Crack growth analysis and fatigue life estimation in the piston rod of a Kaplan hydro turbine

摘要

Linear elastic fracture mechanics (LEFM) approaches are seldom used to analyze fatigue problems in hydraulic turbomachinery, where extremely simplified fatigue models are normally considered. In one of the largest Kaplan hydraulic turbines in the world, the piston rod, which is an important load transfer component, was found broken twice at the same location. This event, which leads to extremely high direct reparation costs and indirect costs, is a very useful case for research of fatigue problems in hydraulic turbines by means of LEFM. For this purpose, the crack growth progress in the piston rod is simulated with numerical LEFM techniques. A probabilistic model for the initial flaw size is considered. A reliability criterion, consisting in the probability of not having a critical crack or a fracture in the piston rod after a specific service time, is defined. Based on this model, the influence of initial crack models and operating services of the machine on the reliability of the unit is discussed. The calculated crack path and predicted number of loading cycles as a function of the initial crack size agree with the actual fracture section and operating time to fracture.