The Effect of Pile-Up on Indentation Hardness Evaluation of Neutron-Irradiated 15Ch2MFA Steel

Abstract

Hardness testing is an efficient means for monitoring the change in mechanical properties of irradiated materials. The evaluation of the indentation load-displacement data is commonly based on the Oliver-Pharr method, which estimates the projected contact area between the indenter tip and the material surface using Sneddon's elastic contact solution. This simplified method can lead to significant errors when the indented elastic-plastic material exhibits extensive pile-up around the indenter, which is typical for a material with a smaller strain-hardening exponent n and a smaller yield strength-to-elastic modulus ratio σy/E. Since both these mechanical properties are influenced by the neutron irradiation, one must be careful with the interpretation of measured indentation hardness. In this study, a finite element simulation was used to investigate the effect of pile-up on indentation hardness evaluation. Load-displacement curves, contact areas evaluated by both Oliver-Pharr method and finite element nodes in contact and the corresponding hardnesses were obtained for the 15Ch2MFA (15Cr2MoV) tempered bainitic steel in non-irradiated and neutron-irradiated state. The Oliver-Pharr method underestimates the true contact area, and therefore overestimates hardness by factor of 1.32 and 1.40 for non-and neutron-irradiated state, respectively. Despite this discrepancy, the Oliver-Pharr method as well as direct observation of the contact area are able to indicate the increase of hardness due to neutron-irradiation (15% vs. 8% increase in hardness).