Structural Health Monitoring Of Square Tube Based On Lamb Wave

Engineering plants in areas of civil construction, large mechanicalequipment, and special chemical industry can provide long period ofservice and have high values, which has a huge effect on the nationaleconomic production and life. Therefore, whether these engineeringstructures are healthy has very strong realistic meaning and itsrequirements of Structural Health Monitoring are extremely urgent. Thetechnology of Structural Health Monitoring has caught great attention ofengineering and academic world for its advantages of real-time, on lineand wide range of detection. Detection method based on ultrasonicguided wave is main means of realization of Structural Health Monitoringand related research results are fruitful. However at present the appliedtargets of research of guided wave technology are restricted to structuresthat own simple boundary conditions like massive plats and squaretubes. This situation restricts the Structural HealthMonitoring technology to spread in real engineering structures. Therefore,further study of guided waves applied in complex structures, such as pressure vessels, truss, square tube, etc. is the only way for StructuralHealth Technology. Square tube structures as an important form ofengineering structures, widely used in ships, vehicles, aerospaceequipment, and other fields, which both have plate’s shape and have itsown particularity. Research on guided waves applied in the field ofsquare tube can not only benefit tube’s health, but also has significantpossibility.In this paper, all work is introduced according to the work step. First,basic theory of guided wave is elaborated and different forms of guidedwaves under different boundary are deduced based on physics equation,including the model of waves propagating in square tube. According tothe dispersion equation of guided wave equation, dispersion curves aredrawn, which can be used for choosing center frequency and testingmodes we needed. From these dispersion phenomena, the importance ofexact elastic constants become very obviously, concerning the test’sreliability. Therefore, based on the basic symmetric mode of Lamb wavesS0, an inverse algorithm was proposed for calculating the elasticproperties of different kind of materials. The inverse algorithm was testedin estimating the elastic parameters of thin aluminum plate, rotors andbogie, the results demonstrate the feasibility and generality of theproposed method. After all the basic preparation were done, a finiteelement simulation model is built and the basic process of simulation work preparation is described in detail, such as modeling of the structuredetected, modeling of the actuator and sensor, meshing and unit selection,setting load set, and setting solver and step. In the end, simulation ofpropagation of the guided wave in the square tube structure will be donemany times.Unlike plate and beam structure, guided waves in the square tube arevery complex because of the existence of boundary, and serious overlapand mode transform lead to hard detection work. In this paper, a new wayis adopted to reduce the kinds of waves propagated in the structure undera single PZT. Meanwhile, Mode transform is founded by analysis ofsignal components and time of flight of Lamb wave mode A0and S0. Thelocation and size of cracks have obvious and various influence on guidedwave in square tube, and many experiments are done to figure out theregulation and its characters. Some key conclusions are described detailedin the end of paper. First of them is that the application of SH wave isvery useful in monitoring early crack. Secondly, the method ofdifferent signal cannot be used in field testing and an energy spectramethod has better adaptability. And based on experiments of a bogie, thepaper is going to show how load on the structure influences the testingsignals and to confirm the feasibility of online testing.