Fracturing: Brittleness

Abstract

The previous two chapters on mechanical properties described how we test ceramics, their elastic response, and how under certain conditions they can permanently deform. In this chapter we describe why and how ceramics break. The main topics are

• Fracture

• Toughening

• Fatigue

Some of these topics may already be familiar from classes on metals. The exception is probably toughening. Ceramics are not tough. Toughening makes the material absorb energy during fracture by mechanisms such as local phase transformations, plastic deformation near the crack tip, or crack bridging behind the crack tip. Fracture requires cracks. In fatigue crack growth occurs as a result of cyclic loading—even at small loads.

We begin this chapter by showing some of the key equations. The most important work is that of A.A. Griffith, the “Father of Fracture Mechanics.” Griffith showed the importance of flaws, which act as stress concentrators. Because it is almost impossible to make ceramics without flaws they often are the dominant cause of failure. So there is a link between this chapter and Chapter 16.