Strain rate behavior of pure aluminum in conical indentation with different indenter control methods

Abstract

Strain rate effect of strength is a crucial factor for material characterization. Attempts have been made to evaluate strain rate effect by indentation tests. An indentation causes a non-uniform stress and strain field inside a specimen. This must induce a non-uniform strain rate field. However, little has been reported about strain rate distribution beneath the indenter. So far, various indenter control methods have been used. In previous studies, no direct comparisons were available as to how strain rate distribution was affected by different control methods. In this study, we report on the strain rate effect of indentation with two indenter control methods: Constant loading rate (CLR) and constant indentation strain rate (CISR). The finite element method was designed to reproduce deformation caused by a conical indenter of a half apex angle of 70.3°. Pure aluminum (99.999 mass% purity), which showed high strain rate dependence of strength, was chosen as a specimen. Material properties were obtained from low strain rate (10-4, 10-2/s) to high strain rate (102/s) tests, and results were incorporated into a FEM analysis using the Cowper-Symonds equation. Four constant loading rates (from 0.7 to 350 mN/s) and constant indentation strain rates (from 0.006 to 6/s) were used, and both results were compared. Differences between both indenter control methods were displacement-dependent. Loading curvature, which has been defined as a material constant in the indentation, was calculated from load divided by square of displacement. Although loading curvatures were decreased with increasing displacement for CLR, they were constant for CISR. Results also showed that values of strain rate decreased as displacement increased for CLR, whereas they were the same for CISR. Similarities of both indenter control methods were found as follows. The highest strain rate regions were observed at the edge of the indenter. In addition, higher strain rate region was distributed hemispherically from the edge of the indenter.