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 Title: Effects of prior ductile tearing on cleavage fracture in the ductile-to-brittle transition region Author(s): Tang, Ming Doctoral Committee Chair(s): Dodds, Robert H., Jr. Department / Program: Civil and Environmental Engineering Discipline: Civil and Environmental Engineering Degree Granting Institution: University of Illinois at Urbana-Champaign Degree: Ph.D. Genre: Dissertation Subject(s): Applied Mechanics Engineering, Civil Engineering, Mechanical Abstract: Experimental studies demonstrate a significant effect of specimen size, a/W ratio and prior ductile tearing on cleavage fracture toughness values ($J\sb{c}$) measured in the ductile-to-brittle transition region of ferritic materials. In the lower-transition region, cleavage fracture often occurs under conditions of large-scale yielding but without prior ductile crack extension. The increased toughness develops when plastic zones formed at the crack tip interact with nearby specimen surfaces which relaxes crack-tip constraint (stress triaxiality). In the mid-to-upper transition region, small amounts of ductile crack extension (typically $<$1-2 mm) routinely precede termination of the ${J-\Delta a}$ curve by brittle fracture. Large-scale yielding, coupled with small amounts of ductile tearing, magnifies the impact of small variations in microscale material properties on the macroscopic fracture toughness which contributes to the large amount scatter observed in measured $J\sb{c}$-values.In previous work a micromechanics fracture model to correct measured $J\sb{c}$-values for the mechanistic effects of large-scale yielding was developed. The present research extends the model to also include the influence of ductile crack extension prior to cleavage. Ductile crack extensions of 10-15$\times$ the crack-tip opening displacement at initiation are considered in plane-strain, finite element computations for two three-point bend configurations (a/W = 0.10 and 0.50). These analyses employ a finite-strain plasticity theory based on the Green-Naghdi rate with constitutive updating performed on the unrotated configuration. The crack growth is modeled with an improved node release procedure with growth controlled by constant values of the tearing modulus. Material stress-strain properties are modeled with a Ramberg-Osgood relationship using hardening exponents of n (= 5 and 10) typical of pressure vessel steels. Numerical analyses of stationary cracks in the same SE(B) specimens provide baseline solutions from which the effects of crack growth are assessed. The finite element results demonstrate a significant elevation in crack-tip constraint due to a macroscopic "sharpening" of the extending tip relative to the blunt tip at initiation of growth. However, this effect is offset partially by the increased plastic deformation associated with the increased applied J required to grow the crack. The initial a/W ratio, tearing modulus, strain hardening exponent and specimen size interact in a complex manner to define the evolving near-tip conditions for cleavage fracture. This report 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). Issue Date: 1994 Type: Text Language: English URI: http://hdl.handle.net/2142/22211 Rights Information: Copyright 1994 Tang, Ming Date Available in IDEALS: 2011-05-07 Identifier in Online Catalog: AAI9512567 OCLC Identifier: (UMI)AAI9512567
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