Parents provide their children with a plastic bowl and not a ceramic bowl at mealtime because the ceramic bowl is fragile. Glass and chinaware break to pieces when they fall to the ground, but things like iron are undamaged even when violently handled. Why? Experientially we understand that some materials are brittle, while others have good toughness. But can we describe "brittleness" or "toughness" in a mathematical language? What methods can we use to measure brittle things, or tough things, in a quantitative manner? In Chapter 2, we established the theoretical basis of material damage, introduced some important concepts such as the stress intensity factor and fracture toughness, and obtained the material damage criterion.