Strain hardening and strain rate characteristics of uniaxial cold compacted salt powders

Abstract EN

The influence of strain hardening and strain rate on the cold compaction of a variety of salt powders is investigated in a series of dynamic uniaxial compression tests.

Strain rate range of 10~3 to 105 s'1 is covered for sodium chloride, potassium bromide, calcium phosphate, and copper sulfate powders.

These materials have been assessed using the constants derived from the power law given as: a = Benim where a is the axial stress (pressure) at the induced strain e, e the strain rate, n the strain hardening exponent, m the strain rate sensitivity exponent, and B a constant.

The investigation show that as the strain rate increases the values of the strain hardening exponent n, the strain rate exponent m, the flow stress and the strength coefficient E are increased in a non-linear manner for all materials tested.

These variations are attributed to a change either from ductile to brittle behavior or a reduction in the amount of plastic deformation due to the time dependent nature of the plastic flow.

This, however, is explained in terms of dislocation and diffusion processes responsible for plastic deformation mechanisms during the compaction process.

At low speeds the dislocation mechanisms are dominant, soon replaced by some form of diffusion process at higher speeds (rates).

For the materials tested, the brittle crystalline copper sulfate is found to be the most strain hardening and the least strain rate sensitive material, while potassium bromide is found to be the reverse (i.e.

the least strain hardening and the most strain rate sensitive material).