Stress deduction and fatigue damage evaluation method and system for ship welding seam

Abstract

The invention provides a ship weld joint stress deduction and fatigue damage evaluation method and system, and the method comprises the steps: firstly building a finite element model of a container ship, calculating the inherent frequency of the finite element model and a corresponding vibration mode through employing a Lanczos algorithm, and forming a plurality of free vibration modes of the finite element model; the inherent frequency of each free vibration mode is compared with a preset frequency range, and the free vibration mode in the preset frequency range is reserved and serves as a basic mode; calculating a theoretical stress distribution field of the whole ship structure in each basic mode based on the vibration mode through a strain-displacement relation equation and a material constitutive equation, arranging a plurality of stress monitoring points and weld joint evaluation points, and performing inversion by adopting a mode superposition method to obtain stress values of the weld joint evaluation points; and finally, calculating a total fatigue damage value by adopting a rain flow counting method and combining a fatigue evaluation method, and evaluating whether the total fatigue damage value is reliable or not, thereby facilitating the realization of long-term fatigue life management of the welding seam area.

Description

Technical Field

[0001] The present invention relates to the technical field of ship structure health monitoring, and in particular to a method and system for stress deduction and fatigue damage assessment at ship welds. Background Art

[0002] With the development of the global shipping industry, the safety and economic efficiency of ships have become a focus of attention. Long-term operation of ships under complex marine environmental loads can cause fatigue damage to structural materials, especially welds in welded structures, which are often areas of high stress concentration and fatigue damage.

[0003] However, the number of ship stress sensors is limited by the ship's structure, making them often impossible to install in areas such as welds. Current ship stress monitoring systems are unable to monitor stress at welds, and the use of numerical models and finite element software to calculate weld stress consumes significant time and computing resources, making them inapplicable for long-term stress monitoring of welds. This leads to unclear fatigue wear of ship structures, a lack of data support, and a lack of purpose during operation and maintenance, impacting the economic viability of ship operations and resulting in a waste of resources.

[0004] In summary, the existing ship monitoring system cannot assess and count fatigue damage in all key areas of concern, especially in areas prone to structural failure such as welds. Monitoring equipment cannot be installed, making it difficult to provide effective early warning of possible safety issues. Summary of the Invention

[0005] To address the problems of current ship stress monitoring systems, which are unable to monitor stress at welds and consume significant time and computing resources, making them unsuitable for long-term stress monitoring of welds, the present invention provides a method for stress deduction and fatigue damage assessment at ship welds. This method, based on data from a limited number of local stress monitoring devices, accurately calculates stress in the weld region, consumes minimal computing resources, and facilitates long-term fatigue life management. The present invention also relates to a system for stress deduction and fatigue damage assessment at ship welds.

[0006] The technical solutions of the present invention are as follows:

[0007] A method for stress deduction and fatigue damage assessment at a ship weld, characterized by comprising the following steps:

[0008] Free vibration mode calculation steps: Use finite element analysis software to establish a finite element model of the container ship. Based on the stiffness matrix and mass matrix of the finite element model, the Lanczos algorithm is used to calculate the natural frequency and corresponding vibration mode of the finite element model. Based on the natural frequency and vibration mode, multiple free vibration modes of the finite element model are composed. The natural frequency of each free vibration mode is then compared with the preset frequency range. The free vibration modes within the preset frequency range are retained and used as the basic modes.

[0009] Steps for arranging monitoring points and weld assessment points: Based on the vibration mode of each basic mode, the theoretical stress distribution field of the entire ship structure under each basic mode is calculated using the strain-displacement relationship equation and the material constitutive equation. Based on the theoretical stress distribution field, the key areas of the ship with the most sensitive structural response or stress concentration, as well as the midship area where the stress response of the ship under external loads is most intense, are determined. Multiple stress monitoring points are arranged in the key areas of the ship, and a weld assessment point is selected at a weld in the midship area as the fatigue assessment area.

[0010] Matrix construction and weld stress deduction steps: Based on the theoretical stress distribution field, the theoretical stress value of each stress monitoring point at the current moment under each basic mode is extracted to construct a coefficient matrix, and at the same time, the theoretical stress value of the weld evaluation point at the current moment under each basic mode is extracted to construct a modal stress vector; then, the stress monitoring equipment is used to collect the ship length direction stress data of each stress monitoring point at the current moment to construct a target matrix; based on the coefficient matrix and the target matrix, the modal superposition method is used to inversely calculate the weight coefficient of each basic mode; the weight coefficient is subjected to a vector inner product operation with the modal stress vector to deduce the stress value of the weld evaluation point at the current moment, and the stress values ​​of the weld evaluation point at the current moment and multiple moments after the current moment are continuously obtained to generate a weld stress time series;

[0011] The total fatigue damage value calculation steps are as follows: multiple stress cycles are extracted from the weld stress time series using the rain flow counting method, and the SN curve data of the material at the weld is obtained from the database. The fatigue damage value of each stress cycle is calculated based on the stress amplitude of each stress cycle and the SN curve data. The fatigue damage values ​​of all stress cycles are accumulated using the Miner criterion to obtain the total fatigue damage value of the weld assessment point;

[0012] Fatigue damage assessment steps: Calculate the relative error based on the total fatigue damage value and the preset theoretical loss value, and compare the relative error with the preset threshold to evaluate whether the calculated total fatigue damage value of the weld assessment point is reliable. If the relative error is less than the preset threshold, the calculated total fatigue damage value of the weld assessment point is judged to be reliable.

[0013] Preferably, in the step of arranging the monitoring points and weld assessment points, calculating the theoretical stress distribution field of the entire ship structure under each basic mode by using the strain-displacement relationship equation and the material constitutive equation specifically includes:

[0014] Based on the vibration mode and the strain-displacement relationship equation, the strain field of the entire ship structure under each basic mode is calculated. Then, based on the strain field and the material constitutive equation, the theoretical stress distribution field of the entire ship structure under each basic mode is calculated.

[0015] Preferably, in the total fatigue damage value calculation step, calculating the fatigue damage value of each stress cycle based on the stress amplitude of each stress cycle and in combination with the SN curve data specifically includes:

[0016] The SN curve data is used to find the fatigue life corresponding to the stress amplitude of each stress cycle, and the fatigue damage value of each stress cycle is calculated based on the fatigue life.

[0017] Preferably, in the free vibration mode calculation step, the basic modes include first-order vertical bending mode, second-order vertical bending mode, third-order vertical bending mode, first-order torsional mode, second-order torsional mode, third-order torsional mode and first-order lateral bending mode.

[0018] Preferably, in the step of arranging the monitoring points and weld assessment points, the key areas of the ship include the bow, midship area and stern of the ship; multiple stress monitoring points are arranged in the key areas of the ship, specifically arranged as follows: stress monitoring points are arranged at the deck position of the bow, the deck position of the stern, and the position near the waterline, the deck position and the side position of the midship, and each monitoring point is provided with a stress monitoring sensor;

[0019] In the matrix construction and weld stress deduction steps, the stress monitoring device is a long baseline strain sensor.

[0020] A ship weld stress deduction and fatigue damage assessment system is characterized by comprising a free vibration mode calculation module, a monitoring point and weld assessment point arrangement module, a matrix construction and weld stress deduction module, a total fatigue damage value calculation module, and a fatigue damage assessment module connected in sequence.

[0021] The free vibration mode calculation module uses finite element analysis software to establish a finite element model of the container ship. Based on the stiffness matrix and mass matrix of the finite element model, the natural frequency and corresponding mode shape of the finite element model are calculated using the Lanczos algorithm. Multiple free vibration modes of the finite element model are composed based on the natural frequencies and mode shapes. The natural frequencies of each free vibration mode are then compared with a preset frequency range, and the free vibration modes within the preset frequency range are retained as basic modes.

[0022] The monitoring point and weld assessment point arrangement module calculates the theoretical stress distribution field of the entire ship structure under each basic mode based on the vibration mode of each basic mode and through the strain-displacement relationship equation and the material constitutive equation. Based on the theoretical stress distribution field, the module determines the key areas of the ship where the structural response is most sensitive or stress is concentrated, as well as the midship area where the stress response of the ship under external loads is most intense. Multiple stress monitoring points are arranged in the key areas of the ship, and a weld assessment point is selected at a weld in the midship area as the fatigue assessment area.

[0023] The matrix construction and weld stress deduction module extracts the theoretical stress value of each stress monitoring point at the current moment under each basic mode based on the theoretical stress distribution field to construct a coefficient matrix, and simultaneously extracts the theoretical stress value of the weld assessment point at the current moment under each basic mode to construct a modal stress vector; then uses the stress monitoring equipment to collect the ship length direction stress data of each stress monitoring point at the current moment to construct a target matrix; based on the coefficient matrix and the target matrix, the modal superposition method is used to inversely calculate the weight coefficient of each basic mode; the weight coefficient is subjected to a vector inner product operation with the modal stress vector to deduce the stress value of the weld assessment point at the current moment, and the stress values ​​of the weld assessment point at the current moment and multiple moments after the current moment are continuously obtained to generate a weld stress time series;

[0024] The total fatigue damage value calculation module uses the rain flow counting method to extract multiple stress cycles from the weld stress time series, and obtains the SN curve data of the material at the weld from the database. The fatigue damage value of each stress cycle is calculated based on the stress amplitude of each stress cycle and the SN curve data. The fatigue damage values ​​of all stress cycles are accumulated using the Miner criterion to obtain the total fatigue damage value of the weld assessment point;

[0025] The fatigue damage assessment module calculates a relative error based on the total fatigue damage value and a preset theoretical loss value, and compares the relative error with a preset threshold to evaluate whether the calculated total fatigue damage value of the weld assessment point is reliable. If the relative error is less than the preset threshold, the calculated total fatigue damage value of the weld assessment point is determined to be reliable.

[0026] Preferably, in the monitoring point and weld assessment point arrangement module, calculating the theoretical stress distribution field of the entire ship structure under each basic mode through the strain-displacement relationship equation and the material constitutive equation specifically includes:

[0027] Based on the vibration mode and the strain-displacement relationship equation, the strain field of the entire ship structure under each basic mode is calculated. Then, based on the strain field and the material constitutive equation, the theoretical stress distribution field of the entire ship structure under each basic mode is calculated.

[0028] Preferably, in the total fatigue damage value calculation module, calculating the fatigue damage value of each stress cycle based on the stress amplitude of each stress cycle and in combination with the SN curve data specifically includes:

[0029] The SN curve data is used to find the fatigue life corresponding to the stress amplitude of each stress cycle, and the fatigue damage value of each stress cycle is calculated based on the fatigue life.

[0030] Preferably, in the free vibration mode calculation module, the basic modes include first-order vertical bending mode, second-order vertical bending mode, third-order vertical bending mode, first-order torsional mode, second-order torsional mode, third-order torsional mode and first-order lateral bending mode.

[0031] Preferably, in the monitoring point and weld assessment point arrangement module, the key areas of the ship include the bow, midship area and stern of the ship; multiple stress monitoring points are arranged in the key areas of the ship, specifically arranged as follows: stress monitoring points are arranged at the deck position of the bow, the deck position of the stern, and the position near the waterline, the deck position and the side position of the midship, and each monitoring point is provided with a stress monitoring sensor;

[0032] In the matrix construction and weld stress deduction module, the stress monitoring device is a long baseline strain sensor.

[0033] The beneficial effects of the present invention are:

[0034] The present invention provides a stress deduction and fatigue damage assessment method for ship welds. First, a finite element model of a container ship is established using finite element analysis software. Based on the stiffness matrix and mass matrix of the finite element model and the Lanczos algorithm, the natural frequency and corresponding vibration mode of the finite element model are calculated to form multiple free vibration modes. Through precise modeling and the Lanczos algorithm, the key dynamic characteristics (natural frequency and vibration mode) of the ship structure can be quickly and accurately identified, providing basic data for subsequent stress analysis and helping to determine which areas are more susceptible to fatigue damage. The natural frequencies of each free vibration mode are then compared with a preset frequency range, and the frequencies within the preset range are retained. The free vibration modes within the frequency range are set as the basic modes. By screening out the modes related to the actual operating conditions, unnecessary calculation complexity is reduced, the calculation efficiency is improved, and the subsequent analysis is ensured to focus on the vibration modes that have the greatest impact on the actual operation of the ship. Then, based on the vibration shapes of each basic mode and through the strain-displacement relationship equation and the material constitutive equation, the theoretical stress distribution field of the entire ship structure under each basic mode is calculated. The detailed stress distribution information is provided through theoretical calculation to help identify the key areas where the structural response is most sensitive or stress is concentrated, providing a basis for the optimal arrangement of sensors. Then, according to the theoretical stress distribution field, multiple stress monitoring points are arranged in the key areas of the ship, and in the midship area A weld assessment point is selected at a weld in the domain as the fatigue assessment area to optimize the location selection of the monitoring point and ensure that all possible high-risk areas are covered, especially the welds that are prone to fatigue damage, thereby improving the effectiveness and pertinence of the monitoring system; then, based on the theoretical stress distribution field, the theoretical stress value of each stress monitoring point at the current moment under each basic mode is extracted to construct a coefficient matrix, and at the same time, the theoretical stress value of the weld assessment point at the current moment under each basic mode is extracted to construct a modal stress vector, laying the foundation for subsequent modal superposition method calculations, so that the stress state of areas that are difficult to directly measure (such as welds) can be inverted through the data of a small number of measurement points; and then the stress monitoring point is used to calculate the stress state of areas that are difficult to directly measure (such as welds). The measuring equipment collects the stress data in the ship's longitudinal direction at each stress monitoring point at the current moment to construct a target matrix. Based on the coefficient matrix and the target matrix, the modal superposition method is used to inversely calculate the weight coefficients of each basic mode, realizing real-time monitoring of the stress state of the ship structure. The modal superposition method effectively converts the measured data into the stress distribution inside the structure, improving the real-time and accuracy of the monitoring. The weight coefficient is then subjected to a vector inner product operation with the modal stress vector to deduce the stress value of the weld assessment point at the current moment. This can achieve accurate calculation of the stress in the weld area based on data from a limited number of local monitoring devices, consumes less computing resources, and facilitates long-term fatigue life management.The stress values ​​of the weld assessment point at the current moment and multiple moments after the current moment are continuously obtained to generate a weld stress time series. By continuously monitoring the stress at the weld, potential signs of fatigue damage can be discovered in a timely manner, preventive measures can be taken in advance, the service life of the ship can be extended, and maintenance costs can be reduced. The rain flow counting method is then used to extract multiple stress cycles from the weld stress time series, and the SN curve data of the material at the weld is obtained from the database. The fatigue damage value of each stress cycle is calculated based on the stress amplitude of each stress cycle and the SN curve data. The fatigue damage values ​​of all stress cycles are accumulated using the Miner criterion to obtain the total fatigue damage value of the weld assessment point. The fatigue damage degree of the weld is quantified through scientific methods, providing a basis for formulating a reasonable maintenance plan and avoiding cost waste or safety hazards caused by excessive or insufficient maintenance. Finally, the relative error is calculated based on the total fatigue damage value and the preset theoretical loss value. The relative error is compared with the preset threshold to evaluate the reliability of the calculated total fatigue damage value of the weld assessment point, ensuring the accuracy of the fatigue damage assessment results, improving the reliability of the decision-making process, and supporting more accurate risk management and maintenance strategy formulation.

[0035] Furthermore, in the steps of arranging monitoring points and weld assessment points, the strain field of the entire ship structure under each basic mode is calculated based on the vibration mode and through the strain-displacement relationship equation, and then the theoretical stress distribution field of the entire ship structure under each basic mode is calculated based on the strain field and through the material constitutive equation. By calculating the strain field and the theoretical stress distribution field step by step, the accurate deduction from vibration mode data to structural stress is achieved, providing more accurate basic data for subsequent weld stress deduction, and improving the physical significance and engineering practicality of the stress field calculation.

[0036] Furthermore, in the total fatigue damage value calculation step, the SN curve data is used to find the fatigue life corresponding to the stress amplitude of each stress cycle, and the fatigue damage value of each stress cycle is calculated based on the fatigue life. The stress amplitude is directly linked to the fatigue life, making the fatigue damage calculation more consistent with the fatigue characteristics of the material, thereby improving the accuracy and reliability of fatigue assessment.

[0037] Furthermore, in the free vibration mode calculation step, the basic modes include first-order vertical bending mode, second-order vertical bending mode, third-order vertical bending mode, first-order torsional mode, second-order torsional mode, third-order torsional mode and first-order lateral bending mode. By limiting the specific types of basic modes, the main vibration forms of the ship are covered, ensuring that the response characteristics of the ship structure under various excitations can be fully captured in the modal superposition method, thereby enhancing the adaptability of the method to complex working conditions and the comprehensiveness of the evaluation results.

[0038] Furthermore, in the steps of arranging monitoring points and weld assessment points, the key areas of the ship include the bow, midship area and stern of the ship; multiple stress monitoring points are arranged in the key areas of the ship, specifically: stress monitoring points are arranged at the deck position of the bow, near the waterline position of the stern, and near the waterline position, deck position and side position of the midship, and each monitoring point is equipped with a stress monitoring sensor; by setting monitoring points at typical positions in key areas, a monitoring network covering the main stress concentration areas of the ship is formed, which effectively captures the stress changes in the sensitive areas of the structural response and provides sufficient measured data support for weld stress deduction.

[0039] The present invention, also known as a method for stress deduction and fatigue damage assessment at ship welds based on strain monitoring data, relates to a hull structure stress fatigue assessment technique based on the modal superposition method. This technique is used for fatigue life management of ship structures. By integrating multiple methods, including finite element analysis, modal analysis, stress monitoring, and fatigue damage assessment, the stress response at welds is derived through multi-step computational inversion and deduction, enabling comprehensive and efficient health monitoring of key ship components, particularly welds. After completing real-time stress deduction and calculation for different regions of the ship, data is collected and collated. The stress response cycles for the corresponding structural regions are extracted using the rainflow counting principle, and then accessed from a fatigue curve database for different structural materials on the ship to complete fatigue damage assessment and calculation, with statistical management performed on a per-segment basis. The stress inversion calculation process does not require the use of additional finite element software. Instead, stress deduction and fatigue calculations can be independently performed based on the program's built-in database and stress equipment monitoring data. Therefore, the method can be easily integrated into ship monitoring systems. Furthermore, the calculation process requires minimal computing resources, the inversion is highly timely, and the deduction and calculation delay for real-time stress data does not exceed 1 second. By using data from a limited number of local stress monitoring devices, accurate stress calculations can be achieved in the weld area, consuming less computing resources and facilitating long-term fatigue life management. This hull structure stress fatigue assessment technology will significantly improve the comprehensiveness and timeliness of ship fatigue damage assessments in actual engineering, providing reliable statistical data to help optimize decisions during ship design, manufacturing, and operation. Furthermore, the calculation process does not require interaction with the finite element model, allowing the fatigue assessment calculation program to be smoothly embedded in the ship monitoring system and meeting the computational timeliness requirements of intelligent ship navigation with minimal computer resources. Combining fatigue damage statistics from past voyages with ship structure material data can effectively provide early warnings of safety hazards caused by fatigue damage, ensuring the safety of personnel and property during ship operation. Furthermore, this hull structure stress fatigue assessment technology can achieve real-time stress monitoring at ship structure welds, calculate and assess fatigue damage to important ship structures, especially welds, provide timely early warnings of structural failures during navigation, and record ship structure fatigue losses based on voyages, providing reliable data support for ship operation and maintenance. The relative error between the deduced results of stress at welds and the calculated results of numerical models by this hull structure stress fatigue assessment technology is within 10%, and it can accurately reflect the characteristics of stress time history changes; it can accurately extract stress cycles based on stress time history data, and the error between the calculated structural fatigue damage and the theoretical value is within 10%. This invention not only improves the monitoring capabilities of ship structural safety, but also significantly reduces operating costs and reduces the risks caused by structural failure. In addition, it promotes the scientific and refined management of maintenance work, which is of great significance for improving the safety and economic benefits of the entire shipping industry.

[0040] The present invention also relates to a system for stress deduction and fatigue damage assessment at ship welds. This system corresponds to the above-mentioned method for stress deduction and fatigue damage assessment at ship welds and can be understood as a system for implementing the above-mentioned method for stress deduction and fatigue damage assessment at ship welds. The system includes a free vibration mode calculation module, a monitoring point and weld position arrangement module, a matrix construction and weld stress deduction module, a total fatigue damage value calculation module, and a fatigue damage assessment module, which are connected in sequence. The modules work together to achieve comprehensive and efficient health monitoring of key parts of the ship, especially the welds, by comprehensively applying multiple methods such as finite element analysis, modal analysis, stress monitoring, and fatigue damage assessment. This not only improves the monitoring capability of ship structural safety, but also significantly reduces operating costs and reduces the risks caused by structural failure. In addition, by accurately calculating the stress in the weld area based on data from a limited number of local stress monitoring devices, less computing resources are consumed, facilitating long-term fatigue life management, and promoting scientific and refined management of maintenance work, which is of great significance for improving the safety and economic benefits of the entire shipping industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 The present invention is a flow chart of the stress deduction and fatigue damage assessment method for ship welds.

[0042] Figure 2 It is a schematic diagram of the stress deduction and fatigue damage assessment method at ship welds of the present invention.

[0043] Figure 3 It is a schematic diagram of the time history curve of the stress deduction result of the present invention.

[0044] Figure 4 It is a schematic diagram of the arrangement of multiple stress monitoring points and weld assessment points of the present invention. DETAILED DESCRIPTION

[0045] The present invention will be described below with reference to the accompanying drawings.

[0046] The present invention relates to a method for stress deduction and fatigue damage assessment at ship welds. The purpose is to establish a fatigue damage assessment model for welds on large-opening container ships based on the modal superposition method and a finite element model. Using measured data from a limited number of stress monitoring devices installed on the ship, the fatigue damage of the weld region structure without sensors installed under external loads is calculated and deduced for fatigue life management of the ship structure. The vibration generated when a ship is subjected to wave loads is composed of various modes of various vibration modes. The modal superposition method can decouple the various vibration orders of the ship and obtain the stress response of the entire hull by linearly superimposing the stress distribution of each mode with its weight coefficient. The parameters required for ship structure stress deduction and fatigue damage calculation are integrated into an independent calculation program in the form of a database. This allows for real-time and rapid stress deduction, cycle extraction, and fatigue damage assessment calculations on equipment that does not rely on finite element calculation software. The ship structure fatigue assessment technology of the present invention can achieve fatigue damage calculation and assessment of each ship weld using a small number of monitoring devices, with low computing resource requirements and high computational efficiency. The present invention can complete the calculation and statistics of fatigue damage at the welds of ship structures under different sea conditions based on very few monitoring equipment, providing certain guidance for ship health monitoring and safety assessment. Figure 1 As shown, the following steps are included in sequence:

[0047] 1. Free vibration mode calculation steps: Use finite element analysis software to establish a finite element model of the container ship, based on the stiffness matrix and mass matrix of the finite element model, and use the Lanczos algorithm to calculate the natural frequency and corresponding vibration mode of the finite element model, and form multiple free vibration modes of the finite element model based on the natural frequency and vibration mode; then compare the natural frequency of each free vibration mode with the preset frequency range, and retain the free vibration mode within the preset frequency range as the basic mode. This step is based on the calculation of the free vibration mode based on the finite element model, which can be understood as follows: Figure 2 The database construction and calculation preparation steps shown involve establishing a finite element model of the entire ship, analyzing the ship's response characteristics, calculating the free vibration modes, establishing a basic model for subsequent stress analysis, and determining the vibration mode-related parameters.

[0048] Specifically, a finite element model of the entire ship is first established. According to the structural drawings of a 14,000 TEU container ship, a finite element model of the 14,000 TEU container ship is established using finite element analysis software (such as femap) to analyze the response characteristics of the ship. Among them, the plate and keel structure of the container ship are simulated using quadrilateral or triangular plate and shell elements, and the longitudinal bones and beams are simulated using two-node beam elements. According to the materials used in the actual ship (such as steel), the corresponding elastic modulus, Poisson's ratio, density and other parameters are assigned to the corresponding units, that is, the corresponding structural materials are assigned based on the material properties of the actual ship structure, and the ship weight distribution is adjusted (in order to accurately reflect the actual load distribution of the ship, the model needs to be corrected for mass distribution) and boundary conditions are set (simulating the constraints of the ship in actual operation, such as how the part of the hull bottom in contact with the water surface is subjected to force). Among them, the object of the fatigue assessment calculation test is a 14,000TEU container ship with a length of 330 meters, a width of 51 meters, and a depth of 30 meters. The test sea conditions are 14.1m draft and oblique waves, and the ship speed is 19.7kn.

[0049] Then calculate the free vibration mode. Based on the stiffness matrix and mass matrix of the finite element model, and using the Lanczos algorithm to calculate the natural frequency (square root of the eigenvalue) and the corresponding vibration mode (also called eigenvector or displacement mode shape) of the finite element model, multiple free vibration modes of the finite element model are composed based on the natural frequency and vibration mode; then the natural frequency of each free vibration mode is compared with the preset frequency range, and the free vibration mode within the preset frequency range is retained and used as the basic mode; that is, the free vibration mode with a natural frequency in the free vibration range of 0-50hz is retained, and the retained free vibration mode is used as the basic mode. Among them, the basic modes include first-order vertical bending mode, second-order vertical bending mode, third-order vertical bending mode, first-order torsional mode, second-order torsional mode, third-order torsional mode and first-order lateral bending mode.

[0050] 2. Steps for arranging monitoring points and weld assessment points: Based on the vibration mode of each basic mode, and through the strain-displacement relationship equation and the material constitutive equation, calculate the theoretical stress distribution field of the entire ship structure under each basic mode, and determine the key areas of the ship where the structural response is most sensitive or stress is concentrated according to the theoretical stress distribution field, as well as the midship area where the stress response of the ship is most intense under external loads; arrange multiple stress monitoring points in the key areas of the ship, and select a weld assessment point at a weld in the midship area as the fatigue assessment area. This step arranges monitoring equipment based on the deformation characteristics of the ship, which can be understood as follows: Figure 2 The ship-side equipment arrangement steps shown involve analyzing the ship's deformation characteristics and arranging stress monitoring equipment. By analyzing the ship's deformation characteristics, several suitable locations are determined for arranging stress monitoring equipment and other measuring points.

[0051] Specifically, first, based on the vibration shape of each basic mode, the strain field ∈ of the entire ship structure under each basic mode is calculated through the strain-displacement relationship equation in continuum mechanics, and the calculation is performed according to the following formula:

[0052]

[0053] In the above formula, φ is the vibration mode. The vibration mode is a description of the shape of the structure under a specific vibration mode, which reflects the relative displacement relationship of each point of the structure when the mode vibrates. For ship structures, different basic modes (such as vertical bending, torsion, lateral bending and other modes) correspond to different vibration modes. For example, in the first-order vertical bending mode, the ship will present a shape similar to an upward or downward arch in the middle, and this shape feature is characterized by the corresponding vibration mode φ. The above strain-displacement relationship equation describes the relationship of calculating the strain field through the vibration mode, It is a gradient operator. By calculating the gradient of the vibration mode and operating according to this formula, the strain distribution of the structure is obtained. It is based on the theory of continuum mechanics and reflects the intrinsic connection between structural deformation and displacement.

[0054] Then, based on the strain field and the material constitutive equation, the theoretical stress distribution field of the entire ship structure under each basic mode is calculated according to the following formula:

[0055] σ=C ∈

[0056] In the above formula, C represents the material stiffness matrix. The above material constitutive equation describes the stress-strain relationship of the material. Different materials have different stiffness matrices C. Through this material constitutive equation, when the strain field ∈ is known, the stress field σ of the structure can be calculated, reflecting the influence of the material's own mechanical properties on the structural stress.

[0057] According to the theoretical stress distribution field, the key areas of the ship where the structural response is most sensitive or the stress is concentrated, as well as the midship area where the stress response of the ship under external loads is most intense, are determined. Multiple stress monitoring points are arranged in the key areas of the ship (the bow, midship area and stern of the ship) (each monitoring point is equipped with stress monitoring equipment such as stress monitoring sensors, so it can also be called arranging multiple stress monitoring equipment), and a weld evaluation point is selected at a weld in the midship area as the fatigue evaluation area, that is, Figure 3As shown in the figure, measuring points are arranged at 1 / 4L of the bow (L is the ship length), midship and 1 / 4L of the stern of the ship, including 7 stress monitoring points P1 to P7 near the waterline, on the deck and on the side (stress monitoring points P1, P2, P3, P4, P5, P6, and P7), and a weld evaluation point W1 is selected at a weld in the midship area as the fatigue assessment area. Preferably, the specific arrangement is as follows: the stress monitoring point P1 is arranged at the deck position at the stern 1 / 4L, and the rib position is FR95; the stress monitoring point P2 is arranged at the deck position at the bow 1 / 4L, and the rib position is FR313; the stress monitoring points P3-P7 are arranged in the midship area, and the rib positions are all FR221, among which P3-P7 cover the midship waterline, deck, and side positions respectively; W1 is located at a weld in the midship area, and the rib position is FR221. It is the range where the stress response of the ship under external loads is most intense, and the weld area is the key area of ​​concern where fatigue damage accumulation leads to structural failure. The good inversion effect in this area is more representative and can prove the reliability of the fatigue assessment method.

[0058] 3. Matrix construction and weld stress deduction steps: Based on the theoretical stress distribution field, the theoretical stress value of each stress monitoring point at the current moment under each basic mode is extracted to construct a coefficient matrix, and at the same time, the theoretical stress value of the weld evaluation point at the current moment under each basic mode is extracted to construct a modal stress vector; then the stress monitoring equipment is used to collect the ship length direction stress data of each stress monitoring point at the current moment to construct a target matrix; based on the coefficient matrix and the target matrix, the modal superposition method is used to inversely calculate the weight coefficient of each basic mode; the weight coefficient is vector-inner-producted with the modal stress vector to deduce the stress value of the weld evaluation point at the current moment, and the stress values ​​of the weld evaluation point at the current moment and multiple moments after the current moment are continuously obtained to generate a weld stress time series. This step is based on the modal superposition principle to deduce the stress response at the weld, which can be understood as follows. Figure 2 The stress response deduction steps shown involve calculating the basic modal coefficients, modal superposition, and obtaining the weld stress deduction value through linear superposition. Based on the previous modal calculation results, the basic modal coefficients are calculated and the weld stress value is obtained using the modal superposition method.

[0059] Specifically, taking a sampling moment during the test sea conditions as an example, a calculation example of stress deduction at the weld is performed. P1, P2, P3, P4, P5, P6, and P7 are stress monitoring points. Using a stress deduction technique based on the modal superposition method, the stress response of point W1 at the weld under the current sea conditions is deduced. Based on the theoretical stress distribution field, the theoretical stress values ​​of each stress monitoring point at the current moment under each basic mode are extracted to construct a coefficient matrix A, i.e., the basic modal stress matrix A for each stress monitoring point. The coefficient matrix A (MPa) is shown in Table a. Different rows in the coefficient matrix A represent the theoretical stress values ​​of each stress monitoring point under each basic mode, and different columns represent the theoretical stress values ​​of each stress monitoring point under a certain basic mode.

[0060] Table a

[0061]

[0062] At the same time, based on the theoretical stress distribution field, the theoretical stress value of the weld evaluation point W1 at the current moment under each basic mode is extracted to construct the modal stress vector A'. The modal stress vector A' (MPa) is shown in Table b.

[0063] Table b

[0064]

[0065] Then, using stress monitoring equipment (i.e., long-baseline strain sensors), we collect stress data in the ship's longitudinal direction at each stress monitoring point at the current moment to construct a target matrix B, as shown in Table C. The data transmission frequency of the long-baseline strain sensors is 10 Hz. In this paper, stress refers to unidirectional stress in the ship's longitudinal direction.

[0066] Table c

[0067]

[0068]

[0069] The distribution of each stress monitoring sensor and the rib position in the verification area is shown in Table d.

[0070] Table d

[0071]

[0072] Then, based on the coefficient matrix A and the target matrix B, the modal superposition method is used to solve the linear equation group AX=B, and the weight coefficient X of each basic mode is inversely calculated. The weight coefficient X of each basic mode is shown in Table e.

[0073] Table e

[0074]

[0075] Then, the weight coefficient X is vector-inner-producted with the modal stress vector A', i.e., A'X = B', and the stress value B' of the weld evaluation point at the current moment is deduced. The stress values ​​of the weld evaluation point at the current moment and at multiple moments after the current moment are continuously obtained to generate the weld stress time series, i.e., the stress values ​​of the weld evaluation point at the current moment and at multiple moments after the current moment are continuously obtained to generate the weld stress time series. k Repeat the above calculation to obtain the weld stress time series.

[0076] For example, suppose the stress value B' of the weld assessment point W1 at the current moment is 37.69 MPa. According to the monitoring equipment data, the theoretical stress response value at W1 at the current moment is 35.36 MPa, so the relative error of the stress deduction technology at the current moment is 6.61%. Based on the above calculation process, the stress response at W1 within 5 minutes is deduced and calculated, and the stress inversion result time history curve is as follows: Figure 4 As shown in the figure, the relative error of stress deduction at each moment is an average of 4.69% within 5 minutes. The test shows that the stress inversion link based on the modal superposition method can calculate the stress response of the ship area without detection equipment, and can more accurately reflect the time domain characteristics of the stress response of the ship structure under external loads. It can be applied to the health monitoring and safety assessment of actual ship structures.

[0077] 4. Calculation steps of total fatigue damage value: Use the rain flow counting method to extract multiple stress cycles from the weld stress time series, and obtain the SN curve data of the material at the weld from the database. Then calculate the fatigue damage value of each stress cycle based on the stress amplitude of each stress cycle and the SN curve data, and use the Miner criterion to accumulate the fatigue damage values ​​of all stress cycles to obtain the total fatigue damage value of the weld assessment point. This step can be understood as follows: Figure 2 The fatigue loss calculation based on stress history data in the weld fatigue assessment step shown is to use stress history data and calculate the fatigue loss of the weld under different stress cycles based on relevant theories (such as Miner's linear cumulative damage theory or Miner's criterion) to obtain the total fatigue damage value.

[0078] Specifically, all local maxima and minima in the weld stress time series are first identified to form a peak-valley sequence. For example, from the weld stress time series to the peak-valley sequence: [30, 25, 40, 10, 50] → [30 (valley), 40 (peak), 10 (valley), 50 (peak)]. The three-point rainflow counting method is then used to sequentially extract closed stress cycles from the peak-valley sequence. In other words, the three-point rainflow counting method is used to extract stress cycles from the stress history data, and the computational reliability of the rainflow counting method is verified using a small segment of stress history data. All extreme values ​​in this segment of stress history data are shown in Table 1.

[0079] Table 1

[0080]

[0081] The manual calculation results based on the rain flow principle and formula and the calculation results of the rain flow program of the present invention are shown in Table 2.

[0082] Table 2

[0083]

[0084] It can be seen from the above table that the calculation results of stress cycle extraction based on the rainflow counting method in the method of the present invention are correct and reliable.

[0085] Then, the SN curve data (i.e., fatigue curve data) of the material at the weld is obtained from a database (such as a ship structure fatigue strength database), and the fatigue damage value of each stress cycle is calculated based on the stress amplitude of each stress cycle and the SN curve. That is, the fatigue life corresponding to the stress amplitude of each stress cycle is found using the SN curve, and the fatigue damage value of each stress cycle is calculated based on the fatigue life; and the Miner criterion is used to accumulate the fatigue damage values ​​of all stress cycles to obtain the total fatigue damage value.

[0086] 5. Fatigue damage assessment step: Calculate the relative error based on the total fatigue damage value and the preset theoretical loss value, and compare the relative error with the preset threshold to evaluate whether the calculated total fatigue damage value of the weld assessment point is reliable. If the relative error is less than the preset threshold, the calculated total fatigue damage value of the weld assessment point is considered reliable. This step can be understood as follows: Figure 2 In the weld fatigue assessment steps shown, fatigue life statistics and evaluation are performed based on the voyage section. After obtaining the total fatigue damage value, the fatigue damage value is further converted into fatigue life statistics and evaluation based on the operating conditions of different voyage sections of the ship (such as speed, sea conditions, etc.), and it is determined whether the fatigue state of the weld meets the requirements.

[0087] The reliability of fatigue loss calculation was verified by performing multiple loadings with different stress amplitudes and different numbers of cycles. The first six loadings were used to compare the fatigue loss calculation results of this method with the theoretical fatigue loss calculation results, as shown in Table 3. The reliability criticism criterion is the relative error between the loss calculated by this method and the theoretical loss value provided in the literature. The calculation formula is: relative error = (calculated loss - theoretical loss value) / theoretical loss value.

[0088] Table 3

[0089]

[0090]

[0091] As can be seen from the above table, the error between the calculated structural fatigue loss value obtained by this method and the theoretical loss value does not exceed 10%, which has good reliability and can be used for fatigue damage calculation and evaluation of ship structures.

[0092] Therefore, this method is used to calculate the fatigue damage at the weld for the stress history fluctuation within 3 hours of the example sea condition. According to the stress curve at the weld obtained by the numerical model and the stress curve obtained by deduction, the data obtained by stress cycle extraction and fatigue damage calculation are shown in Table 4.

[0093] Table 4 Fatigue damage error table

[0094]

[0095] According to the data in the above table, it can be concluded that the deduction and calculation results of stress fatigue damage at ship structure welds by the present invention have good reliability, and the error with the results obtained by long-term calculation of the numerical model does not exceed 20%, which has practical engineering value.

[0096] The present invention also relates to a stress deduction and fatigue damage assessment system for ship welds. This system corresponds to the above-mentioned stress deduction and fatigue damage assessment method for ship welds and can be understood as a system for implementing the above-mentioned method. The system includes a free vibration mode calculation module, a monitoring point and weld assessment point arrangement module, a matrix construction and weld stress deduction module, a total fatigue damage value calculation module, and a fatigue damage assessment module, which are connected in sequence. It can calculate the free vibration mode based on the finite element model, arrange monitoring equipment based on the ship deformation characteristics, deduce the stress response at the weld based on the modal superposition principle, and complete the fatigue damage assessment based on the stress history data. Specifically,

[0097] The free vibration mode calculation module uses finite element analysis software to establish a finite element model of the container ship. Based on the stiffness matrix and mass matrix of the finite element model, the natural frequency and corresponding mode shape of the finite element model are calculated using the Lanczos algorithm. Multiple free vibration modes of the finite element model are composed based on the natural frequencies and mode shapes. The natural frequencies of each free vibration mode are then compared with a preset frequency range, and the free vibration modes within the preset frequency range are retained as basic modes.

[0098] The monitoring point and weld assessment point arrangement module calculates the theoretical stress distribution field of the entire ship structure under each basic mode based on the vibration mode of each basic mode and through the strain-displacement relationship equation and the material constitutive equation. Based on the theoretical stress distribution field, the module determines the key areas of the ship where the structural response is most sensitive or stress is concentrated, as well as the midship area where the stress response of the ship under external loads is most intense. Multiple stress monitoring points are arranged in the key areas of the ship, and a weld assessment point is selected at a weld in the midship area as the fatigue assessment area.

[0099] The matrix construction and weld stress deduction module extracts the theoretical stress value of each stress monitoring point at the current moment under each basic mode based on the theoretical stress distribution field to construct a coefficient matrix, and simultaneously extracts the theoretical stress value of the weld assessment point at the current moment under each basic mode to construct a modal stress vector; then uses the stress monitoring equipment to collect the ship length direction stress data of each stress monitoring point at the current moment to construct a target matrix; based on the coefficient matrix and the target matrix, the modal superposition method is used to inversely calculate the weight coefficient of each basic mode; the weight coefficient is subjected to a vector inner product operation with the modal stress vector to deduce the stress value of the weld assessment point at the current moment, and the stress values ​​of the weld assessment point at the current moment and multiple moments after the current moment are continuously obtained to generate a weld stress time series;

[0100] The total fatigue damage value calculation module uses the rain flow counting method to extract multiple stress cycles from the weld stress time series, and obtains the SN curve data of the material at the weld from the database. The fatigue damage value of each stress cycle is calculated based on the stress amplitude of each stress cycle and the SN curve data. The fatigue damage values ​​of all stress cycles are accumulated using the Miner criterion to obtain the total fatigue damage value of the weld assessment point;

[0101] The fatigue damage assessment module calculates a relative error based on the total fatigue damage value and a preset theoretical loss value, and compares the relative error with a preset threshold to evaluate whether the calculated total fatigue damage value of the weld assessment point is reliable. If the relative error is less than the preset threshold, the calculated total fatigue damage value of the weld assessment point is determined to be reliable.

[0102] Preferably, in the monitoring point and weld assessment point arrangement module, the theoretical stress distribution field of the entire ship structure under each basic mode is calculated by the strain-displacement relationship equation and the material constitutive equation, specifically including:

[0103] Based on the vibration mode and the strain-displacement relationship equation, the strain field of the entire ship structure under each basic mode is calculated. Then, based on the strain field and the material constitutive equation, the theoretical stress distribution field of the entire ship structure under each basic mode is calculated.

[0104] Preferably, in the total fatigue damage value calculation module, calculating the fatigue damage value of each stress cycle based on the stress amplitude of each stress cycle and in combination with the SN curve data specifically includes:

[0105] The SN curve is used to find the fatigue life corresponding to the stress amplitude of each stress cycle, and the fatigue damage value of each stress cycle is calculated based on the fatigue life.

[0106] Preferably, in the free vibration mode calculation module, the basic modes include first-order vertical bending mode, second-order vertical bending mode, third-order vertical bending mode, first-order torsional mode, second-order torsional mode, third-order torsional mode and first-order lateral bending mode.

[0107] Preferably, in the monitoring point and weld assessment point arrangement module, the key areas of the ship include the bow, midship area and stern of the ship; multiple stress monitoring points are arranged in the key areas of the ship, specifically arranged as follows: stress monitoring points are arranged at the deck position of the bow, the deck position of the stern, and the position near the waterline, the deck position and the side position of the midship, and each monitoring point is provided with a stress monitoring sensor;

[0108] In the matrix construction and weld stress deduction module, the stress monitoring device is a long baseline strain sensor.

[0109] The present invention provides an objective and scientific method and system for stress deduction and fatigue damage assessment at ship welds. By comprehensively applying multiple methods such as finite element analysis, modal analysis, stress monitoring and fatigue damage assessment, it achieves comprehensive and efficient health monitoring of key parts of ships, especially welds. This not only improves the monitoring capability of ship structural safety, but also significantly reduces operating costs and reduces the risks caused by structural failure. In addition, by using data from a limited number of local stress monitoring devices, it can accurately calculate the stress in the weld area, consume less computing resources, facilitate long-term fatigue life management, and promote the scientific and refined management of maintenance work. It is of great significance to improving the safety and economic benefits of the entire shipping industry.

[0110] It should be noted that the specific embodiments described above can enable those skilled in the art to more fully understand the present invention, but do not limit the present invention in any way. Therefore, although this specification has described the present invention in detail with reference to the drawings and embodiments, those skilled in the art should understand that the present invention can still be modified or replaced with equivalents. In short, all technical solutions and improvements that do not depart from the spirit and scope of the present invention should be included in the scope of protection of the patent for the present invention.

Claims

1. A method for stress deduction and fatigue damage assessment at ship welds, characterized in that: The following steps are involved: Free vibration mode calculation steps: Use finite element analysis software to establish a finite element model of the container ship. Based on the stiffness matrix and mass matrix of the finite element model, the Lanczos algorithm is used to calculate the natural frequency and corresponding vibration mode of the finite element model. Based on the natural frequency and vibration mode, multiple free vibration modes of the finite element model are composed. The natural frequency of each free vibration mode is then compared with the preset frequency range. The free vibration modes within the preset frequency range are retained and used as the basic modes. Steps for arranging monitoring points and weld assessment points: Based on the vibration mode of each basic mode, the theoretical stress distribution field of the entire ship structure under each basic mode is calculated using the strain-displacement relationship equation and the material constitutive equation. Based on the theoretical stress distribution field, the key areas of the ship with the most sensitive structural response or stress concentration, as well as the midship area where the stress response of the ship under external loads is most intense, are determined. Multiple stress monitoring points are arranged in the key areas of the ship, and a weld assessment point is selected at a weld in the midship area as the fatigue assessment area. Matrix construction and weld stress deduction steps: Based on the theoretical stress distribution field, the theoretical stress value of each stress monitoring point at the current moment under each basic mode is extracted to construct a coefficient matrix. At the same time, the theoretical stress value of the weld evaluation point at the current moment under each basic mode is extracted to construct a modal stress vector. Then, the stress monitoring equipment is used to collect the ship's longitudinal stress data of each stress monitoring point at the current moment to construct a target matrix; Based on the coefficient matrix and the target matrix, the weight coefficients of each basic mode are calculated by inversion using the modal superposition method; Performing a vector inner product operation on the weight coefficient and the modal stress vector to deduce the stress value of the weld evaluation point at the current moment, and continuously obtaining the stress values ​​of the weld evaluation point at the current moment and multiple moments after the current moment to generate a weld stress time series; The total fatigue damage value calculation steps are as follows: multiple stress cycles are extracted from the weld stress time series using the rain flow counting method, and the SN curve data of the material at the weld is obtained from the database. The fatigue damage value of each stress cycle is calculated based on the stress amplitude of each stress cycle and the SN curve data. The fatigue damage values ​​of all stress cycles are accumulated using the Miner criterion to obtain the total fatigue damage value of the weld assessment point; Fatigue damage assessment steps: Calculate the relative error based on the total fatigue damage value and the preset theoretical loss value, and compare the relative error with the preset threshold to evaluate whether the calculated total fatigue damage value of the weld assessment point is reliable. If the relative error is less than the preset threshold, the calculated total fatigue damage value of the weld assessment point is judged to be reliable.

2. The method for stress deduction and fatigue damage assessment at ship welds according to claim 1, characterized in that: In the step of arranging the monitoring points and weld assessment points, the theoretical stress distribution field of the entire ship structure under each basic mode is calculated by the strain-displacement relationship equation and the material constitutive equation, specifically including: Based on the vibration mode and the strain-displacement relationship equation, the strain field of the entire ship structure under each basic mode is calculated. Then, based on the strain field and the material constitutive equation, the theoretical stress distribution field of the entire ship structure under each basic mode is calculated.

3. The method for stress deduction and fatigue damage assessment at ship welds according to claim 1, characterized in that: In the total fatigue damage value calculation step, the fatigue damage value of each stress cycle is calculated based on the stress amplitude of each stress cycle and combined with the SN curve data, specifically including: The SN curve data is used to find the fatigue life corresponding to the stress amplitude of each stress cycle, and the fatigue damage value of each stress cycle is calculated based on the fatigue life.

4. The method for stress deduction and fatigue damage assessment at ship welds according to any one of claims 1 to 3, characterized in that: In the free vibration mode calculation step, the basic modes include a first-order vertical bending mode, a second-order vertical bending mode, a third-order vertical bending mode, a first-order torsional mode, a second-order torsional mode, a third-order torsional mode and a first-order lateral bending mode.

5. The method for stress deduction and fatigue damage assessment at ship welds according to claim 1 or 2, characterized in that: In the step of arranging the monitoring points and weld assessment points, the key areas of the ship include the bow, midship area, and stern of the ship; multiple stress monitoring points are arranged in the key areas of the ship, specifically: stress monitoring points are arranged at the deck position of the bow, the deck position of the stern, and the position near the waterline, the deck position, and the side position of the midship, and each monitoring point is provided with a stress monitoring sensor; In the matrix construction and weld stress deduction steps, the stress monitoring device is a long baseline strain sensor.

6. A stress deduction and fatigue damage assessment system for ship welds, characterized in that: It includes the free vibration mode calculation module, monitoring point and weld assessment point arrangement module, matrix construction and weld stress deduction module, total fatigue damage value calculation module and fatigue damage assessment module, which are connected in sequence. The free vibration mode calculation module uses finite element analysis software to establish a finite element model of the container ship. Based on the stiffness matrix and mass matrix of the finite element model, the natural frequency and corresponding mode shape of the finite element model are calculated using the Lanczos algorithm. Multiple free vibration modes of the finite element model are composed based on the natural frequencies and mode shapes. The natural frequencies of each free vibration mode are then compared with a preset frequency range, and the free vibration modes within the preset frequency range are retained as basic modes. The monitoring point and weld assessment point arrangement module calculates the theoretical stress distribution field of the entire ship structure under each basic mode based on the vibration mode of each basic mode and through the strain-displacement relationship equation and the material constitutive equation. Based on the theoretical stress distribution field, the module determines the key areas of the ship where the structural response is most sensitive or stress is concentrated, as well as the midship area where the stress response of the ship under external loads is most intense. Multiple stress monitoring points are arranged in the key areas of the ship, and a weld assessment point is selected at a weld in the midship area as the fatigue assessment area. The matrix construction and weld stress deduction module extracts the theoretical stress value of each stress monitoring point at the current moment under each basic mode based on the theoretical stress distribution field to construct a coefficient matrix, and simultaneously extracts the theoretical stress value of the weld assessment point at the current moment under each basic mode to construct a modal stress vector; Then, the stress monitoring equipment is used to collect the ship's longitudinal stress data of each stress monitoring point at the current moment to construct a target matrix; Based on the coefficient matrix and the target matrix, the weight coefficients of each basic mode are calculated by inversion using the modal superposition method; Performing a vector inner product operation on the weight coefficient and the modal stress vector to deduce the stress value of the weld evaluation point at the current moment, and continuously obtaining the stress values ​​of the weld evaluation point at the current moment and multiple moments after the current moment to generate a weld stress time series; The total fatigue damage value calculation module uses the rain flow counting method to extract multiple stress cycles from the weld stress time series, and obtains the SN curve data of the material at the weld from the database. The fatigue damage value of each stress cycle is calculated based on the stress amplitude of each stress cycle and the SN curve data. The fatigue damage values ​​of all stress cycles are accumulated using the Miner criterion to obtain the total fatigue damage value of the weld assessment point; The fatigue damage assessment module calculates a relative error based on the total fatigue damage value and a preset theoretical loss value, and compares the relative error with a preset threshold to evaluate whether the calculated total fatigue damage value of the weld assessment point is reliable. If the relative error is less than the preset threshold, the calculated total fatigue damage value of the weld assessment point is determined to be reliable.

7. The ship weld stress deduction and fatigue damage assessment system according to claim 6, characterized in that: In the monitoring point and weld assessment point arrangement module, the theoretical stress distribution field of the entire ship structure under each basic mode is calculated by the strain-displacement relationship equation and the material constitutive equation, specifically including: Based on the vibration mode and the strain-displacement relationship equation, the strain field of the entire ship structure under each basic mode is calculated. Then, based on the strain field and the material constitutive equation, the theoretical stress distribution field of the entire ship structure under each basic mode is calculated.

8. The ship weld stress deduction and fatigue damage assessment system according to claim 6, characterized in that: In the total fatigue damage value calculation module, the fatigue damage value of each stress cycle is calculated based on the stress amplitude of each stress cycle and combined with the SN curve data, specifically including: The SN curve data is used to find the fatigue life corresponding to the stress amplitude of each stress cycle, and the fatigue damage value of each stress cycle is calculated based on the fatigue life.

9. The ship weld stress deduction and fatigue damage assessment system according to any one of claims 6 to 8, characterized in that: In the free vibration mode calculation module, the basic modes include first-order vertical bending mode, second-order vertical bending mode, third-order vertical bending mode, first-order torsional mode, second-order torsional mode, third-order torsional mode and first-order lateral bending mode.

10. The ship weld stress deduction and fatigue damage assessment system according to claim 6 or 7, characterized in that: In the monitoring point and weld assessment point arrangement module, the key areas of the ship include the bow, midship area, and stern of the ship; multiple stress monitoring points are arranged in the key areas of the ship, specifically: stress monitoring points are arranged at the deck position of the bow, the deck position of the stern, and the position near the waterline, the deck position, and the side position of the midship, and each monitoring point is equipped with a stress monitoring sensor; In the matrix construction and weld stress deduction module, the stress monitoring device is a long baseline strain sensor.

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