4. In general, materials fracture when the external acting force is greater than the bond strength. (20)
1) Briefly state why fracture usually occurs at a strength much lower than the theoretical strength. (5)
2) Write why most ceramic materials exhibit brittle fracture, whereas most metals exhibit ductile fracture. (5)
3) List and briefly explain two examples of how external stresses or environmental conditions can cause fracture even at stresses below the yield or tensile strength of a typical material.