Equivalent single layer theories for composite and sandwich structures: A review

This article describes one type of plate theories called equivalent single layer theories that are characterized by a single approximation of the displacements through the thickness. This approach is also used for beams and for shells. The presentation focuses on the basic assumptions of the theories to bring out similarities and differences and to organize the existing approaches in a logical manner. Two broad categories are retained. In the first one the displacement field is expressed in terms of polynomials functions of the transverse variables while in the second category, non-polynomial functions are used. In each category, theories are grouped in terms of the number of unknown variables to be determined. This is a very active research area and it is shown that most theories retain only three, four or five variables even though more terms can be added.