Investigations of the Mechanical Behavior of Graphite/PEEK (APC-2) Composits

The purpose of this research is to investigate the effects of two phenomena that are specific to thermoplastic resin composites. The phenomena are:

(a) the significant nonlinear creep, that occurs especially at high temperatures, which affects the residual thermal stresses in a geometrically constrained structure.

(b) the spatially nonuniform transverse crystallinity which develops in some fiber-reinforced thermoplastic composites, which introduces spatial variability and nonlinearity in the stiffness of the composite.

The first part of the dissertation is related to the former phenomenon. It presents a computational scheme for the optimal temperature path for APC-2 composite laminate, modeled as nonlinear, thermorheologically complex, viscoelastic material. The laminate is assumed to be sufficiently this to obviate the accounting for temperature nonuniformities across the thickness. An iterative, computational scheme is developed for the particular case of a cross-ply lay-ups laminate, which can be generalized to other symmetric lay-up with minor modifications. As prototype cases which can provide the guidance, as well as verification checks, for the iterative numerical scheme, analytic solutions are developed for two idealized relaxation models.

The second part of the dissertation, which concerns the latter phenomenon, presents analytical/computational results for the inplane stresses in a moderately thick, cross-ply, graphite/PEEK (APC-2) laminate accounting for effects of nonuniform PEEK crystallinity across the plate's thickness. These effects are incorporated as crystallinity-induced variations in the longitudinal modulus E_L, including its nonlinear dependence on strain.