NUMERICAL INVESTIGATION OF 3-D CONSTRAINT EFFECTS ON BRITTLE-FRACTUREIN SE(B) AND C(T) SPECIMENS

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.