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This fifth Jerry L. Swedlow Memorial Lecture begins with a brief philosophical and historical overview of applied fracture mechanics, particularly as it pertains to the safety of pressure vessels. It then progresses to a more-or-less chronological panorama of experimental and analytical results pertaining to the important subject of constraint, a fundamental aspect of fracture mechanics in which Jerry Swedlow had a keen interest and to which he made valuable contributions. To be truly useful and dependable in application to the safety analysis of real structures, new analysis developments must be physically realistic. That means that they must accurately describe physical cause and effect. Consequently, before useful mathematical modeling can begin, a pattern of cause and effect must be established from experimental data. This can be a difficult and time consuming process, but it is worth the effort. Accordingly, a central theme of this paper is that, consistent with the scientific method, the search for patterns is constant and vital. This theme is well illustrated historically by the development of small, single-specimen, fracture toughness testing techniques. It is also illustrated, at the end of the present paper, by the development, based on two different published large-strain, elastic-plastic, three-dimensional finite-element analyses, of a hypothesis concerning three-dimensional loss of constraint. Specifically, it appears that, at least in standard compact specimens, when a generalization of Irwin's thickness-normalized plastic-zone parameter, β, reaches a value close to 2π, the through-thickness contraction strain at the apex of the near-tip logarithmic-spiral slip-line region becomes the dominant negative strain accommodating crack opening. Because slip lines passing from the midplane to the stress-free side surfaces do not have to curve, once these slip lines are established, stresses near the crack tip are only elevated by strain hardening and constraint becomes significantly relaxed. This hypothesis, based on published three-dimensional elastic-plastic analyses, provides a potentially valuable means for gaining additional insight into constraint effects on fracture toughness by considering the roles played by the plastic strains as well as the stresses that develop near a crack tip.
Fracture mechanics, pressure vessels, fracture toughness, small specimen testing, flawed structural components, thickness effects, constraint
Research Specialist, Oak Ridge National Laboratory, Oak Ridge, Tennessee