University of Illinois Engineering Experiment Station. College of Engineering. University of Illinois at Urbana-Champaign.
Abstract
Two complementary methodologies are described to quantify the effects of crack-tip stress triaxiality
(constraint) on the macroscopic measures of elastic-plastic fracture toughness, J and CTOD. In the continuum
mechanics methodology, two parameters, J and Q, suffice to characterize the full range of near-tip
environments at the onset of fracture. J sets the size scale of the zone of high stresses and large deformations
while Q scales the near-tip stress level relative to a high triaxiality reference stress state. The material's
fracture resistance is characterized by a toughness locus, Jc(Q), which defines the sequence of J-Q values
at fracture determined by experiment from high constraint conditions (Q=O) to low constraint conditions
(Q < 0). A micromechanics methodology is described which predicts the toughness locus using crack-tip
stress fields and critical J-values from a few fracture toughness tests. A robust micromechanics model
for cleavage fracture has evolved from the observations of a strong, spatial self-similarity of crack-tip principal
stresses under increased loading and across different fracture specimens. The micromechanics model
employs the volume of material bounded within principal stress contours at fracture to correlate Jc values
for different specimens and loading modes. This report explores the fundamental concepts of the J-Q description
of crack-tip fields, the fracture toughness locus and micromechanics approaches to predict the
variability of macroscopic fracture toughness with constraint under elastic-plastic conditions. Computational
results are presented for a surface cracked plate containing a 6: 1 semi-elliptical, a=t/4 flaw subjected
to remote uniaxial and biaxial tension. Crack-tip stress fields consistent with the J-Q theory are demonstrated
to exist at each location along the crack front. The micromechanics model employs the J-Q description
of crack-front stresses to interpret fracture toughness values measured on laboratory specimens for
fracture assessment of the surface cracked plate. The computational results suggest only a minor effect of
the biaxial loading on the crack tip stress fields and, consequently, on the propensity for fracture relative
to the uniaxial loading.U.S. Nuclear Regulatory Commission. Office of Nuclear Regulatory Research. Division of Engineering.Contract No. N61533-90-K-0059Contract No. N00167-92-K-003
