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The stresses and strains in a cracked solid are in a complicated three-dimensional state. There are three two-dimensional limiting cases: plane strain, plane stress, and Dugdale's strip yielding case. The thickness of a plate relative to crack tip plastic zone size determines which one of these three limiting cases is predominant. The characteristics of the plane-stress and the Dugdale strip yielding crack-tip fields were investigated with the moire method together with the finite-element calculations. The state of the crack-tip stress and strain fields approaches that of plane strain for very thick plates. For a very thin plate with a very large crack-tip plastic zone, the Dugdale strip yielding model is applicable, and the crack opening displacement, crack-tip opening displacement, and thickness contraction are related to K or J and they can be used for fracture toughness measurements. When a plate thickness is in between these two extremes, there exists a characteristic plane stress crack-tip field. The correspondence of the planestress crack-tip fields in small-scale yielding and in general yielding was established, and the J value of a small specimen in general yielding can be obtained from the established correspondence. The value of the applied J-field of a small specimen in general yielding can be measured in terms of the applied stress and the specimen elongation.
nonlinear fracture mechanics, fracture toughness, small-scale yielding, general yielding, crack opening displacement, crack-tip opening displacement, thickness contraction, elastic-plastic fracture
Professor of materials science, Syracuse University, Syracuse, N.Y.
Hanford Engineering Development Laboratory, Westinghouse Corp.,
Fairchild Republic Co.,