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|Title:||Characterizing hypervelocity impact-induced damage using linear and nonlinear features of acousto-ultrasonic waves : theoretical modeling, sensor development and experimental validation||Authors:||Liu, Menglong||Degree:||Ph.D.||Issue Date:||2017||Abstract:||Hypervelocity impact (HVI) is a scenario involving an impacting velocity usually in excess of 1 km/s. Ubiquitous in low Earth orbit, paradigms of HVI are typified by the collision between meteoroids and orbital debris (MOD) and spacecraft. HVI poses immense threat to the safety of orbiting spacecraft. With this motivation, prompt, accurate and quantitative HVI monitoring and evaluation of HVI-engendered damage is studied in this PhD research. To start with, a new breeze of nanocomposite-inspired, lightweight, flexible, broadband ultrasonic sensor is developed in this PhD study, for in situ acquisition of acousto-ultrasonic waves in the extremely adverse space environments. The sensor well accommodates the contingent restrictions on the weight and volume addition to spacecraft due to the installation of sensors and associated apparatus. With carbon black (CB) as the filler and polyvinylidene fluoride (PVDF) as the matrix, the sensor is demonstrably responsive to the acousto-ultrasonic waves in a wide frequency band, from static to up to 400 kHz. Experiment has spotlighted the superior capability of the nanocomposite sensor for either passive acoustic emission (AE)-based or active guided waves-based damage identification and structural health monitoring. The sensor outperforms conventional acousto-ultrasonic transducers when attempting to capture HVI-induced signals, by compromising "sensing cost" with "sensing effectiveness". Different from low- and high-velocity impact, HVI features transient, localized, and extreme material deformation in an adiabatic process, under which the induced AE waves present unique yet complex features. Conventional numerical modeling methods, such as Finite Difference or Finite Element (FE), exhibit somewhat inefficiency and inaccuracy to deal with the extreme material deformation and distortion induced by HVI. To address such a context, a dedicated modeling and numerical simulation approach, based on the smoothed-particle hydrodynamics (SPH) in conjunction with the use of FE analysis, is developed, showing capability of balancing calculation efficiency and accuracy. With the model, an insight into the characteristics of HVI-induced AE wave propagation is gained. Different HVI scenarios are considered using the model, including normal and oblique incidence at different impact velocities. Using specific HVI facilities, intensive HVI tests are conducted, in which a typical two-layer shielding structure (including a thinner outer layer and a thicker inner layer) undergoes HVI. Prior to the test, a sensor network with a combined use of lead zirconate titanate (PZT) sensors and the developed CB/PVDF nanocomposite sensors is surface-mounted on the outer shielding layer, to acquire HVI-generated AE waves. Quantitative coincidence in results between simulation and HVI experiment has demonstrated the effectiveness and accuracy of the modeling and simulation. With understanding of HVI-induced AE waves, an enhanced delay-and-sum-based diagnostic imaging algorithm is developed, for localizing HVI spot and projecting identification results to pixelated images.
Upon the penetration of the thinner outer shielding layer under HVI, the shattered projectile, together with the jetted shielding material, further impacts the sequent thicker inner layer, manifesting numerous craters and cracks disorderedly scattered over a wide region. Targeting quantitative evaluation of this sort of damage (multitudinous damage within a singular inspection region), a new characterization strategy, allying linear with nonlinear features of guided waves, is proposed. Linear-wise, the changes in the signals features in the time domain (such as time-of-flight and energy dissipation) are extracted, for detecting gross damage whose characteristic dimensions are comparable to the wavelength of the probing wave; nonlinear-wise, the changes in the signal features in the frequency domain (such as the second harmonic generation) are used, which are proven to be more sensitive than their linear counterparts to small-scale damage (such as barely visible closed crack), for further characterizing undersized pitting damage in HVI-induced damage area with characteristic dimensions much less than the probing wavelength. To understand the nonlinearity in HVI-damaged inner shielding layer, a comprehensive and in-depth interrogation of various nonlinearity sources is performed, in virtue of a two-dimensional (2-D) and three-dimensional (3-D) finite element models, corroborated by experiment. With the models, the accumulation of nonlinear second harmonic wave in several cases, including phase matching and different degrees of mis-matching, is analyzed, and the influence of wave diffraction on the accumulative feature of the second harmonic is explored. Based on the 2-/3-D numerical models and experimental discovery, a nonlinear index is introduced to define the degree of nonlinearity. Residing on the above analysis, two detection approaches, using linear and nonlinear features of guided waves, are developed to characterize HVI-induced damage to the inner shielding layer, in both of which the second harmonic wave (nonlinear feature) shows higher sensitivity to damage compared to the fundamental wave (linear feature). Both approaches, combining a path-based probability imaging algorithm with defined linear and nonlinear indices, are able to identify HVI-induced damage precisely and intuitively. In conclusion, starting from sensor development for in situ signal acquisition, combining in-depth research on passive HVI-generated AE waves and active linear/nonlinear features of guided waves using theoretical modeling and experiment validation, HVI-induced pitting damage, a highly specific type of damage in spacecraft, is characterized, quantitatively and accurately, based on the use of acousto-ultrasonic waves. This study has provided a cost-effective solution for in situ HVI perception and quantitative evaluation of HVI-induced damage. The identification results can greatly benefit subsequent remediation to damage space structures, which is crucial to the success of a space mission.
|Subjects:||Hong Kong Polytechnic University -- Dissertations
Materials -- Dynamic testing
|Pages:||xxix, 247 pages : color illustrations|
|Appears in Collections:||Thesis|
View full-text via https://theses.lib.polyu.edu.hk/handle/200/9189
Citations as of May 22, 2022
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