Study On Fatigue Crack Damage Monitoring Technique Of Z2CND18.12N Steel

As the main type steel of nuclear power plant pressurizer surge pipe, Z2CN18.12N is suffering extremely fatigue damage during service. It is significant for detection and monitoring the propagation of fatigue crack to prevent disaster. However, due to the thickness of pipeline and elastic anisotropy of coarse austenitic grain, conventional detection methods have more limitations, and can not satisfy the requirement of online monitoring. It is urgent to select suitable detection methods to meet the request of nuclear power pipeline safe operation. In this paper, nuclear power austenitic stainless steel Z2CN18.12N base metal and welded joint were regarded as research objects, and electro-mechanical impedance (EMI) combining Lamb wave were used as structural health monitoring technology. The crack damage evolution process of fatigue crack initiation, propagation and fracture was studied. In the study, the specimens were loaded with low cycle fatigue loading. The corresponding relationship among crack length, crack position and damage index was investigated, which is expected to provide reference for online monitoring of nuclear power pipeline mechanical damage. The major conclusions are as follows:(1) EMI technique was used for online monitoring of the entire fatigue process on both base metal and welded joint specimens. All the specimens were machined with175mm length and3mm thickness. The damage indexes root mean square deviation (RMSD) and Δf were extracted from electrical impedance signals to quantitatively characterize fatigue crack damage. RMSD values were more sensitive to initial fatigue damage than Δf. When the macroscopic submillimeter crack was formed, both RMSD and Δf showed increasing tendency; during the stage of crack propagation, the trend of RMSD and Δf increased monotonically with fatigue cycles, both the damage indexes had good corresponding relationship with the fatigue crack length. At the fracture stage, the curve of RMSD turned flat even falled off, while Δf rised sharply. The difference of trend between base metal fracture and welded joint fracture were obvious. According to the difference, fatigue crack damage of base metal and welded joint can be effectively distinguished.(2) Lamb wave technology can effectively make up for the inadequacy of EMI technology in the aspect of fatigue crack positioning. Lamb wave were optimized in respects of wave number, excitation voltage, incentives, center excitation frequency, etc. The optimized Lamb wave was used in the study of online monitoring fatigue crack process for base metal and welded joint specimens. The damage index spectrum peak amplitude variation ΔA which was extracted from the crack reflection Lamb wave differential signal showed good agreement with the change tendency of the fatigue crack length. So ΔA can be used for the quantitative characterization of crack length. The comparison between ΔA and RMSD show that EMI technology is more sensitive to fatigue initial damage while Lamb wave technology is more suitable for fatigue crack propagation stage.(3) Lamb wave technology was used in the study of fatigue crack positioning with the help of wavelet transform method. For the two1mm or so length fatigue cracks generated at67.5mm and75.5mm far from the PZT sensor, the detection error were-1.03%and-1.15%respectively. The results show that Lamb wave technique can realize precise localization of fatigue crack.