CONTINUUM AND MICROMECHANICS TREATMENT OF CONSTRAINT IN FRACTURE

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.