Study On The Mechanisms Of Dislocation Emission From Crack Tip Induced By Hydrogen

Hydrogen often enter into materials in the process of production,machining and usage of crystalline materials.Once hydrogen enter into materials,the internal stress is quite massive when they diffuse outwards.In such a large internal stress,the damage to crystalline materials is very large.Hydrogen in crystal materials,can make the distortion of crystal lattice of the metal and metal began to slip,so as to decrease the mechanical properties of crystals,such as tensile strength decreased,the decrement of toughness of brittle materials,the phenomenon known as 'hydrogen embrittlement'.The essence of hydrogen atmosphere embrittlement is hydrogen atmosphere in the materials can affect the basic process of deformation and fracture,therefore lead to the change of material properties.Thus,Study on the mechanisms of dislocation emission from crack tip induced by hydrogen has an important scientific value and practical significant.This thesis takes the nanocrystalline materials as the object of research.The model of hydrogen atmosphere and edge dislocation in the vicinity of crack tip are established.The interaction between hydrogen atmosphere and edge dislocation in the vicinity of crack tip are investigated under mode I and mode II loading conditions.Using elastic complex potential method,we obtained the closed form solutions of stress intensity factor for edge dislocation emission from crack tip,and the effect of length of crack,the radius of curvature at the crack tip,temperature and hydrogen atmosphere concentration on the stress fields on the crack tip and the degree of difficulty of dislocation emission are studied through numerical curve.The main achievements obtained in this article are expressed as follows:1.A model is established to study the interaction between hydrogen atmosphere,the finite length and edge dislocation.The analytical expressions of stress fields,the image force and the stress intensity factor for the dislocation emission from the finite length crack are derived.The influence of temperature,hydrogen concentration and crack length on the critical stress intensity factor for dislocation emission are discussed.The results show that,dislocation emission from crack tip takes place more difficultly at higher hydrogen concentration,but dislocation emission is more easily at higher temperature.Increasing the crack length can decrease barrier for dislocation emission.2.A model is established to study the interaction between hydrogen atmosphere,the elliptically blunted length and edge dislocation.The analytical expressions of the stress intensity factor for the dislocation emission from the elliptically blunted crack is obtained.The influence of temperature,hydrogen concentration,crack length and crack tip curvature radius on the critical stress intensity factor for dislocation emission is discussed.The results indicate that dislocation emission from crack tip takes place more difficultly at higher hydrogen concentration,but dislocation emission is more easily at higher temperature.Dislocation emission takes place more difficultly with the increment of crack length and radius of curvature.3.A model is established to study the interaction between hydrogen atmosphere,the elliptically blunted and edge dislocation.And the surface stress is considered.The analytical expressions of the stress intensity factor for the dislocation emission from the elliptically blunted crack is evaluated.The influence of temperature,hydrogen concentration,crack length and crack tip curvature radius on the critical stress intensity factor for dislocation emission is discussed.The results show that,dislocation emission from crack tip takes place more difficultly at higher hydrogen concentration,but dislocation emission is more easily at higher temperature.Positive surface stress can prevent dislocation emission from crack tip,negative surface stress can contribute to dislocation emission.Increasing the crack length or radius of curvature can increasing the threshold for dislocation emission.For some particular materials,if the negative surface stress is considered,the critical normalized SIFs decreases to a minimum and then increases with increasing the radius of curvature.