Date of Award
Doctor of Philosophy (PhD)
Mechanical & Aerospace Engineering
S. G. Capschalk
J. C. Newman
K. N. Shivakumar
The purpose of this study was to develop a three-dimensional, elastic-plastic, finite element analysis to investigate crack extension and closure under cyclic loading. The initial study concentrated on the behavior of a straight through crack in a finite-thickness plate subjected to tensile loading (middle-crack tension specimen). The finite element model was composed of 8-noded (linear-strain) isoparametric elements. In the analysis, the material was assumed to be elastic-perfectly plastic. Zienkiewicz's "initial-stress" method, von Mises' yield criterion, and Drucker's normality condition, under small-strain assumptions, were used to account for plasticity. The three-dimensional analysis is capable of extending the crack and changing boundary conditions under cyclic loading. Initially, the crack was assumed to grow as a straight-through crack.
The analysis was applied to determine crack-opening and closure stresses for a middle-crack specimen used in an ASTM Committee E24 task group activity on crack closure. First, imposing proper boundary conditions on the three-dimensional model, plane-strain conditions were simulated and then the model was subjected to a cyclic stress level of 0.25 σys, where σys is the yield stress of the material. Next, a complete three-dimensional analysis of the specimen was made at stress levels of 0.2 σys. and 0.25 σys. . For the highest stress level considered, the ratio of crack-opening stress to maximum applied stress for the outer and inner regions of the model were found to be 0.56 and 0.34, respectively.
The crack-surface displacements for the outer and inner regions of the model, and the variations of normal stresses in the x and y direction are plotted as functions of coordinate location for the stress level of 0.25 σys. The behavior of normal stress in the z direction, through the specimen thickness, for specified elements along the crack plane was also presented. In all of these results, the three-dimensional constraint effect caused higher crack-surface displacements and higher normal stresses on the inner region of the model than those for the outer region. Crack-opening stresses were also determined using displacements at specified points along the crack plane using a load-reduced-displacement technique.
Chermahini, Rahmatollah G..
"Three-Dimensional Elastic-Plastic Finite Element Analysis of Fatique Crack Growth and Closure"
(1986). Doctor of Philosophy (PhD), Dissertation, Mechanical & Aerospace Engineering, Old Dominion University, DOI: 10.25777/cxb0-9j03