Weibull analysis of ceramics under high stress gradients

The Weibull parameter in of the strength distribution of ceramics under
high stress gradients differs from that for moderate stress gradients.
This is shown for contact loading. Bars were loaded by oppositely
concentrated forcers via rollers. For most investigated materials,
measured contact strengths showed strongly reduced Weibull exponents
compared with those from 4-point bending tests. This was the reason for
a study, in which the effective volumes and surfaces for the two tests
were compared and the influence of the strong stress gradients was
considered. Under the assumption of the Weibull theory being valid, the
effective surfaces and volumes were computed for the normal stress and
the energy release rate criteria. In the second part, it will be shown
that the strongly nonhomogeneous stresses lead to a reduced Weibull
exponent.