A method to determine the spherical indentation contact boundary diameter in elastic–plastic materials

•The spherical indentation contact edge was studied for various materials.•FEA models exhibit the same deformation characteristics as experiments.•The contact boundary coincides with peaks on the indentation slope-rate profile.•A method to determine the spherical contact boundary is proposed.•The contact boundary can be directly measured from an unloaded indentation.

Significant measurement differences in Brinell spherical hardness test results have been continually observed worldwide largely due to the edge of the indentation not being a distinct boundary. The boundary is instead a curved surface from either material piling up (pile-up) or sinking in (sink-in), making it difficult to clearly resolve the edge of the indentation and thus to determine the indentation diameter from the optical microscope measurement. The indenter/material contact boundary under the test force should be the basis for the Brinell spherical indentation diameter; however, the contact boundary cannot be observed using an optical microscope after the indenter is removed as is required by the test methods. It is also a critical issue to derive the contact diameter from load depth relations in the widely used instrumented indentation. In this study, finite element analysis (FEA) was used to study the contact location at the indentation boundary. Meanwhile, Brinell hardness indentations were made and measured. The characteristics of the indentation profiles from the experimental measurement showed the same trend with that from FEA models. Various parameters that would affect the indentation pile-up or sink-in conditions, including the material’s strain hardening, ratio of Young’s modulus to yielding stress, indentation depth and friction, were studied from the FEA model. A physical measurement method is developed to determine effectively the indentation contact position. Applying the new method, the deviation of the measured indentation diameter from the actual contact diameter was estimated for each indentation.