Synergistic Crack-Resistant Reinforcement Method of Ultrasonic Impact Testing and CFRP for Diaphragms of Orthotropic Steel Bridges

Abstract

This invention provides a method for synergistic crack-resistant reinforcement of orthotropic steel bridge diaphragms using ultrasonic impact and CFRP (Crystal Reinforced Plastic) treatment. The method comprises the following steps: drilling arresting holes at the tips of fatigue cracks in the diaphragm; performing ultrasonic impact treatment on the damaged area of ​​the diaphragm to induce localized plastic deformation and physical closure of the crack surface, while simultaneously introducing residual compressive stress at the edges of the arresting holes; and symmetrically bonding CFRP plates to both sides of the diaphragm in the ultrasonically impacted area. This invention integrates the active strengthening mechanism of ultrasonic impact with the passive load-sharing mechanism of bonded angle steel. The synergistic effect of these two methods overcomes the limitations of single repair methods, achieving restoration of out-of-plane stiffness of damaged nodes and a significant extension of fatigue life.

Description

Technical Field

[0001] This invention relates to bridge structure fatigue life extension technology, specifically to a method for synergistic crack resistance reinforcement of orthotropic steel bridge diaphragms using ultrasonic impact and CFRP. Background Technology

[0002] Orthotropic steel bridge deck structures, welded from longitudinal stiffeners, diaphragms, and a top plate, offer significant advantages such as light weight, high ultimate bearing capacity, and ease of prefabrication and installation, making them a core form in modern bridge engineering. However, under the frequent alternating effects of vehicle wheel loads and complex local out-of-plane bending and in-plane shear multiaxial stress fields, these structures are highly susceptible to fatigue failure. Extensive surveys of in-service steel bridge defects show that among the many fatigue-prone locations in orthotropic plate structures, fatigue cracks occur most frequently at the arc-shaped cuts of the diaphragms, accounting for approximately 60% of all cracks. Once these cracks initiate and propagate laterally, they not only severely weaken the overall lateral stiffness of the bridge deck system but also induce severe local stress redistribution, posing a significant threat to the overall service life and traffic safety of steel bridge structures.

[0003] Currently, the engineering community mainly uses the following cold repair and reinforcement methods to address arc-shaped cut cracks in diaphragms, but each method has its own clear technical limitations: One method is conventional drill-and-arrest (DSP). This method, used as an emergency repair solution, involves drilling a circular hole at the tip of an existing fatigue crack to alter the crack's geometry, thereby reducing localized stress concentration. It is convenient to implement, requires no traffic closure, and is suitable for the narrow and complex spaces inside orthotropic steel bridges. However, DSP alone is a temporary protective measure. Physical drilling inevitably weakens the effective load-bearing section of the diaphragm, and the hole edges remain under extremely high stress gradients and alternating stress amplitudes under external vehicle loads. In actual service, secondary fatigue microcracks are highly likely to initiate at the edges of the DSP holes (some components experience secondary cracking after approximately 15,000 fatigue cycles), posing a significant risk of secondary failure.

[0004] Another approach is the use of drill-in-place crack arrest combined with carbon fiber reinforced polymer (CFRP) composite panel bonding reinforcement. This technique utilizes the extremely high tensile strength, excellent fatigue resistance, and durability of CFRP material. By bonding the panels to both sides, it distributes the load in the cracked area, restoring the overall local stiffness of the structure to some extent and reducing the local stress amplitude under load. However, traditional CFRP panel reinforcement is mostly a passive load-bearing system, and its construction process cannot fundamentally eliminate or reverse the high residual tensile stress field introduced by welding or initial cracking at the edge of the crack arrest hole and on the surface of existing cracks. Under the repeated action of long-term alternating heavy loads, the high-stress area at the hole edge remains a fatigue weak point in the entire system. Once secondary microcracks re-emerge at the hole edge, it will trigger a severe redistribution of local internal forces, causing stress concentration in the adhesive layer between the CFRP composite panel and the steel substrate at the ends. This can easily induce severe interfacial shear delamination failure, leading to premature failure of the entire composite reinforcement system.

[0005] In summary, existing cold repair solutions for diaphragms all face technical bottlenecks such as "inability to improve the initial stress field at the hole edge and crack" and "easy to induce secondary cracking and peeling of the reinforcement interface," making it difficult to achieve permanent and highly reliable repair of fatigue cracks in orthotropic steel bridge diaphragms. Summary of the Invention

[0006] To address the aforementioned problems in the prior art, this invention aims to provide a method for synergistic crack-resistant reinforcement of orthotropic steel bridge diaphragms using ultrasonic impact and CFRP, which can significantly extend fatigue life.

[0007] To achieve the above objectives, the present invention provides a method for synergistic crack-resistant reinforcement of orthotropic steel bridge diaphragms using ultrasonic impact and CFRP, characterized by comprising the following steps: Step 1: Drill a crack arresting hole at the tip of the fatigue crack in the diaphragm; Step 2: Perform ultrasonic impact treatment on the damaged area of ​​the diaphragm to induce local plastic deformation of the crack surface and achieve physical closure, while introducing residual compressive stress at the edge of the crack arrest hole; Step 3: In the area treated with ultrasonic impact, CFRP sheets are symmetrically bonded to both sides of the diaphragm using structural adhesive.

[0008] According to the above technical solution, before the ultrasonic impact treatment, the process parameters of the ultrasonic impact treatment are determined by finite element simulation.

[0009] According to the above technical solution, the specific steps for determining process parameters using finite element simulation include: Establish a finite element model that includes the main body of the diaphragm, prefabricated cracks, and crack arresting holes; Simulation analysis was performed using different ultrasonic impact velocities. Residual stress distribution data were extracted, and the optimal impact velocity was determined based on the maximum surface compressive stress amplitude and the absence of tensile stress rebound across the entire thickness.

[0010] According to the above technical solution, the different ultrasonic impact velocities are 2-5 m / s.

[0011] According to the above technical solution, before step one, there is also a step of arranging strain gauges on the surface of the diaphragm to collect stress data before and after repair.

[0012] According to the above technical solution, the strain gauges are arranged as follows: strain gauges are symmetrically arranged on both sides at a certain distance from the edge of the crack arresting hole along the horizontal extension line of the pre-fabricated crack and the crack arresting hole.

[0013] According to the above technical solution, the ultrasonic impact treatment includes pre-crack closing impact and crack arrest hole edge strengthening impact; the pre-crack closing impact continues until the crack opening is completely physically closed; the crack arrest hole edge strengthening impact is uniformly impacted along the circumferential direction of the hole wall until a uniformly transitioning plastic deformation band is formed at the edge of the hole, and after impact, a grinding tool is used to smooth out the small flash.

[0014] According to the above technical solution, the CFRP board is a single-layer high-strength and high-elasticity CFRP board with a thickness of 2-4mm.

[0015] According to the above technical solution, when pasting the CFRP board, the adhesive used is an epoxy structural adhesive, and the thickness of the adhesive layer is precisely controlled by uniformly incorporating micro steel balls.

[0016] According to the above technical solution, before pasting the CFRP board, a pretreatment step is also included, which involves grinding, removing rust, and solvent cleaning the predetermined pasting area of ​​the partition and the surface of the CFRP board.

[0017] Compared with the prior art, the present invention has the following beneficial effects: First, it achieves a dual synergistic reinforcement of "active + passive". This invention integrates the active strengthening mechanism of ultrasonic impact with the passive load-sharing mechanism of bonded angle steel. Ultrasonic impact uses high-frequency impact to plastically close cracks and introduce beneficial residual compressive stress, thus cutting the crack propagation dynamics at the microscopic level; angle steel, on the other hand, significantly improves the stiffness of the joint at the macroscopic level and restricts out-of-plane deformation. The synergistic effect of the two overcomes the limitations of single repair methods, thereby restoring the out-of-plane stiffness of damaged joints and significantly extending their fatigue life.

[0018] Secondly, the process parameters were optimized through a combination of simulation and experimentation. This invention established a quantitative relationship between ultrasonic impact velocity and repair effect using finite element simulation. The optimal impact velocity was determined to be 5 m / s, with the maximum surface compressive stress amplitude and no tensile stress rebound across the entire thickness as the selection criteria. Simulation verification showed that using this optimized parameter, the fatigue notch coefficient of the composite repair system could be reduced by more than 60%, avoiding the blind application of process parameters in actual engineering.

[0019] Third, it effectively prevents secondary cracking and interface delamination. Because ultrasonic impact treatment eliminates residual tensile stress concentration sources at the edges of the anti-crack holes, secondary microcracks are unlikely to initiate at the hole edges even under long-term alternating loads. This avoids shear delamination failure of the adhesive layer at the CFRP plate-steel substrate interface caused by severe redistribution of local stress, ensuring the long-term durability of the reinforcement system. Attached Figure Description

[0020] Figure 1 The dimensions are those of the diaphragm simulation model in this embodiment of the invention.

[0021] Figure 2 This describes the mesh and boundary condition settings for the anti-crack hole model in this embodiment of the invention.

[0022] Figure 3 This is a simulation diagram of the crack impact path in the ultrasonic impact embodiment of the present invention.

[0023] Figure 4 This is a simulation diagram of the impact path of the crack-arresting hole in the ultrasonic impact embodiment of the present invention.

[0024] Figure 5 This is a simulation setting for the cohesion of the CFRP board in an embodiment of the present invention.

[0025] Figure 6 The model selection for the diaphragm specimen in the embodiments of the present invention.

[0026] Figure 7 This is a schematic diagram of different repair methods for the diaphragm in an embodiment of the present invention.

[0027] Figure 8 This is a diagram showing the arrangement of strain gauges in an embodiment of the present invention.

[0028] Figure 9 This is a diagram of the transverse residual stress in the ultrasonic impact crack arresting hole in an embodiment of the present invention.

[0029] Figure 10 This is a tensile test stress distribution of impact cracks at different speeds in an embodiment of the present invention.

[0030] Figure 11 This is a stress distribution diagram of tensile tests on crack-arresting holes at different impact speeds in an embodiment of the present invention.

[0031] Figure 12 This is the stress-load curve at the measuring point on the edge of the crack-stopping hole in an embodiment of the present invention.

[0032] Figure 13 The stress distribution at the measuring point in this embodiment of the invention is shown (P=128.6kN).

[0033] Figure 14 This is a diagram illustrating the fatigue repair mechanism of different schemes in the embodiments of the present invention. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. However, it should be understood that the specific embodiments of this invention are only for explaining the invention and are not intended to limit the scope of protection of this invention.

[0035] This embodiment provides a method for synergistic crack-resistant reinforcement of orthotropic steel bridge diaphragms using ultrasonic impact and CFRP, characterized by the following steps: Step 1: Drill a crack arresting hole at the tip of the fatigue crack in the diaphragm; Step 2: Perform ultrasonic impact treatment on the damaged area of ​​the diaphragm. The ultrasonic impact speed is 2-5 m / s, preferably 3 m / s, to induce local plastic deformation on the crack surface and achieve physical closure, while introducing residual compressive stress at the edge of the crack arresting hole. The ultrasonic impact treatment includes pre-crack closure impact and crack arresting hole edge reinforcement impact. The pre-crack closure impact continues until the crack opening is completely physically closed. The crack arresting hole edge reinforcement impact is performed uniformly along the circumferential direction of the hole wall until a uniformly transitioned plastic deformation band is formed at the edge of the hole. After impact, a grinding tool is used to smooth out any small flash.

[0036] Step 3: Pre-treatment steps including grinding, rust removal, and solvent cleaning of the predetermined bonding area of ​​the diaphragm and the surface of the CFRP board.

[0037] Step 4: In the area treated with ultrasonic impact, CFRP sheets are symmetrically bonded to both sides of the diaphragm using structural adhesive. The CFRP sheets are single-layer high-strength, high-elasticity CFRP sheets with a thickness of 2-4mm, preferably 3mm. When bonding the CFRP sheets, epoxy-based structural adhesive is used, and the adhesive layer thickness is precisely controlled by uniformly incorporating micro steel balls.

[0038] Before pasting the CFRP board, a pretreatment step is also included, which involves grinding, removing rust, and solvent cleaning the predetermined pasting area of ​​the diaphragm and the surface of the CFRP board.

[0039] This invention solves the following three core technical problems: 1. Addressing the problem that single-hole crack arrest methods fail to eliminate stress concentration and easily induce secondary cracking. Existing single-hole crack arrest methods can only change the stress distribution by physically removing the affected area, without restoring the cross-sectional stiffness of the cracked region. The stress concentration factor at the edge of the crack arrest hole remains at a high level. Under long-term cyclic wheel loads, the high stress amplitude at the hole edge can easily exceed the fatigue threshold of the material, leading to secondary fatigue cracking failure of the component in a short period of time.

[0040] 2. Addressing the issues of traditional CFRP-coated passive reinforcement failing to improve initial residual stress and prone to premature interface delamination. While existing "drilling to arrest cracks + bonding CFRP plates" technology can distribute the load and restore cross-sectional stiffness through CFRP, it is a passive reinforcement method and cannot fundamentally eliminate the initial residual tensile stress field at the edge of the crack arrest hole and on the surface of existing cracks. Under long-term alternating heavy loads, once secondary microcracks initiate at the hole edge, it easily triggers severe local stress redistribution. This stress abrupt change leads to extremely high shear stress at the CFRP-steel substrate interface (adhesive layer), inducing premature interface delamination failure, resulting in a service life of the reinforcement system far shorter than expected.

[0041] 3. This invention addresses the lack of a synergistic mechanism between "active crack closure and local reinforcement" and a basis for optimizing process parameters in existing cold repair technologies. Existing purely mechanical analysis and conventional finite element techniques struggle to accurately predict secondary failure processes under multiple repair mechanisms, and lack refined assessment methods that couple residual stress field reconstruction with interface peeling damage. This invention aims to fill this technological gap by providing a multi-faceted synergistic protection method that integrates "active crack closure, residual compressive stress reinforcement at the hole edge, and global stiffness enhancement." Simultaneously, through physical static testing, it solves the technical challenges of blindly applying composite repair processes and the inability to quantitatively assess repair effects in practical engineering, effectively ensuring the safe service life of steel bridge structures.

[0042] This embodiment also provides methods for verifying the effectiveness of the method of the present invention, including the following steps: I. Simulation Optimization of Process Parameters (1) To optimize the ultrasonic impact effect, a USP (anti-crack hole) finite element model was first established. The model consists of three parts: the diaphragm body, the pre-existing crack, and the anti-crack hole. The diaphragm's geometric parameters are based on the typical design of long-span bridges, using a standard thickness of 12mm. Considering the need to eliminate the singularity of the crack tip, Seam elements were used to simulate the pre-existing crack, with the crack propagation direction consistent with the original direction. The diameter of the anti-crack hole was 16mm. The diaphragm was simulated using C3D8R hexahedral reduced integral elements, with a mesh accuracy of 5mm. In the local area around the anti-crack hole, the mesh was refined to 1mm. The diaphragm material was Q345qD steel, and the material parameters are shown in Table 1. The dimensions of the diaphragm and the boundary conditions of the anti-crack hole are shown in Table 1. Figure 1 and Figure 2 As shown.

[0043] Table 1 Material Parameter Settings

[0044] In the ultrasonic impact simulation, the ultrasonic impact needle was 3mm in diameter, and there was one impact needle. The impact coverage was fixed at 100%. Impact velocity was a variable factor: by adjusting the impact needle velocity to 2 m / s, 3 m / s, 4 m / s, and 5 m / s, the effects of impact on crack location and impact-induced crack arrestor hole location were investigated. The simulation groups for ultrasonic impact coverage are shown in Table 2.

[0045] Table 2 Simulation Groups of Ultrasonic Impact Velocity

[0046] The area affected by ultrasonic impact is one-third of the anti-crack portion opposite the pre-existing crack (considering computational costs). The ultrasonic impact crack path is shown below. Figure 3 The ultrasonic impact crack path is shown in Figure 4 The diaphragm was simulated using C3D8R hexahedral reduced integral elements, with a mesh precision of 5 mm, and the mesh was refined to 1 mm in local areas around the crack-arresting holes. A linear softening mode was adopted, and a small viscosity coefficient was set to improve convergence. After ultrasonic impact, the model after ultrasonic impact was used as the initial conditions.

[0047] II. Establishment of CFRP Board Bonding Simulation Model After ultrasonic impaction, the CFRP plate bonding process was simulated. The CFRP plate was modeled as a single layer, with dimensions of 300mm × 70mm × 3.0mm, and symmetrically arranged on both sides of the diaphragm perpendicular to the crack direction. To accurately simulate the stress transfer at the reinforced interface, a solid adhesive layer with a thickness of 2.5mm was established between the CFRP plate and the diaphragm. The main body of the diaphragm was still made of Q345qD steel, set as an elastoplastic constitutive model. The CFRP plate was made of high elastic modulus carbon fiber, defined as a linear elastic orthotropic material in the simulation to simulate its high strength characteristics along the fiber direction. The adhesive layer material was given specific cohesive constitutive properties.

[0048] For element generation, both the diaphragm and the CFRP plate use C3D8R eight-node hexahedral reduced integral elements. The mesh generation strategy remains consistent: the global size is 5 mm, and local mesh refinement is performed around the crack arresting holes and in the critical stress areas where the CFRP is bonded, reducing the mesh size to 1 mm to capture drastic stress gradient changes.

[0049] To simulate potential delamination failure at the interface between the CFRP sheet and the steel substrate, a surface-based cohesive contact property was introduced into the model at the contact surface between the diaphragm and the CFRP sheet. The simulation settings are as follows: Figure 5 As shown, its core settings are as follows: Linear elastic behavior (Elasticity): Defines the interface stiffness matrix, including the normal stiffness Knn and the two shear stiffnesses Kss and Ktt. The stiffness values ​​are usually calculated based on the elastic modulus, shear modulus, and actual thickness of the adhesive layer.

[0050] Damage Initiation Criterion: A quadratic nominal stress criterion is used. Damage to the adhesive layer is determined to begin when the sum of the squares of the ratios of the nominal stresses at the interface in the normal and two shear directions reaches 1.

[0051] Damage evolution law: Damage evolution is defined based on fracture energy release rate. An energy-based evolution law and a linear softening mode are selected, with inputs of Type I (tensile) and Type II (shear) fracture energies GIC and GIIC. Simultaneously, a viscous stability coefficient is introduced to improve the numerical convergence of the model during interface cracking.

[0052] A full displacement constraint (Ux=Uy=Uz=0) was applied to the lower clamping area of ​​the diaphragm, while an axial tensile load of 128.6 kN was applied to the upper loading area. By analyzing the damage contour map (SDEG) of the adhesive layer under load and the stress level at key measuring points of the diaphragm, the degree of improvement of the local stress field of the crack arrestor hole by CFRP bonding and the reliability of its interface were quantitatively evaluated.

[0053] III. Simulation Results and Optimization of Process Parameters With 100% ultrasonic impact coverage, the data on the transverse residual stress extraction of the impact-arrested hole at different impact velocities (2 m / s, 3 m / s, 4 m / s, and 5 m / s) are as follows: Figure 9 As shown in the figure, residual stress analysis reveals that the compressive residual stress on the surface of the crack-arresting hole exhibits a significant increasing trend with increasing impact velocity. The lower-velocity groups 2-100 and 3-100 only introduce approximately -180 MPa and -250 MPa compressive stresses on the surface, respectively, with limited strengthening capabilities for deeper layers. In contrast, the strengthening effect of the high-velocity groups is significantly amplified, with the 5-100 group performing the best. It introduces the highest amplitude compressive stress barriers of -425 MPa and -350 MPa on the 0 mm and 12 mm surface layers, respectively, achieving a safe, stress-free coverage across the entire thickness of the plate section, and preventing any self-balancing tensile stress rebound in the core that could damage the structure. This parameter provides the best surface fatigue strengthening effect while ensuring overall structural safety; therefore, 5-100 is ultimately selected as the optimal process parameter.

[0054] The stress distribution in tensile tests after impact at different impact velocities (2 m / s, 3 m / s, 4 m / s, and 5 m / s) for cracks and crack arresters is shown below. Figure 10 and Figure 11 As shown.

[0055] Table 3 Calculation results for different impact velocities

[0056] In the group targeting "impact cracks", when the impact velocity is at a low level (2m / s and 3m / s), the decrease in fatigue notch coefficient (Kf) hovers around 56.1%; while when the velocity is increased to a high level (4m / s and 5m / s), the decrease in Kf increases dramatically, reaching the range of 60.9% to 61.1%.

[0057] A similar pattern was observed in the group targeting "impact-induced crack arresting holes". Although some fluctuations occurred in nominal stress concentration at 3 m / s (Kf decreased by only 55.827%), the maximum principal stress at 5 m / s was most effectively controlled (reduced to 179.713 MPa) as the impact kinetic energy increased, with a Kf decrease of 60.754%.

[0058] Combining the above two sets of data, it can be seen that, excluding minor fluctuations caused by nonlinear contact in individual simulation meshes, a clear trend emerges: within the selected process parameter range, the higher the ultrasonic impact velocity, the better the composite repair effect. The underlying mechanical mechanism is that a higher impact velocity means a greater input of mechanical kinetic energy, which can induce deeper plastic flow on the steel surface, thereby generating higher levels of beneficial residual compressive stress at the hole edge or crack surface, maximally offsetting the tensile stress concentration caused by external loads. Therefore, based on the combined results of residual stress analysis and tensile test stress distribution, an impact velocity of 5 m / s and an impact coverage of 100% are selected as the optimal combination of process parameters.

[0059] Part Two: Specimen Preparation and Sensor Deployment IV. Experimental Model Design This embodiment verifies the practical implementation and effectiveness of the composite cold repair technology through static tensile tests using a physical model. Steel diaphragm specimens with pre-existing cracks are grouped and numbered, such as... Figure 6 As shown. The four groups of specimens were treated separately, as follows: Figure 7 As shown.

[0060] The specimens were divided into four groups: Drilling Crack Arrest Only (DSP), Drilling Crack Arrest + Ultrasonic Impact (DSP+UIT), Drilling Crack Arrest + Bonded CFRP Plate (DSP+CFRP), and Drilling Crack Arrest + Ultrasonic Impact + Bonded CFRP Plate (DSP+UIT+CFRP).

[0061] V. Strain Gauge Installation Use a grinder to remove the metal oxide layer from the surface of the strain gauge mounting area, and clean it with alcohol. Along the horizontal extension line of the pre-existing crack and the crack arrestor hole, symmetrically arrange strain gauges on both sides at positions 2mm, 15mm, 30mm, 60mm, and 90mm from the edge of the crack arrestor hole. Figure 8 As shown. Seal with adhesive and allow to solidify.

[0062] Part Three: Drilling Fracturing Arrest and Ultrasonic Impact Treatment VI. Drilling Crack Prevention A crack arresting hole with a diameter of 16 mm is drilled at the tip of the fatigue crack in the diaphragm.

[0063] VII. Implementation of Ultrasonic Shock Enhancement under Optimized Parameters After the crack arresting holes were drilled, a handheld ultrasonic impact device (equipped with three 3mm impact needles and a working frequency of 17.5±2kHz) was used to reinforce and repair the damaged areas on both sides of the diaphragm.

[0064] (a) Pre-crack closure impact. Align the edge of the impact needle with the crack opening and move it slowly along the crack surface. The operation is completed when the pre-cracks on both sides undergo significant plastic deformation and the crack surface is completely physically closed.

[0065] (ii) Impact reinforcement at the edge of the anti-crack hole. Impact evenly along the circumference of the hole wall until a uniformly transitioning plastic deformation zone forms at the hole edge, eliminating cutting defects and introducing deep residual compressive stress. After impact, use a grinding tool to smooth out any minor flash.

[0066] Part Four: CFRP Board Bonding Process 8. Refined Installation Process of CFRP Composite Board The mechanical properties of the steel used in the diaphragm were tested according to GB / T 228.1-2021 "Metallic materials, tensile testing—Part 1: Test methods at room temperature". According to the manufacturer's data, the elastic modulus and tensile strength of the CFRP sheet are 230 GPa and 4000 MPa, respectively. The structural adhesive used to bond the CFRP sheet is NT epoxy-type carbon fiber adhesive NT-300, with an elastic modulus and tensile strength of 3.11 GPa and 54 MPa, respectively. The mechanical properties of the steel sheet, CFRP sheet, and structural adhesive are shown in Table 4.

[0067] Table 4 Mechanical properties of Q345 steel, CFRP sheet and structural adhesive

[0068] (a) Surface pretreatment. Grind the area to be bonded until the metal luster is exposed (rust removal to St3.0 or Sa2.5 grade), and perform the operation within 4 hours after solvent cleaning. Roughen and clean the surface of the single-layer high-strength and high-elasticity CFRP board (thickness 3.0mm).

[0069] (II) Adhesive Application and Bonding. Epoxy-based carbon fiber reinforced plastic (CFRP) adhesive is applied evenly by roller, with 2.5mm diameter micro-steel balls placed within the adhesive layer to precisely control its thickness. After symmetrically bonding the CFRP boards on both sides, a scraper is used to press and remove air bubbles from the center outwards.

[0070] (iii) Differentiated finishing. Apply the excess adhesive around the edges to form a 45° beveled transition arc to reduce stress concentration at the ends and the risk of peeling.

[0071] (iv) Curing. Then apply uniform pressure using woodworking clamps and let it stand for 7 days until fully cured.

[0072] Part 5: Static Tensile Test and Effect Verification IX. Static Tensile Test The specimen was stably anchored in the MTS electro-hydraulic servo testing machine (the specimen clamping end was equipped with a 100mm×100mm thickened pad for anti-slip and anti-eccentricity). Axial static tensile loads of 0~128.6kN were applied in stages, with gradients of 12.9, 20, 40, 60, 80, 100, 120, and 128.6kN respectively. Data were recorded after each step load was stabilized. The stress concentration factor (Kt) and fatigue notch factor (Kf) were calculated by extracting the stress data at the hole edge to cross-validate the accuracy of the simulation model.

[0073] X. Test Results and Analysis (I) Stress-load relationship. The stress test results of diaphragm specimens repaired under different schemes during static tensile stress to fatigue peak load are as follows: Figure 12 As shown in the figure, P represents the tensile load, σ represents the stress, and x represents the distance from the measuring point to the edge of the crack-arresting hole. From... Figure 12 It can be seen that the stress at the test point near the crack arrest hole in each group of specimens increases linearly with the load, indicating that the steel is basically in an elastic state. Compared with the DSP specimen (cracking arrest only through drilling), the three reinforcement schemes of DSP+UIT, DSP+CFRP, and DSP+UIT+CFRP all effectively reduced the stress at the hole edge.

[0074] Because the CFRP composite plate was bonded to the DSP+CFRP specimen, the cross-sectional stiffness of the cracked area of ​​the diaphragm was significantly improved, and the local load was distributed. Therefore, the slope of the stress-load curve of the DSP+CFRP specimen was significantly smaller than that of the DSP specimen. For the DSP+UIT specimen, ultrasonic impact (UIT) was applied to the drilled foundation, which introduced residual compressive stress at the hole edge through physical plastic deformation, resulting in a certain decrease in the overall stress level. However, the improvement in cross-sectional stiffness was limited, so the slope of its stress-load curve was closer to that of the DSP specimen. For the DSP+UIT+CFRP specimen, based on the combination of ultrasonic impact and bonded CFRP plate, both pre-compression stress was introduced through UIT (significantly reducing the overall stress baseline), and the local stiffness was increased through the CFRP plate (significantly reducing the slope of the curve). Under the triple synergistic effect, the DSP+UIT+CFRP specimen had the most significant effect in mitigating stress growth, and the overall stress level was kept at a minimum.

[0075] (II) Stress Distribution. When P = 128.6 kN, the stress distribution at each measuring point is as follows: Figure 13 As shown, extremely high stress concentration exists at the edge of the crack arrestor holes. For specimen DSP repaired only with crack arrestor holes, the local stress within a distance of less than 60 mm from the hole edge begins to exceed the average stress of the section (σnom = 61.13 MPa), and increases sharply as it approaches the hole edge, reaching a maximum stress of approximately 375 MPa. After reinforcement using different methods, the peak stress at the hole edge under fatigue peak load was effectively controlled: the stress in specimen DSP+UIT decreased to approximately 310 MPa, the stress in specimen DSP+CFRP decreased to approximately 255 MPa, and the stress in specimen DSP+UIT+CFRP decreased significantly to approximately 140 MPa. Simultaneously, the range where local stress on the diaphragm exceeds the average stress of the section (i.e., the stress concentration zone) was also significantly improved, especially in specimen DSP+UIT+CFRP, where this range was drastically reduced from 60 mm from the hole edge in specimen DSP to approximately 20-30 mm, greatly alleviating the local stress deterioration of the diaphragm.

[0076] (III) Repair Mechanism Analysis. The repair mechanisms of each scheme under fatigue load are as follows: Figure 14 As shown in the figure, the value at the top is the local maximum peak stress σmax, and the value in the middle, Δσ, is the stress amplitude.

[0077] The repair mechanism of the bonded CFRP composite panel (DSP+CFRP group) is mainly through significantly increasing the structural stiffness of the cracked area of ​​the diaphragm, and the CFRP panel cooperating to share part of the external load, thereby effectively reducing the stress response of the cracked area under load. In terms of data, the stress amplitude Δσ is significantly reduced (from 344.1MPa in the baseline group to 240.96MPa).

[0078] The repair mechanism of ultrasonic impact treatment (DSP+UIT group) involves generating intense plastic rheology in the arrestor holes and crack surface, thereby introducing high levels of beneficial residual compressive stress deep within the structure. This compressive stress can offset some of the tensile stress generated by the external load, and its core function is to shift the overall local tensile stress level downward (the maximum peak stress is significantly reduced from 374.1 MPa to 313.1 MPa). However, since this method does not fundamentally change the cross-sectional stiffness of the structure, its effect on limiting the stress amplitude Δσ is relatively limited.

[0079] For the novel multi-synergistic repair scheme (DSP+UIT+CFRP group), ultrasonic impact and CFRP composite plate exerted an excellent synergistic effect. Under this condition, the CFRP plate significantly limited the stress fluctuation range under alternating loads, while the residual compressive stress introduced by UIT further suppressed the overall tensile stress peak downward. The synergistic effect of the two resulted in the maximum principal stress at the hole edge dropping sharply to 138.2 MPa, and the stress amplitude being greatly reduced to 133.462 MPa.

[0080] Overall, the ultrasonic impact group (DSP+UIT), the composite plate group (DSP+CFRP), and the novel multi-synergistic repair group (DSP+UIT+CFRP) showed significantly improved effects on stress concentration at cracked sites and reduction of overall stress response, in that order.

[0081] (iv) Analysis of stress concentration factor and fatigue notch factor. Based on the measured stress at the edge of the crack arrest hole in different specimens... Select stress concentration factor K t and fatigue notch coefficient K f K represents the repair effect of each reinforcement method. t and K f The calculation formula is as follows:

[0082]

[0083] in: The nominal stress of the section under load is taken here as the average stress of the cracked section of the diaphragm; Let be a characteristic constant of the material, taken as 0.45; The radius of the stop hole is set to 8 mm.

[0084] The calculation results are summarized in Table 5. It can be seen that the ultrasonic impact method (specimen DSP+UIT), the bonded CFRP composite plate (specimen DSP+CFRP), and the new multi-synergistic repair scheme (specimen DSP+UIT+CFRP) can all effectively improve the stress concentration near the crack arrest hole.

[0085] Table 5. Hole edge stress and fatigue notch coefficient

[0086] Note: η is the percentage reduction in fatigue notch coefficient of each specimen compared to the DSP of the specimen repaired by the crack arrest hole.

[0087] The stress concentration factor (Kt) and fatigue notch factor (Kf) at the hole edge of the DSP+UIT (ultrasonic impact group) specimen were reduced by 16.34% (from 6.12 to 5.12) and 16.19% (from 5.88 to 4.93), respectively, compared to the DSP (blank control group) specimen. The stress concentration factor at the hole edge and fatigue notch factor of the DSP+CFRP (composite plate group) specimen were reduced by 32.35% (from 6.12 to 4.14) and 32.08% (from 5.88 to 3.99), respectively, compared to the DSP specimen.

[0088] Compared to the simple borehole arrest method, the DSP+UIT+CFRP specimen employing a novel multi-layered synergistic repair mechanism significantly improved the stress state, reducing its fatigue notch coefficient by 62.54%. In a horizontal comparison, the fatigue notch coefficient (Kf=2.20) of the DSP+UIT+CFRP specimen is approximately half that of the DSP+CFRP specimen (Kf=3.99), and far lower than that of the DSP+UIT specimen (Kf=4.93). It can be seen that the DSP+UIT+CFRP specimen integrates a triple synergistic protection mechanism of "active crack closure - residual stress reconstruction at the borehole edge - global stiffness enhancement." Its comprehensive control of borehole edge stress (σmax reduced to 138.25MPa) and reduction of the fatigue notch coefficient are significantly superior to the ultrasonic impact scheme (DSP+UIT specimen) which only introduces residual compressive stress, and the traditional plate-mounting scheme (DSP+CFRP specimen) which only improves local stiffness.

[0089] In summary, this invention utilizes optimized ultrasonic impact to induce active physical crack closure and reconstruct the compressive stress field at the hole edge. Combined with a CFRP composite plate applied using a specific process (2.5mm steel ball thickness control + 45° anti-stripping chamfer), a synergistic protection system of "active closure - compressive stress strengthening - global stiffness improvement" is successfully constructed. The repair effect is significant: cross-validation confirms that the method of this invention can substantially reduce the peak principal stress at the hole edge (e.g., from 374MPa to 138MPa), and the fatigue notch coefficient can be reduced by more than 62%. This fundamentally solves the problems of easy secondary cracking of single crack-arresting holes and premature interface peeling of traditional plates, significantly extending the fatigue life of orthotropic steel bridge diaphragms.

[0090] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art can make various improvements and modifications without departing from the spirit and principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for synergistic crack-resistant reinforcement of orthotropic steel bridge diaphragms using ultrasonic impact and CFRP, characterized in that, Includes the following steps: Step 1: Drill a crack arresting hole at the tip of the fatigue crack in the diaphragm; Step 2: Perform ultrasonic impact treatment on the damaged area of ​​the diaphragm to induce local plastic deformation of the crack surface and achieve physical closure, while introducing residual compressive stress at the edge of the crack arrest hole; Step 3: In the area treated with ultrasonic impact, CFRP sheets are symmetrically pasted on both sides of the diaphragm.

2. The method for ultrasonic impact and CFRP synergistic crack-resistant reinforcement of orthotropic steel bridge diaphragms according to claim 1, characterized in that: Prior to the ultrasonic shock treatment, the process parameters for the ultrasonic shock treatment are determined through finite element simulation.

3. The method for synergistic crack-resistant reinforcement of orthotropic steel bridge diaphragms using ultrasonic impact and CFRP as described in claim 2, is characterized in that... The specific steps for determining process parameters using finite element simulation include: Establish a finite element model that includes the main body of the diaphragm, prefabricated cracks, and crack arresting holes; Simulation analysis was performed using different ultrasonic impact velocities. Residual stress distribution data were extracted, and the optimal impact velocity was determined based on the maximum surface compressive stress amplitude and the absence of tensile stress rebound across the entire thickness.

4. The method for synergistic crack-resistant reinforcement of orthotropic steel bridge diaphragms using ultrasonic impact and CFRP as described in claim 3, characterized in that: The different ultrasonic impact velocities are 2-5 m / s.

5. The method for synergistic crack-resistant reinforcement of orthotropic steel bridge diaphragms using ultrasonic impact and CFRP as described in claim 1, characterized in that: Before step one, there is also a step of placing strain gauges on the surface of the diaphragm to collect stress data before and after the repair.

6. The method for synergistic crack-resistant reinforcement of orthotropic steel bridge diaphragms using ultrasonic impact and CFRP as described in claim 5, is characterized in that... The strain gauges are arranged symmetrically on both sides at a certain distance from the edge of the crack arresting hole, along the horizontal extension of the pre-fabricated crack and the crack arresting hole.

7. The method for synergistic crack-resistant reinforcement of orthotropic steel bridge diaphragms using ultrasonic impact and CFRP as described in claim 1, characterized in that, The ultrasonic impact treatment includes pre-crack closing impact and crack arrest hole edge strengthening impact; the pre-crack closing impact continues until the crack opening is completely physically closed; the crack arrest hole edge strengthening impact is performed uniformly along the circumferential direction of the hole wall until a uniformly transitioning plastic deformation band is formed at the hole edge, and after impact, a grinding tool is used to smooth out the small flash.

8. The method for synergistic crack-resistant reinforcement of orthotropic steel bridge diaphragms using ultrasonic impact and CFRP as described in claim 1, characterized in that, The CFRP board is a single-layer high-strength, high-elasticity CFRP board with a thickness of 2-4mm.

9. The method for synergistic crack-resistant reinforcement of orthotropic steel bridge diaphragms using ultrasonic impact and CFRP as described in claim 1, characterized in that, When bonding the CFRP board, an epoxy structural adhesive is used, and the thickness of the adhesive layer is precisely controlled by uniformly incorporating micro steel balls.

10. The method for synergistic crack-resistant reinforcement of orthotropic steel bridge diaphragms using ultrasonic impact and CFRP as described in claim 1, characterized in that, Before pasting the CFRP board, a pretreatment step is also included, which involves grinding, removing rust, and solvent cleaning the predetermined pasting area of ​​the partition and the surface of the CFRP board.

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