An analytical model for power law impression creep

Impression Creep Test (ICT) employing a rectangular indenter is a novel miniaturized creep testing technique popularly used to characterize the secondary creep properties of materials. The test holds numerous advantages, including less deformation constraint compared to the conventional ICT systems with circular indenters, the requirement for small amounts of materials for sampling compared to conventional uniaxial creep tests, and the capability to perform several stepped tests on the same specimen. Nonetheless, data interpretation from such tests remains a key challenge due to the requirement to perform extensive FE analyses to correlate the measured impression creep response to the uniaxial creep behaviour. To alleviate this challenge, the present work established, for the first time, a mechanistic-based theoretical framework to represent impression creep deformation behaviour and convert ICT data into equivalent uniaxial creep properties using closed-form analytical solutions. The model was formulated based on the expanding cavity theory for power-law creep solids and following the principles of energy conservation for a semi-infinite medium, which can be applied to a finite medium via a correction function for practical impression specimen geometry. The model was calibrated and validated through numerical analysis and experimental data obtained from actual impression creep tests at high temperatures. Our model reasonably captured the global deformation behaviour and produced uniaxial creep parameters closely matching those obtained from standard uniaxial creep tests, indicating that it can be applied to conveniently extract the uniaxial power-law creep parameters for a given material from the experimentally measured impression creep response. The proposed analytical approach also supports ICT standardization and enhances the design of efficient testing programs.