“…There are several well-known simplified versions of second gradient theory, such as the couple stress theory ( Koiter, 1964;Mindlin & Tiersten, 1962;Toupin, 1962 ) corresponding to letting the micro-deformation gradient be only rotation gradient, Kleinert strain gradient theory ( Kleinert, 1989 ) corresponding to considering the gradient of volumetric strain in addition to rotation gradient as the micro-deformation gradient, etc. Some other models, like Aifantis ( Altan & Aifantis, 1997;Gao & Park, 2007 ) and Wei & Hutchinson's strain gradient elasticity theory ( Song, Liu, Ma, Liang, & Wei, 2014;Wei, 2006;Wei & Hutchinson, 1997 ), use strain gradient or second gradient of displacement as the micro-deformation gradient and define different constitutive relations. The objective of developing these high order continuum theories is to characterize the size effect well observed when size of structure is decreasing to micro or nano scale ( Fleck, Muller, Ashby, & Hutchinson, 1994;Kouzeli & Mortensen, 2002 ), the microstructure comes into play in this case which is unable to be accounted for by Cauchy continuum theory without microstructure ( Hu, Liu, & Lu, 2005 ).…”