We consider the mechanical response of a quasi-brittle material
under a general, three-dimensional stress state. The material is assumed
to contain a large number of penny-shaped cracks with different
sizes and orientations. As in previous work (Addessio and
Johnson, 1990; Zuo et al., 2006), the size distribution of the cracks
is assumed to remain isotropic (i.e., independent of the crack orientation)
and exponential during loading. Under such assumptions,
the probability density function (pdf) of the crack numbers can
be written as (Addessio and Johnson, 1990)