Author: Luo, Guojie
Title: A framework based on flexible transducers and zero group velocity lamb mode for the evaluation of impact damage
Advisors: Su, Zhongqing (ME)
Degree: M.Sc.
Year: 2022
Subject: Composite materials -- Impact testing
Lamb waves
Structural health monitoring
Hong Kong Polytechnic University -- Dissertations
Department: Department of Mechanical Engineering
Pages: 119 pages : color illustrations
Language: English
Abstract: Impact damage, which is manifested as the structure is subjected to the impact of solid particles at a certain speed and angle caused material plastic deformation and cutting. In particular, spacecraft, and satellites operate in regions filled with tens of thousands of natural micrometeoroids and orbital debris (MODs). These MODs are flying around the orbit at speed over the first cosmic velocity which is 7.9 km/s. When spacecraft collided with MODs, the relative impact velocity often exceeds 5000 ms-1, leading to a potential and tremendous threat to the safe operations of spacecraft. Guided-wave-based non-destructive evaluation (NDE) and structural health monitoring (SHM) techniques have been successfully implemented for the detection and localization of hypervelocity impacts (HVI). However, with the development and widespread use of multilayer structures or composites in aerospace, guided wave techniques are difficult to achieve this goal. Therefore, some SHM methods based on laser-induced locally sensitive zero group velocity (ZGV) lamb waves have been found to be effective for detecting the internal health of materials in multilayer structures. However, because the experimental setup for the laser ultrasonic technique is bulky, susceptible to external conditions influence, and other shortcomings. These limitations make it more challenging to achieve real-time, in-situ health monitoring of impact damage to composite materials. Against this background, an SHM method based on the use of flexible, light sensors for excitation and detection of ZGV lamb waves is developed in this study, with an objective to evaluate the impact damage of aviation structures in a precise, online, and real-time manner.
To start with, a theoretical analysis is proposed, which depicts the principle of ZGV lamb wave generation and attenuation in a single-layer homogeneous material. On this basis, different size damages caused by HVI are added, linked with different ZGV amplitude attenuation and mode conversion due to damages, a preliminary framework of HVI evaluation based on ZGV changes is proposed. In virtue of a 2-D finite element model, the generation of ZGV resonance is clearly displayed, and the amplitude attenuation law and mode conversion due to HVI is also analyzed by acquiring numerical signals. The simulation results are in good coincidence with those obtained from calculation showing the validity of the proposed theoretical basis.
To experimentally verify the feasibility of the proposed method, flexible and light polyvinylidene difluoride (PVDF) transducers are selected and mounted on the surface of the aluminum plate instead of lead zirconate titanate (PZT) or laser technique, for excitation and acquisition of signals. ZGV features of the captured signals are extracted to get the influence of damage on ZGV waves by doing signal processing. The generation of ZGV resonance could be effectively excited and captured by PVDF sensors, and different ZGV power attenuation trends and mode conversions are demonstrated, in which the crack size and plate thickness dominate this variation. The consistency of theoretical analysis, simulation, and experimental results verify the effectiveness of modeling and simulation.
Residing on the above analysis, the developed methods are further applied to evaluate the health status of selected engineering structures with complex constructions. Carbon fiber-reinforced polymers (CFRP) Composite plates with pitting damage and indentation which is caused by an impact are considered in this study. Through theoretical calculation, the ZGV frequency of CFRP composites is obtained. In virtue of a 3-D finite element model, the generation of ZGV and the influence of damage on ZGV waves are analyzed by processing simulation signals. Compared to the single-layer structure, the increase of layers leads to the generation and interaction of multiple ZGV resonances, bringing abnormal attenuation law and thickness resonances. Signals for healthy and damaged areas are obtained through a pair of properly grounded PVDF sensors mounted on the backside of the pitting and dent areas. On this basis, the pitting damage and imperceptible damage inside can be highlighted and characterized using flexible sensors and special ZGV lamb waves.
In summary, we explain the generation of ZGV resonance and its decay principle from the aluminum homogeneous model and combine in-depth numerical and experimental studies to establish a characterization framework based on the damage-induced ZGV anomalous decay and Lamb wave mode conversion. HVI on metal plates and latent impact damage in CFRP composites can be detected and evaluated in a portable and accurate manner. This study has proposed a method for effective in situ and real-time detection and evaluation of impact damage, with the potential to greatly reduce the complexity of an SHM method while maintaining its validity and generating considerable economic benefits.
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Access: restricted access

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