Comparative assessment of failure strain predictions using ductile damage criteria for warm stretch forming of IN718 alloy

It is vital to envisage accurately the fracture limits of high strength superalloys when noticeable localized necking or thinning tendency is not observed during sheet-metal stretch forming process. The present study mainly focuses on fracture limits evaluation of Inconel 718 alloy (IN718) in the effective plastic strain (EPS) vs. average triaxiality space. First, uniaxial tensile test, to analyze the material properties, were instigated at different test temperatures (RT-700 °C). Subsequently, stretch forming is performed to evaluate forming and fracture forming limit diagrams (FLD and FFLD) of IN718 using Nakazima test. It is observed that forming and fractured limits of IN718 are significantly influenced by variation of processing temperatures (with approximately 65–70% improvement in major safe and fracture strains) in all deformation regions (with respect to RT). In average triaxiality (η) vs effective plastic strain (EPS) space, higher fracture limits of IN718 are noticed in the entire triaxiality path of deformation region. Seven different ductile fracture models, namely McClintock (M-Mc), Brozzo, Rice-Tracey (R-T), Ko, Oh, Cockcroft and Latham (C-L), and Clift, are formulated so as to foresee the fracture loci of IN718 in EPS vs. triaxiality space. Overall, Oh model, showed best predictability at all temperatures with least Average absolute error (AAE < 13.5%).