Fracture Behavior Of HR2 Steel Under Dynamic Loading

The investigations on fracture behavior of materials under dynamic loading are important in both industrial and military fields.HR2 steel is a typical hydrogen-resistant brittle austenitic stainless steel.It is usually applied in millitay fields,due to its good comprehensive mechanical properties,high temperature oxidation resistance,and good hydrogen embrittlement resistance.At present,the research on the fracture behavior of HR2 steel under dynamic loading,especially the dynamic fracture behavior under detonation loading,is still very limited.In this work,the HR2 steel cylinder shells with different wall thickness and different surface processing methods were explosive implored and the fragments were collected after explosion.The morphology of the fragments,fracture microscopic morphology,and distribution of cracks and shear bands in the cross section of fracture fragments were characterized systematically to analysis the fracture behavior and the fracture mechanism.The effects of wall thickness and surface processing on dynamic fracture behavior were discussed In order to further study the effects of different microstructures and loading strain rates on the fracture behavior and mechanism of HR2 steel,two kinds of HR2 steels with different microstructures were performed in quasi-static loading and impact loading fracture experiments.The fracture morphology and cross-section microstructure were characterized to analysis the fracture behavior and the fracture mechanism of HR2 steel,was analyzed.The main results and conclusions are as follows:(1)The fracture toughness of the as annealed and as drawn samples under quasi-static loading were s 589 KJ·m^-2 and 453 KJ·m^-2.The dimple size and surface undulation of the cracked steady-state expansion zone of the annealed sample are significantly larger than that of the drawn state sample.This is attribuilt to the good ductility of the annealed samples,which benefits for the formation of a large number of deep dimples and effectively consumes crack growth energy.At the same time,the better plasticity of the annealed sample also caused the formation of larger shear lips on the sides of the fracture.The impact energy of the annealed and drawn HR2 steel under impact loading is 116 J and 95 J.The shear lip size of the as-pulled specimen under impact loading is significantly increased compared with that of the shear lip formed in quasi-static loaded samples,because the plastic deformation of the as-drawn specimen are highly the strain rate sensitive and the plastic deformation ability enhanced when increasing the deformation strain rate.(2)Explosive experiments were carried out on four samples of surface-polished thin-walled cylindrical shell,surface-polished thick-walled cylindrical shell,surface-turned thin-walled cylindrical shell and surface-turned thick-walled cylindrical shell.The initial structure of the four cylindrical shells is the same,the surface of the surface-polished cylindrical shell sample is smooth and flat,the surface of the surface-turned cylindrical shell sample has a processing texture,and a processed deformation layer appears in the grain near the surface.The fracture fragments of the thin-walled cylindrical shell samples show a pure shear fracture mode,and the fractured residual fragments of the thick-walled cylindrical shell samples show a stretch-shear mixed fracture mode.This is because when the wall thickness of the cylindrical shell is large,the outer surface of the cylindrical shell is in a state of circumferential tensile stress,so that the outer surface forms a tensile crack,and when it meets the shearing instability zone of the inner wall,a stretch-shear mixing fracture is formed.When it meets the shearing instability zone of the inner wall,a stretch-shear mixing fracture is formed.(3)The distribution directions of the fracture shear sections of all the recovered fragments of the four cylindrical shells were statistically analyzed.The results show that the surface-turned thin-walled cylindrical shell fragments have a one-way superior distribution.After the explosive loading,the distribution of shear bands and cracks in the fracture fragments of the cylindrical shells were observed,and it was found that a large number of shear bands and microcracks along the shear band nucleus appeared in the fracture fragments,and both are evenly distributed in both directions.The deformation near the inner surface of the fractured piece is the most serious.The shear band near the inner surface is the most dense,and there is no obvious crack initiation.Only a small amount of shear cracks extend along the shear band to the surface,a large number of cracks were observed on the outer surface of the fractured piece,and a one-way dominant distribution along the turning direction.The uniform initiation of the crack along the shear zone and the one-way dominant initiation along the outer surface result in a one-way dominant distribution of the fracture fragments of the HR2 steel cylinder shell,showing an approximate single-spin shear fracture mode.