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Title: Accurate descriptions of the ansiotropic plastic yielding behaviour of various metallic sheets
Author: Alharthi, Hamzah
ISNI:       0000 0004 6348 6343
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2016
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This thesis focuses on two vital requirements - yield criterion and flow curve - to obtain reliable numerical results in sheet metal forming simulations. First, this thesis generally aims to explore the potential accuracy of the Taylor models, namely the full constraint and pancake, for replacing complicated mechanical tests involved in the defining process of advanced yield functions for aluminium alloys. The exploration process resulted in a simple and efficient yield locus description denoted as CTF. This model correlates with the texture-based model (Taylor full constraint) and the phenomenological model (BBC2005) for the considered aluminium alloys. Based on this newly proposed model (CTF), a hybrid solution, denoted as Method II, is suggested. Consequently, the demands associated with the extensive and difficult tests required for calibrating the advanced yield function can be reduced. A remaining issue related with the identification procedure of the plastic anisotropy parameters associated with advanced yield criteria was addressed in this thesis by applying the trust region approach for identifying the material coefficients, and its performance was compared with the line-search approach. The applied algorithms were tested for various aluminium and steel alloys with different levels of anisotropy. Second, this research sought to develop an accurate determination of the biaxial flow curve for various aluminium and steel alloys when a continuous and in-line thickness measurement system, such as the digital image correlation (DIC) system, is absent. In certain sheet metal forming processes, it is more appropriate to determine a flow curve using biaxial stress condition tests, such as the hydraulic bulge test, than a uniaxial test because hardening proceeds higher strains before necking occurs. In a uniaxial test, higher strains are extrapolated, which might lead to erroneous results. Usually, the bulge test coupled with the DIC system is used to obtain stress–strain data. In the absence of the DIC system, analytical methods are instead employed to estimate hardening. Typically, such models incorporate a correction factor to achieve correlation with the experimental data. An example is the Chakrabarty and Alexander method that utilises a correction factor based on the n-value. Here, the Chakrabarty and Alexander approach was modified with a correction factor based on normal anisotropy. When compared with DIC data, the modified model was found to be able to predict the hardening curves better for the materials examined in this study. Based on the fact that a biaxial flow curve is required to compute the biaxial yield stress, an essential input to advanced yield functions, the effects of various approaches to biaxial stress–strain data on the shape of the BBC2005 yield loci were also investigated. The proposed method could accurately predict the magnitude of biaxial yield stress, when compared with DIC data, for all materials that were investigated in this study.
Supervisor: Not available Sponsor: Jāmiʻat Umm al-Qurá
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General)