M. Nevalainen et Rh. Dodds, NUMERICAL INVESTIGATION OF 3-D CONSTRAINT EFFECTS ON BRITTLE-FRACTUREIN SE(B) AND C(T) SPECIMENS, International journal of fracture, 74(2), 1996, pp. 131-161
Specimen size and geometry effects on cleavage fracture of ferritic st
eels tested in the ductile-to-brittle transition region remain an impo
rtant technological impediment in industrial applications of fracture
mechanics and in the on-going development of consensus fracture testin
g standards. This investigation employs 3-D nonlinear finite element a
nalyses to conduct an extensive parametric evaluation of crack front s
tress triaxiality for deep notch SE(B) and CO specimens and shallow no
tch SE(B) specimens, with and without side grooves. Crack front condit
ions are characterized in terms of J-Q trajectories and the constraint
model for cleavage fracture toughness proposed previously by Dodds an
d Anderson. An extension of the toughness scaling model suggested here
combines a revised 'in-plane' constraint correction with an explicit
thickness correction derived from extreme value statistics. The 3-D an
alyses provide 'effective' thicknesses for use in the statistical corr
ection which reflect the interaction of material flow properties and s
pecimen aspect ratios, a/W and W/B, on the varying levels of stress tr
iaxiality over the crack front. The 3-D computational results imply th
at a significantly less strict size/deformation limit, relative to the
limit indicated by previous plane-strain computations, is needed to m
aintain small-scale yielding conditions at fracture by a stress-contro
lled, cleavage mechanism in deep notch SE(B) and C(T) specimens. Moreo
ver, the analyses indicate that side grooves (20 percent) should have
essentially no net effect on measured toughness values of such specime
ns. Additional new results made available from the 3-D analyses also i
nclude revised eta-plastic factors for use in experimental studies to
convert measured work quantities to thickness average and maximum(loca
l) J-values over the crack front. To estimate CTOD values,new m-factor
s are included for use in the expression J = m sigma(flow)delta.