  This paper focuses on the isogeometric vibration analysis of curvilinearly
stiffened composite panels. The stiffness matrices and the mass matrices are
derived using the first-order shear deformation theory (FSDT). The present
method models the plate and the stiffener separately, which allows the
stiffener element nodes to not coincide with the plate shell-element nodes. The
stiffness and mass matrices of a stiffener are transformed to those of the
plate through the displacement compatibility conditions at the plate/stiffener
interface by interpolation using NURBS basis functions. Cutouts are modeled
using a single NURBS patch generated by creating a ruled surface between two
curves. The proposed formulation is first validated by comparing it with
available literature. The effects of width-to-thickness ratio, fiber
orientation, ply layups, shape and size of the cutouts and the boundary
conditions on the response of stiffened composite plates are then analyzed and
the numerical results are used to derive useful conclusions.
