Observations of strain rate-sensitive pile-up during nanoindentation in single-crystal and polycrystalline tungsten

Instrumented indentation offers several fast and convenient ways to characterize the strain rate sensitivity of rate-sensitive materials. One such method involves measuring the indentation hardness at increasing indentation strain rates. A key assumption that is frequently made in these measurements is that the contact area does not depend significantly on the indentation strain rate. In this study, we measure and report the strain rate sensitivity of four samples of single-crystalline and polycrystalline tungsten and show that the strain rate sensitivity is, in fact, influenced by the rate sensitivity of the degree of pile-up and thus the measured contact areas. Calculations show that in this material, there is a direct relationship between the rate sensitivity of the measured contact areas and the error associated with assuming that the contact area does not depend on the indentation strain rate.