ISSUES IN DIAPHRAGM FORMING OF CONTINUOUS FIBER REINFORCED THERMOPLASTIC COMPOSITES

A literature survey is presented on sheet-forming methods and mechanisms for continuous fiber reinforced thermoplastics. The diaphragm forming process is shown to be one of the more promising fabrication routes for complex-curvature structures. The primary deformation mechanisms involved in the sheet-forming processes are identified and discussed. Earlier approaches to develop mathematical models either have been kinematically based or have treated only one of the primary deformation mechanisms. A promising constitutive model for the highly anisotropic behavior of the composite at forming temperature is examined. The composite sheet is assumed to behave as a transversely isotropic Newtonian fluid that is both incompressible and inextensible in the fiber direction. The second section of the paper treats the experimental development of the polymeric diaphragm forming process for thermoplastic composites. The viscoelastic properties of the diaphragm material are characterized by dynamic mechanical analysis. The rate sensitivity of the phenomenon of shear-buckling during forming of certain cross-ply and quasi-isotropic composite laminates is investigated, using a shallow female mold. Finally, the interface condition between the diaphragm and the composite during forming is examined.