Failure Criterion for Hydrogen Induced Cracking Based on Fracture Mechanics Theory and Molecular Dynamics Simulation

Author:Fu Lei

Supervisor:fang hong yuan


Degree Year:2019





Hydrogen-induced cracking(HIC),known as the delayed cracking,has been reported in engineering for many years.The fracture criterion obtained before is only based on the test result of some certain materials,so it does not give sufficient physical mechanism and has no broad applicability.The new failure criterion with hydrogen given in this paper is based on the theory of fracture mechanics,and applied to the test results.The new criterion has been further modified using molecular dynamics(MD)simulation.Thus,the new criterion has universal applicability and clear physical mechanism.Moreover,the relevant experiment has been designed to verify the rationality of the new criterion.It has important practical significance for engineering control of HIC formation and growth.Firstly,the effects of hydrogen on steels were systematically studied using electrochemical charged method.The hydrogen in material had a loading effect to promote crack propagation,and an embrittlement effect to reduce crack resistance.The loading effect of hydrogen was the molecular pressure of residual hydrogen in the crack cavity.On the other hand,the embrittlement effect of diffusible hydrogen caused the plastic deformation ability to decrease and the fracture morphology to transform.However,the diffusible hydrogen had no significant effect on yield strength and tensile strength.For special conditions,the decrease in broken strength was due to the formed HIC.In addition,plastic deformation promoted the hydrogen damage behavior for increasing the hydrogen trap density and hydrogen charged effect.Secondly,the regularity of fracture toughness with hydrogen was studied using compact tensile tests.The exponential function was used to characterize the variation of fracture toughness with hydrogen for Q235 steel and 45 steel.The method of characterizing HIC sensitivity was obtained.Through the analysis of fracture morphology and load-displacement curves,it was confirmed that the reason for fracture toughness reduced by hydrogen was the decreasing plastic deformation ability rather than the decreasing fracture strength.Thirdly,a new fracture criterion based on the critical conditions for crack instability expansion of fracture mechanics was given.The new criterion consideredthe loading effect and embrittlement effect of hydrogen to adapt to the condition of the HIC expansion.The loading effect of residual hydrogen was characterized using the stress intensity factor of hydrogen pressure,and the fracture toughness with hydrogen was used to characterize the embrittlement effect of the diffusible hydrogen.According to the relevant theories,the specific theoretical representation of the new fracture criterion was further obtained.The theoretical relationships between average diffusion hydrogen content and two physical quantities were given.The two physical quantities were stress intensity factor of hydrogen pressure and fracture toughness with hydrogen.The theoretical relationship between the average diffusion hydrogen concentration around the crack and the fracture toughness was obtained using the Griffith energy theory modified by Orowan,Jolk’s plasticity model around the crack tip,Hirth’s heat balance theory and the diffusion equation driven by stress gradient.The theoretical relationship between the diffusion hydrogen content and the stress intensity factor of hydrogen pressure under thermal equilibrium was obtained using the Sievert’s law and the K factor theory of fracture mechanics under conditions of linear elastic model and elastoplastic model,respectively.Fourthly,MD simulation and finite element(FE)method were used to modify and supplement the theoretical characterization of the new criterion.For the theoretical relationship of fracture toughness with hydrogen,a quadratic polynomial function was given using MD simulation to correct the linear relationship between hydrogen coverage and surface energy of fracture surface during theoretical reasoning.Moreover,through the tensile model and the cracking model using MD simulations,it was found that the embrittlement mechanism of diffusible hydrogen was caused by the inhibition effect of hydrogen atoms on dislocation formation.On the other hand,through comparing various FE simulation methods,the applicability of the displacement method was determined to calculate the stress intensity factor.In addition,since hydrogen pressure did not satisfy the premise of the stress method and the J-integration method,both methods were not suitable to calculate the stress intensity factor of hydrogen pressure.Finally,the coupling device of hydrogen and external load was designed to verify the rationality of the new criterion.The purpose of the designed experimentunder the external load simultaneously with hydrogen was to simulate the conditions of HIC formation in the actual engineering.The difference in tensile loads caused different hydrogen contents required for crack growth.The rationality and accuracy of the new criterion had been verified through applying the criterion to the HIC formation process under different loads.