High-Throughput Bending Tests for Investigating Creep Behavior Affected by Structural and Microstructural Inhomogeneities

Abstract

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 homogeneous systems showing tension-compression symmetry, digital image correlation (DIC) can provide a high-throughput evaluation of creep parameters. Multiple stress-strain rate pairs could be obtained 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 and tension-compression asymmetry. In the first example, bending integrated with digital image correlation (DIC) and finite element analysis (FEA) demonstrates that creep deformation in a material primarily occurs in approximately 30% of the cantilever volume near the fixed end, especially when the power-law exponent ranges from 6 to 7. As an approach to minimize the amount of material required for testing, the concept of fabricating composite cantilevers has been proposed and validated in this study. The composite cantilever consists of an ‘active’ creeping portion of T22 boiler steel (2.25Cr-1Mo) and an additively extended ‘passive’ non-creeping portion of IN-718. The reduction process involved varying the length of the T22 section, a, while keeping the total sample length, L, constant. The DIC measurements conducted at 600 °C to assess the creep behavior revealed that non-linear analytical expressions for cantilevers could aptly predict the monolithic constitutive steady-state creep laws from the composite cantilever behavior if measurements are made at a critical distance away from the interface. FEA indicates that accurate stress estimations enable predicting monolithic behavior using composite samples with smaller “a/L” ratios (e.g., 5%) also, with regions closer to the interface. Using such an approach, the creep degradation in boiler steel that has been in service for ~ 240,000 h is ascertained using composite cantilevers with a small volume of “active” material. The implications of the observations for estimating the residual life of in-service high-temperature components are discussed.

The second example comes from a study of the influence of microstructurally textured regions (MTRs) in a rolled commercial 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 [101̅0] MTRs along the rolling direction (RD) leads 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. In addition, microstructure correlations have been made using polarized light microscopy (PLM) 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 complex engineering material systems, such as weldments, composites, hybrid joints, etc. and residual life assessment and structural health monitoring.