Reliable, flaw-tolerant brittle materials are produced by incorporating
layers under residual compression on the surface and throughout the bulk
of the material that act to trap and contain the propagation of otherwise
catastrophic cracking. The residual compression within these layers acts
to reduce the stress intensity of the cracks, thereby causing them to
arrest until further loading is provided. This highly desirable stable,
subcritical crack growth mode persists with increased loading until the
applied stress is large enough to drive the crack completely through
compressive region, after which failure occurs. The exact level of stress
needed to accomplish this is dictated by the architectural design of the
compressive layers such that the material can be designed to have any
minimum strength desired, within the limits of the materials system used.
This results in a truncation of the strength distribution, such that there
is virtually zero probability of failure below this minimum value, i.e.
the material possesses a threshold strength. Consequently, sensitivity to
flaws that would ordinarily cause catastrophic failure at stresses below
the threshold strength is eliminated. Furthermore, the potential exists
for the complete elimination of the strength variability, hence improving
reliability, through the creation of nearly deterministic, i.e.
single-valued, strengths by increasing the threshold strength above the
stresses at which failure normally initiates from intrinsic flaws.
