Abstract: A fifteen-variables quasi-conforming triangle element technique including transverse shear effect for penalty-finite element method is used in this paper to work out the postbuckling incremental equilibrium equation the corresponding tangent stiffness matrix and nonlinear eigen-equation of composite laminates with inital imperfection. By using numerical calculations, the precision and convergence of the element, the existence of the secondary bifurcation point the effect of boundary conditions, the loading forms and ply configurations on the secondary buckling and failure load are also studied. An effective method is proposed for engineering designers to analyse the instability and even the failure load of structures.

Key words: composite laminates, secondary bifurcation, failure load, quasi-conforming penalty-finite element