Study On Fracture Behavior For A Type Of High Strgenth Steel At Different Temperature

In this paper,Fracture Mechanism of High Strength Steel in Tensile Specimens Tested,peripheral shear,3PB,4PB,in situ oberservation at Various Temperatures are investigated.The new results obtained are as follows:(1)Tensile properties are almost the same for three type specimens With test temperature decreasing,yield strength,the ultimate tensile strength and the true fracture stress show a increasing trend;At -196℃for T specimens,in layers composed of boundaries of original austenite grains, a longitudinal crack initiates from the center of the necking region and propagates along the tensile direction to the regions close to both ends of the necking.It then changes the orientation and develops into two transverse cracks which propagate into two opposite directions on two cross-sections;In the range of -30℃～-20℃for T specimens and in the range of -150℃～-20℃for CZT,the fracture surfaces of specimens are composed of typical center-fibrous-crack-initiation,middle shear-radical propagation and outer shear lip forming;In the range of -150℃～-60℃for T specimens,a transverse crack initiates at the center of the necking region. The shear stresses produced by the transverse crack and produced by the necking initiate and propagate two shear cracks;(2)This high strength steel possessed superior toughness with a transit temperature around -150℃. Even though at a very low temperature(-196℃),the cleavage cracking dominates the fracture process,it can not propagate straight forward through entire ligament and shows the pop-in extension in macroscopic tests and the quasi-cleavage mode in microscopic feature.It is found that the strong resistance which hinders the crack propagation is provided by three barriers:original austenite grain boundaries,bainite colony boundaries and inter-layers between bainite laths.These barriers manifest themselves by tear ridges with dimples on the fracture surfaces;(3)At higher temperatures,the fracture mechanism is dominated by fibrous cracking which makes dimple fracture surface with some individual quasi-cleavage facets.For notch bending specimens,ductile fracture is dominitaed at above -100℃,brittle quasi-cleavage fracture is domainated at -196℃.Stress concentration of notch root for notch specimens is smaller than that of COD precrack specimens,much elastic energy are accumulated before crack initiation,so it induces crack propagatin faster to fracture,no "pop-in" phenomena appear even at -196℃;(4)After much plastic deformation is produced at the notch root,plastic crack is initiated.With increasing applied stress,plastic crack is further propagated under the shear stress and normal stress.With increasing applied stress,plastic cracks are initiated, propagated and blunted continuously,finally the specimens fracture.