| This study couples the ASTM E1921 procedure to characterize the ductile-tobrittle toughness of ferritic steels in terms of KJc (or Jc) values with the Weibull stress model, i.e ., the "local approach" for fracture at the micro-scale. The E1921 procedures assume that uniform, small-scale yielding (SSY) conditions exist at fracture along the full crack-front. Plasticity induced constraint loss (crack-front triaxiality) frequently invalidates these assumptions. The non-dimensional functions, g(M = bsigma 0/J), derived from application of the Weibull stress approach for a specific specimen and material, describes the evolution of constraint loss effects on the fracture toughness relative to a plane-strain, SSY reference condition. For specific materials, cleavage fracture assessments employing the Weibull stress model depend on the calibration of several parameters. This work proposes that the Weibull stress scale parameter, sigma u, increases with temperature to reflect the increasing microscale toughness of ferritic steels caused by local events that include plastic shielding of microcracks, microcrack blunting, and microcrack arrest. The Weibull modulus, m, then characterizes the temperature invariant, random distribution of microcrack sizes in the material. Direct calibration of sigma u values at temperatures over the DBT region requires extensive sets of fracture toughness values. A more practical approach developed here utilizes the so-called Master Curve standardized in ASTM Test Method E1921-02 to provide the needed temperature vs. toughness dependence. Over the mid-to-upper region of the DBT, the brittle transgranular cleavage and ductile tearing are competing failure mechanisms in ferritic steel. At metallurgical scales (<50mum), the formation and growth of the voids driving ductile crack extension likely alter the local stress fields acting on the smaller inclusions that trigger cleavage fracture. Here we study the effects of void growth on cleavage fracture by modeling discrete cylindrical voids lying on the crack plane ahead of the crack tip within a small-scale yielding (SSY) boundary layer model. We develop a new non-dimensional function, h( J), to represent the stress concentration effects on the Weibull stress in a convenient framework (J= J/Dsigma 0 denotes a non-dimensional loading for SSY analyses). |
