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, 2016, Mechanical Engineering

In this dissertation, a comprehensive study of ductile damage of metallic materials is presented, covering constitutive modeling, numerical implementation and model calibration and verification. As the first part of this dissertation, a pressure-insensitive plasticity model, expressed as a function of the second and third invariants of the stress deviator (J2 and J3), is presented. Depending on whether the power of the J3 term is odd or even, the proposed model can capture either the tension-compression strength-differential (S-D) effect or the torsion-tension strength-differential effect of the material. The plasticity model with an odd power to the J3 item has been calibrated and validated using measured experimental data of a ß-treated Zircaloy-4 with a wide range of triaxiality and Lode parameter values. Results show that this model captures the strong strength-differential (S-D) effect in the material. The plasticity model with an even power to the J3 item is able to capture the isotropic plastic behavior of a stainless steel Nitronic 40, under various stress states with good accuracy and computational efficiency. Next, the effect of the material's plasticity behavior on the ductile damage process is studied by conducting a series of unit cell analyses of a void-containing representative material volume (RMV), where the plastic response of the matrix material is governed by the J2-J3 dependent plasticity model. To simulate the ductile damage process in anisotropic materials, a new constitutive model, which combines the models proposed by Zhou et al. (2014) and Stewart and Cazacu (2011), is developed and employed to study the plasticity and ductile fracture behavior of a commercially pure titanium (CP Ti). In particular, a Gurson-type porous material model is modified by coupling two damage parameters, accounting for the void damage and the shear damage respectively, into the yield function and the flow potential. The plastic anisotropy and tension-compre (open full item for complete abstract)

Committee: Xiaosheng Gao Dr. (Advisor); Yalin Dong Dr. (Committee Member); Chang Ye Dr. (Committee Member); Ernian Pan Dr. (Committee Member); Kevin Kreider Dr. (Committee Member) Subjects: Mechanical Engineering; Mechanics

Doctor of Philosophy, University of Akron, 2014, Civil Engineering

Numerical simulation plays an irreplaceable role in reducing time and cost for the development of aerospace and automotive structures, such as composite fan cases, car roof and body panels etc. However, a practical and computationally-efficient methodology for predicting the performance of large braided composite structures with the response and failure details of constituent level under both static and impact loading has yet to be developed. This study focused on the development of efficient and sophisticated numerical analysis modeling techniques suitable for two-dimensional triaxially braided composite (TDTBC) materials and structures under high speed impact. A new finite element analysis (FEA) based mesomechanical modeling approach for TDTBC was developed independently and demonstrated both stand alone and in the combined multi-scale hybrid FEA as well. This new mesoscale modeling approach is capable of considering the detailed braiding geometry and architecture as well as the mechanical behavior of fiber tows, matrix, and the fiber tow interface, making it feasible to study the details of localized behavior and global response that happen in the complex constituents. Furthermore, it also accounts for the strain-rate effects on both elastic and inelastic behavior and the failure/damage mechanism in the matrix material, which had been long observed in experiments but were neglected for simplicity by researchers. It is capable of simulating inter-laminar and intra-laminar damage and delamination of braided composites subjected to dynamic loading. With high fidelity in both TDTBC architecture and mechanical properties, it is well suited to analyze high speed impact events with improved simulation capability in both accuracy and efficiency. Special attention was paid to the applicability of the method to relatively large scale components or structures. In addition, a novel hybrid multi-scale finite element analysis method, entitled Combined Multiscale (open full item for complete abstract)

Committee: Wieslaw Binienda Dr. (Advisor); Ernian Pan Dr. (Committee Member); Guo-Xiang Wang Dr. (Committee Member); Robert Goldberg Dr. (Committee Member); Qindan Huang Dr. (Committee Member); Kevin Kreider Dr. (Committee Member) Subjects: Aerospace Engineering; Aerospace Materials; Automotive Engineering; Engineering; Mechanical Engineering