We can't use a tensile test to determine the strength of ceramics, because it is difficult to prepare specimens accurately, specimens often break in the grips and ceramics break after 0.1% strain, meaning that the specimen must be perfectly aligned to avoid any bending stresses.
Usually ceramic strength is tested using a four-point bend test.
Ceramics have compressive strengths about ten times higher than their tensile strength. The tensile strength of ceramics and glasses is low because the existing flaws (internal or surface cracks) act as stress concentrators. This is because flaws do not propagate under compression. As a result of this, ceramics are usually used in applications where loads are compressive.