Damage Identification Of Thin-plate-like Structures Based On Lamb Waves

The structural damage and breakdown is usually originated by a small-size crack at the early stage,and the safty and lifespan of the structure is tied up with the development of the location,size and the shape of the structural damage.With an ability to travel a relative long distance with low energy attenuation at a given frequency in large-scale thin-plate or thin-shell structures and a high sensitivity to damages,Lamb waves are widely employed to identify damage in structural health monitoring of thin-plate and thin-shell structures.In recent years,the studies of Lamb wave-based damage detection mainly focus on damage index-based damage assessment and ultrasound imaging based damage locating.However,the studies of damage detection with a small size at early-stage and damage shape identification are paid little attention,and the efficiency and accuracy of the traditional ultrasound imaging based damage locating is low.Therefore,this dissertation mainly focuses on the damage shape and size identification,and imaging based damage location detection based on Lamb waves by combining the methods of theoretical analysis,numerical simulation and experimental study.This dissertation is aimed to solve the problem of early-stage damage detection with a small size,and to investigate how to improve the accuracy and efficient of imaging based damage locating.The main work of this dissertation is concluded as below:?1?Based on the theory of Lamb waves,the multi-modes and dispersive characteristics of Lamb waves are investigated by using the method of numerical simulation,and the rules for exciting a single mode Lamb wave is propsed based on the tunned Lamb wave theory.The simuation results indicate that there are only S₀ and A₀modes of Lamb waves in a certain range of the procduct of the frequency and the half-depth of the structure,and the group velocity of the S₀ is much larger than that of A₀mode,which can be employed to separate the modes of Lamb waves.The results also indicate that when the piezoelectric ceramics are employed to activate Lamb waves,there are a series of tunned frequencies at which there is only one mode of Lamb wave.?2?The influence of the early-stage damage with a small size on the ultrasonic energy diffusion is investigated and a new damage size detection algorithm based on ultrasound energy diffusion is proposed to detect the damage size.The equations of ultrasound energy diffusion are derived,and the equations indicate that the ultrasound energy diffusion coefficient is inversely proposional to square of the diameter of the damage of a hole.Based thess characteristics of ultrasound energy diffusion,the small change of the damage at early-stage can be detected by the ultrasound diffusion coefficient.To study the characteristics ultrasound energy diffusion,an experimental setup is designed and fabricated.The experimental results reveal that the existence of the damage changes the energy transport between the actuator and the sensor,and both the ultrasonic energy diffusion coefficient and the ultrasonic energy dissipation coefficient change if there is a damage.In addition,when there is only one dominant mode of Lamb wave excited in the structure,the ultrasound energy diffusion coefficient decreases with the diameter of the through-hole damage while the ultrasonic energy dissipation coefficient increases with the diameter of the through-hole damage.However,when two or more modes of Lamb waves are excited,due to the existence of difference of group velocity between different modes,the energy transport of different modes is not synchronous,the ultrasonic energy diffusion dose not increases with the size of the damage.For ultrasound energy diffusion based damage size detection in a thin-plate-like structure,the center frequency of excitation pulse should be carefully chosen to excite only one dominant mode of Lamb wave?3?A multi-delay-and-sum?multi-DAS?imaging algorithm is proposed to cancel the artifacts and to increase the efficient and accuracy of defect detecting in thin-plate-like structures.This proposed method makes best use of the signals which are detected by the sensors,and obtains the damage image by drawing both ellipses and hyperbolas with transmitter-sensor pair signals.A reflection coefficient is introduced to the new imaging method to attenuate the signals reflected by the boundary and to cancel the artifacts caused by the boundary reflection signals.The experimental results,which are conducted on an aluminum alloy plate,demonstrate that the proposed multi-DAS imaging algorithm can identify the location of a bonded mass damage efficiently and accurately,and this algorithm also decreases the number of artifacts greatly.?4?A delay-and-Boolean-ADD?DABA?imaging algorithm,which obtains a better defect image by employing Boolean ADD operation instead of addition or multiplication operation in the DAS algorithm,is developed for defect detection of plate-like structures.This new method brings no artifacts in the damage image and the widely used envelop-detection method based on Hilbert-transformation is replaced by a new envelop-detection technique based on a local maximum value to increase the accuracy of locating.The results of the experiments conducted on an aluminum plate indicate that the proposed DABA imaging algorithm combining with the new techniques can detect a bonded mass defect accurately and efficiently.?5?A new algorithm to detect damage size and shape using PZT transducer array for a plate structure is developed.During the process of detection,the pre-determined PZT transducers are chosen as actuators to generate Lamb waves,and others are chosen as sensors to detect the Lamb waves.When there is a damage,the Lamb wave reflects at the boundary of the damage.The reflection point is on an ellipse with two foci at the actuator and sensor,and this ellipse is tangent to the boundary curve of the damage at the reflection point.The reflection point can be obtained by finding a special point on the ellipse,and this point has the shortest distance to the center of the curvature of the damage boundary curve.As a result,the damage boundary curve can be determined by finding various reflection points,and the envelope of the identified reflection points is employed to represent the shape of the damage.The experimental results indicate that the proposed method can obtain the size and shape of bonded masses with a relative small error.The investigations of damage detection based on Lamb waves,including damage location detection,damage shape detection and early-age micro damage identification,build a foundation for the practical application of damage detection.