Channel fracture model and criterion

The experimental study results of the deformation and fracture properties are represented for austenitic 18Cr-9Ni steel (analog of AISI 304 steel) irradiated at 400 °C up to damage dose of 15 dpa. The fracture properties are obtained for smooth round bars and notched round bars under uniaxial tension over temperature range from 20 °C up to 500 °C, and the fracture surfaces are examined by SEM to analyze the main fracture mechanisms with emphasis on specific fracture mechanism of irradiated austenitic steels – channel fracture. Fractions of various fracture modes are estimated depending on stress triaxiality and test temperature. It is shown that the channel fracture fraction depends significantly on the stress triaxiality and increases when the stress triaxiality increases. The main features of the channel fracture surface and the channel fracture facets are studied. It is found that the channel fracture surface is characterized by flat shear facets terraced on which the exits of secondary deformation channels are revealed that locate regularly on the distance of 1–2 μm each other. On the basis of the obtained experimental data the channel fracture model is proposed and the channel fracture criterion is formulated for irradiated austenitic steels. Procedure for the criterion parameter determination is developed on the basis of the test results of standard tensile round bar and notched round bar tested at one temperature over the channel fracture temperature range. Numerical values of the criterion parameters are found from the test results obtained. The proposed channel fracture criterion is verified as applied to the irradiated 18Cr-9Ni steel by comparison of the experimental and calculated values of the fracture strain for specimens with various stress triaxialities and for specimens tested at different temperatures.