Theses and Dissertations

Issuing Body

Mississippi State University

Advisor

Daniewicz, S. R

Committee Member

Schneider, J. A.

Committee Member

Newman, J. C., Jr.

Date of Degree

8-2-2003

Document Type

Dissertation - Open Access

Major

Mechanical Engineering

Degree Name

Doctor of Philosophy

College

James Worth Bagley College of Engineering

Department

Department of Mechanical Engineering

Abstract

For the characterization of crack advance in mechanical components and specimens under monotonic and fatigue loading, many engineering approaches use the assumption that the plastic deformation at the crack tip is isotropic. There are situations when this assumption is not correct, and the modeling efforts require additional correction factors that account for this simplification. The goal of this work is to study two cases where the plastic anisotropy at the crack tip is predominant and influences the magnitude crack-tip parameters, which in turn determine the amount of crack advance under applied loading. At the microstructural level, the small crack issue it is a long-standing problem in the fatigue community. Most of the small crack models consider that the plastic deformation at the crack tip is isotropic. The proposed approached for analyzing small crack growth is to perform finite element simulation of small cracks growing in a material that is assigned single crystal plastic properties. The nature of the plastic deformation of the material at the crack tip in the intra-granular regions could be accurately described and used for modeling small crack growth. By employing finite element analyses for stationary and growing cracks, the main characteristics of the plastic deformation at the crack tip, such as plastic zone sizes and shapes, crack-tip opening displacements, crack-tip opening stresses, are quantified and crack growth rates are determined. Ultimately, by using this crystal plasticity model calibrated for different microstructures, important time and financial resources for real experiments for the study of small cracks can be spared by employing finite element simulations. At macroscale, it is widely known that the manufacturing processes for aluminum alloys results in highly anisotropic microstructures, known as textures. The plastic behavior of these types of materials is far from isotropic and even the use of classical anisotropic yield criteria, such as that on Hill (Hill, 1950), is far from producing accurate results for describing the plastic deformation. Two of these anisotropic yield functions are implemented into finite element code ANSYS and stationary cracks are studied in a wide variety of textures. Significant variations of the plastic deformation at the crack due to the anisotropy are revealed.

URI

https://hdl.handle.net/11668/21122

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