Lamb Wave-based Damage Identification Techniques For Aircraft Composite Plate Structures

Recent developments in the area of smart materials and structures have made it possible to obtaininformation about the state of structures and to identify the structural safety status online withadvanced actuator/sensor network integrated in the structures. It is important and crucial for impedingthe structural health monitoring system （SHMS） to obtain the information about the characteristics,extent, distribution and progress of the damages.This dissertation studies several methods and techniques in structural health monitoring ofaircraft structures.The active methods is applied to monitor the structures continuously and online toensure the structural safety. These techniques can indentify the damage loction and damage size, andconsider the effect of actual operational environment on damage detection and identification. After abrief introduction of the background of this study in Chapter1, the main contents of this dissertationare as follows:（1） In Chapter2, the problem of damage identification for composite plate structure is studied. Lambwave time reversal method is a new and tempting baseline-free damage detection technique forstructural health monitoring. With this method, damage can be detected without baseline data. In thepaper an online damage detection and identification method is presented using time reversal Lambwaves method and ultrasonic tomography for damage diagnosis of composites. The principle andcharacters of the time reversal lamb waves in a composite plate have been introduced firstly. Then thetime reversal method has been adapted to detect the local defects in composite plate structures, byusing an active sensing system mounted on a composite plate to excite and receive Lamb waves. Thismethod can identify the location and size of the damage in a composite plate quickly without relyingon past baseline date. The image that indicates the damage can be obtained by the ultrasonictomography algorithm. Experimental study results demonstrate the applicability and effectiveness ofthe proposed method.（2） In Chapter3, the multiple modes and dispersion nature of ultrasonic Lamb waves are investigatedin plate structure, the propagation of Lamb waves in damaged plate is simulated using the finiteelement method, and the electromechanical coupling behavior of piezoelectric ceramic piece and theinduced strain model are studied for the activation and sensation of Lamb waves; the matchingpursuits is introduced in detail, a fast implement method is proposed for matching pursuits methodwith a Chirplet dictionary, it can decompose the Lamb signals quickly and extract the characteristic information, and it is demonstrated that the Chirplet atoms can match the distorted narrow band pulsesaccurately; a damage identification method is proposed based on Lamb waves and matching pursuits,and it is verified by the results of finite element method, the simulation results show that matchingpursuits method can identify the modes of Lamb waves and extract the damage informationeffectively, it can locate the damage accurately, and an experiment device is established to verify thismethod, the experiment results show that the matching pursuits method can distinguish the overlappedscattered signals from several damages, and locate the damage accurately; finally the damageidentification method is employed to detect the impact damage in honeycomb sandwich of carbonfiber composite structure, the ellipse location method and damage imaging method are used to locatethe impact damage.（3） In chapter4, the problem of damage detection and identification for composite structures underenvironmental changes is studied. At first, reconstruction algorithm for probabilistic inspection ofdamage, which is a two step identification method, is proposed to firstly judge whether there isdamage in the structure, and secondly identify the location and size of the damage. In this method,signal processing methods are applied to extract the features of the Lamb wave signals. The differencecoefficient, which is called as damage index （DI）, is obtained by comparing the features of thereference and present signals. Then the probabilistic method is used to judge that the DI is caused byeither the structural damage or the environmental factors and decide whether the damage occurs. Andfinally, the damage imaging algorithm is applied to obtain a tomogram for damage identification. Inorder to verify the feasibility and effectiveness of this method, some structural damage identificationexperiments are carried out on composites. The experimental identification results are accurate andthe image is clearness.This study is partially supported by the National Natural Science Foundation of China GrantNo.11172128, and the Funds for International Cooperation and Exchange of the National NaturalScience Foundation of China （No.61161120323）, the Jiangsu Province for Six Kinds of ExcellentTalent of China （No.2010-JZ-004） and Jiangsu Graduate Training Innovation Project （CX09B₀70Z）.