that, The application relates to a crack arrestor, which comprises connecting plates arranged on a main pipe and a branch pipe and shape memory alloy plates arranged on the connecting plates, wherein the connecting plates are two groups of first connecting plates and second connecting plates; the first connecting plates are welded on the main pipe, and the second connecting plates are welded on the branch pipe; the first connecting plates and the second connecting plates are located in the same plane, the plane is perpendicular to a crack, and the extension line of the plane passes through a crack tip; two ends of the shape memory alloy plates are respectively installed on the first connecting plates and the second connecting plates through fastening assemblies; directional excitation is applied to the shape memory alloy plates to make the shape memory alloy plates generate a targeted recovery stress perpendicular to the crack propagation direction through a shape memory effect. The materials of the first connecting plates and the second connecting plates are Q355 steel; the welding positions between the main pipe and the first connecting plates and between the branch pipe and the second connecting plates are all greater than or equal to 75 mm away from the crack. The shape memory alloy plates are two groups, and the two groups of shape memory alloy plates are respectively installed on two sides of the first connecting plates and the second connecting plates; the thickness of each group of shape memory alloy plates is 3-5 mm, the length is greater than or equal to 240 mm, and the height is 40-60 mm. The fastening assembly comprises a bolt, a nut and a gasket, wherein the bolt passes through a through hole on the first connecting plate or the second connecting plate, a first group of shape memory alloy plates, a second group of shape memory alloy plates and the gasket in sequence and is connected with the nut.

2. The K-joint crack SMA plate target prestressing reinforcement apparatus of claim 1, wherein, The first connecting plate is a triangular structure, and the two side angles of the bottom of the triangular structure are chamfered; the length of the bottom side of the first connecting plate is 90-110 mm, the height is 90-100 mm, and the thickness is 20 mm.

3. The K-joint crack SMA plate target prestressing reinforcement apparatus of claim 2, wherein, The second connecting plate is also a triangular structure, and the length of the bottom side of the second connecting plate is 80-100 mm, the height is 90-100 mm, and the thickness is 20 mm.

4. The K-joint crack SMA plate targeted prestressing reinforcement apparatus according to claim 3, characterized in that, Through holes are formed in the centers of the first connecting plate and the second connecting plate, and the diameters of the through holes are 14-16 mm.

5. The K-joint crack SMA plate target prestressing reinforcement apparatus of claim 1, wherein, Bolt holes with diameters of 14-16 mm are symmetrically formed in the length direction of the shape memory alloy plates; the distances between the centers of the bolt holes and the two sides of the length direction of the shape memory alloy plates are less than or equal to 28 mm.

6. The K-joint crack SMA sheet target prestressing reinforcement apparatus of claim 5, wherein, The shape memory alloy plates need to be subjected to annealing treatment and pre-stretching processing.

7. A SMA plate targeted prestressing reinforcement method for the SMA plate targeted prestressing reinforcement device of the K-joint crack according to any one of claims 1 to 6, characterized by, The application further discloses a crack arrestor manufacturing method, which comprises the following steps: A grinder is used to polish and remove the coating and impurities on the crack tip and the welding area; A steel plate with the same size as the shape memory alloy plate is installed on the first connecting plate and the second connecting plate through a fastening assembly, then the first connecting plate is welded on the main pipe, the second connecting plate is welded on the branch pipe, and the first connecting plate and the second connecting plate are located in the same plane, the plane is perpendicular to the crack, and the extension line of the plane passes through the crack tip; After the temperature of the first connecting plate and the second connecting plate is cooled, the steel plate is removed, and the shape memory alloy plate is installed on the first connecting plate and the second connecting plate through the fastening assembly. The shape memory alloy plate is excited to cause the shape memory alloy plate to generate a targeted recovery stress through a shape memory effect, and the targeted recovery stress is converted into a targeted compressive stress field perpendicular to a crack propagation direction by the fastening assembly, and the targeted compressive stress field can actively inhibit a stress intensity factor of a crack tip to prevent crack propagation.

Images