Accelerated Creep Testing of Inconel 718 Using the Stepped Isostress Method (SSM)

Abstract

Abstract This paper demonstrates the stepped isostress method (SSM), an accelerated creep test (ACT) for the rapid assessment of metallic materials. The SSM test is based on the time-temperature-stress superposition principle in which temperature, stress, or both, are step increased to accelerate the time to rupture. The SSM test has proven successful for the ACT of polymers and polymeric composites but has yet to be proven for metallic materials. In this study, new test matrix design rules for the SSM of metals are established based on deformation mechanism, time-temperature transformation, and time-temperature precipitation maps. A test matrix of SSM and conventional creep tests (CCTs) are executed for alloy Inconel 718 at 750°C (1,382°F) with stress levels ranging from 100 to 350 MPa. Validation CCT data are gathered from the Japan National Institute of Material Science. Material constants for the Sine-hyperbolic (Sinh) constitutive model are calibrated using the SSM data and employed to predict the conventional creep response. When blindly compared with the CCT data, the SSM-calibrated Sinh model can accurately predict the conventional creep response across logarithmic decades and, thus, accelerate the capture of conventional creep data. Fractography indicates ductile fracture by transgranular microvoid coalescence with the same fracture mode observed in both SSM and CCT specimens. Creep cavitation is indicated by the population, smoothness, and size of microvoids.