Author:  Cheng, Chiho 
Title:  Formability of tailorwelded blanks for ratedependent materials 
Degree:  Ph.D. 
Year:  2010 
Subject:  Hong Kong Polytechnic University  Dissertations Materials Laser welding 
Department:  Dept. of Industrial and Systems Engineering 
Pages:  xxxii, 346, 50 p. : ill. ; 30 cm. 
InnoPac Record:  http://library.polyu.edu.hk/record=b2392633 
URI:  http://theses.lib.polyu.edu.hk/handle/200/5882 
Abstract:  The primary objective of the thesis is to carry out a prediction analysis on the formability of tailorwelded blanks (TWBs) for ratedependent materials. This includes (i) the development of a modified vertex theory for the forming limit prediction of ratedependent sheet metals, and (ii) the establishment of a systematic approach for the formability analysis of TWBs taking fully into account their structural behaviors and weldment effects. In the study, the modified vertex theory is implemented substantially into the finite element simulation for the process analysis and failure prediction of TWB forming. The scope of the research involves experimental investigations, theoretical modeling and numerical simulations. The study results, including the modified vertex theory and the developed analysis on TWBs, are applicable not only to the TWBs made of ratedependent material but also to the TWBs made of rateindependent material. First of all, the study was motivated by the apparent discrepancies between the measured forming limit diagrams (FLDs) of ratedependent sheet metals (e.g., AKDQ steel) and those predicted values using the conventional vertex theory. Strictly speaking, the localized necking criterion based on the conventional vertex theory is only rigorous to the rateindependent strainhardening metals, such as aluminum alloys. Therefore, in this thesis, a modification to the vertex theory is proposed to extend the capability of forming limit prediction to the ratedependent metals. This can be achieved by considering the ratedependent powerhardening material rule in the modified vertex theory. A novel form of quasilinear stressstrain relation based on the powerhardening rule is analytically derived to model the constitutive behavior of ratedependent materials. The vertex theory assumes that localized necking of sheet metals occurs simultaneously with the initiation of a vertex on the yield surface. The stressstrain relation of ratedependent material is then coupled into the vertex theory to deduce the critical conditions for localized necking (i.e., forming limits on both sides of the FLD). The localized necking criteria for rateindependent materials can be considered as a special case of the theory. A typical ratedependent metal, AKDQ steel, was chosen for the validation of the modified theory. The stressstrain curves of AKDQ were tested on a MTS under different constant strainrates, covering a wide range of strainrates from 10{205}{197}/s to 10/s. The material ratedependency of AKDQ was measured for the calculation of FLD, while the hardening law of AKDQ was found to depend greatly on the strainrate. Forming limit test was performed for the AKDQ sheets to measure the FLD. The FLD was compared to the prediction based on the modified theory and good agreement was observed. Case studies were also carried out to illustrate the implementation of the modified theory for failure prediction in finite element simulations of the sheet metal forming process. A much better description on the forming performance and failure behavior for the ratedependent material could therefore be obtained by using the modified theory as a localized necking criterion. Subsequently, the thesis presents an analysis of the formability of TWBs for ratedependent materials by applying the modified vertex theory for forming limit prediction. A TWB is composed of several flat metal sheets of different thicknesses or strengths, which are welded together prior to the forming operation. However, the forming performance of a TWB depends critically on its distinctive structural design and the properties of its welded material. Due to the structural nature of TWB, the general formability measures, such as FLD, are not simply applicable to analyzing the formability of TWB. Therefore, a systematic approach for the formability analysis of TWBs by considering the effects of their structure and weldment properties has been proposed here. In this proposed approach, an experimental measurement system was established to acquire the distinctive material properties and ratedependent mechanical data for each material region in a TWB, especially for the weldment in miniature size. The experimental data were then implemented into the forming simulations of TWB in a generalpurposed finite element package. In the simulations, the thickness and geometry of the TWB were modeled concretely, while the weldment and HAZs were also constructed and defined carefully. This was found to be particularly important to those TWBs with weaker weldment properties, which led to a weld failure during forming. The modified vertex theory was, hence, employed to deduce the forming limits of each material in TWB, including the weldment. Once the stressstrain data of selected elements satisfy the localized necking criteria of the modified vertex theory, the critical status and location of localized necking on TWBs could be identified. The modified theory was found to be applicable to both ratedependent and independent materials, whereas the strainrate data were involved during the calculation of ratedependent material. The simulation results were verified with the experimental one using both AKDQ and AlTWBs for the case studies of ratedependent and rateindependent materials analysis respectively. Furthermore, investigations of the structural effects (e.g., different thickness combinations, weldment orientations and specimen widths) on the formability of the TWBs were carried out on both types of TWBs. 
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