STIFFNESS OPTIMIZATION OF FIBEROUS COMPOSITE LAMINATES

Abstract

Design of fibrous composite laminates is governed by many parameters that g•ve the designer high flexibility but at the same time represents tremendous difficulty to select the best combination out of these parameters. This includes the fiber and matrix materials, the fiber volume fraction, the ply thickness, the number of plies and the fiber orientation angles. This thesis investigates the best combination of fiber orientation angles that maximizes the stiffness of fiber reinforced composites by using strain-energy based optimization.

The fiber orientation angles were chosen as the only set of variables in maximizing the stiffness of laminates because selection of the remaining design parameters is rather obvious to the designed. In particular, the laminate stiffness is directly proportional to the fiber volume fraction. Consequently, carbon IM fiber­ reinforced epoxy is selected for the fibrous composite laminas since it is known to have the highest stiffness among the available reinforcing fibers. Moreover, the ply stiffness increases as the fiber volume fraction increases. The highest practical volume fraction of 0.6 was selected in the present study. Since the stiffness of a layered medium is expected to increase as the number of layers increase, this number tends to go to infinity if considered as a parameter in the optimization problem under consideration. Therefore, the number of layers is taken as a constant which is determined from other design considerations. In addition, the thickness of a unidirectional ply was assumed constant, such that thick plies of a particular fiber orientation will be represented by multiple layers with identical orientation

The composite laminate consists of six plies each of thickness I mm subjected to a combination of forces and moments. Mori-Tanaka's model was used to compute its overall properties. The overall properties are then used to calculate the ply stiffness matrix of Carbon IM fiber reinforced plastic composite. The classical lamination theory (CLT) uses the overall properties to model deformation of a laminate under membrane forces and bending moments giving strains and curvatures. CLT assumes that the composite plate consists of orthotropic laminae bonded together with a thickness much smaller than the lengths along the plate edges.