EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

It is vitally important to consider time dependent (or “viscous”) deformation behaviors when formulating material models, particularly when elevated temperature conditions are present in the intended application. In many macroscopic modelling attempts, time dependencies are limited to “plastic” regions of deformation. Under such modelling paradigms stresses will, under periods of strain hold (for example), relax back to the yield locus and no further. This is in direct conflict with experimental observations, wherein “sub-yield” stress relaxation is easily demonstrated. Limitations in how materials are typically tested in the laboratory, and thus how material model calibration data is interpreted, result in important macroscopic phenomena being neglected. The chromium steel P91 is considered in the present work and an an-isothermal elastic-viscoelastic-viscoplastic material model is developed over an industry relevant temperature range (400°C – 600°C). The developed model is grounded using the generalized standard materials and thermodynamics of irreversible processes formalisms. Calibration of material parameters is achieved through the application of targeted isothermal waveforms in uniaxial loading conditions. An-isothermal predictive capabilities are confirmed through application of the model to non-standard in-phase and out-of-phase thermo-mechanical waveform, in which strain and temperature rates are varied within a cycle itself. Strain rates between 0.1%/s and 0.001%/s are considered, with corresponding temperature rates ranging from 16.5°C/s to 0.17°C/s The additional effort of calibrating viscoelastic terms is justified by contrasting the model results with those generated by a simpler elastic-viscoplastic model. Many critical components are designed such that they experience extended periods of operation at modest load. Long term time dependencies excited by these conditions, as indicated by full multiaxial implementations of the two models in commercial FEA packages.