A Micromechanics Approach Incorporating Plastic Strain Effects to Predict the Influence of Specimen Geometry on Cleavage Fracture Toughness for a Pressure Vessel Steel
Claudio Ruggieri1; Robert H. Dodds2
1 University of Sao Paulo (USP); 2 University of Illinois at Urbana-Champaign
doi:10.20906/CPS/COB-2015-0052
Resumo
Structural integrity assessment procedures for commercial 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 becomes necessary when severe limitations exist on material availability such as, for example, in nuclear irradiation embrittlement studies. A recently developed procedure to characterize fracture toughness data (Jc or KJc ) over the ductile-to-brittle transition (DBT) region, often known as the Master Curve approach , makes extensive use in practice of PCVN specimens to determine a reference temperature, T0, and the associated median fracture toughness applicable to a wide range of structural ferritic steels. However, as the specimen size is reduced (relative to the standard 1-T specimen), the evolving crack-tip plastic zones developing from the free surfaces with increased loading affects strongly the crack-front size over which high levels of near-tip stress triaxiality (constraint) are maintained. These changes in the crack-tip stress fields over a relatively small thickness in connection with a smaller sampling volume for cleavage fracture influence the measured toughness values, including their statistical scatter and mean value. Such features clearly affect the fracture toughness dependence on temperature and, consequently, produce potential differences in Jc-values measured using small size specimens and larger fracture specimens. This work describes a modified Weibull stress (Sw) approach incorporating the strong effects of plastic strain on cleavage fracture. The primary purpose of this study is to explore application of the micromechanics model incorporating effects of plastic strain on cleavage fracture to determine the reference temperature for pressure vessel steels from precracked Charpy (PCVN) specimens. Fracture toughness testing conducted on an A285 Grade C and A515 pressure vessel steels provides the cleava
Palavras-chave: cleavage fracture; local approach; Weibull stress; plastic strain; probabilistic fracture mechanics