The typical experimental data from literature that show the abnormal grain size dependence. The dashed straight lines
in this figure represent Eq.(2) for those of the data that exhibit the normal Hall-Petch relation in the relative large grain size range.
The curves are calculated by Eq.(8) based on the coherent polycrystal model developed in this work for those of the data that
display the so-called inverse Hall-Petch relation.