Study On Monitoring Technique Of CT Specimen Fatigue Crack Growth Rate Based On Direct Current Potential Drop Method

In the long-term service process of important mechanical structures such as nuclear power pressure vessels,cracks will occur under the action of periodic alternating load due to the original defects of materials,processing,or environment.Accurate monitoring of the crack propagation process is of great significance for structural integrity evaluation and life prediction.In view of the fact that the direct current potential drop(DCPD)method has a wide range of application values in the monitoring of crack growth,and this method can accurately monitor the crack growth process in real-time.In this thesis,304 austenitic stainless steel,which was commonly used in important mechanical structures such as nuclear power,was taken as the research object,and the fatigue crack growth monitoring technology was studied at room temperature.A calibration method based on DCPD technology was proposed to effectively characterize the relationship between potential drop and crack length,and it was used for monitoring tests of compact tensile(CT)specimens.At the same time,the virtual crack closure technology(VCCT)was used to simulate the fatigue crack propagation to supplement and verify the crack monitoring technology.The main research contents of this thesis are as follows:The fatigue crack expansion process was analyzed based on the fracture mechanics theory,and the DCPD method was used to monitor fatigue cracks according to the test standard.The test method was analyzed and a reasonable design plan for the monitoring test was proposed.And the construction of the static and dynamic crack expansion monitoring test platform was completed.The finite element model of CT specimen was established by using AB AQUS simulation software,and the trend of potential change during crack expansion was observed by analyzing the factors affecting the crack expansion monitoring process.The exact characterization of crack length and potential drop parameters was realized and verified with static crack growth monitoring platform.The propagation model along the predefined path was established combining with VCCT to simulate the fatigue crack propagation process.The stress field state of the crack tip under different cyclic loads was analyzed,and the effect law of different stress ratios on the amount of crack expansion was studied.The fatigue life was studied combining with the results of crack growth monitoring test.The fatigue crack propagation test of 304 austenitic stainless steel was carried out by dynamic loading crack monitoring test platform.Four sets of tests under different stress ratios were compared to analyze their fatigue life,and the fracture morphology was simply analyzed.Combined with the simulation results of fatigue crack growth,the feasibility of monitoring technology scheme was verified.