CN116933580B - Submarine pipeline steel hydrogen induced fatigue crack propagation cycle cohesive force model prediction method - Google Patents
Submarine pipeline steel hydrogen induced fatigue crack propagation cycle cohesive force model prediction method Download PDFInfo
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Abstract
The invention discloses a submarine pipeline steel hydrogen fatigue crack propagation cyclic cohesion model prediction method, which adopts finite element model processing, and after loading cyclic stress on the finite element model, the processing process comprises the following steps: carrying out elastoplastic analysis on a crack tip area in the finite element model of the test piece by a finite element analysis method; performing hydrogen diffusion analysis on the material; performing hydrogen-induced material performance degradation analysis on the material; carrying out cohesive force analysis by using a cyclic cohesive force model to obtain damage indexes of cohesive units in the current time step, deleting the corresponding cohesive units if the damage indexes are larger than 1, enabling cracks to expand forwards, updating boundary conditions, and otherwise, entering the next time increment to return to finite element analysis; and extracting the crack length and the cyclic stress cycle times under each time step, and calculating the crack expansion rate under the current loading condition to realize the prediction of the hydrogen induced fatigue crack expansion process. The invention can realize the accurate prediction of the hydrogen induced fatigue crack growth of the bottom pipeline steel.
Description
Technical Field
The invention relates to the technical field of submarine pipeline steel hydrogen induced fatigue crack growth prediction, in particular to a submarine pipeline steel hydrogen induced fatigue crack growth cycle cohesive force model prediction method.
Background
As the installed capacity of offshore wind farms increases, the concentration of large-scale offshore wind farms results in a gradual increase in grid balance costs. In addition, offshore wind power energy storage is difficult, and the traditional technologies such as electrochemical energy storage, electromagnetic energy storage and physical energy storage have higher running cost, so that the development requirements of a large amount of storage and pure green energy sources cannot be met.
Therefore, the conversion of the undegraded wind power generation energy into hydrogen by the water electrolysis technology for storage and transportation gradually becomes an important direction for large-scale green development, storage and utilization of offshore wind energy. The offshore wind power hydrogen production project formed by combining offshore wind power with hydrogen production is an important direction of current offshore new energy development technology research. Long hydrogen pipelines on the sea floor are important devices to support this project. The mechanical properties of the metallic materials used in subsea pipelines are severely degraded, i.e. hydrogen embrittlement, due to the influence of hydrogen. The failure load of the structure where hydrogen embrittlement occurs may be much lower than in an inert atmosphere or air, which may lead to unexpected failure. The degradation of materials caused by hydrogen has been a hotspot in submarine pipeline environment-assisted cracking research.
At present, no mature and unified explanation exists for the action mechanism of hydrogen embrittlement. The mainstream view is that hydrogen embrittlement is not a result of a single mechanism, but a result of coexistence and mutual promotion of multiple mechanisms. However, in most cases, the idea that the theory of hydrogen enhancement of cohesion is the dominant mechanism is being demonstrated. Serebrinsky et al propose a quantum mechanical based hydrogen embrittlement continuum model that successfully simulates the phenomenon of hydrogen embrittlement in high strength steels. The agreement between the simulation results and the experimental results suggests that the theory of hydrogen enhanced cohesion may be the main cause of hydrogen embrittlement. Wang et al established a relationship between fracture toughness and gaseous hydrogen concentration based on the theory of hydrogen enhanced cohesion, which is well in agreement with experimental data.
The cyclic cohesion model provides a tool with great development potential for fatigue crack growth simulation in a hydrogen environment. The physical basis for constructing cohesive bands in cohesive forces is the interaction between molecules or atoms, i.e. the bond energy. Therefore, the cohesive force model has better consistency with the hydrogen enhanced cohesive force theory. Moriconi et al propose a cohesive model based on hydrogen lattice diffusion and implement the model numerically. The effect of low-concentration hydrogen on fatigue crack growth can be qualitatively predicted by comparing the model with experimental data. However, the sub-model makes it difficult to predict a significant increase in fatigue crack growth rate at hydrogen concentrations. Colombo et al establish a cohesive model coupled with the diffusion and stress fields. The sensitivity analysis was performed on AISI4140 steel using this model and the effect of material, environment and experimental input parameters on hydrogen induced fatigue crack growth was discussed.
CN113916705A provides a parameter acquisition and simulation method based on crack propagation of a cyclic cohesion model, and the method is based on a material crack closure measurement experiment, and develops a cyclic cohesion model damage parameter fitting method considering crack closure effect. And finally, constructing a crack propagation simulation calculation frame based on the cyclic cohesion.
CN107832492B provides a steel structure corrosion fatigue damage calculation method based on a cohesive force model, which comprises the following steps: defining a cohesive force unit model; the problems of corrosion defect formation and expansion are solved based on the pitting theory and the Monte Carlo method; performing time-course analysis on the structure to be researched by using the cohesive force model defined by the structure; dynamically updating the to-be-researched structural model grid; and processing the real-time stress by using a real-time rain flow counting method, and calculating the equivalent stress amplitude and the accumulated damage amount.
However, the following drawbacks exist in the prior art: the fatigue crack growth rule and characteristics of the pipeline steel under the influence of hydrogen are difficult to be reflected correctly, namely, the sudden increase and saturation of the fatigue crack growth rate under the high hydrogen concentration environment; the structural stress response is predicted only based on the cohesive force model, and finally the fatigue damage prediction is still required based on the traditional rain flow counting method and the accumulated fatigue damage rule, and the prediction cannot be effectively performed on the fatigue crack expansion rate; the influence of material performance degradation caused by hydrogen embrittlement on the fatigue crack propagation process is not considered only for normal states or only for structural defect cases caused by corrosion.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a cyclic cohesive force model prediction method for hydrogen-induced fatigue crack growth of a self-submarine pipeline steel, which is used for predicting the hydrogen-induced fatigue crack growth of a steel structure in a hydrogen environment, and the reasonable prediction of the hydrogen-induced fatigue crack growth of the submarine pipeline steel is realized by supplementing the defect that the known cohesive force model cannot analyze the hydrogen-induced material performance degradation by adding cohesive strength and accumulated cohesive length degradation into the known cyclic cohesive force model frame.
The invention is realized in such a way that a method for predicting the cyclic cohesive force model of the hydrogen induced fatigue crack growth of the self-subsea pipeline steel adopts finite element model processing, after setting the initial condition of hydrogen concentration in the finite element model, the finite element model is loaded with cyclic stress, and the processing process comprises the following steps:
s1, acquiring a crack tip area in a finite element model of a test piece;
S2, carrying out elastoplastic analysis on the crack tip area by a finite element analysis method to obtain the hydrostatic stress in the stress field of the crack tip area under the current time step;
s3, carrying out hydrogen diffusion analysis on the material based on the hydrogen concentration to obtain a hydrogen diffusion analysis result;
S4, performing hydrogen-induced material performance degradation analysis based on a hydrogen diffusion analysis result to obtain a degradation analysis result;
S5, carrying out cohesive force analysis by utilizing a cyclic cohesive force model based on a degradation analysis result, obtaining damage indexes of cohesive units in the current time step, and judging whether the damage indexes are larger than 1; if yes, deleting the corresponding cohesive unit in the finite element model, enabling the crack to expand forwards, updating the hydrogen concentration boundary condition, and returning to the step S1; if not, entering the next time increment, and returning to the step S2;
s6, extracting the crack length and the cycle times of the cyclic stress of the analysis object under each time step, and calculating the crack expansion rate under the current loading condition, so as to predict the hydrogen induced fatigue crack expansion process.
Wherein, the damage index of the cohesive unit is calculated by the following formula:
，
In the method, in the process of the invention, Representing fatigue damage to the cohesive zone at the current time step,An index of damage indicating the cohesive unit is shown,Is the normal separation vector of the crack,Indicating the amount of change in normal separation of the crack,Is thatThe increment at the current time step,As a cohesive force limit coefficient of the resin,As a function of the Heaviside,Indicating the normal traction in the cohesive zone,Represents the cohesive strength under the influence of hydrogen,Represents the maximum cohesive length corresponding to cohesive strength,Represents the cumulative cohesive length after degradation under the influence of hydrogen.
The hydrogen diffusion analysis is carried out on the pipeline material based on the hydrostatic stress to obtain a hydrogen diffusion analysis result, wherein the hydrogen diffusion analysis result comprises the step of obtaining the hydrogen concentration in the pipeline material; the hydrogen concentration in the metal material is obtained based on a mass diffusion equation of hydrogen in the metal considering the influence of the strain rate, and the coupling relation between the hydrogen diffusion field and the stress field is established through the mass diffusion equation; the mass diffusion equation is as follows:
，
Wherein the method comprises the steps of Is the concentration of the lattice hydrogen,Is the concentration of the trapped hydrogen and,Is the diffusion coefficient of the crystal lattice,Is the average molar volume of hydrogen and,Is the constant of the gas which is used to produce the gas,Is the absolute temperature of the water in the water,Is the static water stress in the stress field,Is hamiltonian.
The hydrogen-induced material performance degradation analysis is performed based on the hydrogen diffusion analysis result, and is realized through a degradation model which is established based on an inverse logistic function and is influenced by the hydrogen concentration, wherein the degradation model is expressed as follows:
，
In the method, in the process of the invention, Indicating the initial cumulative cohesive length of the film,And (3) withThe material parameters controlling the degradation model, respectively.
Wherein the cohesive strength under the influence of hydrogen is obtained by the following formula:
，
，
In the method, in the process of the invention, Is the initial cohesive strength of the material,For total hydrogen coverage in metal, the total hydrogen concentration C H and the difference between the Gibbs free energy of any microstructure interface and the surrounding materialAnd (5) determining.
Wherein, the relation between the initial cohesive strength and the normal traction force in the cohesive area is determined by traction separation law, and the normal traction force is regarded as a function of the normal separation vector of the crack in the cohesive area of the crack tip of the material; the equation for the exponential traction separation law is:
。
The crack propagation rate under the current loading condition is calculated by using da/dN, wherein a represents the crack length, N represents the cycle times of the cyclic stress, and d represents the differential sign.
When cyclic stress is loaded on the finite element model, the loading condition is limited by the loading ratio and the loading frequency.
The method comprises the steps of predicting hydrogen induced fatigue crack growth of submarine pipeline steel by adopting a compact tensile CT test piece, establishing a hydrogen diffusion model and a compact tensile CT test piece finite element model based on the material property of the pipeline steel to be tested, arranging a layer of cohesive units with 0 thickness at the symmetrical axis position of the compact tensile CT test piece finite element model, encrypting grids within the area range of 1mm of the crack tip length, and then carrying out a test by loading cyclic stress.
When loading cyclic stress for test, loading equal-sized and opposite cyclic load P in a round hole of a compact tensile CT test piece according to preset loading conditions, limiting the time increment length to 1/40 of the loading period, calculating the hydrostatic stress in a stress field of a crack tip area under the current time step from an initial time step t 0, completing hydrogen diffusion analysis based on the hydrostatic stress in the stress field, and calculating the damage index of a cohesive unit under the current time step through a cyclic cohesive model.
In the hydrogen induced fatigue crack growth prediction process, the influence of the degradation of cohesive strength on the fatigue crack growth is considered, the influence of the degradation of accumulated cohesive strength on the fatigue crack growth is considered, the reliability of the hydrogen induced fatigue crack growth of the pipeline steel is effectively improved, and the fatigue crack growth rule and characteristics of the pipeline steel under the influence of hydrogen can be accurately captured.
Drawings
FIG. 1 is a flow chart of a method for predicting hydrogen induced fatigue crack growth cycle cohesion model of subsea pipeline steel according to an embodiment of the present invention.
FIG. 2 is a schematic illustration of a compact tensile test piece structure according to an embodiment of the present invention.
FIG. 3 is a schematic diagram of a compact tensile test piece finite element model and crack tip grid encryption details of an embodiment of the present invention.
FIG. 4 is a graph of comparison of X42 steel fatigue crack growth rate experiments and model predictions in accordance with an embodiment of the present invention.
Detailed Description
The invention is described in further detail below with reference to the drawings and the specific examples. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
According to the submarine pipeline steel hydrogen induced fatigue crack propagation prediction method based on the cyclic cohesion model, a hydrogen diffusion field and stress field coupling analysis model is built through a finite element platform based on a mass diffusion model/equation of hydrogen in metal. To take into account the effect of hydrogen induced material property degradation, two important parameters (cohesive strength and cumulative cohesive length) are introduced into the traction separation law of the internal cohesive model as degradation of hydrogen diffusion.
Based on the first principle, considering the degradation of hydrogen induced cohesive strength, and based on a degradation model, considering the degradation of accumulated cohesive length, introducing the degradation process into the damage evolution law of the cyclic cohesive model for correction. And coupling the cohesion model with a hydrogen diffusion field and a stress field based on the modified damage evolution law to obtain a cyclic cohesion analysis model under multi-field coupling.
Based on fatigue crack propagation experiments, key parameters of a traction separation law and an injury evolution law of the cyclic cohesion model are obtained. And setting cohesive units (also called cohesive units) in the finite element model of the test piece, and judging whether the crack is expanded according to the damage condition of the cohesive units, so as to realize the prediction of the hydrogen-induced fatigue crack expansion of the pipeline steel material.
Referring to FIG. 1, a method for predicting hydrogen induced fatigue crack growth cycle cohesion model of submarine pipeline steel is provided, which adopts finite element model processing, and after establishing a finite element model, hydrogen concentration initial conditions are set in the finite element modelAfter loading the finite element model with cyclic stress (initial lattice hydrogen concentration), the subsequent processing steps include the steps of:
s1, acquiring a crack tip area in a finite element model of a test piece;
S2, performing elastoplastic analysis on the obtained crack tip region by a finite element analysis method to obtain the hydrostatic stress in the stress field of the crack tip region at the current time step;
s3, carrying out hydrogen diffusion analysis on the material based on the hydrogen concentration to obtain a hydrogen diffusion analysis result;
S4, performing hydrogen-induced material performance degradation analysis based on a hydrogen diffusion analysis result to obtain a degradation analysis result;
S5, carrying out cohesive force analysis by utilizing a cyclic cohesive force model based on a degradation analysis result, obtaining damage indexes of cohesive units in the current time step, and judging whether the damage indexes are larger than 1; if yes, judging that the corresponding cohesive unit fails, deleting the corresponding cohesive unit in the finite element model, enabling the crack to expand forwards, updating the hydrogen concentration boundary condition, and returning to the step S1; if not, entering the next time increment, and returning to the step S2;
S6, extracting the crack length and the cycle times of the cyclic stress of the cohesive unit under each time step, and calculating the crack expansion rate under the current loading condition, so as to predict the hydrogen induced fatigue crack expansion process.
The multiple cohesive units are connected in a straight line, the multiple cohesive units are paved on a preset crack propagation path in a finite element model of a test piece, and the crack tip area is an area with a preset length on the cohesive units, such as an area with a length of 1mm, as shown in fig. 3.
The invention establishes a finite element model containing a cyclic hydrogen induced fatigue crack growth prediction cohesive force model based on a finite element analysis framework, lays cohesive units on a preset crack growth path, sets response grid types according to finite element analysis requirements at other materials, and carries out coupling treatment on a hydrogen diffusion field, a stress field and the cyclic cohesive force model to realize the submarine pipeline steel hydrogen induced fatigue crack growth prediction based on a multi-field coupling cyclic cohesive force model.
According to the invention, the hydrogen diffusion field, the stress field and the cyclic cohesion model are subjected to coupling treatment, so that the submarine pipeline steel hydrogen induced fatigue crack growth prediction based on the multi-field coupling cyclic cohesion model is realized. The coupling process is programmed into the finite element computation model in the form of UMAT material subroutines written in the FORTRAN language. And carrying out elastoplastic mechanical analysis by a finite element analysis method, loading cyclic stress on the finite element model, calculating the hydrostatic stress of the crack tip area under the current time step, introducing the information into a material subroutine, feeding back to the hydrogen diffusion model, and calculating the current damage index of the cohesive unit by the cyclic cohesive force model.
As an embodiment, the calculation of the damage index of the cohesive unit may be calculated by using the following formula, which is obtained based on the conventional cohesive model damage evolution law and by adding the degradation of cohesive strength and accumulated cohesive length;
，
In the method, in the process of the invention, Representing fatigue damage to the cohesive zone at the current time step,An index of damage indicating the cohesive unit is shown,Is the normal separation vector of the crack,Indicating the amount of change in normal separation of the crack,Is thatThe increment at the current time step,As a cohesive force limit coefficient of the resin,As a function of the Heaviside,Indicating the normal traction in the cohesive zone,Represents the cohesive strength under the influence of hydrogen,Represents the maximum cohesive length corresponding to cohesive strength,Represents the cumulative cohesive length after degradation under the influence of hydrogen.
As one example, wherein the hydrogen diffusion analysis is performed on the material based on the hydrogen concentration, a hydrogen diffusion analysis result is obtained, including obtaining the hydrogen concentration inside the metal material. The hydrogen concentration in the metal material is obtained based on a mass diffusion equation of hydrogen in the metal considering the influence of the strain rate, and the coupling relation between the hydrogen diffusion field and the stress field is established through the mass diffusion equation; the mass diffusion equation is as follows:
，
Wherein the method comprises the steps of Is the lattice hydrogen concentration, which is determined by the material lattice density and hydrogen coverage rate,Is the trap hydrogen concentration, is determined by the microcosmic trap density of the material and the hydrogen coverage rate,Is a lattice diffusion coefficient, which can be obtained by the existing experiments or literature,Is the average molar volume of hydrogen and,Is a general-purpose gas constant and is,Is the absolute temperature of the water in the water,Is the static water stress in the stress field,Is hamiltonian. The coupling relation between the hydrogen diffusion field and the stress field is established through the method, so that the hydrogen concentration in the material can be calculated based on the finite element elastoplastic analysis resultAnd (3) with. The diffusion of hydrogen in the stress field is described based on the mass diffusion equation of hydrogen in the metal taking into account the strain rate effects.
As one embodiment, the hydrogen-induced material performance degradation analysis is performed based on the hydrogen diffusion analysis result, and is implemented by a degradation model, which is established based on an inverse logistic function and is formed by that the cumulative cohesive length of the material is affected by the hydrogen concentration, and the degradation model is expressed as follows:
，
In the method, in the process of the invention, Indicating the initial cumulative cohesive length of the film,And (3) withThe material parameters for controlling the degradation model can be obtained based on the existing experimental technology.
As an example, wherein the cohesive strength under the influence of hydrogen is obtained by:
，
，
In the method, in the process of the invention, Is the initial cohesive strength, i.e. the cohesive strength when the metallic material properties have not degraded,For total hydrogen coverage in metal, the total hydrogen concentration C H and the difference between the Gibbs free energy of any microstructure interface and the surrounding materialAnd (5) determining.
As an embodiment, wherein the relationship of the initial cohesive strength to the normal traction in the cohesive zone is determined by a traction separation law, the normal traction is considered as a function of the crack normal separation vector in the cohesive zone of the metallic material crack tip; the equation for the exponential traction separation law is:
。
As an embodiment, the calculating the crack growth rate under the current loading condition is implemented by using da/dN, where a represents the crack length, N represents the number of cycles of the cyclic stress, and d represents the differential sign.
As an embodiment, when the finite element model is loaded with cyclic stress, the loading condition is defined by the loading ratio and the loading frequency.
As one embodiment, a compact tensile CT test piece is adopted to predict the hydrogen induced fatigue crack extension of the submarine pipeline steel, a hydrogen diffusion model and a compact tensile CT test piece finite element model are established based on the material properties of the pipeline steel to be tested, a layer of 0-thickness cohesive unit is arranged at the symmetrical axis position of the compact tensile CT test piece finite element model, the grids within the area range of the crack tip length of 1mm are encrypted, and then the test is carried out by loading and loading cyclic stress.
As an embodiment, when loading cyclic stress for test, loading equal-sized and opposite cyclic load P in a round hole of a compact tensile CT test piece according to a predetermined loading condition, wherein the time increment length is limited to 1/40 of the loading period, calculating the hydrostatic stress in a stress field of a crack tip region at the current time step from an initial time step t 0, completing hydrogen diffusion analysis based on the hydrostatic stress in the stress field, and calculating the damage index of a cohesive unit at the current time step through a cyclic cohesive model.
The hydrogen fatigue crack growth prediction process of the pipeline steel according to the present invention will be described below by taking the case of developing the hydrogen fatigue crack growth prediction of the X42 pipeline steel as an example.
Referring to fig. 2, fig. 2 shows a schematic diagram of a compact tensile CT test piece. Wherein W is the reference distance from the bottom end (left side of FIG. 2) of the test piece to the center of the opening, the allowable size of the test piece is determined by the multiple of W, the allowable distance W+/-0.005W from the center of the opening of the test piece to the bottom end of the test piece, the allowable distance W+/-0.010W from the bottom end of the test piece to the front end (right side of FIG. 2) of the test piece, the diameters of two openings on the test piece are 0.25W, the allowable deviation is within 0.05 of the aperture Dia, a is the crack length of the test piece, a n is the distance from the tip of the crack to the center of the opening, the initial value ﹡ a n is 0.20W, the allowable length of half of the upper end to the lower end of the test piece is 0.6W+/-0.005W, namely the allowable length from the upper end and the lower end of the center of the slot of the test piece to the center of the slot of the test piece is 0.6W+/-0.005W, and the allowable length from the center of the opening of the test piece is 0.275 W+/-0.005W.
First, a hydrogen diffusion model and a CT test piece finite element model were established based on the compact tensile CT test piece shown in FIG. 2 and the X42 pipeline steel material properties shown in Table 1, and yield strengths in Table 1 were expressed asUltimate strength is expressed as/>. For the CT test piece finite element model, as shown in FIG. 3, a layer of 0-thickness cohesive unit (or called cohesive unit, black line straight line shown in FIG. 3) is arranged at the symmetry axis position of the CT test piece finite element model, and the grid within the area range of the crack tip length of 1mm is encrypted, so that the length of the crack can be recorded conveniently. Subsequently, an equal and opposite cyclic load P was applied to the round hole of the compact tensile CT test piece, and the hydrogen induced fatigue crack growth prediction example loading conditions are shown in Table 2, with the time increment length limited to 1/40 of the loading period.
And (3) carrying out elastoplastic mechanical analysis by a finite element analysis method, calculating the static water stress of the crack tip area under the current time step from an initial time step t 0, introducing a material subroutine, completing hydrogen diffusion analysis based on the static water stress of the crack tip area under the current time step, and calculating the damage index D of the cohesive unit under the current time step by a cyclic cohesive force model. If the damage index D is not greater than 1, entering the next time increment analysis, updating the hydrogen concentration boundary condition based on the hydrogen diffusion analysis result, and recalculating the crack tip area hydrostatic stress information and the damage index; if the damage index D is greater than or equal to 1, judging that the current cohesive unit fails, deleting the cohesive unit, enabling the crack to expand forwards through loading stress, obtaining a forward expanded crack tip area, and repeating the substeps for the next adjacent cohesive unit. By recording the changes of the crack length a and the cycle number N in the finite element analysis process, the crack expansion rate da/dN under the current loading condition can be calculated, so that corresponding prediction is realized.
The hydrogen induced fatigue crack growth rate predicted based on the model of the present invention was compared with the hydrogen induced fatigue crack growth rate of the X42 steel, and the results are shown in fig. 4. The model prediction result of the invention is well matched with the experimental result, so that the model of the invention can better capture the evolution characteristics and the trend of the hydrogen induced fatigue crack growth of the pipeline steel.
It should be noted that although the above examples only show hydrogen induced fatigue crack growth predictions for X42 steel, the model of the present invention may also be used for hydrogen induced fatigue crack growth predictions for any pipeline steel, such as Grade B pipeline steel, X52 pipeline steel, etc., and is not limited to X42 steel.
TABLE 1
| Material properties | Value of |
| Yield strength (MPa) | 371 |
| Ultimate strength (MPa) | 539 |
| Density (kg/m 3) | 7850 |
| Young's modulus E (GPa) | 206 |
| Poisson ratio v | 0.3 |
TABLE 2
| Loading parameters | Value of |
| Load ratio R L | 0.1 |
| Loading frequency f (Hz) | 1 |
It should be noted that, the cohesive strength and accumulated cohesive length degradation model in the hydrogen-induced fatigue crack growth prediction cohesive force model established by the invention can be replaced by other mathematical models, such as a linear degradation model, an exponential degradation model or various mathematical models based on direct fitting of experimental data.
The multi-field coupling cohesive force analysis model established by the invention can be subjected to decoupling treatment, namely, the finite element analysis process is completed only based on a unidirectional data transmission mode from a stress field to a diffusion field and then to the cohesive force model, so that the hydrogen induced fatigue crack growth prediction is realized.
The invention can realize the fatigue crack growth prediction in the hydrogen environment, and can accurately capture the evolution rule of the hydrogen-induced fatigue crack growth of the pipeline steel by introducing the influence of the hydrogen-induced material performance degradation.
The method can simultaneously consider the influence of cohesive strength degradation and accumulated cohesive length degradation on the hydrogen-induced fatigue crack growth, so as to obtain a hydrogen-induced fatigue crack growth rate prediction result which is closer to experimental data.
The degradation model of the accumulated cohesive strength of the pipeline steel, which is established based on the inverse logistic function and is under the influence of the hydrogen concentration, can reasonably reflect the performance degradation result of the hydrogen-induced material, and related parameters can be corrected based on test data, so that the method is suitable for different materials.
The foregoing is merely a preferred embodiment of the present invention and it should be noted that modifications and adaptations to those skilled in the art may be made without departing from the principles of the present invention, which are intended to be comprehended within the scope of the present invention.
Claims (9)
1. The method for predicting the cyclic cohesive force model of the hydrogen induced fatigue crack growth of the submarine pipeline steel is characterized by adopting finite element model processing, after setting initial conditions of hydrogen concentration in the finite element model, loading cyclic stress on the finite element model, wherein the processing process comprises the following steps:
s1, acquiring a crack tip area in a finite element model of a test piece;
S2, carrying out elastoplastic analysis on the crack tip area by a finite element analysis method to obtain the hydrostatic stress in the stress field of the crack tip area under the current time step;
s3, carrying out hydrogen diffusion analysis on the material based on the hydrostatic stress to obtain a hydrogen diffusion analysis result;
S4, performing hydrogen-induced material performance degradation analysis based on a hydrogen diffusion analysis result to obtain a degradation analysis result;
S5, carrying out cohesive force analysis by utilizing a cyclic cohesive force model based on a degradation analysis result, obtaining damage indexes of cohesive units in the current time step, and judging whether the damage indexes are larger than 1; if yes, deleting the corresponding cohesive unit in the finite element model, enabling the crack to expand forwards, updating the hydrogen concentration boundary condition, and returning to the step S1; if not, entering the next time increment, and returning to the step S2;
s6, extracting the change of the crack length and the cycle times of the cyclic stress under each time step, and calculating the crack expansion rate under the current loading condition so as to predict the hydrogen induced fatigue crack expansion process;
the damage index of the cohesive unit is calculated by adopting the following formula:
，
In the method, in the process of the invention, Representing fatigue damage of cohesive zone at the current time step,/>Index of damage indicating cohesive unit,/>Is the normal separation vector of cracks,/>Representing the variation of normal separation of cracks,/>For/>Increment at the current time step,/>Is cohesive endurance limit coefficient,/>As a Heaviside function,/>Representing normal traction in cohesive zone,/>Represents cohesive strength under the influence of hydrogen,/>Represents the maximum cohesive length corresponding to cohesive strength,/>Represents the cumulative cohesive length after degradation under the influence of hydrogen.
2. The method for predicting the hydrogen induced fatigue crack growth cycle cohesive force model of the submarine pipeline steel according to claim 1, wherein the hydrogen diffusion analysis is performed on the material based on the hydrostatic stress to obtain a hydrogen diffusion analysis result, and the method comprises the steps of obtaining the hydrogen concentration inside the material; the hydrogen concentration in the material is obtained based on a mass diffusion equation of hydrogen in metal considering the influence of strain rate, and the coupling relation between a hydrogen diffusion field and a stress field is established through the mass diffusion equation; the mass diffusion equation is as follows:
，
Wherein, Is lattice hydrogen concentration,/>Is the trap hydrogen concentration,/>Is the lattice diffusion coefficient,/>Is the average molar volume of hydrogen,/>Is a gas constant,/>Absolute temperature,/>Is the hydrostatic stress in stress field,/>Is hamiltonian.
3. The method for predicting the hydrogen induced fatigue crack growth cycle cohesive force model of the submarine pipeline steel according to claim 2, wherein the hydrogen induced material performance degradation analysis is performed based on the hydrogen diffusion analysis result, and is realized through a degradation model which is established based on an inverse logistic function and is influenced by the hydrogen concentration, wherein the degradation model is expressed as follows:
，
In the method, in the process of the invention, Representing the initial cumulative cohesive length,/>And/>The material parameters controlling the degradation model, respectively.
4. A subsea pipeline steel hydrogen induced fatigue crack growth cycle cohesive model prediction method according to claim 3, characterized in that the cohesive strength under the influence of hydrogen is obtained by:
，
，
In the method, in the process of the invention, Is the initial cohesive strength,/>For total hydrogen coverage in metal, the difference/>, between the total hydrogen concentration C H and the Gibbs free energy of any microstructure interface and surrounding materialsAnd (5) determining.
5. The method for predicting the cyclic cohesive force model of the hydrogen induced fatigue crack growth of the subsea pipeline steel according to claim 4, wherein the relation between the initial cohesive strength and the normal traction force in the cohesive zone is determined by the traction separation law, and the normal traction force is regarded as a function of the normal separation vector of the crack in the cohesive zone of the crack tip of the material; the equation for the exponential traction separation law is:
。
6. The method for predicting the hydrogen induced fatigue crack propagation cyclic cohesion model of the subsea pipeline steel according to claim 1, wherein the calculation of the crack propagation rate under the current loading condition is realized by using da/dN, wherein a represents the crack length, N represents the number of cycles of cyclic stress, and d represents a differential sign.
7. The method for predicting the cyclic cohesion model of hydrogen induced fatigue crack growth of subsea pipeline steel according to claim 1, wherein the loading condition of the finite element model is defined by the loading ratio and the loading frequency when the cyclic stress is loaded.
8. The method for predicting the hydrogen induced fatigue crack propagation cyclic cohesive force model of the submarine pipeline steel according to claim 1, wherein a compact tensile CT test piece is adopted for predicting the hydrogen induced fatigue crack propagation of the submarine pipeline steel, a hydrogen diffusion model and a compact tensile CT test piece finite element model are established based on the material property of the to-be-detected pipeline steel, a layer of cohesive units with the thickness of 0 is arranged at the symmetrical axis position of the compact tensile CT test piece finite element model, a grid within the area range of the crack tip length of 1mm is encrypted, and then the test is carried out by loading and loading cyclic stress.
9. The method for predicting the cyclic cohesion model of hydrogen induced fatigue crack growth of subsea pipeline steel according to claim 8, wherein when loading cyclic stress is tested, an equal and opposite cyclic load P is loaded in a round hole of a compact tensile CT test piece according to a predetermined loading condition, the time increment length is limited to 1/40 of the loading period, from an initial time step t 0, the hydrostatic stress in a stress field of a crack tip region in a current time step is calculated, hydrogen diffusion analysis is completed based on the hydrostatic stress in the stress field, and at the same time, the damage index of a cohesive unit in the current time step is calculated by the cyclic cohesion model.
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