Improving Forming of Aerospace Composite Components through Process Modelling

Abstract: In the aerospace industry there is a constant effort to reduce the weight of aircraft. Since weight reduction has a direct impact on fuel consumption. Reducing the fuel consumption leads to botheconomical benefits through less money spent on fuel and environmental benefits through reduced CO2 emissions. One way that weight savings have been achieved in the last couple of decades is by replacing metals with carbon fiber composites in structural components, where a common choice is unidirectional pre-impregnated (UD prepreg) carbon fiber. Traditionally manufacturing is done by hand lay-up where one ply at a time is laid up on a tool. However the need to make large production volumes feasible has led to a need of automated manufacturing processes. One way to rationalize production is to form the whole laminate at once instead of layer by layer. This is done presently with the single and double diaphragm forming techniques. The challenge with forming of stacked laminates is that the individual plies interact with each other as they conform to the geometry increasing the likelihood of defects to develop. This thesis investigates the effect of forming method and process parameters on the development of manufacturing faults and on the geometry of the finished formed part and studies if these faults can be predicted in numerical simulations. First a method for forming stacked laminates using an industrial robot with methods inspired by human forming techniques is presented. Using this system the effect of different forming sequences on the appearance of wrinkles can be investigated. Forming simulations were done to relate the appearance of wrinkles to ply strains detected in the simulated forming process. The method is used to manufacture joggled spars with a length of 1.4 m and a laminate consisting of 20 plies. Thereafter process simulation of hot drape forming (HDF) is used to determine why wrinkling occurs when plies with specific fiber directions are combined with each other in a stack. This study is supported by an experimental study where plies using two different material systems were mixed in the stack to promote or suppress different types of wrinkles. This leads to the discovery that the wrinkles observed could be divided into two main types: global wrinkles were the whole laminate is under compression due to the geometry, and local wrinkling were wrinkling is initiated by compression of one layer due to interaction with surrounding layers. In the fifth paper the impact of forming method on radius thinning is investigated. By comparing hand lay-up and HDF it is shown that a majority of radius thinning of a laminate can occur already in the forming step if HDF is used. In the last study inter-ply shear of prepreg under a variety of different testing parameters is investigated, including different relative fiber directions between the plies. The study shows that the relative fiber direction is an important parameter to take into account when characterizing inter-ply shear as the force required to shear an interface that has a difference of fiber direction of 0° is significantly higher than the force required to shear interfaces with a difference of 45° and 90°. Taking the difference into account also has a significant impact on the results of forming simulations where models that included the difference in inter-ply shear behavior showed a higher tendency for in-plane wrinkling.