Miniaturized ring-type creep testing: a synthesis of theory and practice

• Presents the first unified framework for converting ring-type specimen creep data to uniaxial properties. • Introduces Equivalent Gauge Length as a novel metric for superior sensitivity in miniature testing. • The two-bar specimen uniquely bridges secondary creep and full rupture life prediction. • A novel comparative selection matrix guides optimal specimen choice for fracture and life assessment. This work provides a comprehensive synthesis of bending-tensile-dominated, ring-type miniature specimen creep testing methods, including closed rings, open (C-shaped) rings, and two bar structure. Based on the published work, we present a unified theoretical framework, based on the theory of the reference stress method, for converting measured creep responses of non-standard miniaturized ring-type specimens into equivalent uniaxial creep properties, enabling derivation of geometry-independent conversion parameters via the critical concept of Equivalent Gauge Length (EGL) for general non-uniaxial specimen types. The framework is rigorously validated against experimental data for high-temperature steels and nickel-based superalloys, demonstrating universal efficacy for determining both minimum creep strain rates and full-stage creep rupture (fracture) properties. Practical implementation guidelines and a comparative selection matrix are provided to facilitate optimal specimen choice based on specific testing objectives and material constraints, serving as an essential reference for researchers and engineers engaged in fracture and life assessment applications across energy and aerospace sectors.