Dynamic Behavior of Initially-Stressed and Post-Buckled Laminated Composite Beams

Abstract

The vibration response (natural frequencies and damping) of a preand post-buckled laminated composite beam as a result of an axial compression force is studied both analytically as well as experimentally. In the analytical development, the simply-supported laminated composite beam is modeled using a nonlinear Timoshenko beam theory and solved using the Galerkin method. It was shown that the post-buckled beam vibrational behavior is inverse the behavior of the pre-buckled beam vibrational behavior. Thus, as one applies an increasing compression load to a prebuckled beam, the natural frequencies decrease, while the modal damping increases from underdamped to overdamped. As this load approaches the buckling load, the beam natural frequency goes to zero, while the modal damping approaches infinity. Once the beam deforms into its post-bucked state, then further increases in the compression loading causes an increase in the beam bending natural frequency and a corresponding decrease in the modal damping from overdamped back to underdamped. The natural frequency and damping of the post-buckled beam approach that of the initially unloaded beam as the beam deformation is fully collapsed. Experimental testing of a compressively loaded graphite/epoxy beam validate these trends for both the pre-buckled and post-buckled conditions. These results are of interest to designers of compressively loaded composite structures that are subject to buckling, for example fuselage and wing skins that are subjected to dynamic loads.