Method and system for predicting corrosion fatigue life of steel box girder bridge

Abstract

The invention discloses a steel box girder bridge corrosion fatigue life prediction method and system, and relates to the technical field of steel structure bridge life prediction. The method comprises the following steps: establishing an erosion pit initiation time calculation model considering a cyclic load acceleration effect; a short crack propagation time calculation model is established, and the model represents the corrosion environment intensity based on actually measured corrosion current and is coupled with the influence of the stress level; a long crack propagation time calculation model is established, and the model represents the corrosion environment intensity based on actually measured corrosion current and is coupled with the influence of the stress level; and adding the corrosion pit initiation time, the short crack propagation time and the long crack propagation time to obtain the total corrosion fatigue life of the steel box girder bridge in the corrosion environment. The problem that the corrosion and fatigue synergistic effect is difficult to quantify in the prior art can be solved.

Description

Technical Field

[0001] This invention relates to the field of steel structure bridge life prediction technology, and in particular to a method and system for predicting the corrosion fatigue life of steel box girder bridges. Background Technology

[0002] Steel box girder bridges (referred to as "steel bridges") are widely used in harsh corrosive environments such as those spanning the sea and rivers due to their advantages of being lightweight, high-strength, and easy to construct. However, steel bridges exposed to the marine atmosphere for extended periods are prone to corrosion fatigue damage under the coupled effects of chloride ion erosion, acid rain corrosion, and cyclic traffic loads, significantly shortening their service life and even leading to catastrophic accidents (such as the collapse of the Yibin Xiaonanmen Jinsha River Bridge due to stress corrosion of its suspenders).

[0003] Corrosion fatigue of steel bridges is a complex process in which electrochemical corrosion and mechanical fatigue mutually promote each other: the corrosive medium destroys the passivation film on the steel surface, inducing pitting corrosion; the pits cause stress concentration, accelerating the initiation of fatigue cracks; and the propagation of fatigue cracks provides a penetration channel for the corrosive medium, especially at the crack tip, which intensifies local corrosion, leading to the accumulation of corrosion products and enhanced electrochemical reactions, further weakening the crack tip strength, forming a vicious cycle of mutually accelerating corrosion and fatigue, and significantly shortening the fatigue life of the structure.

[0004] Currently, fatigue life assessment of steel bridges mainly employs the SN curve method, fracture mechanics method, or probabilistic assessment methods based on reliability theory. Among these, the SN curve method relies on numerous targeted experiments, making it difficult to reflect the corrosion evolution process. While the fracture mechanics method can predict crack propagation life, it typically assumes the presence of an initial crack, neglecting the contribution of the initial stage of corrosion pit formation to the total life. More importantly, existing models generally fail to explicitly incorporate the corrosive environment as measurable electrochemical parameters during the crack propagation stage, making it difficult to effectively quantify the corrosion-fatigue coupling effect.

[0005] Although existing studies have explored corrosion fatigue behavior through experiments and numerical simulations, a unified computational theory covering the entire process from pit initiation to short crack propagation to long crack propagation has not yet been established. Current bridge design codes do not consider the corrosion-fatigue coupling effect, leading to an overestimation of fatigue life and insufficient structural safety. Therefore, there is an urgent need to integrate electrochemical corrosion and fracture mechanics theories to establish a method for predicting the corrosion fatigue life of steel bridges that considers both pit initiation and crack propagation stages. Summary of the Invention

[0006] The purpose of this invention is to provide a method for predicting the corrosion fatigue life of steel box girder bridges, so as to solve the problem that it is difficult to quantify the synergistic effect of corrosion and fatigue (i.e. corrosion-fatigue coupling effect) in the prior art.

[0007] To achieve this objective, this invention uses the measurable electrochemical parameter—corrosion current—as a key input to characterize the intensity of the corrosive environment, and couples it with mechanical parameters to construct a corrosion fatigue life prediction model that covers the entire process of pit initiation, short crack propagation, and long crack propagation, thereby improving the applicability and prediction accuracy of the corrosion fatigue life prediction method under different corrosive environments.

[0008] Specifically, the present invention adopts the following technical solution:

[0009] A method for predicting the corrosion fatigue life of a steel box girder bridge includes the following steps:

[0010] S1. Establish a calculation model for pit initiation time that considers the acceleration effect of cyclic loading;

[0011] S2. Establish a calculation model for short crack propagation time. The model is based on measured corrosion current to characterize the intensity of the corrosion environment and coupled with the influence of stress level. The starting length of the short crack is the critical length for the transformation of corrosion pit into crack (i.e., the initial crack length), and the ending length is the critical length for the transformation of short crack into long crack.

[0012] S3. Establish a calculation model for the propagation time of long cracks. The model is based on the measured corrosion current to characterize the intensity of the corrosion environment and coupled with the influence of stress level. The starting length of the long crack is the critical length for the transformation from a short crack to a long crack, and the termination length is the critical length (i.e., the critical fracture length) when the long crack propagates to failure.

[0013] S4. Add the pit initiation time, short crack propagation time, and long crack propagation time to obtain the total corrosion fatigue life of the steel box girder bridge under corrosive environment.

[0014] Furthermore, the calculation model for the pit initiation time is based on Faraday's law and combines normalized stress amplitude to characterize the accelerating effect of cyclic loading on the corrosion rate.

[0015] Furthermore, the short crack propagation time calculation model is based on Paris's law, and the parameter obtained by dimensionless processing of the corrosion current is used as a measurable input quantity, which, together with the stress intensity factor range, determines the short crack propagation rate.

[0016] Furthermore, the long crack propagation time calculation model is based on Paris's law, and the parameter obtained by dimensionless processing of corrosion current is used as a measurable input quantity, which, together with the stress intensity factor range, determines the long crack propagation rate.

[0017] The dimensionless processing of the corrosion current includes dividing the corrosion current by the unit corrosion current to make it dimensionless.

[0018] Furthermore, the calculation model for the pit formation time is as follows:

[0019]

[0020] In the formula, Indicates the time period of pit formation; Indicates the depth of corrosion pits; Indicates the number of electrons released; Denotes Faraday's constant; Indicates the density of steel; This represents the acceleration factor of cyclic loading on the corrosion rate; Indicates the normalized stress amplitude. , Indicates stress amplitude, Indicates the unit stress amplitude; Indicates the molar mass of iron; The current coefficient representing corrosion; Indicates activation energy; Represents the universal gas constant; This indicates absolute temperature.

[0021] Furthermore, the calculation model for the short crack propagation time is as follows:

[0022]

[0023] In the formula, Indicates the time of the short crack propagation stage; This indicates the critical length at which a short crack transforms into a long crack; This represents the critical length at which a corrosion pit transforms into a crack (i.e., the initial crack length). Indicates the frequency at which fatigue loads are applied; Indicates the depth of corrosion pits; This represents the acceleration factor of cyclic loading on the corrosion rate; This represents the sensitivity coefficient of stress amplitude to corrosion acceleration factors; Indicates the normalized stress amplitude. , Indicates stress amplitude, Indicates the unit stress amplitude; Indicates corrosion current; Indicates the unit corrosion current; This represents the fatigue coefficient of short cracks; Indicates the range of stress intensity factor; The fatigue index represents the fatigue index of short cracks.

[0024] Furthermore, the calculation model for the long crack propagation time is as follows:

[0025]

[0026] In the formula, Indicates the time of long crack propagation stage; This represents the critical length at which a long crack propagates to failure (i.e., the critical fracture length). This indicates the critical length at which a short crack transforms into a long crack; Indicates the frequency at which fatigue loads are applied; Indicates the depth of corrosion pits; This represents the acceleration factor of cyclic loading on the corrosion rate; This represents the sensitivity coefficient of stress amplitude to corrosion acceleration factors; Indicates the normalized stress amplitude. , Indicates stress amplitude, Indicates the unit stress amplitude; Indicates corrosion current; Indicates the unit corrosion current; This represents the fatigue coefficient of a long crack; Indicates the range of stress intensity factor; The fatigue index represents the fatigue index of long cracks.

[0027] Furthermore, the total corrosion fatigue life is calculated using the following formula:

[0028]

[0029] In the formula, Indicates the total corrosion fatigue life; This indicates the time of pit formation (i.e., pit formation time). This indicates the time of the short crack propagation stage (i.e., the short crack propagation lifetime). This indicates the time of the long crack propagation stage (i.e., the long crack propagation life).

[0030] Step S4 includes: adding the calculation expressions for pit initiation time, short crack propagation life (time), and long crack propagation life (time) to obtain the prediction model for the total corrosion fatigue life of the steel box girder bridge as follows:

[0031]

[0032] Among them, parameters This indicates the depth of the pit, which is the vertical distance from the original surface of the steel to the bottom of the pit.

[0033] Another object of the present invention is to provide a corrosion fatigue life prediction system for steel box girder bridges, which includes a storage unit and a computing unit. The storage unit stores a corrosion fatigue life prediction program for steel box girder bridges, and the computing unit runs the corrosion fatigue life prediction program for steel box girder bridges to perform the steps in the corrosion fatigue life prediction method for steel box girder bridges as described above.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] This invention divides the fatigue life of steel box girder bridges in corrosive environments into three continuous stages with different physical mechanisms: pit initiation, short crack propagation, and long crack propagation. Corresponding computational models are established for each stage, overcoming the limitations of traditional fatigue life assessment methods that either ignore the initial corrosion damage evolution process or uniformly treat crack propagation behavior. This three-stage architecture more realistically reflects the complete damage evolution path of corrosion fatigue from surface pitting to structural failure, significantly improving the physical completeness and engineering applicability of life prediction.

[0036] In the pit formation stage, this invention establishes a calculation model that considers the accelerating effect of cyclic loading, which can effectively characterize the promoting effect of alternating stress on the formation and development rate of corrosion pits. It solves the problems that the existing SN curve method is difficult to reflect the dynamic evolution of corrosion and the unreasonableness of the fracture mechanics method directly assuming the existence of initial cracks.

[0037] During the crack propagation stage, this invention clearly distinguishes between the propagation behavior of short and long cracks: For short cracks (from the initial crack length to the critical length for the short-to-long crack transition), a propagation time calculation model considering the coupling effect of the corrosive environment and stress level is established, fully reflecting the nonlinear interaction between the corrosive medium and fatigue load at the microcrack scale; while for the long crack stage, an independent propagation time calculation model is established based on the coupling mechanism of the corrosive environment and stress level, and according to the principles of fracture mechanics, until the crack reaches the critical fracture length. This segmented modeling strategy avoids the prediction bias caused by treating the short / long crack propagation mechanisms interchangeably, and is particularly suitable for engineering practice where there are significant differences in the propagation rates of short and long cracks under corrosive environments.

[0038] Ultimately, by directly adding the three-stage lifespans to obtain the total corrosion fatigue life, this invention provides a life prediction method with a clear structure, analytical capability, and embeddability into engineering evaluation systems. Compared to the simplified methods in current bridge design codes that do not consider the corrosion-fatigue coupling effect, this scheme can more accurately assess the actual service life of steel box girder bridges in harsh environments such as the marine atmosphere, effectively avoiding safety risks caused by overestimating lifespan, and providing reliable theoretical support for durability design and scientific maintenance decisions for major steel bridge projects such as those spanning seas and rivers.

[0039] Furthermore, since the corrosion current can be directly obtained through electrochemical testing or converted from the corrosion rate, this invention can introduce the changes in the actual corrosion environment into the crack propagation life calculation in the form of quantifiable input parameters, reducing the uncertainty caused by the traditional method relying solely on empirical coefficient fitting, and improving the applicability and stability of corrosion fatigue life prediction under different corrosion environments and service conditions. Attached Figure Description

[0040] Figure 1 This is a comparison chart of the corrosion fatigue life calculation model and the fitted SN curve of the salt spray corrosion-fatigue test in the example;

[0041] Figure 2 This is a standard cross-sectional view of the main beam of the steel box girder of the cable-stayed bridge in the embodiment;

[0042] Figure 3 This is a schematic diagram showing the details of the top plate-U rib weld construction in the embodiment;

[0043] Figure 4 This is a comparison chart of the corrosion fatigue life model in the example with the GB 50017-2017 and IIW (125) SN curves;

[0044] Figure 5 This is a schematic diagram illustrating the relationship between critical crack size and fatigue life in the embodiment.

[0045] Figure 6 This is a schematic diagram illustrating the relationship between the critical stress intensity factor value and fatigue life in the embodiment;

[0046] Figure 7 This is a schematic diagram illustrating the relationship between the shape of the pits and the corrosion fatigue life in the embodiments. Detailed Implementation

[0047] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0048] During service, steel bridges are often subjected to the coupled effects of alternating stress and corrosive environment, and their fatigue performance degradation mechanism is corrosion fatigue.

[0049] Corrosion fatigue in steel bridges refers to the phenomenon where the material properties of steel gradually degrade and eventually fail under the combined action of alternating stress (fatigue load) and a corrosive environment. This failure mechanism is more complex and severe than simple fatigue or static corrosion.

[0050] Corrosion fatigue in steel bridges can generally be divided into two key stages: the initial damage stage (crack initiation stage) and the crack propagation stage. In the early stages of the initial damage stage, the electrochemical effects of corrosive media (such as moisture and salt spray) lead to changes in the microstructure of the steel surface and crack surfaces, including increased hydrogen ion concentration, changes in chemical elemental composition, and alterations in grain size. This damages the protective layer on the steel surface (such as oxide films or coatings), ultimately leading to the formation of corrosion defects (pitting corrosion). The electrochemical reaction formula for this corrosion process is as follows:

[0051] anode:

[0052]

[0053]

[0054] cathode:

[0055]

[0056] The products obtained from the reaction formulas (1) and (2) at the anode and It will react with the product obtained in the cathode reaction (3) The process involves bonding, and further complex electrochemical reactions occur on the surface of the steel to generate rust. The electrochemical reaction formula for rust formation is as follows:

[0057]

[0058]

[0059]

[0060]

[0061] A large amount of chloride ions in the corrosive medium in the corrosive environment ( Chloride ions accelerate the dissolution of the steel anolyte, leading to a faster corrosion reaction on the steel surface. Furthermore, chloride ions... It can penetrate the corrosion layer and react with the steel substrate, leading to localized acidification of the substrate and causing corrosion on the steel surface. concentration and The increased concentration further accelerates the dissolution of the steel anode. The electrochemical reaction equation for this part is as follows:

[0062]

[0063]

[0064]

[0065] Under cyclic loading, localized stress concentration occurs at the pitting defects, causing the pits to transform into microcracks. After crack formation, the corrosion fatigue of the steel bridge enters the crack propagation stage. At this point, the corrosive medium penetrates into the crack interior, weakening the material at the crack tip through electrochemical anodic dissolution. Simultaneously, hydrogen atoms are generated in the cathodic reaction, penetrating to the crack tip and triggering hydrogen embrittlement, which leads to a sharp decrease in the material's toughness. Furthermore, under continuous cyclic loading, fatigue cracks periodically open and close, further tearing the steel. When the crack propagates to a critical size, the remaining cross-section cannot withstand the load, causing the structure to suddenly fracture.

[0066] Therefore, accurately quantifying the entire crack propagation process under the interaction of corrosion and fatigue is the key to achieving reliable prediction of the corrosion fatigue life of steel bridges.

[0067] Based on the above-mentioned corrosion fatigue mechanism, this embodiment provides a staged method for predicting the corrosion fatigue life of steel box girder bridges, which includes the following steps:

[0068] S1. Based on Faraday's law and combined with the normalized stress amplitude to characterize the acceleration effect of cyclic loading, a calculation model for pit initiation time is established.

[0069] S2. Based on Paris's law, a calculation model for the propagation time of short cracks is established by introducing the range of corrosion current and stress intensity factor, taking into account the coupling effect of corrosion environment and stress level. The starting length of the short crack is the critical length for the transformation of corrosion pit into crack (i.e., the initial crack length), and the ending length is the critical length for the transformation of short crack into long crack.

[0070] S3. Based on Paris's law, a calculation model for the propagation time of long cracks is established by introducing the range of corrosion current and stress intensity factor, taking into account the coupling effect of corrosion environment and stress level. The starting length of the long crack is the critical length for the transformation from short crack to long crack, and the termination length is the critical length (i.e., the critical fracture length) when the long crack propagates to failure.

[0071] S4. Add the pit initiation time, short crack propagation time, and long crack propagation time to obtain the total corrosion fatigue life of the steel box girder bridge under corrosive environment.

[0072] Based on the above steps, the total corrosion fatigue life of a steel box girder bridge can be expressed as the sum of the pit initiation time, the short crack propagation time, and the long crack propagation time, i.e.:

[0073]

[0074] In the formula: This indicates the total corrosion fatigue life of the steel bridge; Indicates the time period of pit formation; Indicates the time of the short crack propagation stage; This indicates the time elapsed during the long crack propagation stage.

[0075] The specific calculation methods for each component are as follows.

[0076] Step S1 (the pit formation stage) specifically includes:

[0077] According to Faraday's law, the rate of increase in the volume of an erosion pit can be expressed as:

[0078]

[0079] In the formula: The volume of the pit is represented by t; time is represented by t. To represent the molar mass of iron, take... kg / mol; The current coefficient representing corrosion; Indicates the number of electrons released; Denotes Faraday's constant; Indicates the density of steel; Indicates activation energy; Represents the universal gas constant; This indicates absolute temperature.

[0080] Generally, the shapes of corrosion pits include cones, semi-ellipsoids, hemispheres, and cylinders. In this embodiment, to simplify calculations, we assume that all corrosion pits during the corrosion development process are hemispherical. The volume formula for a hemispherical corrosion pit is:

[0081]

[0082] In the formula: This indicates the depth of the pit (i.e., the radius of the pit on a hemispherical surface).

[0083] Substituting formula (13) into formula (12), considering the accelerating effect of cyclic loading on the corrosion rate, and simplifying it, the pit initiation time can be expressed by the following formula:

[0084]

[0085] In the formula: The factor representing the acceleration of corrosion rate by cyclic loading is taken in this embodiment. ; This represents the normalized stress amplitude to ensure that the exponential term is dimensionless. ; Indicates stress amplitude; Represents the unit stress amplitude, taken as MPa.

[0086] Determination of the critical size of a pit: When the stress intensity factor at the bottom of the pit reaches a certain value... Reaching the critical stress intensity factor At this point, the pitting transforms into a crack. According to the stress intensity factor criterion, the following conditions must be met for a pit to transform into a crack:

[0087]

[0088] In the formula: Indicates the stress intensity factor at the bottom of the pit; This represents the critical stress intensity factor. It can be represented as:

[0089]

[0090] In the formula: This represents the stress concentration factor of the pitting. The value of is related to the shape of the pit. It is related to the ratio of the width to the length of the pit. It can be represented by the following expression:

[0091]

[0092] Since the shape of the erosion pit is assumed to be hemispherical in this embodiment, the shape ratio of the erosion pit (the ratio of width to length) is... According to formula (17), the result can be obtained. To simplify the calculation, the stress concentration factor of the erosion pit can be approximated. According to the stress intensity factor criterion and formula (15), let At that time, the critical size at which corrosion pits transform into cracks. This can be expressed by the following formula:

[0093]

[0094] Step S2 (short crack propagation stage) specifically includes:

[0095] To account for the intensity of environmental corrosion, the corrosion current is associated. Considering the nonlinear amplification effect of stress on corrosion rate, a stress amplitude correlation exponent is introduced. According to Paris's law, the equation for the short crack propagation rate can be expressed by the following formula:

[0096]

[0097] In the formula: The sensitivity coefficient of stress amplitude to corrosion acceleration factor is used to quantify the nonlinear amplification effect of stress amplitude on corrosion-fatigue coupling effect. In this embodiment, k=0.1. Indicates corrosion current; Indicates the unit corrosion current; This represents the fatigue coefficient of short cracks; This indicates the range of stress intensity factor, according to the stress intensity factor handbook. Considering multiaxial stress correction, take ; The fatigue index indicates the number of short cracks; This indicates the lifespan of the short crack propagation stage.

[0098] The expression for the time of the short crack propagation stage can be given as:

[0099]

[0100] In the formula: Indicates the frequency at which fatigue loads are applied; This represents the critical size at which a short crack transforms into a long crack, taken as... mm.

[0101] Step S3 (long crack propagation stage) includes:

[0102] Assuming that the corrosion current and stress amplitude remain unchanged throughout the crack propagation stage, the crack propagation rate equation during the long crack propagation stage can be expressed as:

[0103]

[0104] In the formula: This represents the fatigue coefficient of a long crack; The fatigue index indicates the length of a crack. This indicates the lifespan of the long crack propagation stage.

[0105] The propagation time of a long crack can be expressed as:

[0106]

[0107] In the formula: This represents the critical dimension at which a long crack propagates to failure, and it is defined as half the thickness of the structural member.

[0108] Step S4 (Corrosion Fatigue Life Calculation Model) specifically includes:

[0109] Substituting the times of each corrosion fatigue stage obtained from formulas (14), (20), and (22) into formula (11), the corrosion fatigue life calculation model can be expressed as:

[0110]

[0111] Based on the SN curve, considering the influence of the corrosive environment, the number of corrosion fatigue cycles for the steel bridge is... The expression can be represented as:

[0112]

[0113] In the formula: This indicates the number of fatigue load cycles (i.e., fatigue life) corresponding to the fatigue failure of the steel bridge. Parameters indicating the corrosion fatigue performance of steel. This represents the material fatigue parameter; in this embodiment, we take... ; This indicates the stress amplitude.

[0114] Equation (24) represents the stress amplitude. and fatigue life The relationship between them is a power function, and after logarithmic transformation, it becomes linear in a double logarithmic coordinate system.

[0115]

[0116] In the formula: The opposite of the slope of a straight line; Represents the intercept of the line. .

[0117] This embodiment introduces a dimensionless influence term of corrosion current when calculating crack propagation life, so that the intensity of the corrosive environment participates in the calculation of crack propagation rate and life in the form of a measurable input quantity, thereby improving the applicability of the corrosion fatigue life prediction method.

[0118] Since corrosion current can be obtained through electrochemical testing or corrosion rate conversion, the above method can reflect changes in the corrosion environment as an input quantity in the crack propagation life calculation, reduce the uncertainty caused by relying solely on empirical coefficient fitting, and improve the applicability of corrosion fatigue life prediction under different corrosion environments and service conditions.

[0119] To verify the accuracy of the corrosion fatigue life calculation model described in this embodiment, its prediction results were compared with salt spray corrosion-fatigue test data.

[0120] This experiment simulated an acid rain corrosion environment. The corrosive solution was prepared by mixing sulfuric acid, nitric acid, hydrochloric acid, and sodium hydroxide in specific proportions, with the pH controlled at 4. The concentrations of sulfate, nitrate, and chloride ions in the solution were ensured to meet typical industrial atmospheric corrosion conditions. The test specimen was an equal-angle steel structure. Under cyclic loading at a frequency of 4 Hz, periodic salt spraying was applied simultaneously: each cycle lasted 2 hours, with continuous spraying for 24 minutes followed by a 96-minute pause, until the specimen fractured.

[0121] The material parameters, corrosion environment conditions, and loading regime from the above experiments were substituted into the model of this embodiment for calculation. The comparison results are as follows: Figure 1 As shown in Table 1.

[0122]

[0123] Figure 1 A comparison is made between the corrosion fatigue life calculation model established for this embodiment and the SN curve fitted by the salt spray corrosion-fatigue test. Figure 1The comparison results show that the two SN curves exhibit consistent trends and are quite compatible. Furthermore, the experimental curve shows a smaller error compared to the model curve at lower stress amplitudes than at higher stress amplitudes. This reflects the good accuracy of the model established in this embodiment. Table 1 compares the model and experimental SN expressions with the equivalent fatigue strength after 2 million cycles. Specifically, according to the results in Table 1, the equivalent fatigue strength after 2 million cycles of the corrosion fatigue life calculation model and the salt spray corrosion-fatigue test are 100 MPa and 107 MPa, respectively, with the model calculation value being 7% higher than the experimental value. The error is within 10%, further demonstrating that the steel bridge corrosion fatigue life calculation model established in this embodiment can accurately calculate the fatigue life of the steel bridge under the coupled effects of corrosion and fatigue.

[0124] To verify the applicability of the method in this embodiment in actual engineering, it was applied to the prediction of corrosion fatigue life of a steel box girder of a long-span cable-stayed bridge across the sea.

[0125] This cross-sea bridge is a long-span cable-stayed bridge, its main structural feature being the use of steel box girders as the main beams. The entire bridge consists of seven spans, with specific span arrangements of 98 meters, 120 meters, 132 meters, 938 meters, 132 meters, 120 meters, and 98 meters. The height and width of the steel box girders are 45 meters and 47 meters, respectively. To meet traffic demands, the main bridge is designed with eight lanes in both directions. Figure 2 The standard cross-sectional view of the main girder of the cable-stayed bridge is shown. The steel box girder bridge deck adopts the classic design of orthotropic steel bridge deck, using Q345C steel with a strength grade of 14mm, a top plate thickness of 8mm, a diaphragm thickness of 8mm, a U-rib height of 280mm, a thickness of 8mm, and a center-to-center distance of 600mm. Details of the U-rib-top plate welded structure are shown below. Figure 3 As shown. Given the structural characteristics and geographical location of this long-span cable-stayed bridge, characterized by its large span, wide deck, and high traffic volume, the pavement and waterproofing layers are prone to cracking and damage under the constant pressure of heavy vehicles. Furthermore, because the bridge spans the sea, high temperatures allow chloride-containing water vapor to directly contact the orthotropic steel deck, leading to corrosion. In addition, during icy and snowy weather, de-icing salt is applied to ensure smooth traffic flow; the harmful chloride ions in this salt can seep into the deck through cracks, further exacerbating corrosion fatigue damage. Therefore, corrosion fatigue is a significant problem for this bridge.

[0126] Figure 2In the middle section: A1 and A2 are both road crowns; B1 and B2 are both light lanes 1; C1 and C2 are both light lanes 2; D1 and D2 are both heavy lanes 3; E1 and E2 are both heavy lanes 4; F1 and F2 are both emergency lanes. a1 and a2 are both 1750mm; b1 and b2 are both 4×3750=15000mm; c1 and c2 are both 3250mm; d1 and d2 are both 500mm; e1 and e2 are both 3500mm; f1 and f2 are both 18500 / 2=9250mm; g1 and g2 are both 11815mm; h1 and h2 are both 2935mm; j1 and j2 are both 48000 / 2=24000mm; k is 4500mm.

[0127] Figure 3 In the diagram: m is 1000mm; n is 350mm; p is 300mm; q is 350mm; s is 8mm; r is 40mm (radius); u is 106mm; v is 14mm; w is 280mm; x is 65mm; y is 280mm; i1 is 12.7mm; i2 is 0.5mm; i3 is 14mm; i4 is ≥0.8t, where t is the thickness of the U-rib (8mm), i4 is taken as 6.4mm; i5 is 8mm; i6 is 45° (angle).

[0128] Based on the above engineering conditions, the parameters required for the corrosion fatigue life calculation model (including key parameters for pit initiation and crack propagation stages (short crack propagation and long crack propagation stages)) are determined, as detailed in Table 2 below.

[0129]

[0130] Specifically, the parameter values ​​are as follows:

[0131] molar mass of iron The value is 5.6 × 10 -2 kg / mol;

[0132] Number of electrons released The value is 2;

[0133] Faraday constant The value is 9.65 × 10 4 C / mol;

[0134] Material density The value is 7850 kg / m³. 3 ;

[0135] gas constant The value is 8.314 (J / mol)·K;

[0136] Corrosion current coefficient The value is 1.0 × 10 -7C / s;

[0137] absolute temperature The value is 293 K;

[0138] activation energy The value is 1.55 × 10 4 J / mol;

[0139] Critical stress intensity factor The value is 2 MPa·m 0.5 ;

[0140] Short crack fatigue coefficient The value is 9.38 × 10 -13 ;

[0141] Long crack fatigue coefficient The value is 2.7 × 10. -11 ;

[0142] Long crack fatigue index The value is 2.88;

[0143] Short crack fatigue index The value is 3.

[0144] In this embodiment, the fatigue load input uses a constant amplitude stress amplitude and a loading frequency; for example, the stress amplitude is taken as... =194MPa, frequency =3Hz.

[0145] Substitute the above parameter values ​​into formula (23) to calculate the corrosion fatigue life of the welded joint of the U-rib-top plate of the orthotropic steel bridge deck. Then, plot the SN curve according to formulas (24) and (25). Figure 4The corrosion fatigue life calculation model is shown, and the SN curves provided by the Chinese Steel Structure Design Code GB50017-2017 and the International Institute of Welding (IIW) (125) are compared. The slopes in commonly used fatigue codes are generally fixed. IIW (125) uses m=3 for both welded components and base metal, while GB50017-2017 specifies m=4 for non-welded structures and m=3 for welded structures. Therefore, in this embodiment, the slope of the SN curves in both GB50017-2017 and IIW (125) is set to 3. It is worth noting that the expressions for the SN curves in commonly used fatigue codes are all for fatigue stress amplitude under cyclic symmetrical loading, which is an ideal state of cyclic loading. However, the cyclic loading of actual bridge structures is often asymmetrical. Therefore, the SN curve expressions in current codes cannot be directly used to calculate the fatigue life of the structure; it is necessary to convert the average stress amplitude under symmetrical loading to the equivalent stress amplitude under asymmetrical cyclic loading for calculation. Currently, the conversion between SN curves under symmetric and asymmetric cyclic loading is usually achieved using the Goodman equation, as shown in the following expression:

[0146]

[0147] In the formula: This represents the stress amplitude under asymmetric cyclic loading; This represents the stress range under symmetrical cyclic loading, where and Their fatigue life is the same; This represents the average stress under asymmetric cyclic loading. This represents the ultimate strength of Q345C steel; in this embodiment, it is taken as... MPa.

[0148] Transforming formula (26) yields formula (27), whose expression is as follows:

[0149]

[0150] Substituting the relationship (27) between asymmetric and symmetric cyclic loads into the logarithmic expression (25) of the SN curve, we can obtain the equivalent SN curve expression under asymmetric cyclic load:

[0151]

[0152]

[0153] like Figure 4As shown, the SN curves of the corrosion fatigue life calculation model, GB50017-2017, and IIW (125) specifications show that the SN curve specified in the Chinese steel structure design code GB50017-2017 is below the SN curve specified in the International Institute of Welding (IIW) (125). This indicates that the GB50017-2017 specification is more conservative than the IIW (125) specification when calculating fatigue strength. The SN curve of the corrosion fatigue life calculation model established in this embodiment is significantly lower than the SN curves of the two specifications mentioned above. This indicates that the fatigue strength of the steel bridge structure under the coupled effect of corrosion and fatigue is significantly lower than that under the effect of a single fatigue factor. This shows that corrosion significantly reduces the fatigue resistance of the steel bridge. Therefore, during the service of the steel bridge, it is necessary to regularly check the corrosion status of the service components, especially the key nodes bearing fatigue loads. Once corrosion occurs, appropriate anti-corrosion treatment should be carried out in a timely manner to avoid the coupling effect of corrosion and fatigue. Table 3 shows the SN curve expressions of the corrosion fatigue life calculation model and the two specifications, as well as the equivalent fatigue strength after 2 million cycles. In terms of specific numerical values, considering the coupling effect of corrosion and fatigue, the equivalent fatigue strength after 2 million cycles according to the IIW(125) standard and the GB50017-2017 standard are 100.00 MPa, 137.40 MPa, and 123.03 MPa, respectively. The equivalent fatigue strength after 2 million cycles considering the coupling effect of corrosion and fatigue is reduced by 27.22% and 18.72% compared to the IIW(125) standard and the GB50017-2017 standard, respectively.

[0154] To further clarify the sensitivity of key parameters in the model of this embodiment to corrosion fatigue life, a systematic analysis was conducted on factors such as critical crack size, critical stress intensity factor, and initial pit shape.

[0155] Critical crack size (primarily referring to length) is one of the key parameters affecting crack life. Critical crack sizes for short and long cracks. It is a random variable with a mean of 1 mm. Because the critical crack size varies, it can introduce some error into the fatigue life calculation. To investigate the influence of the critical size of short and long cracks on fatigue life, while keeping other parameters constant, we take the critical size at which a short crack transforms into a long crack. Fatigue life analysis was performed for crack sizes of 0.2 mm, 0.6 mm, 1.0 mm, 1.4 mm, and 1.8 mm. Critical crack size. Effects on fatigue life, such as Figure 5 As shown.

[0156] Figure 5 The critical crack size parameters are shown. The relationship with fatigue life. (From...) Figure 5It can be seen that when the critical crack size is less than 1.0 mm, the fatigue life increases rapidly with the increase of the critical crack size. When the critical crack size is greater than 1.0 mm, the fatigue life increases gradually and slowly with the increase of the critical crack size. A nonlinear fitting was performed on the fatigue life cycle count corresponding to different critical crack sizes, and the fitting curve is a curve with a goodness of fit R0. 2 The curve is a power function of 0.998. Therefore, the fatigue life cycle count initially increases rapidly with increasing critical crack size, then increases slowly, and finally gradually stabilizes. Specifically, the critical size at which a short crack transforms into a long crack... The fatigue life increased by 5.03%, 6.72%, 7.55%, and 8.11% respectively compared to the critical crack size of 0.2 mm when the critical crack size was 0.6 mm, 1.0 mm, 1.4 mm, and 1.8 mm. This phenomenon occurs because when the critical crack size is 0.2 mm... As the critical crack size increases, the lifespan of the short crack propagation stage increases accordingly, while the lifespan of the long crack propagation stage decreases accordingly. Therefore, the total lifespan increases with the critical crack size. The increase was slight, but the overall change was small.

[0157] The critical stress intensity factor is one of the key parameters affecting service life. The fatigue crack critical stress intensity factor is an important indicator of the fatigue resistance of steel structures. When the applied stress intensity factor exceeds the fatigue crack critical stress intensity factor, the crack will continue to propagate until the structure suddenly fractures. The fatigue crack critical stress intensity factor values ​​of steels with different microstructures under different stress ratios were experimentally determined. The results show that the critical stress intensity factor value of Q345C steel is approximately 2 MPa·m. 0.5 ~8MPa·m 0.5 To investigate the effect of the critical stress intensity factor on fatigue life under corrosion-fatigue coupling, other parameters of the steel box girder in the above example were kept constant, and the critical stress intensity factor values ​​were taken as follows. 2MPa·m 0.5 4MPa·m 0.5 6MPa·m 0.5 and 8MPa·m 0.5 Comparative analysis was conducted. Critical stress intensity factor values. Effects on fatigue life, such as Figure 6 As shown.

[0158] Figure 6 The critical stress intensity factor value is shown. The relationship with fatigue life. (From...) Figure 6 It can be seen that fatigue life first decreases and then increases with the increase of the critical stress intensity factor. Specifically, when the critical stress intensity factor is less than 4 MPa·m... 0.5At this point, fatigue life decreases as the critical stress intensity factor increases. The critical stress intensity factor increases from 2 MPa·m. 0.5 Increased to 4 MPa·m 0.5 At that time, its corresponding fatigue life decreased by 48.42%. When the critical stress intensity factor value is greater than 4 MPa·m 0.5 At this point, fatigue life increases rapidly with increasing critical stress intensity factor. When the critical stress intensity factor increases from 4 MPa·m... 0.5 Increased to 6 MPa·m 0.5 At that time, its corresponding fatigue life increased by 55.41%. And when the critical stress intensity factor value increased from 6 MPa·m... 0.5 Increased to 8 MPa·m 0.5 At a lower critical stress intensity factor value, the corresponding fatigue life increased by 281.33%. This trend occurs because, at lower critical stress intensity factor values, the reduction in short crack propagation time dominates the change in total life. Although the pit initiation time increases, the increase is insufficient to offset the reduction in short crack propagation time, thus the total life decreases. At higher critical stress intensity factor values, the exponential increase in pit initiation time becomes the dominant factor. Although the short crack propagation time continues to decrease, its impact is significantly weakened, thus the total life rebounds. Therefore, this leads to a trend of first decreasing and then increasing fatigue life. It is noteworthy that this trend of first decreasing and then increasing corrosion fatigue life differs significantly from the trend of fatigue life under single cyclic loading. Under single cyclic loading, the higher the critical stress intensity factor value, the slower the crack propagation rate, and the longer the crack failure time, resulting in an increase in the fatigue life of the steel. This phenomenon reveals the complex interaction between pit initiation and crack propagation processes in corrosion fatigue.

[0159] The shape of corrosion pits is one of the key parameters affecting service life. Corrosion is a process with randomness and complexity. To simplify calculations, this embodiment assumes that all corrosion pits in the corrosion development process are hemispherical. Generally, the shapes of corrosion pits include cones, semi-ellipsoids, hemispheres, and cylinders. Different pit shapes lead to different corrosion initiation times, ultimately affecting the corrosion fatigue life of the steel structure. To analyze the impact of different pit shapes on the fatigue life of steel bridges, this embodiment selects cone-shaped, semi-ellipsoidal, hemisphere-shaped, and cylindrical corrosion pits for comparative analysis. The formulas for calculating the volume of cone-shaped, semi-ellipsoidal, and cylindrical corrosion pits are as follows.

[0160] Formula for calculating the volume of a semi-ellipsoidal pit:

[0161]

[0162] In the formula: This indicates the shape ratio of the erosion pit (the ratio of width to depth). Indicates the depth of the pit.

[0163] Formula for calculating the volume of a conical pit:

[0164]

[0165] Formula for calculating the volume of a cylindrical pit:

[0166]

[0167] Among them, the diameter-to-depth ratio of the corrosion pits in the corroded steel is mainly distributed between 1 and 10. Conical pits are relatively deep and narrow, while cylindrical pits are relatively wide and narrow. Therefore, in this embodiment, the shape ratios of cones, semi-ellipsoids, and cylinders are taken as 1.0, 3.0, and 6.0, respectively, for research. Substituting the pit volume formulas (29), (30), and (31) for cones, semi-ellipsoids, and cylinders into formula (12) respectively, the corrosion initiation time can be obtained. Then, the corrosion fatigue life of different pit shapes can be calculated according to the corrosion fatigue life model, such as Figure 7 As shown.

[0168] Figure 7 This demonstrates the relationship between different pit shapes and corrosion fatigue life. Figure 7 It is known that conical pits have the longest corrosion fatigue life, followed by hemispherical and semi-ellipsoidal pits, while cylindrical pits have the shortest. Specifically, the fatigue lives of semi-ellipsoidal and hemispherical pits are 16.67% and 38.14% shorter than those of hemispherical pits, respectively, while the fatigue life of conical pits is 56.17% longer than that of hemispherical pits. This phenomenon occurs because the shape of the pit directly affects the stress concentration factor and the critical pit size, thus determining the competition between crack initiation and propagation. Deep and narrow pits (such as conical pits) significantly improve fatigue life by delaying crack propagation initiation, while wide and shallow pits (such as cylindrical pits) cause a sharp drop in life due to accelerated propagation. This trend is consistent with the physical mechanism of corrosion fatigue, indicating the crucial influence of pit shape on the fatigue life of steel bridges.

[0169] The method described in this embodiment successfully constructed a corrosion fatigue life prediction model for steel box girder bridges, covering the entire process of pit initiation, short crack propagation, and long crack propagation. The model was validated by comparison with salt spray corrosion-fatigue test data, showing an equivalent fatigue strength prediction error of only 7% under 2 million cycles, indicating good accuracy. Further analysis revealed that under the coupled effects of corrosion and fatigue, the fatigue strength of the structure was significantly lower than the traditional assessment results that did not consider corrosion effects (a decrease of 18.72%–27.22%), highlighting the risk that existing simplified methods may overestimate the service life of the structure.

[0170] Furthermore, parameter sensitivity analysis revealed that the critical crack size, material fracture toughness (characterized by the critical stress intensity factor), and initial pit geometry have a significant impact on corrosion fatigue life. For example, when the critical short crack size increases from 0.2 mm to 1.8 mm, the predicted fatigue life increases by approximately 8.11%; the critical stress intensity factor increases from 2 MPa·m 0.5 Increased to 8 MPa·m 0.5 At that time, the corrosion fatigue life showed a non-monotonic trend of first decreasing and then increasing; the fatigue life corresponding to deep and narrow corrosion pits (such as conical ones) was 56.17% higher than that of wide and shallow corrosion pits (such as cylindrical ones). These results indicate that it is crucial to reasonably consider the initial morphology of corrosion damage and the fracture characteristics of materials to improve the accuracy of fatigue life prediction.

[0171] In summary, the method provided in this embodiment can effectively reflect the fatigue performance degradation law of steel bridges under the combined effect of corrosion and cyclic loading, and provides reliable technical support for the durability design and scientific maintenance decision-making of steel box girder bridges in harsh environments such as sea crossings and river crossings.

[0172] This invention is not limited to the above embodiments. Those skilled in the art will understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of this invention. The scope of this invention is defined by the claims and their equivalents.

Claims

1. A method for predicting the corrosion fatigue life of a steel box girder bridge, characterized in that, Includes the following steps: S1. Establish a calculation model for pit initiation time that considers the acceleration effect of cyclic loading; S2. Establish a calculation model for short crack propagation time. The model is based on measured corrosion current to characterize the intensity of the corrosion environment and coupled with the influence of stress level. The starting length of the short crack is the critical length for the transformation of corrosion pit into crack, and the ending length is the critical length for the transformation of short crack into long crack. S3. Establish a calculation model for the propagation time of long cracks. The model is based on the measured corrosion current to characterize the intensity of the corrosion environment and coupled with the influence of stress level. The starting length of the long crack is the critical length for the transformation from a short crack to a long crack, and the termination length is the critical length for the long crack to propagate to failure. S4. Add the pit initiation time, short crack propagation time, and long crack propagation time to obtain the total corrosion fatigue life of the steel box girder bridge under corrosive environment.

2. The method for predicting the corrosion fatigue life of steel box girder bridges according to claim 1, characterized in that, The model for calculating the pit initiation time is based on Faraday's law and incorporates normalized stress amplitude to characterize the accelerating effect of cyclic loading on the corrosion rate.

3. The method for predicting the corrosion fatigue life of steel box girder bridges according to claim 1, characterized in that, The short crack propagation time calculation model is based on Paris's law, and uses the parameter obtained by dimensionless processing of corrosion current as a measurable input quantity, which together with the stress intensity factor range determines the short crack propagation rate.

4. The method for predicting the corrosion fatigue life of steel box girder bridges according to claim 1, characterized in that, The long crack propagation time calculation model is based on Paris's law, and uses the parameter obtained by dimensionless processing of corrosion current as a measurable input quantity, which together with the stress intensity factor range determines the long crack propagation rate.

5. The method for predicting the corrosion fatigue life of steel box girder bridges according to claim 1 or 2, characterized in that, The calculation model for the pit formation time is as follows: In the formula, Indicates the time period of pit formation; Indicates the depth of corrosion pits; Indicates the number of electrons released; Denotes Faraday's constant; Indicates the density of steel; This represents the acceleration factor of cyclic loading on the corrosion rate; Indicates the normalized stress amplitude; Indicates the molar mass of iron; The current coefficient representing corrosion; Indicates activation energy; Represents the universal gas constant; This indicates absolute temperature.

6. The method for predicting the corrosion fatigue life of steel box girder bridges according to claim 1 or 3, characterized in that, The calculation model for short crack propagation time is as follows: In the formula, Indicates the time of the short crack propagation stage; This indicates the critical length at which a short crack transforms into a long crack; This indicates the critical length at which corrosion pits transform into cracks; Indicates the frequency at which fatigue loads are applied; Indicates the depth of corrosion pits; This represents the acceleration factor of cyclic loading on the corrosion rate; This represents the sensitivity coefficient of stress amplitude to corrosion acceleration factors; Indicates the normalized stress amplitude; Indicates corrosion current; Indicates the unit corrosion current; This represents the fatigue coefficient of short cracks; Indicates the range of stress intensity factor; The fatigue index represents the fatigue index of short cracks.

7. The method for predicting the corrosion fatigue life of steel box girder bridges according to claim 1 or 4, characterized in that, The calculation model for the long crack propagation time is as follows: In the formula, Indicates the time of long crack propagation stage; This indicates the critical length at which a long crack propagates to failure. This indicates the critical length at which a short crack transforms into a long crack; Indicates the frequency at which fatigue loads are applied; Indicates the depth of corrosion pits; This represents the acceleration factor of cyclic loading on the corrosion rate; This represents the sensitivity coefficient of stress amplitude to corrosion acceleration factors; Indicates the normalized stress amplitude; Indicates corrosion current; Indicates the unit corrosion current; This represents the fatigue coefficient of a long crack; Indicates the range of stress intensity factor; The fatigue index represents the fatigue index of long cracks.

8. A corrosion fatigue life prediction system for steel box girder bridges, comprising a storage unit and a computing unit, wherein the storage unit stores a corrosion fatigue life prediction program for steel box girder bridges, and the corrosion fatigue life prediction program for steel box girder bridges is run by the computing unit to execute the steps in the corrosion fatigue life prediction method for steel box girder bridges according to any one of claims 1-7.

Images