Effect of local anisotropy on fatigue crack initiation in a coarse grained nickel-base superalloy

TL;DR

This study investigates how local grain orientation and anisotropy influence fatigue crack initiation in a coarse-grained nickel-base superalloy, combining theoretical modeling with experimental validation.

Contribution

It introduces a model extending Schmid's law to include local elastic anisotropy effects and validates it with experimental SEM-EBSD analysis.

Findings

Grain orientation significantly affects crack initiation sites.

The $E\cdot m$ parameter correlates with shear stress amplitude.

Theoretical predictions align with experimental observations.

Abstract

In the present work, theoretical approaches, based on grain orientation dependent Young's modulus and Schmid factor are used to describe the influence of local grain orientation on crack initiation behaviour of the coarse grained nickel base superalloy Ren\'e80. Especially for strongly anisotropic crystal structures with large grain size, such as the investigated material, the local elastic properties must be taken into account for assessment of fatigue crack initiation. With an extension of Schmid's law, the resulting shear stress amplitude, which triggers local cyclic plastic deformation, can be calculated depending on local Young's modulus and Schmid factor. A Monte Carlo simulation with 100,000 samples shows how random grain orientation affects these parameters. Furthermore, the product of Young's modulus and Schmid factor (called $E\cdot m$) is used as a parameter to determine how grain orientation influences resulting shear stress amplitude for given total strain amplitude. In addition to the theoretical work using that approach, this model is also validated using isothermal LCF experiments by determining local grain orientation influence on the crack initiation site using SEM-EBSD analyses.