CN-121994595-A - Evaluation method for fracture toughness of metal material in hydrogen environment

CN121994595ACN 121994595 ACN121994595 ACN 121994595ACN-121994595-A

Abstract

The invention discloses an evaluation method and system for influencing fracture performance of a metal material by hydrogen content under stress-strain synergism, wherein the evaluation method comprises the following steps of (1) a hydrogen diffusion and mechanical property coupling experiment, and respectively obtaining a rule of hydrogen diffusion parameters along with stress-strain co-evolution and fracture toughness data under different hydrogen-stress-strain coupling states through an in-situ tensile hydrogen permeation experiment and a compact tensile experiment. (2) And establishing and verifying a multi-physical field coupling numerical model, constructing a hydrogen diffusion-mechanical coupling model based on stress-driven diffusion and strain propagation trap theory, and establishing a quantitative correlation function of 'stress-strain-hydrogen concentration-hydrogen induced cracking J integral' through experimental data calibration. (3) And carrying out safety assessment on the defective pipeline, establishing an actual pipeline defect model, calculating J integral at the defect and predicted cracking J integral of the metal material under the working condition, and carrying out quantitative risk classification by defining a safety margin coefficient to realize early warning and management and control of hydrogen induced cracking risk.

Inventors

ZHANG ZHUWU
Wu Diefan

Assignees

福州大学

Dates

Publication Date: 20260508
Application Date: 20260205

Claims (6)

1. The method for evaluating the fracture toughness of the metal material in the hydrogen environment is characterized by comprising the following steps of: Step S1, determining hydrogen concentration distribution under stress-strain synergism, systematically researching diffusion and enrichment behaviors of hydrogen in a material under different stress and strain states through a metal material in-situ stretching hydrogen permeation experiment, and determining a synergism mechanism of stress-driven diffusion and strain proliferation traps to provide key input parameters for subsequent multi-field coupling simulation; S2, testing breaking performance in a hydrogen environment, and measuring J integral of the material and a hydrogen induced cracking J integral J IH under different stress-strain-hydrogen environment coupling conditions through a metal material compact stretching experiment; Step 3, based on a multi-physical field coupling theory, establishing a numerical model comprising a stress field, a strain field, a hydrogen diffusion field and a damage field, simulating the behavior of a compact tensile sample under different loads and hydrogen concentrations, verifying the accuracy of an experimental result, and constructing a quantitative association function of stress-strain-hydrogen concentration-hydrogen induced cracking J integral; And S4, applying the established quantitative association function to engineering safety evaluation of the pipeline with the defects, and comparing the J integral value of the actual defect with the predicted hydrogen induced cracking J integral value of the material under the working condition to realize quantitative judgment of the hydrogen induced cracking risk and provide basis for pipeline maintenance decision.
2. The method according to claim 1, wherein the in-situ stretching hydrogen permeation test of the metal material in step S1 specifically comprises the following steps: step S11, selecting a proper metal material sample; Step S12, polishing, cleaning, dehydrating and drying the sample before the electrochemical experiment; Step S13, installing a sample in an experimental device capable of independently controlling load and displacement, and placing the sample in an electrochemical hydrogen charging system of a double electrolytic cell; Step S14, processing a hydrogen permeation current density-time curve by adopting a constant concentration model, calculating hydrogen permeation parameters including surface hydrogen concentration C 0 , effective hydrogen diffusion coefficient D eff , reversible hydrogen trap density N T and the like, respectively establishing an enhancement relation of stress to the hydrogen diffusion coefficient and an evolution relation of plastic strain to the reversible hydrogen trap density by analyzing permeation behavior differences of an elastic stage and a plastic stage, thereby providing staged physical parameter input for a subsequent coupling model, wherein the functional relation of the reversible hydrogen trap density evolving along with equivalent plastic strain is shown as follows: 。
3. The method of evaluation according to claim 1, wherein the method of compact stretching experiments of step S2 comprises the steps of: step S21, selecting a proper metal material sample; step S22, polishing, cleaning, dehydrating and drying the sample before the uniaxial tension experiment, and installing the sample in a stress ring; step S23, performing compact stretching experiments in an experimental device with controllable hydrogen environment, applying load to a metal material steel sample at a constant displacement rate, synchronously recording an accurate load-displacement curve, maintaining a constant hydrogen concentration environment through an electrochemical hydrogen charging system in the experimental process, and monitoring the dynamic behavior of a crack tip in real time by a strain gauge; Step S24, the system changes the hydrogen charging concentration, the load level and the pre-strain level, a plurality of groups of compact stretching experiments are carried out, J integral data under different stress-strain-hydrogen coupling conditions are obtained, and the hydrogen induced cracking J integral value J IH of the material under different working conditions is determined by analyzing the trend of a J-R curve.
4. The method of evaluating according to claim 1, wherein the simulating the behavior of the compact tensile specimen under different loads and hydrogen concentrations in step S3 comprises the steps of: S31, establishing a compact tensile sample model by a finite element method, and setting proper grid density to ensure the calculation accuracy of a crack tip region; step S32, based on a stress-driven diffusion theory, taking the influence of hydrostatic stress and concentration gradient on hydrogen diffusion into consideration, and establishing a hydrogen transport control equation: Wherein D L is the lattice diffusion coefficient of hydrogen atoms, R is the ideal gas constant, 8.314J/(mol ∙ K), T is the temperature; Is the chemical potential gradient of the hydrogen atoms among the crystals, H 2×10 -6 m 3 /mol;σ h Is hydrostatic stress, which is the partial molar volume of hydrogen atoms in steel; Step S33, coupling a 'solid mechanics' interface with a 'dilute substance transfer' interface in finite element simulation software, substituting a stress-driven diffusion coefficient function and a strain propagation trap density function obtained in the step S1 into corresponding control items in a coupling model respectively, so as to realize simulation of stress and strain staged and synergistic action mechanisms in terms of values; Step S34, calculating a corresponding J IH integral value based on J integral theory by using the verified multi-physical field coupling model Wherein w is the strain energy density, T i is the stress component acting on the integral path, u i is the corresponding displacement component, ds is the unit length on the loop, Γ is any anticlockwise loop around the crack tip, starts at the crack lower surface, and ends at the crack upper surface; Meanwhile, considering a hydrogen induced cracking mechanism, namely increasing the concentration of an external hydrogen environment aggravates stress strain concentration at the tip of a crack, promoting the concentration of internal hydrogen to be increased through the actions of stress-driven diffusion and plastic strain proliferation and trapping, and increasing the hydrogen coverage rate theta to further reduce the critical energy release rate, wherein the reduction of the cracking J integral value is finally shown as the following formula: Where χ represents the hydrogen damage coefficient, θ represents the hydrogen coverage, The increase of the hydrogen coverage rate can lead to the decrease of G c , reduce the capability of the material for resisting crack propagation and finally lead to the decrease of the crack initiation J integral value; Comparing and verifying the simulated cracking J integral value with experimental test results to ensure the accuracy of the model in describing hydrogen induced cracking behavior; and step 35, establishing a parameterized analysis and correlation function, and changing the load and hydrogen environment by the system, researching the coupling influence of the stress state, the plastic strain and the local hydrogen concentration on the damage evolution, and finally establishing a quantitative correlation function of stress-strain-hydrogen concentration-hydrogen induced cracking J integral.
5. The evaluation method according to claim 4, wherein in the hydrogen transport control equation, dislocation hydrogen concentration and hydrogen trap trapping hydrogen concentration are expressed as: Wherein θ L ,θ T is the occupancy rate of lattice sites and hydrogen trap sites, and N L ,N T is the site density of lattice sites and hydrogen trap sites, respectively, wherein N L ,N T has the expression: Wherein the reversible hydrogen trap density is a function of equivalent plastic strain, and characterizes the proliferation effect of the plastic strain on the hydrogen trap, and then the equilibrium formula of lattice sites and hydrogen trap sites is: Wherein K T is an equilibrium coefficient, W B is the binding energy of a metal material, and the concentration relation between the concentration of dislocation hydrogen and the concentration of hydrogen trap trapped hydrogen after simplification is expressed as: Therefore, the total hydrogen conservation equation driven by the concentration gradient, the hydrostatic stress gradient and the equivalent plastic strain can be obtained as follows: The hydrogen atom diffusion equation is expressed as: the effective hydrogen diffusion coefficient is expressed as: 。
6. The method according to claim 1, wherein the engineering safety assessment of the defect-containing pipe in step S4 comprises the following steps: S41, establishing a finite element model of a pipeline section containing real defects, calculating load and strain fields according to conditions such as actual operating pressure, soil constraint and the like, simulating a local strain field and a steady-state hydrogen concentration field of a pipeline defect area under a specified service hydrogen partial pressure environment by using the hydrogen diffusion coupling model calibrated in S32, and obtaining multi-field coupling data at the most dangerous point in a key way; S42, extracting strain and hydrogen concentration data of the most dangerous point of the defect, substituting an established stress-strain-hydrogen concentration-hydrogen induced cracking J integral correlation function, and calculating to obtain a predicted hydrogen induced cracking J integral of the pipeline under the working condition; And S43, calculating a J integral value of the defect under the current load by adopting a fracture mechanics method based on the stress field of the defect obtained in the step S41, directly comparing the J integral value with a hydrogen induced cracking J integral value of the material under the current working condition predicted by a correlation function, judging that the pipeline is safe and can maintain operation or optimize a detection maintenance scheme if the J integral value is smaller than the J integral value, and judging that the hydrogen induced cracking risk exists if the J integral value is larger than or equal to the J integral value, and immediately early warning and adopting wind risk control measures such as depressurization operation, repair replacement or on-line monitoring are needed.

Description

Evaluation method for fracture toughness of metal material in hydrogen environment Technical Field The invention relates to the technical field of energy storage and transportation, in particular to a method for evaluating fracture toughness of a metal material in a hydrogen environment. Background The pipeline transportation is used as a main transportation mode of petroleum, natural gas, hydrogen, ammonia, alcohol and other energy sources, and the safety and the long-term service performance of the pipeline transportation are of great importance. However, when the metal material is in service in a hydrogen-containing environment, hydrogen embrittlement is very easy to occur, so that the toughness of the material is reduced, crack initiation and expansion resistance is reduced, and the structural integrity and operation safety of the pipeline are seriously threatened. The pipeline bears internal pressure and external load in the actual service hydrogen environment, and complex stress fields and strain fields are generated in the pipeline at the same time. When the stress field is used as a main driving force, the diffusion of hydrogen atoms to a defect area is obviously accelerated, and the strain field is used as a hydrogen trap through multiplication of crystal defects such as dislocation, micro holes and the like, and the local trapping and enrichment of hydrogen are dominant. These two phases are sequentially coordinated, coupled to each other, which together determine the final distribution of hydrogen and the hydrogen embrittlement effect. However, most studies attribute hydrogen embrittlement to static hydrogen concentration, and fail to deeply reveal the staged, synergistic mechanisms of influence of stress and strain fields on the hydrogen induced cracking process under complex service loads of pipelines. Specifically, the remarkable driving action of stress fields on hydrogen diffusion in the elastic or small-range yielding stage is ignored, and defects such as dislocation, micro holes and the like induced by strain are used as hydrogen traps in the large plastic deformation stage to capture and locally enrich the hydrogen. The existing evaluation method often depends on an empirical formula or single factor association, and lacks a physical model capable of quantitatively representing a multi-field coupling relation of stress-strain-hydrogen-fracture toughness, so that the cracking behavior of the defective pipeline under the combined action of dynamic load and hydrogen environment is difficult to accurately predict, and the pipeline integrity management is caused to have obvious uncertainty. Disclosure of Invention The invention aims to overcome the defect that the existing hydrogen embrittlement study ignores the synergistic effect of stress and strain, and provides a systematic method integrating experimental characterization, multi-physical field coupling simulation and engineering risk assessment. By carrying out in-situ hydrogen permeation and fracture experiments in an experimental device with controllable load and combining numerical simulation, the system reveals how a synergistic mechanism of stress-driven hydrogen diffusion and strain proliferation hydrogen traps affects hydrogen distribution and embrittlement behavior, so that a quantitative relation between stress-strain-hydrogen concentration-fracture toughness is established, and an accurate theoretical basis is provided for safety evaluation of hydrogen-containing pipelines. In order to achieve the above purpose, the invention adopts the technical scheme that: a method for evaluating the fracture toughness of a metal material in a hydrogen environment comprises the following steps: and S1, determining the concentration distribution of hydrogen under the stress-strain synergistic effect, systematically researching the diffusion and enrichment behaviors of hydrogen in the material under different stress and strain states through a metal material in-situ stretching hydrogen permeation experiment, and determining the synergistic action mechanism of stress-driven diffusion and strain proliferation traps to provide key input parameters for subsequent multi-field coupling simulation. And S2, testing the fracture performance in a hydrogen environment, and measuring J integral and hydrogen induced cracking J integral (J IH) of the material under different stress-strain-hydrogen environment coupling conditions through a metal material compact stretching experiment. And step S3, based on a multi-physical field coupling theory, establishing a numerical model comprising a stress field, a strain field, a hydrogen diffusion field and a damage field, simulating the behavior of a compact tensile sample under different loads and hydrogen concentrations, verifying the accuracy of an experimental result, and constructing a quantitative correlation function of stress-strain-hydrogen concentration-hydrogen induced cracking J integral. And S4, applying