A finite element model is performed for the free and forced vibrations of the laminated rectangular composite plate exposed to a steady state hygrothermal environment. The Mindlin plate elements which include the effects of transverse shear and rotatory inertia are used to study the vibration problems of thick composite plates with hygrothermal effects. A parametric study is done for a symmetrically [0°, +45°, -45°, 90°]2s laminated, T300/5208 graphite-epoxy plate with both simply supported and clamped boundary conditions. The results show that the steady state hygrothermal environment has degradational effects on the natural frequencies of a laminated composite plate.
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