Doctor of Philosophy, University of Akron, 2018, Mechanical Engineering

This dissertation aims to develop valid numerical approaches to investigate the micromechanics of ductile fracture process and predict the ductile material failure under various loading conditions. As the first portion of this work, a layered unit cell micromechanics model is proposed. This model consists of three void containing material units stacked in the direction normal to the localization plane. Localization takes place in the middle material unit while the two outer units undergo elastic recovery after failure occurs. Thus, a failure criterion is established as the material is considered failure when the macroscopic effective strain of the outer material units reaches the maximum value. Comparisons of the present model with several previous models suggest that the present model is not only easy to implement in finite element analysis but also more suitable to robustly determine the failure strain. A series of unit cell analyses are conducted for various macroscopic stress triaxialities and Lode parameters to investigate the dependency of failure strain on stress state. The analysis results also reveal the effect of the stress state on the deformed void shape within and near the localization band. Additionally, analyses are conducted to demonstrate the effect of the voids existing outside the localization band. Next, the unit cell model is utilized to investigate the effect of hydrogen on ductile fracture demonstrated by its influence on the process of void growth and coalescence. The evolution of local stress and deformation states results in hydrogen redistribution in the material, which in turn changes the material's flow property due to the hydrogen enhanced localized plasticity effect. The result shows that hydrogen reduces the ductility of the material by accelerating void growth and coalescence, and the effect of hydrogen on ductile fracture is strongly influenced by the stress state experienced by the material, as characterized by the stress tr (open full item for complete abstract)

Committee: Xiaosheng Gao Dr. (Advisor); Chang Ye Dr. (Committee Member); Gregory Morscher Dr. (Committee Member); Ernian Pan Dr. (Committee Member); Chien-Chung Chan Dr. (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, University of Akron, 2013, Mechanical Engineering

This thesis sought to investigate and develop valid numerical approaches to predict ductile fracture under different stress state and loading conditions. As the first portion of this work, the plastic flow and fracture behaviors of three aluminum alloys (5083-H116, 6082-T6 and 5183 weld metal) under the effects of strain rate and temperature were studied through a series of experiments and finite element analyses. The fracture behavior under the influential factor of stress triaxiality was also studied. The applicability of the Johnson-Cook plasticity and fracture models were investigated with mixed results. For all three materials, the dependency of the failure strain on triaxiality is adequately described.The stress state effect on plasticity and ductile fracture behaviors was further explored for aluminum alloy 5083-H116 through tests on plane strain specimens and torsion specimens, focusing on the third deviatoric stress invariant (lode angle). A stress state dependent plasticity model, J2-J3 model, together with the Xue-Wierzbicki fracture criterion which defined the damage parameter as a function of the stress triaxiality and the Lode angle, was implemented and calibrated with the test data. The calibrated model was utilized to study the residual stress effect on ductile fracture resistance, using compact tension specimens with residual stress fields generated from a local out-of-plane compression approach. Fracture tests with positive and negative residual stresses were conducted on the C(T) specimens. Both experimental and finite element results showed significant effect of residual stress on ductile fracture resistance.In an attempt to predict ductile fracture under shear-dominated conditions, this study combined the damage mechanics concept with the Gurson-Tvergaard-Needleman porous plasticity model that accounts for void nucleation, growth and coalescence. The GTN model was extended by coupling two damage parameters, representing volumetric damage and she (open full item for complete abstract)

Committee: Xiaosheng Gao Dr (Advisor); Shing-Chung Wong Dr. (Committee Member); Gregory Morscher Dr. (Committee Member); Ernian Pan Dr. (Committee Member); Kevin Kreider Dr. (Committee Member) Subjects: Mechanical Engineering; Mechanics

Doctor of Philosophy, University of Akron, 2012, Mechanical Engineering

It has been shown that the plastic response of many materials, including some metallic alloys, depends on the stress state. Based on plasticity analysis of three metal alloys, a series of new plasticity models with stress state effect is proposed. The effect of stress state on plasticity and the general forms of the yield function and flow potential for isotropic materials are assumed to be functions of the first invariant of the stress tensor (I1) and the second and third invariants of the deviatoric stress tensor (J2 and J3). Finite element implementation, including integration of the constitutive equations using the backward Euler method and formulation of the consistent tangent moduli, are presented in this thesis. A 5083 aluminum alloy, Nitronic 40 (a stainless steel), and Zircaloy-4 (a zirconium alloy) were tested under tension, compression, torsion, combined torsion-tension and combined torsion-compression at room temperature to demonstrate the applicability of proposed I1-J2-J3 dependent models. It has shown that the output produced by the proposed model have better agreement with experimental data than those produced by the classical J2 plasticity theory for the tested loading conditions and materials. Furthermore, the Gurson-Tvergaard-Needleman porous plasticity model, which is widely used to simulate the void growth process of ductile fracture, is extended to include the effects of hydrostatic stress and the third invariant of stress deviator on the matrix material. The experimental and numerical work presented in this thesis reveals that the stress state also has strong effects on the ductile fracture behavior of an aluminum 5083 alloy. For the ductile fracture analysis, The Goluganu-Leblond-Devaux (GLD) model is employed to describe the porous plasticity behavior of aluminum 5083. The effect of stress triaxiality and Lode angle is analyzed and fracture locus is calibrated as a criterion for void coalescence. The GLD model combined with the fracture (open full item for complete abstract)

Committee: Xiaosheng Gao Dr. (Advisor); Fred Choy Dr. (Committee Member); Gregory Morscher Dr. (Committee Member); Ernian Pan Dr. (Committee Member); Kevin Kreider Dr. (Committee Member) Subjects: