TIME DEPENDENT DEFORMATION DURING INDENTATION TESTING B. N. Lucas*, W. C. Oliver*, G. M. Pharr**, and J-L. Loubet*** *Nano Instruments, Inc., 1001 Larson Drive, Oak Ridge, TN 37830, nano@nanoinst.com **Rice University, Department of Materials Science, Houston, TX, 77005 ***Ecole Central de Lyon, LTDS, URA CNRS 855, BP 163, F-69131 Ecully Cedex, France ABSTRACT Constant loading rate/load indentation tests (i/P dP/dt) and constant rate of loading followed by constant load (CRL/Hold) indentation creep tests have been conducted on high purity electropolished indium. It is shown that for a material with a constant hardness as a function of depth, a constant (1/P dP/dt) load-time history results in a constant indentation strain rate (1/h dh/dt). The results of the two types of tests are discussed and compared to data in the literature for constant stress tensile tests. The results from the constant (1/P dP/dt) experiments appear to give the best correlation to steady-state uniaxial data. INTRODUCTION The response of a material to an applied stress will in general be a function of several parameters, e.g., the prior strain, the imposed strain rate, the microstructure and the temperature. At a given temperature and stress, given enough time, there is strong evidence that a steady state strain rate and microstructure can be reached in some materials. Any perturbations in these parameters will typically result in a transient period during which the microstructure will evolve to a new state representative of the new set of conditions. Given enough time, steady state conditions can again be reached. While these transient periods are very important, they are not very well understood. The purpose of this study is to contribute to the understanding of how to use indentation testing to measure the time dependent mechanical properties of materials. The indentation strain rate is defined as the instantaneous descent rate of the indenter (dh/dt) divided by the displacement at that instant in time (h) [1]. Experimental observations indicate that the indentation strain rate may not be sufficient to completely predict the measured hardness. This is the expected result for any material for which the strength is not uniquely related to the strain rate as described above. The strain-time history that precedes the strain rate and resulting hardness measurements appears to be important as well. In most types of indentation creep tests, the indentation strain rate and hardness continuously change during the experiment. For instance, in a constant load creep test the indenter is loaded at a specified rate and the load is then held constant for a period of time while the displacement is monitored. The indentation strain rate changes both during the loading and the subsequent hold segment under constant load. While the deformation under a pyramid shaped indenter has typically been viewed as being geometrically similar from a time-independent point of view, this type of test does not yield geometrical similarity from a rate-dependent point of view. That is to say, as the deformation proceeds under the indenter, the strain-rates experienced by geometrically similar portions of material change as the deformation proceeds. It is therefore desirable to perform an indentation experiment during which the indentation strain rate and therefore hardness remain constant. DEVELOPMENT OF A CONSTANT INDENTATION STRAIN RATE EXPERIMENT The technique for conducting constant indentation strain rate experiments can be developed from the equation for the hardness of a material, P P A ch 2 (1) 233 Mat. Res. Soc. Symp. Proc. Vol. 436 01997 Materials Research Society