The quality of continuous fiber reinforced polymer matrix composite parts and structures depends strongly on the friction during the composite forming process. The two major types of friction that cause deformations during this process are ply-ply friction and tool-ply friction. One of the challenges in the composite forming process is the occurrence of wrinkling and shape distortion of the fabric caused by the surface differences between the forming tool and surface of the laminate. Frictional measurements of composites can vary widely depending on processing parameters, measurement technique, and instruments used. In this study, a commercial rheometer was used to evaluate tool-ply friction of unidirectional carbon fiber epoxy prepreg at various contact pressures, temperatures and sliding velocities. Viscoelastic properties such as the complex viscosity (η*), storage modulus (G'), loss modulus (G"), and loss factor (tan δ) were used to determine the critical transition events (such as gelation) during cure. An understanding of changes in viscoelastic properties as a function of time, temperature, and cure provides insight for establishing a suitable processing range for compression forming of prepreg systems. Surface imaging results were coupled with rheological results to qualitatively examine the effects of processing parameters on prepreg distortions. Changes in gap height over the measurement interval qualitatively describe the changes in contact area and contact mechanisms between the tool-ply surfaces. The results indicate that friction behavior of the prepreg system is a contribution of adhesive and frictional forces, where increase in viscosity, reduction in gap height, and cure of the sample correlate to higher friction values. / Master of Science / The quality of composite parts and structures depends strongly on the friction present during the composite forming process. One of the major challenges in the forming process is the occurrence of wrinkling and shape distortions of the fabric caused by the surface differences between the forming tool and material. The presence of these defects can compromise the final material property and lead to failure when in use. Frictional measurements of composites can vary widely depending on processing parameters, measurement technique, and instruments used. The extent of interaction between the tool and surface of the material depends on the tooling height, and by extension, contact area, which cannot easily be monitored with traditional test designs. A commercial rheometer was used in this study to evaluate tool-ply friction of unidirectional carbon fiber epoxy prepreg at various contact pressures, temperatures, and sliding velocities. Gap height and torque were monitored to provide information on the frictional dependence of processing parameters. In addition, surface-imaging results were coupled with rheological results to examine the relationship between friction and fiber distortions. The understanding of changes in material property with respect to the tooling process is the key to optimizing the composite forming process.

Understanding the interactions of polymer interfaces is essential to improve polymer-based designs, as the properties of the interface are often different than those of the bulk material. This thesis explores the interfacial interactions of polymer interfaces for two classes of materials, additive manufacturing materials and fiber-reinforced composites. Additive manufacturing (AM) refers to a number of processes which rely on data generated from computer-aided design (CAD) programs to construct components by adding material in a layer-by-layer fashion. AM continues to generate a substantial amount of interest to produce fully functional products while reducing tooling costs associated with traditional manufacturing techniques such as casting and welding. Recent advancements in the field have led to the production of multi-material printing that has the potential to create products with enhanced mechanical properties and additional functionality. This thesis attempts to characterize the fracture resistance of AM materials produced by the PolyJet process. Test standards established for mode I fracture testing of adhesive joints are adapted to evaluate the fracture resistance and interface between two printed acrylic-based photopolymers. Significant differences in fracture energy and loci of failure between the selected test configurations were observed depending on the print orientation. Failures were nominally seen to occur at the interface, alternating from one adherend interface to another in a random fashion. Results demonstrated a decreasing trend in fracture energy at slower crack propagation rates, indicating that such dependency is associated with the fracture resistance of the interface. T-peel tests conducted on specimens prepared with both constant and graded interlayers revealed enhanced peel resistance with gradient interlayers, suggesting design opportunities of enhanced fracture toughness by implementing intricate material patterns at the interface of the two photopolymers. Fiber reinforced composite (FRCs) materials have become increasingly desirable in a number of industrial applications where weight reduction is critical for increased payloads and higher performance. When manufacturing structures from these materials, the presence of friction in the composite forming process is seen to have a major effect on the finished quality. Friction between the plies, or between the composite laminate and forming tool, can be undesirable as shape distortions such as wrinkles can appear and compromise the structural integrity of the finished product. To evaluate these frictional processes, a standard rheometer is used to evaluate tool-ply friction on dry textile fabrics and graphite/epoxy prepregs over a range of temperatures, pressures, and sliding velocities. The results provide some general insights into the frictional response of composite prepregs as a function of the manufacturing environment. The materials tested are shown to have different mechanisms that govern the frictional processes. In particular, the results of friction testing on the prepreg indicate that friction comes from a contribution of both Coulomb and viscous-related mechanisms, the latter which become especially at higher temperatures. / Master of Science