Fracture behaviour of Alloy 718 at high strain rates, elevated temperatures, and various stress triaxialities

A methodology for fracture characterisation at strain rates up to 1000 s−1, temperatures up to 650 °C, and various stress triaxialities is presented. High-speed photography combined with digital image correlation is used to evaluate the strain at fracture. The methodology was successfully demonstrated on aged nickel based Alloy 718, commonly used in the containment structure of aircraft engines. Tensile specimens with four different geometries were loaded to get a wide range of positive stress triaxialities. All specimens originated from one single heat and batch to ensure consistent mechanical properties. The results showed evident stress state dependency on the failure strain, where lower failure strains were observed at higher stress triaxialities for all combinations of temperatures and strain rates. A coupled relationship between the temperature and the stress triaxiality controlling the fracture strain was found. However, any clear dependency on strain rate was hard to detect.