HYGROSCOPIC EFFECT ON THE NATURAL FREQUENCY OF COMPOSITE PLATES

Abstract

The present work deals with the study of effect of moisture absorption on the natural frequency of composite laminated plates. Composite materials are being used in many critical application where they are subjected to adverse environmental condition. As such, a knowledge of the deterioration in the performance of these structures is a subject of great importance. In the present work the composite lami1riate has been--- modelled as a thin plate. Moisture diffusion in the plate is assumed to be governed by Fick"s law. The resultant hygroscopic (moisture induced) strain is used in the formulation of the plate vibration problem. A finite element formulation with three noded plate bending element has been carried out. Results have been presented for simply supported and clamped square plate of T300/5208 graphite-epoxy laminates. Stacking sequences are taken to be (0/+45/90)ZS (0/90)4S and ( +45)4s Variation in lowest natural frequency of the laminate with moisture absorption time has been plotted for different equilibrium moisture concentration\ in the laminate. Results presented in this work indicate that effect of moisture absorption on natural frequency of laminates become significant after about 100 hours of exposure. For low equilibrium moisture concentration a steady state value for the natural. frequency is obtained whereas for higher values of equilibrium" moisture concentration natural frequency drops down to zero, indicating hygroscopio buckling of plate. Laminates with more layers in the directions of plate edges are less sensitive to moisture absorption. From the work presented, it is conclu,a-ed that with moisture diffusion, dynamic performance level of the composite laminate deteriorates and there is a need for accounting the hygroscopic effects in a realistic design of composite structures subjected to adverse environmental condition. (