Assessment of a semielliptical crack in the interface between ferritic and austenitic material on the basis of the Gurson model

In safety analyses of nuclear reactor pressure vessels, hypothetical cracks are often postulated in the ferritic base material beneath the austenitic cladding that is used to protect against corrosion. The criticality of the hypothetical cracks is strongly influenced by the integrity of the cladding. If the cladding is intact, the crack tip loading is significantly reduced compared with the case where the cladding is assumed to be broken. The assessment of the load-carrying capacity of such a cladding cannot be done on the basis of the J-integral concept since its application is problematic at the interface of two materials and steep gradients in the material properties in the heat-affected zone cannot be characterized by fracture mechanics tests meaningfully. Moreover, the shape of cracks is usually assumed as semielliptical and the conventional J-concept has to be modified to take into account the effect of the constraint, changing along the crack front, on the crack resistance behavior. To avoid these problems, the behavior of a flaw in the interface between ferritic and austenitic material has been analyzed by a micromechanical material model based on the Gurson flow function. A three-point-bend specimen consisting of a ferritic block with an austenitic cladding and a semielliptical crack in the ferritic base material beneath the cladding was tested and evaluated. The global (that is, load versus displacement curve) and local (that is, ductile crack extension in ferrite and austenite) behaviors of this specimen were predicted by means of two- and three-dimensional finite element analyses with the Gurson parameters determined for different material zones. The material characterization was done by utilizing subsized tension tests with improved evaluation and fracture mechanics tests with precracked Charpy-type specimens.