High-Throughput Bending for Investigating Creep Behavior Affected by Structural and Microstructural Inhomogeneities
Prof. Praveen Kumar and Prof. Vikram Jayaram
18th September, 2023 (Monday), 03:00 PM (India Standard Time)
K I Vasu Auditorium, Department of Materials Engineering
The bending of cantilevers is an alternate technique for the extraction of power-law creep parameters of materials. It has been shown earlier that in simple homogeneous systems, digital image correlation (DIC) can provide a high-throughput evaluation of creep parameters by obtaining multiple stress-strain rate pairs from a single test in a geometry that can accommodate small samples and simple fixturing. The present thesis attempts to extend the usefulness of this technique to more complex systems which display large or small-scale microstructural gradients.
In the first example, bending is used in conjunction with DIC and finite element analysis (FEA) to show, that creep deformation in T22 (2.25Cr-1Mo) steels is localized to ~30% of the beam volume near the constrained end for power law exponents ~ 5-7. Therefore, the fabrication of composite cantilevers is proposed as an alternative for minimising the volume of material for testing. The ‘structural inhomogeneity’ is defined in the form of a composite consisting of an active creeping portion (T22) and an additively extended passive non-creeping portion (IN-718). The optimisation process involved varying T22 steel length, ‘a’ while keeping the total sample length, ‘L’, constant. High-temperature DIC (600°C) was used to measure creep for varying a/L ratios. An excellent match between monolithic and composite beam creep behaviour was observed. Moreover, the 5% a/L composite beam showed a good match with monolithic behaviour after stress estimation from FEA while taking care of chamfer effects.
The second example comes from a study of the influence of microstructurally textured regions (MTRs) in a commercial rolled Ti-6Al-4V alloy. Such macroscopic inhomogeneities lead to long-range strain localisation owing to the non-uniform deformation. The well-known tension-compression asymmetry in titanium during the c+a dislocation glide is captured using the small-scale bending of samples at room temperature. The presence of extended MTRs along the rolling direction (RD) lead to a shift in the neutral axis towards the other half of the cantilever. Thus, the anisotropy and asymmetry in creep are readily brought out uniquely in a single experiment via the geometry of bending creep. Polarized light microscopy (PLM) was established as a unique quantative technique to identify and characterise MTRs in large areas. Microstructure correlations have been made using integrated PLM and DIC as a high throughput method to correlate the presence of such MTRs with the creep behaviour at room temperature.
Overall, these experiments serve as a foundation for the study of creep in weldments, joints and other complex material combinations in future.
Acknowledgements: Pratt & Whitney and IMPRINT (Ministry of Education and Ministry of Power)