Damage Detection Based On Auxiliary Mass Method And Fractal Dimension Method

Beams and other basic structures have been widely used in the fields of aerospace,infrastructure and other fields,the damage detection on these structures is very important.Vibration-based damage detection method has obvious advantages compared with other methods: overall situation,flexible sensor layout,real-time monitoring,etc.and become an important research topic.In this paper,the damage detection of cracked beam is analyzed,the thesis mainly contains following contents:Firstly,the research status of vibration damage detection domestic and abroad is summarized.The vibration-based damage detection methods are classified into the following major categories: natural frequency-based method,mode shape-based method,frequency response function method,computer optimization method etc.The advantages of moving auxiliary mass method is obvious: it does not need the data information of the health structure,does not need modal analysis,the laboratory equipment is simple etc.Secondly,the free vibration equation of the damaged beam is briefly analyzed.Taking the clamped-clamped beam as an example,the effectiveness of the curvature mode and the curvatures mode shape difference method is discussed when the damage located at the noneffective damage locations.In addition,experimenting with the cracks which are simulated by masses,and a new procedure is suggested to identify the damage positions.Then,the model of the moving auxiliary mass-damage beam system is established,and the vibration equation is deduced and the natural frequency curve is solved.the effect of the crack on the natural frequency is amplified when the auxiliary mass is moved to the vicinity of the crack.The damage information is included in the natural frequency curve.Three kinds of fractal dimension method(Higuchi FD,Katz FD,Sevcik FD)are given to analyze the natural frequency curve,and the fractal dimension curve is taken as the damage index.And the influence of crack location,crack depth,and the size of auxiliary mass on damage index is discussed.Finally,the modal experiment is performed on the cantilever beam single cracked with single crack.The experimental results show that the crack damage can be well detected with the modal curvature as the damage index.The auxiliary mass method experiments of cantilever beam with single crack and clamped-clamped beam with double cracks are also performed.The measured natural frequency curves are investigated by the central difference method and three fractal dimension methods,respectively.The results show that the effect of the central difference method is not obvious,and the three fractal dimension methods are more sensitive to single crack damage.The Sevcik FD method is most sensitive for clamped-clamped beam with double cracks.