Study On Fatigue Law And Fracture Mechanism Of Drill Steel

Through electrochemical testing,coupon test,fatigue test to study the corrosion mechanism of S135 drill pipe in 3.5% NaCl solution and the effect of Pre-corrosion and Notch on Fatigue Life and Fracture Mechanism of Materials.The influence of pre-corrosion and notch type on the fatigue life of S135 drill pipe was studied by fatigue test.The fracture morphology was studied by macroscopic and microscopic analysis.The results showed that: S135 drill pipe steel having anodic dissolution profile in 3.5%NaCl solution,no passivation phenomenon;With the extension of the etching time,the open-circuit potential of the S135 drill pipe decreases,the charge transfer resistance decreases,the corrosion current density increases,and the corrosion rate of the sample increases.With the corrosion time,the surface of the sample corrosion pits more and more,more and more deep,corrosion products are more loose,easy to fall off from the surface and can not protect the steel.In the air,the S135 drill pipe has the fatigue limit characteristic,the fatigue crack initiation is on the surface of the specimen,the fatigue fracture is composed of the fatigue source area,the fatigue crack stability extension zone and the transient area,The fatigue source area is characterized by cleavage fracture,and the crack zone is mainly fatigue zone.As the loading stress decreases,the number of fatigue strips increases.Pre-corrosion and the presence of notches reduce the fatigue life of the material.With the prolongation of the pre-corrosion time,the fatigue life of the S135 drill pipe is significantly reduced,and the fatigue crack initiation occurs at the corrosion pits on the surface of the specimen or near the corrosion pits.When the loading stress is high,the crack propagation zone is characterized by cleavage fracture.When the loading stress is low,the crack propagation zone is fatigue strip.The existence of the gap significantly reduces the fatigue life of the material,resulting in premature fatigue fracture,fatigue crack initiation at the specimen surface of the gap,the crack extension to cleavage fracture as the main feature.With the decrease of loading stress,a small amount of fatigue bands appear in the fatigue crack growth zone.