Compression

Compression
The strength of ceramic materials measured in compression is much larger than that measured in tension? As a result the tensile strength is generally cited as the critical parameter for purposes of design. However, there are a number of applications including biaxially prestressed engine bearings, armor, and bioceramic components for bone replacement, where the main loading mode is in compression. The successful utilization of ceramic components in these applications requires reliable estimates of compressive strength.
Compressive failure is thought to involve the combination of damage in the form of microcracks and microvoids, this damage is thought to be generated by localized microplasticiity arising from twinning and slip.
Failure occurs by structural collapse when the density of the damage zone reaches a critical size. Because this size appears to depend only upon intrinsic material characteristics, the compressive strength is generally thought to be independent of specimen size. For the case of uniaxial testing, the compressive stress ?c is given simply by the applied load P divided by the cross-sectional area A. However, the high stresses required for failure under compressive loading have created a number of problems in the design of test fixtures and specimen geometries required for successful compression strength measurement. In particular, problems related to improper alignment and load block stress concentrations can lead to the generation of tensile stresses in the specimen sufficient to cause failure. Therefore, the measured compressive strength will underestimate the true value. The possible error sources are.
(1) load block/specimen size mismatch, (2) load block/specimen agreement mismatch, (3) surface irregularities, and (4) unusual