The mechanical behavior of fiber-reinforced composite thermoplastic sh
eets during forming processes is modelled as a viscous fluid with inex
tensibility and incompressibility constraints. Techniques of linear st
ability analysis are used to study the growth or decay of initial impe
rfections in plane sheets reinforced by two families of fibers subject
ed to biaxial strains. This theory delimits situations when buckles ca
n be expected to form during forming operations.