In the process of static mechanical testing (tension, bending or
compression) ceramic materials as a rule suffer a brittle fracture at
stress lower than yield stress. In other words, stress-strain curves
can not be constructed by static mechanical tests.
Along with this stress-strain curves for ceramics can be obtained
under shock wave loading. In the process of shock wave loading a
certain delay of macroscopic fracture is observed, and modern
technique permits to register the curves of volume compression which
are practically the stress-strain curves at high loading rate and are
indicative of the availability of a certain microplasticity in
ceramics.
At the same time static indentation makes possible the plastic
deformation of ceramics.
On this ground the stress-strain curves for ceramics can be
constructed by indentation using diamond pyramidal indenters with
different angles at the tip by the method developed by the author with
coworkers.
It is known that in metals an essential dependence of hardness and
yield stress on the deformation rate is observed, if mechanical
testing is carried out at room temperature. However, as it follows
from the author`s theory of the temperature dependence of yield
stress, the dependence of yield stress on strain rate for ceramic
materials at room temperature shall be weak because the characteristic
deformation temperature T* for them is very high (usually higher than
1000 C), and at room temperature the processes of thermal activation
have no important influence on dislocation mobility and
microplasticity. Therefore the comparison of hardness measured under
static loading with dynamic yield stress obtained under shock wave
loading can be considered to be correct.
Indeed, the comparison of our stress-strain curve for SiC ceramic
obtained by indentation method with stress strain curve of R.Feng,
Y.Gupta and G.Yuan obtained by shock wave loading has shown a very
good correlation.
The values of Hugonio Elastic Limit (HEL) determined from both curves
are very closely related.
It was shown that determination of Vickers hardness HV with
simultaneous determination of the plasticity characteristic f obtained
by indentation makes possible to estimate the dynamic yield Y by the
equation Y/HV = A -- Bxf, where A =~ 0.48 and B =~ 0.27. In common
case the ratio HV/Y decreases with the diminution of f, i.e. with
increasing the share of elastic deformation, HV/Y =~ 2 for purely
elastic deformation and H/Y =~1 for elastic deformation in conformity
with indentation theories.

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