A model for the diaphragm forming process

The diaphragm forming process is a potential manufacturing method for advanced thermoplastic matrix composites. This process involves stacking continuous fiber reinforced thermoplastic prepregs in an arbitrary sequence to form a laminate. Polymeric diaphragms are placed on top and bottom of the prepreg layers to hold the material. The diaphragms are larger than the prepreg so that the composite can move freely between the diaphragms. The diaphragms are clamped around their edges. The material is then heated above the melting temperature of the matrix and pressure is applied to force the material onto a one-sided tool. The formability of this process is difficult to predict. Some doubly-curved shapes can be made, while others cannot be formed or always wrinkle.Unlike previous models, which were either based on the kinematic constraint of the fibers or considered one layer only, this model is based on force and moment equilibrium equations and is able to deal with multiple layers with substantial slip between individual layers. In this model incremental large deformation of arbitrary shapes with double curvature is assumed. For the thermoplastic material in the molten state, a Newtonian fluid behavior is assumed. The composite material is highly anisotropic. Its behavior depends on the fiber orientation of the individual composite layer. Ericksen's transversely isotropic fluid model is used for the constitutive law of the composite. Isotropic, rate-dependent material behavior is assumed for the diaphragms. The global mechanical behavior of each individual layer is assumed to be a general shell.Each layer is modeled as a separate shell, with the kinematic assumption is that the adjacent points of two separate layers have the same normal velocity. Shear force due to interlayer slip is assumed to be proportional to the relative tangential velocities of the layers. The squeezing behavior of the composite layers is important, so transverse stress in the direction normal to the membrane surface is retained in the governing equations. Thus a set of differential equations are formed that govern the forming process. Special boundary conditions are used to treat the points where layers are not continuous. Also a fill-factor function is introduced to capture the movement of layer edges during processing.The finite difference method is used for the computation. A two-dimensional computation is done to demonstrate the feasibility of this model. With this model we are able to see the deformed shape, stress distribution, thickness change and relations between parameters like forming load, forming rate and geometry.U of I OnlyETDs are only available to UIUC Users without author permissio