Under the far-field uniform tensile load,the crack tip will generate stress concentration,and the grain boundary adjacent to the crack tip will bear large shear stresses to cause nanograin boundary slip.The effects of hydrogen and nanoboundary slip on the crack nucleation,the critical stress intensity factor and the shielding action were investigated.The theoretical solution of the model was given with the continuous distribu-ted dislocation method.The results show that,the wedge cracks preferentially germinate along direction DC of the grain boundary triple junction and grain boundary BD due to the plugging of the dislocation at the grain boundary triple junction and the tip of the slip plane.Moreover,hydrogen decreases the total energy of crack initiation.When hydrogen concentration increases by 1％,the total energy of the most stable crack initiation will decrease by about 1.86％.Although the grain boundary slip increases the critical stress intensity factor and the shielding action at the crack tip,hydrogen will decrease the critical stress intensity factor.Finally,accord-ing to the hydrogen enhanced decohesion(HEDE)theory,the influence of hydrogen on surface energy was studied.With every 1％increase of the hydrogen concentration,the surface energy will decrease by 5％.This theoretical work provides new information on the microscopic fracture mechanics of materials caused by hydro-gen and grain boundary slip,and helps to explain the microscopic mechanism of metal fracture.
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