Cracks are one of the most important demonstrations of structural damages.Under the effect of complicated external loads,positions of cracks inside structures are often complex and changeable and their vibratory responses tend to change according to such positions.To study characteristics of vibrations of cracked structures,people have to first develop an appropriated crack analysis model.Equivalent drop section based model,local flexibility based model and model based on principle of a beam with consistent cracks are currently three commonly used methods for crack simulations.Among them,local flexibility based model is widely adopted due to its strong theoretical properties and clear physical significance.