On the validity of interface indentation technique : Experiments and simulations

Abstract

Indentation is an extremely useful tool to indirectly estimate the strength of materials for nearly a century now since the invention of Brinell method. Though it is confined to indirect measurement of strength till few decades, the advent of “instrumented indentation” made it possible to obtain the elastic properties like elastic modulus but still elusive to determine the characteristic properties like yield strength, ductility, and work hardening exponent which requires a comprehensive understanding of deformation zone underneath the indentation. The experimental determination of subsurface deformation zone by plastic strain mapping proposed by Chaudhri and co-workers is extremely time consuming (because of the several steps involved in it) and sometime may not even possible to perform on brittle materials like ceramics, bulk metallic glasses etc. Interface indentation experiments can be an alternative to this as they are relatively easy to perform on all classes of materials. A large number of these experiments is performed in recent years on bulk metallic glasses also reveals the deformation mechanism which otherwise not possible by conventional constrained indentation experiments. However, there is no validation of the interface indentation experiments barring few early experimental studies by Mullhearn, Tabor, and others who has shown that the plastic zone shape of interface indentation samples is like the conventional ones. Again, there are no systematic investigations clearly describing the role of interface width, state of the material (annealed or work hardened) on the shape and size of the plastic zone. In this work, the subsurface deformation zones of constrained and interface indentation samples under a spherical indentation is investigated. Further the hardness mapping is used to compare the differences in deformation zone size in the work hardened samples. Finite element simulations are performed to understand the shape and size of deformation zone and compared with the experiments. Oxygen-free high-conductivity (OFHC) copper is used in the experiments as it is too easy generate a range of strain hardening exponents by changing the deformation history in these samples. In addition to this OFHC an attractive model material as a wealth of information is available regarding constitutive modelling, microstructure response, mechanical properties and sensitivity to strain hardening to assess the plastic strains.