Process development and technological advances in double diaphragm forming of advanced and uniform short fiber composites using fixed and reconfigurable tooling.

摘要

This thesis discusses research related to advances in automated composites forming by (1) developing process and simulation capabilities for forming composite parts incrementally with a pin-based reconfigurable tool using Double Diaphragm Forming (DDF) and (2) investigating the formability and analytical modeling of a new Stretch Broken Carbon Fiber (SBCF) material from Hexcel Corporation.; DDF is a method where a flat, uncured composite laminate is sandwiched between two compliant diaphragms, heated to below its curing temperature, and then vacuum formed over a part mold. The reconfigurable tool used in this research is comprised of 96 individually controlled pins that can be reconfigured into an infinite number of mold shapes---subject to tool resolution limits---incrementally in real time. When they are coupled together, the reconfigurable tool and DDF technology form a reconfigurable forming system that can be used for rapid prototyping or low production runs of parts that are not formable through any other means. The benefits of this system are reduced cost of designing, building and storing primary molds for composite part forming and, more importantly, that the system has the ability to vary the strain path on the composite lay-up by changing the shape of the mold during the forming process in real time. No other forming system known has this ability.; The SBCF material used in this research is a continuous carbon fiber material whose fibers have been intentionally broken to give it extensibility. Tests show that the SBCF material retains up to 80% of the cured tensile strength and 60% of the cured compressive strength of the unbroken material. In its uncured state the SBCF material exhibits axial tensile behavior of relaxation, creep, test length (i.e. gauge length) dependence, and softening.; Since the original reconfigurable tool was initially equipped with single diaphragm forming, a new DDF retrofit was designed and built for the tool. Single and multiple zone heating blankets were designed and integrated into the DDF setup, and the process of forming with the tool has been developed to the point where it is almost production ready.; Forming trials using lay-ups made with the SBCF and continuous materials were performed over fixed and reconfigurable tools using DDF. The forming results revealed that double convex curvature shapes form better over the reconfigurable tool and combined double convex/concave shapes form better over a fixed tool. It was also shown that using the reconfigurable tool's ability to vary the strain path during forming increases the range of parts that can be formed with the system. Finally, using a zonal heating blanket that was designed for the DDF process had a positive effect on suppression of wrinkles during forming.; A comparison between parts formed using continuous and SBCF material was inconclusive. The tensile forces required to stretch the SBCF material cannot be generated using the current DDF system unless the material is continuously clamped around its edge. The type and severity of wrinkles in the parts were in many cases identical but in certain circumstances, the SBCF material defects were more severe.; Finite element simulation capabilities for forming SBCF and continuous composite materials have only been partially realized. The ability to simulate shape change from the reconfigurable tool and DDF has been demonstrated, but long processing time and lack of an adequate material model are currently limiting factors. Off-the-shelf material models proved to be inadequate for modeling the SBCF material, so a representative volume model is being developed to use as a user-defined material model.