Method for calculating fatigue by adopting near-field dynamics under complex ice condition based on propeller icebreaking

Abstract

The invention provides a method for calculating fatigue by adopting near-field dynamics under a complex ice condition based on propeller icebreaking, which is used for processing excitation data of an ice-induced load on a propeller of an icebreaker, and comprises the following steps: firstly, constructing a sea ice environment, an elastic propeller constructed by a plurality of units is simulated by adopting a finite element method, and the continuous interaction between the elastic propeller and ice blocks in the rotating process is considered. The contact algorithm adopts an element surface finite element (FEM) method to process force transmission from a force bearing point to a force bearing surface so as to influence the force bearing state of each node of the propeller, the force is transmitted to element stress through the node effect, and the ice load borne by the propeller structure is obtained and verified.

Description

Technical Field

[0001] The present invention relates to a simulation calculation fatigue method under complex ice conditions based on the collision characteristics between a propeller and ice, belonging to the field of ocean engineering. Background Art

[0002] In various studies on ice-propeller interaction carried out abroad for the prediction and evaluation of the structural fatigue strength of ice region propellers, the main method relied on is model tests. With the continuous development of ice tank model test technology and computing power, in recent years, foreign research on issues related to ice-propeller interaction has made great progress. However, the theories and methods for accurately predicting the hydrodynamic performance of propellers under ice-propeller interaction still need to be studied. For the existing research on the fatigue strength of propellers, it mainly starts from the fatigue tests of propeller materials, and there are few literatures that systematically study the fatigue of propellers from the perspectives of theoretical analysis and corresponding software calculations. In addition, existing research often focuses on the fatigue damage assessment of propellers under wave loads, and there are obvious differences between the ice loads caused by sea ice on propellers and wave loads in terms of the load forms of wave loads. There is not only a lack of calculation of propeller fatigue damage, but also empirical formulas or experimental measurements are often adopted in the calculation. The methods adopted not only have uncertainties in data, but also are difficult to apply to different propellers under different ice-breaking conditions. Considering the current situation that the finite element method does not have good rationality and correctness in dealing with fracture problems, there is a certain possibility that existing research is incorrect and needs to be further determined by comparing with new methods. Therefore, the research on the fatigue analysis theory and software of propellers under ice-induced fatigue loads needs to be carried out urgently. Summary of the Invention

[0003] Object of the Invention: In order to overcome the deficiencies existing in the prior art, the present invention provides a fatigue simulation method for a propeller during ice-breaking under complex ice conditions based on finite element and peridynamics, which can realize the simulation of ice loads under complex ice conditions based on the physical characteristics of propeller-ice collision, and obtain the corresponding fatigue conditions of the propeller through important calculation data such as forces, torques, stress and strain obtained, solve the shortcomings of finite element calculation of fracture fatigue, and provide a reference for the ice load prediction and safety check of polar engineering structures.

[0004] Technical Solution: To solve the above technical problems, a peridynamics calculation fatigue method for a propeller during ice-breaking under complex ice conditions includes the following steps:

[0005] (1) According to the normal complex polar ice conditions, construct an equivalent and similar complex ice condition geometric model, mainly including bottom ice and broken ice, and divide the ice model into multiple parts;

[0006] (2) Establish a numerical model for the interaction between the propeller and sea ice at a local scale. Specifically, set the external contact area of 3R / 4 - R of the propeller as the ice contact area, mainly including: the propeller numerical model and the sea ice numerical model; set the initial structural stress in the propeller and sea ice models to zero;

[0007] (3) Set the polar ship propeller to move forward at a constant speed of V a and rotate at a constant angular velocity of ω a . The length of the local ice field is L a , then the load time history of this local - scale propeller is t a = L a / V a ;

[0008] (4) Carry out the coupling collision analysis of the propeller based on elements and ice blocks based on discrete ice particles using finite - element analysis and peridynamics. Set the propeller model to be divided into multiple elements based on finite - element analysis and the sea ice model to be discretized into multiple ice particles based on peridynamics. Output the position, velocity, acceleration, stress - strain parameters of the discrete element nodes of the propeller model and the positions of the ice particles as initial data, and save them to the initial read - in file of the program;

[0009] (5) When the time is 0, read the file in step (4) as the initial data of the ice particles and the propeller at time 0, and as the model data at the moment when there is no change in the first time step of the program initial;

[0010] (6) Based on the basic data of the velocity, acceleration, and position of the ice particles and the propeller nodes, use the explicit step - by - step integration method in the discrete method to calculate the velocity, acceleration, and position of the next moment of the particles and nodes. Calculate the average velocity through the displacement change of the particles, further find the force exerted by the particles on the propeller surface through the velocity change, and then synthesize the forces at each time step to calculate the maximum forward force and backward force. After ensuring that they are not greater than the maximum forward force and backward force in the empirical formula of the ice - region propeller respectively, use the unit nodal shape - function load distribution method to process them into the nodal forces on the element surface, and then calculate the elastic changes of compression and tension of the element from the nodal forces as the element's force condition. Output the stress on each element of each propeller at this step. Finally, save the data of the velocity, acceleration, and position of the particles and the velocity, acceleration, and position of the propeller nodes as the calculation data for the next moment;

[0011] (7) If tt < t a , tt = tt + 1, repeat step (6); if tt = t a , enter step (8);

[0012] (8) Output from time 0 to t aProgram stress and applied force data at all time steps of a moment. In the case of no fracture, an area taking more than 80% of the maximum stress or more than 80% of the maximum cyclic variation N in the entire time period is used as the research area. The calculation method of fatigue fracture is based on the S-N curve of the propeller material, which can be obtained mainly through query or experiment. Specifically, the fatigue test of the propeller material can be consulted, and the stress coefficient D is taken from it. h For the research objective, where:

[0013]

[0014] When D h Greater than 1 indicates over-fatigue fracture;

[0015] N L Is the total number of cycles of the stress range within the entire time period considered, m and A are S-N curve parameters obtained from fatigue experiments, q is the scale parameter of the long-term distribution of the stress range, α is the shape parameter of the long-term distribution of the stress range, and Γ is the gamma function.

[0016] Preferably, the propeller is an open propeller, and the maximum backward force F b And the forward force F f The calculation formulas are as follows:

[0017] When D < D limit

[0018]

[0019] When D > D limit

[0020]

[0021] Where: C 1 Is a coefficient, taken as 1.1 for icebreakers, S ice And H ice Selected according to the table

[0022] Icebreaker <![CDATA[H ice (m)]]> <![CDATA[S ice (-)]]> PC1 4.0 1.2 PC2 3.5 1.1

[0023] D is the propeller diameter, d is the outer diameter of the hub at the propeller plane, EAR is the disk area ratio, Z is the number of blades, n is the rated speed during free operation without ice at the maximum continuous power for controllable pitch propellers; for fixed pitch propellers, it is 85% of the rated speed during free operation without ice at the maximum continuous power.

[0024] In the present invention, the ice conditions in the polar region are set with each large piece of ice as the main body, and each piece of ice is discretized into multiple particles; the data of the particles, such as velocity, acceleration, and position, are initialized and stored. The propeller is processed by finite element unitization, and the numbers of the neighboring particles of each particle of each piece of ice are stored. The relatively small hexahedron is used as the unit of the propeller and linked according to the relatively high strength of the finite element to ensure that it cannot be damaged. The impact contact force between the unit and the ice particles is calculated by the calculation method of the unit surface contacting the particles. The particle unit surface position and the contact force are processed into the nodal force of the unit surface, and then the nodal force is used as the force condition of the unit to calculate the elastic changes such as compression and tension of the unit. According to the elastic compression and bending and other conditions generated after the force of each part of the unit, the original velocity and position are ensured to change due to the external force to realize the elastic condition of the propeller; this time, the explicit time integration method is used. According to the speed and angular velocity of the polar ship propeller, the nodes of the propeller hexahedron unit move according to their speeds, and the ice particles move continuously according to the speed of the ice block. Both of them have a neighborhood-based link to realize the solid state of the object. According to the FEM finite element method and peridynamics, a numerical calculation model of ice-structure coupling is established to calculate the position and velocity changes of the generated particles. Thus, the actual ice block is crushed, squeezed, etc. due to the contact with the propeller movement, and with time, the important relevant data such as the stress, strain, total force, and total bending moment of the propeller in response are continuously calculated and stored.

[0025] In the present invention, according to the normal and complex polar ice conditions, an equivalent and similar complex ice condition geometric model is constructed, mainly including bottom ice and broken ice. The bottom ice has a length five times the diameter of the propeller, a width ten times its height, and a thickness three times its radius. The broken ice has a size from about one-twentieth to one-fifteenth of the radius of the propeller. The ice conditions are simulated with multiple ice models.

[0026] ①: At the moment of t = 0, the file in (4) is read to determine the position, velocity, and acceleration of the ice particles at the 0 moment; the position, velocity, acceleration, and other data of the propeller nodes are used as the model data at the moment when no change occurs in the first time step of the program initial. When the moment is not 0, the next step is carried out through the particle position and other data calculated in the previous moment.

[0027] ②: Through the basic data such as the velocity, acceleration, and position of the ice particles and the propeller nodes, the basic data such as the velocity, acceleration, and position of the particles and nodes at the next moment are calculated. The propeller rotates and advances according to the calibrated rotational speed and forward speed of the polar ship propeller set initially. Each node moves according to the propeller speed. If each ice particle at the current moment does not contact and impact with the propeller surface calculated by the nodes after calculation, its node simply rotates around the y-axis and advances along the y-axis according to the original propeller speed.

[0028] ③: In the continuous movement of the propeller and ice, the propeller is inevitably in contact with the ice in terms of design. Taking the th moment as an example, after the continuous movement of the propeller nodes, the nodes penetrate the ice particles on the calculated propeller surface. The contact calculation between the particles and the surface is realized by the particle penetration correction method, and the new position of the particles is obtained. The average velocity is calculated from the displacement change of the particles, and further, the force exerted by the particles on the propeller surface is found through the velocity change.

[0029] ④: Then, the forces of all particles at each time step are synthesized to calculate the maximum forward force and backward force at this time step. After ensuring that they are not greater than the maximum forward force and backward force in the empirical formula of the propeller in the ice area respectively, according to the unit nodal shape function load distribution method, the acting position and force of the particles on each unit surface are processed as the nodal forces of the unit surface. Then, the nodal forces are used to calculate the elastic changes such as unit compression and tension of the unit, and the stress on each unit of each propeller at this time step is output. At the node, based on the acceleration, velocity, position and other parameters at the th moment, after calculating its position and velocity at the th+1 moment, the data of the node is stored and updated. In addition, to ensure that the propeller conforms to the normal movement of polar ships, it is necessary to fix the units at the position of 0~R / 2, that is, they will not be affected by ice particles and move at the original speed.

[0030] In the present invention, the relevant data such as the velocity, acceleration and position of the particles are initialized and stored. The propeller is processed into finite elements. The numbers of the neighboring particles of each particle of each piece of ice are stored. The relatively small hexahedron is used as the unit of the propeller and linked according to the relatively high strength of the finite element to ensure that it cannot be damaged. And the impact contact force between the unit and the ice particles is calculated by the calculation method of the unit surface contacting the particles. The contact judgment and the calculation of the contact force are calculated by the following formula:

[0031] x t+Δt =x t +v t Δt+dn

[0032]

[0033] where x t+Δt is the position assigned to the particle after contact, x t is the original position. The contact judgment is whether the particle enters the interior of the unit body. v t is the moving velocity of the unit contact surface, d is the distance from the particle to the unit surface, and n is the normal vector of the unit surface; is the average velocity of the particle during the contact process, v t+Δt is the velocity of the particle that does not penetrate into the unit interior in the hypothesis. ρ and V are the density and volume of the particle respectively. F t+ΔtIs the calculated contact force.

[0034] The particle element surface position and the contact force are processed into the nodal forces of the element surface according to the unit nodal shape function load distribution method, and then the nodal forces are used to calculate the elastic changes such as compression and tension of the element according to the force condition of the element as follows Figure 6 :

[0035] According to the elastic compression, bending, etc. generated after the force of each part of the element, and then ensuring that the original velocity position changes due to the external force to realize the elastic situation of the propeller; among them, the FEM finite element calculation method is used to calculate the force, and the main formulas are as follows:

[0036]

[0037] Where N I Is the shape function of node I, Is the internal force matrix of the node, Is the external force matrix of the node, M IJ Is the structural mass matrix, a IJ Is the nodal acceleration matrix, and the specific calculation forms of each matrix are as follows:

[0038]

[0039] In the formula, B is the elastic matrix of the element, b is the applied body force, t is the applied surface force, ρ 0 Is the material density.

[0040] In this case, the explicit time integration method is used. According to the speed and angular velocity of the polar ship propeller, the nodes of the propeller hexahedron element move according to its speed, and the ice particles move continuously according to the speed of the ice block. Both of them have a neighborhood-based link to realize the solid state of the object. According to the FEM finite element method and peridynamics, a numerical calculation model of ice-structure coupling is established to calculate the position and velocity changes of the generated particles. Thus, the actual ice block is crushed, squeezed, etc. due to the contact with the propeller movement, and over time, the important relevant data such as the stress, strain, total force, and total bending moment of the propeller that respond are continuously calculated and stored.

[0041] Beneficial effects: The present invention is an ice load simulation calculation and damage method under complex ice conditions of the propeller-ice collision characteristics of polar ships. Based on the peridynamics for dealing with large fracture deformations and the characteristics of stress-strain continuity in the finite element FEM numerical method, the research on the ice load simulation method is carried out. And the fatigue damage of the propeller is calculated through the cumulative damage theory and empirical formula. It can realize the fatigue safety test of the structure on the basis of truly simulating the failure of sea ice, with the advantages of high efficiency and accurate calculation. The present invention can also realize the calculation of stress-strain loads under complex ice conditions based on the physical characteristics of propeller-ice collision, solve the shortcoming that the existing commercial software simulation does not have excellent calculation ability for large block damage, and provide a reference for the ice load prediction and safety check of polar engineering structures. Description of the Drawings

[0042] Figure 1 It is a flow chart of the present invention.

[0043] Figure 2 It is a schematic diagram of typical complex ice conditions of the present invention.

[0044] Figure 3 It is a simulation diagram of typical complex ice conditions of the present invention.

[0045] Figure 4 It is a process diagram of the ice-breaking working condition of the implementation example of the present invention.

[0046] Figure 5 It is the ice load excitation result of the implementation example of the present invention.

[0047] Figure 6 It is a schematic diagram of the unit nodal shape function load distribution method of the ice load. Detailed Embodiments

[0048] The present invention will be further described below with reference to the drawings.

[0049] As Figures 1 to 5As shown, based on the characteristics of stress-strain continuity in the finite element FEM numerical method and the excellent performance of peridynamics in dealing with problems such as fracture and breakage, the present invention conducts research on the stress-strain simulation method of ice load for propellers. It can, on the basis of truly simulating the failure of sea ice, achieve the prediction of ice load on structures, and calculate the fatigue damage of propellers, with the advantages of high efficiency and accurate calculation. The present invention first constructs a particle connection model of peridynamics for local small-scale broken ice and thick ice at the bottom layer, an interaction model with the finite element unit model of the propeller, and obtains the ice load and stress-strain parameters of the propeller; then outputs the relative positions of the ship propeller unit and sea ice particles and the stress-strain parameters of the propeller obtained from this example to a file; then uses the above output file as the input parameter for the next example to form a new interaction model of sea ice and propeller, and conducts contact and collision analysis of ice particles and propeller units on this basis; repeats the above step until sufficient long-term ice load stress-strain is obtained to ensure the accuracy of the research. The present invention can realize the simulation of stress-strain of ice load under complex ice conditions based on the physical characteristics of propeller-ice collision, obtain its safety check for fatigue damage, solve the disadvantage that existing commercial software basically adopts finite element analysis and has no advantage in fracture analysis and calculation, and provides a reference for ice load prediction and safety check of polar engineering structures.

[0050] A fatigue simulation calculation method under complex ice conditions based on propeller-ice collision characteristics using peridynamics, comprising the following steps:

[0051] (1) According to normal complex polar ice conditions, construct an equivalent and similar complex ice condition geometric model, mainly including bottom ice and broken ice. Among them, the bottom ice has a length five times the diameter of the propeller, a width ten times its height, and a thickness three times its radius, and the broken ice is about one-twentieth to one-fifteenth of the radius of the propeller. Simulate the ice conditions with multiple ice models;

[0052] (2) Establish a numerical model of the interaction between the propeller and sea ice at a local scale. Specifically, set the external contact area of 3R / 4 - R of the propeller as the ice contact area. It mainly includes: a propeller numerical model and a sea ice numerical model; set the initial structural stress in the propeller and sea ice models to zero;

[0053] (3) Set the polar ship propeller to move forward at a uniform speed of V a and rotate at a uniform angular velocity of ω a . The length of the local ice field is L a . Then the load time history of this local-scale propeller is t a = L a / V a ;

[0054] (4) Based on the finite element analysis, the coupling collision analysis of the propeller based on elements and the ice block based on discrete ice particles by peridynamics simulation is carried out. It is set that based on the finite element analysis, the propeller model is divided into multiple elements, and the sea ice model is discretized into multiple ice particles by peridynamics. The positions, velocities, accelerations, and stress-strain parameters of the discrete unit nodes of the propeller model and the positions of the ice particles are output as the initial data of the input program and saved to the initial read file of the program;

[0055] (5) Based on the data in the file of step (4), determine its position, velocity, and acceleration as the ice particles at time 0; the data such as the position, velocity, and acceleration of the propeller nodes are used as the model data at the moment when there is no change in the first time step of the program initial. When the time is not 0, the basic data such as the velocity, acceleration, and position of the next moment of the particles and nodes are calculated by the explicit step-by-step integration method using the data such as the particle position calculated at the previous moment. When the particle penetrates the propeller surface and is judged to penetrate by the particle penetration correction method, contact calculation is carried out. Through the correction of the particle position, the change in velocity is caused by the change in particle position, and the change in acceleration is caused, and thus the point force on the surface of each unit of the propeller can be obtained, which is the force acting on the points on the surface due to the action of the ice particles. By synthesizing the maximum forward and backward forces, failure is judged by the empirical formula. Then, according to the unit node shape function load distribution method, the data of the point force acting on the unit surface, that is, the key factors of magnitude, direction, and acting point, are transformed into the acting forces on the unit surface nodes, and based on this, the stress of the unit is processed by using data such as the elastic modulus of the unit, and the elastic changes of the propeller such as bending and compression are established to realize the elastic situation of the propeller under the ice load and output the stresses of each part of the propeller at this moment.

[0056] (6) If tt < t a , tt = tt + 1, repeat steps (3)-(5); if tt = t a , output the program stress data at all time steps from time 0 to t a moment. In the case of no fracture, consider whether the maximum stress situation of the propeller occurs, and then carry out fatigue fracture calculation, where more than 80% of the maximum stress in the entire time period is taken as the research area. The calculation method of fatigue fracture is based on the S-N curve of the propeller material as the basic research, and the stress coefficient D h is taken as the research target, where:

[0057]

[0058] N L is the total number of cycles of the stress range during the entire time period considered, m, A are the S-N curve parameters obtained from fatigue experiments, q is the scale parameter of the long-term distribution of the stress range, α is the shape parameter of the long-term distribution of the stress range, and Γ is the gamma function.

[0059] When D h When it is greater than 1, it indicates fatigue excessive fracture.

[0060] (7)End.

[0061] In view of complex ice conditions, the present invention studies the ice load simulation method based on the stress-strain continuity characteristics in the finite element numerical method and the large deformation fracture failure calculation characteristics in the near-field dynamics. It can realize the ice load prediction and continuous simulation of channel navigation of polar ship propellers under complex ice conditions on the basis of truly simulating sea ice failure, and based on the stress-strain, total force and total bending moment of the propeller, it realizes the safety research on fatigue damage of the propeller under the action of ice load.

[0062] It is worth noting that the basic data of the propeller finite element of this invention is processed through the unit division of the finite element software, so the modeling information of the propeller is required and imported into the corresponding finite element analysis software to extract the unit information according to the hexahedron output and then import it into the program.

[0063] It is worth noting that the FEM method is used in the program to process finite element units, and all related units are processed jointly. The actual conditions such as compression and stretching on the hexahedral unit are studied and calculated with the contact of each face of the hexahedral unit. The calculation is carried out through the unit surface contact calculation and study, and the contact force is converted from a point force to the force of the four unit nodes on the surface by using the shape function load distribution method, and then applied to the unit itself to ensure that the finite element can be realized through unit processing in the program in dealing with slight deformations similar to propellers and the calculation advantages of stress and strain.

[0064] It is worth noting that when dealing with polar ice conditions of propellers, the present invention adopts milling of the bottom of the propeller and breaking of the ice on the hull to produce crushed ice of different sizes and shapes. Therefore, in the simulation, this study adopted ten sizes and shapes of random crushed ice shapes, but the smallest is one-tenth of the propeller radius length, and the largest is one-fifth of the propeller radius length, and the crushed ice continues to be randomly generated after passing through the propeller range. A rectangular ice block with a fixed bottom side is used for research at the bottom, and during the movement of the propeller, the length and width of the rectangular ice block can meet the situation that the propeller will not move out of the ice block during the movement of the propeller; all ice blocks are discretized into ice particles with a length of one thousandth of the propeller radius, and the principle of peri-field dynamics is used to link the ice particles with the surrounding particles within the range of influence. The method uses conventional type peri-field dynamics, and the basic calculation formula for linking force is:

[0065]

[0066] Among them: a, b, d, Λ are all PD-specific parameters; s is the elongation of the bond, x′-x is the relative position of the particle in the affected range before deformation, y′-y is the relative position of the particle after deformation, θ′ and θ are the expansion deformation of the material points x′ and x, respectively.

[0067] It is worth noting that the time step in the calculation of this study needs to be adjusted through calculation to ensure that the time step of calculating the position and velocity of the ice particles using explicit time integration is consistent with the time step of the propeller unit itself, so as to ensure that when the two are combined for calculation, there will be no errors in the calculated data due to inconsistent time steps.

[0068] It is worth noting that after calculating the force on the propeller unit, propeller stress and strain and other important calculation parameters, this study conducted research from two perspectives. One is to calculate through an empirical formula, which requires the total force and torque on the propeller to be output as follows:

[0069] When D <D limit

[0070]

[0071] When D>D limit

[0072]

[0073] When the force on the propeller is greater than F b It is judged that the fluctuating ice load excitation caused fatigue damage to the propeller.

[0074] The other method is to use the stress and strain at each time step. This study takes the stress as 50% of the maximum stress iteration of the propeller, and fluctuates up and down on this stress in multiple cycles. The stress at this stage is used as the calculation data. Through the cumulative damage theory of the propeller and the MINER linear damage theory, the stress is imported into the SN diagram for study. When the number of fluctuations is the y-axis of the graph, that is, it appears more times than the graph, the propeller is considered to have fatigue damage.

[0075] Example 1

[0076] This example uses a typical icebreaker propeller to sail in a complex ice field composed of bottom ice and crushed ice. The propeller adopts the basic design of R-class propeller, with a radius length of 4.12m and 4 blades. Figure 1As shown, taking the bottom ice and broken ice as examples, based on the above ice area ship navigation conditions, a long-term simulation method of ice load under complex ice conditions with propeller-ice collision characteristics is adopted to calculate the propeller stress, and a fatigue damage calculation method based on MINER is used to study the fatigue of the propeller, including the following steps:

[0077] 1. Determine the complex ice field under the propeller navigation conditions. According to the streamline of typical polar navigation ships, the velocities of multiple broken ice model blocks are output. Each part is generated in a random shape as the shape of the broken ice. For the bottom ice, the radius range of the bottom ice occupied by the propeller is obtained according to the actual position where the propeller contacts the ice (the value here is the external contact area of 3R / 4 - R of the propeller). Based on the above conditions, the boundary conditions of the ice are set.

[0078] 2. Construct a numerical model with the propeller finite element. Investigate the basic data of the propeller, perform finite element processing with finite element software, obtain multiple elements and export the position information of each element node. Then, adopt the force state calculation method of FEM to construct the interacting forces of multiple elements to create the actual elastic propeller action situation.

[0079] 3. Based on peridynamics, discrete the corresponding particles from the multiple ice block model data in 1, and use the basic parameters of the ice as the link force state between the particles in the basic state type of peridynamics.

[0080] 4. In this study, the relative position of the propeller rotates clockwise around the y-axis in the negative y-axis direction. Each ice block moves from the positive y-axis direction to the negative direction. Set the speed of the propeller through the operating speed of the polar ship and the rotational speed of the propeller. Use the sea condition and the moving speed of the ice block after being impacted by the ship as the initial speed of the ice block. The internal forces of the propeller and the ice are processed by the finite element method FEM and peridynamics respectively. When they come into contact, the particle contact changes are processed by the FEM method, and the corresponding particle motion changes are calculated by the explicit time motion method. After the particles enter the propeller and are allocated positions, the velocity and position changes of the ice particles can be studied respectively. The velocity and position changes of the particles are converted into contact forces acting on the unit surface, and the forces on the unit surface are processed as the forces on the unit nodes, which are then converted into the motion deformation of the propeller unit. Record and output the stress of the propeller, the total force, and the total bending moment of the propeller at each time step.

[0081] 5. For the obtained data, conduct research through the general calculation damage methods of linear damage theory and cumulative damage theory, and assist with empirical formulas:

[0082] When D < D limit

[0083]

[0084] When D > D limit

[0085]

[0086] According to the above analysis, important calculation data such as the stress, force, and bending moment of the propeller of polar ships under complex ice conditions can be obtained, and whether there is damage to the propeller can be studied therefrom.

[0087] The core schematic diagram of the long-term ice load simulation method of the present invention is as Figure 2 shown.

[0088] The ice-breaking working condition diagram of the embodiment of the present invention is as Figure 3 shown.

[0089] The long-term ice load result of the embodiment of the present invention is as Figure 4 shown.

[0090] The above are only the preferred embodiments of the present invention. It should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A fatigue calculation method based on propeller icebreaking in complex ice conditions using peridynamics, which is characterized by the following steps: (1) Based on the normal and complex polar ice conditions, an equivalent and similar complex ice condition geometric model is constructed, with the bottom ice and broken ice as the main components, and the ice model is divided into multiple parts; (2) Establish a local-scale numerical model of the interaction between propeller and sea ice. Specifically, the external contact area of ​​the propeller's 3R / 4-R is set as the ice contact area, which mainly includes: propeller numerical model, sea ice numerical model; set the initial structural stress in the propeller and sea ice models to zero; (3) Setting the polar ship propeller to V a The speed is uniform, with ω a The angular velocity of the ice rink is uniform, and the length of the local ice rink is L a , then the load history of the local scale propeller is t a =L a / V a ; (4) Based on finite element analysis and peridynamics, a unit-based propeller and a discrete ice particle-based ice block coupling collision analysis is carried out. The propeller model is divided into multiple units based on finite element analysis, and the sea ice model is discretely divided into multiple ice particles based on peridynamics. The discrete unit node positions of the propeller model and the ice particle positions, velocities, accelerations, and stress-strain parameters are output as initial data and saved in the initial read file of the program. (5) When the time is 0, the file in step (4) is read as the initial data of ice particles and propellers at 0, and as the model data at the time when no changes occur in the first time step of the program; (6) Based on the basic data of velocity, acceleration and position of ice particles and propeller nodes, the explicit step-by-step integration method in the discrete method is used to calculate the velocity, acceleration and position of the particles and nodes at the next moment. The average velocity of the particles is calculated through the displacement change, and the force of the particles on the propeller surface is further found through the velocity change. The forces at each time step are synthesized to calculate the maximum forward force and backward force. After ensuring that the two are not greater than the maximum forward force and backward force in the empirical formula of ice zone propellers, they are processed into the node force of the unit surface according to the unit node shape function load distribution method. The node force is then used as the unit force to calculate the unit compression and tension elastic changes, and the stress of each unit of each propeller at this step is output. Finally, the velocity, acceleration and position data of the particles and the velocity, acceleration and position data of the propeller nodes are saved as the calculation data for the next moment. (7) If tt <t a , tt=tt+1, repeat step (6); if tt=t a , proceed to step (8); (8) Output from time 0 to t a The program stress and force data at all time steps at the moment, in the case of no fracture, take more than 80% of the maximum stress in the entire time period as the research area, the calculation method of fatigue fracture is based on the SN curve of the propeller material. The SN curve can be obtained mainly through query or experiment. For details, you can refer to the fatigue test of the propeller material, and take the stress coefficient as D h The research objectives include: When D h When it is greater than 1, it indicates excessive fatigue fracture; N L is the total number of cycles of the stress range in the entire time period considered, m and A are the SN curve parameters obtained from fatigue experiments, q is the scale parameter of the long-term distribution of the stress range, α is the shape parameter of the long-term distribution of the stress range, and Γ is the gamma function.

2. The method for calculating fatigue using peridynamics in complex ice conditions based on propeller icebreaking according to claim 1 is characterized in that: The propeller is an open propeller, and the maximum backward force F b With forward force F f The calculation formula is as follows: When D <D limit When D>D limit in: C1 is the coefficient, which is 1.1 for icebreakers, S ice With H ice Select from table D is the propeller diameter, d is the hub outer diameter at the propeller plane, EAR is the disk ratio, Z is the number of blades, n is the rated speed when free running without ice at maximum continuous power for controllable pitch propeller; it is 85% of the rated speed when free running without ice at maximum continuous power for fixed pitch propeller.

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