A method for predicting fatigue crack propagation life of a metallurgical crane metal structure

Abstract

The present application relates to the technical field of metallurgical crane detection, in particular to a metallurgical crane metal structure fatigue crack propagation life prediction method, comprising: collecting structure stress spectrum, stress concentration area strain monitoring time series data, hoisting operation spectrum and temperature and humidity data under service environment, constructing initial crack parameter model, calculating stress intensity factor change spectrum containing crack tip stress intensity factor amplitude corresponding to each load cycle, calculating single cycle crack propagation increment and accumulating to obtain crack size evolution data, setting crack propagation critical size threshold value combined with structure integrity evaluation criterion, and predicting fatigue crack propagation life by comparing evolution data with threshold value. The method refines the stress intensity factor calculation dimension, restores the crack size dynamic evolution process, fits the actual service load characteristics of the metallurgical crane, and optimizes the accuracy of the crack propagation life prediction.

Description

Technical Field

[0001] This invention relates to the field of metallurgical crane equipment testing technology, and in particular to a method for predicting the fatigue crack propagation life of metal structures in metallurgical cranes. Background Technology

[0002] Conventional fatigue crack propagation life prediction technologies for metal structures in metallurgical cranes often employ simplified load spectra for stress intensity factor calculation. They determine the crack tip stress intensity factor by fitting the entire load spectrum piecewise, without individually matching the corresponding crack tip stress intensity factor amplitude for each independent load cycle within the load spectrum. Crack propagation life is frequently derived directly from empirical formulas, without quantitative analysis of the real-time changes in crack size with the number of load cycles. Existing prediction methods lack a stress intensity factor calculation system that precisely corresponds to a single load cycle, and the calculation of crack propagation increments relies solely on macroscopic load characteristics, failing to achieve refined calculation of crack propagation parameters under successive load cycles.

[0003] Traditional prediction methods have application defects: the stress intensity factor has a low degree of matching with the actual load cycle; the characterization of the stress state at the crack tip cannot closely match the real changes under a single load; the calculation results of crack propagation increment are greatly affected by the load dispersion; the dynamic change process of crack size cannot be fully recorded by successive accumulation; and the life prediction results deviate significantly from the crack propagation law under the actual service state of metallurgical cranes.

[0004] This invention aims to accurately calculate the amplitude of the stress intensity factor at the crack tip corresponding to each load cycle in the load spectrum and form a unique variation spectrum. By accumulating the crack propagation increment in a single cycle, the evolution data of crack length with the number of load cycles is obtained. Combined with the structural integrity assessment criteria of metallurgical cranes, a critical size threshold for crack propagation is set to accurately determine the fatigue crack propagation life. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and propose a method for predicting the fatigue crack propagation life of metal structures in metallurgical cranes.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a method for predicting the fatigue crack propagation life of a metal structure for a metallurgical crane, comprising:

[0007] The load condition data of the metallurgical crane under service environment is collected. The load condition data includes structural stress spectrum, strain monitoring time series data of stress concentration area, lifting operation spectrum of metallurgical production process and temperature and humidity monitoring data of working environment.

[0008] An initial crack parameter model of the metal structure of a metallurgical crane is constructed. Based on the load condition data and the initial crack parameter model, the stress intensity factor variation spectrum is calculated. The stress intensity factor variation spectrum includes the stress intensity factor amplitude at the crack tip corresponding to each load cycle in the load spectrum.

[0009] Based on the stress intensity factor variation spectrum, the single-cycle crack propagation increment of the initial crack under the load condition is calculated.

[0010] The incremental crack propagation of a single cycle is cumulatively calculated to obtain crack size evolution data, which records the change in crack length with the number of load cycles.

[0011] Based on the structural integrity assessment criteria for metallurgical cranes, a critical crack propagation size threshold is set. By comparing the crack size evolution data with the critical crack propagation size threshold, the fatigue crack propagation life of the metal structure of the metallurgical crane is predicted.

[0012] As a further aspect of the present invention, the construction of the initial crack parameter model for the metal structure of the metallurgical crane includes:

[0013] The initial crack parameter model includes the initial crack location, initial crack size, crack morphology classification, and main crack propagation direction defined based on the initial non-destructive testing results.

[0014] Extract defect signal features from the initial nondestructive testing results. The defect signal features include defect echo amplitude, defect depth indication, and the projection length of the defect on the structural surface.

[0015] Pattern recognition is performed on the defect signal features to classify the defect morphology into surface cracks, buried cracks, or corner cracks, and the name of the structural component and local coordinates corresponding to the initial position of the crack are determined.

[0016] Based on the defect echo amplitude and defect depth indication, the initial size of the crack is calculated through a preset conversion relationship. The initial size of the crack includes the crack length, depth, and opening width.

[0017] Based on the stress direction history of the metal structure of the metallurgical crane and the finite element analysis results of the stress concentration area of ​​the structure, the propagation direction of the main crack is inferred.

[0018] The initial crack location, crack morphology classification, initial crack size, and main crack propagation direction are integrated into the data structure of the initial crack parameter model.

[0019] As a further aspect of the present invention, the stress intensity factor variation spectrum is calculated based on the load condition data and the initial crack parameter model, including:

[0020] From the load condition data, the local hot spot stress change sequence in the structural stress spectrum is extracted, and the local hot spot stress change sequence corresponds to the stress history in the region near the crack tip;

[0021] Based on the initial crack size, crack morphology classification, and main crack propagation direction in the initial crack parameter model, a stress intensity factor calculation function matching the metal structure material and crack type of the metallurgical crane is selected.

[0022] Each stress value of the local hot spot stress change sequence is input into the stress intensity factor calculation function to calculate the opening stress intensity factor, sliding stress intensity factor and tearing stress intensity factor of the crack tip at the corresponding moment.

[0023] The opening stress intensity factor, the sliding stress intensity factor and the tearing stress intensity factor are weighted and synthesized according to the main crack propagation direction to obtain the composite stress intensity factor corresponding to the current load point.

[0024] For the composite stress intensity factor within a complete load cycle in the load spectrum, calculate the difference between its maximum and minimum values, and use it as the amplitude of the crack tip stress intensity factor corresponding to the complete load cycle.

[0025] By iterating through all load cycles, a stress intensity factor variation spectrum is generated, consisting of the magnitude of the stress intensity factor at the crack tip for each cycle.

[0026] As a further aspect of the present invention, based on the stress intensity factor variation spectrum, the single-cycle crack propagation increment of the initial crack under the loading condition is calculated, including:

[0027] Query the fatigue crack propagation database of metal structure materials for metallurgical cranes to obtain the correspondence between the crack propagation rate and the stress intensity factor amplitude of the material. The correspondence includes the crack propagation threshold value, the Paris power law constant, and the fracture toughness value.

[0028] Read the stress intensity factor amplitude at the crack tip during the current load cycle from the stress intensity factor variation spectrum;

[0029] Determine whether the amplitude of the stress intensity factor at the crack tip is greater than the crack propagation threshold value;

[0030] If the magnitude of the stress intensity factor at the crack tip is greater than the crack propagation threshold, the crack propagation rate under the current load cycle is calculated according to the Paris power law relationship.

[0031] Multiplying the crack propagation rate by the time step of a single load cycle yields the crack propagation increment per cycle.

[0032] If the magnitude of the stress intensity factor at the crack tip is less than or equal to the crack propagation threshold, then the single-cycle crack propagation increment is determined to be zero.

[0033] As a further aspect of the present invention, the incremental crack propagation in a single cycle is cumulatively calculated to obtain crack size evolution data, including:

[0034] The initial crack size in the initial crack parameter model is used as the starting point for crack propagation calculation;

[0035] Following the order of the load spectrum, starting from the first load cycle, the single-cycle crack propagation increment calculated in each cycle is added to the current crack length.

[0036] After each accumulation step, record the current accumulation cycle number and the corresponding accumulation crack length;

[0037] When the cumulative crack length reaches the preset size recording point, record the cumulative number of cycles, crack length, and the amplitude of the crack tip stress intensity factor recalculated based on the current crack length.

[0038] The accumulation and recording process continues until the stress intensity factor amplitude at the crack tip approaches the fracture toughness value of the material, or the cumulative crack length reaches the preset simulation termination length, forming a data sequence containing the corresponding relationship between the cumulative number of cycles, crack length, and stress intensity factor amplitude, i.e., the crack size evolution data.

[0039] As a further aspect of the present invention, the step of setting a critical crack propagation size threshold based on the structural integrity assessment criteria for metallurgical cranes includes:

[0040] Obtain structural design drawings, material mechanical property parameters, and failure safety factors for the metal structure of metallurgical cranes;

[0041] Based on the structural design drawings, identify the structural components containing the initial crack, and determine the maximum design load and the most unfavorable load combination that the structural components will bear under normal working conditions;

[0042] Based on the material mechanical property parameters and the most unfavorable load combination, calculate the residual strength of the structural member in the presence of cracks;

[0043] Based on the comparison between the remaining strength and the structural working load, and by applying the failure safety factor, the maximum allowable crack size to ensure that the structure does not fail can be derived.

[0044] The calculated maximum crack size that ensures the structure does not fail is set as the critical crack propagation size threshold.

[0045] As a further aspect of the present invention, the fatigue crack propagation life of the metal structure of the metallurgical crane is predicted by comparing the crack size evolution data with the crack propagation critical size threshold, including:

[0046] Extract the corresponding sequence of crack length and cumulative cycle number from the crack size evolution data;

[0047] In the sequence of crack length and cumulative number of cycles, find the cumulative number of cycles corresponding to the first time the crack length is equal to or greater than the critical size threshold for crack propagation;

[0048] The accumulated number of cycles found is recorded as the predicted number of load cycles required from the initial crack propagation to the critical size, i.e., the fatigue crack propagation life;

[0049] If the crack size evolution data has ended due to the simulation termination condition before reaching the critical crack propagation size threshold, the corresponding sequence of crack length and cumulative cycle number is extrapolated by curve fitting to predict the number of load cycles required to reach the critical crack propagation size threshold, and this predicted value is used as the fatigue crack propagation life.

[0050] As a further aspect of the present invention, it also includes a step of correcting the stress intensity factor calculation function by taking into account the temperature and humidity monitoring data of the working environment:

[0051] Extract the average ambient temperature and relative humidity for the corresponding load condition time interval from the monitoring data of the working environment temperature and humidity;

[0052] Based on the average ambient temperature, the relationship curve between the material's elastic modulus and temperature is consulted to obtain the corrected elastic modulus of the material at the current temperature.

[0053] Based on the relative humidity, query the correction coefficient for the influence of ambient humidity on the material crack propagation threshold;

[0054] The modified elastic modulus is used to update the material elastic constants in the stress intensity factor calculation function;

[0055] The crack propagation threshold is dynamically adjusted using a correction coefficient that measures the effect of ambient humidity on the material crack propagation threshold.

[0056] The stress intensity factor variation spectrum and subsequent crack propagation parameters are recalculated using the updated stress intensity factor calculation function and the adjusted crack propagation threshold.

[0057] As a further aspect of the present invention, a maintenance decision generation step based on crack size evolution data is also included:

[0058] After predicting the fatigue crack propagation life, the fatigue crack propagation life is compared with the planned maintenance cycle of the metallurgical crane.

[0059] Based on the comparison results, the number of load cycles required for the current crack to expand from its initial size to a warning size requiring intervention is calculated, where the warning size is less than the critical size threshold for crack expansion.

[0060] Based on historical operation data of metallurgical cranes, the average number of load cycles per unit time in the future is estimated, and the required number of load cycles is converted into the corresponding calendar time.

[0061] Based on the calendar time and the planned maintenance cycle, a suggested time window for the next non-destructive testing and maintenance recommendations are generated.

[0062] As a further aspect of the present invention, a load spectrum compression and simplification step is also included:

[0063] The collected stress spectrum of the structure was processed using the rainflow counting method to extract the complete stress cycle;

[0064] The extracted stress cycles are classified and merged according to the magnitude of the stress amplitude and the average stress.

[0065] The frequency of occurrence of various stress cycles is statistically analyzed to generate a simplified block spectrum, which contains a finite number of representative combinations of stress amplitude, average stress, and cycle number.

[0066] In subsequent calculations of stress intensity factor variation spectrum and crack propagation, the simplified block spectrum is used instead of the original continuous load spectrum to reduce computational load.

[0067] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0068] For each independent load cycle in the load spectrum, the corresponding stress intensity factor amplitude at the crack tip is calculated one by one to construct a dedicated stress intensity factor variation spectrum. This allows the calculated parameters of the stress intensity factor to form a one-to-one matching relationship with the actual load cycle. The quantitative characterization of the stress state at the crack tip can closely match the real-time variation law under single-cycle load action. The value of the stress intensity factor amplitude can accurately reflect the actual action characteristics of the load cycle during the service of the metallurgical crane, reduce the parameter error caused by the segmented fitting of macroscopic loads, and ensure that the basic stress parameters for crack propagation calculation are highly consistent with the actual stress state of the metal structure.

[0069] By progressively accumulating the crack propagation increment in a single cycle, a complete record of crack size evolution data is generated, documenting the change in crack length with the number of load cycles. This evolution data is then directly compared with the critical crack propagation size threshold set in the structural integrity assessment criteria for metallurgical cranes. This provides a complete picture of the dynamic crack propagation process under load cycles, enabling progressively quantified recording of crack size changes. The setting of the critical size threshold directly aligns with the specifications for assessing the structural integrity of metallurgical cranes. The life prediction results are based on the actual crack size evolution process, avoiding the dispersion problem caused by traditional empirical formula estimations. This ensures that the life prediction results are consistent with the actual process of crack propagation to the critical state in metal structures. Attached Figure Description

[0070] Figure 1 This is a flowchart of a method for predicting fatigue crack propagation life of a metal structure for a metallurgical crane, as described in this invention.

[0071] Figure 2 This is a flowchart for calculating the stress intensity factor variation spectrum;

[0072] Figure 3 Core visualization of crack propagation life;

[0073] Figure 4 A temperature and humidity monitoring chart for the service environment of a metallurgical crane;

[0074] Figure 5 This is a spectrum showing the variation of the stress intensity factor. Detailed Implementation

[0075] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0076] In the description of this invention, it should be understood that the terms "length," "width," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships, are based on the orientation or positional relationships shown in the accompanying drawings and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, in the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0077] See Figure 1 This invention provides a method for predicting the fatigue crack propagation life of metal structures in metallurgical cranes, the specific method including:

[0078] Load condition data of the metallurgical crane under service environment were collected. This data included structural stress spectrum, strain monitoring time-series data of stress concentration areas, hoisting operation spectrum during metallurgical production, and temperature and humidity monitoring data of the working environment. An initial crack parameter model of the metal structure of the metallurgical crane was constructed. Based on the load condition data and the initial crack parameter model, the stress intensity factor variation spectrum was calculated. This spectrum included the stress intensity factor amplitude at the crack tip corresponding to each load cycle in the load spectrum. Based on the stress intensity factor variation spectrum, the single-cycle crack propagation increment of the initial crack under load conditions was calculated. The single-cycle crack propagation increment was cumulatively calculated to obtain crack size evolution data, which recorded the change in crack length with the number of load cycles. Combining the structural integrity assessment criteria of the metallurgical crane, a critical crack propagation size threshold was set. Based on the comparison between the crack size evolution data and the critical crack propagation size threshold, the fatigue crack propagation life of the metal structure of the metallurgical crane was predicted.

[0079] In one embodiment of the present invention, the initial crack parameter model includes the initial crack location, initial crack size, crack morphology classification, and main crack propagation direction defined based on the initial non-destructive testing results. Defect signal features are extracted from the initial non-destructive testing results, including defect echo amplitude, defect depth indication, and the projected length of the defect on the structural surface. Pattern recognition is performed on the defect signal features to classify the defect morphology into surface cracks, buried cracks, or corner cracks, and the structural component name and local coordinates corresponding to the initial crack location are determined. Based on the defect echo amplitude and defect depth indication, the initial crack size is calculated through a preset transformation relationship, including crack length, depth, and opening width. The main crack propagation direction is inferred based on the stress direction history of the metallurgical crane's metal structure and the finite element analysis results of the stress concentration area. The initial crack location, crack morphology classification, initial crack size, and main crack propagation direction are integrated into the data structure of the initial crack parameter model.

[0080] In practice, constructing an initial crack parameter model is the starting point for life prediction. The data for the initial crack parameter model comes from a comprehensive initial non-destructive testing of the metal structure of the metallurgical crane. This testing may use ultrasonic testing, magnetic particle testing, or radiographic testing methods. The testing range covers known stress concentration areas such as the main beam web, end beam welds, and pulley block support structure. The testing operation will generate a record of the original test results, including the defect location, waveform characteristics, and amplitude.

[0081] Extracting defect signal features from initial nondestructive testing results is a fundamental task. These features include defect echo amplitude, defect depth indication, and the projected length of the defect on the structural surface. The defect echo amplitude is directly read from the A-scan waveform of the ultrasonic flaw detector and recorded in decibels. The defect depth indication is automatically calculated by the flaw detector based on the propagation time and velocity of sound in the material. The projected length of the defect on the structural surface is determined by moving the probe across the surface and measuring the start and end points of the defect signal. Pattern recognition of the defect signal features involves comparing the original signal features with a feature database of known crack types. The pattern recognition process utilizes the proportional relationship between the defect depth indication and the projected length of the defect on the structural surface, as well as the dynamic waveform characteristics of the defect echo, to classify the defect morphology into surface cracks, buried cracks, or corner cracks. Simultaneously, by recording the probe's position in the structural grid coordinate system, the structural component name and local coordinates corresponding to the initial crack position are determined.

[0082] The initial crack size is calculated based on the defect echo amplitude and defect depth indication. This step is achieved through a preset conversion relationship, which can be a set of empirical formulas or a calibrated lookup table. For example, for a surface crack, the formula for its depth 'a' is:

[0083]

[0084] Where: a represents crack depth, D represents defect depth indication, k and b are material and detection method related constants calibrated through experiments, crack length c is obtained by combining the projection length of the defect on the structural surface with the probe beam width, and the opening width can be assigned a default statistical value according to the crack type or estimated based on empirical correlation.

[0085] It is understandable that inferring the direction of the main crack propagation requires combining historical load information with simulation analysis results. Based on the historical stress direction of the metal structure of the metallurgical crane, the information comes from the crane's operation records. It is identified that the main beam is mainly subjected to bending stress. Combined with the finite element analysis results of the stress concentration area of ​​the structure, the finite element analysis can show the direction of the maximum principal stress inside the component under the main bending load. By combining the historical stress direction with the direction of the maximum principal stress revealed by the finite element analysis, it is inferred that the direction of the main crack propagation is perpendicular to the direction of the maximum principal stress.

[0086] The initial crack location, crack morphology classification, initial crack size, and main crack propagation direction are integrated into a data structure for the initial crack parameter model. In practice, this data structure can be implemented using a standardized data object or database record row. The data structure contains fields for storing component number, local three-dimensional coordinates, crack morphology classification code, crack length value, crack depth value, opening width value, and a direction cosine vector representing the main crack propagation direction. This complete data structure serves as the geometric and positional input for all subsequent crack propagation calculations.

[0087] In one embodiment of the present invention, see [reference] Figure 2 From the load data, local hot spot stress variation sequences are extracted from the structural stress spectrum. These sequences correspond to the stress history near the crack tip. Based on the initial crack size, crack morphology classification, and main crack propagation direction in the initial crack parameter model, a stress intensity factor calculation function matching the metal structure material and crack type of the metallurgical crane is selected. Each stress value from the local hot spot stress variation sequence is input into the stress intensity factor calculation function to calculate the opening, sliding, and tearing stress intensity factors at the crack tip at the corresponding time. These factors are then weighted and synthesized according to the main crack propagation direction to obtain the composite stress intensity factor corresponding to the current load point. For the composite stress intensity factor within a complete load cycle in the load spectrum, the difference between its maximum and minimum values ​​is calculated and used as the crack tip stress intensity factor amplitude for the complete load cycle. This process is repeated for all load cycles to generate a stress intensity factor variation spectrum composed of the crack tip stress intensity factor amplitudes for each cycle.

[0088] In practical implementation, the stress change sequence of local hot spots in the structural stress spectrum is extracted from the load condition data. This local hot spot stress change sequence corresponds to the stress-time history of the region near the crack tip, obtained by converting strain monitoring time-series data through elastic mechanics relationships. Based on the initial crack size, crack morphology classification, and main crack propagation direction in the initial crack parameter model, a stress intensity factor calculation function that matches the metal structure material and crack type of the metallurgical crane is selected.

[0089] The stress value at each discrete time point of the local hot spot stress change sequence is input into the selected stress intensity factor calculation function to calculate the opening stress intensity factor, sliding stress intensity factor, and tearing stress intensity factor at the crack tip at the corresponding time. In a specific implementation, an example of the opening stress intensity factor calculation function for a given surface crack is as follows:

[0090]

[0091] Where: symbol The opening stress intensity factor at the crack tip is represented by the symbol [symbol missing]. The boundary correction factor is related to the crack geometry, component size, and loading method. (Symbol: ...) This represents the local hotspot stress value at the input calculation time, with the symbol... This represents the current crack depth. The calculation of the slip-type and tear-type stress intensity factors uses a similar function with different boundary correction coefficients.

[0092] For tearing cracks, an example of the tearing stress intensity factor calculation function is as follows:

[0093]

[0094] Among them, symbols The tearing stress intensity factor at the crack tip is represented by the symbol [symbol missing]. This represents the tear-type boundary correction factor related to crack geometry, component size, and loading method, with the symbol... The input calculation time corresponds to the local hot spot shear stress value of the tearing deformation, and the symbol is... This indicates the current crack depth. This formula provides a specific technical means to calculate the tear stress intensity factor, enabling those skilled in the art to perform calculations based on measured shear stress data and crack size.

[0095] An example of the function for calculating the slip stress intensity factor for a slip crack is as follows:

[0096]

[0097] Among them, symbols The slip-type stress intensity factor at the crack tip is represented by the symbol [symbol missing]. This represents the slip-out boundary correction factor related to crack geometry, component size, and loading method, with the symbol... The input calculation time corresponds to the local hot spot shear stress value of the slip-out deformation, and the symbol is... This indicates the current crack depth. This formula provides a specific technical means for calculating the slip-type stress intensity factor. Combined with the open-type stress intensity factor calculation formula disclosed in the specification, it enables those skilled in the art to fully perform the calculation of the stress intensity factor at the tip of a complex crack.

[0098] The calculated stress intensity factors for the opening, sliding, and tearing types are weighted and synthesized according to the main crack propagation direction to obtain the composite stress intensity factor corresponding to the current load point. In specific implementations, the weighted synthesis is performed by vector synthesis based on the projection components of each type of stress intensity factor on a plane perpendicular to the main crack propagation direction. For the composite stress intensity factor within a complete load cycle in the load spectrum, the difference between its maximum and minimum values ​​is calculated as the amplitude of the crack tip stress intensity factor corresponding to the complete load cycle. In some embodiments, a load cycle is defined as a complete change process from the lowest stress point through the highest stress point and back to the second lowest stress point. The amplitude of the crack tip stress intensity factor is denoted as... Its value is the maximum value of the composite stress intensity factor within this cycle. and minimum value The difference is calculated. All load cycles identified by the rainflow counting method are traversed, generating a stress intensity factor variation spectrum consisting of the crack tip stress intensity factor amplitude for each cycle. Optionally, the stress intensity factor variation spectrum is stored as a data list or sequence, where each item in the list records the load cycle number and its corresponding crack tip stress intensity factor amplitude. It can be understood that the traversal process is implemented iteratively by a computer program, repeatedly performing the above operations of calculating the composite stress intensity factor and obtaining the amplitude for each identified load cycle. Optionally, the stress intensity factor variation spectrum data can also be represented as a histogram of the distribution of the crack tip stress intensity factor amplitude relative to the number of cycles.

[0099] In one embodiment of the present invention, a fatigue crack propagation database of metallurgical crane structural materials is queried to obtain the correspondence between the crack propagation rate and the stress intensity factor amplitude. This correspondence includes a crack propagation threshold value, a Paris power law constant, and a fracture toughness value. The stress intensity factor amplitude at the crack tip during the current load cycle is read from the stress intensity factor variation spectrum, and it is determined whether this amplitude is greater than the crack propagation threshold value. If the amplitude is greater, the crack propagation rate under the current load cycle is calculated according to the Paris power law. The crack propagation rate is then multiplied by the time step of a single load cycle to obtain the crack propagation increment per cycle. If the amplitude is less than or equal to the crack propagation threshold value, the crack propagation increment per cycle is determined to be zero.

[0100] The initial crack size in the initial crack parameter model is used as the starting point for crack propagation calculation. Following the load spectrum sequence, starting from the first load cycle, the crack propagation increment calculated in each cycle is accumulated and added to the current crack length. After each accumulation step, the current cumulative cycle number and the corresponding cumulative crack length are recorded. When the cumulative crack length reaches a preset size recording point, the cumulative cycle number, crack length, and the stress intensity factor amplitude at the crack tip recalculated based on the current crack length are recorded. This accumulation and recording process continues until the stress intensity factor amplitude at the crack tip approaches the material's fracture toughness value, or the cumulative crack length reaches a preset simulation termination length, forming a data sequence containing the correspondence between the cumulative cycle number, crack length, and stress intensity factor amplitude—that is, crack size evolution data.

[0101] In practical implementation, the fatigue crack propagation database for metallurgical crane structural materials is consulted to obtain the correspondence between the crack propagation rate and the stress intensity factor amplitude. This correspondence includes the crack propagation threshold, the Paris power-law constant, and the fracture toughness value. The crack propagation database stores material performance data obtained through standard fatigue tests. The stress intensity factor amplitude at the crack tip during the current load cycle is read from the stress intensity factor variation spectrum. It is then determined whether the crack tip stress intensity factor amplitude exceeds the crack propagation threshold. The crack propagation threshold is a material-related constant; for Q345B steel, the typical range of the crack propagation threshold is 5.5 to 7.0 MPa·m. 1 / 2 If the stress intensity factor at the crack tip is greater than the crack propagation threshold, the crack propagation rate under the current load cycle is calculated according to the Paris power law. The Paris power law is expressed as follows:

[0102]

[0103] Where: symbol Represents the crack propagation rate, symbol and These are the Paris power-law constants obtained from the fatigue crack propagation database, with symbols... This represents the amplitude of the crack tip stress intensity factor during the current load cycle, read from the stress intensity factor variation spectrum. For the aforementioned Q345B steel, this is a constant. The typical value is approximately 3.5 × 10⁻⁶. -11 ,constant The typical value is approximately 3.2. It can be understood that after calculating the crack propagation rate, multiplying the crack propagation rate by the time step of a single load cycle yields the crack propagation increment per cycle.

[0104] The initial crack size in the initial crack parameter model is used as the starting point for crack propagation calculation. Following the order of the load spectrum, starting from the first load cycle, the single-cycle crack propagation increment calculated in each cycle is accumulated and added to the current crack length. In specific implementation, the load spectrum is a simplified block spectrum processed by the rainflow counting method, and the calculation process is performed sequentially according to the different stress levels in the block spectrum. After each accumulation step, the current cumulative cycle number and the corresponding cumulative crack length are recorded. When the cumulative crack length reaches a preset size recording point, the current cumulative cycle number, crack length, and the crack tip stress intensity factor amplitude recalculated based on the current crack length are recorded. The preset size recording point can be set to record data every 0.5 mm. It can be understood that recalculating the crack tip stress intensity factor amplitude based on the current crack length requires using the updated crack length, the current load level, and the stress intensity factor calculation function. The process of continuously accumulating and recording data continues until the stress intensity factor amplitude at the crack tip approaches the fracture toughness value of the material, or the cumulative crack length reaches a preset simulation termination length. This forms a data sequence containing the correspondence between the cumulative number of cycles, crack length, and stress intensity factor amplitude, i.e., crack size evolution data. Optionally, the simulation termination condition is set to the stress intensity factor amplitude at the crack tip reaching 90% of the material's fracture toughness value. In some embodiments, the preset simulation termination length can be set to 80% of the component plate thickness. Optionally, the crack size evolution data is also used to plot the relationship curve between crack length and cumulative number of cycles. This curve visually illustrates the crack propagation process.

[0105] See Figure 3 This is a core visualization of crack propagation life, intuitively showing the cumulative evolution of crack length in the metal structure of a metallurgical crane with the number of load cycles. It serves as the final output basis for fatigue life prediction and maintenance decisions. The initial crack length is approximately 1 mm, and it grows at an accelerated rate with increasing load cycles. From 0 to 25,000 cycles, the crack propagates slowly, increasing in size from 1 mm to approximately 10 mm. From 25,000 to 100,000 cycles, the crack propagation rate accelerates dramatically, with the size surging from 10 mm to approximately 110 mm, exhibiting typical fatigue crack "late-stage acceleration" characteristics. The critical size remains constant at approximately 10 mm, which is the failure warning threshold set in structural integrity assessment: when the crack length reaches this value, the remaining strength of the structure will not meet safety requirements, necessitating immediate intervention. The crack first reaches the critical size of 10 mm at approximately 25,000 load cycles, meaning that the predicted fatigue life from the initial crack state to the failure warning is 25,000 cycles. After more than 25,000 cycles, the crack size far exceeds the critical threshold, and the structure enters a high-risk failure stage, at which point it may break at any time.

[0106] In one embodiment of the present invention, structural design drawings, material mechanical property parameters, and failure safety factors of the metal structure of a metallurgical crane are obtained. Based on the structural design drawings, structural members containing initial cracks are identified, and the maximum design load and the most unfavorable load combination that the structural members will bear under normal operating conditions are determined. Based on the material mechanical property parameters and the most unfavorable load combination, the residual strength of the structural members in the presence of cracks is calculated. Based on the comparison between the residual strength and the structural working load, the failure safety factor is applied to deduce the maximum allowable crack size to ensure structural failure. The calculated maximum allowable crack size to ensure structural failure is set as the critical crack propagation size threshold. From the crack size evolution data, a sequence of corresponding crack lengths and cumulative cycle counts is extracted. In the sequence of corresponding crack lengths and cumulative cycle counts, the cumulative cycle count corresponding to the first time the crack length is equal to or greater than the critical crack propagation size threshold is found. The found cumulative cycle count is recorded as the predicted load cycle number from the initial crack propagation to the critical size, i.e., the fatigue crack propagation life. If the crack size evolution data ends due to the simulation termination condition before reaching the critical crack propagation size threshold, the number of load cycles required to reach the critical crack propagation size threshold is predicted by extrapolating the corresponding sequence of crack length and cumulative cycle count through curve fitting, and this predicted value is used as the fatigue crack propagation life.

[0107] In practice, the structural design drawings, material mechanical property parameters, and failure safety factors of the metal structure of the metallurgical crane are obtained. The structural design drawings define the geometric dimensions, connection forms, and design loads of the components. The material mechanical property parameters include data on the yield strength, tensile strength, fracture toughness, and fatigue performance of the material. The failure safety factor is selected based on the crane design specifications and service safety level. Based on the structural design drawings, structural components containing initial cracks are identified, and the maximum design load and the most unfavorable load combination that the structural components will bear under normal operating conditions are determined. In practice, the maximum design load is extracted from the hoisting operation spectrum and the structural stress spectrum. The most unfavorable load combination considers the simultaneous action of multiple load conditions such as hoisting dynamic load, running impact, and off-center load. It can be understood that the determination of the load combination follows relevant crane design standards.

[0108] Based on the material's mechanical properties and the most unfavorable load combination, the residual strength of the structural member in the presence of cracks is calculated. In some embodiments, the calculation of residual strength is based on the net section yield criterion or fracture mechanics criterion. An example formula based on net section stress is as follows:

[0109]

[0110] Where: symbol Represents the net cross-sectional stress of a component, symbol This represents the total load acting on the cross section of the cracked member under the most unfavorable load combination, with the symbol [symbol missing]. This represents the net cross-sectional area of ​​the component after deducting the projected area of ​​the crack. Based on the comparison between the residual strength and the structural working load, the failure safety factor is applied to deduce the maximum allowable crack size to ensure structural failure. In specific implementation, the condition for ensuring structural failure is expressed as follows: the yield strength of the material (or the critical stress corresponding to the fracture toughness) divided by the failure safety factor should be greater than or equal to the working stress calculated under the maximum allowable crack size. It can be understood that by iteratively calculating or analytically solving for the crack size that satisfies this condition, the maximum allowable crack size to ensure structural failure is obtained. The calculated maximum allowable crack size to ensure structural failure is set as the critical crack propagation size threshold, which is used as a fixed value or a function related to the load level for subsequent life prediction.

[0111] From the crack size evolution data, the corresponding sequence of crack length and cumulative cycle count is extracted. Within this sequence, the cumulative cycle count corresponding to the first time the crack length equals or exceeds the critical crack propagation size threshold is located. In practice, the crack size evolution data is stored in a data table, and the lookup is performed through sequential comparison or interpolation. See Table 1 for a simplified sequence of crack length and cumulative cycle count.

[0112] Table 1: Correspondence between crack length and cumulative number of cycles

[0113]

[0114] The accumulated number of cycles found is recorded as the predicted number of load cycles required for the initial crack to propagate to the critical size, i.e., the fatigue crack propagation life. In some embodiments, if the critical size threshold for crack propagation is set to 10.0 mm, then looking up the example in the table above, the crack length is 8.95 mm at 90,000 cycles and 10.05 mm at 105,000 cycles. Through linear interpolation, the fatigue crack propagation life can be predicted to be approximately 103,000 cycles. Optionally, if the crack size evolution data terminates due to reaching the critical size threshold for crack propagation before reaching the simulation termination condition, then the corresponding sequence of crack length and accumulated cycle count is extrapolated by curve fitting to predict the number of load cycles required to reach the critical size threshold for crack propagation, and this predicted value is used as the fatigue crack propagation life. It can be understood that curve fitting can use exponential functions, polynomials, or other models to fit the existing data sequence, and then substitute the critical size threshold for crack propagation into the fitting formula to solve for the corresponding number of cycles.

[0115] See Figure 4This is a temperature and humidity monitoring chart of the service environment for a metallurgical crane. It shows the changing trends of temperature and humidity in the service environment of the metallurgical crane under different cumulative load cycles, and is a key visualization for environmental impact analysis in fatigue crack propagation life prediction. The highly synchronized positive correlation between temperature and humidity indicates that in this service scenario, the more operation cycles, the more the environment tends to be "high temperature and high humidity." High temperature reduces the elastic modulus and yield strength of metallic materials, while high humidity accelerates material corrosion and lowers the crack propagation threshold. The combined effect of these two factors accelerates the fatigue crack propagation rate, which is a key factor that must be corrected in life prediction. The synchronous increase of temperature and humidity with load cycles suggests a correlation between operational intensity and environmental conditions, requiring the introduction of an environmental correction coefficient into the life prediction model to avoid underestimating the crack propagation risk. The high temperature and high humidity stage corresponds to the crack propagation acceleration period, which can be used to adjust the frequency of non-destructive testing and maintenance windows, providing early warning of structural failure risks.

[0116] In one embodiment of the present invention, the average ambient temperature and relative humidity for the corresponding load condition time interval are extracted from the ambient temperature and humidity monitoring data. Based on the average ambient temperature, the relationship curve between the material's elastic modulus and temperature is retrieved to obtain the corrected elastic modulus of the material at the current temperature. Based on the relative humidity, the correction coefficient for the influence of ambient humidity on the material's crack propagation threshold is retrieved. Using the corrected elastic modulus, the material's elastic constants in the stress intensity factor calculation function are updated. Using the correction coefficient for the influence of ambient humidity on the material's crack propagation threshold, the crack propagation threshold is dynamically adjusted. Using the updated stress intensity factor calculation function and the adjusted crack propagation threshold, the stress intensity factor variation spectrum and subsequent crack propagation parameters are recalculated.

[0117] After predicting the fatigue crack propagation life, it is compared with the planned maintenance cycle of the metallurgical crane. Based on the comparison results, the number of load cycles required for the current crack to expand from its initial size to the warning size requiring intervention is calculated, where the warning size is less than the critical crack propagation size threshold. The average number of load cycles per unit time in the future is estimated based on historical operating data of the metallurgical crane, and the required number of load cycles is converted into corresponding calendar times. Based on the calendar times and the planned maintenance cycle, a suggested time window for the next non-destructive testing and maintenance recommendations are generated. The collected structural stress spectrum is processed using the rainflow counting method to extract complete stress cycles. The extracted stress cycles are classified and merged according to stress amplitude and average stress. The frequency of occurrence of each type of stress cycle is statistically analyzed to generate a simplified block spectrum, which contains a finite number of representative combinations of stress amplitude, average stress, and cycle number. In subsequent calculations of the stress intensity factor variation spectrum and crack propagation, the simplified block spectrum is used instead of the original continuous load spectrum.

[0118] In practical implementation, the average ambient temperature and relative humidity for the corresponding load condition time interval are extracted from the ambient temperature and humidity monitoring data. The average ambient temperature is obtained by calculating the arithmetic mean of all temperature monitoring values ​​within the specified time interval, and the relative humidity is processed in the same way. Based on the average ambient temperature, the relationship curve between the material's elastic modulus and temperature is retrieved to obtain the corrected elastic modulus of the material at the current temperature. The relationship curve between the material's elastic modulus and temperature is obtained through high-temperature performance tests and stored in a data table. Based on the relative humidity, the correction coefficient for the influence of ambient humidity on the material's crack propagation threshold is retrieved. The correction coefficient is obtained from a pre-stored humidity influence relationship table. In practical implementation, the correction coefficient for the influence of ambient humidity on the material's crack propagation threshold... With relative humidity The relationship can be expressed by the following empirical relation:

[0119]

[0120] Where: symbol This represents a correction factor indicating the effect of ambient humidity on the crack propagation threshold of a material, with the symbol... The coefficient of sensitivity of a material to humidity, with the symbol [symbol missing]. The extracted average relative humidity value is represented by the symbol. This represents the reference relative humidity. As you can understand, it's a coefficient. and reference relative humidity It is determined by fatigue tests of the material under specific humidity conditions.

[0121] The material elastic constants in the stress intensity factor calculation function are updated using a modified elastic modulus. In some embodiments, the elastic modulus of the material is included as a parameter in the stress intensity factor calculation function, and the elastic modulus value in the function is replaced with the queried modified elastic modulus. The crack propagation threshold is dynamically adjusted using a correction coefficient for the influence of ambient humidity on the material crack propagation threshold. The adjusted crack propagation threshold is the original threshold value minus the correction coefficient. The product of stress intensity factor and subsequent crack propagation parameters is recalculated using the updated stress intensity factor calculation function and the adjusted crack propagation threshold. In practice, this means that the entire crack propagation life prediction process is re-executed from the step of calculating the stress intensity factor variation spectrum, but using material parameters corrected for temperature and humidity.

[0122] After predicting the fatigue crack propagation life, it is compared with the planned maintenance cycle of the metallurgical crane, which is determined according to the equipment maintenance procedures. Based on the comparison results, the number of load cycles required for the current crack to expand from its initial size to a warning size requiring intervention is calculated, where the warning size is less than the critical crack propagation size threshold. In practice, the warning size can be set as a percentage of the critical crack propagation size threshold, such as 80%. The average number of load cycles per unit time is estimated based on historical operating data of the metallurgical crane, and the required number of load cycles is converted into corresponding calendar times. This can be understood as the average number of load cycles per unit time being obtained by statistically analyzing the typical number of cycles per day or week in historical lifting operation patterns. Based on the converted calendar times and the planned maintenance cycle, a suggested next non-destructive testing (NDT) time window and maintenance recommendations are generated. Optionally, the suggested next NDT time window can be set to a point in time before the predicted crack reaches the warning size.

[0123] The collected structural stress spectrum is processed using the rainflow counting method to extract complete stress cycles. In specific implementations, the rainflow counting method decomposes the complex stress-time history into a series of independent closed stress cycles. The extracted stress cycles are classified and merged according to stress amplitude and mean stress, and the classification can be based on preset stress amplitude ranges and mean stress ranges. The frequency of occurrence of each type of stress cycle is statistically analyzed to generate a simplified block spectrum, which contains a finite number of representative combinations of stress amplitude, mean stress, and cycle number. In some embodiments, the simplified block spectrum is presented in list form, with each row containing a stress amplitude level, an mean stress level, and its corresponding cycle number. In subsequent calculations of the stress intensity factor variation spectrum and crack propagation, the simplified block spectrum is used instead of the original continuous load spectrum to reduce computational load. Optionally, when using the simplified block spectrum, the calculation process follows the stress level order defined in the block spectrum, sequentially calculating the crack propagation increment corresponding to each stress level and multiplying it by the cycle number of that stress level.

[0124] See Figure 5This is a stress intensity factor variation spectrum, used to illustrate the variation of the stress intensity factor amplitude at the crack tip in the metal structure of a metallurgical crane during load cycling. It is a key basis for predicting fatigue crack propagation life. Throughout the entire cycle, the stress intensity factor amplitude remains above the crack propagation threshold, indicating that the crack is continuously propagating throughout the service life, with no "crack arrest" interval. The stress intensity factor amplitude is much lower than the fracture toughness, indicating no immediate fracture risk at this stage, and the structural failure mode is primarily fatigue crack propagation. The fluctuations in the stress intensity factor amplitude directly determine the stage-specific differences in the crack propagation rate: during the 0-20000 cycle phase, the amplitude increases, and the crack propagation rate gradually accelerates; during the 20000-45000 cycle phase, the amplitude decreases, and the crack propagation rate gradually slows down; after 45000 cycles, the amplitude slightly rebounds, and the crack propagation rate increases again.

[0125] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments that can be applied to other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for predicting the fatigue crack propagation life of a metal structure in a metallurgical crane, characterized in that, The method includes: The load condition data of the metallurgical crane under service environment is collected. The load condition data includes structural stress spectrum, strain monitoring time series data of stress concentration area, lifting operation spectrum of metallurgical production process and temperature and humidity monitoring data of working environment. An initial crack parameter model of the metal structure of a metallurgical crane is constructed. Based on the load condition data and the initial crack parameter model, the stress intensity factor variation spectrum is calculated, including: From the load condition data, the local hot spot stress change sequence in the structural stress spectrum is extracted, and the local hot spot stress change sequence corresponds to the stress history in the region near the crack tip; Based on the initial crack size, crack morphology classification, and main crack propagation direction in the initial crack parameter model, a stress intensity factor calculation function matching the metal structure material and crack type of the metallurgical crane is selected. Each stress value of the local hot spot stress change sequence is input into the stress intensity factor calculation function to calculate the opening stress intensity factor, sliding stress intensity factor and tearing stress intensity factor of the crack tip at the corresponding moment. The opening stress intensity factor, the sliding stress intensity factor and the tearing stress intensity factor are weighted and synthesized according to the main crack propagation direction to obtain the composite stress intensity factor corresponding to the current load point. For the composite stress intensity factor within a complete load cycle in the load spectrum, calculate the difference between its maximum and minimum values, and use it as the amplitude of the crack tip stress intensity factor corresponding to the complete load cycle. Iterate through all load cycles to generate the stress intensity factor variation spectrum, which consists of the magnitude of the stress intensity factor at the crack tip for each cycle. The stress intensity factor variation spectrum includes the stress intensity factor amplitude at the crack tip corresponding to each load cycle in the load spectrum; Based on the stress intensity factor variation spectrum, the single-cycle crack propagation increment of the initial crack under the load condition is calculated. The incremental crack propagation of a single cycle is cumulatively calculated to obtain crack size evolution data, which records the change in crack length with the number of load cycles. Based on the structural integrity assessment criteria for metallurgical cranes, a critical crack propagation size threshold is set. By comparing the crack size evolution data with the critical crack propagation size threshold, the fatigue crack propagation life of the metal structure of the metallurgical crane is predicted. The method further includes the step of correcting the stress intensity factor calculation function by taking into account the temperature and humidity monitoring data of the working environment. Extract the average ambient temperature and relative humidity for the corresponding load condition time interval from the monitoring data of the working environment temperature and humidity; Based on the average ambient temperature, the relationship curve between the material's elastic modulus and temperature is consulted to obtain the corrected elastic modulus of the material at the current temperature. Based on the relative humidity, query the correction coefficient for the influence of ambient humidity on the material crack propagation threshold; The modified elastic modulus is used to update the material elastic constants in the stress intensity factor calculation function; The crack propagation threshold is dynamically adjusted using a correction coefficient that measures the effect of ambient humidity on the material crack propagation threshold. The stress intensity factor variation spectrum and subsequent crack propagation parameters are recalculated using the updated stress intensity factor calculation function and the adjusted crack propagation threshold.

2. The method for predicting fatigue crack propagation life of metal structures in metallurgical cranes according to claim 1, characterized in that, The initial crack parameter model for constructing the metal structure of the metallurgical crane includes: The initial crack parameter model includes the initial crack location, initial crack size, crack morphology classification, and main crack propagation direction defined based on the initial non-destructive testing results. Extract defect signal features from the initial nondestructive testing results. The defect signal features include defect echo amplitude, defect depth indication, and the projection length of the defect on the structural surface. Pattern recognition is performed on the defect signal features to classify the defect morphology into surface cracks, buried cracks, or corner cracks, and the name of the structural component and local coordinates corresponding to the initial position of the crack are determined. Based on the defect echo amplitude and defect depth indication, the initial size of the crack is calculated through a preset conversion relationship. The initial size of the crack includes the crack length, depth, and opening width. Based on the stress direction history of the metal structure of the metallurgical crane and the finite element analysis results of the stress concentration area of ​​the structure, the propagation direction of the main crack is inferred. The initial crack location, crack morphology classification, initial crack size, and main crack propagation direction are integrated into the data structure of the initial crack parameter model.

3. The method for predicting fatigue crack propagation life of metal structures in metallurgical cranes according to claim 1, characterized in that, Based on the stress intensity factor variation spectrum, the single-cycle crack propagation increment of the initial crack under the load condition is calculated, including: Query the fatigue crack propagation database of metal structure materials for metallurgical cranes to obtain the correspondence between the crack propagation rate and the stress intensity factor amplitude of the material. The correspondence includes the crack propagation threshold value, the Paris power law constant, and the fracture toughness value. Read the stress intensity factor amplitude at the crack tip during the current load cycle from the stress intensity factor variation spectrum; Determine whether the amplitude of the stress intensity factor at the crack tip is greater than the crack propagation threshold value; If the magnitude of the stress intensity factor at the crack tip is greater than the crack propagation threshold, the crack propagation rate under the current load cycle is calculated according to the Paris power law relationship. Multiplying the crack propagation rate by the time step of a single load cycle yields the crack propagation increment per cycle. If the magnitude of the stress intensity factor at the crack tip is less than or equal to the crack propagation threshold, then the single-cycle crack propagation increment is determined to be zero.

4. The method for predicting fatigue crack propagation life of metal structures in metallurgical cranes according to claim 3, characterized in that, The cumulative calculation of the single-cycle crack propagation increment yields crack size evolution data, including: The initial crack size in the initial crack parameter model is used as the starting point for crack propagation calculation; Following the order of the load spectrum, starting from the first load cycle, the single-cycle crack propagation increment calculated in each cycle is added to the current crack length. After each accumulation step, record the current accumulation cycle number and the corresponding accumulation crack length; When the cumulative crack length reaches the preset size recording point, record the cumulative number of cycles, crack length, and the amplitude of the crack tip stress intensity factor recalculated based on the current crack length. The accumulation and recording process continues until the stress intensity factor amplitude at the crack tip approaches the fracture toughness value of the material, or the cumulative crack length reaches the preset simulation termination length, forming a data sequence containing the corresponding relationship between the cumulative number of cycles, crack length, and stress intensity factor amplitude, i.e., the crack size evolution data.

5. The method for predicting fatigue crack propagation life of metal structures in metallurgical cranes according to claim 4, characterized in that, The structural integrity assessment criteria for metallurgical cranes include setting a critical crack propagation size threshold, including: Obtain structural design drawings, material mechanical property parameters, and failure safety factors for the metal structure of metallurgical cranes; Based on the structural design drawings, identify the structural components containing the initial crack, and determine the maximum design load and the most unfavorable load combination that the structural components will bear under normal working conditions; Based on the material mechanical property parameters and the most unfavorable load combination, calculate the residual strength of the structural member in the presence of cracks; Based on the comparison between the remaining strength and the structural working load, and by applying the failure safety factor, the maximum allowable crack size to ensure that the structure does not fail can be derived. The calculated maximum crack size that ensures the structure does not fail is set as the critical crack propagation size threshold.

6. The method for predicting fatigue crack propagation life of metal structures in metallurgical cranes according to claim 5, characterized in that, Based on the comparison between the crack size evolution data and the crack propagation critical size threshold, the fatigue crack propagation life of the metal structure of the metallurgical crane is predicted, including: Extract the corresponding sequence of crack length and cumulative cycle number from the crack size evolution data; In the sequence of crack length and cumulative number of cycles, find the cumulative number of cycles corresponding to the first time the crack length is equal to or greater than the critical size threshold for crack propagation; The accumulated number of cycles found is recorded as the predicted number of load cycles required from the initial crack propagation to the critical size, i.e., the fatigue crack propagation life; If the crack size evolution data has ended due to the simulation termination condition before reaching the critical crack propagation size threshold, the corresponding sequence of crack length and cumulative cycle number is extrapolated by curve fitting to predict the number of load cycles required to reach the critical crack propagation size threshold, and this predicted value is used as the fatigue crack propagation life.

7. The method for predicting fatigue crack propagation life of metal structures in metallurgical cranes according to claim 1, characterized in that, It also includes a maintenance decision generation step based on crack size evolution data: After predicting the fatigue crack propagation life, the fatigue crack propagation life is compared with the planned maintenance cycle of the metallurgical crane. Based on the comparison results, the number of load cycles required for the current crack to expand from its initial size to a warning size requiring intervention is calculated, where the warning size is less than the critical size threshold for crack expansion. Based on historical operation data of metallurgical cranes, the average number of load cycles per unit time in the future is estimated, and the required number of load cycles is converted into the corresponding calendar time. Based on the calendar time and the planned maintenance cycle, a suggested time window for the next non-destructive testing and maintenance recommendations are generated.

8. The method for predicting fatigue crack propagation life of metal structures in metallurgical cranes according to claim 7, characterized in that, It also includes a load spectrum compression and simplification step: The collected stress spectrum of the structure was processed using the rainflow counting method to extract the complete stress cycle; The extracted stress cycles are classified and merged according to the magnitude of the stress amplitude and the average stress. The frequency of occurrence of various stress cycles is statistically analyzed to generate a simplified block spectrum, which contains a finite number of representative combinations of stress amplitude, average stress, and cycle number. In subsequent calculations of stress intensity factor variation spectrum and crack propagation, the simplified block spectrum is used instead of the original continuous load spectrum to reduce computational load.

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