Doctor of Philosophy
Faculty of Engineering
Glass, Bradley Smyth, Continuum mechanics approaches to the study of fracture and fatigue in metals, PhD thesis, Faculty of Engineering, University of Wollongong, 2004. http://ro.uow.edu.au/theses/265
This thesis investigates continuum mechanics based means of metal failure assessment. A basic science approach was employed throughout the study to examine the fundamental relationships responsible for metal failure. The extension of previously existing continuum mechanics based theories to encompass a wider range of application was considered in this thesis. Research was conducted as two separate studies which examine specific aspects of the metal failure spectrum, namely failure due to monotonic loading, and fatigue failure due to cyclic loading. The failure due to monotonic loading research was conducted to examine the influence of hydrostatic stress on metal ductility. A fundamental relationship in the form of a monotonic failure criterion was proposed based on a relationship between equivalent plastic fracture strain and hydrostatic stress. An experimental program incorporating uniaxial tensile testing of notched specimens was conducted to examine the proposed relationship for the hydrostatic tensile stress range. Finite element analyses were produced to confirm the mechanical properties and obtain the stress-strain state present at specimen failure. A good correlation was established between the load-displacement results obtained from experiment and finite element analysis, providing confirmation of the stress-strain data. The stress-strain results confirmed the existence of a relationship between hydrostatic stress and ductility in the form of a monotonically decreasing value of equivalent plastic fracture strain with increasing hydrostatic tensile stress. The relationship determined was in accordance with the trend indicated by various researchers for the hydrostatic compressive stress range. The potential application of such a criterion to finite element methods was amply demonstrated from this research. The fatigue failure due to cyclic loading research examined the application of energy based methods to fatigue life characterisation. Based on the hypothesis that irreversible damage may be attributed entirely to plastic deformation, the application of the plastic strain energy approach to the entire fatigue life spectrum was pursued. For application to high cycle fatigue, a thermodynamic approach was developed to allow plastic strain energy determination beyond the range of application of conventional mechanical measurement. Thermodynamic models consisting of varying degrees of free surface contribution to heat dissipation were developed as possible representations of the high cycle fatigue damage process. An experimental program was conducted incorporating mechanical and thermodynamic means of measurement. Thermodynamic measurement was achieved via an experimental apparatus incorporating precision temperature measurement and thermal isolation at the specimen surface. Assuming an appropriate thermodynamic model, a finite difference analysis allowed a quantitative determination of plastic strain energy. Close agreement was indicated from comparison of the low cycle fatigue plastic strain energy results obtained from mechanical and thermodynamic measurement. A qualitative determination of plastic strain energy for high cycle fatigue was achieved, subject to confirmation of the thermodynamic model. The qualitative assessment verified the existence of measurable plastic strain energy during high cycle fatigue.
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