Question

Regarding the fracture mechanism of metals, polymers and ceramics

(a) Explain why FCC metals show a ductile fracture even at low temperatures while BCC metals do not.

(b) What is crazing? What can you say about the Poisson’s ratio of the crazed material?

(c) Generally, in ceramics, intergranular fracture is preferred to cleavage fracture; however, in metals, transgranular fracture is the preferred mechanism for brittle failure. Why? Use a physical reason supported with an equation in your answer.

Answer 1

a) Ductility means the ability to get plastically deformed. Plastic deformation happens by movement of slip plane in some favourable slip direction. The number of slip systems in ACC is 12 and in BBC it's 48. As the number of such slip system increases, the dislocation moments are interfered or obstructed in BBC. Hence they are more harder than fcc crystals.

b) Carazing is found in plastic like acrylic nitrile styrene(abs) when stressed. A poisson ratio of 1btween 0.3 to 0.5 is found.

c) The rupture of atomic bonds alone some specific crystallography direction is cleavage. This happens through the grains and is transgranular. This type of fracture is predominantly brittle in nature. Consider a ductile low carbon steel and a cast iron . As the interstulitial carbon contents increases, dislocatons are not able to move around grain boundaries, so a movement of dislocation through grains happens leading to transgranular mechanism