A local criterion for cleavage fracture of a nuclear pressure vessel steel
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I.
INTRODUCTION
THEassessment of the integrity of any flawed mechanical structure requires the development of approaches which can deal not only with simple situations (symmetrical Mode I loading under small scale yielding) but also with more complex situations (large plasticity, nonisothermal loading, etc.). Several approaches to deal with this problem are possible. The "global approach" which results directly from linear elastic fracture mechanics or elastic-plastic fracture mechanics phrases the fracture resistance in terms of global parameters such as Klc or J~c. This approach is most appropriate in Mode I isothermal loading conditions. Another possible approach relies upon the fact that it is possible to model macroscopic fracture behavior in terms of "local" fracture criteria. In recent years, this procedure has received the attention of many authors (for a review, see, e.g., Reference 1). By local criterion we mean a criterion based on the elastic-plastic stress-strain history calculated at the point where fracture takes place in conjunction with the utilization of a micromechanistically based model for a given physical process of fracture. For simple loading conditions, the stress-strain history can be easily obtained especially when a single parameter such as//1 is a description of the entire field. For more complex situations, information required for the application of local criteria is now available at ever decreasing expense through the development of finite element calculations performed on medium-size computers. The present study was instigated to derive a local criterion applicable to cleavage fracture of a low alloy steel used for the fabrication of pressurized water reactors, A508 class 3 steel. Before presenting the details of the study it is well to F.M. BEREMIN is a research group including ANDRE PINEAU and FRANCOIS MUDRY, both with Centre des Matrriaux de l'Ecole des Mines de Paris, B, P. 87, 91003 Evry Crdex, ERA CNRS N~ 767, France; JEAN-CLAUDE DEVAUX, with Centre de calcul de la division des fabrications de Framatome, TM/C-Usine Framatome, B.P. 13, 71380 Saint-Marcel, France; and YANNICK D'ESCATHA and PATRICK LEDERMANN, both at Bureau de Contr61e de la Construction Nucldaire, 3, rue Devosges, 21000 Dijon, France. Manuscript submitted October 14, 1982.
METALLURGICALTRANSACTIONS A
remember the salient features related to cleavage fracture in quenched and tempered ferritic steels. In these materials, the physical events involved in the cleavage fracture are fairly well understood, at least from a macroscopic point of view. A review of experimental evidence for understanding of cleavage fracture can be found in some recent review papers (see, e.g., References 1 and 2). For the following discussion we will oversimplify by assuming a simple two-step process and by dealing with only slip initiated cleavage. First microcracks are formed due to an inhomogeneous distribution of plastic deformation within the grains.3'4 In mild steels it is believed that cracked grain boundary carbides originate t
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