Determination of R-curves for thermal aged 16MND5 bainitic forging steel using 3D constraint-based fracture mechanics

Abstract

The current paper presents the application of three-dimensional (3D) constraint-based fracture mechanics for the determination of R-curves (J-R curves and CTOD-R curves) of thermal aged 16MND5 bainitic forging steel. First, the fracture toughness test results of 16MND5 steel using different sized clamped single edge notched tension (SENT) specimens under various thermal aging durations were summarized. Then, 3D finite element method (FEM) was applied to determine constraint parameters for the tested clamped SENT specimens. From the analysis results, it is shown that the T11 and the independent portion of T33 normalized by the nominal stress, named F11 and V33, can accurately quantify the in-plane and out-of-plane constraint levels, respectively. In addition, for all thermal aging condition, the thickness strain and V33 have a good linear relationship, which proves the applicability of V33 to quantify out-of-plane constraint effect from the experimental point of view. Next, the in-plane and out-of-plane constraint dependent R-curves were developed. Results showed that the developed R-curves can reproduce the experimental J-R curves and CTOD-R curves of thermal aged RPV steel obtained by clamped SENT specimens with generally good agreements. The combined effects of in-plane constraint, out-of-plane constraint and thermal aging on the fracture toughness were revealed and quantified. Finally, the R-curves of 16MND5 steel with various in-plane and out-of-plane constraint levels, and thermal aging durations were predicted, and compared to available test data.