Two complementary methodologies are described to quantify the effects
of crack-tip stress triaxiality (constraint) on the macroscopic measur
es of elastic-plastic fracture toughness J and Crack-Tip Opening Displ
acement (CTOD). In the continuum mechanics methodology, two parameters
J and Q suffice to characterize the full range of near-tip environmen
ts 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 le
vel relative to a high triaxiality reference stress state. The materia
l's fracture resistance is characterized by a toughness locus J(c)(Q)
which defines the sequence of J-Q values at fracture determined by exp
eriment from high constraint conditions (Q almost-equal-to 0) to low c
onstraint conditions (Q < 0). A micromechanics methodology is describe
d which predicts the toughness locus using crack-tip stress fields and
critical J-values from a few fracture toughness tests. A robust micro
mechanics model for cleavage fracture has evolved from the observation
s of a strong, spatial self-similarity of crack-tip principal stresses
under increased loading and across different fracture specimens. We e
xplore the fundamental concepts of the J-Q description of crack-tip fi
elds, the fracture toughness locus and micromechanics approaches to pr
edict the variability of macroscopic fracture toughness with constrain
t under elastic-plastic conditions. Computational results are presente
d for a surface cracked plate containing a 6:1 semi-elliptical, a = t/
4 flaw subjected to remote uniaxial and biaxial tension. Crack-tip str
ess 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 tou
ghness values measured on laboratory specimens for fracture assessment
of the surface cracked plate.