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Hardness-induced elastic, plastic, cracking observations

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A useful stress – strain method is illustrated for comparison of the elastic, plastic, and cracking behaviors exhibited in hardness tests done on a number of crystal, polycrystal, and composite materials [1]. Hertzian-type elastic modulus values are able to be easily determined from either loading or unloading curves obtained in continuous nano-indentation hardness tests as compared with macro-indentation test results [2]. And for nano-indentation testing, there is the issue of determining the highest Hertzian pressure that a material can support before plastic flow is initiated. For indentation-induced plastic deformation behavior, there is the opportunity stemming from early micro-hardness test results obtained on MgO (001) crystal surfaces to probe dislocation pile-up stress concentrations and crack-forming reactions [3]. Such crack size dependence on hardness load connects with specification of indentation fracture mechanics parameters and, based on comparison with conventional fracture mechanics test results, prediction of material failure conditions [4].


1. R.W. Armstrong and W.L. Elban, “Macro-to-nano-indentation hardness stress-strain aspects of crystal elastic, plastic, and cracking behaviors”, Exp. Mech., 50 (2010) 545-552. 2. L. Ferranti, Jr., R.W. Armstrong, and N.N. Thadhani, “Elastic/plastic deformation behavior in a continuous ball indentation test”, Mater. Sci. Eng. A, 371 (2004) 251-255. 3. R.W. Armstrong and C. Cm. Wu, “Lattice misorientation and displaced volume for microhardness indentations in MgO crystals”, J. Amer. Ceram. Soc., 61 (1978) 102-106. 4. R.W. Armstrong and O. Cazacu, “Indentation fracture mechanics toughness dependence on grain size and crack size: Application to alumina and WC-Co”, Intern. J. Refract. Met. & Hard Mater., 24 (2006) 129-134.

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