https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1048&context=usnavyresearch

Review of fracture toughness (G, K, J, CTOD, CTOA) testing and standardization

Based on the asymptotic stress fields including higher order terms at the crack tip of a non-linear material, conditions under which single or double parameter controlled onset of fracture initiation occur are discussed. A set of experimental data under cleavage fracture was analysed. It is shown that under Mode I plane strain conditions two parameters J and A2 can be utilized to predict the onset of fracture with J setting the loading level and A2 relating to the constraint. Theoretical meanings of fracture toughness values corresponding to conditions of small scale yielding and the HRR stress distribution are discussed. A procedure of applying this concept to the evaluation of flawed structures is outlined.

On the fracture of solids characterized by one or two parameters: Theory and practice

Specimen size and geometry effects on cleavage fracture of ferritic steels tested in the ductile-to-brittle transition region remain an important technological impediment in industrial applications of fracture mechanics and in the on-going development of consensus fracture testing standards. This investigation employs 3-D nonllinear finite element analyses to conduct an extensive parametric evaluation of crack front stress triaxiality for deep notch SE(B) and C(T) specimens and shallow notch SE(B) specimens, with and without side grooves. Crack front conditions are characterized in terms of J-Q trajectories and the constraint model for cleavage fracture toughness proposed previously by Dodds and 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 analyses provide ‘effective’ thicknesses for use in the statistical correction which reflect the interaction of material flow properties and specimen aspect ratios, a/W and W/B, on the varying levels of stress triaxiality over the crack front. The 3-D computational results imply that a significantly less strict size/deformation limit, relative to the limit indicated by previous plane-strain computations, is needed to maintain small-scale yielding conditions at fracture by a stress-controlled, cleavage mechanism in deep notch SE(B) and C(T) speciments. Moreover, the analyses indicate that side grooves (20 percent) should have essentially no net effect on measured toughness values of such specimens. Additional new results made available from the 3-D analyses also include revised η-plastic factors for use in experimental studies to convert measured work quantities to thickness average and maximum (local) J-values over the crack front. To estimate CTOD values, new m-factors are included for use in the expression 131-1.

/pdf/numerical-investigation-of-3-d-constraint-effects-on-brittle-3y7fu62w6p.pdf

S. Yang

Papers

Higher order asymptotic crack tip fields in a power-law hardening material

Higher order crack tip fields and its implication for fracture of solids under mode II conditions