Evaluation Of Fracture Characteristics Of Reactor Pressure Vessel Steel 20mnmoni55 Using Damage Mechanics Approach

Abstract

Material of Reactor Pressure Vessel (RPV) should have good enough fracture toughness to ensure safe operation of nuclear reactor. In order to be on safe side the fracture characteristics of RPV material have to be quantified. In this present work the fracture characteristics of 20MnMoNi55 steel – the material used for Indian PHWR – have been found using damage mechanics approach. Fracture resistance of a ductile material is conventionally characterized by J-R curves. The original idea was that a unique fracture resistance curve would suffice to characterize the material. However, testing of different type of specimens revealed considerable different J-R curves due to different levels of triaxility imposed on the specimens. This raises the question of transferring fracture parameters from specimens to component level.

Under such circumstances it becomes advisable to use Damage Mechanics. The difficulty of geometry-dependency is largely overcome by Damage Mechanics, which models the drop in load carrying capacity of material with increase in plastic strain. Such modeling is done considering nucleation, growth and coalescence of the voids in a material following large scale plasticity. The constitutive model introduced by Gurson and modified by Tvergaard and Needleman is used in this work.

The model parameters are determined by using hybrid methodology of different tensile tests subsequent numerical analysis and curve fitting and finally comparison of analytical and experimental results. A parametric study on different Gurson parameters was carried out to experience the effect of different parameters on load ΔD curves for RNTS and J-R curves CT specimens. Later the model parameters were calibrated by fitting the analytical results in the experimental results.