A low-temperature fatigue crack propagation life prediction method and system based on a steel bridge deck

Abstract

The present application belongs to the technical field of industrial detection, and provides a low-temperature fatigue crack propagation life prediction method and system based on a steel bridge deck, comprising: judging whether the crack direction deflection risk assessment needs to be started through the analysis of the sharp drop of the plasticity of steel and the non-uniformity of the local stress gradient; if so, the crack direction deflection risk is assessed through the analysis of the grain orientation difference and the strengthening phase in the weld and HAZ area; if the risk is high, the stress gradient gentle area is determined through the weld toe stress gradient cloud chart, the resistance-gradient coupling equation is established according to the minimum resistance principle and combined with the stress gradient, and the crack deflection path is determined; the crack propagation path prediction accuracy and the fatigue life prediction evaluation accuracy are improved by dividing the crack deflection path according to the crack crossing area, combining the path proportion of each stress gradient area after the division and the stress gradient distribution, calculating the actual crack propagation rate, and correcting the life prediction output result.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of industrial detection, and in particular relates to a low-temperature fatigue crack propagation life prediction method and system based on a steel bridge deck. BACKGROUND

[0002] In a laboratory fatigue crack propagation test, the stress field of a standard sample (such as a CT sample) is a preset "uniform gradient distribution", and the crack propagation direction is fixedly constrained by the sample geometry and the load direction; however, in actual service of a steel bridge deck, a low-temperature environment can aggravate the non-uniformity of the stress gradient of local structural details, causing the crack propagation direction to deflect as the temperature decreases, and the existing life prediction model all assumes that the crack propagates in a preset direction (such as perpendicular to the load direction), which ultimately causes a large deviation between the predicted life and the actual life. For example, after the longitudinal rib and the top plate of the steel bridge deck are welded, there are micro defects such as "weld toe undercutting" and "unfused" at the root of the weld, and due to the superposition of the welding residual stress and the bridge deck bending stress, a "local high stress gradient zone" (the stress rapidly decreases from 300 MPa at the weld toe to 100 MPa in the base material zone, with a gradient difference of 200 MPa / mm) is formed. At room temperature: the steel has good plasticity, and after the crack is initiated from the weld toe defect, it will stably propagate along the "perpendicular to the principal stress direction" (i.e., the thickness direction of the bridge deck), which is consistent with the preset propagation direction of the laboratory model; however, at low temperature (such as -60°C): the plasticity of the steel decreases sharply, and the dislocation movement is blocked, making it difficult for the crack tip to propagate along the principal stress direction through "plastic blunting". At this time, the non-uniformity of the local stress gradient is amplified - if there are small "grain orientation differences" or "strengthening phase aggregation zones" near the weld toe, the crack will deviate from the principal stress direction and deflect towards the "subgrain boundary or weakened zone with a more gentle stress gradient" (such as propagating along the length direction of the longitudinal rib), and this deflection causes the crack to continuously cross "different stress gradient zones" during propagation, and the propagation rate (da / dN) fluctuates with the gradient, while the model calculates the propagation rate da / dN based on the "uniform stress gradient", which is constant, ultimately causing a deviation between the predicted life and the actual life.

[0003] To this end, the application provides a low-temperature fatigue crack propagation life prediction method and system based on a steel bridge deck. SUMMARY

[0004] In order to make up for the deficiencies of the prior art and solve at least one technical problem raised in the background.

[0005] The technical scheme adopted by the application to solve the technical problems is: a low-temperature fatigue crack propagation life prediction method based on a steel bridge deck, comprising the following steps:

[0006] Step S10: through the correlation analysis of the sharp decrease in plasticity of the steel and the non-uniformity of the local stress gradient, it is determined whether the crack direction deflection risk assessment needs to be started.

[0007] Step S20: If necessary, the steel bridge crack direction deflection risk is evaluated by analyzing the grain orientation difference and the strengthening phase in the weld and HAZ area;

[0008] Step S30: If the steel bridge crack deflection risk is high, the stress gradient cloud chart of the weld toe obtained by the finite element simulation is used to determine the stress gradient gentle area, the resistance-gradient coupling equation is established according to the minimum resistance principle and the stress gradient, and the crack deflection angle is calculated, the crack path propagation length is predicted based on the crack deflection angle analysis, and the crack deflection path is determined.

[0009] Step S40: Combined with the finite element stress gradient, the stress gradient area of the crack crossing area determined according to the crack deflection path is divided, the actual propagation rate of the crack is calculated through the path proportion and the stress gradient distribution of each stress gradient area, and the life prediction output result of the original fatigue life prediction model is corrected according to the actual propagation rate of the crack.

[0010] As a further technical solution of the application, the analysis process of the sharp decrease of the plasticity of the steel material is:

[0011] Tensile specimens are drawn from the base material and the butt weld, and after heat preservation at each temperature node, the specimens are loaded to fracture, the elongation at break δ at each temperature is calculated, and the δ-temperature curve is fitted, and the critical temperature of the sharp decrease of the plasticity is determined in the temperature curve according to the preset elongation at break;

[0012] The process of the local stress gradient non-uniform analysis is:

[0013] The residual stress of the weld specimen is tested to obtain the longitudinal residual stress and the transverse residual stress, strain gauges are pasted at the longitudinal rib-top plate weld of the actual steel bridge, traffic loads are collected, and the bending stress is obtained through strain-stress conversion.

[0014] The weld model is established, the residual stress and the bending stress are applied, the stress distribution at the weld toe under different temperatures is extracted, and the stress gradient is calculated.

[0015] As a further technical solution of the application, the process of determining whether the crack direction deflection risk evaluation needs to be started is:

[0016] At the critical temperature of the sharp decrease of the plasticity, the δ drop and the stress gradient increase are obtained;

[0017] If the δ drop and the stress gradient increase are greater than or equal to the preset threshold value, it indicates that the stress gradient non-uniformity is significant, and the crack direction deflection risk evaluation needs to be started.

[0018] As a further technical solution of the present application, the process of analyzing the grain orientation difference and the strengthening phase in the weld and HAZ area is:

[0019] Take the weld and HAZ sample, scan after pretreatment to obtain a grain orientation map, and count the proportion of high-angle grain boundaries in the grain orientation map;

[0020] Take a thin section around the weld toe, make a TEM sample by double-spray electrolytic thinning, observe with a transmission electron microscope, count the density of strengthening phase particles, determine the strengthening phase aggregation area according to the density of strengthening phase particles, and measure the distance from the weld toe;

[0021] If the proportion of high-angle grain boundaries and the distance of the strengthening phase aggregation area from the weld toe are both greater than or equal to the preset threshold, it indicates that the risk of steel bridge crack direction deflection is high.

[0022] As a further technical solution of the present application, the process of determining the stress gradient gentle area from the weld toe stress gradient cloud map obtained by finite element simulation is:

[0023] Based on the weld model, the grid around the weld toe area is encrypted, and after the residual stress and bending stress at the critical temperature of plastic sharp drop are applied, the weld toe stress gradient cloud map is extracted, and the stress area that meets the preset requirements of the stress gradient is determined according to the weld toe stress gradient cloud map, which is the stress gradient gentle area.

[0024] As a further technical solution of the present application, the process of establishing a resistance-gradient coupling equation according to the principle of minimum resistance and combining finite element stress gradient data, and calculating and predicting the crack deflection angle is:

[0025] Obtain the crack propagation resistance of different crystal orientations in the grain orientation map, and construct a resistance-gradient coupling equation combining the finite element stress gradient:

[0026] The crack deflection angle θ = arctan (stress gradient gentle area gradient / main stress direction gradient) × (main stress direction resistance / stress gradient gentle area crystal direction resistance), wherein the main stress direction is the initial crack propagation direction.

[0027] As a further technical solution of the present application, the process of obtaining the crack path propagation length based on the crack deflection angle analysis and determining the crack deflection path is:

[0028] Use an in-situ fatigue stage of a microscope to apply cyclic load to the weld sample, record the whole process of crack initiation, deflection, and expansion to the stress gradient gentle area in real time, and take multiple crack morphologies;

[0029] Fit the crack path of each shot, calculate the crack deflection angle and crack path propagation length at each stage, and obtain the dynamic curve of the deflection angle-propagation length.

[0030] Traverse all data points on the curve, extract the crack propagation length corresponding to the predicted crack deflection angle, and integrate into the crack propagation sequence;

[0031] Calculate the coefficient of variation of the crack propagation sequence, then average the crack propagation length in the crack propagation sequence to obtain the predicted crack propagation length under the crack deflection angle, and vice versa, then perform crack propagation sequence clustering through K-means algorithm, and the crack propagation average length corresponding to the cluster with the highest proportion of data points in the cluster is selected as the predicted crack propagation length.

[0032] According to the crack deflection direction and the predicted crack propagation length, the crack deflection path is determined.

[0033] As a further technical solution of the application, the stress gradient area division result of the crack crossing area determined according to the crack deflection path is a high gradient area, a medium gradient area and a low gradient area.

[0034] Perform fatigue crack propagation test on the stress gradient area at the plastic sharp drop critical temperature to obtain da / dN-ΔK curve, wherein da / dN is the crack propagation rate, and ΔK is the stress intensity factor amplitude, fit Paris formula, and calculate the crack propagation rate of different stress gradient areas.

[0035] According to the path of the stress gradient area, the path proportion of the stress gradient area is calculated.

[0036] Combine the crack propagation rate of different stress gradient areas and the path proportion to calculate the actual propagation rate of the crack.

[0037] As a further technical solution of the application, the process of correcting the life prediction output result of the original fatigue life prediction model according to the actual propagation rate of the crack is:

[0038] Substitute the actual propagation rate into the original fatigue life prediction model to obtain the corrected life prediction output result, and calculate the proportion of the corrected life prediction output result and the life prediction output result of the original fatigue life prediction model to obtain the correction coefficient.

[0039] A low-temperature fatigue crack propagation life prediction system based on a steel bridge deck includes the following modules:

[0040] Crack deflection analysis module: through the correlation of steel plastic sharp drop and local stress gradient non-uniform analysis, it is judged whether the crack direction deflection risk assessment needs to be started;

[0041] Crack deflection evaluation module: if necessary, the risk of steel bridge deck crack deflection is evaluated by analyzing the grain orientation difference and strengthening phase of the weld and HAZ area;

[0042] Crack deflection path determination module: if the risk of steel bridge deck crack deflection is high, the stress gradient cloud atlas of the weld toe obtained by finite element simulation is used to determine the stress gradient gentle area, the resistance-gradient coupling equation is established according to the minimum resistance principle and combined with the stress gradient, the crack deflection angle is calculated, the crack path extension length is analyzed and predicted based on the crack deflection angle, and the crack deflection path is determined;

[0043] Fatigue life prediction correction module: combined with the finite element stress gradient, the stress gradient area of the crack crossing area determined according to the crack deflection path is divided, the actual expansion rate of the crack is calculated through the path proportion and stress gradient distribution of each stress gradient area divided, and the life prediction output result of the original fatigue life prediction model is corrected according to the actual expansion rate of the crack.

[0044] The beneficial effects of the present application are as follows: the present application mainly focuses on the low-temperature fatigue crack life prediction of steel bridge weld, and a risk judgment-evaluation-path prediction-rate calculation-life correction closed loop process is constructed: first, the plasticity sharp decline characteristics of steel are quantified through low-temperature tensile test, the plasticity and local stress gradient of steel are analyzed combined with the measured residual stress by X-ray diffraction and finite element simulation, and when the plasticity index decreases and the stress gradient increases, the crack deflection risk evaluation is started; the crack deflection risk is determined by characterizing the grain orientation and strengthening phase of the weld and HAZ area, when the risk is high, the stress gradient gentle area is located relying on the finite element weld toe stress gradient cloud atlas, the resistance-gradient coupling equation is established according to the minimum resistance principle to calculate the deflection angle, the crack deflection path is determined, the high, medium and low stress gradient areas are divided, the actual expansion rate is calculated through the Paris formula fitting and weighted calculation through the low-temperature fatigue test, and finally the prediction result of the original fatigue life model is corrected and the correction coefficient is output, which improves the crack expansion path prediction accuracy and improves the fatigue life prediction evaluation accuracy. BRIEF DESCRIPTION OF DRAWINGS

[0045] The present application will be further described below in conjunction with the drawings.

[0046] Figure 1 is a step flow chart of the low-temperature fatigue crack expansion life prediction method based on steel bridge surface according to the embodiment of the present application;

[0047] Figure 2 is a logic judgment schematic diagram of the low-temperature fatigue crack expansion life prediction method based on steel bridge surface according to the embodiment of the present application;

[0048] Figure 3A program block diagram of a low-temperature fatigue crack propagation life prediction system based on a steel bridge deck is provided in the embodiments of the present application. DETAILED DESCRIPTION

[0049] In order to make the technical means, creative features, purposes and effects achieved by the present application easy to understand, the present application will be further described below in conjunction with specific embodiments.

[0050] Embodiment 1: Please refer to Figures 1-2 As shown in the drawings, the low-temperature fatigue crack propagation life prediction method based on a steel bridge deck provided in the embodiments of the present application includes the following steps:

[0051] Step S10: Determine whether the crack direction deflection risk assessment needs to be started by performing the associated steel plasticity sharp decline and local stress gradient non-uniformity analysis;

[0052] In step S10, the analysis process of the steel plasticity sharp decline is as follows:

[0053] According to GB / T228.1-2021 "Metallic Materials Tensile Test Part 1: Room Temperature Test Method", standard tensile specimens are processed from the base material and the butt weld, and after being kept at each temperature node, they are loaded to fracture, and the elongation at break δ at each temperature is calculated;

[0054] The δ-temperature curve is fitted by Origin software, and the plasticity sharp decline critical temperature is determined in the temperature curve according to the preset elongation at break (50%);

[0055] For example: According to GB / T228.1-2021 "Metallic Materials Tensile Test Part 1: Room Temperature Test Method", standard tensile specimens (diameter 10mm, gauge length 50mm) are processed from Q345qD / Q370qE base material and butt weld (welding process: submerged arc welding, welding wire H08MnA, welding flux HJ431), and 15 specimens (5 temperature nodes, 3 parallel samples for each node) are prepared for each material;

[0056] A MTSC64.104 electro-hydraulic servo tensile testing machine is used, which is matched with a TH-800 low-temperature environmental chamber (temperature control accuracy ±1℃), and a data acquisition system with a sampling frequency of 100Hz. The specimen is placed in the environmental chamber and is stabilized at -60℃, -40℃, -20℃, 0℃, room temperature (23±2℃), respectively, and is kept for 30min. The specimen is loaded to fracture at a tensile rate of 5mm / min, and the elongation at break δ at each temperature is calculated (such as δ=22% for Q345qD weld at room temperature and δ=9% at -60℃). The "δ-temperature" curve is fitted by Origin software, and when the δ drop amplitude exceeds 50%, the corresponding temperature is the "plasticity sharp decline critical temperature" (Q345qD weld -45℃, Q370qE base material -50℃).

[0057] It can be understood that the core role of the plasticity sharp drop critical temperature is to lock the key temperature node of material plasticity degradation, the material will be transformed from toughness to brittleness at low temperature, and the "plasticity sharp drop" is the core sign of the transformation (such as the sharp increase of the plasticity index of Q345qD steel at about-60℃), and the critical temperature is determined, which provides an environmental prerequisite for subsequent judgment of stress gradient non-uniformity;

[0058] In step S10, the process of local stress gradient non-uniformity analysis is:

[0059] According to GB / T 7704-2017 "Nondestructive Testing X-ray Stress Measurement Method", the residual stress of the weld sample is tested to obtain the longitudinal residual stress and the transverse residual stress;

[0060] In the actual steel bridge deck longitudinal rib-top plate weld, HBM1-LY11-3 / 350 strain gauges are pasted, traffic load data is collected, and bending stress is obtained through strain-stress conversion;

[0061] A 1:1 weld model is established, residual stress and bending stress are applied, stress distribution at the weld toe at different temperatures is extracted, and stress gradient is calculated;

[0062] For example, X-ray diffractometer (Bruker D8 Discover) is used, GB / T 7704-2017 "Nondestructive Testing X-ray Stress Measurement Method" is used to test the residual stress of the weld sample, the test points are distributed along the center line of the weld toe on both sides of each 5mm, and 1 point is taken every 0.5mm, the longitudinal residual stress (210MPa at the weld toe, 80MPa in the base material area) and the transverse residual stress (140MPa at the weld toe, 60MPa in the base material area) are obtained.

[0063] In the actual steel bridge deck longitudinal rib-top plate weld, HBM1-LY11-3 / 350 strain gauges are pasted, 3-day traffic load data (morning peak 7:00-9:00, evening peak 17:00-19:00, flat peak period) is collected, and bending stress (static 90MPa, dynamic peak value 170MPa) is obtained through strain-stress conversion (elastic modulus 206GPa at room temperature, 215GPa at-60℃).

[0064] A 1:1 weld model is established by ABAQUS (element type C3D8R, mesh size: 0.1mm in the weld toe area, 1mm in the base material area);

[0065] Residual stress (as initial stress) and bending stress (as external load) are applied;

[0066] Extract the stress distribution at the weld toe at different temperatures, calculate the stress gradient, such as the stress at the weld toe (x=0) 300MPa, the stress at x=1mm 180MPa, the stress gradient=(300-180) / 1mm=120MPa / mm; the stress at x=0 at-60℃ 330MPa, the stress at x=1mm 110MPa, the stress gradient=220MPa / mm);

[0067] In step S10, it is judged whether the process of crack direction deflection risk assessment needs to be started:

[0068] At the critical temperature of plastic sharp drop, the δ drop and the stress gradient increase are obtained;

[0069] If the δ drop and the stress gradient increase are both greater than or equal to the preset threshold value, it indicates that the stress gradient non-uniformity is significant, and the crack direction deflection risk assessment needs to be started;

[0070] If the δ drop and the stress gradient increase are both less than the preset threshold value or not all greater than or equal to the preset threshold value, it indicates that the stress gradient non-uniformity is not significant, and the crack direction deflection risk assessment does not need to be started;

[0071] The preset threshold values corresponding to the δ drop and the stress gradient increase are both obtained by referring to the steel structure low temperature fatigue design specification;

[0072] Step S20: If needed, the crack direction deflection risk of the steel bridge deck is evaluated by analyzing the grain orientation difference and the strengthening phase in the weld and HAZ area;

[0073] In step S20, the process of analyzing the grain orientation difference and the strengthening phase in the weld and HAZ area is:

[0074] Take the weld and HAZ sample, scan after pretreatment, get the grain orientation map, and count the proportion of high-angle grain boundaries in the grain orientation map;

[0075] Take a thin section around the weld toe, make a TEM sample by double-spray electrolytic thinning, observe with Tecnai G2F20 TEM (Transmission Electron Microscope), randomly select the field of view, count the density of strengthening phase particles, determine the strengthening phase aggregation area according to the density of strengthening phase particles, and measure the distance from the weld toe;

[0076] If the proportion of high-angle grain boundaries and the distance of the strengthening phase aggregation area from the weld toe are both greater than or equal to the preset threshold value, it indicates that the crack direction deflection risk of the steel bridge deck is high;

[0077] If the proportion of high-angle grain boundaries and the distance of the strengthening phase aggregation area from the weld toe are both less than the preset threshold value or not all greater than or equal to the preset threshold value, it indicates that the crack direction deflection risk of the steel bridge deck is low;

[0078] Exemplarily, a weld and (heat-affected) HAZ sample (size 10 mm x 10 mm x 5 mm) is sanded (400#-2000#), polished (diamond polishing paste 1 μm), and electrolytically polished (electrolyte: perchloric acid + ethanol = 1:9, voltage 20 V);

[0079] The ZEISS Sigma 500 SEM (with an EBSD accessory) is used to obtain a grain orientation map, with an acceleration voltage of 20 kV, a step size of 0.1 μm, and a scanning area of 5 mm x 3 mm at the weld toe;

[0080] Channel 5 software is used for analysis, and the proportion of high-angle grain boundaries (orientation difference > 15°) is counted (32% in the weld toe HAZ area and 12% in the base material area);

[0081] A 3 mm x 3 mm x 0.5 mm thin section is taken around the weld toe, and a double-jet electrolytic thinning process (electrolyte same as EBSD, temperature -20°C) is performed to make a TEM sample;

[0082] Tecnai G2 F20 TEM (acceleration voltage 200 kV) is used for observation, and 5 random fields of view (each field of view area 1 μm²) are selected to count the strengthening phase (NbC, TiN) particle density (7 x 10³ / μm² in the weld toe HAZ area and 2.5 x 10³ / μm² in the base material area) and the distance from the weld toe (the HAZ area aggregation zone is 45 μm away from the weld toe, and the base material area aggregation zone is 120 μm away from the weld toe);

[0083] The aggregation zone is determined by counting the average density of strengthening phase particles ρ_avg and the standard deviation σ in all fields of view;

[0084] If the strengthening phase particle density in the field of view area is > ρ_avg + 2σ, it is the aggregation zone;

[0085] Step S30: If the steel bridge deck crack deflection risk is high, the weld toe stress gradient cloud map obtained by finite element simulation is used to determine the stress gradient gentle area, a resistance-gradient coupling equation is established according to the minimum resistance principle and combined with the stress gradient, the crack deflection angle is calculated, the crack path propagation length is predicted based on the crack deflection angle analysis, and the crack deflection path is determined;

[0086] In step S30, the process of determining the stress gradient gentle area from the weld toe stress gradient cloud map obtained by finite element simulation is as follows:

[0087] Based on the weld model, the mesh around the weld toe is encrypted, and after the residual stress and bending stress at the critical temperature of plastic sharp drop are applied, the weld toe stress gradient cloud map is extracted, and the stress area where the stress gradient meets the preset requirements is determined according to the weld toe stress gradient cloud map, that is, the stress gradient gentle area;

[0088] Exemplarily, in the weld model, a 5mm*5mm area around the weld toe is meshed to 0.05mm, a stress load at-60℃ (residual stress+ bending stress) is applied, a stress gradient nephogram is extracted, and an area with a stress gradient <100MPa / mm is found, which is located in the transition zone between the weld HAZ and the base metal (200-300μm away from the weld toe in the longitudinal rib length direction, 1-2mm in the thickness direction), and the area is marked as a stress gradient flat area, and the stress gradient flat area coordinates are (x=250μm, y=1.5mm, z=0-5mm);

[0089] In step S30, a resistance-gradient coupling equation is established according to the minimum resistance principle and in combination with the finite element stress gradient data, and the process of calculating the crack deflection angle is as follows:

[0090] The crack propagation resistance of different crystal orientations in the grain orientation map is obtained, and a resistance-gradient coupling equation is constructed in combination with the finite element stress gradient:

[0091] The crack deflection angle θ=arctan (stress gradient flat area gradient / main stress direction gradient) x (main stress direction resistance / stress gradient flat area crystal orientation resistance), wherein the main stress direction is the initial crack propagation direction;

[0092] Exemplarily, the crack propagation resistance of different crystal orientations (85MPa*m^(1 / 2) along the

[110] crystal orientation, 130MPa*m^(1 / 2) along the

[100] crystal orientation) is obtained, and a resistance-gradient coupling equation is established in combination with the finite element stress gradient data (stress gradient of the flat area 80MPa / mm, main stress direction gradient 220MPa / mm):

[0093] θ=arctan (80 / 220) x (130 / 85)≈15°x1.53≈23°, that is, the crack is deflected by 23° from the main stress direction and propagates along the longitudinal rib length direction;

[0094] In step S30, the crack path propagation length is obtained based on the crack deflection angle analysis, and the process of determining the crack deflection path is as follows:

[0095] An in-situ fatigue stage of a scanning electron microscope (SEM) or an optical microscope (OM) is used to apply a cyclic load (residual stress+bending stress) to the weld sample, and the whole process of crack initiation, deflection and propagation to the stress gradient flat area is recorded in real time, and multiple crack morphologies are obtained by shooting (shooting once every 100 load cycles);

[0096] Through an Image-Pro image analysis software, the crack path of each shooting is fitted, the crack deflection angle (relative to the main stress direction) and the crack path propagation length at each stage are calculated, and a dynamic curve of the deflection angle-propagation length is obtained;

[0097] An exemplary fitting method for each photographed crack path can be: when fitting the crack path with Image-Pro, selecting piecewise curve fitting (rather than polyline) - in the high gradient area (such as 0-4.5 mm), the crack path is tortuous, and is fitted with a 3rd order polynomial; in the low gradient area (such as 4.5-15 mm), the path is gentle, and is fitted with a 2nd order polynomial;

[0098] Traverse all data points on the curve (such as a total of 100 points, corresponding to a crack propagation length of 0-15 mm), extract the crack propagation length corresponding to the predicted crack deflection angle, and integrate it into a crack propagation sequence;

[0099] Calculate the coefficient of variation of the crack propagation sequence. If the coefficient of variation is less than the coefficient of variation threshold, it indicates that the crack propagation length is stable at the predicted crack deflection angle, and the crack propagation length contained in the crack propagation sequence is subjected to mean value processing to obtain the predicted crack propagation length at the crack deflection angle;

[0100] If the coefficient of variation is greater than or equal to the coefficient of variation threshold, it indicates that the crack propagation length is unstable at the predicted crack deflection angle, and clustering analysis is performed;

[0101] The clustering analysis includes the following steps:

[0102] A1, determine the number of clusters K, wherein the number of clusters K is set according to the physical law of crack propagation and the predicted crack deflection angle, for example, in the case of a crack deflection angle of 23°, it is usually concentrated in a single interval that transitions from the high stress gradient area to the medium gradient area, rather than multiple intervals, and the number of clusters K is set to 2 (to avoid over-splitting);

[0103] A2, use the K-means algorithm to perform crack propagation sequence clustering, divide the data into two classes, and calculate the intra-class average length and the proportion of intra-class data points, for example, class 1: 3.1, 3.2 mm (40% of the proportion, average length 3.15 mm); class 2: 3.5, 3.8, 3.6 mm (60% of the proportion, average length 3.63 mm);

[0104] A3, select the cluster with the highest proportion of intra-class data points (class 2, 60%), and the intra-class crack propagation average length (3.63 mm) is the predicted crack propagation length;

[0105] Determine the crack deflection path according to the crack deflection direction and the predicted crack propagation length;

[0106] Step S40: in combination with the finite element stress gradient, the stress gradient area of the crack crossing region determined according to the crack deflection path is divided, the actual propagation rate of the crack is calculated through the path proportion of each stress gradient area and the stress gradient distribution after the division, and the life prediction output result of the original fatigue life prediction model is corrected according to the actual propagation rate of the crack;

[0107] In step S40, the stress gradient area division result of the crack crossing region determined according to the crack deflection path is a high gradient area, a medium gradient area and a low gradient area;

[0108] For example, the high gradient area: 0-4.5mm, gradient 200-220MPa / mm;

[0109] The medium gradient area: 4.5-10.5mm, gradient 100-200MPa / mm;

[0110] The low gradient area: 10.5-15mm, gradient <100MPa / mm;

[0111] In step S40, the process of calculating the actual propagation rate of the crack through the path proportion of each stress gradient area and the stress gradient distribution after the division is as follows:

[0112] The fatigue crack propagation test of the stress gradient area is carried out according to GB / T3075-2008 “Metal Material Fatigue Test Axial Force Control Method” at the critical temperature of plastic sharp decline, the da / dN-ΔK (crack propagation rate-stress intensity factor amplitude) curve is obtained, the Paris formula (da / dN=C(ΔK)^m) is fitted (Paris formula), C is a constant, m is the expansion index, and the crack propagation rate of different stress gradient areas is calculated;

[0113] The path proportion of the stress gradient area is calculated according to the stress gradient area path, wherein the path proportion of the stress gradient area = stress gradient area path / predicted crack propagation length, for example, the high gradient area: 0-4.5mm, the medium gradient area: 4.5-10.5mm, and the low gradient area: 10.5-15mm, and the path proportions are 30%, 40% and 30% respectively;

[0114] In combination with the crack propagation rate of different stress gradient areas and the path proportion, the actual propagation rate of the crack is weighted calculated, which is specifically:

[0115] ;

[0116] For example:

[0117] In the high gradient region: C1 = 1.2 × 10⁻¹¹, m1 = 3.2, when ΔK = 30 MPa・m^(1 / 2), da / dN1 = 1.2 × 10⁻¹¹ × 30³.² = 1.8 × 10⁻ 6 mm / cycle;

[0118] In the medium gradient region: when C2 = 0.8 × 10⁻¹¹, m2 = 2.9, and ΔK = 25 MPa・m^(1 / 2), da / dN2 = 0.8 × 10⁻¹¹ × 25². 9 =1.1×10⁻ 6 mm / cycle;

[0119] In the low gradient region: when C3 = 0.5 × 10⁻¹¹, m3 = 2.6, and ΔK = 20 MPa・m^(1 / 2), da / dN3 = 0.5 × 10⁻¹¹ × 20². 6 =0.6×10⁻ 6 mm / cycle;

[0120] ④ Weighted average actual expansion rate da / dN = 0.3 × 1.8 × 10⁻ 6 + 0.4×1.1×10⁻ 6 + 0.3×0.6×10⁻ 6 =1.1×10⁻ 6 mm / cycle;

[0121] In step S40, the process of correcting the life prediction output of the original fatigue life prediction model based on the actual crack propagation rate is as follows:

[0122] Substitute the actual expansion rate into the original fatigue life prediction model to obtain the corrected life prediction output. Calculate the ratio of the corrected life prediction output to the original fatigue life prediction model's life prediction output to obtain the correction coefficient (corrected life / original model life). This coefficient is used to quickly correct the original fatigue life prediction model's life prediction output in subsequent engineering applications.

[0123] For example, the original fatigue life prediction model uses da / dN1=1.8×10⁻ in the high gradient region. 6 Based on the mm / cycle calculation, as the crack propagates from 0.5 mm to 20 mm, the life prediction output N1 of the original fatigue life prediction model is 1.1 × 10⁻¹⁰. 6 In the next cycle, based on the actual expansion rate of 1.1 × 10⁻ 6 The corrected lifetime prediction output N² is 0.7 × 10⁻⁶ based on the mm / cycle calculation. 6 If the cycle continues, the correction factor = N2 / N1 = 0.64;

[0124] The technical scheme of the embodiment of the present application is as follows: the present application mainly focuses on the low-temperature fatigue crack life prediction of the steel bridge deck, and a risk judgment-evaluation-path prediction-rate calculation-life correction closed loop process is constructed: first, the plasticity sharp drop characteristics of the steel material are quantified through the low-temperature tensile test, the residual stress is measured by X-ray diffraction, and the finite element simulation is combined to analyze the plasticity and local stress gradient of the steel material; when the plasticity index decreases and the stress gradient increases, the crack deflection risk evaluation is started; the crack deflection risk is judged by characterizing the grain orientation and strengthening phase in the weld and HAZ area; when the risk is high, the stress gradient gentle area is located by relying on the finite element weld toe stress gradient cloud chart, the deflection angle is calculated by establishing the resistance-gradient coupling equation according to the minimum resistance principle, the crack deflection path is determined, the high (0-4.5mm, 30%), medium (4.5-10.5mm, 40%) and low (10.5-15mm, 30%) gradient areas are divided, the low-temperature fatigue test is performed, the Paris formula is fitted and the actual expansion rate is calculated by weighting, and finally the prediction result of the original fatigue life model is corrected by the actual expansion rate and the correction coefficient is output, so that the crack expansion path prediction accuracy is improved, and the fatigue life prediction evaluation accuracy is improved.

[0125] Embodiment 2, please refer to Figure 3 The low-temperature fatigue crack propagation life prediction system based on the steel bridge deck according to the embodiment of the present application comprises the following modules:

[0126] The crack deflection analysis module: the steel material plasticity sharp drop and local stress gradient non-uniformity analysis are performed to determine whether the crack direction deflection risk evaluation needs to be started;

[0127] The crack deflection evaluation module: if needed, the crack direction deflection risk of the steel bridge deck is evaluated by analyzing the grain orientation difference and strengthening phase in the weld and HAZ area;

[0128] The crack deflection path determination module: if the crack deflection risk of the steel bridge deck is high, the stress gradient gentle area is determined by the weld toe stress gradient cloud chart obtained by the finite element simulation, the resistance-gradient coupling equation is established according to the minimum resistance principle and combined with the stress gradient, the crack deflection angle is calculated, the crack path expansion length is analyzed and predicted based on the crack deflection angle, and the crack deflection path is determined;

[0129] The fatigue prediction life correction module: the crack crossing area determined according to the crack deflection path is divided into stress gradient areas by combining the finite element stress gradient, the actual expansion rate of the crack is calculated according to the path proportion and stress gradient distribution of each stress gradient area, and the life prediction output result of the original fatigue life prediction model is corrected according to the actual expansion rate of the crack.

[0130] The above shows and describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above-mentioned embodiments, and the above-mentioned embodiments and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for predicting the low-temperature fatigue crack propagation life of steel bridge decks, characterized in that: Includes the following steps: Step S10: By performing correlation analysis of the sharp decrease in steel plasticity and the non-uniformity of local stress gradient, determine whether it is necessary to initiate a crack direction deflection risk assessment. Step S20: If necessary, assess the risk of crack direction deflection in the steel bridge deck by analyzing the differences in grain orientation and strengthening phases in the weld and HAZ region. Step S30: If the risk of crack deflection in the steel bridge deck is high, the stress gradient gradient cloud map obtained by finite element simulation is used to determine the stress gradient flat zone. Based on the principle of minimum resistance and combined with the stress gradient, a resistance-gradient coupling equation is established, and the crack deflection angle is calculated. Based on the crack deflection angle, the crack path propagation length is analyzed and predicted, and the crack deflection path is determined. The process of establishing the resistance-gradient coupling equation based on the principle of minimum resistance and combined with finite element stress gradient data, and calculating and predicting the crack deflection angle, is as follows: Obtain the crack propagation resistance for different crystal orientations in the grain orientation diagram, and construct the resistance-gradient coupling equation by combining the finite element stress gradient: Crack deflection angle θ = arctan(gradient of stress gradient in gentle region / gradient of principal stress direction) × (resistance of principal stress direction / resistance of crystal orientation in gentle stress gradient region), where the principal stress direction is the initial crack propagation direction; The process of determining the crack deflection path by obtaining the crack path propagation length based on crack deflection angle analysis is as follows: Using an in-situ fatigue stage of a microscope, cyclic loading was applied to the weld specimen, and the entire process of crack initiation, deflection, propagation to the stress gradient flattening zone was recorded in real time. Multiple crack morphologies were also captured. The crack path of each shot is fitted, and the crack deflection angle and crack path propagation length at each stage are calculated to obtain the dynamic curve of deflection angle-propagation length. Traverse all data points on the curve, extract the crack propagation length corresponding to the predicted crack deflection angle, and integrate them into a crack propagation sequence. Calculate the coefficient of variation of the crack propagation sequence, then average the crack propagation lengths contained in the crack propagation sequence to obtain the predicted crack propagation length under the crack deflection angle. Otherwise, perform crack propagation sequence clustering through the K-means algorithm, and select the average crack propagation length corresponding to the cluster with the highest proportion of data points in the cluster as the predicted crack propagation length. The crack deflection path is determined based on the crack deflection direction and the predicted crack propagation length. Step S40: Combining the finite element stress gradient, the crack crossing region determined by the crack deflection path is divided into stress gradient zones. The actual crack propagation rate is calculated based on the path proportion and stress gradient distribution of each divided stress gradient zone. The life prediction output of the original fatigue life prediction model is corrected based on the actual crack propagation rate.

2. The method for predicting the low-temperature fatigue crack propagation life based on steel bridge decks according to claim 1, characterized in that: The analytical process for the sharp decrease in the plasticity of the steel is as follows: Tensile specimens were prepared from the base material and butt weld, and after being kept at each temperature node, they were loaded until fracture. The elongation after fracture δ at each temperature was calculated, and the δ-temperature curve was obtained by fitting. The critical temperature at which the plasticity drops sharply was determined from the temperature curve based on the preset elongation after fracture. The process of analyzing the non-uniform local stress gradient is as follows: Residual stress tests were performed on the weld specimens to obtain longitudinal and transverse residual stresses. Strain gauges were attached to the weld joint between the longitudinal rib and the top plate of the actual steel bridge deck to collect traffic loads and obtain bending stress through strain-stress conversion. A weld model is established, residual stress and bending stress are applied, the stress distribution at the weld toe at different temperatures is extracted, and the stress gradient is calculated.

3. The method for predicting the low-temperature fatigue crack propagation life based on steel bridge decks according to claim 2, characterized in that: The process for determining whether a crack direction deflection risk assessment needs to be initiated is as follows: At the critical temperature where plasticity decreases sharply, the decrease in δ and the increase in stress gradient are obtained; If both the decrease in δ and the increase in stress gradient are greater than or equal to the preset threshold, it indicates that the non-uniformity of stress gradient is significant, and a crack direction deflection risk assessment needs to be initiated.

4. The method for predicting the low-temperature fatigue crack propagation life based on steel bridge decks according to claim 3, characterized in that: The process of analyzing the grain orientation differences and strengthening phases in the weld and HAZ region is as follows: Weld and HAZ samples were taken and scanned after pretreatment to obtain grain orientation maps. The proportion of high-angle grain boundaries in the grain orientation maps was statistically analyzed. Thin slices were taken from around the weld toe and thinned by double-spray electrolysis to prepare TEM samples. The samples were observed using a transmission electron microscope, and the density of the reinforcing phase particles was statistically analyzed. The reinforcing phase aggregation area was determined based on the particle density, and the distance from the weld toe was measured. If the proportion of high-angle grain boundaries and the distance from the strengthening phase aggregation region to the weld toe are both greater than or equal to the preset threshold, it indicates a high risk of crack direction deflection in the steel bridge deck.

5. The method for predicting the low-temperature fatigue crack propagation life based on steel bridge decks according to claim 4, characterized in that: The process of determining the stress gradient flattening region using the weld toe stress gradient cloud map obtained through finite element simulation is as follows: Based on the weld model, the mesh around the weld toe is refined. After applying residual stress and bending stress at the critical temperature of sharp plasticity decrease, the stress gradient cloud map of the weld toe is extracted. The stress region where the stress gradient meets the preset requirements is determined according to the stress gradient cloud map of the weld toe, which is the stress gradient flat region.

6. The method for predicting the low-temperature fatigue crack propagation life based on steel bridge decks according to claim 1, characterized in that: The stress gradient region division of the crack crossing region determined according to the crack deflection path is divided into a high gradient region, a medium gradient region, and a low gradient region. Fatigue crack propagation tests were conducted on the stress gradient region at the critical temperature of rapid decrease in plasticity to obtain the da / dN-ΔK curve, where da / dN is the crack propagation rate and ΔK is the stress intensity factor amplitude. The Paris formula was fitted to calculate the crack propagation rate in different stress gradient regions. Calculate the path percentage of the stress gradient region based on the path in the stress gradient region; The actual crack propagation rate is calculated by weighting the crack propagation rate and path proportion in different stress gradient zones.

7. The method for predicting the low-temperature fatigue crack propagation life based on steel bridge decks according to claim 6, characterized in that: The process of correcting the fatigue life prediction output of the original fatigue life prediction model based on the actual crack propagation rate is as follows: Substitute the actual extension rate into the original fatigue life prediction model to obtain the corrected life prediction output. Then, calculate the ratio of the corrected life prediction output to the life prediction output of the original fatigue life prediction model to obtain the correction coefficient.

8. A low-temperature fatigue crack propagation life prediction system based on steel bridge deck, characterized in that: The system is used to perform the method described in any one of claims 1-7, and the system includes the following modules: Crack deflection analysis module: By performing correlation analysis of sharp decrease in steel plasticity and non-uniform local stress gradient, it determines whether a crack direction deflection risk assessment needs to be initiated. Crack deflection assessment module: If necessary, the risk of crack deflection in the steel bridge deck can be assessed by analyzing the differences in grain orientation and strengthening phases in the weld and HAZ region. Crack deflection path determination module: If the risk of crack deflection in the steel bridge deck is high, the stress gradient gradient cloud map obtained by finite element simulation is used to determine the stress gradient flat zone. Based on the principle of minimum resistance and combined with the stress gradient, a resistance-gradient coupling equation is established, and the crack deflection angle is calculated. Based on the crack deflection angle, the crack path propagation length is analyzed and predicted to determine the crack deflection path. Fatigue life prediction correction module: Combining finite element stress gradient, the stress gradient region is divided into stress gradient zones based on the crack deflection path. The actual crack propagation rate is calculated based on the path proportion and stress gradient distribution of each stress gradient zone. The life prediction output of the original fatigue life prediction model is corrected based on the actual crack propagation rate.

Images