Ductile fracture properties of 16MND5 bainitic forging steel under different in-plane and out-of-plane constraint conditions: Experiments and predictions

Abstract

Bridging the gap between small-scale laboratory specimens and engineering full size engineering structures plays a critical role in integrity assessment of reactor pressure vessels (RPVs). In the present work, in-plane and out-of-plane constraint effects are both considered to determine ductile fracture properties under various low constraint conditions. The fracture behavior of 16MND5, a bainitic forging steel was investigated using clamped single edge notched tension (SENT) specimens with different crack sizes, specimen thicknesses and span lengths. Various fracture properties, including R-curves (J-integral, CTOD) and initiation fracture toughness (JSZW, J0.2, CSZW and C0.2) were determined. The results showed that the fracture properties are highly dependent on the in-plane (crack depth) and out-of-plane (specimen thickness) constraint conditions. The lower in-plane and out-of-plane constraint levels will introduce higher fracture properties. To further understand and quantify the impact of in-plane and out-of-plane constraint effects, the finite element method (FEM) and fracture surface analysis were conducted. Constraint dependent R-curves and initiation fracture toughness are developed for 16MND5 steel using a newly developed three-parameter fracture mechanics methodology. In particular, two independent constraint parameters, one for in-plane and one for out-of-plane, were used to quantify the 3D constraint effect. Finally, the present method was used to predict fracture properties for other specimens or low-constraint RPV cracked structures with different in-plane and out-of-plane constraint conditions.