“…We have also extended their applicability to study free vibrations of piezoelectric structures for which the electric and the mechanical fields are coupled. It is shown that the numerical results obtained with the CQ4 element agree well with those reported in the literature and obtained by such methods as the boundary element method (Manolis and Beskos, 1988), the FEM (Petyt, 2010;Miranda et al, 2008;Dai and Liu, 2007), mesh-free methods (Bui et al, 2011a,b;Bui and Nguyen, 2011;Kosta and Tsukanov, 2014;Sadeghirad et al, 2009;Cui et al, 2010;Gu and Liu, 2005), and the isogeometric analysis (Valizadeh et al, 2013;Cottrell et al, 2006).…”