Numerical Simulation Analysis Of Vibration Damage For Small Branch Pipes In Nuclear Power Plant

There are a lot of small pipes in nuclear power plants. Vibration fatigue usually induces cracks in the socket weld joint, leading to leakage of radioactive fluid in the pipeline. It has brought great threat for the safe operation of the nuclear power plant. Therefore, in order to prevent the fatigue failure of small branch pipes caused by vibration, it is necessary to study the vibration damage characteristics of small branch pipes.The main contributions of the thesis are summarized as follows:(1) The research of damage identification based on natural frequency change. The natural frequency variation characteristics of a small branch pipe with three constraint conditions were calculated by ABAQUS. It was found that the natural frequency variation is a shock curve with the crack location, and it increases with the crack depth.(2) The distribution of stress intensity factor for weld toe cracks was studied. The distribution of the stress and stress intensity factor was calculated by the ABAQUS and Zencrack. The result shows that the vibration direction along with the stress direction, and the stress of the vibration direction is higher than the direction of vertical vibration. The SIF distributes as an upside down "U" alphabet, and the maximum value of SIF is at the deepest point of the crack. When the crack depth is the same, the length of the circumferential crack gradually expanded to a large crack.(3) The local flexibility of cracked pipes was calculated. The effect of crack size, position and load on the local flexibility of pipe were discussed based on strain energy release rate. It shows that the local flexibility of pure shear load is maximum, and the propagation rate of crack caused by the bending vibration is the fastest. The results can provide the theoretical basis for the establishment of the spring hinge model.(4) The vibration model of fatigue crack growth for small branch pipes was deduced. The dynamic response characteristics of cantilever vibration model were analysed. It was found that the exciting force, excitation frequency and modal values have a significant effect on the vibration fatigue life, and the resonance fatigue life is the shortest. Moreover, the vibration propagation formula was deduced. The results can provide technical basis for the online operation and the fatigue life prediction of small branch pipes.