Sigmoid-FGM rule has two different relations for top and bottom surfaces. Hence, the value of the volume fraction index depends on the studied section. For example at the top surface of the FGM plate (when: 0
It is necessary that the properties of metal and Ceramics are identical (K in this case will have no influence), there are models (material properties) where never a sigmoid plate will be brought to a homogeneous plate.
One solution may be useful, is to change the expression of your model (material properties).