(a) Both cleavage and twinning in iron are favored by increasing the strain rate. Rationalize this statement. (b) Describe the development of a cup?and?cone fracture in a tensile specimen.

Question

What will be an ideal response?

Answer 1

Solution
Both fracture and twinning have effective stress levels which must be exceeded in order for them to occur. These stress levels are normally lower than the stress required for slip. When slip occurs, it tends to relax the stress and keep it from reaching the fracture stress or the stress required to form twins. On the other hand, the slip process is aided by thermal activation. In other words, the applied stress is assisted by the thermal energy in overcoming obstacles that impede the motion of dislocations on their slip planes. At high strain rates, the stress needed to make the metal deform has to increase because the time that a dislocation waits at an obstacle is reduced, and the thermal contribution to the energy needed to overcome the obstacles is less. In simple words, increasing the strain rate increases the flow stress for slip. This increase in the stress level favors twinning and fracture.