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Title: Characterizing undersized damage using nonlinear features of guided ultrasonic waves : from theoretical modeling, numerical simulation and experimental validation to applications
Authors: Wang, Kai
Degree: Ph.D.
Issue Date: 2019
Abstract: Undersized damage, as typified by small cracks and pitting corrosion with their characteristic dimensions being remarkably smaller than the wavelength of a probing wave, is ubiquitous in engineering structures. It imposes immense threat to the health, integrity and durability of the engineering structures which are subjected to cyclic loading or thermal variation. It is fairly challenging to monitor and characterize the undersized damage using traditional non-destructive evaluation (NDE) and structural health monitoring (SHM) techniques. With this motivation, a SHM method based on the use of nonlinear features in guided ultrasonic waves (GUWs) is developed in this study, with an objective to evaluate undersized damage in a precise, quantitative and real-time manner. To start with, a dedicated analytical model is proposed, and in conjunction with a modal decomposition method and a variational principle-based algorithm, the model analytically depicts the generation of contact acoustic nonlinearity (CAN) induced owing to the interaction of probing GUWs with a "breathing" crack in a two-dimensional (2-D) scenario. On this basis, a set of damage indices, linked with CAN embodied in acquired GUW signals, is proposed to evaluate the severity of the crack quantitatively. The method, in principle, does not entail a benchmarking process against baseline signals. In virtue of a 2-D finite element model, the generation of second harmonics due to the "breathing" crack is numerically explored, and good coincidence between the results from the simulation and those obtained analytically validates the proposed analytical model. To fulfill quantitative evaluation of a fatigue crack in practice, particularly at an initiation stage, the analytical model from the 2-D scenario is expanded to a three-dimensional (3-D) scenario. Influence of key parameters of the crack (i.e., crack length and orientation) on the CAN is investigated analytically, whereby a nonlinearity index associated with these parameters is defined to orientate and evaluate the severity of the "breathing" crack. With insight into the fatigue crack-induced CAN, a probability diagnostic imaging algorithm is exploited to localize the fatigue crack and project identification results to pixelated images. Validation for the proposed method is obtained via numerical simulation using a 3-D finite element model. To experimentally validate the proposed method, the fatigue test of an aluminum plate is conducted using a fatigue test machine, and the actuators and sensors, mounted on the surface of the plate, are utilized to excite and capture waves when the plate is in intact and damaged status, respectively. Nonlinear features of the captured signals are extracted to calculate the defined index, and a trend embracing three stages (i.e., increasing, decreasing and reaching a plateau) is demonstrated, in which the influence of the crack length, crack opening displacement and crack tip dominates, respectively. Compared with the slight increasing in nonlinear features related with the material property, the nonlinear features engendered by the CAN manifest a remarkable intensification. Coincidence in results between theoretical analysis, simulation and experiment has corroborated the effectiveness of the modeling and simulation.
Residing on the above analysis, the developed methods are further applied to evaluate the health status of selected real engineering structures which bear undersized damage generated in different circumstances. A specific damage considered in this study is the pitting damage which is caused by a hypervelocity impact (HVI) in a shielding plate. In virtue of a 2-D and 3-D finite element models, the material nonlinearity-associated accumulation of second harmonics in several scenarios, including phase matching and different degrees of mis-matching, is analyzed and compared with the CAN-induced nonlinearity in a plate bearing pitting damage, exhibiting the dominance of the CAN-induced nonlinearity. In sensing paths traversing the pitted region, conspicuous intensification in high-order harmonics can be discerned via a sensor network consisting of a set of lead zirconate titanate (PZT) wafers. On this basis, the pitting damage can be intuitively highlighted and quantitatively evaluated using a diagnostic imaging algorithm and image fusing technique. Despite that the physical nature of nonlinear ultrasonic waves has been well interpreted, the evaluation of material acoustic nonlinearity in experiments is still extremely challenging, because the precise measurement of nonlinear features of GUW is prone to contamination from various factors. To tackle this deficiency, an approach is proposed, based on an analytical interpretation of the temperature-dependency of nonlinear ultrasonic waves. By measuring the fluctuation of nonlinear ultrasonic waves in response to a thermal variation, the material acoustic nonlinearity is evaluated in an interference-free manner, thereby advancing the practical application of the damage characterization methods previously developed using nonlinear features in GUWs. In conclusion, starting from the analytical modeling for interpreting the generation of nonlinear features induced by a "breathing" crack in a 2-D scenario to the analysis in a 3-D scenario, combining in-depth numerical and experimental study on the characterization framework using damage-induced nonlinear features of GUWs, the undersized damage, a prevailing yet insidious defect in engineering structures, is detected and evaluated, quantitatively and accurately. This study has provided a solution for sensitive in situ identification and quantitative evaluation of undersized damage, with potential to greatly improve the sensitivity of existing SHM methods and generate immense economic benefit.
Subjects: Hong Kong Polytechnic University -- Dissertations
Ultrasonic testing
Structural health monitoring
Pages: xxiii, 178 pages : color illustrations
Appears in Collections:Thesis

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