Apalowo, Rilwan Kayode
(2019)
Wave propagation and interaction with damage in periodic composite structures.
PhD thesis, University of Nottingham.
Abstract
A number of engineering structures such as aircraft fuselages, nuclear reactors, boilers and pressure vessels are subject to severe conditions, during operation, which can endanger the operation and lead to possible catastrophic failure. Such failure consequently has a huge impact on economy and environment. These engineering structures, especially aerospace and automotive structures, generally consist of composite materials due to their excellent mechanical properties. Therefore, there is need to develop numerical tools for constant non-destructive monitoring and assessment of composite structures in order to avoid sudden failure of these engineering components during operation. Various studies have been conducted on wave propagation, and distortion of the wave properties to examine damage/failure within structures. In the first theme of this study, a brief review of the analytical and numerical methodologies for wave propagation and interaction modelling is presented.
The second theme of this study considers two common conditions, namely temperature and pressure, that aerospace and automotive structures are subject to during operation. The first part presents a wave finite element (WFE) methodology for predicting wave propagation properties within periodic layered structures. The computed wave properties are then coupled to the finite element (FE) model of a coupling joint, which bears structural damage, in order to quantify wave interaction with the damage. Second part of the theme considers the impact of temperature on the wave interaction with damage in composite structures. A thermal-mechanical analysis is conducted to experimentally measure mechanical characteristics of a composite layered panel over a wide temperature range. The measured thermomechanical characteristics are applied to study and analyse the effect of temperature on wave propagation and interaction properties within the composite panel. The significance of the panel's glass transition range on the measured and calculated properties is emphasized. In the third part, the developed methodology is extended to incorporate pre-stress effect in wave propagation and interaction with damage within pressurized layered structures. The pre-stress effect is mostly introduced in engineering structures by pressurization, which is among the severe conditions they are exposed to during operation. Pre-stress impact is analysed by comparison of results under pressurized and non-pressurized scenarios.
The third theme proposes an inverse wave finite element approach for identifying the geometric and material characteristics of layered composite structures. The non-destructive approach is able to recover the thickness, density, as well as all independent mechanical characteristics such as the tensile and shear moduli for each layer of the composite structure under investigation. It is emphasized that the success of the approach is independent of the employed excitation frequency regime, meaning that both structural dynamics and ultrasound frequency spectra can be employed.
In the fourth theme of this study, a FE based computational scheme for quantifying guided wave interaction with Localized Nonlinear Structural Damage (LNSD) within structures of arbitrary layering and geometric complexity is developed. The computed harmonic wave
interaction coefficients are established as indices for structural damage mode identification/classification. Finally, in the fifth theme, a numerical scheme is presented
for quantifying wave interaction with structural damage within two-dimensional multilayer
composite structures. Wave interaction coefficients for different damage modes and structural parameters are analysed in order to establish an optimum basis for detecting and identifying damage, as well as assessing the orientation and extent of the detected damage.
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