The hardness and/or yield stress of a material are measures of its resistance to local deformation. For plastic materials the two parameters are directly
K = (C„ + C22 + C33 + 2C12 + 2C23 + 2C13)/9 G = (Cu + C22 + C33 — £-12 — C23 — Q3 + + 3 CS5 + 3CJ/15
related, the yield stress being one-third of the hardness. While hardness is generally measured by means of indentation, yield stress may be determined by more indirect methods, e.g. data from compaction studies. Both will be discussed in this section.
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