Abstract
The strength of a brittle material is not a singular value, but a distributed set of values that reflect the large variety of flaws (types and sizes) that are incorporated during processing. The distribution of strength values obtained during testing of ceramic specimens, all made at one time, are generally characterized by statistical parameters that vary with the processing method and processing period, That is, most manufactures cannot control the nature of the flaws they inadvertently incorporate during processing. Proof testing, i.e., the pre-stressing of a component to a specific value, can be used to truncate the statistical distribution, therefore defining a minimum, or threshold strength for components that do not fail the proof test. Although there is added cost to proof testing, it allows the designer to ensure reliability.
As reviewed below, a threshold strength can also be obtained by incorporating periodic compressive layers within a brittle material. It is shown that the compressive layers can stop cracks larger than a given size, and an increasing stress must be applied to 'push' the crack through the compressive layers to cause catastrophic failure. Factors that affect the threshold strength are reviewed.