Date of Award

Fall 2020

Project Type

Dissertation

Program or Major

Mechanical Engineering

Degree Name

Doctor of Philosophy

First Advisor

Yannis P Korkolis

Second Advisor

Jinjin Ha

Third Advisor

Brad L Kinsey

Abstract

The implementation of new materials for light-weighting purposes in the automotive industry has often been hindered due to the low ductility of these materials, as well as inadequate empirical knowledge about their fracture behavior and inadequate material modeling techniques. This thesis addresses these issues through extensive experimental and numerical study of plastic anisotropy and ductile fracture of several aluminum alloys and a stainless-steel. The test materials used for this study include AA365 die-casting, AA6013 and AA6111 aluminum sheets, AA6260 aluminum tube and SS304L stainless-steel microtube. The plastic anisotropy is assessed using uniaxial tension, plane-strain tension and disc compression experiments for the die-casting and the sheets; and using biaxial experiments for the tubes. These experiments are then used to model the anisotropic plastic behavior of the test materials using advanced non-quadratic anisotropic yield criteria including Yld2000-2D and Yld2004-3D.

The fracture behavior of the casting and sheets is investigated using conventional notched tension and central hole specimens, as well as novel specimen designs for shear and biaxial stress states. These improved specimen designs exhibit stress states that develop at the neighborhood of the fracture initiation point to remain proportional throughout the loading history. Likewise, the fracture behavior of the tubes is assessed by loading them under axial force and internal pressure along different stress paths. The ability to control the force/pressure ratio enables probing the fracture behavior under proportional and non-proportional loading paths.

Fracture oftentimes initiate at the interior (for example through-thickness mid-plane) of the specimens and thus direct measurement of fracture parameters i.e., stress triaxiality, Lode angle parameter and equivalent plastic strain is not possible from experiments alone. Instead, these parameters at the onset of fracture are obtained in this work using finite element modeling with the material modeling framework using anisotropic yield criteria described above. The loading path and the resulting fracture locus are found to be sensitive to the constitutive model employed, which underscores the importance of an appropriate modeling of plastic anisotropy in ductile fracture studies. Based on the finite element results, the fracture locus is represented by numerous fracture initiation criteria common in literature (e.g., Oyane, Johnson-Cook, Hosford-Coulomb and DF2015), as well as a newly proposed one, created during the course of this research, that is shown to offer better agreement with the experiments, without additional calibration or implementation cost.

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