Damage detection in FRP-strengthened concrete structures utilising nonlinear guided waves
Date
2024
Authors
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Ng, Alex Ching Tai
Smith, Scott T
Smith, Scott T
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Thesis
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Abstract
Externally bonded fibre-reinforced polymer (FRP) plates have been widely used to enhance the tensile strength of concrete beams and slabs over recent decades. The FRP strengthening intervention can however suffer from wear and tear, as well as damage from external factors such as aggressive environments and impacts. The necessity for a reliable inspection method therefore arises to ensure the safety and integrity of FRP-strengthened concrete structures. Non-Destructive Testing (NDT) is a technique that evaluates the integrity and properties of structural components without compromising the integrity of the underlying structure. One such NDT method, utilising guided waves, is a promising tool for damage detection. The primary objective of the research presented in this dissertation is to develop a novel NDT method, utilising the nonlinear features of guided waves (i.e., the second harmonic generation and combinational harmonic generation), to effectively detect damage within FRP-strengthened concrete structures. Chapter 1 provides a concise introduction to the essential concepts of guided waves and offers background information on FRP materials. It also includes a comprehensive review of existing literature, particularly related to the utilisation of guided waves for damage detection. Chapter 2 presents a study on debonding detection in FRP-strengthened concrete structures utilising the combination harmonic generation of nonlinear guided wave mixing. A three-dimensional (3D) finite element (FE) model was built to simulate guided wave propagation at the FRP-concrete interface. The simulated results in both the time and frequency domains were experimentally validated on two concrete blocks with different debonding sizes. Parametric studies were conducted utilising the experimentally verified FE model, where a nonlinear crack growth parameter was proposed. The results demonstrated that the combinational harmonic, especially the sum harmonic generated by mixed-frequency waves, exhibited sensitivity to debonding. In Chapter 3, a debonding locating imaging technique (DLIT) utilising the sum harmonic generation of guided wave mixing was developed to predict the location of debonding. The proposed DLIT employs a piezoceramic transducer network, where one transducer serves as an actuator while the remainder serves as sensors. A time-frequency analysis was used to analyse the data collected by each actuator-sensor pair. Cross-correlation analysis between the signals at fundamental frequencies and at the sum harmonic frequency, which is induced by debonding, was applied to construct an image revealing the centroid of debonding. The predicted centroid of debonding was compared with the actual debonding centroid as derived from the numerical simulation and the experimental specimens. The results showed that the proposed DLIT can satisfactorily predict the location of debonding. Chapter 4 focuses on the detection of thermal damage using the second harmonic generation of guided waves. Thermal damage is considered a potential cause of debonding, in FRP-strengthened concrete structures. A user-defined subroutine was programmed into the FE model to simulate nonlinear elastic behaviour of the FRP plate. This subroutine allows effective simulation of thermal damage on the FRP surface by adjusting nonlinear elastic coefficients of the FRP material. Experimental studies were conducted to compare the amplitudes of second harmonics at different thermal cycles. The results confirmed the reliability of utilising the second harmonic generation of guided waves for detecting thermal damage in FRP-strengthened concrete structures. Overall, the outcomes of this thesis verify the proof of concept and provide an effective tool to evaluate damage in FRP-strengthened concrete structures. The successful application of nonlinear guided wave-based approaches offers promising prospects for proactive maintenance and minimally invasive strategies, ultimately ensuring the long-term performance of FRP-strengthened concrete structures.
School/Discipline
School of Architecture and Civil Engineering
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Architecture and Civil Engineering, 2025
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