A nonlinear finite element analysis of buckling and postbuckling behavior of I-and hat-stiffened laminated composite flat panels is presented. Progressive failure analysis is also conducted to predict failure characteristics and postbuckling compressive strength. In the nonlinear finite element formulation, the updated Lagrangian scheme based on the second Piola-Kirchhoff stress tensor and the Green strain tensor is used. For the finite element modeling of stiffened panels, the eight-node degenerated shell element is utilized. For the progressive failure analysis, the maximum stress criterion is applied to the average stress in each layer of all the finite elements and stiffness modulus and stress corresponding to the each failure mode are reduced to zero. The behaviors of stiffened panels loaded in axial compression with various stiffener spacings and cap thicknesses of I-stiffener are investigated. The postbuckling compressive strength of stiffened panels is much higher than their initial buckling stress. The initial buckling stress and postbuckling compressive strength of hat-stiffened panels are higher than those of I-stiffened panels since local buckling deformations in skin and stiffener of hat-stiffened panels are small. Once first compression fiber failure occurs in the stiffener, the stiffness of stiffened panel decreases abruptly and catastrophic failure occurs. While increasing cap thickness of I-stiffener increases only initial buckling stress, decreasing stiffener spacing increases both the initial buckling stress and postbuckling compressive strength.
