Using a simplified version of Johnson's core model analysis of the plastic zone, one may determine the plastic zone size around a contact or, alternatively, determine the yield strength by measuring the plastic zone. The theoretical model contains three parameters: indentation load, yield strength and zone size so that knowing any two gives the third. This is experimentally demonstrated for a series of single crystals (Fe-3wt%Si, tungsten, zinc) and polycrystals (1100-0 Al, copper and 2024-T6 aluminum). In addition, two of these are evaluated in several work-hardened states. Plastic zone sizes are estimated principally by atomic force microscopy and Zygo interferometry imaging with some verification by transmission electron microscopy. On the theoretical side, verification of the relationship is obtained by elastic-plastic numerical analysis of a bi-material system based upon an Fe-3wt%Si single crystal with a thin oxide film. It is shown that in general predictions are reasonable down to nanometer level contacts except for two cases where an indentation size effect may dominate. The proposed relationship is suggested to be an alternative measure of yield strength compared to the often cited value of H/3, particularly at light contacts.
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The authors would like to thank Drs Jack Houston and Neville Moody of Sandia Albuquerque and Livermore as well as Karl Yoder at Minnesota for their latest thoughts on indentation size effects. We also wish to thank K. W. Hipps for use of experimental facilities and C. M. Watkins for experimental assistance at WSU. This research was supported by the Center for Interfacial Engineering under grant NSF/CDR-8721551 for five of us (HH, MK, JN and JR) and by the Navy under Grants E-25-T46-51/ONR and N00014-91-J-1998 for four of us (DK, AW, DB and WWG).