Strain-rate sensitivity (SRS) of nickel by instrumented indentation

Abstract

For materials which exhibit a power-law relationship between stress and strain rate, it is theoretically possible to evaluate the exponent (m) which governs the relationship by means of instrumented indentation. However, in practice, tests at small strain rates take so long that the results can easily be dominated by thermal drift. A new test method is developed in which several constant strain rates are examined within a single indentation test by switching strain rates as the indenter continues to move into the material. Switching strain rates within a single test overcomes the problem of long testing times by examining large strain rates first and transitioning to smaller strain rates as the test proceeds. The new method is used to test a sample of fine-grained nickel sold by NIST as a standard reference material for Vickers hardness. The strain-rate sensitivity of this sample is measured to be m = 0.021. This value is in good agreement with values obtained by others on fine-grained nickel using both instrumented indentation and uniaxial creep testing. © The Society for Experimental Mechanics, Inc. 2013.