Effects of residual stress and surface roughness on measurement of mechanical properties of IN718 by instrumented indentation testing

Material surface state greatly impacts the mechanical properties of materials. Ignoring the factors that influence the surface state may lead to considerable errors in the measurement of material behavior, especially when using instrumented indentation testing (IIT). In this paper, experimental and numerical methods were used to investigate the effects of residual stress, surface roughness, and their combinations on the mechanical response of a nickel-based superalloy IN718 during indentation. Results show that residual stress significantly influences parameters such as the load-displacement curve, pile-up/sink-in patterns and indentation hardness. An empirical formula was then proposed to predict the influence of residual stress on indentation hardness. It is shown that all predictions fall within a ±8% error band in comparison with simulation results. Surface roughness markedly affects the scatter of indentation results. As surface roughness increases, the hardness tends to decrease. To ensure the accuracy of the IIT, the normalized surface roughness Sa/h should be kept below 0.02. The combined effects of surface roughness and residual stress on the scatter of indentation results are additive, causing notable changes in the load-displacement curves. Moreover, their interaction does not follow a simple linear additive pattern, with surface roughness predominantly determining the indentation hardness. It is recommended to adopt a graded management approach for surface roughness and to provide necessary explanations and specifications for residual stress in the current international standard ISO 14577.