SUMMARY

Spherical indentation has been shown to be a reliable high throughput alternative to capture the complete elastic-plastic response of polycrystalline metal alloys when using the Pathak-Kalidindi (P-K) protocol. However, yielding initiates subsurface and due to the hydrostatic pressure associated with the constraint of the surrounding material, this occurs at a higher stress than it would for the same material under uniaxial load. Hence, the indentation stress is higher than uniaxial stress and the two are related by a scaling factor, referred to as the constraint factor. In the current work, some of the open questions in the use of spherical indentation to extract uniaxial stress-strain curves are addressed. The dependence of the mechanical properties of the material and the indentation strain on the constraint factor is investigated using FEA and experiments. Based on the P-K indentation strain definition, revisions are proposed to the classical equations for: 1) the representative uniaxial strain, which relates the indentation strain to an equivalent uniaxial strain and 2) the non-dimensional strain, which relates the indentation strain to the constraint factor. From these revised definitions, an inverse method has been developed using FEA simulations to estimate the uniaxial stress-strain from spherical indentation stress-strain curves. The inverse method is verified with FEA generated indentation stress-strain curves and experiments conducted on Al7050 and Al6061 samples of varying strengths and hardening behaviors. Finally, spherical indentation of a material exhibiting anisotropic mechanical response is studied. The uniaxial and spherical indentation mechanical response of samples of Inconel 718 that were additively manufactured using electron beam melting are critically evaluated. The variation of the experimentally obtained elastic spherical indentation response as a function of the orientation of the material is verified using FEA simulations.