13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Fracture Toughness Predictions Using the Weibull Stress Model with Implications for Estimations of the T 0 Reference Temperature Claudio Ruggieri1,*, Rafael G. Savioli1, Robert H. Dodds Jr.2 1 Department of Naval Architecture and Ocean Engineering, University of São Paulo, 05508-030, Brazil 2Dept. Of Civil Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA * Corresponding author: claudio.ruggieri@poli.usp.br Abstract This work presents a numerical and experimental investigation into the applicability of precracked Charpy specimens to determine the T0 reference temperature for pressure vessel steels. A central objective is evaluate the effectiveness of the Weibull stress model to correct effects of constraint loss in PCVN specimens which serves to determine the indexing temperature T0 based on the Master Curve methodology. Fracture toughness testing conducted on an A285 Grade C pressure vessel steel provides the cleavage fracture toughness data needed to estimate T0. For the tested material, the Weibull stress methodology yields estimates for the reference temperature from small fracture specimens which are in good agreement with the corresponding estimates derived from testing of much larger crack configurations. Keywords Cleavage Fracture, Constraint Effects, Weibull Stress, Master Curve, PCVN Specimen 1. Introduction Standard compact tension and three-point bend specimens containing deep, through cracks ( ≥ a W 0.5) are commonly employed in fracture toughness testing of ferritic steels in the ductile-to-brittle transition (DBT) region. The primary motivation to use deeply cracked specimens is to guarantee SSY conditions leading to high crack-tip constraint with limited-scale plasticity. Under these conditions, fracture toughness values (such as the J-integral at cleavage instability, cJ , or the elastic-plastic stress intensity factor, Jc K ) prove effective to characterize the essentially stress-controlled failure by a transgranular cleavage mechanism. Current structural integrity assessment procedures for reactor pressure vessels (RPVs) focus on the utilization of small fracture specimens to facilitate experimental measurements of fracture toughness data. In particular, three-point bend testing of precracked Charpy (PCVN) specimens become necessary when severe limitations exist on material availability, for example, in nuclear irradiation embrittlement studies, as this specimen configuration is predominant within surveillance capsule programs. However, the measuring capacity of these specimens for fracture toughness prior to constraint loss may be insufficient for moderate strength pressure vessel and structural steels. Once constraint loss occurs, measured values of cleavage fracture toughness ( cJ , Jc K ) increase markedly as the global plastic deformation interacts with the local crack front fields (governed by J) thereby relaxing the level of stress triaxiality. Moreover, cleavage fracture is a highly localized phenomenon which exhibits strong sensitivity to material characteristics at the microlevel. In particular, the random inhomogeneity in local features of the material causes large scatter in measured values of cleavage fracture toughness. The coupled effects of constraint loss and inherent scatter of toughness values in the DBT region greatly complicate the development of fracture mechanics assessments based on small specimen data. Motivated by these observations, this work explores application of a micromechanics model to determine the 0T reference temperature for pressure vessel steels from precracked Charpy (PCVN) specimens. A central objective is evaluate the effectiveness of the Weibull stress model to correct effects of constraint loss in PCVN specimens which serves to determine the indexing temperature 0T based on the Master Curve methodology. Fracture toughness testing conducted on an A285
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