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Title: Ultrasonic guided wave structural health monitoring and its application to anisotropic composite material
Author: Putkis , Osvaldas
ISNI:       0000 0004 5915 2172
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2014
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Composite materials, such as Carbon Fibre Reinforced Polymer (CFRP), are strong, lightweight and corrosion-resistant and therefore are increasingly used across industries such as automotive, aerospace, wind energy and pressure vessel. However, they are currently over-engineered in order to withstand typical damage that might occur during their life cycle so that they can maintain the required strength. This is particularly disadvantageous in aerospace industry, where strength to mass ratio is very important. Therefore, Nondestructive Evaluation (NDE) and Structural Health Monitoring (SHM) techniques have to be developed for inspection and monitoring of composite materials in order to increase the confidence of using them and allow building lighter structures. This thesis investigates possibilities of using ultrasonic guided waves for damage monitoring in CFRP and other composite structures in general. Guided waves can travel long distances along the plate-like structures, potentially offering rapid large area monitoring with a relatively low sensor density or monitoring from a remote location. Composite materials. unlike most metals, are anisotropic and attenuative, leading to a more complex guided wave propagation. Therefore. ultrasonic guided wave propagation in highly anisotropic and attenuative composite plates was investigated, theoretically and experimentally, and concepts such as Minimum Resolvable Distance (MRD) and amplitude sensitivity maps were applied for performance evaluation of guided wave modes for NDE/SHM applications on such materials. These concepts help determining the optimum guided wave mode, operational frequencies and sensor positioning for particular NDE/SHM applications. In this thesis, it is demonstrated how the knowledge of guided wave propagation characteristics in CFRP materials can be used in the informed design process of SHM systems. Prototype SHM systems were built and appropriate damage detection strategies were implemented for detection of stringer disbond and impact damage in CFRP plates. As a side project, possibilities of using energy velocity measurements of guided waves for estimation of elastic constants were explored with promising results for highly anisotropic CFRP plates. Elastic constants can be used for prediction of guided wave propagation or evaluation of manufacturing quality of the materiaL As SHM systems continuously monitor structures with permanently installed sensor networks, they are exposed to varying environmental conditions. Changing environmental conditions can have a significant impact on guided wave propagation and the monitoring system itself that can in turn jeopardize damage detection capabilities. Therefore, the effect of environmental valiations, such as temperature changes, has to be understood and damage detection strategies that can cope with environmental variations have to be developed. Response of a particular guided-wave-based SHM system deployed on CFRP plates to temperature variations was studied. Amplitude and spectral changes of recorded guided wave signals were observed that were determined to be dominated by the response of transduction system (transducers and adhesive). Anisotropic guided wave velocity changes with changing temperature were observed. Velocity valiation has implications to the baseline subtraction performance, which is a widely adopted damage detection technique in guided wave SHM. A relatively simple model, which relates CFRP material properties to temperature was established, which enabled calculation of velocity-temperature response and prediction of the baseline subtraction performance. A novel algorithm for a practical SHM that addresses the issues associated with acquisition of a baseline set, required for compensation of environmental effects, was developed. It lifts the requirement of acquiring baselines corresponding to different environmental conditions on a damage-free structure before the monitoring phase can be started. It makes the baseline acquisition an evolutionary process integrated in the monitoring phase. Finally, the performance of an SHM system developed at "QinetiQ pic" for monitoring an aircraft component was investigated and an appropriate damage detection strategy was established.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available