A steel pipe joint fatigue life prolonging method based on stress field optimization

Abstract

The application discloses a steel pipe node fatigue life prolonging method based on stress field optimization and belongs to the technical field of steel structure fatigue reinforcement. The method first carries out regional planning and surface treatment on a steel structure pipe node to be reinforced to determine a reinforcing range covering an original welding heat affected zone; then a metal additive manufacturing technology is adopted to print a smooth transition section from a branch pipe to a main pipe and an integrated anchoring platform in the reinforcing range; subsequently, a prefabricated anchoring piece is welded and fixed on the anchoring platform; FRP cloth is arranged and tensioned in stages to apply active prestress to the node; and finally, surface protection treatment is carried out. The application reduces stress concentration of the node from the root by additive manufacturing, actively restrains crack propagation in combination with prestressed FRP, realizes synergistic enhancement of geometric shape optimization and active prestress, reduces hot spot stress of the node by more than 40%, prolongs fatigue life by 12-75 times, and is suitable for round steel pipe nodes and square steel pipe nodes, and has the advantages of convenient construction, less material consumption and high reliability.

Description

Technical Field

[0001] This invention relates to the field of fatigue strengthening technology for steel structures, and in particular to a method for extending the fatigue life of steel pipe joints based on stress field optimization. Background Technology

[0002] In recent years, steel tube structures have been widely used in engineering structures such as bridges, offshore platforms, industrial plants, and stadiums due to their advantages such as light weight, high rigidity, and aesthetically pleasing appearance. However, fatigue failure of steel tube structures under long-term cyclic dynamic loads has become increasingly prominent, becoming a key factor affecting structural safety and service life.

[0003] Pipe joints in steel structures are core load-bearing components. Due to the abrupt geometric changes at the intersection of branch pipes and main pipes, and the dense weld seams, stress concentration is significant. Simultaneously, the deterioration of material properties in the heat-affected zone generated during welding further reduces the joint's fatigue resistance. Under repeated loads from vehicles, wind, and waves, pipe joints are highly susceptible to fatigue cracks initiating at the weld toe, which gradually propagate until fracture. This not only affects the normal use of the structure but may also lead to major safety accidents.

[0004] To address the fatigue failure of existing steel structure pipe joints, commonly used reinforcement techniques include welding stiffeners, bonding steel plates, and bonding FRP fabric. Welding stiffeners increases the load-bearing capacity by enhancing joint stiffness, but it cannot eliminate the geometric stress concentration in the original joint, and the welding process can cause secondary thermal damage to the original structure. Bonding steel plates has disadvantages such as high self-weight, susceptibility to corrosion, and complex construction. Ordinary FRP reinforcement is mostly passively stressed; FRP only begins to function after joint deformation and crack opening, and it cannot actively inhibit crack initiation and propagation.

[0005] While prestressed FRP reinforcement technology can offset some tensile stress by actively applying prestress, its application in steel pipe joints has obvious limitations: the surface of steel pipe joints is mostly a complex curved surface, lacking a flat anchoring installation surface, which makes the anchoring and tensioning of prestressed FRP very difficult. Traditional bonded anchoring is prone to slippage, resulting in large prestress loss and making it difficult to fully utilize the high strength characteristics of FRP.

[0006] Additive manufacturing technology (3D printing) offers the advantage of customized molding of complex geometries, enabling the precise fabrication of arbitrary curved surfaces. However, currently, the application of additive manufacturing technology in the steel structure field mainly focuses on the integral manufacturing of new nodes or the production of connectors. There is no systematic method specifically designed to combine it with prestressed FRP technology for fatigue strengthening of existing steel pipe nodes. Therefore, there is an urgent need to develop a fatigue strengthening method for steel pipe nodes that can reduce stress concentration at its source through geometric optimization, effectively suppress crack propagation through active prestressing, and provide reliable anchoring and convenient construction. Summary of the Invention

[0007] The purpose of this invention is to overcome the above-mentioned shortcomings in the prior art and provide a fatigue life extension method for steel pipe joints based on stress field optimization. This method deeply integrates metal additive manufacturing technology with prestressed FRP technology, realizing the synergistic enhancement of geometric shape optimization and active prestress, which can significantly improve the fatigue life of steel pipe joints.

[0008] To achieve the above objectives, the technical solution of the present invention is implemented as follows: A fatigue life extension method for steel pipe joints based on stress field optimization includes the following steps: S1. Area planning and surface treatment: Determine the additive manufacturing reinforcement area, which completely covers the original weld heat-affected zone of the node, and perform surface treatment on the stress concentration area of ​​the steel structure pipe node to be reinforced. S2. Printed transition section and anchoring platform: A smooth node transition section is printed in the reinforced area using metal additive manufacturing technology, and an anchoring platform for welding prestressed anchors is reserved on the transition section and / or pipe wall. The smooth transition section and the anchoring platform are integrally formed. S3. Install anchoring components: Weld and fix the prefabricated anchors to the anchoring platform; S4. Arrange and tension the FRP fabric: Anchor one end of the FRP fabric to the precast anchor, apply prestress using a graded tensioning method, and then anchor it to the other precast anchor. S5. Surface protection treatment: Cover the reinforced area with a protective coating.

[0009] Furthermore, in step S2, the smooth transition section is fitted with a curved surface according to the intersection line of the pipe node and the actual geometric characteristics of the connecting pipe to form a smooth rounded transition from the branch pipe to the main pipe, so as to reduce stress concentration; the anchoring platform is a planar structure reserved during the printing process, and its surface flatness meets the welding requirements of the anchor.

[0010] Furthermore, if the steel structure pipe node to be reinforced is a round pipe, then an anchoring platform is printed on both the transition section and the pipe wall. The bottom of the anchoring platform on the pipe wall is designed to fit the curvature of the pipe wall. If the steel structure pipe node to be reinforced is a square pipe, then an anchoring platform is printed only on the transition section, and the pipe wall utilizes its original plane without the need for a platform.

[0011] Furthermore, in step S2, the printing process adopts a layered stacking method. After each layer is printed, it is naturally cooled to room temperature before the next layer is printed, so as to avoid thermal deformation of the pipe node base material caused by the heat input of printing. The printing material is a metal material that matches the mechanical properties of the pipe node base material.

[0012] Furthermore, the prefabricated anchor includes an anchor base, a pressure plate, and fastening bolts; the bottom of the anchor base is provided with a groove to enhance mechanical engagement, and the top is provided with an opening for the bolt to pass through; the pressure plate matches the groove on the upper part of the anchor base, and the fastening bolts are tightened by nuts and provide preload by limiting and pressing the pressure plate and the FRP cloth at the bottom of the pressure plate.

[0013] Furthermore, in step S3, the prefabricated anchor is fixed to the anchoring platform by fillet weld; welding materials that match the printing material are used, welding heat input is strictly controlled, and a symmetrical welding sequence is used during welding to control welding deformation; after welding, the weld is ground to make its surface smooth.

[0014] Furthermore, in step S4, symmetrical operation is performed during tensioning to avoid eccentricity, and the bolts are tightened after the prestress value stabilizes; the direction of the anchor point connection is perpendicular to the expected direction of fatigue crack propagation, and the reinforcement path formed by the connection of the centers of each anchor point is located in the central symmetry plane of the node.

[0015] Furthermore, step S4 specifically includes: S41. FRP Path Confirmation: Check the location of the anchoring components and confirm the laying path of the FRP cloth between the anchoring points to avoid forced bending at the corners of the nodes. S42. Laying and initial anchoring: Inject structural adhesive into the groove of the anchor base at one end, embed one end of the FRP cloth into the groove, cover it with the pressure plate and tighten the fastening bolts to complete the initial fixation, and then lay the FRP cloth along the preset path and pass through the anchor point at the other end. S43. Tensioning and final anchoring: After the structural adhesive at the starting point has fully cured, clamp the free end of the FRP cloth and perform graded tensioning. After the load check shows no slippage and the prestress loss is within the allowable range, inject structural adhesive into the groove of the final anchoring base, embed the FRP cloth and fix it. S44. Quality Inspection: Check whether the weld is cracked, whether the bolts are loose, and whether the FRP cloth is flat and wrinkle-free.

[0016] Furthermore, in step S5, the protective coating includes an anti-corrosion coating, and a fireproof coating may be selected according to the fire resistance requirements of the structure; the anti-corrosion coating is selected from epoxy resin or polyurethane coatings, and the fireproof coating is selected from intumescent fireproof coatings; the coating needs to extend at least 50mm beyond the anchoring end of the FRP cloth.

[0017] Furthermore, in step S1, the stress concentration area includes the intersection of the pipe joints and the saddle point and crown point of the circular pipe joints; the surface treatment includes removing surface oil, rust and old coatings, polishing until the metal luster is exposed, and blowing away dust with compressed air.

[0018] Beneficial Effects: This invention is the first to systematically combine metal additive manufacturing technology with prestressed FRP technology for fatigue strengthening of existing steel pipe joints, achieving a dual synergistic effect of "geometric optimization + active prestress suppression". The smooth transition section printed by additive manufacturing eliminates the geometric stress concentration of the original joint from the source, reducing the stress at the most dangerous saddle point hotspot by more than 50%; the prestressed FRP actively applies compressive stress, offsetting the remaining tensile stress and inhibiting crack propagation. The combined effect of both increases the fatigue life of the joint by 12-75 times, far exceeding the effect of single strengthening technologies.

[0019] This invention solves the technical challenge of anchoring traditional prestressed FRP (fiberglass reinforced plastic) on complex curved steel pipe joints by integrally printing a standardized anchoring platform using additive manufacturing. The anchoring platform and transition section are integrally formed, eliminating the need for subsequent drilling and welding of additional components, thus avoiding secondary damage to the original structure. At the same time, it allows the anchors to be manufactured as universal standard parts, significantly reducing construction difficulty and cost.

[0020] The additive manufacturing reinforcement area of ​​this invention precisely covers the original weld heat-affected zone, achieving the effect of strengthening the weak areas of the original structure and improving the mechanical properties of the original structure; the printing process adopts a layered stacking and natural cooling process, strictly controlling heat input to prevent thermal deformation of the base material.

[0021] This invention designs a composite anchoring system consisting of a grooved base, a pressure plate, bolts, and structural adhesive. Through the dual action of mechanical interlocking and adhesive force, it effectively prevents the slippage of FRP fabric under fatigue load, ensuring the long-term effectiveness of prestress.

[0022] This invention consumes fewer materials and has a high cost-performance ratio. The steel used in additive manufacturing is only 1.3% of the total steel used in the node. The construction process is standardized and can be implemented quickly on site. It is also applicable to two common node types, round steel pipe and square steel pipe, and has strong versatility. It can be widely used in fatigue reinforcement projects of existing steel structures such as bridges, offshore platforms, and industrial plants. Attached Figure Description

[0023] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a flowchart illustrating the steps of a method for extending the fatigue life of steel pipe joints based on stress field optimization, as described in an embodiment of the present invention. Figure 2 This is a schematic diagram of the overall reinforcement of the pipe node according to the present invention; Figure 3 This is a magnified view of a node as described in this invention; Figure 4 This is a cross-sectional view of the smooth transition section printed according to the present invention; Figure 5 This is a schematic diagram of the prefabricated anchor described in this invention; Figure 6 This is a cross-sectional view of the anchoring base described in this invention; Figure 7 Print stress contour plots of nodes after applying prestress to smooth transition sections and saddle points; Figure 8 This is a stress-distance curve near the weld toe.

[0024] In the diagram, 1-branch pipe; 2-main pipe; 3-transition section; 4-anchoring platform; 5-prefabricated anchor; 6-FRP cloth; 7-weld; 8-original weld heat-affected zone; 51-anchoring base; 52-pressure plate; 53-fastening bolt; 54-nut. Detailed Implementation

[0025] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0026] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0027] Example 1 See Figure 1-6 A fatigue life extension method for steel pipe joints based on stress field optimization includes the following steps: S1. Surface treatment and area planning: Determine the additive manufacturing reinforcement area, which completely covers the original weld heat-affected zone 8 of the node, and perform surface treatment on the stress concentration area of ​​the steel structure pipe node to be reinforced. S2, Printed transition section 3 and anchoring platform: A smooth node transition section is printed in the reinforced area using metal additive manufacturing technology, and an anchoring platform 4 for welding prestressed anchors is reserved on the transition section 3 and / or pipe wall. The smooth transition section 3 and the anchoring platform 4 are integrally formed. S3. Install anchoring components: Weld and fix the prefabricated anchor 5 to the anchoring platform 4; S4. Arrange and tension the FRP fabric: Anchor one end of the FRP fabric 6 to the precast anchor 5, apply prestress using a graded tensioning method, and then anchor it to the other precast anchor 5. S5. Surface protection treatment: Cover the reinforced area with a protective coating.

[0028] This embodiment first removes impurities and precisely delineates reinforcement boundaries through surface treatment to ensure reliable bonding between the subsequent printed layer and the base material. Utilizing the customized molding capabilities of additive manufacturing, the manufacturing of "geometric optimization (transition section)" and "anchoring foundation (platform)" is completed in one go, overcoming the technical shortcomings of traditional methods. Reliable anchoring of FRP is achieved through prefabricated anchors, and active prestress is applied through graded tensioning to form a dual reinforcement system of "geometric stress reduction + active compressive stress". Finally, surface protection isolates the system from environmental erosion, ensuring the long-term durability of the reinforcement system.

[0029] This embodiment achieves a systematic and synergistic reinforcement of "metal additive manufacturing geometry optimization + prestressed FRP active suppression", which solves the inherent defects of traditional single reinforcement technology. It eliminates the geometric stress concentration of steel pipe nodes from the root, while actively offsetting tensile stress and suppressing crack propagation, reducing hot spot stress at the nodes by more than 40% and increasing fatigue life by 12-75 times. It forms a standardized construction process that is applicable to the fatigue reinforcement of most existing steel pipe nodes and has strong engineering practicality.

[0030] In a specific example, in step S2, the smooth transition section 3 is fitted with a curved surface based on the intersection line of the pipe node and the actual geometric characteristics of the connecting pipe to form a smooth rounded transition from the branch pipe 1 to the main pipe 2, so as to reduce the stress concentration factor; the anchoring platform 4 is a planar structure reserved during the printing process, and its surface flatness meets the welding requirements of the anchor.

[0031] It should be noted that the stress concentration at the original welded joint is essentially a force flow deflection caused by a geometrical abrupt change. By fitting a curved surface, the sharp intersection line is transformed into a continuous rounded transition, allowing the force flow to be transmitted smoothly and avoiding stress peaks at the weld toe. The flatness of the anchoring platform directly affects the welding quality and stress uniformity of the anchor. The reserved planar structure can ensure that the anchor fits tightly with the platform, avoiding stress concentration and anchor failure caused by welding gaps.

[0032] This embodiment can reduce the hot spot stress at the node saddle point by more than 50% simply by additive manufacturing the transition section (subsequent verification shows that, for example, the stress at the main pipe saddle point under uniaxial tension is reduced from 536.56MPa to 242.61MPa); it ensures the accuracy and reliability of the anchor installation and lays the foundation for the effective application of subsequent prestress.

[0033] In a specific example, if the steel structure pipe node to be reinforced is a round pipe, then the anchoring platform 4 is printed on both the transition section 3 and the pipe wall. The bottom of the anchoring platform 4 on the pipe wall is designed to fit the curvature of the pipe wall. If the steel structure pipe node to be reinforced is a square pipe, then the anchoring platform 4 is printed only on the transition section 3, and the pipe wall does not need to be printed with a platform due to its original flat surface.

[0034] In this embodiment, the surface of the circular tube is a continuous curved surface with no naturally flat mounting surface. Therefore, a planar platform that precisely matches the curvature needs to be printed on the tube wall to achieve stable welding of the anchors. The square tube wall itself is flat, which naturally meets the welding requirements of the anchors. No additional printing platform is required; printing is only needed on the arc-shaped curved surface of the transition section.

[0035] This embodiment significantly improves the versatility of the method, and is applicable to the most common round steel pipe and square steel pipe joints in engineering; it avoids unnecessary additive manufacturing consumables, and the amount of steel used for printing square pipe joints can be further reduced, thus improving the cost-effectiveness of the project.

[0036] In a specific example, in step S2, the printing process adopts a layered stacking method. After each layer is printed, it is naturally cooled to room temperature before the next layer is printed, so as to avoid thermal deformation of the pipe node base material caused by the heat input of printing. The printing material is a metal material that matches the mechanical properties of the pipe node base material.

[0037] It should be noted that since additive manufacturing generates a large amount of heat input, if continuous printing is not cooled, the accumulated heat will cause thermal deformation of the base material and even reduce its mechanical properties. Layered natural cooling can effectively control heat input and avoid secondary damage to the original structure. Matching the mechanical properties of the printing material with those of the base material can ensure the continuous transfer of force between the transition section and the base material, and avoid the formation of new stress concentrations at the interface due to differences in material strength.

[0038] This embodiment effectively controls thermal deformation during the printing process, ensuring the geometric accuracy of the nodes and the mechanical properties of the original structure; it also ensures the integrity of the printed transition section and the base material, avoiding the risk of cracking at the interface.

[0039] In a specific example, the prefabricated anchor 5 includes an anchor base 51, a pressure plate 52, and a fastening bolt 53; the anchor base 51 has a groove at the bottom to enhance mechanical engagement and an opening at the top for the fastening bolt 53 to pass through; the pressure plate 52 matches the groove at the top of the anchor base 51, and the fastening bolt 53 is fastened by a nut 54 and provides preload force to limit and compress the pressure plate 52 and the FRP cloth 6 at the bottom of the pressure plate 52.

[0040] In this embodiment, the groove at the bottom of the anchoring base forms a mechanical interlock with the FRP cloth, which, together with the adhesive force of the structural adhesive, can significantly improve the anchoring force of the FRP; the pressure plate can evenly transfer the tightening force of the bolts to the surface of the FRP cloth, avoiding fiber breakage caused by local stress concentration; the bolt tightening method can achieve precise locking of prestress, ensuring the long-term effectiveness of prestress.

[0041] This embodiment solves the problems of easy slippage and large prestress loss in traditional bonded anchorage, and effectively improves the fatigue slip resistance of the anchorage system; the anchors can be prefabricated as standard parts, eliminating the need for on-site customization, which greatly improves construction efficiency.

[0042] In a specific example, in step S3, the prefabricated anchor 5 is fixed to the anchoring platform 4 by fillet weld; welding materials that match the printing material are used, welding heat input is strictly controlled, and a symmetrical welding sequence is used during welding to control welding deformation; after welding, the weld 7 is ground to make its surface smooth.

[0043] It should be noted that fillet welds are simple to operate and provide sufficient shear strength, making them suitable for on-site construction; matching welding materials ensures that the mechanical properties of the weld and the printing material are consistent, preventing the weld from becoming a new weak point; symmetrical welding can balance residual welding stress and effectively control welding deformation; post-weld grinding can eliminate sharp edges on the weld surface and prevent new stress concentration.

[0044] This embodiment avoids the impact of the welding process on the mechanical properties of the printed area, reduces welding residual stress by more than 40%, ensures the connection strength between the anchor base and the platform, and prevents weld cracking under fatigue load.

[0045] In a specific example, in step S4, symmetrical operation is performed during tensioning to avoid eccentricity, and the bolts are tightened after the prestress value stabilizes; the direction of the anchor point connection is perpendicular to the expected direction of fatigue crack propagation, and the reinforcement path formed by the connection of the centers of each anchor point is located in the central symmetry plane of the node.

[0046] It should be noted that staged tensioning can avoid FRP fracture or node deformation caused by one-time tensioning, while ensuring uniform distribution of prestress; symmetrical tensioning can prevent eccentric stress at the nodes and avoid triggering additional bending moments; the anchorage line is perpendicular to the crack propagation direction, which allows the prestress to act directly on the crack tip, maximizing the effect of suppressing crack opening; the reinforcement path in the central symmetry plane can ensure uniform stress at the nodes and avoid local stress concentration.

[0047] In a specific example, step S4 specifically includes: S41, FRP Path Confirmation: Check the position of the anchoring components and confirm the laying path of FRP cloth 6 between anchoring points to avoid forced bending at the corners of the nodes; S42. Laying and initial anchoring: Inject structural adhesive into the groove of the anchor base at one end, embed one end of the FRP cloth 6 into the groove, cover it with the pressure plate 52 and tighten the fastening bolts 53 to complete the initial fixation, and then lay the FRP cloth 6 along the preset path and pass through the anchor point at the other end. S43. Tensioning and final anchoring: After the structural adhesive at the starting point has fully cured, clamp the free end of the FRP cloth 6 and perform graded tensioning. After the load check shows no slippage and the prestress loss is within the allowable range, inject structural adhesive into the groove of the final anchoring base, embed the FRP cloth 6 and fix it. S44. Quality Inspection: Check whether weld 7 is cracked, whether bolts are loose, and whether FRP cloth 6 is flat and wrinkle-free.

[0048] The path confirmation in this embodiment can avoid fiber damage caused by the forced bending of FRP at the node corners; Anchoring one end first and then tensioning the other end ensures that the FRP is laid straight and avoids wrinkles; load-bearing inspection can promptly detect problems such as anchor slippage, ensuring the reliability of prestressing application; final quality inspection can comprehensively identify construction defects and ensure reinforcement quality.

[0049] This embodiment achieves standardization and normalization of FRP tensioning construction, which can significantly reduce the construction error rate; it effectively avoids common construction problems such as FRP fiber breakage and anchor slippage, and improves the reliability of the reinforcement system.

[0050] In a specific example, in step S5, the protective coating includes an anti-corrosion coating, and a fireproof coating may be selected according to the fire resistance requirements of the structure; the anti-corrosion coating is selected from epoxy resin or polyurethane coatings, and the fireproof coating is selected from intumescent fireproof coatings; the coating needs to extend at least 50mm beyond the anchoring end of the FRP cloth.

[0051] The epoxy resin or polyurethane anti-corrosion coatings of this embodiment can effectively isolate corrosive media such as water and oxygen, protecting metal parts and FRP fabric from corrosion; the intumescent fireproof coating will expand to form a heat insulation layer during a fire, protecting metal parts from being softened by high temperatures; the coating extends beyond the anchoring end to form a closed protection, preventing corrosive media from entering from the gaps at the anchoring end.

[0052] In a specific example, in step S1, the stress concentration area includes the intersection of pipe nodes and the saddle point and crown point of circular pipe nodes; the surface treatment includes removing surface oil, rust and old coatings, polishing to expose the metallic luster, and blowing away dust with compressed air.

[0053] This embodiment precisely delineates stress concentration zones and heat-affected zones, ensuring that the reinforcement area covers all weak points and avoids omissions; polishing to expose the metallic luster removes the oxide layer, oil, and rust from the surface, ensuring a good bond between the printed layer and the base material; compressed air dust removal thoroughly removes surface dust, preventing any impact on the bonding strength of the printed layer.

[0054] Effect verification In practice, the steel structure pipe joints to be reinforced adopt X-shaped circular steel pipe joints. The main pipe diameter is 112mm, the main pipe wall thickness is 6mm, the branch pipe diameter is 68mm, the branch pipe wall thickness is 4mm, and the length of both the main pipe and the branch pipe is 500mm. They are made of S960 steel. Under unidirectional stress, the branch pipe bears an axial tensile force of 64kN. Under bidirectional stress, both branch pipes bear a tensile force of 64kN simultaneously.

[0055] To achieve a smooth connection between the branch pipe and the main pipe, additive manufacturing technology was used to print a smooth transition section on the node surface. The transition section is connected to the surface of the branch pipe and the main pipe with a smooth rounded corner, the chord width is 25mm, and the amount of steel used is only 1.3% of the amount of steel used in the node.

[0056] To further strengthen the nodes, prestress was applied successively at the saddle point and the crown point. The prestress at the saddle point was set to 10 kN, and the prestress at the crown point was set to 2.5 kN.

[0057] Numerical calculations of the nodes before and after reinforcement were performed using the finite element simulation software ABAQUS, such as... Figure 7 As shown, four finite element models were established: Model 1 is the original welded joint without reinforcement; Model 2 is the joint with a smooth transition section printed using only additive manufacturing; Model 3 is the joint with additive manufacturing and saddle point prestressing; Model 4 is the joint with additive manufacturing, saddle point prestressing, and crown point prestressing.

[0058] Based on the stress-distance curve near the weld toe ( Figure 8 The hot spot stress at the nodes is calculated using the linear extrapolation method, and the calculation formula is as follows:

[0059] In the formula, 0.4t and 1.0t refer to the distance from the weld toe, and t is the pipe wall thickness.

[0060] The calculated results of the hot spot stress at the reinforced nodes are shown in Table 1. As can be seen from Table 1, through the dual reinforcement of additive manufacturing and prestressing, the hot spot stress at the nodes is reduced by more than 40%. Among them, the hot spot stress at the saddle point of the main pipe under uniaxial tension is reduced from 536.56 MPa to 141.60 MPa, a reduction of 73.6%.

[0061] Table 1. Hot spot stress at nodes (unit: MPa)

[0062] According to the CIDECT fatigue design specification, the fatigue life of the above four types of nodes can be predicted based on hot spot stress and pipe wall thickness. The calculation formula is as follows:

[0063] In the formula, N f For fatigue life, S hsThe stress is the hot spot stress, and t is the pipe wall thickness.

[0064] The fatigue life of a joint follows the "weakest link principle," meaning the fatigue life of the entire joint is equal to the life of the shortest-lived component. Under uniaxial tension, the main pipe saddle point is always the most stressful and dangerous part, thus the joint fatigue life is controlled by the hot spot stress at the main pipe saddle point. Under biaxial tension, in the unreinforced and additive-reinforced stages, the stress at the main pipe saddle point is the highest, making it the control point. When prestressing is applied to the saddle point, the stress at the main pipe saddle point decreases significantly, and the stress at the main pipe crown point becomes the new peak value. Therefore, the joint fatigue life is then controlled by the hot spot stress at the main pipe crown point.

[0065] The predicted fatigue life of the reinforced joints is shown in Table 2. As can be seen from Table 2, through additive manufacturing and prestressing reinforcement, the fatigue life of the joints can be increased by 12-75 times: under uniaxial tension conditions, the fatigue life increases from 10... 4.64 The next upgrade to 10 6.52 This represents an increase of approximately 75 times; under biaxial tensile conditions, fatigue life increases from 10... 5.04 The next upgrade to 10 6.12 This improvement of approximately 12 times demonstrates the effectiveness of the method of the present invention.

[0066] Table 2. Fatigue life prediction of nodes (unit: number of stress cycles)

[0067] The above description is only 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.

Claims

1. A fatigue life extension method for steel pipe joints based on stress field optimization, characterized in that, Includes the following steps: S1. Area planning and surface treatment: Determine the additive manufacturing reinforcement area, which completely covers the original weld heat-affected zone of the node, and perform surface treatment on the stress concentration area of ​​the steel structure pipe node to be reinforced. S2. Printed transition section and anchoring platform: A smooth node transition section is printed in the reinforced area using metal additive manufacturing technology, and an anchoring platform for welding prestressed anchors is reserved in the transition section. The smooth transition section and the anchoring platform are integrally formed. S3. Install anchoring components: Weld and fix the prefabricated anchors to the anchoring platform; S4. Arrange and tension the FRP fabric: Anchor one end of the FRP fabric to the precast anchor, apply prestress using a graded tensioning method, and then anchor it to the other precast anchor. S5. Surface protection treatment: Cover the reinforced area with a protective coating.

2. The fatigue life extension method for steel pipe joints based on stress field optimization according to claim 1, characterized in that, In step S2, the smooth transition section is fitted with a curved surface based on the intersection line of the pipe node and the actual geometric characteristics of the connecting pipe to form a smooth rounded transition from the branch pipe to the main pipe, so as to reduce stress concentration; the anchoring platform is a planar structure reserved during the printing process, and its surface flatness meets the welding requirements of the anchor; the connecting pipe includes the main pipe and the branch pipe.

3. The fatigue life extension method for steel pipe joints based on stress field optimization according to claim 1, characterized in that, If the steel structure pipe node to be reinforced is a round pipe, then an anchoring platform is printed on both the transition section and the pipe wall. The bottom of the anchoring platform on the pipe wall is designed to fit the curvature of the pipe wall. If the steel structure pipe node to be reinforced is a square pipe, then an anchoring platform is printed only on the transition section. The pipe wall utilizes its original plane and does not require a platform to be printed.

4. The fatigue life extension method for steel pipe joints based on stress field optimization according to claim 1, characterized in that, In step S2, the printing process adopts a layer-by-layer stacking method. After each layer is printed, it is naturally cooled to room temperature before the next layer is printed to avoid thermal deformation of the pipe node base material caused by the heat input of printing. The printing material is a metal material that matches the mechanical properties of the pipe node base material.

5. The fatigue life extension method for steel pipe joints based on stress field optimization according to claim 1, characterized in that, The prefabricated anchor includes an anchor base, a pressure plate, and fastening bolts; the bottom of the anchor base is provided with a groove to enhance mechanical engagement, and the top is provided with an opening for the bolt to pass through; the pressure plate matches the groove on the upper part of the anchor base, and the fastening bolts are tightened by nuts and provide preload by limiting and pressing the pressure plate and the FRP cloth at the bottom of the pressure plate.

6. The fatigue life extension method for steel pipe joints based on stress field optimization according to claim 5, characterized in that, In step S3, the prefabricated anchor is fixed to the anchoring platform by fillet weld; welding material matching the printing material is used, welding heat input is strictly controlled, and a symmetrical welding sequence is used during welding to control welding deformation; after welding, the weld is ground to make its surface flat and smooth.

7. The fatigue life extension method for steel pipe joints based on stress field optimization according to claim 1, characterized in that, In step S4, symmetrical operation is performed during tensioning to avoid eccentricity, and the bolts are tightened after the prestress value stabilizes; the direction of the anchor point connection is perpendicular to the expected direction of fatigue crack propagation, and the reinforcement path formed by the connection of the centers of each anchor point is located in the central symmetry plane of the node.

8. The fatigue life extension method for steel pipe joints based on stress field optimization according to claim 1, characterized in that, Step S4 specifically includes: S41. FRP Path Confirmation: Check the location of the anchoring components and confirm the laying path of the FRP cloth between the anchoring points to avoid forced bending at the corners of the nodes. S42. Laying and initial anchoring: Inject structural adhesive into the groove of the anchor base at one end, embed one end of the FRP cloth into the groove, cover it with the pressure plate and tighten the fastening bolts to complete the initial fixation, and then lay the FRP cloth along the preset path and pass through the anchor point at the other end. S43. Tensioning and final anchoring: After the structural adhesive at the starting point has fully cured, clamp the free end of the FRP cloth and perform graded tensioning. After the load check shows no slippage and the prestress loss is within the allowable range, inject structural adhesive into the groove of the final anchoring base, embed the FRP cloth and fix it. S44. Quality Inspection: Check whether the weld is cracked, whether the bolts are loose, and whether the FRP cloth is flat and wrinkle-free.

9. The fatigue life extension method for steel pipe joints based on stress field optimization according to claim 1, characterized in that, In step S5, the protective coating includes an anti-corrosion coating and a fireproof coating is selected according to the fire resistance requirements of the structure. The anti-corrosion coating is an epoxy resin or polyurethane coating, and the fireproof coating is an intumescent fireproof coating. The coating needs to extend at least 50 mm beyond the anchoring end of the FRP cloth.

10. The fatigue life extension method for steel pipe joints based on stress field optimization according to claim 1, characterized in that, In step S1, the stress concentration area includes the intersection of pipe nodes and the saddle point and crown point of circular pipe nodes; the surface treatment includes removing surface oil, rust and old coating, polishing until the metal luster is exposed, and blowing away dust with compressed air.

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