Research On Fatigue Crack Propagation Behavior Of Metal Based On SEM-DIC

In engineering practice,fatigue fracture is the main cause of component and structural failure.The number of casualties and economic losses caused by fatigue failure is countless every year,so the problem of fatigue fa ilure has attracted much attention.During the research of fatigue fracture,the measurement of displacement field and strain field at crack tip can be used to study the fatigue damage mechanism and crack propagation law,which is of great significance for improving the reliability and service life of engineering structures.The Digital Image Correlation?DIC?method is widely used in the research of fatigue problems due to its high precision,full field,non-contact measurement,etc.,especially in the research of crack tip deformation.In this paper,the DIC method is combined with the in-situ scanning electron microscope?SEM?experimental system,ie SEM-DIC research method,with high-entropy alloys CoCrFeNi and CoCrFeNiMo_0.2(referred to as Mo₀ and Mo_0.2)as the research object.Research on fatigue crack growth behavior and crack tip deformation studies,which helps to evaluate the material's resistance to crack growth and the crack resistance of the crack tip.The work of studying the fatigue crack growth and evaluating the crack propagation resistance based on SEM-DIC method is mainly divided into the following three aspects:?1?Research on digital speckle production methods based on different scales,including macroscopic,microscopic and cross-scale speckle production methods,which lays a foundation for the research of digital image correlation methods at different scales.The static tensile curves were measured by the extensometer method and the digital image correlation method in the static tensile test of 316L stainless steel.The results verified the correctness of the digital image correlation method.?2?The fatigue crack growth rate of Mo₀ and Mo_0.2 was studied by using the in-situ SEM fatigue test system as the experimental platform.The experi mental method based on SEM-DIC to measure the closed stress was designed to measure the crack closure level of Mo₀ and Mo_0.2,so as to obtain the effective crack growth rate curve with the crack closure effect removed.Thus,the crack propagation resistanc e of Mo₀ and Mo_0.2 was measured.?3?Based on the SEM-DIC method,the crack propagation resistance of Mo₀ and Mo_0.2 was investigated from the viewpoint of strain.Through the design experiment,the plastic strain field of the crack tip of Mo₀ and Mo_0.2 was measured,and the plastic cumulative strain at the crack tip was quantitatively analyzed,and the strain distribution from the average plastic cumulative strain growth rate and the crack propagation direction to Mo₀ and The crack tip resistance of Mo_0.2 was analyzed.By observing the microstructure of Mo₀ and Mo_0.2,the reasons for the different crack propagation resistance of Mo₀ and Mo_0.2 are briefly analyzed.