Study On Ultrasonic Guided Waves Propagation Characteristics In Square Tube Structures With The Detection Of Damages By Guided Waves

Square tube structures are widely used in construction,machinery,electrical power,chemical factory and many other fields.They play an important role in national economic and social development.During the service life,due to the influence of various loads and environmental factors,square tube structures are prone to suffer various damages,i.e.,cracks and corrosion.The timely detection of these defects is of great importance to the prevention of accidents.At present,the inspection of square tubes mainly adopts the traditional ultrasonic method.The traditional ultrasonic method is based on point-to-point inspection style.It is time consuming and quite inefficient when dealing with square tubes with a long distance.Therefore,it is particularly necessary to develop a method to timely,efficiently and accurately inspect square tube structures.In this study,ultrasonic guided wave technique is proposed to efficiently detect damages in square tube structures.The dispersion characteristics of ultrasonic guided wave in square tube structure,the energy distribution characteristics at edges and surfaces of a square tube along the circumferential direction,and the scattering characteristics of ultrasonic guided wave interacting with defects in square tube structure have been comprehensively studied.The inspection system for square tubes by ultrasonic guided wave is designed and established.Due to the relatively complex shape of the cross section,it is difficult and impossible to solve the dispersion equations in a square tube by analytical method.The semi-analytical finite element method is proposed to solve this problem.The cross section of a square tube is discretized by finite element method.The displacement field is assumed harmonic propagating along the axial direction.The phase and group velocity dispersion curves in a square tube structure are plotted.The influences of varying wall thickness and radial size on the dispersion characteristics in square tube structure are studied.The dispersion characteristics of square tube structures are analyzed.The dispersion curve of L(0,6)mode at the frequency range from around 54kHz to 74kHz is relative flat and it has highest group velocity values.L(0,6)mode is thus selected as the ultrasonic guided wave inspection mode.A square tube structure has four edges and planes.Unlike the circular hollow cylinders,this nonuniform distributed structure along the circumferential direction makes the unclear ultrasonic guided wave energy distribution characteristics along the circumferential direction.To investigate the energy distribution characteristics of ultrasonic guided wave finite element simulation method is used.L(0,6)ultrasonic guided wave mode is excited in a square tube structure.By observing the contours of stress filed of the propagation of ultrasonic guided waves,The stress field distribution characteristics at edges and in planes of a square tube are derived.Different receiving points are placed along the circumferential direction at the left end of a square tube.The reflected ultrasonic guided waves from the right end are captured at each receiving point.The displacement amplitudes of the reflected ultrasonic guided waves along the circumferential direction of a square tube are obtained.The energy distribution characteristics of the propagation of ultrasonic guided waves in square tubes are revealed.The scattering characteristics of ultrasonic guided wave interacting with defects also need to be investigated.Finite element simulation method is also employed.L(0,6)mode ultrasonic guided waves interacting with damages at different positions and with different types in square tubes are studied.The contours of stress field distribution when the ultrasonic guided wave interacting with the defects are observed and analyzed.The receiving points are placed along the circumferential direction at the left end to capture the reflected ultrasonic guided waves from the damages.The amplitudes of reflected waves from the damages at all monitoring points are derived.The sensitivities of ultrasonic guided waves interacting with damages at different positions and with different types are obtained.Finally,based on the obtained dispersion characteristics,energy distribution characteristics and the scattering characteristics,the ultrasonic guided wave based inspection system for square tube structures is designed and established.The impedance matching of the piezoelectric wafer transducers in the system is conducted to improve the signal-to-noise ratio of the received signal.At the same time,the wavelet analysis is used to process the received reflected waves to improve the inspection accuracy of the system.The system is applied to detect damages at different positions,with different size,with different types and at different orientations in square tube structures.The results show that ultrasonic guided wave technique is a accurate,efficient and promising method for the detection damages in square tube structures.