Effects of Prior Ductile Tearing on Cleavage Fracture Toughness in the Transition Region
Final rept. Jun 1992-Nov 1993
ILLINOIS UNIV CHAMPAIGN
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Previous work by the authors described a micromechanics fracture model to correct measured J sub c-values for the mechanistic effects of large- scale yielding. This new work extends the model to also include the influence of ductile crack extension prior to cleavage. Ductile crack extensions of 10-15 X the initial crack tip opening displacement at initiation are considered in plane-strain, finite element computations The finite element results demonstrate a significant elevation in crack-tip constraint due to macroscopic sharpening of the extending tip relative to the-blunt tip at the initiation of growth. However this effect is offset partially by the additional plastic deformation associated with the increased applied J required to grow the crack. The initial aW ratio, tearing modulus, strain hardening exponent and specimen size interact in a complex manner to define the evolving near-tip conditions for cleavage fracture. The paper explores development of the new model, provides necessary graphs and procedures for its application and demonstrates the effects of the model on fracture data sets for two pressure vessel steels A533B and A515. J- integral, Constraint, Scaling model, Ductile-brittle, Crack growth effects.