Memmolo, Vittorio (2017) Structural Health Monitoring of complex structures based on propagation and scattering of Guided Ultrasonic Waves in composite media. [Tesi di dottorato]


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Item Type: Tesi di dottorato
Lingua: English
Title: Structural Health Monitoring of complex structures based on propagation and scattering of Guided Ultrasonic Waves in composite media
Date: 11 December 2017
Number of Pages: 187
Institution: Università degli Studi di Napoli Federico II
Department: dep11
Dottorato: phd046
Ciclo di dottorato: 30
Coordinatore del Corso di dottorato:
Ricci, FabrizioUNSPECIFIED
Date: 11 December 2017
Number of Pages: 187
Uncontrolled Keywords: damage detection; wave propagation, aerospace structures
Settori scientifico-disciplinari del MIUR: Area 09 - Ingegneria industriale e dell'informazione > ING-IND/04 - Costruzioni e strutture aerospaziali
Date Deposited: 06 Jan 2018 14:13
Last Modified: 20 Mar 2019 12:06


The major concern of aerospace and transportation engineering during the last years has been related to increase performances and safety with energy savings. Composite materials have been indeed widely adopted with the aim to design high performance and lighter components. However, random events such as certain low velocity impacts may induce barely visible or not visible failure due to their complex mechanics behavior. Impact induced damages in stiffened composite structures are usually accommodated with constrained design and strictly maintenance tasks which increase operational costs above all else and decrease the advantages for which composites have been massively introduced. To overcome such drawbacks, an integrated structure providing monitoring of critical components appears a reasonable solution. A condition based approach could be able to relax the maintenance strategy minimizing aircraft downtime as well. Moreover the design constraints would be avoided further increasing the structural performance with a more ecological friendly aircraft. Although this is a very long time perspective, for the first demand Structural Health Monitoring (SHM) systems, providing information about the structural efficiency, appear to be the best solution. Within this context, the work deals with detection, localization and size assessment of sudden failures like delamination and disbondings with on-line monitoring technique by permanently attached piezoelectric transducers (PZT) capable to excite and sense guided ultrasonic waves. Composite stiffened structures typically designed for aircraft wing-box are mainly investigated. Delamination between several layers and disbondings are carried out inducing low velocity impacts with the attempt to analyze different and complex damage scenarios. Two different approaches are mainly proposed. Anti-symmetric propagation based technique (global approach) using the A0 Lamb wave mode for interrogation demand is first presented to obtain a fast end effective localization of damage, no matter the failure type is. A reliable solution is also provided using a multi-parameter approach to increase the probability of detection. The obtained algorithm is implemented in a graphic user interface developed and coded by the author. Different features, including customized selection of ultrasonic time histories, statistical threshold definition and interactive tools for self-diagnosis of sensors and complex geometry representation are developed to obtain an effective tool for global damage diagnosis. Although the promising results obtained with a fewer computational time required respect to classic tomographic approaches, size and severity of damage can be assessed only statistically relating the metric adopted to the flaw dimension. However, their deterministic assessment is crucial where the stringers adopted to reinforce thin walled structures are affected by not visible disbondings. This fact leads to the separation between the stiffener and the hosting structure preventing the collaboration between parts with a dangerous drawback for loading absorbing. For the last demand, usually disbonding stoppers are designed to prevent critical decrease of load carrying capacity below the limit design loads. Towards a condition based approach, a novel detection technique capable to predict arrival time of guided waves redirectedbby stringers to detect any possible change in a specific scattering area is presented. Hence, the reflections of wave interacting with stiffeners can be analyzed to improve the diagnostic leading to a point by point interrogation (local approach). Theoretical aspects are investigated to correctly exploit the technique leading to a geometrical reduction to describe the propagation of the A0 mode including boundary (stringer) reflections. The model is capable to return the optimal design of the system achieving a high performance diagnostic. Several measurements are carried out to validate the adopted propagation model and a promising result in agreement with classic ultrasonic nondestructive testing is thus obtained and discussed. Furthermore it is shown that accounting Lamb wave reflections improves the localization accuracy respect a general purpose reconstruction algorithm while making use of fewer number of sensors possible or increase the probability of detection combining both methodologies. The mentioned results are achieved with large experimental campaigns as well as comprehensive numerical simulations with the aim to better understand the physics of wave propagation in composite media typically adopted in the aerospace field. Several strategies have been investigated using finite element methods with the attempt to reduce the experimental costs as well as system validation efforts. Using an effective equivalent single layer approach, the flexural behavior of the A0 mode has been modeled correctly. The proposed simulated environment allows to correctly relate the propagation and damage interaction behavior to effective features to detect damage with fewer efforts possible. Moreover, a preliminary and strategic analysis for the global approach is indeed carried out with the attempt to verify the feasibility of a numerical framework for system performance assessment using a (model assisted) probability of detection approach. The concluding part of the work focuses on a preliminary investigation of a transducer self-diagnostic procedure, aimed to in-situ monitoring of the sensors and actuators used in the SHM methodologies presented. Due to the large number of distributed PZTs needed to correctly nterrogate the structure, confirming if sensors/actuators are functioning properly during operation is a crucial process to minimize false alarms in the diagnosis. The procedure verified is based on the capacitive analysis of piezoelectric sensors, which is manifested in the imaginary part of the measured electrical admittance. Even though a further investigation is needed to study more complex sensor fault scenarios, the final algorithm is able to simultaneously sort sensors and detect damages with information gathered from healthy transducers.


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