Chapter 9 - Strength and engineering design

Summary

To ensure the use of mechanically reliable ceramic components in technological applications, it is critical to establish an approach that can be incorporated into the engineering design process. In this chapter, the emphasis will be on the use of strength data in designing reliable ceramic components. After briefly describing strength measurement techniques, the use of failure statistics will be introduced. Finally, the time dependence of strength will be considered. As seen in the previous chapter, cracks can grow sub-critically in ceramics, causing strength to decrease with service time.

Strength testing

A common technique for the strength determination of a ceramic component is the bend (flexure) test. This approach has been popular as it involves simple specimen shapes. This is particularly useful when the specimen is machined from larger production units. The loading configuration is usually either three- or four-point bending and American Society for Testing and Materials (ASTM) standards are available for both approaches (ASTM C 1161, 1990). Assuming the material fails in tension, the bend strength is determined from the maximum applied tensile stress, using Eq. (4.6). The bending configuration is statically indeterminate and, thus, the stress equations assume that the material is linearly elastic. Four-point is often preferred over three-point bending as the specimen has a larger region under the maximum stress.