What is the effect of strain rate on yield pressures that are obtained from heckel plots ?
Dear Aditya Singaraju , since the yield pressure value 1/k in the Heckel plot is a material constant of the solid material to be compacted, it would be reasonable to assume, at least in principle, a similar effect of the strain rate as known in forming for similar materials. In hot metal forming increasing strain rates lead to higher flow stresses as well as yield stresses, whereby in cold metal forming this dependency is negligible. Since the Heckel plot is derived for powder compaction and the material changes desity, it could turn out, that a suitable 1/k value requires a dependency of the pressure as well. It might be helpful to mention about the material you want to look at.
Possible, the paper "The 3-D model: Does time plasticity represent the influence of tableting speed?" by Katharina M. Picker (AAPS PharmSciTech., 2003, Vol. 4, N 4 ; DOI: 10.1208/pt040466) will be useful for you. In this paper 3-D data plots were prepared and analyzed using pressure, normalized time, and porosity according to Heckel.
Best regards
To my knowledge Heckel plot shows the relation between volume reduction of the powder compact and the applied pressure. It implies that the pores become smaller (or you get better compaction) as the applied pressure increases. Pore size reduction is primarily dependent on the mass transfer at the powder/binder/powder interfaces which is mainly governed by diffusion. With the assumption that the pressure rate is being increased at an appropriately high temperature, up to the saturation limit of pressure the mass transfer rate increases which implies that the compact size reduction rate also increases (this is a measure of strain rate in the compact). In short, as the applied pressure rate is increased, the slope of Heckel line (region II) also increases, up to a saturation limit.
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