Study On Crack Damage Identification Method Of Simply-Supported Beam Based On Variational Modal Decomposition

With the acceleration of economic construction and the gradual growth of urban and rural traffic demand,all kinds of bridges have been built.At the same time,the bridge is damaged by material aging,overload,erosion and earthquake to varying degrees,resulting in the bearing capacity,safety and durability of the bridge are constantly weakened.Therefore,it is of great practical significance to develop a convenient and effective bridge damage detection method to discover the defects of bridges in time.Aiming at the common crack damage of Bridges,this paper considers the two excitation modes of moving load and excitation load.Based on the variational modal decomposition(VMD)algorithm,damage-sensitive indexes are constructed to realize the effective identification of crack damage of simply supported beam.The main contents of this paper are as follow:Firstly,according to the basic principle of variational modal decomposition algorithm,a reasonable parameter assignment method of VMD algorithm is proposed.1.The number of modal decomposition K is determined by the number of frequency bands of the signal in the frequency domain.2.The penalty factor α was determined according to the optimal iteration number of the algorithm.Through the decomposition of the test signal,the advantages of variational modal decomposition algorithm over empirical modal decomposition(EMD)algorithm are highlightedSecondly,a crack identification method not based on the complete finite element model is proposed for a simple beam model with a crack under dynamic load.The method only needs to analyze and deal with the acceleration response in the middle span of the simply supported beam,which reduces the work of loading,unloading and maintenance of sensors in practical engineering.At the same time,based on the model,the analytical expression of mid-span acceleration of simply supported cracked beam is derived.On the basis of theoretical derivation,new instantaneous indexes are constructed by using variational mode decomposition and Hilbert transformation,which can effectively identify small cracks with 5%crack depth.Then,the influence of different wheel load size,damage degree and noise interference on crack damage identification is further analyzed.Thirdly,taking the simply supported beam model with breathing crack under harmonic excitation as the research object,the breathing crack is established by using ANSYS contact pair element in 3D solid model.The correctness of the model is verified by using the contact compressive stress at the crack position.At the same time,based on the theoretical analysis,combined with variational modal decomposition algorithm,the corresponding nonlinear measurement indexes are established,including nonlinear index D1,harmonic energy ratio A/A,total energy ratio A/α1 and velocity displacement two-dimensional phase plane.The location of breathing crack damage is successfully identified by using nonlinear damage indexes.Finally,the influences of different factors on damage identification of simply supported beams with breathing cracks is analyzed.The influences of excitation frequency and damping are studied by using velocity displacement two-dimensional phase plane.When the excitation frequency is 1/2 fundamental frequency,the nonlinear effect of structural vibration is the most obvious.With the increase of damping ratio,the nonlinear effect of structural vibration decreases gradually.Furthermore,the damage identification of semi-penetrating breathing cracks in the cross section of the solid model is carried out,and the degree of nonlinearity of different semi-penetrating forms and the minimum crack identification degree are studied.