Submarine pipeline steel hydrogen-induced fatigue crack propagation prediction method based on hydrogen-induced dislocation pinning-unpinning mechanism

Abstract

The invention discloses a subsea pipeline steel hydrogen-induced fatigue crack propagation prediction method based on a hydrogen-induced dislocation pinning-unpinning mechanism, and the method comprises the steps: constructing a four-stage nonlinear constitutive model of a pipeline steel material in a given hydrogen environment; the four-stage nonlinear constitutive model comprises constitutive relation equations of different stages, which are respectively constructed in four stages of pinning hardening, platform, nail removal and post-nail-removal hardening based on elastic-plastic mechanical behaviors of the pipeline steel material; and endowing the pipeline steel material in the constructed finite element model with the four-stage nonlinear constitutive model, and carrying out pipeline steel hydrogen-induced fatigue crack propagation prediction. According to the method, the staged nonlinear elastic-plastic constitutive model is constructed on the basis of the mechanical property change of the pipeline steel in the hydrogen environment due to hydrogen-induced dislocation pinning and nail disengaging, and the model is used for predicting the hydrogen-induced fatigue crack propagation of the subsea pipeline steel, so that the prediction precision of the crack propagation behavior of the material in the hydrogen environment is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of safety analysis of submarine pipeline steel structures, and in particular to a method for predicting hydrogen-induced fatigue crack growth in submarine pipeline steel based on a hydrogen-induced dislocation pinning-depinning mechanism. The method can be applied in the field of predicting hydrogen-induced fatigue crack growth in pipeline steel under various pure hydrogen or hydrogen-doped gaseous environments. Background Art

[0002] In recent years, power-to-hydrogen projects that combine offshore wind power with hydrogen production have received increasing attention. The core of large-scale development of power-to-hydrogen projects lies in efficient and economical long-distance hydrogen transportation. Among various transportation routes, long-distance submarine pipeline transportation is considered to be the most promising method. However, hydrogen in the internal environment of submarine pipelines can easily penetrate and diffuse into the pipeline steel. Local enrichment of hydrogen in steel can lead to severe deterioration of the mechanical properties of pipeline steel, namely hydrogen embrittlement. The coupling effect between hydrogen embrittlement and fatigue loads means that even when the load size is far below the design conditions, it will cause the initiation and rapid expansion of fatigue cracks in the material, thereby causing the submarine pipeline to fail unexpectedly within its design life. Therefore, hydrogen-induced fatigue has been confirmed as one of the main failure mechanisms of submarine hydrogen transmission pipelines.

[0003] To rationally explain the characteristic behavior of hydrogen-induced fatigue crack growth in submarine pipelines, predictive models have been developed based on fracture mechanics and mainstream hydrogen embrittlement theory. For example, Cheng and Chen proposed a model that relates hydrogen embrittlement to cracks based on fracture mechanics and hydrogen-enhanced depolymerization (HEDE) theory. This model first derived expressions for the threshold stress intensity factor (SIF) and critical frequency and applied them to predict the fatigue crack growth behavior of pipeline steels such as X65 and X70 in a gaseous hydrogen environment. The predicted results were highly consistent with experimental data. Subsequently, Cheng and Chen used this model to establish a two-component model for predicting corrosion fatigue, which includes anodic dissolution and hydrogen embrittlement. The proposed model can accurately predict the crack growth behavior in corrosion fatigue. However, due to the lack of a unified and clear conclusion on the intrinsic mechanism of hydrogen-induced fatigue, models based on traditional fracture mechanics and hydrogen embrittlement theory still have significant limitations.

[0004] In order to effectively simulate the hydrogen-induced fatigue crack growth process, a key phenomenon worthy of attention is the effect of hydrogen on the plastic zone at the crack tip, with dislocation mobility being one of the key parameters. Nagumo reviewed various hydrogen embrittlement theories in steel and found that the evolution of strain-induced microdefects (such as vacancies and dislocations) is affected by hydrogen based on energy stabilization and localized plastic deformation. Xie et al. conducted quantitative mechanical tests under an environmental transmission electron microscope, and the results showed that strong dislocation pinning occurred in aluminum metal under conditions of excessive vacancies.

[0005] Chinese patent CN119578189A discloses a method and system for assessing hydrogen-induced fatigue life of bolts based on a Bayesian approach. This method constructs a basic fatigue characteristic curve based on a material-level fatigue constitutive model, and combines it with finite element stress analysis to obtain the stress concentration coefficient of the actual bolt structure. A hydrogen embrittlement correction function is then introduced to correct the environmental effects of the basic model, establishing a modified constitutive model that takes hydrogen embrittlement damage into account. Furthermore, a nonlinear cumulative damage theory is used to construct a life prediction model, breaking through the limitations of traditional linear models. Finally, the Bayesian statistical method is used to integrate experimental data to optimize the model parameters and achieve probabilistic life prediction.

[0006] Chinese patent CN116933580B provides a cyclic cohesive force model prediction method for hydrogen-induced fatigue crack growth in submarine pipeline steel. The method is based on a finite element framework and has the following processing steps: performing elastic-plastic analysis on the crack tip area in the specimen finite element model using the finite element analysis method; performing hydrogen diffusion analysis on the material; and performing hydrogen-induced material performance degradation analysis on the material. Cohesive force analysis is performed using a cyclic cohesive force model to obtain the damage index of the cohesive unit at the current time step. If the damage index is greater than 1, the corresponding cohesive unit is deleted to allow the crack to propagate forward and the boundary conditions are updated. Otherwise, the method enters the next time increment and returns to the finite element analysis. The crack length and the number of cyclic stress cycles at each time step are extracted to calculate the crack growth rate under the current loading conditions to predict the hydrogen-induced fatigue crack growth process.

[0007] Through the above-mentioned related existing technologies, it can be found that the existing technologies are difficult to correctly reflect the saturation effect of hydrogen embrittlement of pipeline steel in a high hydrogen concentration environment and the sudden increase in the hydrogen-induced fatigue crack growth rate; they do not consider the influence of the changes in the elastic-plastic behavior of the material caused by hydrogen-induced dislocation pinning and depinning on the fatigue crack growth process; they only focus on the normal state or only consider the structural defects caused by corrosion, and do not consider the influence of the material performance degradation caused by hydrogen embrittlement on the fatigue crack growth process. Summary of the Invention

[0008] The present invention aims to overcome the shortcomings and drawbacks of existing technologies by providing a method for predicting hydrogen-induced fatigue crack growth in submarine pipeline steel based on the hydrogen-induced dislocation pinning and depinning mechanism. Based on the changes in the mechanical properties of pipeline steel caused by hydrogen-induced dislocation pinning and depinning in a hydrogen environment, the present invention constructs a staged nonlinear elastic-plastic constitutive model and uses this model to predict hydrogen-induced fatigue crack growth in submarine pipeline steel.

[0009] The present invention is achieved in that:

[0010] A method for predicting hydrogen-induced fatigue crack growth in submarine pipeline steel based on the hydrogen-induced dislocation pinning-depinning mechanism comprises:

[0011] A four-stage nonlinear constitutive model for pipeline steel in a given hydrogen environment is constructed. The four-stage nonlinear constitutive model includes constitutive equations for different stages, namely, pinning hardening, platform, de-pinning, and post-de-pinning hardening, based on the elastic-plastic mechanical behavior of the pipeline steel.

[0012] The four-stage nonlinear constitutive model is assigned to the pipeline steel material in the constructed finite element model to predict the hydrogen-induced fatigue crack growth of the pipeline steel;

[0013] The construction of the constitutive model includes the following steps:

[0014] Calculate the hydrogen coverage inside the material;

[0015] When the material true stress is between 0 and the pinning stress, the constitutive equation of the pinning hardening stage of the submarine pipeline steel constitutive model is constructed based on the linear elastic relationship between the material true stress and true strain, according to the material's yield strength and elastic modulus.

[0016] When the true stress of the material reaches the depinning stress, the hydrogen coverage inside the material remains constant while the true strain continues to increase. Therefore, the hydrogen coverage is introduced and the constitutive relationship equation of the plateau stage of the constitutive model is constructed according to the Hollomon strength coefficient and Hollomon index.

[0017] When the true strain growth in the plateau stage reaches the depinning strain range, the constitutive equation of the depinning stage of the constitutive model is constructed;

[0018] When the true strain in the depinning stage is equal to the true strain in the hardening stage after depinning, the constitutive relationship equation of the hardening stage after depinning of the constitutive model is constructed.

[0019] Preferably, the calculation equation for the hydrogen coverage inside the material is as follows:

[0020]

[0021] Among them, θ H is the hydrogen coverage inside the material, C H is the hydrogen concentration inside the material, exp represents the exponential operation with the natural number e as the base. is the Gibbs free energy, is the universal gas constant and T is the absolute temperature.

[0022] Preferably, the constitutive equation of the pinning hardening stage is as follows:

[0023] σ t,ph =Eε t,ph

[0024] Among them, σ t,phis the true stress in the pinning hardening stage, E is the elastic modulus of the material, ε t,ph is the true strain during the pinning hardening stage.

[0025] Preferably, the constitutive equation of the plateau stage is as follows:

[0026]

[0027] Among them, σ t,p is the true stress in the plateau phase, K H is the Hollomon strength coefficient, n is the Hollomon index, σ y is the yield strength of the material, ε t,p is the true strain in the plateau phase, θ H is the hydrogen coverage.

[0028] Preferably, the constitutive equation of the depinning stage is as follows:

[0029] σ t,dp =E(ε t,dp -Δε dp )+σ y

[0030] Δε dp =σ y / E

[0031] Among them, σ t,dp is the true stress in the depinning stage, Δε dp is the depinning strain range, which represents the true strain range covered by the depinning stage, that is, the absolute value of the difference between the true strains at the starting and ending points of the depinning stage, σ y is the yield strength of the material, E is the elastic modulus of the material, ε t,dp is the true strain during the unpinning stage.

[0032] Preferably, the constitutive equation of the hardening stage after nail removal is as follows:

[0033]

[0034] Among them, σ t,pd is the true stress in the hardening stage after nail removal, ε t,pd is the true strain in the hardening stage after nail removal, n is the Hollomon index, K H is the Hollomon strength coefficient, ε t,pd It is obtained by combining the constitutive equation of the de-pinning stage and the constitutive equation of the hardening stage after de-pinning. t,dp =σ t,pd Solve below.

[0035] Preferably, the depinning stress is calculated based on the hydrogen coverage θ H The calculation equation and the constitutive relationship equation of the platform stage are used to obtain the hydrogen concentration C inside the material in the platform stage. H The true stress σ t,p and true strain ε t,p The relationship between the two is then combined with the constitutive equation of the pinning hardening stage to obtain the true strain ε in the pinning hardening stage. t,ph =ε t,p Obtained by solving the following.

[0036] Preferably, the finite element model includes a compact tensile specimen finite element model, in which a layer of zero-thickness cohesive force unit is arranged at the symmetry axis position, the constitutive relationship of the zero-thickness cohesive force unit is characterized by the traction force and opening displacement at the crack tip, and the grid is encrypted within the area of ​​2 mm in length from the crack tip.

[0037] Preferably, the four-stage nonlinear constitutive model is assigned to material units other than the cohesive force unit to predict hydrogen-induced fatigue crack propagation; during the prediction, an equal and reverse cyclic load F is applied to the upper half surface of the upper circular hole and the lower half surface of the lower circular hole of the CT specimen respectively, the calculation equation of the hydrogen coverage rate is introduced into the finite element analysis process, the constructed four-stage nonlinear constitutive model is assigned to the pipeline steel material in the finite element model, and the elastic-plastic analysis is performed in combination with the calculated real-time hydrogen coverage rate to couple the effects of hydrogen and fatigue load; based on the results of the elastic-plastic analysis, the crack propagation rate is predicted.

[0038] Preferably, the crack propagation rate is predicted based on the elastic-plastic analysis results, and the elastic-plastic analysis results are imported into the cyclic cohesion model to calculate the current damage of the cohesive unit. When the damage value of the current unit reaches 1, the current unit is deleted to allow the crack to propagate forward until the crack propagates beyond the mesh refinement area, thereby obtaining the crack propagation rate.

[0039] The present invention constructs a nonlinear constitutive model for pipeline steel to simulate the elastic-plastic mechanical behavior of pipeline steel in a hydrogen environment, and uses the constructed nonlinear constitutive model for pipeline steel in combination with a cyclic cohesion model to predict the crack growth rate, thereby improving the precision and accuracy of the prediction. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a flow chart of a method for predicting hydrogen-induced fatigue crack growth of submarine pipeline steel based on a hydrogen-induced dislocation pinning-depinning mechanism according to an embodiment of the present invention.

[0041] Figure 2 Schematic diagram of the process of constructing a nonlinear constitutive model of submarine pipeline steel based on the hydrogen-induced dislocation pinning-depinning mechanism according to an embodiment of the present invention.

[0042] Figure 3Schematic diagram of constructing a four-stage nonlinear constitutive model of X52 according to an embodiment of the present invention.

[0043] Figure 4 This is a finite element model of a compact tensile specimen and a detailed diagram of the crack tip mesh encryption according to an embodiment of the present invention.

[0044] Figure 5 This is a comparison chart of the experimental results of the hydrogen-induced fatigue crack growth rate of X52 steel in an embodiment of the present invention and the prediction results of the model constructed using the present invention. DETAILED DESCRIPTION

[0045] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0046] Dislocation pinning and depinning have a universal impact on hydrogen-induced fatigue crack growth in various metals. However, there are few models and experimental reports on the dislocation pinning and depinning effects of hydrogen on pipeline steel. Therefore, based on the study of the hydrogen-induced dislocation pinning and depinning mechanism, this paper proposes a method for constructing a nonlinear constitutive model for pipeline steel based on this mechanism to simulate the elastic-plastic mechanical behavior of pipeline steel in a hydrogen environment. This nonlinear constitutive model of pipeline steel is then used in combination with a cyclic cohesive force model to predict crack growth rates.

[0047] In the embodiment of the present application, the nonlinear constitutive model of pipeline steel is a four-stage elastic-plastic constitutive model of submarine pipeline steel in a gaseous hydrogen environment. The model takes into account the influence of hydrogen-induced dislocation pinning and depinning on the elastic-plastic mechanical behavior of pipeline steel, and reflects the change in yield strength of pipeline steel during hydrogen-induced dislocation pinning and depinning and the subsequent strain hardening; after the model is integrated into the cyclic cohesion model, it can effectively predict the hydrogen-assisted fatigue crack growth rate.

[0048] In the exemplary embodiment of the present application, in the hydrogen-induced dislocation pinning-depinning mechanism, the dislocation is a microscopic defect inside the material, and hydrogen atoms gather at the dislocation core under stress to form pinning points, thereby causing the material to harden; when the external energy exceeds a certain critical value, the pinning point fails, the dislocation depins, and the material enters the plastic deformation stage.

[0049] In the embodiment of the present application, the elastic-plastic mechanical behavior of pipeline steel material is divided into four stages (pinning hardening, platform, nail removal, and hardening after nail removal), and the hydrogen coverage θ H The classical elastic-plastic model is modified to quantify the influence of hydrogen concentration on the elastic-plastic behavior of pipeline steel. Combined with the cohesive force model, a cohesive force model for predicting hydrogen-induced fatigue crack growth is formed. This can realize the prediction of hydrogen-induced fatigue crack growth and improve the precision and accuracy of the prediction.

[0050] In the embodiment of the present application, a cohesive force model for predicting hydrogen-induced fatigue crack propagation is used to describe the dynamic distribution of hydrogen in the material and its synergistic effect on crack tip damage through an improved hydrogen diffusion equation and thermodynamic equilibrium relationship. At the same time, based on the cohesive force theory, a constitutive relationship of the material crack tip is established and crack propagation prediction is performed.

[0051] See also Figure 1 As shown, in the implementation of this application, the method for predicting hydrogen-induced fatigue crack growth of submarine pipeline steel based on the hydrogen-induced dislocation pinning-depinning mechanism includes:

[0052] A four-stage nonlinear constitutive model for pipeline steel in a given hydrogen environment is constructed. The four-stage nonlinear constitutive model includes constitutive equations for different stages, namely, pinning hardening, platform, de-pinning, and post-de-pinning hardening, based on the elastic-plastic mechanical behavior of the pipeline steel.

[0053] The four-stage nonlinear constitutive model is assigned to the pipeline steel material in the constructed finite element model to predict the hydrogen-induced fatigue crack propagation of pipeline steel.

[0054] See also Figure 2 As shown, in the embodiment of the present application, the construction of the four-stage nonlinear constitutive model includes the following steps:

[0055] Calculate the hydrogen coverage inside the material;

[0056] When the material true stress is between 0 and the pinning stress, the constitutive equation of the pinning hardening stage of the submarine pipeline steel constitutive model is constructed based on the linear elastic relationship between the material true stress and true strain, according to the material's yield strength and elastic modulus.

[0057] When the true stress of the material reaches the depinning stress, the hydrogen coverage inside the material remains constant while the true strain continues to increase. Therefore, the hydrogen coverage is introduced and the constitutive relationship equation of the plateau stage of the constitutive model is constructed according to the Hollomon strength coefficient and Hollomon index.

[0058] When the true strain growth in the plateau stage reaches the depinning strain range, the constitutive equation of the depinning stage of the constitutive model is constructed;

[0059] When the true strain in the depinning stage is equal to the true strain in the hardening stage after depinning, the constitutive relationship equation of the hardening stage after depinning of the constitutive model is constructed.

[0060] In the embodiment of the present application, the hydrogen coverage inside the material is calculated based on the Langmuir-McLean isotherm equation. Specifically, the calculation equation for the hydrogen coverage inside the material is as follows:

[0061]

[0062] Among them, θ H is the hydrogen coverage inside the material, C H is the hydrogen concentration inside the material, exp represents the exponential operation with the natural number e as the base. is the Gibbs free energy, which is generally taken as 30 kJ / mol for pipeline steel. is the universal gas constant, which is 8.3145 J / (mol·k), and T is the absolute temperature.

[0063] In the embodiment of the present application, during the pinning hardening stage, considering the increase in material strength caused by the hydrogen-induced pinning mechanism, the relationship between the true stress and the true strain is linear elastic, wherein the constitutive relationship equation of the pinning hardening stage is as follows:

[0064] σ t,ph =Eε t,ph

[0065] Among them, σ t,ph is the true stress in the pinning hardening stage, E is the elastic modulus of the material, ε t,ph is the true strain during the pinning hardening stage.

[0066] In the embodiment of the present application, when the true stress σ of the material in the pinning hardening stage is t,ph The magnitude reaches the pinning stress σ for the first time dp When the constitutive relationship equation of the platform stage of the constitutive model is constructed, the stress in the platform stage remains constant or increases only slightly according to the change of hydrogen coverage, while the strain continues to increase. The expression equation of this stage is constructed by modifying the classic Hollomon strain hardening equation by introducing hydrogen coverage. Among them, the constitutive relationship equation of the platform stage is as follows:

[0067]

[0068] Among them, σ t,p is the true stress in the plateau phase, K H is the Hollomon strength coefficient, n is the Hollomon index, σ y is the yield strength of the material, ε t,p is the true strain in the plateau phase, θ H is the hydrogen coverage.

[0069] In the embodiment of the present application, when the true strain ε in the plateau stage is t,p The growth reaches the pinning strain range Δε dp The depinning phase of the constitutive model begins. During this phase, stress unloading occurs due to large-scale dislocation motion. In this phase, the true stress-strain relationship is defined as a linear relationship similar to that in the elastic phase. The constitutive equation for the depinning phase is as follows:

[0070] σ t,dp =E(ε t,dp -Δε dp )+σ y

[0071] Δε dp =σ y / E

[0072] Among them, σ t,dp is the true stress in the depinning stage, Δε dp is the depinning strain range, which represents the true strain range covered by the depinning stage, that is, the absolute value of the difference between the true strains at the starting and ending points of the depinning stage, σ y is the yield strength of the material, E is the elastic modulus of the material, ε t,dp is the true strain during the unpinning stage.

[0073] In the embodiment of the present application, in the post-pinning hardening stage, the material undergoes a certain degree of hardening due to dislocation pinning-pinning, wherein the constitutive equation of the post-pinning hardening stage is as follows:

[0074]

[0075] Among them, σ t,pd is the true stress in the hardening stage after nail removal, ε t,pd is the true strain in the hardening stage after nail removal, n is the Hollomon index, K H is the Hollomon strength coefficient, ε t,pd It is obtained by combining the constitutive equation of the de-pinning stage and the constitutive equation of the hardening stage after de-pinning. t,dp =σ t,pd Solve below.

[0076] In the embodiment of the present application, the depinning stress can be calculated based on the hydrogen coverage θ H The calculation equation and the constitutive relationship equation of the platform stage are used to obtain the hydrogen concentration C inside the material in the platform stage. H The true stress σ t,p and true strain ε t,p The relationship between the two is then combined with the constitutive equation of the pinning hardening stage to obtain the true strain ε in the pinning hardening stage. t,ph =ε t,p Obtained by solving the following.

[0077] In an embodiment of the present application, the finite element model includes a finite element model of a compact tensile specimen, in which a layer of zero-thickness cohesive force unit is arranged at the position of the symmetry axis. The constitutive relationship of the zero-thickness cohesive force unit is characterized by the traction force and opening displacement at the crack tip, and the grid is encrypted within the area of ​​2 mm in length from the crack tip.

[0078] In an embodiment of the present application, a four-stage nonlinear constitutive model is assigned to material units other than the cohesive force unit to predict hydrogen-induced fatigue crack propagation; during the prediction, an equal and reverse cyclic load F is applied to the upper half surface of the upper circular hole and the lower half surface of the lower circular hole of the CT specimen, respectively, and the calculation equation of the hydrogen coverage rate is introduced into the finite element analysis process. The constructed four-stage nonlinear constitutive model is assigned to the pipeline steel material in the finite element model, and an elastic-plastic analysis is performed in combination with the calculated real-time hydrogen coverage rate to couple the effects of hydrogen and fatigue load; based on the results of the elastic-plastic analysis, the crack propagation rate is predicted.

[0079] In the embodiment of the present application, the crack propagation rate is predicted based on the elastic-plastic analysis results, and the elastic-plastic analysis results are imported into the cyclic cohesion model to calculate the current damage of the cohesive unit. When the damage value of the current unit reaches 1, the current unit is deleted to allow the crack to extend forward until the crack extends beyond the mesh refinement area, thereby obtaining the crack propagation rate.

[0080] Next, we constructed a constitutive model for X52 pipeline steel in a given hydrogen environment using the method described in this invention. This model, combined with cohesive force theory, predicted hydrogen-induced fatigue crack growth in the pipeline steel. The effectiveness of the proposed solution was verified by comparing the predicted results with experimental data.

[0081] According to the material properties of X52 pipeline steel shown in Table 1 and the constitutive model construction method of the present invention, according to the yield strength σ of X52 steel y and elastic modulus E, firstly, the pinning hardening stage expression of the constitutive model is constructed; then, according to the Hollomon strength coefficient K H and Hollomon index n to construct the plateau phase expression of the constitutive model; the hydrogen coverage θ H The expression of is substituted into the constructed platform stage constitutive relation equation, and the platform stage constitutive relation equation is converted into the hydrogen concentration C inside the material. H The true stress σ t,p and true strain ε t,p The transformed relationship is combined with the constitutive relationship of the step pinning hardening stage, and the depinning stress σ corresponding to the turning point of the two stages under different hydrogen coverage is solved under the condition of equal strain value. dp ; Calculate the pin removal strain range Δε dp =0.001577; when the true strain ε in the plateau stage t,pThe growth reaches the pinning strain range Δε dp When the pin removal stage of the constitutive model is constructed, the Hollomon strength coefficient K H The expression of the post-depinning hardening stage of the constitutive model is constructed by combining the expressions of the depinning stage and the post-depinning hardening stage to obtain the depinning hardening strain ε corresponding to the turning point of the two stages. pd =0.003041. Combining the expressions of each stage and the turning point, we finally establish the following Figure 3 The different hydrogen coverage θ shown H The four-stage nonlinear constitutive model under the condition of X52 pipeline steel with different hydrogen coverage θ H The relationship between stress and strain inside the material under certain conditions.

[0082] like Figure 3 As shown in the figure, when the hydrogen coverage is 0, the model is equivalent to the traditional Hollomon strain hardening model. However, as the hydrogen coverage increases, the stress-strain relationship of pipeline steel gradually exhibits special elastic-plastic behavior caused by the influence of dislocation pinning and depinning, namely the occurrence of pinning hardening, plateau, depinning, and post-depinning hardening characteristics.

[0083] When the constitutive model constructed above is used to predict hydrogen-induced fatigue crack growth, firstly, based on the cohesive theory, the following Figure 3 The finite element model of the compact tensile (CT) specimen shown in the figure has a layer of zero-thickness cohesive elements arranged at its symmetry axis. This element is a type of finite element whose constitutive relationship is characterized by the traction force and opening displacement at the crack tip. The mesh within the area with a length of 2 mm at the crack tip is encrypted. Then, the different hydrogen coverage ratios θ are constructed based on the present invention. H The following four-stage nonlinear constitutive model is given to material elements other than cohesive elements for prediction.

[0084] During the prediction process, equal and opposite cyclic loads F were applied to the upper half of the upper circular hole and the lower half of the lower circular hole of the CT specimen, respectively. The loading conditions are shown in Table 2. These loading conditions affect the fatigue crack evolution process in the finite element analysis. Based on the UMAT subroutine, the calculation equation of the hydrogen coverage rate was introduced into the finite element analysis process, and the constructed four-stage nonlinear constitutive model was assigned to the pipeline steel material in the finite element model. During the analysis process, the calculated real-time hydrogen coverage rate was combined with the effects of hydrogen and fatigue load to obtain the elastic-plastic analysis results. The elastic-plastic analysis results were then imported into the cyclic cohesion model to calculate the current damage of the cohesive element. When the current element damage value reached 1, the current element was deleted to allow the crack to propagate forward until the crack propagated beyond the mesh refinement area, and the crack propagation rate was obtained.

[0085] The hydrogen-induced fatigue crack growth rate predicted by the constitutive model proposed in the embodiment of the present invention is compared with the fatigue crack growth rate obtained from the hydrogen-induced fatigue crack growth experiment of X52 steel. The results are as follows: Figure 5 As shown in the figure, it can be seen that the model proposed in the present invention is in good agreement with the experimental results when used to predict hydrogen-induced fatigue crack growth, indicating that the model constructed in the present invention can well capture the constitutive relationship characteristics of pipeline steel under the influence of hydrogen-induced dislocation pinning-depinning mechanism and the evolution characteristics of hydrogen-induced fatigue cracks.

[0086] Table 1X52 steel material properties

[0087]

[0088] Table 2 Loading conditions for the prediction of hydrogen-induced fatigue crack growth in X52 steel

[0089]

[0090] It should be noted that, although the above example only describes the elastic-plastic behavior of X52 steel during hydrogen-induced fatigue crack growth, the constitutive model can also be used to simulate the elastic-plastic constitutive behavior of any pipeline steel in a hydrogen environment, such as X60, X65, and X70 pipeline steels. In addition, it will be understood by those skilled in the art that the implementation scheme may only include the factors necessary to implement the embodiment scheme of this specification, and does not necessarily include all the factors shown in the specification.

[0091] The plateau, pin removal, and pin removal hardening stages described in the multi-stage constitutive model established in the embodiments of the present invention are based on modifications and improvements to the classical elastic-plastic model. The classical elastic-plastic model involved can be replaced, not just by the Hollomon strain hardening model. For example, the expressions for the plateau, pin removal, and pin removal hardening stages described in the constitutive model can be constructed based on the perfect elastic-plastic model or the bilinear elastic-plastic model.

[0092] An embodiment of the present invention divides the constitutive model of pipeline steel in a hydrogen environment into multiple stages based on the hydrogen-induced dislocation pinning-depinning mechanism, introduces the internal hydrogen coverage of the material into the constitutive model to dynamically predict the elastic-plastic constitutive behavior of the material in a hydrogen environment, and integrates the constitutive model into a complete finite element framework to realize the evolutionary simulation of fatigue crack propagation. Since the model provided by the present invention can effectively reflect the influence of the hydrogen-induced dislocation pinning-depinning model on the constitutive relationship of pipeline steel, it can significantly improve the prediction accuracy of the crack propagation behavior of the material in a hydrogen environment.

[0093] The classical elastic-plastic model employed in the model provided by the present invention can be flexibly replaced to accommodate different assumptions about material elastic-plastic behavior, thereby more effectively simulating material hardening and degradation. The model provided by the present invention exhibits excellent cross-scale coupling, effectively enabling multi-scale correlation between microscopic dislocation mechanisms and macroscopic crack propagation, thus overcoming the limitations of traditional single-scale models.

[0094] The basic principles, main features and advantages of the present invention are shown and described above. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments and that the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention.

[0095] The embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being defined by the appended claims rather than the foregoing description, and all changes that come within the meaning and range of equivalents of the claims are therefore intended to be embraced therein.

[0096] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A method for predicting hydrogen-induced fatigue crack growth in submarine pipeline steel based on the hydrogen-induced dislocation pinning-depinning mechanism, characterized in that: include: A four-stage nonlinear constitutive model for pipeline steel in a given hydrogen environment is constructed. The four-stage nonlinear constitutive model includes constitutive equations for different stages, namely, pinning hardening, platform, de-pinning, and post-de-pinning hardening, based on the elastic-plastic mechanical behavior of the pipeline steel. The four-stage nonlinear constitutive model is assigned to the pipeline steel material in the constructed finite element model to predict the hydrogen-induced fatigue crack growth of the pipeline steel; The construction of the constitutive model includes the following steps: Calculate the hydrogen coverage inside the material; When the material true stress is between 0 and the pinning stress, the constitutive equation of the pinning hardening stage of the submarine pipeline steel constitutive model is constructed based on the linear elastic relationship between the material true stress and true strain, according to the material's yield strength and elastic modulus. When the true stress of the material reaches the depinning stress, the hydrogen coverage inside the material remains constant while the true strain continues to increase. Therefore, the hydrogen coverage is introduced and the constitutive relationship equation of the plateau stage of the constitutive model is constructed according to the Hollomon strength coefficient and Hollomon index. When the true strain growth in the plateau stage reaches the depinning strain range, the constitutive equation of the depinning stage of the constitutive model is constructed; When the true strain in the depinning stage is equal to the true strain in the hardening stage after depinning, the constitutive relationship equation of the hardening stage after depinning of the constitutive model is constructed.

2. The method for predicting hydrogen-induced fatigue crack growth of submarine pipeline steel based on hydrogen-induced dislocation pinning-depinning mechanism according to claim 1, characterized in that: The calculation equation for the hydrogen coverage inside the material is as follows: Among them, θ H is the hydrogen coverage inside the material, C H is the hydrogen concentration inside the material, exp represents the exponential operation with the natural number e as the base. is the Gibbs free energy, is the universal gas constant and T is the absolute temperature.

3. The method for predicting hydrogen-induced fatigue crack growth of submarine pipeline steel based on hydrogen-induced dislocation pinning-depinning mechanism according to claim 1, characterized in that: The constitutive equation of the pinning hardening stage is as follows: s t,ph =Ee t,ph Among them, σ t,ph is the true stress in the pinning hardening stage, E is the elastic modulus of the material, ε t,ph is the true strain during the pinning hardening stage.

4. The method for predicting hydrogen-induced fatigue crack growth of submarine pipeline steel based on hydrogen-induced dislocation pinning-depinning mechanism according to claim 1, characterized in that: The constitutive equation of the plateau stage is as follows: Among them, σ t,p is the true stress in the plateau phase, K H is the Hollomon strength coefficient, n is the Hollomon index, σ y is the yield strength of the material, ε t,p is the true strain in the plateau phase, θ H is the hydrogen coverage.

5. The method for predicting hydrogen-induced fatigue crack growth of submarine pipeline steel based on hydrogen-induced dislocation pinning-depinning mechanism according to claim 1, characterized in that: The constitutive equation of the depinning stage is as follows: s t,dp =E(e t,dp -No dp )+s y No dp =s y / E Among them, σ t,dp is the true stress in the depinning stage, Δε dP is the depinning strain range, which represents the true strain range covered by the depinning stage, that is, the absolute value of the difference between the true strains at the starting and ending points of the depinning stage, σ y is the yield strength of the material, E is the elastic modulus of the material, ε t,dp is the true strain during the unpinning stage.

6. The method for predicting hydrogen-induced fatigue crack growth of submarine pipeline steel based on hydrogen-induced dislocation pinning-depinning mechanism according to claim 1, characterized in that: The constitutive equation of the post-pinning hardening stage is as follows: Among them, σ t,pd is the true stress in the hardening stage after nail removal, ε t,pd is the true strain in the hardening stage after nail removal, n is the Hollomon index, K H is the Hollomon strength coefficient, ε t,pd It is obtained by combining the constitutive equation of the de-pinning stage and the constitutive equation of the hardening stage after de-pinning. t,dp =σ t,pd Solve below.

7. The method for predicting hydrogen-induced fatigue crack growth of submarine pipeline steel based on hydrogen-induced dislocation pinning-depinning mechanism according to claim 1, characterized in that: The depinning stress is based on the hydrogen coverage θ H The calculation equation and the constitutive relationship equation of the platform stage are used to obtain the hydrogen concentration C inside the material in the platform stage. H The true stress σ t,p and true strain ε t,p The relationship between the two is then combined with the constitutive equation of the pinning hardening stage to obtain the true strain ε in the pinning hardening stage. t,ph =ε t,p Obtained by solving the following.

8. The method for predicting hydrogen-induced fatigue crack growth of submarine pipeline steel based on hydrogen-induced dislocation pinning-depinning mechanism according to claim 1, characterized in that: The finite element model includes a compact tensile specimen finite element model, in which a layer of zero-thickness cohesive force unit is arranged at the symmetry axis position. The constitutive relationship of the zero-thickness cohesive force unit is characterized by the traction force and opening displacement at the crack tip, and the mesh is encrypted in the area within the crack tip length of 2 mm.

9. The method for predicting hydrogen-induced fatigue crack growth of submarine pipeline steel based on hydrogen-induced dislocation pinning-depinning mechanism according to claim 8, characterized in that: The four-stage nonlinear constitutive model is assigned to material units other than the cohesive force unit to predict hydrogen-induced fatigue crack growth. During the prediction, a cyclic load F of equal magnitude and opposite direction is applied to the upper half surface of the upper circular hole and the lower half surface of the lower circular hole of the CT specimen respectively. The calculation equation of the hydrogen coverage rate is introduced into the finite element analysis process. The constructed four-stage nonlinear constitutive model is assigned to the pipeline steel material in the finite element model. The elastic-plastic analysis is performed by coupling the calculated real-time hydrogen coverage rate with the effect of hydrogen and fatigue load. The crack growth rate is predicted based on the results of the elastic-plastic analysis.

10. The method for predicting hydrogen-induced fatigue crack growth of submarine pipeline steel based on hydrogen-induced dislocation pinning-depinning mechanism according to claim 9, characterized in that: The crack growth rate is predicted based on the elastic-plastic analysis results. The elastic-plastic analysis results are imported into the cyclic cohesion model to calculate the current damage of the cohesive unit. When the damage value of the current unit reaches 1, the current unit is deleted to allow the crack to extend forward until the crack extends beyond the mesh refinement area, thereby obtaining the crack growth rate.

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