Accurate predictions of material strength under different loading conditions with large plastic deformation and ductile fracture are of great importance. This dissertation aims to develop an essential understanding of ductile fracture of AISI 4340 steel alloy using both empirical and micromechanics based models. For this purpose, 29 specimens of different geometries with different heat-treatments were designed to investigate the effects of stress states. These specimens are: (a) 13 round bars with different notches (axial symmetric tension); (b) 13 flat grooved specimens with different grooves; (c) 3 small round cylinders. Mechanical tests up to fracture were conducted on these specimens to characterize the influence of hydrostatic stress and Lode angle on material plasticity and fracture. Scanning electron microscopy (SEM) observations were performed on both original and fractured specimens to investigate different micromechanical revelations and features. The plasticity model with pressure and Lode angle effects (PPL model) and the modified Mohr-Coulomb (MMC) fracture criterion were used to predict plastic flow and fracture initiation behaviors under different loading conditions in finite element simulations. A model optimization method using ISIGHT was set up to achieve simulation results that were well correlated with experimental data. The effects of heat-treatment on material strength and ductility of AISI 4340 steel were also discussed. This work was further carried onto the microvoids based metal plasticity theory. The well-known Gurson-Tvergaard-Needleman (GTN) model was calibrated for AISI 4340 steel. It is found that the GTN model is not sufficient in simulating test data for the 16 Rockwell hardness plane strain specimens. Therefore, The GTN model is modified to include the Lode angle dependence on matrix material plasticity. It is also found that using a fixed or constant microvoid volume fraction at failure (ff) for all loading conditions is inadequate. Following a similar derivation of the MMC fracture model, the microvoid volume fraction at failure (ff) becomes a function of both stress triaxiality and Lode angle. This new criterion is named (GTN-MMC). The proposed plasticity and fracture models were implemented into ABAQUS through a user-defined material subroutine (VUMAT) for FE simulations. Good correlations were achieved between experimental results and numerical simulations.
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Doctor of Philosophy (Ph.D.)
College of Engineering and Computer Science
Mechanical and Aerospace Engineering
Length of Campus-only Access
Doctoral Dissertation (Campus-only Access)
Ghazali, Sami, "A Study on the Plasticity and Fracture of AISI 4340 Steel Under Different Loading Conditions Considering Heat Treatment and Micromechanics" (2018). Electronic Theses and Dissertations. 5881.