Answer Happy

12.12. The surface energy of a glass can be measured by carrying out a zero creep experiment, where a glass fiber is suspended in a furnace and is heated. As a result of gravity, the wire will extend to a final equilibrium length leg, beyond which no further increase in length is measured. (a) What do you suppose keeps the fiber from extending indefinitely? (b) Show that at equilibrium leq = 27sv/Preq& where g and r are. respectively, the gravitational constant and the density of the glass; req is the equilibrium radius of the wire. Hint: Write an expression for the total energy of the system that includes grav- itational and surface energy terms, and minimize that function with respect to I. Creep, Subcritical Crack Growth, and Fatigue 439 600 500 400 Stress, (MPa) 300 100 hrs 200 100 0 1200 1400 1600 1800 Temperature (K) Figure 12.18 Fracture mechanism map for SizN4, for which the properties are listed in Prob. 12.11.