Adiabatic Shear Band - Deformation

Deformation

It is necessary to include some basics of plastic deformation to understand the link between heat produced and the plastic work done. If we carry out a compression test on a cylindrical specimen to, say, 50% of its original height, the stress of the work material will increase usually significantly with reduction. This is called ‘work hardening’. During work hardening, the micro-structure, distortion of grain structure and the generation and glide of dislocations all occur. The remainder of the plastic work done – which can be as much as 90% of the total, is dissipated as heat.

If the plastic deformation is carried out under dynamic conditions, such as by drop forging, then the plastic deformation is localized more as the forging hammer speed is increased. This also means that the deformed material becomes hotter the higher the speed of the drop hammer. Now as metals become warmer, their resistance to further plastic deformation decreases. From this point we can see that there is a type of cascade effect: as more plastic deformation is absorbed by the metal, more heat is produced, making it easier for the metal to deform further. This is a catastrophic effect which almost inevitably leads to failure.