Strain Hardening vs Strain Softening

A perfectly plastic material or Stain Venant material is the one where the stress cannot rise above the yield stress and strain can continue to accumulate without any change in the stress level.

Rocks do not generally behave as perfectly Plastic during Plastic deformation. Strain rate is likely to have an effect, and stress level is likely to change during the deformation history. If we have to increase the applied stress for addition strain to accumulate, than we are dealing with the phenomenon called work hardening or strain hardening.

• Strain hardening means that the stress necessary to deform the rock must be increased for strain to accumulate, because the rock becomes stronger and harder to deform.
• Strain hardening is related to the deformation at the atomic scale. Atomic scale defects known as dislocations form and move.
• (metals) just bend a metal wire, and then try to bend it exactly back to its original shape, it will be difficult as the bend part is hardened. It takes more stress to make the original shape

If there is no strain hardening than material keeps deforming without any increase in the applied force or stress, then the process is called creep. Steady-state flow may imply that dislocation movements are quick enough that strain can accumulate at a constant rate for any given stress.

• Strain softening is the case when less stress is required to keep the deformation going.
• Ex- effect of grain size reduction during plastic deformation or mylonitization. Grain size reduction makes the deformation mechanism such as grain boundary sliding more effective because of the increase in grain surface area.
• Other factors can be new and weaker minerals, introduction of fluids and increase in temperature.

Strain hardening/softening are also used in connection with the growth of brittle deformation structures like deformation bands. Strain hardening can occur during deformation of unconsolidated sand or soil, where interlocking of grains may lead to strain hardening.