Quasicontinuum modeling of fracture in bcc materials

Abstract

In this thesis fracture of the bcc materials iron and tungsten has been studied through multiscale simulations. The quasicontinuum (QC) method with embedded atom method (EAM) interatomic potentials are employed. The results from four different research papers on this topic are presented. The thesis includes a review on fracture mechanics including an overview of atomistic and multiscale simulations of fracture in bcc-Fe. In addition dislocation theory for bcc materials and multiscale materials modeling techniques, with a focus on the quasicontinuum method, is presented.

Simulations of a semi-infinite edge crack in bcc-Fe have been carried out assuming different crystallographic orientations and different T-stress under mode I loading. Both anisotropic and isotropic formulations of the modified boundary layer (MBL) approach has here been investigated and compared. The results show that the mechanisms at the crack tip and the critical stress intensity factor KIc are sensitive to both the crystallographic orientation and whether or not the formulation of the boundary conditions are isotropic or anisotropic. Mechanisms such as cleavage crack propagation, twinning, and dislocation emission are observed in the analyses.

Mixed mode fracture of bcc-Fe has been investigated. The analyses have been carried out with four crystallographic orientations and different degrees of mixed mode loading. The results show that the critical stress intensity factor and the mechanisms at the crack tip are sensitive to both the crystallographic orientation and the mode of loading. Phenomena such as crack propagation, twinning, and emission of edge and screw dislocations are observed.

Simulations of three-dimensional (3D) edge cracks in single crystal bcc-Fe have been performed. Crystals with different thickness and crystallographic orientation have been investigated and loaded in modes I, II and III. Incipient plasticity at the crack tip and the onset of crack propagation have been studied, and the critical stress intensity factors for crack propagation and dislocation emission has been calculated. The active slip systems and the character of the dislocations have been identified and compared with the standard 2D behavior. The observed type of propagation has been found to depend on the loading mode and crystallographic orientation. Mechanisms includes twinning, bcc→fcc phase transformation, void formation and dislocation emission.