Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)


Mechanical Engineering

First Advisor

M.P. Wnuk


Existing linear elastic fracture mechanics and J-integral analyses are well suited for safety assessments of high-strength, low-toughness materials. These analyses apply only to the onset of crack growth, which is usually tantamount to crack instability and structural failure in that class of materials. However, this is not the case for the low-strength, high-toughness materials in which crack instability may be preceeded [sic] by extensive stable crack growth under rising loads. Here, a substantial margin of safety may exist even when the onset of crack growth is imminent. This thesis describes research leading to a ductile fracture-mechanics methodology designed to treat two-dimensional, large-scale yielding and stable crack growth problems. The linear plasticity model of moving cracks (Wnuk, 1972-1978) is used to obtain predictions concerning the material toughness associated with the preliminary crack extension (R-curves), and to calculate the critical parameters, e. g., the load and crack size at which a transition to unstable brittle-like fractures will occur. One important finding of this work is that parameters truly reflecting the state of the crack-tip process zone are not functions of the extent of stable crack growth when the mode of fracture (full shear or flat) remains fixed. The possibility exists, therefore, that useful, stable growth parameters can be evaluated from the state of the crack tip at the onset of crack extension.

Library of Congress Subject Headings

Fracture mechanics




South Dakota State University