Abstract :
The present paper proposes ductile failure
criteria in terms of true fracture strain (the
equivalent strain to fracture) as a function of the
stress triaxiality (defined by the ratio of the hydrostatic
stress to the equivalent stress) for the API
X65 steel. To determine the stress-modified fracture
strain, smooth and notched tensile bars with
four different notch radii are tested, from which
true fracture strains are determined as a function
of the notch radius. Then detailed elastic-plastic,
large strain finite element analyses are performed
to estimate variations of stress triaxiality in the tensile
bars, which leads to true fracture strains as a
function of the stress triaxiality, by combining them
with experimental results. Two different failure criteria
are proposed, one based on local stress and
strain information at the site where failure initiation
is likely to take place, and the other based
on averaged stress and strain information over theligament where ductile fracture is expected. As a
case study, ligament failures ofAPI X65 pipes with
a gouge are predicted and compared with experimental
data.
