Preliminary Forming Limit Analysis for Advanced

Advanced composite materials are attractive in situations where high performance materials are required, such as in aerospace, military, medical applications and sporting goods industries. However, because they are difficult to shape into complex geometries, currently, most composite parts are manufactured by the hand lay-up process, which is time-consuming and labor-intensive. The main motivation of this research is to explore new techniques which would lead to automation of the manufacturing process. Diaphragm forming proves to be a cost effective way of producing complex shaped parts with aligned-fiber composite materials. The work presented in this thesis is focused on the limits of the process. The main problem in diaphragm forming is laminate wrinkling which occurs under inappropriate processing conditions. Through a large amount of experiments, it is found that the factors which determine the forming limits are: composite material properties, part geometry, diaphragm properties, temperature, forming rate, etc.. Current analyses show that the conformance of laminates to complex geometries is achieved by viscous shearing mechanisms, among which the two most important such modes are in-plane shear, where adjacent fibers slide past one another, and inter-ply shear where plies slide relative to each other. In order to calculate the shear strains required to form a given part, a kinematics analysis has been carried out for a range of different shapes. A three point bending test is employed to understand the constitutive laws governing the deformation. The nonlinear elastic behavior of the diaphragm materials is modeled by using the established bi-axial stress theory of rubbers. The balance between the mechanisms that cause and prevent wrinkling leads to the preliminary forming limit diagrams, which allow us to predict the occurrence of undesirable modes such as laminate wrinkling.