A unified modified couple stress model for size-dependent free vibrations of FG cylindrical microshells based on high-order shear deformation theoryYuewu Wang, Ke Xie, Tairan Fu and Wei ZhangEur. Phys. J. Plus, 135 1 (2020) 71DOI: https://doi.org/10.1140/epjp/s13360-019-00012-3