Calibration and validation of three fracture criteria for alloy 718 subjected to high strain rates and elevated temperatures

The aerospace industry has an important role to play in lowering greenhouse gas emissions and thereby reducing its ecological footprint. Aerospace manufacturers do therefore push components and materials closer to their limits in order to design for minimal weight. An example of component is the metallic turbine case of the aircraft engine for protection of the surrounding structure in situations such as blade-off events. This implies design, where knowledge regarding mechanical material response from onset of yield to fracture, at both high strain rates and elevated temperatures, are needed. Alloy 718 is a nickel-based superalloy commonly used in the containment structures of the hot parts of the aircraft engines. Three established criteria, Modified-Mohr-Coulomb, Maximum Shear Stress and Magnitude of Stress Vector, have been applied to characterise the fracture behaviour of Alloy 718 supplied to conditions present at a blade-off event. The calibrations of the criteria were based on high-speed tensile tests of thin sheet specimens with different geometries for varying different stress states. The fracture strains were determined using high-speed photography combined with digital image correlation. The temperature and strain rate were varied from 20 to 650 °C and 1 to 1000 s−1 respectively. The calibrated criteria were validated through finite element analyses and reverse impact testing at room temperature up to 650 °C. Discs were fired against a instrumented rod with different shapes of their tips for obtaining varying stress states. It was found that the Modified-Mohr-Coulomb locus showed the best agreement with measured fracture strains in the calibration experiments. This criterion did also predict the fractures of discs in the reverse impact tests fairly well. The possibility to accurately predict fracture facilitates the use of modern numerical software for containment design as a complement to time-consuming and expensive full-scale tests.