2.2. The material properties of S-FGM plates
In the case of adding an FGM of a single power-law function to the multi-layered composite, stress concentrations appear on one of the interfaces where the material is continuous but changes rapidly (Lee and Erdogan, 1995 and Bao and Wang, 1995). Therefore, Chung and Chi (2001) defined the volume fraction using two power-law functions to ensure smooth distribution of stresses among all the interfaces. The two power-law functions are defined by: