Experimental and numerical investigations on the influence of crack-tip constraint on the ductile fracture behavior in nodular cast iron

When determining static fracture mechanics parameters, specimens are typically designed to maximize crack-tip constraint and consequently induce high stress triaxiality at the crack front. This is often achieved using deeply cracked bending specimens with side grooves. The crack resistance curves evaluated from such specimens represent a lower bound, enabling a conservative and reliable assessment of cracks in components. Depending on the geometry of the component and the crack configuration being evaluated, these bounds can be, however, overly conservative. Numerous studies over the past 30 years have investigated the influence of crack- tip constraint on the crack resistance behavior of steels, aiming to incorporate these effects into the evaluation of components. In contrast, there is a significant lack of corresponding data for nodular cast iron, a material widely used in mechanical engineering. This work aims to address this gap by presenting an experimental program complemented by numerical simulations. Single-edge bending SE(B) specimens with varying crack depths and side-groove configurations, as well as middle-cracked tension M(T) specimens are tested. Additionally, some specimens are tested in-situ using synchrotron-radiation computed laminography (SRCL) to gain insights into the processes occurring in the fracture process zone (FPZ). This work also evaluates the applicability of a non-local version of the Gurson-Tvergaard-Needleman (GTN) ductile damage model for predicting the effects of constraint. Correlations between fracture values and constraint-parameters, such as the T-stress, Qparameter and stress triaxiality h are derived. Finally, an empirical relation is proposed to account for the loss of crack-tip constraint in the engineering assessment of cracks in components made of ductile cast iron.