Constraint and Ductile Tearing Effects on the Cleavage Fracture of Ferritic Steel

163 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2004.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 (