13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Stress-Controlled and Stress-and-Strain Controlled Criteria of Brittle Fracture in Local Approach Victoria Shvetsova1,*, Boris Margolin1, Alexander Gulenko1 1 Central Research Institute of Structural Materials “Prometey”, 191015 Saint-Petersburg, Russia * victoria.shvetsova@prometey2.spb.su Abstract Stress-controlled and stress-and-strain controlled criteria are compared in light of prediction of brittle fracture on a macro-scale and in light of adequate description of microstructural features of brittle fracture of irradiated RPV steels. Keywords brittle fracture, local criterion, radiation defects, modelling, RPV steel 1. Introduction At present, several brittle fracture models based on local criteria are developed. The local approach models differ from each other just in the formulation of local criteria. Basically, two types of local criteria may be defined: stress-controlled and stress-and-strain controlled criteria. According to stress-controlled criterion proposed by Ludwik [1] and Yoffe [2] brittle fracture occurs when the maximum principal stress reaches some critical fracture stress SC independent of temperature. Later Davidenkov [3] and Knott [4] added necessary condition to initiate brittle fracture - a requirement of active plastic deformation. This stress-controlled criterion was initially used for low-strength steels. Application of this criterion for middle and high strength steels, in particular, for reactor pressure vessel (RPV) steels, showed that SC varies for specimens with various geometries. It means that stress-controlled criterion does not work, at least, in deterministic statement. The Beremin model [5] based on stress-controlled criterion takes into account a stochastic nature of brittle fracture using the Weibull approach. This model allows one to describe the scale effect and provide the agreement of test results from notched and cracked specimens from RPV steel over narrow temperature range. However to predict the temperature dependence of fracture toughness with the Beremin model for RPV steels the introduction of additional a priori assumption is required. More difficulties arise when using stress-controlled criterion for irradiated RPV steels. To overcome these difficulties, stress-and-strain controlled criterion was proposed near 20 years ago [6]. Application of this criterion allows one not only to provide the transferability of test results from specimens with various geometries, but also to predict known properties of brittle fracture. The application of fracture modelling with a local cleavage fracture criterion (local approach) in principle allows the prediction of the fracture properties on a macro-scale without any additional assumptions. Local approach models may be enlarged to include in the consideration microstructural characteristics, in particular, irradiation-induced defects. Thus, fracture modelling with local fracture criterion unites various scales from irradiation-induced defects on a nano-scale to cleavage microcrack on micro-scale and to fracture on a meso- and macro-scales. This approach may provide reliable (both from mechanical and physical viewpoints) underpinning for engineering methods that are required for RPV integrity assessment. The predictable possibilities of fracture models appear to be substantially determined by local criterion used. The present report compares stress-controlled and stress-and-strain controlled criteria in light of prediction of brittle fracture on a macro-scale and discusses which local criterion provides adequate
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