The material length scale parameter used in couple stress theories is not a material constant

The purpose of this paper is to clarify that a material length scale parameter of the modified couple stress theory is not constant for an especial material and this parameter changes as size of a structure changes. To determine this value, experimental data for all different sizes are required. The experimental results showed that the value of material length scale parameter has been never fixed and is always changing in various dimensions. Therefore, the modified couple stress theory can be applied for micro- and nano-scales, if the material length scale parameter is individually calculated for these scales. Also, hardening and softening effects of the modified couple stress theory are investigated. Material length scale parameters of different structures made of epoxy, copper, titanium and nickel are reported corresponding to their geometries. Results indicate that geometry especially thickness or diameter has significant influence on the material length scale parameter and the softening and hardening behaviors can be satisfied by the modified couple stress theory.