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Chapter VIII Micro-hardness studies…
saucer-shaped configuration, modified into some extent by the presence of
median vents. In certain cases fringes are generated by the interference
between slightly mismatched, simultaneously diffracting portions of crystal
across the crack interfaces. These are considered as the thickness fringes [96].
Hockey and Lawn [96] studied cracking around indentation by electron
microscope in aluminum oxide and silicon carbide (SiC). In semi quantitative
information on geometry of residual cracks about micro-indentations, they
obtained interesting information. The information obtained is related to the
history of the crack propagation and not the initiation of the cracks. The electron
microscopy is not capable to tell how the cracks originally nucleated and
formed. It has been known from the etching examination [92] that median and
lateral-vent initiation both tied up intimately with events within gross deformation
zone immediately surrounding the indenter, unfortunately, this region of lost
resolution does not give clear pattern. The authors have also discussed in
detail the important aspect of crack healing. Lawn and Fuller [97] suggested
that the toughness, Kc can be related to the indenter load P, and radius of
median crack, C, by
315
Kc = c P C 3/2
(8.14)
Where c is the number dependent upon the indenter geometry for Vickers
indenter, it is assumed to be 0.0726.
Evans and Charles [98] have employed the dimensional analysis and
obtained correlation with conventional fracture toughness measurements. In
this analysis the functional dependence on the E/H is introduced to account for
elastic/plastic stress with indentation
Kc = (E/H) 0.4 P C 3/2 (8.15)