Abstract: The strength of metals is closely related with crack propagation.There exists stress concentration at the crack tip during the process of crack propagation,thus the plastic deformation will occur at the crack tip.Experimental and atomistic simulation results show that the plastic deformation occurring at crack tip includes dislocation emission, twinning and phase transformation.The occurrence of the plastic deformation is closely related with the crack tip field,and it may play an important role on the crack propagation behaviors.In this paper,the finite element method and continuum mechanics method are applied to calculate the stress and strain field at the crack tip.We aim to discuss the relation between the crack tip field and the micro structure evolution.Different types of crack in body-centered cubic(bcc)iron are selected while the crack orientations are different.The plane strain condition is applied in our simulation.The results indicate that for {100}<110> and {110}<110>modeⅠcracks,the distribution of crack tip field is symmetric for the crack plane.Thus the structure evolution at the crack tip is symmetric. However,for {111}<110> modeⅠcrack,the crack tip field is unsymmetric.The crack tends to stretch along the direction of 280 degree.The shear stress above the crack plane is different with that below the crack plane,thus martensitic phase transformation occurs above the crack plane,while twinning occurs below the crack plane.We can conclude that the plastic deformation occurring at the crack tip is closely related with the distribution of the crack tip filed.Due to the occurrence of the plastic deformation at the crack tip,the direction of crack propagation would change.Furthermore,we discuss the influence of the martensitic phase transformation at the crack tip on crack propagation according to the finite element method by introducing the plastic analysis.For {110}<110> modeⅠcrack,martensitic phase transformation from bcc to hcp structures will occur at the crack tip when the load is larger enough.The elastic energy at the crack tip transforms to the energy for phase transformation,thus the high stress concentration at the crack tip is released.We conclude that the martensitic transformation improves the fracture toughen in bcc iron…

Key words: Anisotropy; crack tip field; plastic deformation; fracture toughness; FEM

# Finite Element Analysis of the Crack Tip Field and Structural Evolution Behavior of the Crack Tip

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