Microscopic Digital Image Correlation Analysis of Strain Distributions in Stainless Steel 316L Induced by Multiaxial Cyclic Loading

Fatigue damage in materials results in localized strain at the microstructural level. In many engineering components of the cooling circuits of nuclear power plants, where austenitic steels are used, the material experiences multiaxial cyclic loading, either proportional or non-proportional. The load path applied influences the fatigue life of austenitic steels that are characterized by significant additional hardening under non-proportional loading. Under multiaxial loading conditions, the effect of cyclic load paths is primarily observed at the microstructure, leading to different strain distributions at the grain scale. To quantify the strain distributions at the material microstructure in situ, the three-dimensional Digital Image Correlation (3D-DIC) method is required. However, achieving the necessary spatial resolution for capturing the local deformation response at the microstructural scale using 3D-DIC is challenging due to the limitations of available patterning techniques.The work presented here proposes the application of in-situ 3D-DIC analysis to investigate the grain-scale strain distribution on a Stainless Steel 316L using a high-resolution optical imaging system. The limitations regarding the patterning method were overcome by developing a novel approach employing soft-thermal Nanoimprint Lithography (NIL). This patterning method enabled the generation of micro-scale speckle patterns from pre-designed image templates appropriately scaled for the length scale of interest with optical contrast for DIC analysis. By employing the developed patterning methodology and microscopic 3D-DIC measurements, the effect of the load path on the strain distribution in microstructure under uniaxial, multiaxial (axial and shear) proportional, and non-proportional cyclic load paths in the Low Cycle Fatigue regime was investigated in-situ. The DIC data were linked with the microstructural characteristics of the investigated steel using Electron Back Scattering Diffraction. Furthermore, the in-situ 3D-DIC analysis was combined with ex-situ high-resolution DIC (HR-DIC) experiments using Scanning Electron Microscopy performed on the same microstructural region to identify length scale-dependent features and to characterize the slip behavior of Stainless Steel 316L as a function of the applied load path. To achieve this aim, a speckle pattern for HR-DIC measurements was first prepared using a method so-called vapor-assisted gold remodeling. It was followed by the application of a soft-thermal NIL pattern on the top of the gold film for the in-situ-DIC measurements. Once the fatigue experiments and in-situ DIC analysis were performed, the soft-thermal NIL pattern was removed to conduct ex-situ HR-DIC measurements. The in-situ DIC measurements revealed modest differences in the strain distributions between different load paths that would require modeling to better understand their origins. The slip trace analysis based on HR-DIC results revealed that multiple slip system activation is dominant and the Schmid law is mainly followed under uniaxial and multiaxial proportional load paths. While multiple slip systems were also observed in non-proportional loading, the determination of the most probable slip system was not undertaken; indeed, such a determination is not straightforward because it would require a time-dependent analysis of the principal stresses due to the continuous rotation of the principal directions.