Fracture, brittle controlled

Ductility is the plastic response to tensile force. Plastic response, or plasticity, is particularly important when a material is to be formed by causing the material to flow during the manufacture of a component. It also becomes important in components that are subject to tension and compression, at every temperature between the lowest service temperature and the highest service temperature. Ductility is essential for steels used in construction of reactor pressure vessels. Ductility is required because the vessel is subjected to pressure and temperature stresses that must be carefully controlled to preclude brittle fracture. Brittle fracture is discussed in more detail in Module 4, Brittle Fracture. 

In ceramics, brittle fracture is controlled by the extension of small flaws which are dispersed in a material or component s surface, and which behave like cracks. Flaws can arise not only from the production process, but also from handling and service. Some examples of critical flaws are shown in Figures 12.1 and 12.7. 

J. J. Petrovic, and M. G. Mendiratta, Fracture from Controlled Surface Flaws, Fracture Mechanics Applied to Brittle Materials, ASTM STP 678, ed. S. Freiman, (ASTM, West Conshohocken, PA, 1979), pp. 83 102. 

So why aren t today s engines made of ceramics The short answer is that, unlike metals, ceramics cannot bend and deform to absorb impacts. Intense research is currently under way to solve the problem of ceramic brittleness, with some success. Improved resistance to fracturing, for example, can be attained by careful quality control of starting materials and processing. As we shall see in the next section, brittleness can also be combated by compositing ceramics with other materials. 

Aging does not modify the value of E significantly (except in the close vicinity of Tg), so that aging effects on fracture properties are relatively low in the brittle regime, except if defects are created. This is the case in polyesters (osmotic cracks), where durability is controlled by this process rather than by chain scission or any other structural change at the molecular or macromolecular scale. 

---